Can posterior implant removal prevent device-related vertebral osteopenia after posterior fusion in adolescent idiopathic scoliosis? A mean 29-year follow-up study.

PURPOSE

To determine whether posterior implant removal prevents stress-shielding-induced vertebral osteopenia within the posterior fusion area in surgically treated patients with adolescent idiopathic scoliosis (AIS).

METHODS

Eighteen patients with major thoracic AIS (mean age, 43.3 years; range, 32-56 years; mean follow-up, 28.8 years, range, 20-39 years) who underwent posterior spinal fusion (PSF) alone between 1973 and 1994 were included. Participants were divided into implant removal (group R, n = 10, mean interval until implant removal, 50 months) and implant non-removal groups (group NR, n = 8). Bone mineral density was evaluated using the Hounsfield units (HU) of the computed tomography image of the full spine. The HU values of the UIV-1 (one level below the uppermost instrumented vertebra), apex, LIV+1 (one level above the lowermost instrumented vertebra), and LIV-1 (one level below the lowermost instrumented vertebra; as a standard value) were obtained. Stress-shielding-induced osteopenia was assessed as the UIV-1/LIV-1, apex/LIV-1, and LIV+1/LIV-1 HU ratios (× 100).

RESULTS

Overall (median, 25th-75th percentile), the apex (144.7, 108.6-176.0) and LIV+1 (159.4, 129.7-172.3) demonstrated lower HU values than LIV-1 (180.3, 149.2-200.2) (both comparisons, p < .05). Comparison of groups R and NR showed no significant differences in the scoliosis correction rate, bone mineral density of the proximal femur, the HU absolute values of all investigated vertebrae, or in the HU ratios of the investigated vertebrae to LIV-1.

CONCLUSION

Instrumented PSF causes stress-shielding-induced osteopenia of the vertebral body within the fusion area in adulthood, which cannot be prevented by posterior implant removal, probably due to firm fusion mass formation. These slides can be retrieved under Electronic Supplementary Material.