Biomechanics of external fixator of distal radius fracture, a new approach: Mutifix Wrist

The purpose of this study was to assess factors affecting fixator stiffness with a finite element model and an experimental validation with particular attention to a new fixator device named Mutifix Wrist^® (NCS Lab srl, Carpi, MO, Italy).

Mechanical tests were carried out to determine the stiffness of the construct with different configurations. The obtained results were compared to those obtained with the Hoffmann II Compact (Howmedica-Osteonics Inc. Rutherford, NJ, USA). Data were sampled at 20 Hz and test speed was 0.05 mm/s. For each loading condition, tests were performed four times. A FEM campaign was also conducted to analyze how geometrical variables (type of configuration and K-wire diameter) affect both stiffness and stress distribution of the fixator.

Stiffness, axial displacement, magnitude of the displacement, magnitude and localization of the peak stress of the construct were all analyzed. Axial compression tests showed that the axial displacement reached by the machine actuator when the measured force reached 45 N was 0.56 mm ± 0.13 on average. Magnitude of the displacement along with peak stress magnitude and localization varied through the several configuration tested, but it resulted that the distal pin near the fracture gap was the more stressed one in all cases except those in which the fracture line is crossed.

From the tests performed, it emerged that the addiction of a K-wire provides a construct stiffening and a consequent local stress reduction; while span increase reduces stiffness and increase the local stress. If a K-wire is implanted through the fracture site, the axial stiffness is significantly increased.