Disc height and motion patterns in the lumbar spine in patients operated with total disc replacement or fusion for discogenic back pain. Results from a randomized controlled trial

Background context: Fusion is considered the "gold standard" in surgical treatment of degenerated disc disease; the intended postoperative goal is absence of mobility, but treatment may induce degeneration in adjacent segments. Total disc replacement (TDR) aims to restore and maintain mobility by replacing a painful disc. Little is known about the degree and quality of mobility in artificial discs in vivo and whether maintained mobility reduces the stress on adjacent segments that is believed to occur after fusion.

Purpose: To see if surgical goals for respective treatments had been reached, if clinical outcome was related to this, and if differences in disc height at treated segments and adjacent segment motion patterns between groups occurred.

Study design: Comparison of X-ray measurements in a randomized controlled trial between instrumented posterior fusion and TDR for chronic low back pain (CLBP) assumed to be discogenic. Results were compared with clinical outcome.

Patient sample: The material consisted of 152 patients suffering from CLBP assumed to be discogenic. Seventy-two patients were treated with fusion and 80 with TDR.

Outcome measures: Results of X-ray measurements were compared between groups and related to self-reported clinical results regarding back pain and disability.

Methods: Flexion-extension X-rays were analyzed preoperatively and 2 years postoperatively using distortion-compensated Roentgen analysis (DCRA) at treated and adjacent levels, and mobility after fusion and TDR was estimated. Changes in disc height and range of motion (ROM) respective translation in adjacent segments were compared between groups. Results of DCRA measurements were also compared with the clinical outcome.

Results: Preoperative flexion-extension ROM was similar between the fusion and TDR groups, and preoperative disc heights of segments to be treated were between one and two standard deviation less than that previously established in a normative database. Seventy percent of fused patients had no mobility, whereas 85% of TDR patients were mobile. Fulfillment of surgical goals was correlated to neither back pain nor disability. Fused segments were lower and TDR segments were higher than normative values postoperatively. There were also significant differences at adjacent segments, there being more translation and flexion-extension in the fusion group than in the TDR group.

Conclusions: This very accurate X-ray method (DCRA) indicates that surgical goals were reached in most patients. This, however, was not correlated to clinical outcome. Differences between the groups in postoperative disc height at treated segments, respective ROM, and translation at adjacent segments did not affect the clinical outcome after 2 years.