Ventral and Lateral Thoracic and Lumbar Fixation Techniques

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Chapter 146 Ventral and Lateral Thoracic and Lumbar Fixation Techniques

Daniel C. Lu, Gerald E. Rodts, Jr., Regis W. Haid, Jr., Kevin T. Foley, Praveen V. Mummaneni

Surgery on the ventral thoracic and lumbar spine began nearly 100 years ago. Ventral approaches for decompression of spinal pathology were first attempted in the early 1900s. Pioneers such as Royle ¹ excised hemivertebrae for the treatment of scoliosis. Ito² as well as Hodgson and Stock³ refined the ventral (transperitoneal) approach to the thoracolumbar spine for the treatment of Pott disease. These early efforts to decompress ventral spinal pathology were frequently complicated by postoperative mechanical instability and progressive deformity.

The first reports of ventral instrumentation of the spine were from Humphries,⁴ who developed ventral interbody fusion with ventral plates and unicortical screws. These devices provided little biomechanical advantage. Most of these cases were transperitoneal approaches for debridement and stabilization in patients with Pott disease. The transperitoneal approach was eventually replaced by the retroperitoneal or extracavitary approaches for lesions of the lower thoracic and lumbar spine.

Throughout the 1970s, the preferred treatment for traumatic injuries was dorsal fusion and instrumentation, combined with ventral decompression and fusion. The Dwyer ⁵^–⁷ and Zielke⁸ devices were developed as ventrolateral implants that could augment or replace dorsal instrumentation. These consisted of screws that traversed the vertebral body that were interconnected with cable (Dwyer) or threaded rods (Zielke) that could be tightened in tension. These devices had limited ability to fixate two-column traumatic injuries. The Dunn device (developed in the late 1970s) represented a more rigid instrument for use in burst fractures. This double-screw, double-threaded rod device provided excellent strength but was removed from the market in 1986 after reports of great vessel erosion and rupture.⁹

It was not until the 1990s that numerous plate-screw and screw-rod systems were developed. Among the first were the Kaneda, Kostuik-Harrington, I-plate, and University plate systems. Later products included the anterior locking plate, the Z-plate, the Texas Scottish Rite Hospital system, the M-8 dual-rod system, the Expedium Anterior system, and others. Newer systems were developed for lower-profile, rigid ventral fixation of the spine. These systems have the added advantage of easier means of distraction and compression at the moment of plate or rod fixation. Furthermore, shorter segment fusions became more feasible. Some of the early generation ventral plating systems did not provide for a fully rigid screw-plate interface. Bicortical screw purchase was important but did not eliminate the possibility of screw toggle and possible mechanical failure in the early systems. However, most systems on the market by 2003 provided for a rigid screw-plate or screw-rod interface. To analyze the various systems critically, an appreciation of the biomechanics of the thoracolumbar spine is important. The reader is directed to other chapters outlining more detailed biomechanical information. It is also essential to understand some of the general indications for ventral instrumentation and fusion.

Biomechanical Considerations

A complex discussion of ventral thoracolumbar instrumentation is beyond the scope of this chapter; however, recognition of several basic concepts is essential to avoid complication. The anterior and middle columns provide the most resistance to axial loads. The ventral approach to spinal pathology has the distinct advantage of allowing for reconstruction of the vertebral body and intervertebral space with autologous or allogeneic bone or synthetic materials (e.g., methylmethacrylate, ceramic). The dorsal tension band—normally provided by the interspinous ligaments, ligamentum flavum, paraspinal musculature, and facet joints—must not be severely damaged if a ventral construct alone is to provide stability, because these devices cannot effectively withstand tremendous flexion forces. Complications arise when a posterior column injury has gone unrecognized or when too much is expected of a ventral construct. Plate and screw constructs can provide resistance against axial load, distraction, and extension. These implants have a higher success rate when axial loads are shared by a sturdy bone graft and the implant.

Certain biomechanical characteristics of implant systems are important to understand. Rigid implants (e.g., Z-plate, Kaneda systems, Expedium Anterior system, and M-8 dual-rod system) theoretically allow for greater immobilization of the spine. If the implant bears most of the stress, there is a risk of implant breakage or failure. Some plate systems have set screw holes rather than slots, thereby creating a static (nondynamic) condition beginning at the time of plate fixation. Also, stress shielding provided by the rigid fixation may prevent the beneficial compressive forces from enhancing bone fusion (Wolff’s law). Because bone is a biologic, deformable material, repeated stress loading may cause bony erosion and failure at the metallic implant-bone interface.

Indications for Ventrolateral Instrumentation

A variety of infectious, neoplastic, congenital, and traumatic pathologic conditions are suitable for ventral thoracic or lumbar surgery. An initial step in complication avoidance is to determine whether a ventral approach truly provides the safest and most efficacious means of decompression of the neural structures, reconstruction of the anterior and middle column, application of corrective forces for realignment, and placement of an appropriate graft or spacer-implant construct. Conditions in which dorsal neural compression or posterior column bony/ligamentous damage are the predominant findings are best treated by a dorsal approach. Similarly, lesions with three-column pathology may possibly require circumferential treatment.

Anterior and middle column trauma (with preservation of the dorsal elements) may be treated adequately with a ventral approach (Fig. 146-1). Failure to recognize significant posterior column injury may result in delayed kyphotic deformity and neurologic deterioration. There are few clinical outcome data to encourage ventral decompression of trauma patients with complete neurologic loss below the level of the lesion. Ventral approaches, however, may be useful in paraplegic patients with a severe kyphotic deformity. Anterior reconstruction may provide better sagittal balance that could be important for long-term pulmonary function, independent transferring, and upper extremity function.

FIGURE 146-1 Thoracic burst fracture with retropulsed bone fragment and incomplete spinal cord injury suitable for anterior decompression.

Patients with incomplete spinal cord injury, severe vertebral body collapse (≥40%) and kyphosis, and/or significant spinal canal compromise should be considered for ventral decompression and reconstruction. Intact patients with a myelographic block or ventral compression of the spinal cord on MRI may be considered for ventral decompression and stabilization. If the posterior longitudinal ligament (PLL) is intact on MRI and there is 30% or less loss in height of the anterior and middle column, a dorsal approach with reduction by ligamentotaxis could be considered in the intact or incomplete patient with a burst fracture.

