To the Editor:

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We read with interest the recent study published in the Global Spine Journal comparing the rate of lumbar facet injections following total disc replacement (TDR) versus ALIF/LLIF using a national administrative database. While the authors present meaningful long-term follow-up data, we respectfully offer several clarifying points and concerns regarding the study’s rationale and conclusions.

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The authors conclude that TDR is associated with a higher rate of facet injections and suggest this may reflect a greater progression of symptomatic facet arthrosis. However, the use of facet injections as a surrogate for facet arthrosis is problematic. Arthrosis is, by definition, a radiographic diagnosis, and injections—while valuable for symptom management—can be influenced by physician practice patterns, patient expectations, and non-specific axial back pain. Without imaging correlation or patient-level clinical data, interpreting injections as a marker of structural degeneration is speculative at best.

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Importantly, the TDR cohort likely comprised articulating ball-and-socket designs such as the ProDisc, which were referenced by the authors in their discussion. These first-generation implants are known to produce non-physiologic motion and may increase facet loading due to constrained kinematics and lack of shock absorption. This mechanical profile may reasonably explain the observed increase in facet-related interventions, independent of true arthrosis progression.

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In contrast, elastomeric TDRs, such as the AxioMed® Freedom Lumbar Disc—which was not included in this study—feature a monolithic viscoelastic core designed to mimic the damping behavior and multidirectional stiffness of the native disc. By absorbing loads and restoring more natural motion patterns, these designs may mitigate the very facet-related complications highlighted in this analysis. Grouping all TDRs together, without accounting for biomechanical differences, limits the generalizability of the study’s conclusions—particularly in the context of modern, elastomeric technologies.

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Additionally, postoperative segmental lordosis was not addressed. Fusion techniques like ALIF and LLIF inherently restore sagittal alignment through lordotic cage placement, potentially unloading the facets. Most early TDRs were non-lordotic, which could contribute to posterior element stress. Without controlling for alignment or implant geometry, conclusions about long-term facet health remain incomplete.

Finally, while large databases offer valuable population-level insights, they lack granular clinical and radiographic detail necessary to draw conclusions about structural degeneration or implant biomechanics. Future studies should differentiate between implant types, incorporate imaging, and evaluate postoperative alignment to more accurately assess the impact of TDR on facet health.

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We appreciate the authors' contribution to the growing literature on lumbar TDR and hope these points help contextualize the findings for readers and guide future research in the evolving field of motion-preserving spine technologies.

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Sincerely,
Dr. Kingsley R. Chin MD, MBA
KIC Ventures

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