Three-Dimensional Orientation of the Radial Neck Axis Angulation: Implications for Radial Head Replacement

The purpose of this study was to determine the 3-dimensional (3D) orientation of the radial neck axis with respect to the forearm rotation axis and explore its theoretical implications for radial head prosthetic replacement.

Computed tomography scans of the forearm of healthy individuals were used to construct 3D bone models defining the relationship between the forearm rotation and radial neck axes (FRANA). These models were adjusted to achieve neutral forearm rotation. A comprehensive method for measuring FRANA in 3D was developed, and the results were compared with those of clinical simulated 2-dimensional (2D) projections.

In 20 patients, the 3D FRANA angle averaged 5.1° (SD: 2.2°, 95% confidence intervals [CI]: 4.1-6.2°) consistently oriented toward the palmar side. This was strongly correlated (r = 0.87) with 2D measurements in the anterior view with the forearm in neutral rotation (mean: 4.5°, SD: 2.3°, 95% CI: 3.5°-5.6°), but not with the measurements in the lateral view (mean: -0.1°, SD: 2.5°, 95% CI: -1.3° to 1.0°). Angular measurements in the anterior view were strong predictors of 3D FRANA (R² = 0.8).

The axis of the radial neck does not line up precisely with the axis of forearm rotation but points to a location that is approximately 2 cm palmar to the fovea of the distal ulna. Significant differences were found between 3D and 2D lateral measurements, which highlights the benefits of computer-assisted modeling for analyzing proximal radial geometry.

With the forearm in neutral rotation, the proximal radial canal is aligned with the axis of forearm rotation in the sagittal plane but points to a mean of 5° palmarly in the coronal plane. This information can be used to optimize the radial neck cut and direction of canal preparation prior to prosthesis insertion.