THE SIGNIFICANCE OF ANGULAR MISMATCHING BETWEEN VERTEBRAL ENDPLATE AND PROSTHESIS ENDPLATE IN L-TDR

Abstract

The prosthesis anchored to the vertebral body by a large central keel has inherent risk of angular mismatching between vertebral endplate and prosthesis endplate at large lordotic segment such as L5-S1. Theoretically, these angular mismatching can be considered to cause several problems such as segmental hyperlordosis, anterior positioning of upper prosthesis, posterior prosthetic edge subsidence, decreased ROM and poor clinical outcome. The purpose of this study is to assess whether angular mismatching between vertebral endplate and prosthesis endplate in lumbar total disc replacement (L-TDR) with ProDisc-L influence on radiological and clinical outcomes.

We evaluated 64 levels of 56 patients who were implanted with ProDisc-L from 2002 to 2006. Prosthetic levels were 38 levels of L4–5, and 26 levels of L5-S1 (8 patients had 2 level-operations of L4–5 and L5-S1). Mean follow-up was 25.6 (12–49) months. Angle of mismatching between lower endplate of upper vertebral body and upper prosthetic plate, segmental flexion/extension ROM, segmental lordosis angle at extension, distance from the posterior wall of vertebral body to posterior prosthetic edge were measured in the radiographs. Clinically VAS and ODI were evaluated. Angular mismatching between upper vertebra and prosthesis of L4–5 and L5-S1 was 1.6° (range, 0–6°) and 5.6° (0–13°) (p< 0.001) respectively, at final follow-up. Angular mismatching at immediate postoperative radiographs (2.3° in L4–5 and 4.9° in L5-S1) and at final follow-up was not significantly different (p=0.324 in L4–5, 0.620 in L5-S1). Mean segmental ROM of operated levels was 10.6° (4–22°) in L4–5 and 6.1° (2–13°) in L5-S1(p< 0.001). Mean segmental ROM, mean segmental lordosis angle, and mean distance from posterior margin of vertebral body to posterior end of prosthesis in L5-S1 were 6.8° (4–13°), 12.8° (8–17°), 3.8mm (1–6mm) in cases with angular mismatching less than 10°, and 4.6° (2–7°), 21.3° (19–25°), 6.0 mm (2–8mm) in that of 10° or more (p=0.024, < 0.001, 0.039), respectively. In L4–5 angular mismatching of more than 5° were only 2 cases without statistical significance. Clinical outcomes, VAS and ODI, of L4–5 compared with that of L5-S1 and of angular mismatching less than 10° with that of 10° or more in L5-S1 did not have difference between them (p> 0.05). Angular mismatching between lower endplate of upper vertebra with upper prosthesis endplate is more common in L5-S1 than in L4–5. L-TDR at the most lordotic level, L5-S1, implantation of upper prosthesis with mismatched angle seems to be the causes of lessened segmental ROM, increased segmental lordosis, and anterior positioning of prosthesis.