Preoperative CT-derived Hounsfield unit values and mechanical complications after thoracolumbar or lumbar instrumented fusion: a systematic review and meta-analysis

Background/objectives Mechanical complications after thoracolumbar or lumbar instrumented fusion remain a major concern, especially in patients with poor bone quality. CT-derived Hounsfield unit (HU) values may offer an accessible marker of local bone strength, but the evidence is inconsistent. We evaluated its association with postoperative mechanical complications. Methods PubMed, Embase, Web of Science, Scopus, the Cochrane Library, and Google Scholar were searched for studies of adults undergoing instrumented fusion reporting HU and postoperative mechanical outcomes. The primary outcome was overall mechanical complications. Random-effects models were used; continuous per-HU odds ratios, dichotomous low-versus-high estimates, and hazard ratios were analyzed separately. Results Thirty-five retrospective studies were included. In twelve reporting an adjusted per-HU odds ratio, each one-unit increase carried lower odds of mechanical complications (OR 0.983; 95% CI, 0.978–0.989; P < 0.001), with moderate heterogeneity (I 2 = 46.6%) and a 95% prediction interval of 0.974–0.993; per 10 HU the pooled OR was 0.846 (95% CI, 0.802–0.891). The estimate was stable on leave-one-out analysis (range 0.982–0.984) and after excluding the one study at high risk of bias (OR 0.984; 95% CI, 0.980–0.988). Outcome-specific estimates were directionally consistent but imprecise (screw loosening, k = 3, OR 0.980, 95% CI 0.949–1.012; cage subsidence, k = 5, OR 0.982, 95% CI 0.971–0.993). Of 25 studies comparing lower- with higher-HU groups, all placed lower HU at higher risk, but thresholds ranged from approximately 71–369 HU; the 11 with an odds ratio reported by the study or reconstructable from its data were summarized descriptively rather than pooled. Conclusions Lower preoperative HU is associated with a higher risk of postoperative mechanical complications. The per-unit association was small but significant; low-versus-high comparisons were directionally consistent but rested on disparate thresholds and were not combined. HU is best used as an opportunistic marker that prompts further bone-health evaluation and individualized planning, rather than as a validated risk-stratification tool, a fixed per-unit estimate, or a universal threshold. Standardized HU measurement, consistent outcome definitions, and prospective validation are needed. Systematic Review Registration https://www.crd.york.ac.uk/PROSPERO/view/CRD420261376259 , PROSPERO, CRD420261376259.