Range of Motion and Adjacent Level Degeneration After Lumbar Total Disc Replacement**

Introduction: The theoretical rationale for total disc replacement (TDR) is that preservation of motion may prevent the development of adjacent level degeneration (ALD) seen in long-term follow-up of fusions. However there are no published data that indicate how much motion is sufficient to reduce the incidence of ALD. The purpose of this study is to examine the relationship between ROM and the development of ALD after TDR and to establish thresholds for ROM below which the risk of ALD is increased.

Methods: We performed a retrospective review of flexion-extension radiographs of 42 patients with 60 Prodisc TDRs at 9-year follow-up. There were 23 males and 19 females. At time of surgery, mean age was 45.2 ± 8.6 years (25-65) and mean weight was 72.4 ± 13.9 kg (52-102). Twenty-one patients (50%) had prior spine surgery. Twenty-seven patients had TDR at one level, 12 had 2 levels, and 3 had implantation at 3 levels. There was one TDR placed at L2-3, 5 at L3-4, 32 at L4-5, and 22 at L5-S1.

Postoperative flexion-extension lateral radiographs were used to determine flexion-extension ROM and to evaluate for adjacent level degeneration. Cephalad adjacent levels were evaluated for ALD: loss of disc space height e”2mm compared to adjacent normal discs, anterior osteophyte formation, or dynamic flexion-extension instability of >3.5 mm.

Graphical analysis was performed plotting the flexion-extension ROM of the most cephalad TDR (x-axis) against the prevalence of radiographic ALD (y-axis) at 9-year follow-up. Statistical significance testing was performed using two-tailed Fisher’s exact test.

Results: Of 42 patients evaluated, 10 patients (24%) with radiographic ALD were identified. Four patients had loss of disc space height, 3 had anterior osteophyte formation, and 3 had both height loss and osteophytes. None had static or dynamic listhesis >3.5mm. The mean TDR ROM measured was 3.8º ± 2.0º (0-18). Mean ROM was 1.5º at L2-3, 3.8º ± 5.5º (0-12) at L3-4, 4.4º ± 4.4º (0-18) at L4-5, and 3.1º ± 2.7º (0- 10) at L5-S1. The patients with ALD had a ROM of 1.6º ± 1.3º (0-4) whereas the patients without ALD had ROM 4.7º ± 4.5º (0-18), (Student’s t-test, p=0.035).

A clear relationship between TDR ROM and the development of ALD at 9-year follow-up was observed. When patients were stratified by ROM, none with ROM e”5º developed ALD. The overall prevalence of ALD was 24% but was higher in patients with less ROM (FIGURE 1). Patients were divided into those with ROM e”5º (n=13) and those with motion <5 º (n="29)." The prevalence of ALD was 0% in the high ROM group and 34% low (Fisher’s exact p="0.021," odds ratio 13.5).

Conclusions: These data clearly demonstrate that the prevalence of adjacent level degeneration after TDR is higher in patients with ROM <5 º, and suggest that 5º of flexion-extension ROM may represent a protective threshold against the development ALD. This information prove useful in design clinical evaluations nonfusion technologies spine surgery. Longer-term follow-up this other patient cohorts is essential.