37: Minimally Invasive Exposure Techniques of the Lumbar Spine

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Procedure 37 Minimally Invasive Exposure Techniques of the Lumbar Spine

D. Greg Anderson, Christopher K. Kepler

Indications

Conditions requiring decompression of the lumbar spine, wherein a minimally invasive surgery (MIS) technique is desired

MIS techniques covered include:

• Lumbar diskectomy/decompression

• Posterior lumbar fusion (PLF)

♦ Posterolateral (onlay)

♦ Posterior lumbar interbody fusion (PLIF)

♦ Transforaminal lumbar interbody fusion (TLIF)

• Anterior lumbar interbody fusion (ALIF)

Indications Pitfalls

• Any diagnosis making adequate fluoroscopic imaging of bony elements difficult or impossible, such as:

• Severe osteopenia

• Intraabdominal contrast

• Severe obesity, wherein a tubular retractor system is unable to reach bony anatomy

Indications Controversies

• The relative benefits of MIS compared with traditional open approaches continue to be debated.

• Revision surgery, severe deformity, and severe obesity are conditions that make minimally invasive spinal surgery (MISS) more challenging.

Examination/Imaging

Although it is difficult to define the exact boundaries of a percutaneous, mini-open, or traditional “open” surgery, the application of less invasive spinal surgery principles is much more important than the length of the skin incision (Jaikumar et al, 2002; Lehman et al, 2005).

The most important aspect to the success of spinal surgery is proper patient selection.

Before surgery, the surgeon should carefully study the imaging studies (plain radiographs, magnetic resonance imaging [MRI] and/or computed tomography [CT]) and develop a surgical plan, including an optimal workflow for the procedure.

Evaluation of imaging is critical, because all relevant pathologic features must be visualized and addressed to achieve results comparable or superior to an open operation.

Patients with severe osteopenia, obesity, or intraabdominal contrast may be impossible to adequately image with the C-arm. If adequate fluoroscopic images cannot be obtained, an alternative surgical strategy should be employed.

When setting up for percutaneous pedicle instrumentation, the vertebrae should be aligned so that, on an anteroposterior (AP) image, the spinous process is centered between the pedicles, and the superior end plate is parallel to the fluoroscopy beam (the true AP view) (Figure 37-1).

On the fluoroscopic lateral image, the pedicles should be superimposed, and only a single posterior cortex of the vertebral body should be seen (Figure 37-2, arrow). The edges of the superior end plate should be superimposed, forming a single radiopaque line.

FIGURE 37-1

FIGURE 37-2

Treatment Options

• The alternative to any MIS procedure for the lumbar spine is traditional open surgery.

• With experience, MIS can be applied to essentially all degenerative conditions; however, because certain clinical situations may arise that preclude the completion of the surgery with a minimally invasive approach, the surgeon should always be prepared to extend the incision if required to adequately address the spinal pathology.

Surgical Anatomy

The radiographic position of all relevant anatomy should be undertaken before making the initial incision.

The incision should be positioned to allow optimal access to the surgical pathology.

The skin and fascia should be sharply divided.

The muscle tissue should be gently traversed, working between the muscle planes or between muscle fascicles.

The tubular retractor should be docked to the spine to minimize the need to resect muscle tissues to visualize the bony anatomy.

The bony landmarks should be identified before the resection of any bone.

Care should be taken to preserve an adequate amount of the pars interarticularis and inferior articular process if a fusion of the operative level is not planned.

When working in the spinal canal, the epidural fat is a useful clue to localize the plane beneath the ligamentum flavum, adjacent to the dura.

The pedicle is a key landmark to assist the surgeon in localizing the position within the spinal canal. By palpating the pedicle, the surgeon can gauge the amount of bony resection and can also localize migrated disk fragments.

The exiting and traversing nerve root should be decompressed as needed, depending on the nature of the patient symptoms and pathology.

Surgical Anatomy Pearls

• Careful fluoroscopic localization of the surgical incision is mandatory before making the incision.

• A spinal needle inserted along the proposed trajectory of the surgical incision can be used to check the location of the incision using fluoroscopy.

• Careful palpation of surgical planes is useful before using a Kerrison instrument to remove bone from the region of the spinal canal.

Surgical Anatomy Pitfalls

• Avoid “getting lost” by use of both direct anatomic visualization and fluoroscopic confirmation of the position of instruments as needed during surgery. With experience, less fluoroscopic confirmation will be required.

