8 - The Influence of Bone Health on Risk of Mechanical Complications After Spinal Fusion for Degenerative Lumbar Disease: A Systematic Review

Background Context
Patients with degenerative spinal disease often have underlying osteoporosis, which may be a risk factor for complications after surgical treatment. However, the relationships between bone health and specific outcomes of lumbar fusion surgery have yet to be concretely defined. Therefore, we present the first systematic review investigating the association between osteoporosis and development of mechanical complications in patients undergoing spinal fusion for degenerative lumbar disease (DLD).

Methods
PubMed, Embase, and Web of Science databases were searched for original clinical research articles reporting on osteoporosis and outcomes of spinal fusion. Eligible studies comprised at least 10 patients and evaluated measured bone health as a predictor of mechanical complications or need for revision surgery in adults undergoing instrumented lumbar fusion for DLD. Biomechanical and cadaveric experiments were excluded, as were studies involving cervical procedures or those performed for tumor, trauma, or infection. Evidence quality was assessed by two reviewers independently using the GRADE method.

Results
70 articles comprising 12,162 patients (63% female) were ultimately selected for this review. Most had moderate to very-low quality evidence. There was significant variability in study design including surgical indications and approaches, bone health assessment, outcomes evaluated, and analysis of results. 38 authors reported bone mineral density (BMD) measured by dual energy x-ray absorptiometry (DEXA) and 41 performed indirect assessments of bone density or quality using CT or MRI-based methods. Among those with data for comparison, alternative site-specific measurements were often more predictive of surgical outcomes than traditional DEXA T-scores.
Six primary outcomes were evaluated: interbody cage subsidence (24 studies), screw loosening (14 studies), pseudarthrosis (7 studies), junctional disease (22 studies), vertebral fracture (6 studies), and reoperation (5 studies). 31 studies also provided patient-reported metrics of pain or disability to assess the functional implications of these complications. Overall, osteoporosis (T-score < -2.5, or other correlate) was identified as an independent risk factor for instrumentation-related complications like cage subsidence and screw loosening, which were often diagnosed with concomitant pseudarthrosis. Lower BMD was also strongly associated with the development of proximal junctional kyphosis or failure, particularly secondary to vertebral fracture (figure 1).

Conclusions
Our findings suggest that osteoporosis increases the risk for instrumentation failure, pseudarthrosis, vertebral fracture, junctional deformity, and reoperation after lumbar fusion surgery. These results emphasize the importance of an early, preoperative diagnosis in allowing for medical optimization and modification of the surgical strategy to minimize complications in high-risk patients. This review also highlights current practical challenges in osteoporosis surveillance and related clinical research. Routine screening faces obstacles related to cost, access, and technical inaccuracies of DEXA measurements in DLD. CT, MRI, or ultrasound-based techniques may therefore be useful alternatives for detecting osteoporosis preoperatively. Further prospective studies using standardized methods of assessing bone health and reporting complications are necessary to strengthen current evidence and determine optimal practices. In the meantime, an awareness of the surgical implications of osteoporosis and utility of preoperative screening can provide for more informed, effective patient care.