Infection is another indication for ventral decompression, reconstruction, and instrumentation. The primary indication would be severe deformity of the spine, because most spinal infections can be treated without ventral instrumentation. Early ventral approaches for the treatment of Pott disease have been modified and are still very useful in debridement and stabilization of pyogenic, mycobacterial, or fungal infections. Reconstruction with autologous or allogeneic bone is feasible if a complete debridement of all necrotic tissue is accomplished. The risk of persistent infection or implant failure with instrumentation of infected cases is low if a prolonged course of antibiotics is given to these patients.

Metastatic neoplasms commonly affect the vertebral body before the posterior elements and therefore can be palliated with vertebrectomy. The cell kinetics of any malignancy, if known before surgery or determined by frozen section, must be considered when deciding on the material for reconstruction of the axial spine. In cases of a rapidly dividing carcinoma, synthetic spacers such as vertical titanium cages, methylmethacrylate, polyether ether, or carbon fiber cages can provide immediate stability of the vertebral column in patients with a short life expectancy. In more indolent neoplasms, such as breast or prostate cancer, longer survival can be expected, and autogenous or allogeneic bone can be expected to incorporate, thereby avoiding complications such as pseudarthrosis or implant migration. Use of titanium and other nonferromagnetic implants allows for long-term follow-up with MRI.

Treatment of other conditions, such as congenital or developmental scoliosis, iatrogenic lumbar kyphosis (flat back syndrome), and degenerative lumbar scoliosis may involve a ventral approach. Ventral procedures in adult scoliosis with curves greater than 40 to 50 degrees are associated with a higher rate of fusion than dorsal constructs alone. Ventral fusion and instrumentation may also be useful in patients with deficient laminae, facet joints, or pars interarticulari or extremely severe scarring from prior dorsal surgery.

Inadequate radiographic studies before surgery can lead to intraoperative or postoperative complications. Plain radiographs are essential and should include flexion and extension views when there is any suspicion of mechanical instability. In addition, attention should be paid to the density of bone as well as the sagittal and coronal alignment. Patients with scoliosis should have complete 36-inch standing films to assess overall spinal balance. The value of sagittal reconstruction of CT images is often overlooked, particularly after myelographic dye injection. Axial CT may be preferable over MRI to determine the amount of bony spinal canal compromise in trauma. Sagittal MRI often provides excellent views of the PLL. If intact, one may consider use of ligamentotaxis to reduce a burst fracture fragment. MRI has the added advantage of showing signs of soft dorsal tissue injury and hematoma that would commonly go unrecognized with plain radiographs and CT scan alone. Although cost-effectiveness is a primary concern, any patient with complex spinal pathology (and for whom aggressive surgery is contemplated) may require both CT and MRI as part of the workup.

Ventral Surgical Techniques

Positioning

Complication avoidance in the operating room begins with the simplest of steps. In positioning all patients, foam rubber padding is placed over ankles and elbows. For the prone position, gel pads are placed over the supports of the Wilson frame or chest rolls. The knees are flexed 45 to 90 degrees. Electrocardiography electrodes must not be on areas of the chest or trunk that contact the frame or rolls to prevent pressure necrosis. In females, the breasts must be tucked medially between the supports. Pillows are placed under the feet to provide knee flexion and relaxation of the sciatic nerve. Foam rubber rings are commonly used by anesthesiologists to protect the face and eyes, but care must be taken not to place the patient’s neck in too much extension when in the prone position, particularly in one with diffuse spondylosis. It is necessary to double-check the eyes to ensure that there is no direct pressure on the globe. If the arms are not tucked at the patient’s side but raised above the head, one should neither abduct the shoulders more than 90 degrees nor flex the elbow more than 90 degrees to prevent postoperative shoulder or elbow pain or even peripheral nerve ischemia.

Complications arising from the lateral decubitus position can also be averted with due diligence. We have all placed patients on a bean bag, but the bag must not extend into the axilla of the down arm. A roll (a liter bag of IV solution wrapped in a towel suffices) is placed above the edge of the bean bag just below the axilla. The peroneal nerve in the down leg must be protected with foam and/or gel padding over the fibular head. A pillow is placed between the legs, which are flexed 45 degrees at the hip and knees. The coronal plane of the patient’s thorax must be perpendicular to the floor. Wide tape should be used to secure this position to allow rotation of the bed along its long axis (“airplaning”). Establishing this position assists the surgeon in remaining oriented throughout the procedure, especially during the critical steps of decompressing the spinal cord or placing a vertebral body screw. Some tables are equipped with a compass so that the desired neutral position can be recorded and reset by the anesthesiologist if an “airplane” maneuver is necessary. The perpendicular orientation of the coronal patient plane relative to the floor also allows for more efficient manual reduction of a kyphotic deformity by pressing on the back. The authors routinely “break” or flex the table at the level of the pathology to help open the disc spaces laterally and aid in the insertion of the bone graft (Fig. 146-2). Flexing the table also helps open the space between the 12th rib and the iliac crest. Once the bone graft is in place, the anesthesiologist is asked to return the table to the neutral, unflexed position.

FIGURE 146-2 Lateral position for thoracoabdominal or thoracotomy approach with exposed iliac crest harvest site. (Note flexion of table at surgical site.)

We routinely administer suitable gram-positive antibiotic coverage (e.g., cefazolin 1 g or nafcillin 1 g). In cases of traumatic cord contusion or cord compression caused by tumor, we consider the use of methylprednisolone at least 1 hour before surgery. Using the spinal cord contusion protocol, the patient may receive a bolus of 30 mg/kg over 45 minutes followed by continuous infusion of 5.4 mg/kg per hour for 23 hours.^10,¹¹

Paralytic agents are not used after induction to allow for motor response in the event of inadvertent nerve or spinal cord stimulation. The role of somatosensory-evoked potential (SSEP) monitoring is debatable. A decrease in SSEP amplitude of more than 50% and limited or absent intraoperative recovery of amplitude are predictors of a postoperative neurologic deficit.^12,¹³ Despite this reasonable sensitivity and low-false negative rate, SSEP monitoring measures only dorsal column function. False positives are common and often related to anesthetic considerations that can lead to a dangerous desensitization of the surgeon to warnings of intraoperative injury. SSEPs may be useful in deformity cases in which distractive or compressive forces are anticipated and could be reversed.

Motor-evoked potentials may be more accurate than SSEPs in monitoring spinal cord motor function during surgery.¹⁴ This technique is extremely sensitive to anesthetics and requires expertise on the part of the anesthesiologist and monitoring team.