Positioning

For posterior procedures (microdiskectomy, lumbar decompression, PLF, PLIF, TLIF etc.), the patient should be positioned prone on a radiolucent spinal table or frame.

The abdomen should be free of compression (Lehman et al, 2005; Seldomridge and Phillips, 2005).

Careful padding of all vital and bony regions should be confirmed.

Access for fluoroscopy should be confirmed in the operative position.

Access for the operative microscope should be confirmed.

For anterior procedures, the abdomen should be widely draped with access from the xiphoid to the pubis.

For lateral interbody fusion procedures (XLIF, DLIF) the patient should be secured in a “true” lateral position with a slight lateral bend to the lumbar region (away from the operative incision) to improve access to the lateral aspect of the vertebral body.

Positioning Pearls

• Failure to adequately position the patient may result in problems with direct or fluoroscopic access to a critical region of the spinal anatomy and may compromise the results of surgery.

General Aspects to Posterior Tubular Retractor Surgery

The learning curve for MIS techniques must be acknowledged and planned for.

Reconstructive procedures (Figure 37-3) are more difficult compared with decompressive procedures and should be approached farther along the learning curve of the individual surgeon. Additional time should be allotted for surgical cases in the early portion of the surgeon’s learning curve.

The first surgical step is to localize the precise site for all skin incisions using fluoroscopy (Seldomridge and Phillips, 2005).

• After the skin and fascial incisions are made, serial dilation allows parting of the paraspinal muscle fascicles, to minimize tissue damage. This allows placement of the tubular retractor against the bony anatomy (Figure 37-4, A). Fluoroscopic confirmation of tubular retractor position should be performed (Figure 37-4, B).

• To create a working space, a Cobb retractor is placed through the skin incision to achieve subperiosteal elevation of the musculoligamentous envelope.

• Use of an operative microscope provides the best visualization, especially when a decompressive procedure is performed.

The use of an expandable tubular retractor system allows a more generous exposure but has the trade off of increased tissue dissection and soft tissue creep.

• Secure docking of the retractor against the spine can minimize soft tissue creep, which obscures visibility of the spine (Khoo et al, 2002; Tafazal and Sell, 2004).

• Because the angular movement of surgical tools is reduced in longer tubular retractors, the shortest retractor that reaches from the skin to the spine should be used.

• Fluoroscopic confirmation of tubular retractor position should be obtained before commencing with surgery.

• The position of the tubular retractor can be adjusted to optimized access to the region of interest.

Although the instruments used for minimally invasive spine surgery (MISS) procedures are similar to those used with traditional open procedures, longer, bayoneted instruments are useful to ensure that visualization is not obscured by the surgeon’s hands.

• The surgeon generally operates with a suction instrument in one hand and a working instrument (e.g., Kerrison rongeur or curette) in the opposite, dominant hand (Figure 37-5) (Seldomridge and Phillips, 2005).

• In most cases, only minimal assistance is required from a surgical assistant to retract the nerve root when working in the ventral aspect of the spinal canal.

“Wanding” of the tubular retractor is an important technique that allows access outside the initial surgical exposure.

• Wanding is performed by loosening the attachment between the retractor and the operating table and angling the tubular retractor to the new position (Figure 37-6).

• By wanding, it is generally possible to reach both sides of the spine canal at two adjacent vertebral levels through a single skin incision.

In addition to direct visualization through the retractor, tactile “feel” is also an important skill for the MIS spinal surgeon to develop and use.

In the event of a dural laceration, direct suture repair is the authors’ preferred treatment strategy for most tears (Bosacco et al, 2001). However, others have reported successful management of minor dural tears by the use of sealants without direct repair, as long as there is no tendency for nerve rootlet extravasation.

The risk of dural cutaneous fistula, resulting from the small “dead space” in the wound, is reduced with an MIS exposure compared with an open surgical procedure.

FIGURE 37-3

FIGURE 37-4, A-B

FIGURE 37-5

FIGURE 37-6

General Controversies

• The need for a direct repair of “stable” dural lacerations in an MIS procedure is often debated by experts in the field.

Procedure

Step 1

An MIS approach offers an excellent option for correcting localized spinal canal stenosis or treating herniated lumbar disks.

Simple decompressive surgery is generally straightforward and is an appropriate starting point for the novice MISS surgeon.

• It is important to remember that, although MISS decompressions use a smaller skin incision, the same adequate decompression must be achieved to have a good clinical outcome.

• In the case of a disk herniation, any free fragments should be localized on preoperative imaging studies (recent MRI) relative to the pedicles and the disk space.

