A Novel Classification of Sagittal Spinal Alignment to Aid Surgical Planning for Adult Spinal Deformity

For type I patients (PI \< 39.56°), the target LL = -0.13\*PI2+9.36\*PI-134.08; for type II patients (39.56° ≤ PI \< 49.16°), the target LL = 0.45\*PI+26.57; for type III patients (49.16° ≤ PI \< 58.31°), the target LL = -0.15\*PI2+17.09\*PI-420.57; for type IV patients (PI \> 58.31°), the target LL = 0.06\*PI2-7.55\*PI+289.77. Prediction intervals of 95% confidence is adopted as the target LL range.

"This new score is composed of three sagittal parameters (PI-LL, PT and TPA). For these three parameters, points were assigned based on offset with age-adjusted targets 0 points if the parameter was within 10 years of the patient's age (Match). For each 20 years above the age-adjusted target, 1 point was added (e.g., + 1 point between + 10 and + 30, + 2 points between +30 and +50). Conversely, 1 point was subtracted for each 20 years below the age-adjusted target (e.g., -1 point between -10 and -30, -2 points between -30 and -50). SAAS was calculated by adding all 3 components, creating a discreet score that could have a negative value (under corrected) or positive value (over corrected). SAAS was sub-categorized into SAAS-Under if it was less than -1, SAAS-Match if it was between -1 and + 1 or SAAS-Over if it was greater than + 1."