Abstract
This paper presents a methodology for measuring the femoro-pelvic joint angle based on in vivo magnetic resonance imaging (MRI) images taken under weight-bearing conditions. We assess the reproducibility of angle measurements acquired when the subject is asked to repeatedly assume a reference position and perform a voluntary movement.
We scanned a healthy subject in a lying position in a 3T MRI scanner to obtain high resolution (HR) images including two transverse T1-weighted TSE sequence scans at the pelvis and knee and a sagittal T1-weighted dual sense scan at the hip joint. We then scanned the same subject in a weight-bearing configuration in a 0.5T open MRI scanner to obtain related low resolution (LR) images of the femur and acetabulum. Four scan cycles were obtained with the subject being removed and reinserted between cycles in the Open MRI scanner. In each cycle, a block was inserted (up position) and removed (down position) under the subject's foot.
The femur and acetabulum bone models were manually segmented and the models from the LR (sitting) images were registered to the HR (supine) images. The femoroacetabular angles relative to the LR scanning plane for four cycles were calculated. The femoral angle relative to the scanner were quite repeatable (SD < 0.9°), the pelvic angles less so (SD ∼2.6–4.3°). The hip flexion angle ranged from 23°–34° in the down and up positions, respectively, so the block induced a mean angle change in the flexion direction of approximately 11° (SD = 1.7°).
We found that the femoral position could be accurately re-acquired upon repositioning, while the pelvic position was notably more variable. Limb position changes induced by inserting a block under the subject's foot were consistent (standard deviations in the relative attitude angles under 2°). Overall, our measurement method produces plausible measures of both the femoroacetabular angles and the changes induced by the block, and the reproducibility of relative joint changes is good.
ACKNOWLEDGMENTS: Dr. Kang was supported by the National Science and Engineering Research Council of Canada (NSERC) through a Postdoctoral Fellowship and conducted her research at the Centre for Hip Health and Mobility at Vancouver General Hospital, Canada.
