An expandable tubular retractor (X-Tube, Medtronic Sofamor Danek, Memphis, TN) (Fig. 132.1) can be used to accomplish a minimally invasive TLIF. The tube is inserted at a diameter of 26 mm and is expanded in situ to a final working diameter of 44 mm (which can span from pedicle to pedicle). Use of an endoscope or the operating microscope is possible through this tubular retractor (Fig. 132.2A). The basic surgical set is essentially the same as a standard laminectomy/fusion set. It is important to have a high-speed telescoping drill (Midas Rex, Ft. Worth, TX) available as an aid for removing bone. Instruments should be bayoneted so that visualization of the operative field is not occluded down the barrel of the tubular retractor. The tools for disc space preparation prior to graft placement consist of distractors (7–14 mm), rotating cutters, endplate scrapers, and chisel. Many options exist for interbody graft material and can include bone or cages (with autologous bone or BMP-2 [Medtronic Sofamor Danek, Memphis, TN]).

Fig. 132.1 Expandable tubular retractors. The tube is inserted at a diameter of 26 mm and is expanded in situ to a final working diameter of 44 mm (which can span from pedicle to pedicle). Decompression, interbody fusion, and instrumentation are possible through the tube.

Fig. 132.2 (A) The use of the microscope is possible through the tube instead of an endoscope, reducing the learning curve necessary for performing minimally invasive procedures. (B) An interbody allograft is placed directly under visualization through the tubular retractor (C) Pedicle screw instrumentation can also be placed directly through the tube. (D) The ATAVI Flexposure cannula (Endius) system.

The operating room is arranged such that the operating table is in the center of the room, anesthesia at the head and fluoroscopy monitor at the foot. The C-arm base is placed on the side opposite of the TLIF as is the video monitor. Equipment tables are kept behind the surgeon on the operative side and a Mayo stand is situated over the feet to pass instruments in active use. The patient is positioned prone on a radiolucent Wilson frame over a Jackson table.

Localization and exposure

The region of pathology is localized with aid of fluoroscopy and a Steinman pin. Once marked, a stab incision is made 3 cm from midline, and the Steinman pin is inserted until it rests on bone. Ideally, the pin should be on the facet complex of the affected level and the skin incision extended to a length of 3 cm. Sequential tubular dilators are passed over one another and fluoroscopy is used to confirm adequate insertion. The appropriate-length working channel (X-tube) is introduced over all the dilators, brought into line with disc space in a medial orientation and secured to the operating table with a flexible arm clamp. The X-tube working channel is opened to its full capacity, which should span the distance from pedicle to pedicle at the level of interest. Muscle and soft tissue are cleared from the lamina and facet with monopolar cautery. Next, the working channel is angled laterally, and the transverse processes are exposed. The tubular retractor is turned medially to begin the laminotomy and facetectomy. The decompression should extend from pedicle to pedicle in a rostral–caudal direction. Laterally, a total facetectomy is done to provide adequate space for graft placement and to minimize root retraction. Next, the ligamentum flavum is removed. Epidural veins are coagulated with bipolar cautery and divided if necessary. The lateral edge of the dura, the nerve root, and the disc space should be clearly visualized.

The anulus is cut and disc material is removed with pituitary rongeurs. A down-angled curette is helpful to ensure that subligamentous disc fragments and the contralateral disc are properly removed. The disc space is sequentially dilated until disc space height is similar to adjacent levels. The maximum insertable dilator translates into the width of the interbody graft used. Next, the rotating cutter is introduced parallel with the disc space and rotated to start preparing the vertebral body endplates. The endplates are scraped, and debris is removed with a pituitary rongeur. A disc space chisel can be used to better prepare the endplates. A graft of surgeon’s preference can now be placed (see Fig. 132.2B). Medial angulation of the tube will allow for midline graft placement. One should also pack autologous laminofacet bone removed during the decompression, anteriorly into the disc space prior to graft placement. The X-tube can also be repositioned laterally if a unilateral intertransverse fusion is desired.

