Background: Dysfunction of the hip secondary to dysplasia is a complex problem that includes excessive stresses on the articular cartilage, dynamic hip instability, and muscular fatigue, eventually leading to degenerative arthritis if left uncorrected. Mechanical stress on the dysplastic hip has been widely described, but dynamic instability requires further evaluation. The purpose of this study was to investigate dynamic instability of the dysplastic hip with use of triaxial accelerometry.
Methods: We evaluated forty-eight hips of twenty-four patients with unilateral hip dysplasia (a center-edge angle of <25°). All contralateral hips were disease-free with normal radiographic findings. An accelerometer was used to record triaxial acceleration while the patient was walking (x-axis: superoinferior direction, y-axis: anteroposterior direction, and z-axis: mediolateral direction). Sensors were attached to the skin, with adhesive tape, over the greater trochanter and the anterior superior iliac spine bilaterally. The time of heel-strike was confirmed visually and by superoinferior acceleration. The averages of the peak values of the middle three gait cycles were used for data analysis. The overall magnitude of acceleration was calculated to evaluate hip instability. The overall magnitudes of acceleration of the dysplastic and contralateral, normal hips were compared with the radiographic data.
Results: The three directions of acceleration were the same in all cases. The overall magnitude of acceleration of the dysplastic hips was significantly larger than that of the contralateral, normal hips (p < 0.0001). There was a negative correlation between the overall magnitude of acceleration and both the center-edge angle and the acetabular head index, and there was a positive correlation between the overall magnitude of acceleration and both the acetabular roof angle and the Sharp angle.
Conclusions: Hip instability is increased in proportion to the degree of dysplasia. Triaxial accelerometry is helpful in the evaluation of dynamic instability of the dysplastic hip. The center-edge angle can be used as an indicator of hip instability.
Level of Evidence: Diagnostic Level III. See Instructions to Authors for a complete description of levels of evidence.