Adjacent segment disease (ASD) is a common complication following ACDF, occurring at a rate of approximately 3% per year. Traditionally, surgeons treat this with ACDF at the adjacent level using a plate, screws, and bone graft. They frequently have to remove the original plate in order to accomplish this, which requires greater dissection and can be technically difficult with some plate designs. Zero-profile stand-alone cages (SAC) with integrated screws were initially promoted for having lower dysphagia rates, and surgeons subsequently realized they were advantageous in treating ASD as the index plate did not have to be removed to place the device. As such, SACs were rapidly adopted to treat ASD, without much data supporting their use in that application. Many SAC designs include only 2 screws compared to the typical plate with 4 screws, so there is some concern that the SAC could be biomechanically inferior to a plate and screws in the ASD environment where stresses are higher. In order to compare SACs to the traditional plate and screws for ASD, Dr. Gandhi and colleagues from William Beaumont Hospital retrospectively reviewed 46 patients undergoing ACDF for ASD, 17 of whom were treated with plate and screws and 29 of whom were treated with a SAC. They also included a control group of 40 patients undergoing primary single level ACDF with plate and screws. Their primary outcome was fusion rate at a minimum follow-up of 6 months, and this was determined based on bridging bone and/or a lack of motion (<2 mm between spinous processes) on flexion-extension x-rays. Patients suspected of pseudarthrosis were also evaluated with a CT scan. They found an 82% fusion rate in the plate and screw cohort compared to a 69% fusion rate in the SAC cohort, a non-significant difference. The primary ACDF cohort had a fusion rate of 95%, which was significantly higher than the SAC ASD cohort. There were zero revisions in the patients treated with a plate and screws in both the primary and ASD groups, and 14% of the SAC ASD patients underwent revision (2 for pseudarthrosis and 2 for hardware migration).
The authors did a nice job creating a small retrospective cohort study comparing fusion rates between SAC and plate and screw constructs when treating ASD. The study has all of the limitations typical for a small, retrospective study, including lack of power and selection bias. While the plate and screw cohort had a fusion rate 13% higher rate of fusion, which translates to a 72% increase in relative risk of pseudarthrosis, this difference was not significant due to the low numbers involved (3 non-unions in the plate group compared to 9 in the SAC group). Additionally, the groups were likely treated by different surgeons and were different at baseline in both measured and unmeasured ways, so selection bias could be confounding the results. The average duration of follow-up was not reported, and a minimum follow-up of 6 months was likely not sufficient to diagnose pseudarthrosis. It has been shown that radiographic fusion can take up to 2 years to occur, so some patients classified as having non-unions in this study may have gone onto solid fusion over time. While 14 of 86 patients were classified as having non-union in this study, only 2 went onto revision surgery for this indication, so the majority of the non-union patients were likely relatively asymptomatic. The limited and variable follow-up duration also makes it difficult to interpret the revision rates as they undoubtedly would increase with longer follow-up. Despite its significant limitations, this paper does raise the question of whether or not SACs are the best option for ASD given that they may be biomechanically inferior to a plate and screws in this application. Clearly more study is needed to answer the questions, and the authors point out that SACs that incorporate more screws may have better fusion rates. It seems as though a plate and screws may be the best option in situations where the original plate is easily removed. However, in cases where the plate cannot be easily removed (i.e. index 3 level plate, stripped screws, etc.), the SAC may have an important role.
Please read Dr. Gandhi's article on this topic in the June 1 issue. Does this change how you view the use of SACs for ASD?
Adam Pearson, MD, MS
Associate Web Editor