Instrumentation with sextant

In conjunction with the use of the X-tube, pedicle screw placement can be performed percutaneously with use of the sextant instrumentation set (Medtronic Sofamor Danek, Memphis, TN). For use of a single fluoroscopy machine, the ‘bull’s eye’ technique for percutaneous pedicle screw placement is adequate and more straightforward. On the other hand, if triangulation of the pedicle screws is desired, biplanar fluoroscopy is used for placement in a manner similar to a vertebroplasty procedure. Alternatively, image-guided systems can be used depending on surgeon’s comfort and preference. For the ‘bull’s eye’ pedicle screw placement, the C-arm is rotated 90° for a true anteroposterior (AP) view parallel with the disc space. The bone biopsy needle is localized over the pedicle and passed through the soft tissue onto the pedicle so that the needle will appear as a single spot (‘bull’s eye’) in this orientation. The needle is taped into the pedicle with a mallet and position is confirmed by fluoroscopy. Then, with the needle held firmly in the correct orientation, the stylet is removed and a K-wire is drilled approximately 1 cm into the pedicle. The bone needle is removed and fluoroscopy is used to confirm that the K-wire is in the center of the pedicle. The process is repeated for the contralateral pedicle and then for both pedicles at the adjacent affected level. The C-arm is brought to the lateral position for advancement of the K-wires to approximately two-thirds the length of the vertebral body parallel with the endplate. Soft tissue over the K-wires is dilated, the pedicles are taped, and cannulated screws are inserted. Attention is paid to the K-wire as accidental removal or advancement would be undesirable. The sextant device is attached to the screw extenders and pushed through the soft tissue to create a tract for the rod. The rod size is calculated by placing templates on the sextant at this point. The tip of the sextant is replaced with the rod, which is pushed through the soft tissue and both screw heads. The C-arm is brought to the AP orientation to confirm the rod has passed through both screw heads before tightening. The screws are then compressed, tightened, and broken off with a torque wrench. The sextant is disconnected from the rod and removed. The process is repeated on the opposite side.

In essence, if percutaneous pedicle screw placement is not desired, surgical instruments and fixation can all be applied directly through the retractor port (see Fig. 132.2C). Examples of commercial access systems in addition to the METRx Minimal Access System (Medtronic Sofamor Danek; Memphis, TN), include the Access Port (Spinal Concepts; Austin, TX), the Nuvasive system (Nuvasive; San Diego, CA) and the ATAVI System (Endius; Plainville, MA). The central mechanism of each of these systems is fundamentally that of tubular dilation and a cylindrical working portal. Like the METRx Xpand system, the ATAVI Flexposure cannula (Endius) (see Fig. 132.2D) can expand its ultimate working diameter up to 40–60 mm for direct pedicle screw placement.

Laparoscopic anterior lumbar interbody fusion

Prior to the 1980s, laparoscopic procedures were mainly used in the field of gynecology and urology. The transition into general surgery began in the 1980s when the first laparoscopic appendectomy was performed in Germany. In 1987, the first human laparoscopic cholecystectomy was performed in France.³ The widespread acceptance of this minimally invasive approach can best be appreciated by noting the fact that within only 3 years after its introduction, more than 90% of all cholecystectomies were being performed laparoscopically. The significant advantages of transperitoneal laparoscopic surgical treatment include marked reductions in postoperative pain, early hospital discharges, and reduced incidences of postoperative ileus.

Anterior lumbar interbody fusion (ALIF) was initially described by Burns in 1933 for the treatment of spondylolisthesis.⁴ In 1995, Mathews et al.⁵ and Zucherman et al.⁶ described the technique in detail and published preliminary outcome data for laparoscopic anterior lumbar fusion. In 2000, Regan et al.⁷ published a prospective comparative study of open versus laparoscopic anterior lumbar fusion. They demonstrated that the laparoscopy group had a shorter hospital stay and reduced blood loss but had increased operative time. Operative time improved in the laparoscopy group as surgeons’ experience increased. Operative complications were comparable in both groups, with an occurrence of 4.2% in the open approach and 4.9% in the laparoscopic approach. Overall, the device-related reoperation rate was higher in the laparoscopy group (4.7% versus 2.3%). Conversion to open procedure in the laparoscopy group was 10%.

A more recent study did not favor the video-assisted techniques and laparoscopic approach. Escobar et al.⁸ published a comparative analysis focusing on the complications of three techniques (a ‘minilaparotomy’ open extraperitoneal approach through a small midline incision, a transperitoneal video-assisted insufflation technique, and a video-assisted gasless) for anterior lumbar interbody fusion in 135 patients. The study revealed the highest incidence of complications in video-assisted techniques and the laparoscopic approach. Complications are primarily related to surgical exposure of the anterior spine, which can include damage to important vascular structures, the sympathetic plexus, or the abdominal viscera.

