Background

In the early 1900s, Muller recognized that dorsal lumbar fusion techniques were sometimes unsuccessful and attempted to treat patients afflicted with Pott’s disease via an anterior approach.¹ In 1933, Burns successfully performed an interbody fusion for a traumatic L5-S1 spondylolisthesis via the transabdominal approach.¹ In 1944, Briggs and Milligan introduced the posterior interbody approach and placed laminectomy autograft in the interbody space.² In 1946, Jaslow expanded the concept by inserting an excised spinous process into the intervertebral space.³ In 1953, Cloward popularized the technique by using impacted blocks of iliac crest to provide rigid interbody fixation.⁴ The advantage of supplemental instrumentation was soon realized, and in 1961 Humphreys and colleagues were the first to place an anterior lumbar compression plate with unicortical screws.⁵

Technical challenges and device limitations initially restricted the application of interbody procedures until the advent of new interbody fixation devices increased technical ease and efficacy.^6–14 The modern concept of interbody fixation placement was introduced by Wagner and coworkers following the implantation of a bone-filled cage in horses with wobbler syndrome.^15,16 The intervertebral disk space was distracted by placement of an oversized, perforated, stainless steel cylinder (the Bagby basket) filled with autogenous bone graft, ultimately yielding a reported fusion rate nearing 88%.¹⁷

Material evolution and physical modifications of the Bagby basket led to the production of the Bagby and Kuslich (BAK) implant.¹⁸ The popularity of interbody fixation continued to grow as numerous interbody graft options were developed, including autologous iliac crest graft, structural allograft, and bone chips within metallic cages.^11,19–22 More recently synthetic implants including titanium mesh cages,²³ carbon fiber cages, and polyetheretherketone (PEEK) implants have gained recognition as viable options.²⁴ Augmentation of the interbody implant with pedicle screw fixation has further added to fusion success in comparison to stand-alone grafts.^13,25

A circumferential fusion can be achieved via an ALIF (anterior lumbar interbody fusion, PLIF (posterior lumbar interbody fusion), or TLIF (transforaminal lumbar interbody fusion), each of which can be supplemented with posterior segmental fixation through traditional open or minimally invasive means. The choice of approach depends on multiple factors including the age, gender, and comorbidities of the patient, the levels of pathology, the presence of scar tissue from previous procedures, and anatomic considerations.²⁶

The PLIF procedure with pedicle screw fixation can be performed through a single incision, but has been associated with a radiculopathy rate of up to 13%²⁷ and requires violation of the structural integrity of both facet joints to achieve adequate graft placement. The ALIF can achieve the same interbody support as the PLIF, but without manipulation of the dural or posterior neural structures. However, it often requires significant retraction of the iliac vessels, hypogastric nerves, and peritoneum, exposing them to direct manipulation injury. Other complications include an increased risk of deep venous thrombosis, muscular atony, abdominal wall hernias, and retrograde ejaculation in men.^26,28 When combined with posterior fixation, the additional time, expense, and morbidity of the two-stage procedure remains a limiting factor in its widespread usage.²⁹

To circumvent the difficulties associated with the PLIF and ALIF procedures, Harms and Rolinger in 1982 suggested the placement of bone graft and titanium mesh via a transforaminal route.²³ The TLIF approach minimized retraction on the thecal sac and neural structures, enabled lordosis restoration via anterior graft placement, and spared the contralateral lamina, facet, and pars, thus providing increased surface area for fusion along both the anterior and posterior elements.²³ Furthermore, the TLIF can be performed safely in revision cases with significant epidural fibrosis because of its unique posterolateral trajectory.^29–32

The introduction of minimally invasive and laparoscopic techniques represents the most recent modifications to enable lumbar interbody fusion. Laparoscopic and mini-open ALIF procedures aim to minimize the incidence of hernias and abdominal wall atony.³³ The muscle-splitting posterior minimally invasive approaches aim to diminish iatrogenic soft tissue injury, thus reducing intraoperative blood loss, postoperative pain, and hospital stay.^34,35 Recently, the minimally invasive TLIF procedure has become an increasingly popular method of lumbar arthrodesis.^35,36

