Pelvic Width Predicts Hip Joint Center Location In Children

Abstract

A precise estimate of the locations of the hip joint centers (HJC) is necessary when estimating in-vivo joint reaction forces and moments. Errors in timing and magnitude of hip joint moments have been reported to be up to 25% due to a HJC mislocation error of 30mm (Stagni,2000). While methods exist to predict HJC's based on pelvic width in adults, their applicability to children has not been reported. PURPOSE To determine how accurately MRI measures of pelvic width predict HJC locations in children. METHODS MRI's of the pelvic region were acquired from 10 healthy normal girls (9.6±0.9yrs; 141.0±9.4cm; 37.0±7.1kg; mean ± SD). The T1-weighted images were sampled from the coronal plane at 4mm intervals. Two slices were identified and exported into an imaging program for measurement: 1) the slice through the centers of the femoral heads and 2) the most anterior slice in which the left ASIS was visible. The reference frame was defined as: × = positive anterior, Y = positive left, and Z = positive superior. Fiduciary marks on the MRI's were used to calculate a scaling factor from pixels to SI units (1px = 0.5mm). Pelvic width was defined as the distance between the left and right ASIS. Because the MRI's only contained the two HJC's and the left ASIS, bilateral symmetry about the midpoint between HJC's was assumed in calculating the pelvic width. The displacement from the left ASIS to the left HJC was expressed in each direction as a percentage of the calculated pelvic width. The mean percentages in each direction were then used to predict the left HJC location of each subject. T-tests were used to determine if there were differences between the measured and predicted HJC locations in each direction (p<0.05). RESULTS The location of the left HJC relative to the left ASIS was −21.5±2.9%, −14.5±2.6%, and −32.8±4.9% of the pelvic width (18.1±1.4cm) in the X, Y, and Z directions, respectively. The magnitudes of the errors in predicted HJC location based on these percentages were 4.3±2.4mm (p<0.89), 3.5±2.7mm (p<0.89) & 6.8±5.1mm (p<0.91) in the X, Y, and Z directions, respectively for a total error of 9.8±4.3mm. CONCLUSION The location of the HJC relative to the ASIS as a percentage of pelvic width in these children was similar to that determined previously for adults. Our results also agree with Bell (1990) and Seidel (1995) in that the pelvic width best predicted the HJC along the axis it is parallel to. While the HJC prediction error was greater in the × and Z directions, the overall magnitude of the errors was quite small. Prediction of HJC location based on pelvic width is thus appropriate in computing the joint kinetics of children. International Society of Biomechanics Dissertation Grant.