The main disadvantage of the laparoscopic approach is the steep initial learning curve of the surgical team. Additionally, the anterior approach to cage placement is limited in being able to directly decompress the spinal canal. However, with care in patient selection, a stand-alone interbody cage fusion has been successfully demonstrated. In order to evolve to the laparoscopic placement technique of interbody cages, the access surgeon and spine surgeon should work as a team using the open approach but practice placing the particular instrumentation system they plan to use laparoscopically. Lastly, in terms of initial case selection, begin with nondeformity L5–S1 cases, as this represents the ideal case for ALIF (both open and laparoscopic).

Minimally invasive retroperitoneal lumbar fusion

The retroperitoneal approach to the lumbar spine was first described by Iwahara in 1963 and is now being increasingly used for treatment of spondylolisthesis,⁹^–¹¹ degenerative disc disease,¹¹ internal disc derangement,^4,¹² instability,¹⁴ and for reoperations.^14,¹⁵ Endoscopic approaches to the retroperitoneal space, called ‘retroperitoneoscopy,’ were initially described by urological surgeons in the 1990s. Gaur¹⁶ and McDougall et al.¹⁷ first used balloon dissection of the retroperitoneal space to enable laparoscopic visualization of the surrounding anatomy. This eventually gave rise to applications for treatment of lumbar disease. The balloon-assisted endoscopic retroperitoneal gasless(BERG) procedure is a minimally invasive retroperitoneal approach to the anterior lumbar spine. A gasless retroperitoneal approach has further advantages. This procedure is similar to an open spinal procedure, and conventional instruments may be implemented. Valved trocars are not required and the complications involved with carbon dioxide insufflation are avoided. Advances in interbody cage technology and artificial discs have generated a great deal of interest in anterior lumbar fusion. Minimal access techniques to the anterior lumbar spine will be important in optimizing clinical outcomes in addition to preserving posterior load-bearing elements.

Gasless endoscopic ALIF (the BERG approach)

The BERG approach utilizes a balloon to perform the initial dissection followed by a combination of a mechanical lifting arm and fan retractor to distend the abdomen without the use of gas insufflation. This is a true retroperitoneal approach to the anterior lumbar column, which allows for access up to L1. Because there are no pressurized ports with this approach, the surgeon can utilize standard anterior instruments for lumbar fusion surgery. There is no pneumoperitoneum to maintain so loss of exposure due to suction is not a problem. Vascular and bowel retraction are similar to an open anterior retroperitoneal approach.

Surgical technique

The following instruments and equipment are used for the BERG approach to ALIF: clear-ended endoscopic dissecting port, 3 cm flexible nonvalved ports, dissecting balloon and inflator, laprolift mechanical arm, fan retractor, peritoneal balloon retractor, 0° endoscope, two video monitors, and standard set of instruments for anterior lumbar surgery. The patient is positioned supine. Fluoroscopy is used to find the landmarks of the appropriate lumbar level and the skin marked. These are drawn on the lateral aspect of the left abdomen, marking the angles of the appropriate disc spaces.

A transverse, 2 cm left flank incision is made approximately 1 cm above the left iliac crest in the midaxillary line. The dissection is taken down through the external oblique, internal oblique, and transversus muscles under direct vision to the preperitoneal fat layer using a clear-ended, endoscopic dissecting port. As the preperitoneal fat layer is penetrated by the clear-ended dissecting port, there is a color change to yellow. The retroperitoneal space is then gently insufflated with a bulb syringe and digitally dissected into the iliac fossa to allow for balloon insertion. An elliptical-shaped preperitoneal balloon is advanced through the incision until the entire balloon is within the retroperitoneal space.

A 0° angle endoscope is placed through the lumen of the dissection cannula and the balloon is expanded to an approximate volume of one liter. The endoscope is directed toward the anterior abdominal wall for the identification of the peritoneal reflection. The peritoneal reflection is used as a landmark for the anterior working port which is located lateral to the peritoneal reflection on the rectus sheath. A 2–3 cm paramedian incision is made through the anterior abdominal wall and carried down through the fascia, lateral to the peritoneal reflection so that the peritoneal sac is avoided. The balloon is removed after a 1 cm malleable retractor is placed between the two ports under direct endoscopic vision.

