Predicting ideal spinopelvic balance in adult spinal deformity.

OBJECTIVE: Spinopelvic balance is based on the theory that adjacent segments of the spine are related and influenced by one another. By understanding the correlation between the thoracolumbar spine and the pelvis, a concept of spinopelvic balance can be applied to adult deformity. The purpose of this study was to develop a mathematical relationship between the pelvis and spine and apply it to a population of adults who had undergone spinal deformity surgery to determine whether patients in spinopelvic balance have improved health measures.

METHODS: Using values published in the literature, a mathematical relationship between the spine and pelvis was derived where pelvic incidence (PI) was divided by the sum of the lumbosacral lordosis (LL; T12-S1) plus the main thoracic kyphosis (TK; T4-12). The result was termed the spinopelvic constant (r): r = PI/(LL + TK). This was performed in patients in 2 age groups previously defined in the literature as "adult" (18-60 years of age) and "geriatric" (> 60 years). The equation was then constructed to relate an individual's measured PI to his or her predicted thoracolumbar curvature (LL + TK)(p) based on the age-specific spinopelvic constant: (LL + TK)(p) = r/PI. A retrospective review was then performed using cases involving patients who had undergone spine deformity surgery and were enrolled in our spinal deformity database. Sagittal balance, PI, and the sum of the main thoracic and lumbar curves were measured. The difference between the predicted sum of the regional curves (LL + TK)(p), based on the individual's measured PI and the age-specific spinopelvic constant, and the measured sum of the regional curves (LL + TK)(m) was then calculated to determine the degree of spinopelvic imbalance. Health status measures were then compared.

RESULTS: Using the formula r = PI/(TK = LL) and normative values in the literature, the adult spinopelvic constant was calculated to be -2.57, and the geriatric constant -5.45. For the second portion of the study, 41 patients met inclusion criteria (13 classified as nongeriatric adults and 28 as geriatric patients). Application of these constants found a statistically significant decline in almost all outcome categories when the spinopelvic balance showed at least 10° of kyphosis more than predicted. While not statistically significant, the trend was that better outcomes were associated with a spinopelvic balance within 0 to +10° of the predicted value. The final analysis compared and separated outcomes from sagittal balance and spinopelvic balance. For patients to be considered in sagittal balance, they must be within 50 mm (± 50 mm) of neutral. For patients to be considered in spinopelvic balance, they must be within ± 10° of predicted spinopelvic balance. Patients in both sagittal and spinopelvic balance have statistically significant better outcomes than those in neither sagittal nor spinopelvic balance. Except for the mean SF-12 PCS (12-Item Short-Form Health Survey Physical Component Summary), there were no significant differences between those that were either in sagittal or spinopelvic balance, but not the other.

CONCLUSIONS: Restoring a normative relationship between the spine and the pelvis during adult deformity correction may play an important role in determining surgical outcomes in these patients independent of sagittal balance.