Indications For Lumbar Interbody Fusion

The indications for LIF have expanded during the past decade. Current indications include the following: spondylolisthesis (usually grade I or II); DDD causing diskogenic low back pain (with or without radiculopathy); recurrent lumbar disk herniation inducing significant mechanical back pain; postdiscectomy collapse with neural foraminal stenosis and secondary radiculopathy; three or more recurrent lumbar disk herniations with or without back pain; treatment of pseudarthrosis; treatment of postlaminectomy kyphosis; treatment of traumatic instability, and treatment of lumbar coronal and/or sagittal deformities (Table 304-1).²⁶

Table 304-1 Primary Indications for Anterior, Posterior, or Transforaminal Lumbar Interbody Fusion

The relative contraindications for lumbar interbody fusion have also evolved during this period and now include the following: multilevel (three levels) disk disease (in patients without lumbar spinal deformities); single-level disk disease causing radicular pain without symptoms of mechanical low back pain or instability; severe osteoporosis (with attendant risk of graft subsidence through the end plates) (Table 304-2).^6,7,11,26,37

Table 304-2 Relative Contraindications for Lumbar Interbody Fusion

Biomechanical Considerations

Circumferential fusion (360) techniques have distinct theoretical advantages over posterolateral fusion techniques.^4,6,11 As Wolff’s law indicates, fusion potential is enhanced if grafts are placed under compression.²⁶ Interbody fusions place the bone graft in the load-bearing position of the anterior and middle spinal columns, which support 80% of spinal loads and provide 90% of the osseous surface area, thereby maximally enhancing the potential for fusion (Fig. 304-1; Table 304-3).²⁶

FIGURE 304-1 Spinal loads and articular surface area across the lumbar spinal column. Note that 80% of spinal loads and 90% of the articular surface area available for fusion are located in the interbody space.

Table 304-3 Advantages of Lumbar Interbody Fusion Compared with Posterolateral Fusion

Furthermore, supplemental posterior pedicle screw-rod constructs have been shown to increase biomechanical rigidity and decrease pseudoarthrosis rates when associated with an interbody graft.^26,38–41 Lastly, restoration of intervertebral height and segmental lordosis is possible with interbody fusion devices and this restoration has been found to correspond with favorable clinical outcomes.^4,7,11,21

Interbody Grafts

Following a complete interbody discectomy, an interbody device must have design characteristics to not only restore disk height, but create lordosis through the segment, maintain sagittal balance, distract the neuroforaminal space, and restore anatomic weight bearing to the anterior column.^26,42

Over the past 50 years, numerous interbody grafts have been used, including autograft iliac crest, cortical allograft (donor) or synthetic bone, threaded cylindrical titanium cages, impact titanium cages, carbon fiber reinforced or plain PEEK polymer, and impacted carbon fiber reinforced PEEK wedges.^43–46 Osteoinductive materials have also evolved over this time period, from iliac crest bone to bone morphogenetic protein (rhBMP-2).^47,48

The safety and efficacy of iliac crest have been demonstrated over the years by various authors.^5,23,49,50 However, despite its ability to promote interbody fusion, iliac autograft is associated with significant harvest site morbidity in up to 25% of patients.^26,51 This led to the use of tricortical bone allograft for circumferential fusions. Unfortunately, mechanical failures developed because some of these grafts were unable to provide sufficient structural support, thus leading to the open box metal and carbon fiber designs that are routinely used for circumferential fusions in the modern era.²⁴