• Working through a simple laminotomy, the disk fragment can be removed and the compromised neural elements decompressed.

• With spinal stenosis, the location of the obstruction within the spinal canal should be clearly defined before surgery and confirmed by direct visualization of the decompressed neural elements before completing the procedure.

A bilateral lumbar decompression or “laminoplasty” technique can be used to address bilateral stenosis through a single unilateral skin incision.

• With the laminoplasty technique, a wide hemilaminectomy is performed first, after which a medial facet resection can be done to decompress the ipsilateral side of the spinal canal.

• The tube is then angled toward the contralateral side of the spinal canal (using the wanding technique outlined previously), and the spinous process and contralateral lamina are undercut (using a high-speed drill) to provide access to the contralateral side of the spinal canal (Figure 37-7).

• During drilling of bone, it is safer to leave the ligamentum flavum intact to protect the underlying dura.

After the bone drilling is complete, the ligamentum flavum should be removed to allow direct visualization of the nerve roots and decompression of the dura.

FIGURE 37-7

Step 1 Pearls

• Avoid using an overly large diameter tubular retractor for decompressive procedures, because this will push the surgeon away from the midline.

• The authors prefer to use a 14- to 18-mm diameter tube (outer diameter) for diskectomies and an 18- to 20-mm tube for stenosis decompression.

• Leave the ligamentum flavum intact during drilling of bone to protect the dura.

Step 1 Pitfalls

• Ensure an adequate plane over the dura, and watch the dura carefully during the resection of bone to avoid an iatrogenic dural tear.

Step 2

Posterolateral (onlay) fusion

• Posterolateral fusion between the transverse processes of adjacent levels can be achieved, working through a tubular retractor system.

• For in intertransverse (onlay) fusion, an expandable retractor is useful to expose the entire interval between the transverse processes.

• After docking on the transverse processes, the soft tissues are cleared away to allow visualization of the underlying bone that is decorticated.

• The interval between the transverse processes is then filled with a suitable bone graft material, followed by removal of the tubular retractor.

• Take care to avoid violation of the intertransverse membrane, because this may place the underlying nerve root at risk of injury.

Posterior interbody fusion

• A posterior lumbar interbody fusion or transforaminal lumbar interbody fusion is achieved by removal of an adequate amount of the facet joint so that the traversing nerve root requires minimal retraction (German and Foley, 2005; Khoo et al, 2002; Lehman et al, 2005).

• Interbody fusions, theoretically, provide a more favorable fusion environment compared with the intertransverse space and also allow reconstruction of a collapsed disk space.

• When performing a minimally invasive TLIF, the skin incision should be placed at least 4 cm lateral to the midline to allow good access to the contralateral disk space (Figure 37-8).

• Because a TLIF is a more technical procedure, it should be undertaken by surgeons who are already experienced with tubular surgical techniques for lumbar decompression.

• The side of the most significant leg pain is generally chosen for a TLIF exposure.

• With careful protection and retraction of the traversing nerve root, the posterolateral annulus is incised to allow access to the interbody space.

• Curettes and disk shaver instruments are used to perform a thorough diskectomy.

• The bony end plates must not be violated, because this will encourage bleeding and subsidence of the interbody implant.

• Lateral fluoroscopy should be used to determine the depth of instruments introduced into the disk space and reduce the change of inadvertent anterior penetration of the annulus, which can risk a vascular catastrophe.

• The well cleaned out disk space should be sized for the optimal interbody implant.

• The interbody implant should be packed with an appropriate bone graft material, and the space around the implant should also be filled with graft material.

• Great care should be taken to avoid compression of the exiting nerve during insertion of trials or the interbody fusion device.

• The fusion device should be positioned symmetrically on the AP image and toward the front of the disk space on the lateral image.

• Palpation should be used to ensure the absence of compression of the exiting and traversing nerve root at the conclusion of a TLIF procedure.

Anterior lumbar interbody fusion

• ALIF provides an excellent reconstruction of the disk space and eliminates the need to operate adjacent to the dural sac, as with a posterior interbody procedure. This thereby decreases the risk of neural injury or epidural scarring.

• After ALIF, percutaneous pedicle screws can be placed to achieve a posterior tension band construct and provide circumferential stability to the fusion construct.

• A mini-open retroperitoneal approach is preferred for ALIF surgery at the current time.

• This exposure provides wide access to the intervertebral disk, which can be thoroughly resected, allowing reconstruction with a large interbody device.

