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