The influence of bone quality and paraspinal musculature on proximal junctional kyphosis failure mode among patients undergoing lumbopelvic fusion terminating at the thoracolumbar junction

OBJECTIVE Proximal junctional kyphosis (PJK) affects 5%–61% of patients following thoracolumbar fusion. Many patients are asymptomatic, but a plurality require surgical revision at a cost of $75,000 per case. This analysis sought to analyze the degree to which bone quality and paraspinal muscle sarcopenia influence PJK failure mode. METHODS Patients undergoing thoracolumbar instrumented fusion with an upper instrumented vertebra (UIV) at the thoracolumbar junction (T10–L2) were identified and data were gathered on surgery, bone quality, pre- and postoperative sagittal alignment, and paraspinal muscle cross-sectional area (CSA). PJK was defined as a ≥ 10° increase in proximal junctional angle from the first postoperative radiograph. PJK was classified as discoligamentous failure (type 1), bone failure (type 2), or screw-bone interface failure (type 3) according to the Yagi-Boachie system. Bone quality was assessed by Hounsfield units (HUs) and the vertebral bone quality (VBQ) score at the UIV. RESULTS One hundred fifty patients were identified (median age 67 years, 53.3% female), 46 of whom experienced PJK (22 type 1, 13 type 2, 11 type 3). The median time to onset was most rapid for type 2 events (2.6 months). There were no differences between patients experiencing PJK versus controls regarding bone quality (HUs or VBQ score) or paraspinal muscle CSA on univariate comparison. However, subdivision by PJK type showed patients experiencing bone failure (type 2) PJK had significantly lower HUs at the UIV and UIV+1 relative to those experiencing type 1 PJK or no PJK (all p < 0.05). The VBQ score trended toward being significant, with a higher VBQ score (worse bone quality) in those suffering type 2 PJK, but did not reach statistical significance (p = 0.07). Patients experiencing discoligamentous failure PJK (type 1) had small multifidus CSA (390 mm 2 ) relative to patients experiencing type 2 (516 mm 2 ) or type 3 (440 mm 2 ) PJK and patients who did not experience PJK (481 mm 2 ), although the difference did not reach statistical significance. On time-to-event analysis, low HUs of the UIV/UIV+1 predicted type 2 (hazard ratio [HR] 0.81, 95% CI 0.70–0.93; p = 0.002) and type 2/3 PJK (HR 0.87, 95% CI 0.78–0.96; p = 0.006) but not type 1 PJK. Low UIV multifidus CSA trended toward being a significant predictor of type 1 PJK (HR 0.85, 95% CI 0.69–1.05; p < 0.10). CONCLUSIONS The combination of underlying bone quality and paraspinal musculature CSA at the UIV appeared to influence failure mode among patients who underwent lumbosacral instrumented fusion terminating at the thoracolumbar junction.