Session VII - Foot & Ankle

Biomechanical Evaluation of Syndesmosis Fixation with 3.5- and 4.5- Millimeter Stainless Steel Screws

Michael C. Thompson, MD; Dirk Gesink, MD; Karl Hamson, MD; The Phoenix Orthopaedic Residency Training Program, Phoenix, AZ

Purpose: Syndesmosis screws are frequently used in the operative treatment of complex ankle fractures with high fibula fracture or in the presence of syndesmosis diastasis or instability. Although most authors recommend either 3.5-mm or 4.5-mm stainless steel cortical screws for syndesmosis fixation, the optimum screw size has yet to be defined. The present study was designed to biomechanically compare syndesmosis fixation with 3.5-mm and 4.5-mm stainless steel screws.

Methods: Twelve paired, fresh-frozen lower extremities amputated at the knee joint were utilized for testing. To simulate a pronation-external rotation-type ankle injury, the deltoid ligament, anterior and posterior tibiofibular ligaments, and interosseus ligament up to a level 10 cm proximal to the ankle joint were divided. A 3.5-mm (group I) or 4.5-mm (group II) tricortical stainless steel screw was used to stabilize the syndesmosis. Sub-maximal axial ramp (0 to 1200 N) and external rotation/torsional ramp (0 to 5 N-m) loading was performed on each specimen prior to ligament division, following ligament division and following syndesmosis fixation. Axial fatigue testing was performed at 1.5 Hz for a total 100,000 cycles (0 to 900 N). Each specimen was then tested to failure in external rotation.

Results: In external rotation to failure testing, group I failed at a greater angle (38.9 ± 4.1 vs. 32.0 ± 3.8 in group II; p<0.05). Failure torque was slightly higher in group I; however, the difference was not statistically significant (17.8 ± 2.0 N-m vs. 14.3 ± 2.6 N-m in group II; p=0.082). Five of the six specimens in group I failed by screw pullout and five of the six specimens in group II failed by fibula fracture (p=0.061). There were no differences in syndesmosis widening or torsional stiffness during axial and torsional ramp load testing.

Discussion and Conclusion: At sub-failure levels of axial and torsional loading, cadaveric ankle syndesmosis fixation with 3.5-mm and 4.5-mm stainless steel screws demonstrated similar biomechanical properties. In torsional failure testing, ankles repaired with 3.5-mm screws demonstrated greater angular displacement prior to failure when compared to 4.5-mm screws; however, the two screws yielded similar failure torques. The present results suggest that syndesmosis fixation with a 3.5-mm stainless steel screw is biomechanically comparable to fixation with a 4.5-mm stainless steel screw.