Abstract
Purpose
Capitellum hemiarthroplasty is an emerging concept. The current metallic capitellar implants have spherical surface shapes, but the native capitellum is not spherical. This study evaluated the effect of capitellar implant shape on the contact mechanics of the radiocapitellar joint when articulating with the native radial head.
Method
Eight paired radii and humeri were potted in a custom jig. Articular casts were made with medium-viscosity resin while 85 N of axial load was applied to the reduced radiocapitellar joint at 0, 45, and 90 of elbow flexion, and at neutral, 50 pronation and 50 supination at each flexion angle. The native radiocapitellar articulation was compared to capitellar hemiarthroplasties of two surface designs (anatomical and spherical). Contact area and shape (circularity) were determined. Circularity was defined as the ratio of the minor axis and major axis of the shape.
Results
At 0 of flexion, the anatomical hemiarthroplasty had a contact area of 52–70% that of the native articulation (p=0.03), while the spherical hemiarthroplasty had a contact area 40–42% that of the native articulation (p=0.003). At 45 of flexion, both hemiarthroplasties displayed contact area <53% that of the native joint (p<0.007). At 90 of flexion, the hemiarthroplasties had contact areas ranging from 40–70% that of the native articulation (p=0.1). The two capitellar implants had similar contact areas at all flexion angles tested (p>0.05).
The contact shape of the native radiocapitellar articulation was ellipsoid, with a range of circularity values from 0.530.19 to 0.720.16, depending on the flexion and rotation angle. At 0 and 90 flexion, there was no difference in contact shape between the native articulation, the anatomical, or spherical implant (p>0.05). At 45 flexion, the anatomical implant contact was less circular than either the native articulation (p=0.006) or the spherical hemiarthroplasty (p=0.002).
Conclusion
Metallic capitellar hemiarthroplasty causes a significant reduction in contact area at 0 and 45 elbow flexion, which may have important long-term implications for wear of the radial head cartilage. This reduction is similar to previous reports, which have evaluated the effect of metallic radial head hemiarthroplasty articulating with the native capitellum. More compliant alternative materials are needed to improve the contact characteristics of metallic capitellar hemiarthroplasties. Although the anatomical hemiarthroplasty was created from a detailed morphological study of the capitellum, the anatomical implant failed to completely reproduce the contact native shape. The theoretical advantages of a more anatomical capitellar implant shape may not be realized clinically, suggesting a spherical implant, which is easier to manufacture and implant, may be adequate for patient application. Further studies are required to delineate the effect of this altered contact morphology on implant function and radial head wear in-vivo.