Approach and Exposure

The thoracic spine can be approached ventrally by the transmanubrial-transsternal approach, conventional thoracotomy, or thoracoscopic approach. The lumbar spine can be approached by the thoracoabdominal approach, transperitoneal approach, retroperitoneal approach, laparotomy, laparoscopy, balloon-assisted retroperitoneal endoscopy, or low pelvic approaches. These surgical approaches may be performed by the cardiothoracic, general, or vascular surgeon or by the spine surgeon. Detailed preoperative and intraoperative communication about the approach with an approach nonspine surgeon (if used) is important to ensure that not only is the pathologic level exposed but also that the exposure allows the spine surgeon to place instruments perpendicular to the axis of the spine for reconstructive and fixation techniques. Limited exposure may force a screw trajectory in an unsatisfactory cephalad or caudal direction.

Upper Thoracic Spine

Ventral exposure of the rostral levels of the thoracic spine is challenging. The first and second thoracic vertebrae usually can be approached ventrally with a low diagonal or transverse cervical incision. A vertical split of the manubrium often allows exposure down to T3 without sacrificing significant bone. A preoperative sagittal MRI should be obtained and inspected to ensure that the aortic arch does not block ventral access to the T2-3 area. Furthermore, one must be cognizant of the course of the recurrent laryngeal nerve as it emerges dorsal to the brachiocephalic arch to pass between the esophagus and trachea. Although its course is more constant on the left side, low-lying incisions to approach T1 and T2 on the left side put the thoracic duct at risk. This structure is intimately related to the subclavian vein off midline on the left and must be protected. Unrecognized pneumothorax is a complication of this approach because the pleura overlying the medial aspect of the cupola of the lung is adjacent to the spine. Filling the wound with saline and performing a positive pressure inspiration (Valsalva maneuver) at the close of the case is an essential step during closure. An oscillating saw or Gigli saw can be used to remove larger portions of the manubrium, but the retromanubrial space must be palpated to ensure that the brachiocephalic trunk is free. With the patient in the supine position, the upper thoracic spine slopes away from the surgeon, beginning at the T1-2 interspace. It can be difficult to place a ventral plate and screws in this region without a more aggressive removal or splitting of the manubrium or sternum.

Instead of sternotomy, lesions affecting the caudal aspect of T2 to T5 may be approached by a right-sided thoracotomy. The right-sided approach to the upper thoracic spine avoids the aortic arch. One must be cautious, however, to avoid injury to the superior vena cava and supreme intercostal vein. We have found instrumenting the T3-5 area to be easier with a high, right-sided thoracotomy than a midline sternotomy. This experience is particularly true with severe kyphosis (e.g., Scheuermann kyphosis) in this region.

The transaxillary approach is familiar to most vascular surgeons and can be considered for lesions affecting the upper thoracic levels. This approach, however, offers a limited exposure at the base of a cone-shaped cavity and should be reserved for small, more ventrolateral lesions not affecting the entire vertebral body and not requiring (complete) corpectomy or when only open biopsy is necessary. Ventrolateral instrumentation is very difficult because of the limited exposure. The transaxillary approach has associated risks to the lower brachial plexus, long thoracic nerve, and thoracodorsal nerve as well as to vascular structures in the axilla. Splitting of the pectoralis major muscle can also be a source of significant morbidity.

The ventral upper thoracic spine can also be accessed via a third-rib approach in which the patient is positioned in the lateral position with the arm elevated on a rest. The right side is preferable because of the straight course of the brachiocephalic artery. A curved incision is made beginning below the caudal angle of the scapula and ending between the medial scapula and the spinous process of C7. The trapezius and latissimus dorsi muscles are divided medially to minimize denervation, and the scapula is retracted laterally. The dorsal 10 cm of the 2nd, 3rd, and 4th ribs are resected, and the segmental vessels are ligated. The dorsal 3 cm of the 1st rib can also be dissected for additional exposure, but care must be taken not to injure the T1 motor root. The pleura and upper mediastinal structures can then be bluntly dissected for access to the vertebral bodies. Deflation of the lung with a double-lumen endotracheal tube can be very helpful. This approach requires tube thoracostomy placement at the end of the procedure because the parietal pleura is opened and the lung is exposed.

Midthoracic Spine

Lesions involving the midthoracic region are best approached via thoracotomy. Thoracic surgeons are very experienced with this approach. It is recommended that a thoracic surgeon perform the thoracotomy if the spine surgeon is not familiar with this approach. The patient is placed in the lateral decubitus position on a bean bag. The bean bag should not be higher than just below the axilla, and an IV bag or other suitable axillary pad should be placed to protect the brachial plexus and vessels. The area of break in the table should be determined before final positioning so that the desired thoracic level can be placed directly over this area to assist in exposure, opening of the disc spaces, and placement of the graft. Pillows may be placed under the down leg to protect the peroneal nerve and between the legs.

The left side is almost always used for the approach because it is safer and easier to visualize, dissect, and mobilize the aorta and segmental vessels than the vena cava or azygous venous system. It is easier to repair an injured aorta than the vena cava. One should consider obtaining a preoperative axial CT or MRI to assess the location of the aorta. If the aorta is lying very far lateral to the left (Fig. 146-3) or if the pathology is strictly right sided, a right-sided approach can be performed. A standard thoracotomy incision is used beginning approximately two fingerbreadths below the angle of the scapula and coursing ventrally to the midaxillary line. One should select the intercostal space directly over the level of pathology to enter the pleural cavity. We have had satisfactory experience in performing a retropleural thoracotomy. In this procedure, the surgeon separates the endothoracic fascia from the parietal pleura, and the dissection is made down to the rib heads and spine extrapleurally. This is technically more difficult but can obtain a transthoracic approach without the need for a postoperative chest tube. A postoperative radiograph in the recovery room is essential to rule out a significant undetected pneumothorax.

FIGURE 146-3 A, Laterally located aorta in which a right-sided approach was chosen. B, Sagittal view of the same metastatic tumor. C, Postoperative radiograph of this case instrumented with a Z-plate and humerus allograft packed with autologous cancellous bone harvested from the iliac crest.