There are three levels of retraction necessary to access the anterior lumbar spine. The first of these is distraction of the anterior abdominal wall. This is accomplished by the insertion of a fan retractor into the initial flank port. The fan retractor is expanded under direct endoscopic vision. Once expanded, the fan retractor is attached to a mechanical lifting arm. The abdominal wall is elevated by this combination, creating the retroperitoneal space and replacing the need for gas. A flexible nonvalved port, utilized for lateral visualization and retraction, is placed directly below the legs of the fan retractor to provide a clear path for the endoscope.

The second level of retraction is necessary to displace the peritoneal contents past the midline to provide access to the lumbar spine and vascular anatomy. A long retractor with an inflatable end is inserted through the newly created lateral working port in the initial left flank incision to push the peritoneal sac and intra-abdominal contents aside, creating the working space (Fig. 132.3). Once the retractor is in place, the technician stands with his or her abdomen against the retractor handle, leaving two hands free for endoscope operation.

Fig. 132.3 Balloon retractor of the peritoneal contents used for the BERG ALIF procedure.

The remaining level of retraction is vascular. The L5–S1 vascular retraction begins by identifying the right iliac vein and utilizing a vascular retractor to retract the fascia and presacral veins thereby exposing the anterior aspect of the L5–S1 interspace. Through the visualization/retraction port, a standard vein retractor is passed and is used to retract the iliac vein laterally. The L4–5 exposure is more complex. It begins by utilizing an anterior vessel retractor and displacing the vena cava or left iliac vein. This is placed on tension and the iliolumbar vein is identified. If necessary, the iliolumbar vein is ligated using corporeal knot tying. Once the iliolumbar vein is ligated, gentle soft dissection is used to retract the left iliac vein, exposing the L4–5 interspace past the midline. The vascular retraction for L3–4 is performed in a similar way but does not require ligation of the iliolumbar vein.

Following psoas dissection and vessel retraction, fluoroscopy is used to confirm the operative level. The anterior working port allows for both vascular retraction and the introduction of standard spinal instruments such as dissectors, rongeurs, curettes, and endplate elevators. The technique for ALIF is essentially the same as with an open anterior retroperitoneal approach.

THORACIC SPINE

Video-assisted thoracoscopic surgery

The history of thoracoscopy dates back to 1910 when Jacobaeus performed the first thoracoscopic and laparoscopic procedure.¹⁸ In 1990, the introduction of video imaging to standard endoscopy marked the modern era of thoracoscopic surgery. The technique of video-assisted thoracic surgery was first reported in 1993 by Mack et al.¹⁹ Video-assisted thoracic surgery has since played a major role in the treatment of thoracic disc herniations, treatment of spinal deformities requiring anterior release, and corpectomies for the treatment of vertebral body tumors.²⁰^–²² Several published reports demonstrated the efficacy of video-assisted thoracoscopic surgery for excision of thoracic disc herniations.^23,²⁴ Thoracoscopic spine surgery has also made treatment of hyperhidrosis possible in a minimally invasive way. Picetti et al.²⁵ performed corrective surgeries with the thoracoscope in 50 patients who were diagnosed with thoracic scoliosis. Postoperative pain was less, as well as a shorter duration of postoperative analgesic use, in patients treated thorascopically as compared with patients treated with formal open procedures.

In the trauma series reported by Khoo et al,²⁶ 371 patients with fractures of the thoracic and thoracolumbar spine (T3–L3) were treated with a thoracoscopically assisted procedure. Seventy-three percent of the fractures were located at the thoracolumbar junction. In 49% of patients, mobilization of the diaphragm was performed thoracoscopically to expose the fracture site. The severe complication rate was low (1.3%), with one case each of aortic injury, splenic contusion, neurological deterioration, cerebrospinal fluid leak, and severe wound infection. Compared with a group of 30 patients treated with open thoracotomy, thoracoscopically treated patients required 42% less narcotics for pain treatment after the operation. A thoracoscopic approach can only access the anterior and anterolateral aspects of the vertebrae and spinal canal. It cannot adequately expose the posterior elements, the contralateral pedicle, or the transverse process. Thoracoscopic surgery, like endoscopic surgery, will require a steep learning curve but has the advantages of reducing post-thoracotomy pain syndromes and exposure-related morbidity.

