ORIGINAL RESEARCH Syndesmosis Fixation: Analysis of Shear Stress via Axial Load on 3.5-mm and 4.5- mm Quadricortical Syndesmotic Screws Matthew Hansen, DPM, 1 Long Le, DPM, 2 Stuart Wertheimer, DPM, 3 Eric Meyer, BS, 4 and Roger Haut, PhD 5 The effect of shear stress on a fixated distal syndesmosis of the ankle was evaluated with a servohydraulic materials–testing machine. Eighteen syndesmoses were fixated in a quadricortical technique using 3.5-mm cortical and 4.5-mm cortical stainless steel screws. A shear stress was applied via an axial load in an attempt to simulate weightbearing. The 4.5-mm quadricortical screws produced a higher yield load and peak load (484.3  93.8 N and 597.7  81.4 N) when compared with the 3.5-mm quadricortical syndesmotic screws (412.8  55 N, P = .033 and 511.2  64.4 N ). These findings suggest that a larger diameter screw provides greater resistance to an applied shear stress at the distal syndesmosis. ( The Journal of Foot & Ankle Surgery 45(2):65– 69, 2006) Key words: syndesmosis, syndesmotic fixation, distal tibiofibular ligaments S yndesmotic injuries of the ankle commonly occur via an external rotational force applied to the ankle joint. Injuries to the distal tibiofibular ligaments occur in pronation-exter- nal rotation fractures, supination-external rotation fractures, and Maisonneuve fractures (1, 2). Disruption of the syndes- motic ligaments can also occur with severe ankle sprains without fractures, which are termed high ankle sprains (3–5). There are many surgical approaches for placement of syndesmotic fixation in reference to type and number of fixatives, number of cortices, and level of fixative insertion (6–9). Guidelines recommend placement of the syndes- motic screw between 2.0 cm and 3.0 cm superior to the tibial plafond, parallel to the ankle joint, and with 20 to 30° angulation from the frontal plane (10). Fully threaded 3.5-mm or 4.5-mm cortical screws are inserted perpendic- ular to the tibiofibular joint with the neutralization tech- nique, not the lag technique (11, 12). Debate exists whether the screws should purchase 3 or 4 of the available cortices of the tibiofibular joint. Proponents of the tricortical technique suggest that this fixation does not always require removal before full weightbearing (12, 13). The observed presence of micromotion suggests that these screws are more likely to loosen rather than fracture. Re- sorption of the bone surrounding the tricortical screws may help reestablish normal motion within the syndesmosis, thus making routine removal of the tricortical screws unneces- sary (14). Others suggest that the quadricortical technique is more rigid, with less occurrence of syndesmotic widening during healing, and easier removal if screw failure occurs (6, 10, 15, 16). There is also no consensus regarding weightbearing sta- tus after a repair of the syndesmosis with screw fixation. Many believe weightbearing is only allowed after syndes- motic screw removal, because syndesmotic fixation has Address correspondence to: Matthew P. Hansen, DPM, St. John North Shores Hospital, 26755 Ballard Road, Harrison Township, MI, 48045. E-mail mphansen43@hotmail.com 1 Submitted while third year resident, Podiatric Surgical Residency, St. John North Shores Hospital, Harrison Township, MI. 2 Submitted while second year resident, Podiatric Surgical Residency, St. John North Shores Hospital, Harrison Township, MI. 3 Director of Podiatric Education, St. John North Shores Hospital, Har- rison Township, MI. 4 Submitted while graduate student, Mechanical Engineering, College of Engineering, Michigan State University, East Lansing, MI. 5 Director and Professor, Orthopedic Biomechanics Laboratories, Col- lege of Osteopathic Medicine, Michigan State University, East Lansing, MI. Copyright © 2006 by the American College of Foot and Ankle Surgeons 1067-2516/06/4502-0002$32.00/0 doi:10.1053/j.jfas.2005.12.004 VOLUME 45, NUMBER 2, MARCH/APRIL 2006 65