Methods to locate center of gravity in scoliosis.

STUDY DESIGN: Prospective evaluation of the location of the center of gravity during supine, standing, and gait.

OBJECTIVE: Develop methods to quantify center of gravity locations in patients with scoliosis and controls and to evaluate the merit of the quantitative assumptions relative to spinal fusion surgery.

SUMMARY AND BACKGROUND DATA: The center of gravity, or balance point of the body, is generally considered to be the single best estimate of the body's location. To date, investigators have not examined the body's center of gravity location to assist surgical planning to maintain and/or restore coronal and sagittal plane balance, nor have they used center of gravity location to help assess surgical outcomes.

MATERIALS AND METHODS: The whole-body center of gravity (MR-COG) was determined for three subjects from magnetic resonance imaging data obtained supine. The whole-body center of gravity was also determined using subject specific (SS-COG) and literature-based (STD-COG) segment center of gravity locations in conjunction with a video motion capture system obtained supine, standing and during gait.

RESULTS: Differences existed among the three methods of determining COG locations in supine, with the SS-COG and MR-COG being most closely aligned. Results from gait data indicated typical anterior/superior and right/left COG shifts during the gait cycle. The SS-COG method consistently determined a COG location inferior to the STD-COG method; however, variation within the gait cycle was similar. Shifts in COG locations relative to a coordinate system fixed in the pelvis were more than 5 cm in the superior/inferior direction, approximately 4 cm in the anterior/posterior direction, and minimal in the left/right direction.

CONCLUSIONS: Methods have been developed to determine locations of the whole body COG in both preoperative and postoperative subjects undergoing spinal fusion surgery and controls. The methods are robust to include men and women, subjects with and without instrumentation, and subjects in various positions including gait.