Thoracoscopic decompression and fixation

As an alternative approach to thoracotomy or thoracoabdominal approach, thoracoscopic technique can be used to perform thoracic corpectomy to decompress the spinal canal, to reconstruct and fixate the segments affected by destructive disease and trauma. Within the thoracic cavity, thoracoscopic access to the thoracic vertebrae T4–12 can be easily accomplished. By thoracoscopically detaching the diaphragm, L1–3 can be reached so that it is possible to expose the entire thoracolumbar area for a minimally invasive endoscopic procedure (Fig. 132.4A). Thus, the thoracic spine as well as upper lumbar spine from T4 to L3 is accessible endoscopically using the thoracoscopic technique.

Fig. 132.4 (A) Thoracoscopic detachment of the diaphragm allows exposure of the thoracolumbar junction down to L2–3, making stabilization of this area possible in a minimally invasive manner. (B) Thirty degree endoscope used for thoracoscopic procedures. (C) Thoracoscopic instrumentation of the thoracolumbar junction using the MACS-TL system. (D) Thoracoscopic incisions seen at 2 weeks postoperatively.

Surgical technique

Lung isolation with double lumen endotracheal intubation will be required for the thoracoscopic procedure. The patient is placed in a lateral position on a Jackson spinal table. A left-sided approach is preferred for the treatment of pathologies from T4 to T8. A right-sided approach is preferred for exposing the thoracolumbar junction (T9–L3). The upper arm is abducted and elevated so that it does not interfere with the placement and manipulation of the endoscope. The surgeon stands behind the patient. The surgical technique of thoracoscopic spine surgery is described in detail by various authors.²⁶^–²⁸ Early on in the authors’ experience, they often reserved the thoracoscopic approaches for thoracic pathologies involving the T4 to T10 vertebrae. With increasing experience, the authors extended the indications to pathologies involving the thoracolumbar junction.

Localization

The target area is projected onto the skin level under fluoroscopic control. The borders of the involved vertebra are marked on the skin. The working channel is centered over the target vertebra (12.5 mm). The optical channel (10 mm) is placed between two and three intercostal spaces cranial to the target vertebra in the spinal axis. For approaching upper and middle thoracic spine, the optical channel is placed caudal to the target vertebra. The approach for suction/irrigation (5 mm) and retractor (10 mm) is placed approximately 5–10 cm anterior to the working and optical channels.

Placement of portals

The position of the portals in relation to one another and to the operating site on the spine influences the entire course of the operation. The operating portal is the first position to be marked exactly over the target area, and then, corresponding to this, the portal for the optic is drawn in over the spine, two or three intercostals spaces above the mark for the operating portal, for thoracolumbar access, or underneath it for access to the central or upper thoracic spine. Portal for the suction and irrigation instrument is about four fingerbreadths from the operating portal in a ventral and cranial direction. The portal for the diaphragm or lung retractor should be placed as far as possible ventrally to avoid instruments coming into conflict. Lung isolation should start prior to incision. Subsequently, the most cranial portal (optical channel) should be placed first. Through a 1.5 cm skin incision above the intercostal space, a muscle-splitting technique is used to bluntly open into the intercostal space. A 10 mm trocar is inserted into the thoracic cavity. A 30° endoscope is inserted at a flat angle in the direction of the second trocar (see Fig. 132.4B). Perforation of the thoracic wall to insert the remaining trocars is performed under direct intrathoracic visualization through the scope.

Prevertebral dissection and diaphragm detachment

A fan retractor inserted through the anterior port can retract the diaphragm and exposes the insertion of the diaphragm onto the spine. Total detachment of the diaphragm is not necessary for exposure of the thoracolumbar junction. A diaphragmatic opening of about 6–10 cm can expose the entire L2 vertebral body. The anterior circumference of the motion segment can be palpated with a blunt probe. The line of dissection for the diaphragm is ‘marked’ with monopolar cauterization. The diaphragm is then incised using endo-scissors. A rim of 1 cm is left on the spine to facilitate closure of the diaphragm at the end of the procedure. Retroperitoneal fat tissue is now exposed and mobilized from the anterior surface of the psoas insertions. The psoas muscle is dissected very carefully from the vertebral bodies in order not to damage the segmental blood vessels ‘hidden’ underneath. The retractor is now placed into the diaphragmal gap.

