Objective
To assess paraspinal muscularity and bone quality contribution to PJK risk.
Methods
Defining PJK as ≥ 10° increase in proximal junctional angle from first upright radiograph, thoracolumbopelvic fusion patients experiencing PJK were compared to controls. Baseline radiographic parameters, bone quality in CT Hounsfield units (HU), and paraspinal musculature cross-sectional area (CSA) at L3 and the UIV. Patients were subdivided into type 1–3 PJK based upon the Yagi-Boachie scale. Time-dependent analyses with univariable Cox proportional hazards model were performed.
Results
206 patients were included (59.7 % female; median age 67.1); 26.9 % experienced PJK – 52.5 % type 1, 27.8 % type 2, and 19.7 % type 3. Univariable comparisons showed PJK patients had lower HU at the UIV (137 vs 151; p = 0.047) and UIV+ 1 (137 vs 151; p = 0.028); mean multifidus CSA (p = 0.21) was also nonsignificantly smaller. Average HU were lower in type 2 PJK patients relative to non-PJK and type 1 PJK (p < 0.001). Type 2 failure was predicted by UIV/UIV+ 1, UIV, and UIV+ 1 average HU (all p < 0.01) while type 1 failure was predicted by UIV multifidus CSA (p = 0.03); average HU did not predict type 1 failure.
Conclusions
Poor bone quality may be the strongest predictor of PJK; however, subanalysis by PJK type suggests it only increases the odds of bony or implant/bone interface failure. Decreased multifidus CSA appears to confer risk for type 1 (discoligamentous) PJK, suggesting PJK failure mode is dictated by the interplay of UIV bony and soft tissue integrity.
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