No interbody graft device is perfect, and metallic cages have some drawbacks. The modulus of elasticity of titanium is significantly greater than that of bone, thus facilitating subsidence through the vertebral end plates in patients with osteoporosis.²⁶ Furthermore, metal implants have been found to incur micromotion and dislodge debris through the fusion segment, which may result in a cellular reaction with subsequent loosening of the bone and device interface.⁴² Metal implants additionally create imaging artifacts secondary to their opacity and scatter potential, thus limiting postoperative fusion evaluation.⁴²

The modulus of elasticity of bone is more closely approximated by nonresorbable polymers such as carbon fiber, which has an unlimited supply and no risk of viral disease transmission or recipient rejection (unlike allograft).²⁶ The carbon fibers are often embedded in a composite material such as PEEK to prevent their breakdown and release.⁴² The biomechanical drawback of a carbon fiber implant is its relative brittleness, thus allowing the potential for splintering, micromotion, and composite material failure. In several large series, interbody graft collapse, slippage, and migration were reported in 3% to 10% of cases.⁷ To circumvent this confound, newer carbon fiber graft designs have incorporated threaded or ridged interfaces, thus minimizing slippage and migration.⁴²

Bone graft substitutes and extenders are alternatives to autograft and allograft and can be used to fill interbody cages. They are classified into two main categories: (1) biologic agents that induce bone formation from native tissues such as BMP analogues (e.g., rhBMP-2), and (2) bone scaffold substitutes such as calcium phosphate salts, including beta-tricalcium phosphate, hydroxyapatite, and wollanosite.⁵²

Posterior Lumbar Interbody Fusion

Patients are placed prone on a surgical frame to accentuate the lordotic lumbar spine. A midline incision and subperiosteal muscle dissection are performed to expose the levels of interest, following which the posterior spinal elements, including the lamina and facets, are completely removed, revealing the rostral exiting and caudal traversing nerve roots and disk spaces. In some cases, the dorsal third of the interspinous ligament may be preserved to act as a fulcrum for a dural retractor and to preserve the posterior tension band. The thecal sac and traversing nerve roots are gently retracted medially. Exposure of the posterior annulus precedes a complete discectomy using rongeurs, disk shavers, and downbiting curets. Denuding of the cartilaginous end plates creates an environment conducive to a successful fusion. Additionally, disk height may be restored with the use of progressively enlarging distractors, thus placing tension on the annulus fibrosis, and compression on the interbody graft, to aid in the fusion process.

In the case of cylindric cages, specialized tube retractors are used to introduce serial reamers into the disk space followed by a bone tap to allow recessing of the cage below the posterior vertebral body. With rectangular ramp type of cages, a square channel is prepared in the disk space to accept the cage, which is tamped into place to engage the vertebral end plates. Cage devices are routinely filled with osteoinductive or osteoconductive materials to provide a fusion scaffold from end plate to end plate.

When placing interbody devices, care must be taken to preserve the vertebral end plates on which the devices will rest. The insert and rotate technique is similar to the impact and wedge technique, but does not require overdistraction or involve the cutting of any channel through the posterior end plates. The bone graft is afterloaded and placed on either side of the implants.

Most modern techniques rely on supplementary pedicle screw instrumentation to assist in stabilization and deformity correction. Following interbody graft placement, the pedicle screw-rod construct is compressed to restore lumbar lordosis while maintaining disk height. The transverse processes are then decorticated and morselized bone graft is placed for a supplemental posterolateral fusion. A standard closure in layers is performed.

Although the traditional posterior lumbar fusion has demonstrated acceptable rates of fusion, it requires an extensive incision to retract the posterior muscles and adequately expose the transverse processes. Some surgeons employ PLIF procedures through minimal access (keyhole) incisions using impacted interbody devices aided by image-guided technology, as detailed in the following sections.

