Interbody Fusion Approaches and Techniques

Interbody fusion can be performed via several surgical approaches including anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), and transforaminal lumbar interbody fusion (TLIF) ( Fig. 24.2 ). The ideal setting for fusion to develop across the disk space is a clean bone surface across apposing endplates. Therefore regardless of the surgical approach, a total diskectomy is performed at the level of concern through a surgical window in the annulus fibrosus. An osseous substrate is then implanted into the disk space in between the decorticated bleeding surfaces of the adjacent endplates, which serves as a favorable environment for fusion by means of bone formation and healing. The insertion of bone graft material may be conducted with or without interbody spacers (to achieve ligamentotaxis). The implanted osseous substrates placed in the disk space are generally in the form of interbody spacers that are either solid or openwork structures usually filled with bone graft material. The spacer is meant to maintain alignment, provide spinal column support, and promote fusion. Fusion may be attempted at a single or multiple spinal segments.

Lumbar interbody fusion (LIF) approaches. Axial cross-sectional illustration at the level of the lumbar spine demonstrates the trajectory of various anterior and posterior lumbar fusion approaches. ALIF , Anterior LIF; XLIF/LLIF, lateral LIF; PLIF , posterior LIF; TLIF, transforaminal LIF.

In addition, supplemental segmental hardware instrumentation (posterior rods and pedicle screws, plates, and intervertebral implants) may accompany the interbody fusion ( Fig. 24.3 ). Instrumentation itself is not meant to fuse but to stabilize, so that osseous fusion may develop. The aim of the hardware is to immediately stabilize the spine, maintain alignment, limit postoperative segmental motion, and replace excised anatomic components. The hardware limits motion by acting as a rigid internal fixation device and thus provides the stability that favors osseous fusion. An understanding of this point is essential, since it is acknowledged that instrumentation without osseous fusion will be generally unsuccessful. In other words, hardware itself is insufficient for fusion.

Anterior-posterior (A) and lateral (B) 3D reconstructions of the lumbar spine s/p L3–L4 posterior lumbar interbody fusion. Small metallic markers along the anterior and posterior aspects of the disc spacer are present (highlighted in blue ). Posterior fusion hardware including bilateral transpedicular screws at L3 and L4 are in place (also highlighted in blue ).

Anterior Lumbar Interbody Fusion

If an interbody fusion is large enough, it can on its own provide sufficient motion limitation and biomechanical stability to promote fusion. ALIF provides optimal disk space access allowing for complete anterior and central disk removal, which in turn allows for an optimally sized device to be placed. Therefore ALIF is the only approach that does not require supplemental posterior hardware to ensure stability and motion limitation throughout the fusion process. ALIF utilizes single large cages typically supplemented by an anterior plate ( Fig. 24.4 ). The ALIF approach avoids damage to the motion limiting posterolateral vertebral elements, avoids damaging the posterior musculature, and decreases the risk of epidural fibrosis. If delayed arthrodesis is evident after stand-alone ALIF, a second posterior instrumentation procedure can be performed to further promote solid arthrodesis. Importantly, the approach requires skilled surgical expertise to avoid abdominal and vascular complications.

Patient status post L5–S1 anterior lumbar interbody fusion 2 years prior. A lateral radiograph (A) shows metallic hardware in place along the anterior aspect of the disc space (blue arrow) . Mature solid fusion is clearly evident within and surrounding an optimally sized large radiolucent disc spacer on axial (B), sagittal (C), and coronal images (D).

Posterior Lumbar Interbody Fusion

This approach typically requires laminectomies and often facetectomies. The posterior approach allows direct access to the spinal canal allowing for complete decompression of the thecal sac and nerve roots. Retraction of the thecal sac and nerve roots is necessary to access the disk. The size of the devices that can be placed is therefore limited, which in turn reduces the stability of the construct ( Fig. 24.5A ). Supplemental posterior instrumentation is therefore frequently performed and the approach facilitates optimal placement of posterior hardware.

Posterior lumbar interbody fusion (PLIF) versus transforaminal lumbar interbody fusion (TLIF). Axial illustration of the PLIF approach (left) depicts the retraction of the thecal sac and nerve roots necessary to access the disk. Axial illustration of the TLIF approach (right) shows how this approach allows for foraminal decompression with greater access to the disk.

Lumbar Interbody Fusion Devices

There are numerous interbody fusion devices of differing size and morphology. While the nuances of each device are beyond the scope of this discussion, it is important to note that they fall under three general categories: metal devices, composite devices, and biologic devices. Therefore not only do their morphologies differ, but so do their attenuation and visibility on computed tomography (CT) imaging. These include bone graft substitutes such as recombinant human bone morphogenetic protein (rhBMP-2). rhBMP-2 acts as signaling molecule to initiate bone formation and is radiolucent at the time of placement.