Surgical treatment for degenerative lumbar conditions causing axial low back pain can be considered in two broad categories: fusion procedures and motion-preservation techniques. For stable conditions causing low back pain, fusing two vertebrae together will eliminate the pain arising from their articulation. In an attempt to preserve motion, like in the hip or knee, and prevent accelerated degeneration at the adjacent level, motion-preservation strategies have been developed. For more unstable conditions, such as spondylolisthesis or scoliosis, fusion surgery with or without correction of the deformity, is considered the best surgical option.
Spinal fusion is the surgical attempt at bonding two vertebrae together to stop the motion between them and restore the normal anatomical relationships. Fusion procedures are most commonly performed for those who are considered candidates for surgery. There are a variety of fusion techniques that may include the use of instrumentation, the location of fusion (interbody, intertransverse, interspinous, etc.), the approach (posterior, anterior, lateral), and the type of graft material used (e.g. autograft, allograft, osteogenic biologics) or a combination. A detailed account of all of these techniques is beyond the scope of this topic.
The most commonly employed fusion technique is the posterior approach using pedicle screw-rod instrumentation and fusion across the transverse processes or facet joints (Figure 6). Pedicle screw placement is a technically demanding procedure, but it is the most commonly used technique to stabilize the spine. A retrospective study showed that the rate of screw misplacement can reach 6.7 percent, but no major neurological compromise was observed (Jutte and Castelein, 2002). Therefore, pedicle screw fixation is safe and has an acceptable complication rate despite pedicle breach. Spinal fixation can also be performed with a variety of other instrumentation, such as screws alone, hooks, plates, or wires. Non-instrumented fusions remain a viable option, however, they fail to stabilize the spine during the healing process and are associated with higher rates of failure of fusion (pseudarthrosis).
Fig. 6. Posterior and anterior fusion through posterior-based approach.
AP and lateral radiographs of two-level fusion with posterior pedicle screw-rod construct and TLIF at L4-5. (identified by radio-opaque vertical lines).
Anterior fusions through the disc spaces improve our ability to restore the normal anatomy of the anterior column of the spine by restoring normal disc height and curvature. Generally accepted indications for interbody fusions include degenerative disc disease, disc collapse with resultant neuroforaminal stenosis, and the need to restore sagittal and coronal balance. Interbody fusion creates a bond between two vertebral bodies through the disc space and can be done in combination with posterior fusion or as a stand-alone technique. Anterior fusion can be approached via several different routes: posterior, lateral or directly anterior. The posterior approach, most commonly done in association with a posterior fusion and/or decompression, is performed through a posterolateral approach into the disc space similar to removing a herniated disc fragment. There are two commonly used methods for interbody fusion done through a posterior approach: the posterior lumbar interbody fusion (PLIF) and the transforaminal lumbar interbody fusion (TLIF). PLIF is performed bilaterally and uses the same approach as disc fragment removal. A laminotomy or laminectomy is created to allow exposure of the nerve roots, which are carefully retracted and mobilized. Once the disc is identified, a window is created in the disc, the disc material is removed and the vertebral endplates are denuded of cartilage until there is bleeding bone. A prosthetic cage or structural allograft bone filled with bone graft is inserted into the disc space on both sides. TLIF involves resection of the facet and unroofing the neuroforamen on one side only to get to the posterolateral corner of the intervertebral disc. The traversing nerve root requires less retraction with the TLIF since the approach is slightly more lateral than PLIF. Once inside the disc, it is prepared in a similar way as PLIF. A prosthetic cage or structural allograft filled with bone graft is inserted into the disc space only from one side and placed in a central position inside the disc space (Figure 6). The difference between the two is that TLIF entails less neural manipulation to get to the vertebral disc and is done with a unilateral approach so it is more widely practiced. They both take advantage of the commonly used posterior approach to establish access to the anterior column of the spine.
Anterior lumbar interbody fusion (ALIF) approaches the spine directly anteriorly through the abdomen either through a trans-peritoneal or retroperitoneal approach. The rectus abdominus is retracted laterally which makes this approach truly muscle-preserving. The advantage over a posterior interbody approach (i.e. PLIF or TLIF) is ease of clearing out the disc for fusion, the ability to place a large graft for better restoration of normal anatomical height and better fusion rates, and obviating the need to retract the thecal sac or nerve roots. The potential risks include vascular injury, ileus, and retrograde ejaculation in males.
The lateral trans-psoas approach, is a relatively new procedure that has been gaining in popularity (Figure 7). The patient is placed in the lateral position, and with the use of fluoroscopy and nerve monitoring, a safe corridor through the retroperitoneum and psoas muscle is created to access the disc. While the obvious advantages are that it avoids the need for a posterior approach and can correct spinal instabilities or deformities, it cannot be used to access the L5-S1 disc space.
The technology for motion-preservation techniques are developing at an exponential rate and include a wide range of options such as simple as direct pars repair (Figure 8) (for isthmic spondylolisthesis), interspinous spacers, to more complex devices such as disc replacement, facet replacement, and posterior dynamic stabilizations (Figure 9). Because they are relatively novel concepts, there is a lack of long-term clinical studies demonstrating their effectiveness and safety. While disc replacement is indicated primarily for discogenic pathology, facet replacement aims to treat posterior degeneration and dynamic stabilization intends to limit, but not abolish motion in an unstable spine. The purported benefits of lumbar disc replacement, facet replacement, and dynamic stabilization are to maintain normal motion of the lumbar spinal segment and therefore to potentially decreasing the risk of degeneration at the adjacent segments. Mid-term outcomes of single level total disc replacement showed sustained improved outcome measures at an average follow up of 44.9 months in the treatment of DDD (Scott-Young et al, 2011). However, complications reported in literature such as implant subsidence, loosening, early wear, displacement, malposition, and the difficulty with revision surgery, have limited its widespread use.
Fig. 7. Lateral lumbar interbody fusion.
AP and lateral radiographs of the lumbar spine showing lateral trans-psoas interbody fusion at the L2-L3 level with a side plate and interbody fusion mass as depicted by the white markers.
Fig. 8. Pars repair.
AP and lateral radiographs of the lumbar spine showing pars repair of L4 with pedicle screws, hooks, and rods.
Fig. 9. Artificial total disc replacement.
AP and lateral radiographs of an artificial disc replacement at L4-5.
Surgical treatment for low back pain remains controversial largely due to confusion in terminology and the inability of literature to stratify the results based on specific diagnostic indication. Low back pain should be viewed as a symptom, not a disease or diagnosis. When considered only as a diagnosis, study results are mixed and confounded due to the many different causes. Therefore, it is imperative to elucidate the conditions causing low back pain whether structural or non-structural. When stratified into diagnostic subgroups, results of surgery differ. For example, surgery is beneficial for more structural abnormalities, in particular those with more instability such as spondylolisthesis and degenerative scoliosis, as opposed to non-structural conditions which are better treated with non-surgical modalities. While the preferred method of treatment for these degenerative conditions is a non-surgical approach, there are many patients who are candidates for surgery. Although the traditional surgical strategy for structural degenerative conditions is fusion, motion-sparing techniques are showing promise, however, long-term studies are needed. More unstable degenerative conditions benefit more from fusion procedures with correction of deformities. Only with a more refined diagnostic ontology and a better understanding of the pathomechanical processes, can we hope to determine the best treatments available for patients suffering from these conditions.