MOTION OF THE ANKLE IN A SIMULATED SUPINATION-EXTERNAL ROTATION FRACTURE MODEL

An experimental study was undertaken with use of axially loaded, uncon strained cadaver ankles to determine the motion patterns seen with pro gressive stages of the supination-external rotation type of fracture. As described by Lauge-Hansen, these fractures were modeled by transect ion of the anterior aspect of the capsule and the anterior tibiofibula r ligament (stage I), followed by oblique fibular osteotomy ending at the level of the ankle joint (stage II), transection of the posterior aspect of the capsule (stage III), and sequential sectioning of the su perficial and deep fibers of the deltoid ligament (stage IV). Thirteen specimens were tested on an apparatus that allowed for controlled loa ding while the ankle was passed through a physiological range of dorsi flexion and plantar flexion. The ankles were unconstrained about the a xial (internal and external rotation) and coronal (varus and valgus an gulation) axes. Measurements were made throughout the range of motion in these axes in order to define the kinematic behavior. In the intact specimens, maximum plantar flexion was associated with a mean (and st andard deviation) of 1.9 +/- 4.12 degrees of internal rotation of the talus and maximum dorsiflexion, with a mean of 7.2 +/- 3.88 degrees of external rotation. Varus angulation increased slightly with plantar f lexion compared with the value in dorsiflexion (2.4 +/- 2.40 compared with 0.3 +/- 1.96 degrees). Internal and external rotation was not aff ected by fibular osteotomy or by transection of the superficial fibers of the deltoid ligament. Transection of the deep fibers of the deltoi d ligament caused a significant (p < 0.02) increase in external rotati on of the talus at maximum plantar flexion; this was corrected incompl etely by insertion of an anatomical fibular plate. With the numbers av ailable for study, we could not show that varus or valgus angulation w as significantly affected by any combination of sectioning of the delt oid ligament and fibular osteotomy. These experiments were repeated wi th the addition of fixation of the subtalar joint with a talocalcaneal screw. With the number of specimens available, we could detect no sig nificant difference with respect to axial rotation, due to fixation of the subtalar joint. However, along the coronal axis, increased valgus angulation (p < 0.02) was seen during plantar flexion when either the deep or tire superficial fibers of the deltoid ligament had been cut. CLINICAL SIGNIFICANCE: These results indicate that stability of the l oaded ankle is primarily due to the deltoid ligament, which exerts a r estraining influence on external rotation of the talus. Complete fibul ar osteotomy did not cause abnormal motion of the ankle in the absence of a medial injury In the presence of a complete injury lateral recon struction only partially restored the mechanical integrity of the ankl e. The results provide justification for the non-operative treatment o f isolated fractures of the lateral malleolus. The data also suggest t hat a lateral fracture associated with a major injury of the deltoid l igament should be treated with anatomical lateral fixation followed by immobilization without early motion, to allow adequate healing of the deltoid ligament at its resting length.