Grayscale Inversion View Can Improve the Reliability for Measuring Proximal Junctional Kyphosis in Adolescent Idiopathic Scoliosis

BACKGROUND: Proximal junctional kyphosis (PJK) is a common phenomenon after long segmental fusion surgery of adolescent idiopathic scoliosis. However, the inability to reliably identify vertebral endplates on lateral upright radiographs has made an accurate measurement of proximal junctional angle (PJA) technically impossible in many patients. The aim of this study was to determine whether a grayscale inversion view is more reliable to measure PJA and to assess PJK accurately. METHODS: A total of 162 patients with adolescent idiopathic scoliosis who underwent posterior spinal fusion surgeries were included in this study. PJA was measured on preoperative lateral standing films using 3 methods (upper-instrumented vertebrae [UIV] + 1, UIV + 2, and UIV to T2) on both standard view and grayscale inversion view. Two physicians independently measured the PJA twice at a 1-month interval. Intra- and interobserver reliabilities were compared between the 2 radiographic views. Forty patients with preoperative magnetic resonance imaging (MRI) scans were randomly selected. PJA was measured for these patients on both views of lateral standing films and MRI images. The correlation coefficients between PJA obtained on MRI and PJA obtained on radiographs with different views were calculated respectively. RESULTS: The intraclass correlation coefficients were greater in a grayscale inversion view than in a standard view in all 3 methods for both observers, and the intraclass correlation coefficients of interobserver reliabilities also were greater in a grayscale inversion view. The correlation coefficient between PJA obtained on grayscale inversion view and preoperative MRI was greater in all methods compared with standard view. CONCLUSIONS: Grayscale inversion view can be a more reliable tool for the evaluation of PJK as compared with the conventional measurement. We recommend the application of a grayscale inversion view to measure PJA and assess PJK in clinical practice, particularly for patients instrumented to the upper thoracic spine.