Corpectomy and decompression of the spinal canal

The extent of the planned partial vertebrectomy is defined with an osteotome.

The disc spaces are opened to define the borders. After resection of the intervertebral disc(s), the fragmented parts of the vertebra(e) are removed carefully with rongeurs. Resection close to the spinal canal is facilitated with the use of high-speed burrs. If decompression of the spinal canal is necessary, the lower border of the pedicle should first be identified with a blunt hook. The base of the pedicle is then resected in a cranial direction with a Kerrison rongeur and the thecal sac can be identified. Finally, the posterior fragment that occupies the spinal canal can be removed.

Bone grafting/cage placement

Preparation of the graft bed is then completed and the length and the depth of the bone graft are measured with a caliper. A tricortical bone is taken from the iliac crest. The bone graft is prepared for insertion and mounted on a graft holder. The working portal is removed and a speculum is inserted. This allows the insertion of a bone graft up to 1.5 cm in length into the thoracic cavity. If the bone grafts are longer, they are inserted without the use of the speculum, but with the help of Langenbeck hooks. In these cases, they are mounted on the graft holder inside the thoracic cavity. The bone graft is inserted by press-fit into the graft bed. If slight reduction maneuvers are necessary, this can be achieved by manual pressure on the spinous processes of the involved segment, thus creating a segmental lordosis.

Screw/plate/rod placement

Thoracoscopic instrumentation can be performed via available systems (i.e. MACS-TL, Frontier) and invariably involves the use of cannulated top loading screws. Plates or rods can be used to interlink the screws for final stabilization (see Fig. 132.4C).

Closure

The gap in the diaphragm is closed with staples or adaptive sutures using endoscopic technique. The thoracic cavity is irrigated, blood clots are removed, and a chest tube is inserted with the end placed in the costodiaphragmatic recess. The portals are closed with sutures after removal of the trocars (see Fig. 132.4D).

CERVICAL SPINE

Percutaneous endoscopic cervical discectomy and stabilization

Percutaneous endoscopic cervical discectomy (PECD) is a new surgical method for treating soft cervical disc herniations. Specially designed expandable holders can be used as interbody spacers to achieve stability without open discectomy fusion, thus avoiding many approach-related complications seen with open techniques. The goal of the procedure is decompression of the spinal nerve root by percutaneous removal of the herniated mass and shrinkage of the nucleus pulposus under local anesthesia. The minimally invasive PECD under local anesthesia offers an alternative to open therapeutic methods in cervicobrachial neuralgia or radiculopathy due to soft cervical disc herniation. In cases of failure, PECD does not impede conventional surgical approaches, and it offers numerous advantages such as the absence of risk of epidural bleeding and periradicular fibrosis, maintenance of stability of the intervertebral mobile segment, and reduced risk for recurrence after performing an anterior discal window. The procedure provides an excellent cosmetic effect, and the reduced operation time and hospital stay allows the patient to recover to normal daily activity more rapidly.

Although PECD provides an effective and attractive alternative to open discectomy and fusion, it has limitations. For example, PECD is ineffective in the presence of segmental instability or cervical discogenic pain syndromes. Spinal stabilization performed by conventional open procedures, however, often requires an extended pathway through the neck to insert the fusion/spacer devices into the disc space.

Minimally invasive cervical lateral mass screw fixation

Although not without its limitations, posterior cervical instrumentation can also be accomplished via a minimally invasive approach. A ‘novel’ surgical technique of lateral mass screw fixation through a special tunnel retractor has recently been described (Fig. 132.5A). The procedure is performed in the prone position with the use of tubular retractors introduced at two or three levels below the area of pathology at an angle used for placement of the lateral mass screws. Dorsal elevation of the retractor system will provide room for placement of the rod. This technique can be applied to three contiguous cervical levels.

Fig. 132.5 (A) Lateral mass screws placed through a tubular retractor for unilateral jumped facets. (B) Top-loading polyaxial screw design facilitates placement via a minimally invasive technique.

