Prospective multicenter assessment of risk factors for rod fracture following surgery for adult spinal deformity

Object. Improved understanding of rod fracture (RF) following adult spinal deformity (ASD) surgery could prove valuable for surgical planning, patient counseling, and implant design. The objective of this study was to prospectively assess the rates of and risk factors for RF following surgery for ASD. Methods. This was a prospective, multicenter, consecutive series. Inclusion criteria were ASD, age > 18 years, a 5 levels posterior instrumented fusion, baseline full-length standing spine radiographs, and either development of RE or full-length standing spine radiographs obtained at least 1 year after surgery that demonstrated lack of RF. ASD was defined as presence of at least one of the following: coronal Cobb angle >= 20 degrees, sagittal vertical axis (SVA) >= 5 cm, pelvic tilt (PT) >= 25 degrees, and thoracic kyphosis a 60 degrees. Results. Of 287 patients who otherwise met inclusion criteria, 200 (70%) either demonstrated RE or had radiographic imaging obtained at a minimum of 1 year after surgery showing lack of RF. The patients' mean age was 54.8 +/- 15.8 years; 81% were women; 10% were smokers; the mean body mass index (BMI) was 27.1 +/- 6.5; the mean number of levels fused was 12.0 +/- 3.8; and 50 patients (25%) had a pedicle subtraction osteotomy (PSO). The rod material was cobalt chromium (CC) in 53%, stainless steel (SS), in 26%, or titanium alloy (TA) in 21% of cases; the rod diameters were 5.5 mm (in 68% of cases), 6.0 mm (in 13%), or 6.35 mm (in 19%). RF occurred in 18 cases (9.0%) at a mean of 14.7 months (range 3-27 months); patients without RE had a mean follow-up of 19 months (range 12-24 months). Patients with RE were older (62.3 vs 54.1 years, p = 0.036), had greater BMI (30.6 degrees vs 26.7 degrees, p = 0.019), had greater baseline sagittal malalignment (SVA 11.8 degrees vs 5.0 degrees cm, p = 0.001; PT 29.1 degrees vs 21.9 degrees, p = 0.016; and pelvic incidence [Ph-lumbar lordosis [LL] mismatch 29.6 degrees vs 12.0 degrees, p =0.002), and had greater sagittal alignment correction following surgery (SVA reduction by 9.6 degrees vs 2.8 degrees cm, p <0.001; and PI-LL mismatch reduction by 26.3 degrees vs 10.9 degrees, p = 0.003). RE occurred in 22.0% of patients with PSO (10 of the 11 fractures occurred adjacent to the PSO level), with rates ranging from 10.0% to 31.6% across centers. CC rods were used in 68% of PSO cases, including all with RE. Smoking, levels fused, and rod diameter did not differ significantly between patients with and without'RF (p > 0.05). In cases including a PSO, the rate of RE Was significantly higher with CC rods than with TA or SS rods (33% vs 0%, p = 0.010). On multivariate analysis, only PSO was associated with RE (p = 0.001, OR 5.76,95% CI 2.01-15.8). Conclusions. Rod fracture occurred in 9.0% of ASD patients and in 22.0% of PSO patients with a minimum of 1-year follow-up. With further follow-up these rates would likely be even higher. There was a substantial range in the rate of RE with PSO across centers, suggesting potential variations in technique that warrant future investigation. Due to higher rates of RE with PSO, alternative instrumentation strategies should be considered for these cases.