Gap between flexion and extension ranges of motion: a novel indicator to predict the loss of cervical lordosis after laminoplasty in patients with cervical spondylotic myelopathy

OBJECTIVE Kyphotic deformity resulting from the loss of cervical lordosis (CL) is a rare but serious complication after cervical laminoplasty (CLP), and it is essential to recognize the risk factors. Previous studies have demonstrated that a greater flexion range of motion (fROM) and smaller extension ROM (eROM) in the cervical spine are associated with the loss of CL after CLP. Considering these facts together, one can hypothesize that an indicator representing the gap between fROM and eROM (gROM) is highly useful in predicting postoperative CL loss. In the present study, the authors aimed to investigate the risk factors of marked CL loss after CLP for cervical spondylotic myelopathy (CSM), including the gROM as a potential predictor. METHODS Patients who had undergone CLP for CSM were divided into those with and those without a loss of more than 10 degrees in the sagittal Cobb angle between C2 and C7 at the final follow-up period compared to preoperative measurements (CL loss [CLL] group and no CLL [NCLL] group, respectively). Demographic characteristics, surgical information, preoperative radiographic measurements, and posterior paraspinal muscle morphology evaluated with MRI were compared between the two groups. fROM and eROM were examined on neutral and flexion-extension views of lateral radiography, and gROM was calculated using the following formula: gROM (degrees) = fROM & minus; eROM. The performance of variables in discriminating between the CLL and NCLL groups was assessed using the receiver operating characteristic (ROC) curve. RESULTS This study included 111 patients (mean age at surgery 68.3 years, 61.3% male), with 10 and 101 patients in the CLL and NCLL groups, respectively. Univariate analyses showed that fROM and gROM were significantly greater in the CLL group than in the NCLL group (40.2 degrees vs 26.6 degrees, p < 0.001; 31.6 degrees vs 14.3 degrees, p < 0.001, respectively). ROC curve analyses revealed that both fROM and gROM had excellent discriminating capacities; gROM was likely to have a higher area under the ROC curve than fROM (0.906 vs 0.860, p = 0.094), with an optimal cutoff value of 27 degrees. CONCLUSIONS The gROM is a highly useful indicator for predicting a marked loss of CL after CLP. For CSM patients with a preoperative gROM exceeding 30 degrees, CLP should be carefully considered, since kyphotic changes can develop postoperatively.