Newer lateral mass instrumentation systems utilize two rods and variable polyaxial screw islets at each level. These include the Cervi-Fix system (Synthes; Paoli, PA), StarLock System (Synthes; Paoli, PA), Summit System (Depuy Acromed), and Vertex (Medtronic Sofamor Danek; Minneapolis, MN). These systems vary by the angulation of their screws as well as in the degree of the constraint placed at the screw–rod interface. The polyaxial connectors of the screws are able to angle medially, laterally, and straight with varying degrees of rotational freedom in each direction. As such, segmental fixation is more easily achieved via a top-loading approach, thereby making minimally invasive posterior cervical fixation possible (see Fig. 132.5B). With the advent of some of the newer types of expandable access portals (e.g. FlexPosure (Endius; Plainville, MA) and Xpand (Medtronic Sofamor Danek; Memphis, TN), up to three lateral masses can be instrumented through a single exposure.

VERTEBROPLASTY/KYPHOPLASTY

Although not a fusion procedure in itself, vertebroplasty/kyphoplasty may obviate the need for a stabilization procedure via a minimally invasive way. Developed in France in the late 1980s, minimally invasive vertebroplasty involves the percutaneous injection of polymethyl methacrylate (PMMA) into a fractured vertebral body.³⁰ Although this does not reexpand a collapsed vertebra, reinforcing and stabilizing the fracture seems to alleviate pain. The procedure was first used to treat aggressive vertebral hemangiomas³⁰ and was later applied to other lesions that weaken the vertebral body, including osteolytic metastases³¹^–³⁴ and osteoporotic vertebral collapse. Although the European experience with vertebroplasty in the setting of spinal metastases and myeloma is more extensive, the indications for treatment in North America are currently heavily weighted toward osteoporotic bone disease. Percutaneous balloon kyphoplasty is a recent modification of the vertebroplasty technique and involves inflation of a balloon within a collapsed vertebral body to restore height and reduce kyphotic deformity followed by stabilization with PMMA. The risk of cement extravasation is theoretically reduced because the balloon creates a void within the vertebral body into which cement can be injected under relatively low pressure. In addition to PMMA and bone mineral cement, several alternative biological materials have been used in attempts to augment compromised vertebral bodies. The efficacy of osteoinductive growth factors (transforming growth factor-γ, bone morphogenetic protein-2, and bone morphogenetic protein-7) in enhancing arthrodesis is currently being studied among patients undergoing spinal instrumentation.

IMAGING GUIDANCE-ASSISTED SURGERY

Since its introduction, transpedicular screw fixation has been extensively utilized in various spinal disorders to promote fusion and stabilization. Screw misplacement can lead to undesirable neurovascular complications. Pedicle screw placement in patients with deformities carries an even greater risk of serious complications. Weinstein et al.³⁵ reported pedicle cortex violation in close to 20% of these cases. To increase the accuracy of screw placement, various methods have been used to better target the pedicle with respect to the trajectory and depth of screw placement. Image-guided systems are widely used in intracranial surgery and have been adapted to assist with screw placement since the middle 1990s.^36,³⁷ The use of image-guided systems for pedicle screw placement has improved the accuracy of placement. The system relies on precise localization of the pedicles with computed tomography. Furthermore, by replacing direct visualization with radiographic visualization, it has enabled a reduction in surgical exposure, duration, and blood loss. Foley et al.¹ described ‘virtual fluoroscopy’ and its successful use in various spinal procedures including pedicle screw insertion, interbody cage placement, odontoid screw insertion, and atlantoaxial transarticular screw fixation. Nolte et al.³⁷ described the principles of computer-assisted pedicle screw fixation. An infrared camera (Optotrak; Northern Digital, Waterloo, Canada) tracked specific instruments (pedicle probe, awl, and space pointer) equipped with light-emitting diodes. The dynamic reference was fixed to the spinous process of the vertebra to be instrumented. Normal bony landmarks and their correlations with the images confirmed the calibration accuracy. Using that computerized system, Nolte et al.³⁷ reported a pedicle screw misplacement rate of 4.3% under clinical conditions. In contrast, Choi et al.³⁸ reported the use of computer-assisted fluoroscopic targeting for pedicle screw fixation. The authors compared the accuracy of pedicle screw placement with the fluoroscopy-guided system versus the image-guided system and observed no significant differences. Recent development of isocentric C-arm fluoroscopy, which generates CT images using an intraoperative fluoroscope, may offer another means of three-dimensional navigation using a two-dimensional intraoperative imaging source. With increasing familiarity, image-guided surgery will be a very useful adjunct to the further development of minimally invasive surgery.