A Prospective Cohort Analysis of Adjacent Vertebral Body Bone Mineral Density in Lumbar Surgery Patients with or without Instrumented Posterolateral Fusion: A 9 to 12 Year Follow-Up

Purpose: To determine by dual energy x-ray absorptiometry (DEXA) long-term BMD changes that occur at the adjacent three levels above an instrumented posterolateral lumbar fusion or an isolated laminotomy and lumbar discectomy.

Methods: DEXA was performed initially at a mean post-operative follow-up of 4.0 years (range, 2.3 to 5.5 years) and again at a mean of 10.8 years (range, 9.1 to12.4 years). The discectomy group (n=4; mean age, 57.8 years) underwent lumbar hemilaminotomy without fusion, whereas the other group (n=7; mean age, 60 years) underwent pedicle-screw instrumentation and posterolateral lumbar fusion. All patients underwent surgical procedures at the L4-L5 or L5-S1 levels with DEXA analysis being performed on the adjacent three cephalad levels. Peripheral sites were also included in the DEXA analysis.

Results: At the mean 10.8-year follow-up, the fusion group was noted to have at the adjacent level, two levels cephalad, and three levels cephalad normalized BMDs of 1.47, 1.39, and 1.27 respectively. A 14.8%, 10.8%, and 9.5% increase respectively in normalized BMD was observed in the fusion group when compared to the mean 4-year fusion values (p 0.05). No statistically significant difference was noted in hip BMD at the mean 4-year and 10.8-year follow-up (1.05 vs. 1.03), suggesting that the effects were local.

Conclusions: The local BMD adjacent to an instrumented lumbar fusion is increased at a mean of 10.8-years after surgery. There is a gradual decrease in BMD changes with increasing distance from the fusion level. Alterations in fusion site biomechanics and modulus mismatch between the host bone and the spinal instrumentation most likely result in chronic, localized bone remodeling with an increased BMD that decreases the greater the distance from the fusion mass.