In the lower thoracic spine, this usually corresponds to the rib two numbered levels rostral to the desired vertebral body. For instance, a T8 lesion usually corresponds to the horizontal segment of the 6th rib. Commonly, the rib need not be resected unless it is being harvested for bone graft or if unusually lengthy exposure of the spine is needed. Once the lung is deflated via a double-lumen endotracheal tube and the viscera are packed away with moist towels, the ribs are counted from inside the thoracic cavity. The rib identified as at the same level as the pathology is then exposed in a subpleural fashion down to its insertion on the pedicle. The segmental vessels are identified by blunt or scissor dissection in the midportion of each body. The disc spaces represent the “hills,” and the midvertebral section (where the vessels are located) are the “valleys.” The vessels are ligated with silk suture or metal clips in approximately the midbody. Taking the vessels too close to the aorta risks avulsion during this procedure. Conversely, sacrificing the vessels too close to the neural foramen may interfere with the collateral circulation of the spinal cord. Ligation of the vessels should be performed over the lateral aspect of the vertebral body between the aortic branch point and the neural foramen. Most surgeons and the scoliosis literature agree that up to three adjacent segmental arteries may be taken without neurologic sequelae, but the importance of the artery of Adamkiewicz (T10-L2) is still debated. Some surgeons advocate a preoperative spinal angiogram. Once the vessels are ligated and transected, a subperiosteal dissection of the vertebral body is carried out by using an elevator and unipolar cautery. The anterior longitudinal ligament (ALL) is elevated or incised if ventral release is necessary. If left intact during the exposure, the ALL can serve as a tissue barrier between the aorta and the operative site during the procedure. Some surgeons elevate the ALL sharply or use monopolar cautery from the bone and use that potential space to hold a malleable retractor for added safety.

In anticipation of instrumentation to the levels above and below the pathology, the segmental vessels should be taken here as well. Once this step is complete, a periosteal elevator or monopolar cautery is used to complete a subperiosteal exposure of the diseased level and other levels needed for instrumentation. Thus, the rostral end plate and disc space of the level above and the inferior end plate and disc space of the level below the pathologic levels must be clearly and completely visualized. Ventrally, the exposure is limited by the aorta, but with careful mobilization and retraction (i.e., with large malleable retractors) the cortical bone and disc can be dissected close to (but just short of) the midline. It is critical that a thorough exposure be completed dorsally. The dorsal 2 to 3 cm of each rib (level) involved must be removed with a 1⁄2- or 3⁄4-inch osteotome. Rongeurs or a drill may also be useful. Once the heads of the ribs are disarticulated and removed, the pedicles at each level are exposed and the dorsolateral edge of the vertebral body is confirmed by palpation with a Penfield no. 4. Identification of the pedicle and dorsal vertebral body is essential for recognizing the location of the spinal canal and is necessary for safe and accurate identification of landmarks for placement of instrumentation. Frequently, there is a large mass of soft tissue, including the ligated ends of the segmental vessels, that has been swept into the area of the foramen. One should not attempt to cut away or use the monopolar to cauterize this tissue; the patent segments of the vessels often cause annoying bleeding. Shrinking the tissue near the foramen with the bipolar cautery and then placing a single silk suture through the cauterized mass and sewing it in traction to rib periosteum is often useful. This assists in identifying the spinal canal by moving this tissue in a more dorsal direction. Decompression should not be attempted until the limits of the spinal canal are clearly visualized. Catastrophic neurologic injury may result from initially not identifying accurately the borders of the pedicle, foramen, and dorsal vertebral body (i.e., the spinal canal).

Thoracolumbar Junction

For lesions affecting T10 through the upper lumbar spine (L1), a combined thoracoabdominal approach is necessary (Fig. 146-4). This may be true for lesions at L2 that require exposure to the T12-L1 disc space for instrumentation. During a standard thoracotomy, the patient is positioned in the right lateral decubitus position with a bend in the table to facilitate exposure. A double-lumen endotracheal tube is used for ipsilateral lung deflation. The incision is commonly made over the 10th rib and carried from the anterior axillary line to the posterior axillary line and extended as needed. The oblique and transversus abdominis muscles are incised, but care should be taken not to enter the peritoneal cavity. A subperiosteal dissection of the rib allows for efficient resection of the rib. The thoracic cavity is entered via the 10th or 11th rib space, and the diaphragm is immediately identified. The parietal pleura and peritoneal sac are bluntly mobilized by using finger- and sponge-stick dissection. Avoiding the monopolar cautery for most of this stage can prevent inadvertent entry into the peritoneal cavity, lung, or abdominal viscera. The diaphragm is mobilized from its peripheral attachment to the 11th rib. A 2- to 3-cm cuff of diaphragmatic tissue is maintained to allow for reapproximation during closure. The spinal attachments of the diaphragm are taken down sharply or with monopolar cautery. The medial attachment of the lateral arcuate ligament and the lateral attachment of the medial arcuate ligament are divided close to the tip of the transverse process of L1. The crus of the diaphragm is divided 2 to 3 cm away from the vertebral body and should be tagged. At this point, large self-retaining chest retractors can be placed to displace the peritoneal contents and diaphragm. Vessels that require sacrifice should be taken as close to the aorta or vena cava as possible to allow for maximal mobilization of these structures. Coagulation near the neuroforamen should be avoided to decrease the risk of compromising important radicular feeders to the spinal cord. The psoas muscle can be sharply dissected with periosteal elevators or monopolar cautery back to the attachments to the pedicle to maximize exposure of the lumbar vertebral bodies. Gentle retraction can allow exposure from T9 through L3.

FIGURE 146-4 A, T12 burst fracture resulting in incomplete spinal cord injury. B, Decompression and reconstruction was performed with a Synex I expandable cage (Synthes Spine, Paoli, PA) filled with rib autograft and supplemented with the Expedium Anterior Spine (screw-rod) system (DePuy Spine, Raynham, MA) placed via a thoracoabdominal lateral approach. Further fixation was performed with a posterior percutaneous screw fixation (VIPER, DePuy Spine). C, Postoperative lateral radiograph demonstrates restoration of spinal alignment.

Although identification of severe fractures or tumor pathology is often easy, localization for less-obvious lesions at the thoracolumbar junction can be difficult. Although usually accurate, counting the ribs should not be relied on to identify the level. Plain radiographs are recommended and should be repeated with different orientation until the desired levels are confidently identified. With the patient in the lateral decubitus position, cross-table anteroposterior and lateral radiographs often are sufficient for accurate localization of the appropriate level.

