Under-contouring of rods: a potential risk factor for proximal junctional kyphosis after posterior correction of Scheuermann kyphosis

OBJECTIVE Scheuermann kyphosis (SK) could require surgical treatment in certain situations. A posterior reduction is the most widespread treatment so far, although the development of proximal junctional kyphosis (PJK) is one of the possible complications of this procedure. The contour of the proximal part of the rod could influence the occurrence of PJK in SK patients. The objective of this study was to analyze the impact of the proximal rod contour on the occurrence of a PJK complication in SK patients. METHODS This retrospective monocentric study was performed in the Nanjing Spine Surgery Department. All eligible patients had undergone posterior correction surgery with pedicle screws only between 2002 and 2017 and had at least 24 months of follow-up. The presence of PJK was quantified on radiographs using the proximal junctional angle (PJA > 10 degrees at the last follow- up). The authors propose a new radiological parameter to measure the angulation of the proximal part of the instrumentation: the proximal contouring rod angle (PCRA) is the angle between the upper endplate of the upper instrumented vertebra (UIV) and the lower endplate of the second vertebra caudal to the UIV. The patients were analyzed according to the presence or absence of PJK. A t-test, receiver operating characteristic (ROC) curve analysis, and logistic regression analysis were performed for statistical analysis. RESULTS Sixty-two patients treated for SK were included in this study. The mean age was 18.6 +/- 8.5 years, and the mean follow-up was 42.5 +/- 16.4 months. The mean correction rate of global kyphosis was 46.4% +/- 13.7%. At the last follow-up, 17 patients (27.4%) presented with PJK. No significant difference was found between the PJK and non- PJK groups in terms of age and other preoperative variables. A significant difference in the postoperative PCRA was found between the PJK and non- PJK groups (8.2 degrees +/- 4.9 degrees vs 15.7 degrees +/- 6.6 degrees, respectively; p = 0.001). A postoperative PCRA less than 10.1 degrees predicted a significantly higher risk for PJK (p = 0.002, OR 2.431, 95% CI 1.781-4.133). CONCLUSIONS Under-contouring of the proximal part of the rods (lower than 10 degrees) is a risk factor for PJK after posterior correction of SK.