Long-Term Benefits After Lumbar Disc Replacement—Questions Remain

Commentary The article by Marnay et al. reports medium and long-term outcomes for patients with degenerative disc disease (DDD) undergoing lumbar disc replacement. The advantage of this research over existing efforts is threefold: this is one of the largest cohorts with the longest follow-up and was generated from a single institution. While single-center retrospective research will always raise questions of generalizability, the relative standardization of the technique and clinical care in this cohort conferred strength through uniformity. The data showed a substantial postoperative improvement, ranging from 24 to 26 points, in the Oswestry Disability Index (ODI) in patients with or without a prior discectomy who underwent 1 or 2-level lumbar disc replacement. This is well above the published minimum clinically important difference (MCID) thresholds for the ODI, which frequently range from 10 to 15 points1. These overwhelmingly positive results quantify the improvement in the lives of the 1,187 patients included in this cohort. Yet 2 questions still remain unanswered on this research topic. The first is how to select patients who will benefit from this surgery. Table II provides the inclusion criteria, which primarily involve the diagnosis of DDD. This diagnosis is anachronistic and, considering the hallmark study by Boden et al., the majority of the age-matched human population has DDD2. The results of the study by Marnay et al. indicate that the involved surgeons are expert diagnosticians. However, from this angle, the single-center nature of the study becomes a hindrance. Careful patient selection amidst the ubiquity of radiographic findings requires experience and a working relationship alongside a nonoperative team of physiatrists and psychologists that most spine surgeons are not afforded. The second unanswered question is how lumbar disc replacement compares to its counterpart, anterior lumbar interbody fusion (ALIF). In a retrospective study of patients with similar pathology, Guyer et al. demonstrated a similarly impressive 20-point reduction in the ODI with stand-alone ALIF3. Although Marnay et al. discuss and cite the superiority of lumbar disc replacement over ALIF as a foregone conclusion, there is an ample body of literature supporting the equivalency of the 2 procedures4. The consideration of adjacent-level degeneration is germane to research on disc replacement, and the rate of new, adjacent-level surgery following disc replacement in this cohort (1.9%) was impressively low. One potential explanation for this result is the meticulous surgical technique, which is helpfully summarized as "5 fundamental technical rules" in the Discussion section. Interpretation of the adjacent-level results should take note of the cohort attrition (summarized in Table IV), as few patients made it to 21 years of follow-up. In summary, the authors successfully demonstrated meaningful clinical improvement after both 1 and 2-level lumbar disc replacement regardless of whether the patients had previously undergone a discectomy. This benefit was sustained in the long term, which deserves recognition because the length of follow-up is perhaps the most salient variable in any orthopaedic outcomes study. However, the assertion that the lack of long-term data has limited "the wider acceptance of" lumbar disc replacement is specious reasoning. Rather, it is the lack of discrete diagnostic criteria for patient selection and a need for higher-quality data comparing lumbar disc replacement with ALIF and nonoperative study arms that prevent universal adoption.