The biomechanical influence of tibio-talar containment on stability of the ankle joint

Abstract

Chronic ankle instability (CAI) is a frequent sport orthopaedic entity. Although many risk factors have been studied extensively, little is known how it is influenced by the osseous joint configuration. Based on lateral X-rays, the radius of the talar surface and the tibial coverage of the talus (sector alpha) were measured on a DICOM/PACS system in 52 patients with CAI and an age- and sex-matched control group. The talar radius was found to be larger in patients with CAI (21.2 +/- 2.4 mm) than in the control group (17.7 +/- 1.9 mm; P < 0.0001). The tibio-talar sector was smaller in patients with CAI (80 degrees +/- 5.1 degrees ) than in the control group (88.4 degrees +/- 7.2 degrees ; P < 0.0001). The aim of this study is to analyse the biomechanical influence of the clinical data on stability of the ankle joint. A two-dimensional model of the tibio-talar joint in the sagittal plane was developed. The joint configuration was described by the tibio-talar sector (alpha) and the radius (r) of the talus. The force (F = F (BW) tan alpha/2) and energy (E = F (BW) r [1 - cos alpha/2]) to dislocate the talus out of the tibial plafond were deduced. Ankle stability is a function of the tibio-talar sector: the force necessary to dislocate the joint is decreasing with a smaller sector. The clinical data show that the force needed to dislocate the ankle of CAI patients was 14% weaker than the one needed in the case of healthy subjects (P < 0.0001). The energy to dislocate the ankle depends both on the sector and the radius. The clinical data do not show a significant difference between the energy needed to dislocate the joint of CAI patients and the one of healthy subjects. This is because there is a correlation of a small sector and a large radius for CAI ankles. CAI is associated with an unstable osseous joint configuration, which is characterized by a larger radius of the talus and a smaller tibio-talar sector. The findings of the biomechanical model explain the clinical observations and demonstrate how stability of the ankle joint is influenced by the osseous configuration. Surgical ankle ligament stabilization might be more recommended in patients with an unstable osseous configuration as such patients have a disposition for recurrent sprains. Removing anterior osteophytes for anterior impingement should be done carefully in CAI patients because this would decrease the tibial coverage of the talus and thus dispose the talus to dislocate anteriorly. People who have an unstable ankle configuration and who nevertheless engage in activities with high risk of ankle sprains could be asked to wear ankle protecting sports equipment.

Frontal and lateral characteristics of the osseous configuration in chronic ankle instability

“…According to statistical considerations and feasibility in the clinical setting without a DICOM/PACS station, the following parameters can be considered most appropriate to anticipating chronic unstable osseous ankle configuration: (1) a tibiotalar sector ≤82°, (2) a frontal curvature ≥2 mm and (3) anterior TibCOR ≥4 mm. The tibiotalar sector is the most significant parameter because of biomechanical calculations6 and because it is a relative value that is not dependent on radiological magnification.…”

“…So far, our studies have just focused on the bones and neglected the ligaments 4 6. Measuring the TibCOR allows an indirect measurement of the ligament laxity of the ankle joint on radiographs.…”

“…Cutoff criteria varied within 1 SD from both means. Because the tibiotalar sector already included the influence of the radius,6 the radius was not evaluated separately.…”

“…The hypotheses of this study were that (a) a deeper curvature of the talus in the frontal plane facilitates inversion injuries; (b) the lateral malleolus would be longer in CAI, causing a less parallel position of the ATFL to an antero-inferior-orientated luxation force; (c) the talar centre of rotation is not exactly in the tibial axis as has been assumed4 6; and (d) the lateral tibial surface angle would be smaller in CAI. Furthermore, criteria were established to predict the occurrence of an unstable osseous ankle configuration.…”

“…The aim of this study was to describe characteristics of the osseous ankle configuration in the frontal plane as well as to determine further characteristics in the sagittal plane according to biomechanical considerations 6 7. The hypotheses of this study were that (a) a deeper curvature of the talus in the frontal plane facilitates inversion injuries; (b) the lateral malleolus would be longer in CAI, causing a less parallel position of the ATFL to an antero-inferior-orientated luxation force; (c) the talar centre of rotation is not exactly in the tibial axis as has been assumed4 6; and (d) the lateral tibial surface angle would be smaller in CAI.…”