Biomechanical evaluation of syndesmotic screw design via finite element analysis and Taguchi's method

Screw fixation of syndesmotic injuries facilitates ligament healing and restoration of ankle stability, but failure of the screw might threaten the success of the treatment. Screw design parameters, such as outer diameter, inner diameter, thread pitch, leading edge radius, trailing edge radius, leading edge angle, and trailing edge angle, might have effects on the stresses that occur in the screws. This is the first study, to our knowledge, to investigate which geometric screw parameters play key roles in stresses that occur in screws used for syndesmotic fixation.

A three-dimensional finite element model of an ankle was reconstructed. Four different types of titanium screws—4.5-mm malleolar, 4-mm cancellous, 4-mm machine, and 3.5-mm cortical—were placed on this model. Physiologic load was applied to evaluate the stress in the screw. Then the contribution of each design factor to stress in the screws was analyzed systematically by Taguchi's robust design method.

The maximum equivalent ductile failure (von Mises equivalent stress) value was found in the 4-mm cancellous screw (402 MPa). Taguchi's analysis showed that the descending order of contribution of the design factors to stress emerging on the screw is inner diameter, leading edge angle, thread pitch, outer diameter, and trailing edge angle.

Stress that occurs in syndesmotic screws is closely related to their geometry and dimensions. According to the results, a 3.5-mm cortical screw with the ideal screw design regarding optimal parameters to resist against stresses in the syndesmosis seems more reasonable to choose in syndesmotic fixation.