IN VITRO AND IN VIVO DETERMINATION OF ANKLE JOINT AND SUBTALAR JOINT AXES USING THE HELICAL AXIS METHOD

Abstract

INTRODUCTION: In the kinematic analysis of most human joints continuous motion can be simulated by a sequence of finite motion steps of one part of the joint relative to the other part. In previous studies different experimental setups were used in vitro to determine the subtalar joint axis (talo- alcaneo-navicularaxis), and great variability could be shown for both orientation and position. The orientation of the axis can be described as a projection to anatomical planes in terms of deviation – that is, the projection to the transversal plane and as an inclination that is the projection to the sagittal plane. The study objective was to verify the application of the helical axis method to the ankle and subtalar joint in vitro and to transform the method for in vivo investigations. METHODS: In vitro: one fresh frozen ankle specimen was thawed preexperimentally and then fixed in a laboratory frame. The lower leg was fixed about 15 cm proximal to the ankle joint. In the first series we used stereo photogrammetry to determine optimal marker positions. In a second series with ankle, 2 specimen’s stress x-rays were used to compare talar tilt and the helical axis. In vivo: the marker system was fixed to the tibia using adhesive double- sided tape and with a special shoe construction to fix it to the calcaneus. 3D calculation of marker coordinates in finite positions were performed from a four-camera setup using the Peak Performance® system. Twelve subjects were tested in sitting position without foot-ground contact. They moved their feet from a neutral position into dorsiflexion and (while maintaining dorsiflexion) to eversion and inversion position. RESULTS: The in vitro investigation revealed a mean deviation from repeated measurements of 10.6 degrees (± 4.6) and an inclination of 45.7 (± 5.6). The correlation of x-ray talar tilt with the helical axis method was r = 0.86 for the intact joint and 0.9 with ankle joint ligaments cut. From 12 subjects a mean deviation of 23 (± 13) and an inclination of 46 (± 10) were calculated. The correlation of deviation and inclination was r = 0.8. DISCUSSION: This method is applicable to determine subtalar joint axis in vivo. However, the parameters are highly susceptible to measurement errors and noise. The mean results were in accord with previous studies, but the differences among subjects were remarkable. From the correlation of the angles and the relationship of the axis to the other ankle joint stabilizing structures (muscles, tendons) one could conclude that this complex may play an important role with respect to sudden inversion injuries CONCLUSION:. If the axis of the subtalar joint could be identified as risk factor to ankle sprains then in vivo determination of the axis could be used for prevention.