Reliability of the xipho-pubic angle in patients with sagittal imbalance of the spine

BACKGROUND: Proximal junctional kyphosis (PJK) is a frequent complication that compromises the outcomes of spinal surgery, especially for adult deformity. To the date no single risk factor or cause has been identified that explains its occurrence. The purpose of this study was to investigate the test-retest reliability of the radiologic measurements using xipho-pubic angle (XPA) for subjects undergoing surgery for sagittal misalignment of the spine.

METHODS: Retrospective observational cross-sectional study of prospectively collected data. Full-spine standing lateral radiographs of 50 patients who underwent surgery for fixed sagittal imbalance (preoperative and postoperative) were evaluated. Internal consistency, reproducibility, concurrent validity, and discriminative ability of the XPA. Two physicians measured XPA on the 100 randomly sorted and anonymized radiographs on two occasions, one week apart (test and retest conditions), were calculated for inter and intraobserver agreement.

RESULTS: Test-retest reliability of XPA measurement was excellent for pre- (ICC=0.98; P=0.001) and post-surgical (ICC=0.86; P=0.001) radiographs of subjects with sagittal imbalance of the spine. XPA was able to discriminate between preoperative and postoperative radiographs F=17.924, P<0.001) in patients undergoing surgery for fixed sagittal imbalance for both raters. There were significant differences between pre- vs. postoperative XPA, pelvic tilt, lumbar lordosis and sagittal vertical axis values (all P<0.001).

CONCLUSIONS: Xipho-pubic angle had fair to excellent test-retest reliability, and it did possess validity to discriminate between preoperative and postoperative radiographs in patients undergoing surgery for fixed sagittal imbalance.