Adjacent segment disease (ASD) is a well-known phenomenon following fusion of any region of the spine. It remains poorly understood, and the degree to which it is iatrogenic or simply represents the natural history of spinal degeneration is still unclear. Regardless of the etiology, surgeons continue to search for ways to decrease the rate at which it occurs and to prevent the subsequent degradation of clinical outcomes and re-operations that can accompany ASD. While patient factors (i.e. genetic predisposition to degeneration, age, smoking, obesity, etc.) are typically beyond surgeon control, technical factors such as type of fusion (i.e. uninstrumented, instrumented, interbody) and damage to the facet joint cranial to the fusion can be modified. Dr. He and his colleagues from China performed an RCT comparing two different pedicle screw trajectories in order to determine if a more lateral starting point and steeper pedicle screw trajectory decreased the rate of ASD at the level superior to the fusion. They randomized 210 patients with L4-5 or L5-S1 isthmic spondylolisthesis undergoing posterolateral instrumented fusion to one of two pedicle screw trajectories: one with a more medial starting point in the vicinity of the mammillary process and a straighter trajectory and the second with a more lateral starting point at the junction of the transverse process and the lateral border of the facet joint and a more medialized trajectory. They followed all patients for at least 9 years to determine the rate at which they developed radiographic ASD and clinical ASD, the latter defined as needing a re-operation to address symptomatic ASD. They also recorded the final Oswestry Disability Index score. The group with the more lateral starting point developed ASD at a significantly lower rate (52% vs. 72%) and had a significantly lower rate of clinical ASD (0% vs 8%). Final Oswestry scores were similar for the two groups, though there was a trend for the lateral starting point group to do somewhat better (final ODI 20 vs. 24, p=0.07).
This relatively simple study provides reasonably strong evidence that a more lateral starting point results in less radiographic ASD, though it still occurs in the majority of patients. Intuitively, this makes sense, as the superior screw is less likely to cause ASD if it is further away from the unfused adjacent facet joint. Prior studies have shown relatively high rates of facet violation by the superior pedicle screw, and a higher rate of ASD in such a case would be expected.1 Even if the facet is not violated, it seems likely that a screw that is closer to the joint surface will result in increased pressure on the cartilage and potentially accelerate degeneration. The current paper did not perform post-operative CT scans on all patients, and they did not evaluate the rate of facet violation, though it is a reasonable assumption that a more lateral starting point would reduce this. While a more lateral starting point seems preferable in terms of reducing ASD, it can be technically more challenging to insert the screw with such a trajectory, particularly in larger patients. The authors did not comment on the rate of misplaced screws or need for screw revision, but it seems that a more lateral starting point could increase the rate of laterally misplaced screws. This paper does offer further support to the idea of trying to protect the facet joint superior to the fusion as much as possible, and a more lateral starting point seems preferred as long as it can be achieved technically. Given the 52% rate of radioagraphic ASD even with the more lateral starting point, it is clear that more work has to be done. It still remains to be seen if pedicle screw instrumentation improves outcomes in most lumbar degenerative conditions.2 Their use clearly improves fusion rate, but we still do not know if the potentially higher rate of ASD mitigates this benefit.
Please read Dr. He’s article on this topic in the June 15 issue. Will this article change how you place lumbar pedicle screws? Let us know by leaving a comment on The Spine Blog.
Adam Pearson, MD, MS
Associate Web Editor
1. Shah RR, Mohammed S, Saifuddin A, Taylor BA. Radiologic evaluation of adjacent superior segment facet joint violation following transpedicular instrumentation of the lumbar spine. Spine (Phila Pa 1976) 2003;28:272-5.
2. Abdu WA, Lurie JD, Spratt KF, et al. Degenerative spondylolisthesis: does fusion method influence outcome? Four-year results of the spine patient outcomes research trial. Spine (Phila Pa 1976) 2009;34:2351-60.