Segmental Contribution Toward Total Lumbar Range of Motion in Disc Replacement and Fusions A Comparison of Operative and Adjacent Levels

Study Design. Radiographic results from a prospective, randomized, multicenter trial assessing patients who underwent lumbar total disc replacement (TDR) or circumferential arthrodesis for 1-level degenerative disc disease. Objective. To quantify the relative segmental contribution to total lumbar range of motion (ROM) at the operative level at each adjacent level in lumbar TDR and arthrodesis. Summary of Background Data. Although previous studies have evaluated ROM in TDR and fusion, no study has quantified or compared the relative segmental contribution to total lumbar ROM. Further, no study to the best of our knowledge has evaluated the kinematic profile of the more cranial adjacent segments (i.e., the second or third adjacent levels) following implantation of either TDR or fusion. Methods. Radiographic data collected from all randomized 1-level degenerative disc disease patients operated at L4/5 or L5/S1 that participated in the multicenter, prospective, randomized IDE study comparing ProDisc-L with circumferential lumbar arthrodesis were evaluated before surgery and at 24 months. Radiographic measurements were performed independently using custom digitized image stabilization software to generate ROM at the operative level, and at each cranial and caudal adjacent level. Results. There were 200 total patients included (155 ProDisc-L, average age 39 years; 45 arthrodesis, average age 40 years). At 24 months, the L4/5 TDR group experienced a significant improvement in total lumbar ROM from baseline (+6.3 degrees), whereas there was no change seen with L5/S1 TDR or any fusion group. Between-group comparisons from baseline to 24 months postoperatively revealed: (1) significantly more contribution from the operative level towards total lumbar range in TDR at operative level L4/5 (TDR: -2.5%, fusion: -16.8%, P = 0.006), and operative level L5/S1 (TDR: -5.1%, fusion: -15.9%, P < 0.001), and (2) the relative contribution towards total lumbar range of motion from the first cranial adjacent segment to fusion at L5/S1 increased by 12.1%, compared with -1.2% seen in TDR (P = 0.03). There were elevated contributions from the more cranial adjacent levels to a fusion when compared with TDR, however, these differences were not statistically significant. At operative level L4/5, there was significantly increased ROM from the first caudal segment below TDR (6%, P = 0.03), but not below fusion (3.1%, P = 0.59). Conclusion. In conclusion, patients with TDR lost slight relative contribution to total lumbar motion from the operative level which was mostly compensated for by the caudal adjacent level (if operated at L4/5). In contrast, the significant loss of relative range of motion contribution from the operative level in fusions was redistributed among multiple cranial adjacent levels, most notably at the first cranial adjacent level.