An analysis of the usage and limitations of the T1 pelvic angle

The T1 pelvic angle (T1PA) provides a consistent global measure of sagittal alignment independent of compensatory mechanisms and positional changes. However, it may not explicitly reflect alignment goals that correlate with a lower risk of complications. This study assessed the value of T1PA in achieving sagittal alignment goals in patients with an adult spinal deformity (ASD).

Patients aged ≥ 18 years who had undergone surgery for ASD and had complete baseline data and at least two-year postoperative, radiological, and health-related quality of life follow-up were included. A total of 596 patients met the inclusion criteria (mean age 61.5 years (SD 13.4); 78.8% females; mean BMI 27.8 kg/m² (SD 5.9); mean Charlson Comorbidity Index 1.9 (SD 1.8)). The primary outcome was development of mechanical complications. Cohorts were based on postoperative T1PA (T1PA < 10° or > 30° = unfavourable vs T1PA 10° to 30° = favourable). Adjustments for confounders with separate analyses were done using multivariable logistic regression analysis.

Postoperatively, 363 patients (60.9%) had a favourable T1PA and 233 (39.1%) did not. Those with a favourable T1PA had a significantly higher rate of proximal junctional kyphosis (PJK) than those with an unfavourable T1PA (52.0% vs 48.0%; p = 0.035). Having adjusted for confounders, those with a favourable T1PA had a decreased risk of proximal junctional kyphosis (OR 0.532 (95% CI 0.288 to 0.985); p = 0.045).

The T1PA gives valuable information about global alignment, but fails to recognize and adjust for the great variation in patients with ASD. As such, we recommend combining the T1PA with alternative alignment strategies to better inform clinical care.