Lateral transpsoas interbody fusion

• The lateral transpsoas approach is gaining popularity for fusions above the L5-S1 level.

• Advantages of this approach include the absence of required vascular mobilization, the ability to implant a large, mechanically sound interbody implant and the excellent coronal plane correction that is achieved when dealing with a spinal deformity.

• A disadvantage of the lateral approach is the potential risk to the lumbar plexus (Seldomridge and Phillips, 2005).

• Common sequelae of the approach include sensory changes in the region of the upper thigh and hip flexor pain or weakness that are felt to be approach-related although generally transient.

• After positioning in the lateral decubitus position, the C-arm is used to localize the central region of the involved disk space.

• A muscle splitting approach is used to access the retroperitoneal space and psoas muscle.

• The authors prefer to split the psoas between its anterior third and posterior two thirds to reach the lateral disk.

• Neuromonitoring can be used to localize nerve roots within the psoas muscle.

• Fluoroscopic documentation of correct positioning of the retractor relative to the disk space is mandatory.

• A thorough diskectomy should be performed.

• An interbody implant that traverses the disk space from lateral apophyseal ring to lateral apophyseal ring should be chosen.

• The interbody implant and disk space should be packed with an appropriate graft material.

FIGURE 37-8

Step 2 Pearls

• Penetration of the end plate during diskectomy should be avoided by the careful use of sharp instruments within the disk space.

• All cartilaginous material must be removed from the end plates to allow an optimal chance of achieving fusion.

• Avoid undersizing the interbody implants during interbody fusion, which may reduce stability of the construct and allow migration of the implant.

• Any overhanging bone that reduces the opening to the disk space should be removed to allow optimal clearance of disk material and proper sizing of the interbody implant.

Step 2 Pitfalls

• Take great care to avoid violation of the anterior annulus, which risks a major vascular injury.

• Be aware that, rarely, a conjoined nerve root may prevent safe access to the disk space when performing a TLIF or PLIF approach. In such a situation, an alternative fusion method should be undertaken.

Step 3: Instrumentation

Minimally invasive spinal instrumentation has been simplified with the advent of percutaneous cannulated pedicle screw systems.

Surgeons should familiarize themselves with the specifics of the instrumentation system before the procedure.

Percutaneous pedicle screw instrumentation begins with obtaining a true AP fluoroscopic image of the vertebra (Figure 37-9, A).

Incisions should be localized about 1 cm lateral to the lateral margin of the pedicle visualized on the AP image (Figure 37-9, B) (Lehman et al, 2005; Seldomridge and Phillips, 2005).

Preexisting incisions used for a TLIF or PLIF procedure can generally be used for the placement of pedicle screw fixation.

A Jamshidi needle is introduced through the skin incision to dock on the bone directly over the lateral boarder of the pedicle at the 3 o’clock (right) and 9 o’clock (left) position.

After confirming that the tip of the needle is properly positioned (Figure 37-10, A), the tip of the needle is seated a few millimeters into the bone with gentle mallet taps (Figure 37-10, B).

The position of the needle tip is again confirmed with AP fluoroscopy.

Next, the needle shaft is marked 20 mm above the skin edge.

Holding the needle shaft parallel to the end-plate shadow, with about 10 degrees of lateral to medial angulation, the needle is tapped through the pedicle, until the mark on the needle shaft reaches the skin edge (at this point, the needle tip has traversed the isthmus of the pedicle).

An AP image is again taken to assess the position of the tip of the needle, which should lie approximately to of the distance (from medial to lateral) across the pedicle (Figure 37-11).

A guidewire is inserted through the needle shaft and advanced about 15 mm into the vertebral body.

The Jamshidi needle should advance smoothly as it is tapped through the pedicle. If hard bone is encountered, the tip of the needle is most likely medial and striking the cortical surface of the superior articular process. In such an event, the tip of the needle should be repositioned with a more lateral starting point to avoid penetration of the facet joint.

The guidewire should also encounter cancellous bone at the base of the needle shaft. This has a characteristic “feel” that should be confirmed by the surgeon before insertion of the guidewire.

Guidewires are inserted through the Jamshidi needle (Figure 37-12, A and B).

Cannulated instruments, such as awls and taps, are used to prepare the pedicle for screw insertion.

Electromyography can be used to test the tap and ensure that it has not breeched the cortex (Figure 37-13).

Lateral fluoroscopy should be used to monitor the depth of instruments inserted over the guidewires (Figure 37-14).

