Disuse Bone Loss in Fusion Constructs After Multilevel Lumbar Fusion: A Computed Tomography Hounsfield Unit Analysis

Objective: To evaluate long-term bone quality changes within the fusion construct (FC) after 2- to 3-level lumbar fusion using computed tomography (CT)-derived Hounsfield units (HUs).Methods: Among 520 screened patients, 222 who underwent 2- to 3-level posterior lumbar interbody fusion met the inclusion criteria. HU values were measured on CT scans preoperatively, at 1-year postoperative, and at final follow-up. The percentage change in HU (HU [final–pre]%) was calculated for each vertebral level.Results: At the final follow-up, the FC demonstrated a significant decline in HU compared to preoperative values (median [10th–90th percentile], 132.0 [86.5–220.4]; 95% confidence interval [CI], 116.0–142.5 vs. 124.5 [71.0– 210.0]; 109.8–135.1; HU (final–pre)%: -11.0 [-62.0 to 48.5]; -19.9 to -6.1; p<0.001). In contrast, HU increased significantly at the uppermost instrumented vertebra (HU (final–pre)%: median [10th–90th percentile], 28.3 [-19.9 to 102.9]; 95% CI, 21.1–36.4; p<0.001), likely reflecting increased mechanical demands. Subgroup analysis revealed a more pronounced decline in HU in patients with longer follow-up durations, particularly in the FC group (p=0.003).Conclusion: CT-derived HU revealed progressive trabecular bone loss within FC over time after lumbar fusion. In patients with longer postoperative intervals, clinicians should remain aware of the potential weakening of the FC, which has important implications when considering implant removal or planning revision surgery.