Validation of the contralateral side as reference for selecting radial head implant sizes

PURPOSE: In arthroplasty of comminuted radial head fractures, the contralateral radial head diameter can be used as reference for implant selection. However, potential bilateral asymmetry may result in a mismatch of the implant with the native bone. Therefore, our purpose was to evaluate anatomical right-to-left differences of radial head diameters. We also compared conventional two-dimensional (2D) with three-dimensional (3D) measurements. METHODS: We used bilateral CT-scans from 25 intact proximal radius pairs of right-handed adult subjects to obtain 50 3D radial head models. After contralateral matching, diameters were calculated using a 3D-based method using an automated circle-fit in standardized cross-sections at the widest level midway through the radial head. The 3D-based diameters were compared to orthogonal line measurements in standard axial CT-slices. RESULTS: Three-dimensional analysis yielded a radial head diameter of 23.0 ± 1.7 mm. The dominant right side was significantly wider, with right-to-left differences of 0.2 ± 0.4 mm, with a maximum of 0.9 mm. The 2D-based diameter was 22.9 ± 1.7 mm, which was 0.1 ± 0.3 mm smaller compared to corresponding 3D-based diameter. CONCLUSIONS: In healthy radial heads, the diameter was biased to the dominant right side, but individual differences were not larger than 1 mm. Compared to implant designs, in which diameter increments are usually 2 mm, this right-bias is not clinically relevant, as it would not affect implant selection. Therefore, the contralateral side can be considered a suitable reference. In clinical practice, the surgeon could estimate this diameter using standard axial CT slices, since its difference with the 3D-based evaluation was also relatively small compared to implant sizing increments.