Retroperitoneal Approach

The retroperitoneal approach is useful for lesions extending from the inferior surface of L1 to the superior surface of L5. It is necessary to keep in mind that instrumentation and fusion for pathology that actually extends to either the rostral or caudal limits of this exposure requires longer exposure that may not be provided by the retroperitoneal approach alone. As well, the iliac crest prevents true lateral access to the L5 vertebral body for transverse screw placement. To expose the vertebral body of L1 fully, dissection of some of the crural attachments may be necessary. A pneumothorax is a potential complication. At the caudal end, the L5-S1 interspace can be very difficult to expose fully (particularly in large male patients) because of the bulk of the psoas muscle.

Administration of cathartic agents before surgery and placement of a nasogastric tube during induction of anesthesia may facilitate easier retraction of the peritoneal contents during the case. Positioning for the retroperitoneal approach is similar to that for the thoracoabdominal exposure: the patient is in the lateral position with a break in the table at the level of the pathology. Upper lumbar exposure may require resection of the 12th rib. The incision is typically made from the lateral margin of the dorsal longitudinal paraspinal muscles (e.g., iliocostalis, sacrospinalis) and extends ventrally to the lateral border of the rectus abdominis muscles. The external and internal oblique and transversus abdominis muscles are divided with monopolar cautery. Clamp dissection and elevation of these muscles before incision can avoid entry into the peritoneal cavity. Blunt dissection with a sponge-on-a-stick can mobilize the peritoneum away from the spine. Great care must be taken to avoid damage to the ureter, although it is usually safely reflected ventrally with the peritoneal contents. Large, self-retaining table-mounted retractors (e.g., Omni, Thompson-Farley) are used over moist laparotomy sponges to maintain exposure. The transverse processes are palpable through the psoas muscle. This muscle can be dissected from its periosteal attachments by using monopolar cautery. This bovie technique results in less blood loss than a Cobb or periosteal elevator. Vigorous attempts at retraction with a Meyerding or similar retractor may cause laceration of the muscle and excessive bleeding. Careful “toeing-in” of the Meyerding or McElroy retractor is all that is usually necessary to put the psoas on stretch and facilitate subperiosteal exposure. Too much stretch, however, may cause postoperative psoas or iliopsoas weakness and pain. Mobilization of this muscle dorsally to the pedicle allows palpation and visualization of the ventral border of the spinal canal. Key points regarding closure of the retroperitoneal exposure include meticulous closure of the abdominal wall muscular layers to prevent hernia formation. We recommend leaving a large Hemovac drain in the retroperitoneal space for 24 to 48 hours, especially when decortication or resection of vertebral bone causes significant oozing of blood still seen at the time of closure. Another potential complication of this approach is intestinal ileus. Nasogastric suction is continued postoperatively until bowel activity is confirmed.

Transabdominal (Transperitoneal) Approach

Ventral decompression, reconstruction, and fixation of the lower lumbar spine and lumbosacral junction via the transperitoneal approach (open or laparoscopic) are possible.¹⁵^–¹⁷ Threaded interbody titanium cages, bone dowels, and other synthetic implants are available for interbody fusion and/or fixation. Typical thoracolumbar bone screws, plates, and rods are difficult to insert from this approach and have a high profile near vascular structures. The exposure tends to be triangular in shape because the field of view limited by the bifurcation of the iliac vessels and the L5-S1 disc space. Direct ventral screw fixation (e.g., buttress screw) of the upper sacrum is possible after a subperiosteal exposure of S1 immediately caudal to the disc space is performed. Use of Steinmann pins or a table-mounted vascular retractor is useful in retracting the iliac veins and arteries. Blunt dissection and avoidance of the monopolar cautery may avoid damage to the superior hypogastric plexus (and associated retrograde ejaculation). The L5-S1 disc space is readily evacuated, and bone graft and/or implants can be inserted.

Exposure of L4-5 is very feasible, but the surgeon must take extra care to immediately identify the iliolumbar vein. Although its origin is variable, this vein usually branches off the lateral aspect of the left iliac vein. Other times it originates directly from the vena cava. The vein courses to the region of the L5 pedicle and foramen. Heavy blood loss can occur if the left iliac vein is retracted to the patient’s right before the iliolumbar vein is ligated. One may also consider low-dose heparin administration prior to retraction of the bifurcation and vena cava to help prevent thrombosis. Other helpful pearls include use of Steinmann pins to retract the iliac vessels laterally. The Trendelenburg position can facilitate the approach to the sacral angle. Currently, laparoscopic techniques are under investigation that may improve access to the L5-S1 level and allow for decompression, fusion, and instrumentation.

L5 vertebrectomy has been accomplished via the open, laparoscopic, or retroperitoneal approach, but the procedure is difficult and carries a higher risk of vascular injury than discectomy.^18,¹⁹ Furthermore, reconstruction of L5 from the direct anterior transperitoneal approach is also very difficult. Vertebrectomy of L4 is similarly very difficult from the anterior approach and can be more readily accomplished via the retroperitoneal approach. The midlumbar and upper lumbar levels (L3-4, L2-3, L1-2) are more difficult to expose via a transperitoneal approach because of the bulk of abdominal contents that must be retracted. The retroperitoneal approach should be considered.

Lateral Retroperitoneal Transpsoas Approach

This approach allows access to the lumbar spine via a lateral approach that passes through the retroperitoneal fat and psoas major muscle (Fig. 146-5). Variations of this approach include the minimally invasive extreme lateral interbody fusion (XLIF) or direct lateral interbody fusion (DLIF) approaches.

FIGURE 146-5 A and B, Preoperative radiographs demonstrating coronal plane deformity. C and D, Extreme lateral interbody fusion cages (NuVasive, Inc., San Diego, CA) were placed at L2-4 via a lateral retroperitoneal transpsoas approach. Subsequently, a transforaminal lumbar interbody fusion (Concorde cage, DePuy Spine, Raynham, MA) was performed at L5-S1 via a second-stage posterior open approach. In addition, posterior instrumentation at T10-S1 and ilium was performed with the Expedium Anterior Spine system (DePuy Spine).

Ideal patients for this procedure include those with low back pain or flat back syndrome without central canal stenosis. Relative contraindications include significant central canal stenosis, severe rotatory scoliosis, and high-grade spondylolisthesis. Although the XLIF/DLIF can create indirect foraminal decompression via interspace distraction, these approaches do not allow for central canal decompression. Furthermore, in severe rotatory scoliosis cases and high-grade spondylolisthesis cases, the normal anatomy may be skewed and surrounding structures (the iliac vessels or bowels) may be injured.

