Abstract
Summary
The significance of matching radius of curvature of the radial head implant and the capitellum in implant selection is evaluated. A mismatch of radius of curvature could lead to point loading, reducing contact area, creating large contact stresses, resulting in arthritis, pain, and other complications.
Introduction
Radial head (RH) implant size is chosen by reconstruction outside of the radiocapitellar joint capsule measuring the RH diameter and length, which is replicated for implant selection. RH radius of curvature (RC) is rarely part of the decision although important in determining contact area.
Methods
Eleven fresh-frozen cadaver humeri were denuded, with articular cartilage intact. These were held horizontally in an MTS machine; capitellum faced up, and covered with a Tekscan transducer. RH implants were attached by vice to the MTS driving piston. Four different RH implant models were compared: Ascension, Integra Katalyst, Stryker Small and Medium, all CoCr. All implants were 21mm in outer diameter, except the Stryker small implant (18mm). Cyclic axial loading was applied through the RH implant to the capitellum. Contact area and stress concentrations were captured by the transducer. Loading was applied with stroke control until steady state loading occurred between specified values of 115N-65N, within 1N of peak and 5N of base values. Using the Stryker 21mm implant loading at 155N-65N and 195N-105N simulated over-sizings of +2mm and +4mm.
Results
Percent difference between RH and Capitellar RC's were plotted against corresponding Contact Areas of 21mm sized RH implants, and a linear regression done. Negative values corresponded with larger RH than capitellar RC values. The resulting slope was 92.19, showing a significant increase in contact area with decreased RH to capitellar ratios, with an R2 value of 0.8122, showing a linear trend. Total stresses were calculated for all maximum contact areas, using the peak values.
Discussion
Clinical RH implant sizing comes from native head diameter, not curvature. Improper RC could lead to point loading, reducing contact area, creating large contact stresses, resulting in arthritis, pain, and other complications. This can be seen through the linear relation between contact area and RC. With an RH implant RC of greater value than capitellar RC, the contact area decreases significantly, resulting in increased stress. The significance of RC matching in implant selection. Increases in stress are greater for differences in RC values, than for improper sizing of diameter (D=85.7%) or length (+2mm, +4mm). With the decrease in contact area with increased implant RC, and the changes in stress compared to improper sizings (length and diameter), it can be seen that implant RC is an important feature in RH implant selection.
