Review Question - QID 217400

T1 pelvic angle (T1PA) evaluates global deformity by accounting for spinal inclination and pelvic retroversion.

Adult spinal deformity is characterized by coronal and sagittal malalignment caused by degenerative or iatrogenic processes. Sagittal Vertical Axis (SVA) malalignment (> 5 cm) has been associated with poor health-related quality of life metrics. SVA is a plumb line measure, however it can be deceptively normal if patients have a compensated deformity. The T1PA is a global alignment parameter that can be used if plumb lines are impossible to evaluate (e.g., intraoperatively, in deformities with significant pelvic compensation).

Surgical goals are directed at improving radiographic parameters and improving patient quality of life. Surgical goals for T1PA is <14 degrees. Other described surgical goals include TK +LL + PI = 45°, LL = PI, PT < 25°, SVA <5 cm, and GAP Score <3. GAP scores <3 (proportionate alignment)have been associated with lower mechanical failure and revision surgery rates than moderate and highly disproportionate alignment (GAP >3).

Rose et al. retrospectively studied the effect of pelvic incidence and thoracic kyphosis in 40 patients treated with pedicle subtraction osteotomy. They reported a mean decrease in the C7 plumb line from +15 cm (pre-op) to +4.5 cm (2-years post-op) with PI+LL + TK less than or equal to 45 predicting ideal sagittal balance at 24 months. They concluded sagittal correction is equivalent with L2 and L3 PSO and longer fusion constructs preserving correction at 2-year follow-up.

Protopsaltis et al. performed a multicenter, prospective, cross-sectional study assessing the T1 pelvic angle in adult spinal deformity patients. They reported a significant worsening of health-related quality of life scores with increasing T1 pelvic angle with a value >20° associated with severe disability (ODI >40). They concluded T1 pelvic angle is a reliable means to measure sagittal plane deformity that does not require calibration of radiographs (C7 plumb line) and is independent of postural compensatory mechanisms (pelvic tilt).

Yilgor et al. studied 222 patients with adult spinal patients treated with posterior spinal fusion to develop and validate the GAP Score. They reported an AUC of 0.92 for predicting mechanical complications following adult deformity surgery using the GAP score with ideally proportioned patients having a 6% complication rate versus 47% and 95% rate in moderately to severe disproportioned states. They concluded surgical goals based on GAP scores may decrease mechanical complications following adult spinal deformity surgery.

Illustration A is a diagram depicting the T1 pelvic angle. Illustration B is a diagram depicting the criteria used for GAP scoring.

Incorrect answers
Answer 1: T1PA is an angular measurement that does not require radiographic calibration.
Answer 3: The T1PA is not calculated from thoracic kyphosis and pelvic incidence. It is an angle measurement from the center of the T1 vertebral body to the center of the femoral head and then to the center of the superior S1 endplate. It is the sum of T1 spinal inclination and pelvic tilt.
Answer 4: Correcting T1PA to <14 degrees is an ideal surgical goal. A T1PA >20 degrees correlates with an ODI score of 40, which indicates severe disability.
Answer 5: Higher T1PA measurements correlates with higher ODI scores, which is associated with greater disability.