To prepare for XLIF or DLIF, the patient is placed in a 90-degree right lateral decubitus position with the left side elevated and laterally flexed by using a bump or a roll to increase the distance between the iliac crest and the rib cage. A lateral fluoroscopic image is taken with K-wires to demarcate the midpoint of the intended disc exposure. A small incision is created to encompass this location, and the retroperitoneal plane is dissected down to the psoas muscle. A second 2-cm incision may be made posterior to this initial incision between the erector spinae muscles and the abdominal obliques. This incision allows for the insertion of the surgeon’s index finger to identify the retroperitoneal space and psoas muscle and by sweeping the peritoneum anteriorly; the direct lateral incision is palpated with the index finger. Tubular dilators and subsequently an expandable retractor is placed into the direct lateral incision with the assistance of the index finger guiding it safely from the direct lateral incision through the retroperitoneal space to the psoas muscle. The fibers of the psoas muscle are then separated by using dilators and blunt dissection with the concurrent use of electromyographic stimulation or neuromonitoring to assess the proximity of lumbar nerve roots from the advancing dilator. The dilator parts the psoas muscle centrally; therefore, the great vessels should remain anterior and the lumbosacral plexus nerves should be posterior to the retractor. Proper trajectory and position are assessed by fluoroscopy. Once the retractor is inserted over the dilators and its position is confirmed, the rigid articulating arm is attached to the retractor and the surgical table.

Discectomy is then performed using standard instruments. The posterior anulus is left intact, while an anulotomy centered over the anterior half of the disc space allows for discectomy. End-plate arthrodesis and interbody implant and bone graft placement are then completed. The retractor is removed after fluoroscopic confirmation of implant position. A layered closure is then performed.

Decompression

For a vertebrectomy, the discs above and below are incised with a knife blade, or a straight osteotome is used to separate the bulk of the disc from the end plates. One must keep in mind the concave curvature of the dorsal vertebral body in the thoracic spine to avoid the ventral spinal canal. If soft tumor is encountered, large scoop curets or the ultrasonic aspirator is an efficient tool for rapid tissue removal. For stronger bone, a 1- or 2-inch osteotome may be used to make two cuts to remove a large portion of the body. The first cut is 5 to 8 mm dorsal to the ventral-most cortex of the vertebral body in the vertical plane (perpendicular to the disc space) and is approximately 15 to 20 mm deep (toward the opposite side). The second cut is similarly perpendicular to the disc space and is approximately 8 mm ventral to the dorsal cortex of the vertebral body and spinal canal. A curved osteotome or large curet can then remove a large block of bone, leaving a barrier between the aorta and the decompression site and between the spinal canal and the decompression site. The remainder of bone is then removed either piecemeal with smaller curets or with a high-speed drill. Drilling may offer a less traumatic initial removal of bone compared with osteotomy, but blood loss may be less with the faster latter technique. The bone ventral to the canal is the last area to be removed. Once the dorsal cortex and PLL have been removed, the dura is decompressed from the ipsilateral pedicle to the contralateral pedicle. The ventral-most and far lateral cortex are preserved as much as possible to help secure the bone graft. One should be able to visually inspect the contralateral pedicle. The epidural space rostral and caudal is palpated with a double-ender (dental or Woodsen) instrument. One must decompress from pedicle to pedicle to fully ensure that there is no spinal cord compression.

The complication of neurologic injury is best avoided by clearly exposing the ventral spinal canal and dorsal body first. The next step is to create a cavity into which the critical bone fragments or tumor (compressing the spinal cord) can be safely reduced. Thus, one should first complete the bulk of the corpectomy or vertebrectomy (depending on the pathology), leaving the portion in the ventral epidural space last. Then the dorsal vertebral cortex, fracture fragments, or tumor in the epidural space is more safely removed via regular or reverse angle curettage. At times, a diamond bur may be useful. Instruments are best manipulated toward the vertebrectomy defect away from the spinal cord during epidural decompression.

Fracture Reduction

A pathologic or traumatic fracture-dislocation can be reduced with several maneuvers. The most direct technique is manipulation of vertebral bodies with a large Cobb elevator or curet. This can be difficult and runs the risk of having the instrument slip into the canal.

With the patient in the lateral position, the surgeon can push on the back at the apex of the kyphotic deformity. This technique is very effective, and the decompression site can be visualized during the manipulation. If the PLL is intact, then ligamentotaxis can aid to pull fracture fragments ventrally.

Several ventral thoracolumbar instrumentation systems have bolts that (once placed in the adjacent vertebral bodies) can be used with a distractor to accomplish reduction and distraction. Distraction is effective, but the distractor can obscure the surgeon’s view and compromise access to the decompression site during placement of the bone graft. Interspace spreaders have the same limitation, although newer models are longer and more streamlined, allowing greater accessibility to the vertebrectomy site during distraction and reconstruction (Fig. 146-6). When combined with ligamentotaxis from the PLL, anterior distraction maneuvers can be an effective method to reduce posteriorly displaced fractures.

FIGURE 146-6 A, Diagram of intervertebral body spreader and bone graft reconstruction. B, Intervertebral body spreader (Synthes Spine, Paoli, PA) distracting the vertebrae prior to bone graft placement. The thoracic dura is visible posteriorly.

A fourth method to reduce posteriorly displaced fracture fragments is an eggshell osteotomy. The cancellous portion of the fractured vertebral body is removed with a high-speed drill, leaving the cortical remnants. The posteriorly displaced cortical fragments can then be pulled anteriorly by using reverse-angle and straight curets.

Bone Grafts and Vertebral Body Replacements

The choice of bone graft or reconstructive implant depends on the distance the anterior fusion needs to span. The location in the thoracolumbar spine and the patient’s underlying pathology are also important. Factors such as the presence of osteopenia, tumor, previous radiation, diabetes, rheumatoid arthritis, and tobacco use are important. Grafts that span an entire vertebral body length or more must withstand greater axial loads. Allogeneic humerus, tibia, or femur can be used, and they are typically packed with autologous cancellous bone harvested during the vertebrectomy. Rib and iliac crest are also excellent sources of bone to use in and around allograft struts. One should keep in mind that allograft bone takes much longer to incorporate.

Portions of rib tied by suture or cable into a barrel-shaped cylinder can be used as a spacer fusion. Ribs are thought to be high in bone growth factors but provide less strength in resisting axial loads than some of the alternatives.