TransForaminal Lumbar Interbody Fusion

As with the PLIF procedure, patients are placed prone on a surgical frame to facilitate lumbar lordosis. A vertical incision is performed and the muscles and soft tissues are retracted laterally to expose the spinous processes, lamina, and facet joints. At a minimum, a unilateral laminotomy and complete facetectomy are performed on the symptomatic side. If needed, further decompression may be obtained by a full laminectomy and contralateral foraminotomy.⁵³

After adequate decompression of the neural elements has been performed, pedicle screws are placed in the standard fashion. A total discectomy is performed and the disk space is gradually distracted using pedicle screws and interbody dilators.⁵³ To facilitate interbody access, the posterior bony lips of the end plates may be removed with a -inch osteotome while carefully protecting the thecal sac and nerve roots. The interbody graft is then placed, followed by pedicle screw-rod compression to restore lumbar lordosis while maintaining disk height (Fig. 304-2).⁵³ The transverse processes are then decorticated for posterolateral fusion, following which the wound is closed in a standard layered fashion.

FIGURE 304-2 TLIF. A, Anterior-posterior and (B) lateral radiographs revealing a radiolucent carbon fiber cage (Concorde, DePuy Spine, Raynham, MA.) and pedicle screws.

Minimal Access Approach

The minimally invasive approach enables access through a fixed tubular retractor, whereas the mini-open approach involves an expandable tubular system. The minimally invasive approach is associated with the least amount of tissue trauma but conversely offers the least exposure for decompression and instrument manipulation.²⁶

For a minimally invasive TLIF operation, the patient is positioned and draped as previously detailed; a fluoroscopically guided 22-gauge spinal needle is inserted over the disk space of interest. A 2.5-cm skin incision is made approximately 4 cm lateral to the midline on the side of the radicular symptoms. A small muscle dilator is directed through the incision toward the facet complex in a lateral-to-medial trajectory. Serial dilators are passed over the initial dilator to create a muscle-splitting surgical channel. The appropriate length tubular retractor is passed over the dilators, centered over the facet joint, and attached to the table frame via a flexible articulating arm. The remainder of the procedure is performed with an operating microscope, endoscope, or loupe magnification.

Residual soft tissue is removed from the dorsal surface of the facet complex using electrocautery and pituitary rongeurs. A total facetectomy is performed using bayoneted osteotomes or the high-speed drill. The initial cut is made across the medial aspect of the inferior facet, disarticulating it from the superior lamina. It is then carefully separated from the synovial membrane and the ligamentum flavum using curved curets. The remaining superior facet is then partially drilled and removed in piecemeal fashion using a Kerrison rongeur. Care must be taken to avoid breaching the inferior pedicle while using the high-speed drill. The ligamentum flavum and synovium are removed in piecemeal fashion to expose the rostral exiting and caudal traversing nerve roots. Epidural bleeding is controlled using hemostatic foam agents and bipolar cautery.

In contrast to the open approach, pedicle screws are placed on the ipsilateral side after the decompression is complete because the hardware often visually limits an already compact surgical field. In the mini-open approach, the ipsilateral pedicle screws are placed under direct visualization through the expandable tube retractor (Fig. 304-3). The contralateral pedicle screws can be inserted either through a paramedian incision through a second expandable retractor or via a percutaneous screw system. In the minimally invasive approach, percutaneous pedicle screws are placed bilaterally. The pedicle screws contralateral to the interbody exposure are connected to a rod to allow sequential distraction of the interspace. Using a combination of interspace wedges, spreaders, and serial distractors, the increased disk height is maintained by fixing the contralateral pedicle screws in the distracted position on the rod.

FIGURE 304-3 Mini-open TLIF. Bilateral ports enable direct visualization for interbody graft placement on the symptomatic side and pedicle screw placement bilaterally.

Based on the surgeon’s preference, local autologous bone, structural allograft, interbody cages, and/or bone substitutes and extenders can be placed into the interbody space. Ipsilateral percutaneous screws and a rod are then placed and the screw-rod construct is gently compressed, thereby re-creating lordosis and compression of the bone graft.

