Decreased Radial Inclination Without Dorsal Angulation Attenuates the Biomechanical Effects of Ulnar Abutment...

To evaluate, using finite element analysis, how progressive loss of radial inclination (RI) influences ulnar abutment mechanics when volar tilt (VT) is maintained as normal in association with increasing ulnar variance.

Nine fresh-frozen cadaveric forearms were scanned using computed tomography and reconstructed into finite element models. Five models were generated from each specimen: anatomical alignment (RI 0°) and four malunion models with stepwise loss of radial inclination, while maintaining a normal VT; this deformation resulted in a corresponding increase in ulnar variance. Loading conditions included a simulated gripping load applied through axial loading of the metacarpals, equivalent stress, minimum principal stress, and contact force were calculated at the ulnar head, triquetrum, and ulnar side of the lunate.

Decreasing RI reduced ulnar head contact force and lunate stress under both 0 mm and 5 mm simulated distal translation. At 5 mm simulated distal translation, contact force at the ulnar head was numerically reduced from 70.61 ± 5.58 N at RI 0° to 44.29 ± 8.78 N at RI -20° (not statistically significant after Bonferroni correction). Equivalent and minimum principal stresses at the ulnar lunate were significantly lower in models with reduced RI.

In this simplified finite element simulation of ulnar abutment with VT held within the normal range, decreasing radial inclination was associated with reduced ulnar head-carpal contact loading (contact force/stress) under the modeled conditions.

With VT constrained to the normal range, this model suggests that reduced radial inclination can decrease simulated ulnar head-carpal contact loading in a theoretical setting. These results describe a potential biomechanical mechanism within the specified simulation assumptions and do not predict patient-level symptoms or clinical outcomes.