Autologous tricortical iliac crest provides satisfactory axial support. Harvest of large tricortical graft endangers the peritoneal contents. Great care should be taken during periosteal dissection. Significant morbidity can be associated with harvest of autologous iliac crest bone. We typically use a unipolar cautery with the tip angled ventrally to hug the dorsal cortex during stripping of the muscular and fascial attachments. A Cobb elevator is also useful, but it is necessary to take care to avoid plunging into the peritoneum. Vessels perforating the iliac crest must be coagulated or the emissary ostia treated with bone wax to help prevent accumulation of a hematoma. One should always stay one to two fingerbreadths dorsal to the anterior superior iliac spine to avoid injury to the lateral femoral cutaneous nerve. If one desires to reconstruct the defect in the iliac crest, both natural and artificial methods are possible. If rib is available, the ends can be impacted into the harvest site sides of the iliac crest to recreate the superior contour. Steinmann pins or screws (titanium or stainless steel) as a support for methylmethacrylate may be used.²⁰ If the latter technique is used, a postoperative drain is recommended, as the cement will elicit a collection of serous fluid.

Autologous fibula can also be used. In cases of infection, some surgeons also perform microvascular anastomosis to the intercostal arteries to preserve graft blood supply. Meticulous surgical technique must be used to avoid the peroneal nerve near the fibular head and to avoid the ankle joint (syndesmosis). A good rule of thumb is to stay 10 cm or more away from the ankle joint below and the fibular head above. A preoperative lower extremity arteriogram is useful to define the arterial supply to the fibula.

It is important to emphasize that regardless of the source of autologous bone, all soft tissue (e.g., muscular or tendinous attachments, cartilage, fascia) must be cleanly stripped off before implantation to maximize bone surface area for fusion. Leaving cartilaginous material adherent to the vertebral body end plate can result in pseudarthrosis; thus, the vertebral end plates should be meticulously stripped of all disc tissue. Scattered areas of decorticated end plate facilitate fusion, but significant amounts of cortical bone must be spared to allow strong purchase of the bone graft and to prevent impaction of the graft through the end plates during axial loading. Removing (decorticating) too much of the end plate may result in collapse of the bodies above and below with telescoping of the graft. This problem is encountered more often when rigid allograft tibia, fibula, or femur is used.

In addition to autograft or allograft bone, methylmethacrylate and artificial bone spacers are currently being marketed. Vertical mesh titanium cages (Pyramesh, Medtronic Sofamor Danek, Memphis, TN; Harms, DePuy Spine, Raynham, MA; SynMesh System, Synthes Spine, Paoli, PA), carbon fiber cages (Stackable Cage System, DePuy Spine), polyether ether blocks (PEEK, Medtronic Sofamor Danek), and expandable metallic implants (Synex System, Synthes Spine [Fig. 146-7]; VertiSpan, Medtronic Sofamor Danek [Fig. 146-8]) are available.

FIGURE 146-7 The expandable Synex I vertebral body implant demonstrating several possible ratcheted positions of the superior portion of the implant (Synthes Spine, Paoli, PA).

FIGURE 146-8 A, Immediate postoperative radiograph of the VertiSpan expandable cage system (Medtronic Sofamor Danek, Memphis, TN). B, Immediate postoperative coronal CT scan of the expandable cage. Note the central and peripheral placement of cancellous bone from the iliac crest.

The benefit of having a break in the table can be enhanced with the use of an interspace spreader or by manual pressure on the dorsal spine to help open the disc space during distraction and bone impaction. When screw-rod systems are used, direct distraction of vertebral body screws allows for efficient placement of the graft. Once all graft material is in place, the table is returned to the neutral position to help lock in the graft. In addition, compression can be applied across the vertebral body screws and maintained by rod attachment.

The importance of meticulous shaping and “carpentry” with the graft during this step cannot be overemphasized. A spinal metal implant is not a substitute or savior for a poorly shaped or fitted bone graft.

Instrumentation

Ventral or ventrolateral metal implants provide immediate rigidity, allow for compression of bone graft, and help maintain correction of a deformity. Based on individual experience, various amounts of time working with sawbone models or cadavers are needed before use in actual clinical situations. The three basic types of implants are rod, plate, and cable systems.

General Principles of Ventrolateral Instrumentation Systems

Whether rods, plates, or cables are being used, all implants in the thoracolumbar region should be placed on the lateral aspect of the vertebral body. The construct should have a low profile to avoid vascular or visceral injury. To avoid unequal strain and stress on the metallic implant, great care must be taken to maximize the total surface area of metal-to-bone contact. In a method commonly referred to as “gardening” the spine, rongeurs and drills should be used to flatten out the lips of the vertebral end plates, and the heads of the ribs should be removed at all levels to be instrumented. This is particularly important with plate implants.

The dorsal-most points of screw fixation in the vertebral body should be 8 to 10 mm ventral and caudal to the dorsal-rostral corner of the vertebral body at the rostral end of the construct or 8 to 10 mm ventral and rostral from the dorsocaudal corner of the caudal body in the construct. A minimum screw trajectory of 10 degrees ventrally away from the spinal canal is necessary to avoid injury. An awl should be used to begin screw or bolt holes to prevent skating of instruments near the spinal canal or the large vessels and viscera. Screw placement is dictated by the desired forces. To correct a kyphosis, the screws should be placed more ventrally with distraction/compression dual-rod systems (e.g., Kaneda, Expedium Anterior, or M-8 dual-rod systems). With dual-rod systems, the more ventral rod should be longer to correct the kyphosis. The longer segment is distracted initially. One must recognize that even minor distraction forces may cause spinal cord injury via stretch or vascular compromise. In general, when correcting scoliosis, tension forces should be applied on the convex side of the curve, starting at the apex and directed rostrally and caudally in sequential fashion. Bicortical screw penetration is preferred to provide the strongest purchase of the vertebral body, but care must be taken not to penetrate beyond 2 to 3 mm of the distant cortex of the vertebral body.

Cable Systems

The Dwyer cable system was first developed in the early 1960s for application to the convex side of thoracolumbar scoliotic curves. The basic principle of ventrolateral correction of scoliosis is to shorten the convexity by applying compressive forces. A screw with a staple (to maintain screw position and prevent movement) is placed on the convex side of a curve into the vertebral body on the lateral side. A titanium cable is then connected to the screws and tightened to create a compressive force. Excessive force, however, can produce undesirable thoracic kyphosis or, further caudally in the spine, the flattening of a normal lumbar lordosis. Dwyer reported a 43% complication rate, including screw pull-out and vascular or visceral injuries. This system can be used for degenerative lumbar curves, but it is not recommended in flat back or kyphotic deformities. This particular type of implant system has been abandoned by many surgeons in favor of rod and plate systems.

