In a study published in the Journal of Bone and Joint Surgery, Themistocles Protopsaltis, of the New York University School of Medicine, and others propose that the use of the T1 pelvic angle to measure sagittal deformity has several important advantages over established methods of measurement. The authors write that the most notable advantage of measuring the T1 pelvic angle is that increasing the T1 pelvic angle correlates with progressive worsening of health-related quality of life measures and that it is not diminished by postural compensatory mechanisms.
The purpose of the multicentre, prospective, cross-sectional study was to “investigate the relationship of the T1 pelvic angle and other established sagittal alignment measures and to correlate these parameters with health-related quality-of-life measures.”
The authors used the T1 pelvic angle as a novel measure of global sagittal spinal deformity. The study included 559 patients with a mean age of 52.5 years. Inclusion criteria were that the patient must have adult spinal deformity, defined: scoliosis, a Cobb angle of ≥20 degrees, sagittal vertical axis of ≥5cm, thoracic kyphosis of ≥60 degrees, and pelvic tilt of ≥25 degrees. Clinical measures of disability used included the Oswestry Disability Index (ODI), Scoliosis Research Society (SRS) and SF-36. “The T1 pelvic angle was highly correlated with sagittal vertical axis (r=0.837), pelvic tilt (r=0.933), pelvic incidence minus lumber lordosis (r=0.889), and T1 spinopelvic inclination (r=0.589),” the authors explain.
Patients were split into groups based on their T1 pelvic angle – 30 degrees (87). The study found that increasing the T1 pelvic angle resulted in a progressive worsening in sagittal vertical axis, pelvic tilt, and pelvic incidence minus lumbar lordosis, as well as in ODI.
In terms of health-related quality of life, the T1 pelvic angle correlated with the Oswestry Disability Index (r=0.435), Short Form (=–0.445) and Scoliosis Research Society (–0.358) questionnaires. The authors note that: “All the reported correlations were significant at the p<0.05 level.”
The authors also note that the interobserver and intraobserver reliability of the T1 pelvic angle measure was “0.980 and 0.902 respectively.” This compares well with the interobserver and intraobserver reliability for the sagittal vertical axis (0.995 and 0.917), pelvic tilt (0.959 and 0.853) and pelvic incidence (0.909 and 0.866).
The sagittal axis measure can be diminished by postural compensatory mechanisms such as pelvic retroversion, where, for example, a high pelvic tilt can hide a larger spinal deformity. The authors explain that such postural compensations lead to suggestions that “spinopelvic radiographs of patients should be made with the knees extended, a position that may be difficult for patients with severe deformity to maintain.” However, because the T1 pelvic angle measures spinal inclination and pelvic tilt simultaneously, it is not diminished by pelvic retroversion or knee flexion. As such, the authors write: “patients can stand for the spinopelvic radiograph in a position of comfort to maintain horizontal gaze without concern that the deformities will be underestimated.” Furthermore, the T1 pelvic angle is an angular spinopelvic measure that does not require calibration of the radiograph.
When compared to other methods of measurement, Protopsaltis et al claim: “The T1 pelvic angle is intrinsically more intuitive, as it is the sum of the T1 inclination and the pelvic tilt; as the deformity increases, so does the T1 pelvic angle.” The authors go on to conclude that “the T1 pelvic angle has advantages over established measures of sagittal deformity…The T1 pelvic angle measures sagittal deformity independent of many postural compensatory mechanisms and it can be useful as a preoperative planning tool with a target T1 pelvic angle of <14 degrees.”