Anterior Lumbar Interbody Fusion

For a retroperitoneal approach, the patient is positioned supine with the sacrum elevated to displace the abdominal contents rostrally. Depending on the surgical levels (supraumbilical for above L4, infraumbilical for below L4), a paramedian or midline incision is used to enable a broad ventral exposure of the vertebral bodies with sparing of the abdominal wall musculature from transection. The rectus sheath is entered, the muscles retracted, the transversalis fascia exposed and incised, and the plane of dissection to the retroperitoneum is entered. When dissecting toward the upper lumbar spine, it may be necessary to detach the left crus of the diaphragm from the anterior longitudinal ligament at L2. All prevertebral tissues are mobilized together and retracted toward the contralateral side.

A mini-open transperitoneal approach can also be performed to access the lumbar interbody spaces. The incision is made through the subcutaneous tissues, fascia, and peritoneum. The abdominal contents are retracted rostrally with a table-mounted self-retaining retractor while the posterior peritoneum is exposed. The retroperitoneal space is entered by incising the midline posterior peritoneum with caudal extension past the aortic bifurcation, following the course of the right common iliac vessels. Meticulous care must be taken to avoid the ureters lying adjacent to these structures. The prevertebral tissues, including the hypogastric plexus, are identified and mobilized laterally. The middle sacral artery and vein are ligated and divided. Bleeding should be controlled with pressure, bipolar cautery, and vascular clips; monopolar cautery risks superfluous damage to the sympathetic plexus.

The mini-open ALIF has been found to be superior to laparoscopic approaches. Laparoscopic approaches have been reported to have higher complication rates, which are probably related to the two-dimensional view afforded by the laparoscope.^26,28

Irrespective of the invasiveness of the approach, following interbody graft placement, the peritoneum and its contents are returned to their normal anatomic position, any peritoneal openings are closed, and the abdominal wall is closed in layers (Fig. 304-4).

FIGURE 304-4 ALIF. Left, ALIF with threaded cages. Right, Lateral radiograph. ALIF with threaded interbody cages filled with rhBMP-2 and supplemented with posterior percutaneous pedicle screw fixation have been used to produce L5-S1 fusion.

Proposed Benefits Of Minimal Access Approaches

Conventional lumbar fusion surgery is associated with iatrogenic soft tissue injury that can have deleterious short- and long-term effects on patient outcomes. Numerous studies have demonstrated that irreversible muscle injury occurs as a result of muscle stripping and retraction and can be associated with poor clinical results.^54–58 The goal of any minimally invasive procedure is to achieve the same surgical objectives as the corresponding open procedure through a less traumatic exposure.

Schwender and coworkers initially confirmed the feasibility and safety of performing the TLIF procedure through a minimally invasive approach in a series of 49 patients using standardized outcome measures.⁵⁹ Visual analogue scale (VAS) and Oswestry disability index (ODI) scores were equivalent to their open counterparts, although mean hospital stay (1.9 days) and average blood loss (140 mL) were significantly lessened. Jang and Lee also reported significant benefits in their minimally invasive TLIF patients.⁶⁰ Isaacs and coworkers compared endoscopically assisted minimally invasive TLIF patients to their open PLIF counterparts and also found advantages with minimally invasive surgery.⁶¹ Mummaneni and Rodts reported the mini-open TLIF technique to enable bilateral direct visualization of both pedicles for the placement of screws through bilateral expandable retractors.⁶² Despite the growing literature base in these minimally invasive surgical endeavors, the learning curve remains steep, and prospective randomized trials comparing them to their open counterparts remain limited, thus providing guarded conclusions at best to date.

Conclusion

Lumbar spinal fusion techniques have evolved considerably over the years, facilitating a significant broadening of indications, patient candidacy, and employable techniques. Contemporary advances in interbody devices and instrumentation systems, particularly in the minimally invasive extent, have endowed the modern-day spine surgeon with an expanded armamentarium to provide circumferential fusion via anterior, posterior, and transforaminal routes.