The guidewire should always be held when passing instruments over it, to prevent advancement of the guidewire or inadvertent removal.

After tapping the pedicle holes with a cannulated tap, the cannulated pedicle screws can be placed.

The rod can then be introduced and locked into place (Figures 37-15 and 37-16).

Final AP and lateral fluoroscopy (Figure 37-17) should confirm the fusion construct to be in an acceptable position.

The authors prefer to use subcuticular resorbable sutures to close the wound, resulting in a desirable cosmetic result (Figure 37-18).

FIGURE 37-9, A-B

FIGURE 37-10, A-B

FIGURE 37-11

FIGURE 37-12, A-B

FIGURE 37-13

FIGURE 37-14

FIGURE 37-15

FIGURE 37-16

FIGURE 37-17

FIGURE 37-18

Step 3 Pearls

• Obtaining a true AP fluoroscopic image parallel to the superior end plate of the vertebral body is paramount for the success of percutaneous screw placement.

• By marking the depth of insertion through the pedicle on the Jamshidi needle, the surgeon can safely insert the guidewires without having to switch multiple times between the AP and lateral fluoroscopy images.

Step 3 Pitfalls

• Guidewires should always be carefully held when instruments are passed over them.

Postoperative Care and Expected Outcomes

Postoperative care is similar to traditional open lumbar surgery. However, patients generally have less postoperative pain and are able to mobilize earlier in the postoperative period, compared with open surgery.

The authors encourage mobilization and ambulation on the day of surgery and discharge when patients are comfortable on oral medications.

Although all of the complications of open spinal surgery are still applicable to MIS spinal procedures, certain complications, such as infection and heavy bleeding, are much less frequent with MIS procedures.

Optimal long-term outcome depends on proper patient selection and careful, adequate performance of the operation.

Bosacco SJ, Gardner MJ, Guille JT. Evaluation and treatment of dural tears in lumbar spine surgery: a review. Clin Orthop Relat Res. 2001;389:238-247.

Dural lacerations are fortunately not a very common misadventure during spinal surgery, but neglect can lead to significant complications. This review addresses their prevalence and treatment options.

German JW, Foley KT. Minimal access surgical techniques in the management of the painful lumbar motion segment. Spine. 2005;30(Suppl):S52-S59.

This paper reports good results following minimally invasive spinal fusion procedures. Although the preliminary data appear promising, long-term studies are required for critical review.

Jaikumar S, Kim DH, Kam AC. History of minimally invasive spine surgery. Neurosurgery. 2002;51(Suppl):S1-14.

Minimally invasive spine surgery (MISS) appeals to patients because of early recovery and the cosmetic benefits. Advances in this field are influenced by the evolving technologies in lasers, endoscopy, and image guidance. Reviewing the history of MISS is helpful in understanding this emerging technique in spine surgery.

Khoo LT, Palmer S, Laich DT, Fessler AG. Minimally invasive percutaneous posterior lumbar interbody fusion. Neurosurgery. 2002;51(Suppl):S166-S171.

With minimally invasive techniques, a complete posterior lumbar interbody fusion can be achieved safely with good results. However, the efficacy of these procedures remains to be validated by further studies.

Lehman RA, Vaccaro AR, Bartagnoli R, Kuklo TR. Standard and minimally invasive approaches to the spine. Orthop Clin North Am. 2005;36:281-292.

Minimal-access retractors and specialized instruments are being designed to access and treat different spinal pathologies with minimal access surgery. These minimally invasive procedures are safe and effective, and avoid the surgical morbidities and disadvantages associated with standard open techniques.

Seldomridge JA, Phillips FM. Minimally invasive spine surgery. Am J Orthop. 2005;34:224-232.

This paper reviews the rationale of minimally invasive spine surgery (MISS), highlighting its benefits and describing common MISS procedures.

Tafazal SI, Sell PJ. Incidental durotomy in lumbar spine surgery: incidence and management. Eur Spine J. 2004;14:287-290.

This paper describes the incidence and sequelae of an accidental dural laceration as well as the preferred treatment method once a dural tear has been diagnosed.

Wu RH, Fraser JF, Härtl R. Minimal access versus open transforaminal lumbar interbody fusion: meta-analysis of fusion rates. Spine. 2010;35:2273-2281.

A meta-analysis of studies comparing transforaminal lumbar interbody fusion performed using either traditional techniques or minimally invasive spine surgery (MISS) demonstrated similar fusion rates, with a trend toward fewer complications in the MISS group.

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