Rod Systems

Kaneda System

A unique aspect of the Kaneda system (DePuy Spine; Fig. 146-9) is the tetra-plate with four corner spikes that is initially hammered into the center of the lateral aspect of the vertebral body. One should “garden” the spine to ensure flush contact between the staple plate and the bone. Two vertebral (preferably bicortical) screws are placed through each plate. The heads of the screws have a channel through which rods are placed. The design of the staple plate is such that the ventral screw holes are more rostral and caudal in the upper and lower plates, respectively. Thus, the ventral span of the rod is longer than the dorsal. The heads of these screws can be engaged by a distractor for placement of the strut graft. The Kaneda distraction system is very effective and allows ample working space through which the bone graft can be placed. Cross-fixators are available to enhance the rigidity of this implant. This device is very effective for short (one or two) segment fixation for a variety of pathologies. Sources of complication include failure to achieve maximal surface area contact between the staple plate and bone (by allowing rocking of the metal over a bony prominence) and placement of screws that are either too short or too long. Preoperative axial CT slices (bone windows) can be used to indirectly measure the length of screw needed. One can usually feel the screw engaging the opposite lateral cortex. Inadequate countersinking of the graft may make it difficult to apply a cross-link.

FIGURE 146-9 Tetra-plates and eye bolts of the Kaneda system (DePuy Spine, Raynham, MA). Smooth rods are placed through the eye bolts and tightened with a top-loading screw. The bolts can be used for distraction or compression.

Antares Dual-Rod System

The Antares dual-rod system (Medtronic Sofamor Danek) is very similar to the Kaneda system. An anchoring plate with cleats is inserted into the side of the vertebral bodies above and below. Bolts are then placed through holes in the plates and across the vertebral bodies. Rods are slid into the bolts and compressed, and locking screws are tightened. Cross-links are available.

VentroFix System

Texas Scottish Rite System

The Texas Scottish Rite System (TSRH; Medtronic Sofamor Danek), an implant system of variable-angle screws and rods (connected by variable-width offset connectors or eye bolts), is most commonly applied to the dorsal spine but is easily adapted to ventrolateral fixation. Screws of 5.5-mm, 6.5-mm, 7.0-mm, and 7.5-mm diameter width and various lengths are available for bicortical purchase. Screws can be placed in a trapezoidal array similar to the Kaneda device so that the longer span is ventral. Rods of 3⁄16- or 1⁄4-inch or intermediate 5.5-mm diameter are (as with the screws) available in stainless steel and titanium. Distraction and compression forceps are available and connect to the rods below or above the screw-offset connection to apply the desired force. Use of the TSRH for ventrolateral fixation should be done with the knowledge that the heads of the variable-angle screws are somewhat higher in profile than those of other plate and rod systems. Side-tightening screws may be easier to apply than top-tightening ones.

Plate Systems

Z-Plate System

The Z-plate system (Medtronic Sofamor Danek), designed by Zdeblick (Figs. 146-10 to 146-12), combines the low-profile advantage of plates with the distraction and compression capability of vertebral body screws. Bicortical purchase is recommended, and a combination of bone screws and bolts is used to rigidly lock to the plate. Dorsally, screws are placed 10 degrees ventrally away from the canal. The starting point for the rostral body is 8 to 10 mm inferior and ventral to the upper dorsal corner of the vertebral body on lateral exposure. Conversely, the entry point for the caudal bolt is 8 to 10 mm superior and ventral to the lower dorsal corner of the vertebral body. An awl is used to pierce the cortex, but the bone is not tapped. These bolts can be used for distraction and allow for placement of a strut graft and correction of kyphosis. In patients with suboptimal bone strength, it is useful to use (initially) an intervertebral body spreader to apply force over a broader area. When the strut graft is in place, the bolts can be compressed and locked rigidly to the plate. As a last step, cancellous bone screws are placed through the ventral slots in the plate and angled 10 degrees dorsally. These screws are typically 5 mm longer than the dorsally placed bolts because of the configuration of the vertebral body. Complications can arise from placing a plate that is too long and that extends above or below, across, or into a disc space. This may accelerate degeneration at that motion segment. The usual risks of bicortical screw placement also apply. One must also make sure that the bolts, and especially the bone screws, are not angled down or up into the graft or adjacent disc. The latest version of the Z-plate allows for rigid fixation of the screw heads to the plate.

FIGURE 146-10 A, Diagram of the dorsal bolt trajectory (10 degrees away from the spinal canal). B, Placement of a Z-plate bolt (Medtronic Sofamor Danek, Memphis, TN) at the caudal, dorsal corner of the lower vertebral body. (Note dura mater visible through the corpectomy defect.)

FIGURE 146-11 A, Diagram of simultaneous compression across and tightening of bolts in a top-loaded thoracic plate (Medtronic Sofamor Danek, Memphis, TN). B, Operative photograph of tightening of dorsal bolts in the Z-plate system with extended wrenches and long compressor (Medtronic Sofamor Danek).

FIGURE 146-12 Ventral thoracic plate (Z-plate, Medtronic Sofamor Danek, Memphis, TN) with posterior bolts in place just before ventral screw placement. Plastic loops are around the sympathetic chain.

Anterior Thoracolumbar Locking Plate System

The Synthes plate system (Fig. 146-13) is also low profile and composed of titanium (MRI compatible). It differs from the Z-plate system in that distraction forces are not applied to the bolts or screws but are applied directly to the vertebral body end plates. Synthes makes a long-handled distractor with strong but thin blades that allows perhaps the best access to the graft site during placement of the strut. Once the graft is in place, angled screw hole trajectories in the plate cause up to 3 mm of compression as the screws are driven into the bone. This system uses wide, cancellous screws for unicortical purchase, although bicortical purchase is an option. Because placement is unicortical, equal-length screws are used dorsally and ventrally. This system has the advantage of being able to effectively provide distraction and compression, is low profile, and is perhaps the simplest in design. It is limited in the amount of compression (2 to 3 mm) provided by the dorsal compression screws. This system is no longer in widespread use.