Inter/Intra-Observer Reliability of T1 Pelvic Angle (TPA), a Novel Radiographic Measure for Global Sagittal Deformity.

Study Design. Reliability analysis. Objective. To assess intra-and interobserver agreement of the T1 pelvic angle (T1PA), a novel radiographic measure of spinal sagittal alignment. Orthopedic surgeons of various levels of experience measured the T1PA in a series of healthy adult volunteers. The relationship of the TIPA to pelvic position was also assessed. Summary of Background Data. Recent literature suggests that the T1PA is a more reliable measure of global sagittal alignment than traditional measurements (i.e., sagittal vertical axis). Previous research focuses on postoperative patients with known spinal deformity. No published research exists evaluating the use of T1PA on healthy subjects without spinal deformity. The purpose of this study is: (1) to assess the reliability of measurements of the T1PA, (2) to examine its relationship to pelvic position. Methods. Seven evaluators of varying orthopedic experience measured the T1PA in 50 healthy adult volunteers. Subjects were radiographed in each of three pelvic positions: resting, maximal anterior pelvic rotation, and maximal posterior pelvic rotation. After a washout period, the measurement was repeated. Using intraclass correlation coefficients, the intra- and inter-rater agreement for the T1PA was measured. The collected data was also used to determine the accuracy of this measurement and its relationship to pelvic position. Results. A very high level of agreement was found in measurements of the T1PA (intraclass correlation coefficients r = 0.98). At each pelvic position, all examiners had excellent intrarater reliability, > 0.85. The inter-rater reliability, compared with a gold standard, consistently measured the T1PA within +/- 2 degrees. The data also shows that the T1PA changes with pelvic rotation. Conclusion. T1PA is a reproducible and reliable measure of global sagittal alignment regardless of the level of training. The T1PA varies based on pelvic rotation; this variation must be taken into account when assigning an absolute target for correction.