Bony impingement limits design-related increases in hip range of motion

Abstract

Background: Factors affecting risk for impingement and dislocation can be related to the patient, implant design, or surgeon. While these have been studied independently, the impact of each factor relative to the others is not known.

Questions/purposes: We determined the effect of three implant design factors, prosthetic placement, and patient anatomy on subject-specific ROM.

Methods: We virtually implanted hip geometry obtained from 16 CT scans using computer models of hip components with differences in head size, neck diameter, and neck-shaft angle. A contact detection model computed ROM before prosthetic or bony impingement. We correlated anatomic measurements from pelvic radiographs with ROM.

Results: When we implanted the components for best fit to the subject's anatomy or in the recommended orientation of 45° abduction and 20° anteversion, ROM was greater than 110° of flexion, 30° of extension, 45° of adduction-abduction, and 40° of external rotation. Changes in head size, neck diameter, and neck-shaft angle generated small gains (3.6°-6°) in ROM when analyzed individually, but collectively, we noted a more substantial increase (10°-17°). Radiographic measurements correlated only moderately with hip flexion and abduction.

Conclusions: It is feasible to tailor implant placement to each patient to maximize bony coverage without compromising ROM. Once bony impingement becomes the restricting factor, further changes in implant design may not improve ROM. Radiographic measurements do not appear to have value in predicting ROM.

Fig. 1

A computer-generated image shows a representative pelvis and femur implanted with hip arthroplasty components.

Fig. 2

A computer-generated image shows the hip arthroplasty design. Implant design variables included head size, neck diameter, and neck-shaft angle (thick black line).

Fig. 3

A plain AP radiograph of the pelvis shows the measurement of head diameter (dotted circle), neck diameter (N), acetabular depth ratio (= D × 1000/W), the normalized arc length between the tip of greater trochanter and ilium (GT arc), and the arc length between lesser trochanter and ischium (LT arc).

Fig. 4

A graph shows the mean maximum ROM in each direction for anatomic and recommended orientations. Anatomic orientation is when surgeons implant the acetabular component to match each patient’s native acetabular abduction and anteversion; recommended is implanted in 45° acetabular abduction and 20° anteversion.

Fig. 5A–E

Graphs show the results with the acetabular component in the recommended orientation of 45° abduction and 20° anteversion: (A) flexion; (B) extension; (C) abduction; (D) adduction; and (E) external rotation. HD = head diameter; ND = neck diameter, NA = neck angle.

Fig. 6A–B

Graphs show the ROM in the directions considered likely to be at risk for dislocation. (A) Neck diameter and neck angle, but not head size, have an effect on ROM in the direction of maximum risk for anterior dislocation. (B) Neck diameter and neck angle also affect the ROM in the direction of maximum risk for posterior dislocation. However, reducing the neck angle has the opposite effect on posterior dislocation. HD = head diameter; ND = neck diameter, NA = neck angle.