A prospective randomised study on the long-term effect of lumbar fusion on adjacent disc degeneration

Abstract

The existence and importance of an accelerated adjacent segment disc degeneration (ASD) after lumbar fusion have previously not been demonstrated by RCTs. The objectives of this study were, to determine whether lumbar fusion in the long term accelerates degenerative changes in the adjacent disc and whether this affects the outcome, by using a prospective randomised design. A total of 111 patients, aged 18-55, with isthmic spondylolisthesis were randomised to exercise (EX, n = 34) or posterolateral fusion (PLF, n = 77), with (n = 37) or without pedicle screw instrumentation (n = 40). The minimum 10 years FU rate was 72%, with a mean FU time of 12.6 years (range 10-17 years). Three radiographic methods of ASD quantification were used, i.e. two digital radiographic measurement methods and the semi quantitative UCLA grading scale. One digital measurement method showed a mean disc height reduction by 2% in the EX group and by 15% in the PLF group (p = 0.0016), and the other showed 0.5 mm more disc height reduction in the PLF compared to the Ex group (ns). The UCLA grading scale showed normal discs in 100% of patients in the EX group, compared to 62% in the PLF group (p = 0.026). There were no significant differences between instrumented and non-instrumented patients. In patients with laminectomy we found a significantly higher incidence of ASD compared to non laminectomised patients (22/47 vs. 2/16 respectively, p = 0.015). In the longitudinal analysis, the posterior and anterior disc heights were significantly reduced in the PLF group, whereas in the EX group only the posterior disc height was significantly reduced. Except for global outcome, which was significantly better for patients without ASD, the clinical outcome was not statistically different in patients with and without ASD. In conclusion, the long-term RCT shows that fusion accelerates degenerative changes at the adjacent level compared with natural history. The study suggests that not only fusion, but also laminectomy may be of pathogenetic importance. The clinical importance of ASD seems limited, with only the more severe forms affecting the outcome.

Fig. 1

Measurements according to the digital radiographic measurement method: The anterior and posterior disc heights are measured relative to the anterior vertebral body height, and olisthesis relative to the vertebral body antero-posterior depth. The disc heights are measured at a 90° angle to a straight line going through the two lower corners of the vertebra above. The olisthesis is measured from the posterior disc height line to the posterior corner of the vertebra above

Fig. 2

Examples of 2 patients operated on with non-instrumented PLF with total posterior disc height reduction at long term follow up. Arrows indicate the level adjacent to the fusion

Fig. 3

Examples of one physiotherapy patient (EX) and one fused patient with pedicel fixation (PLF) without disc height decrease at long term follow up 13 years later. Arrows indicate the level adjacent to the slip-level and the fusion, respectively