Biomechanical Study Of Subtalar Joint After The Talar Neck Fracture

Objectives: (1) To study the anatomical features of asubtalar joint and trabeculae of talus and calcaneus bone under normal conditions. (2) To study the pressure distribution in the subtalar joint and its clinical significance. (3) To study the pressure distribution and the clinical significance of subtalar with a rotation angle of 5°,10°of the talar head with the talus axis after the talar neck fracture. Methods: (1) Anatomical measurement and observation of the subtalar joint were carried out on 20 pairs of talus using vernier (0.02). The length, width, and depth of the joints were measured. And then anatomical sections were performed and roentgenograms were taken on 20 pairs of talus and calcaneous to observe the trabeculae of bone. (2) On neutral position and under a 600N load, eight fresh cadaveric foot specimens were examined on machine; the contact pressures in the subtalar joints were determined by means of the pressure sensitive film. (3) The above eight fresh cadaveric foot specimens were then made into various types of talar neck fracture models which were examined on machine with neutral position and under a 600N load; The types of the talar neck fracture included: medial circumflex 5°, lateral circumflex 5°and 10°. The contact pressures in the subtalar joints were determined also by means of the pressure sensitive film. Results: (1) The subtalar joint includes anterior, middle, posterior joint surface, the area of the posterior joint surface is about 3-4 times bigger than that of the anterior and the middle. The posterior joint surface takes on "∧"type and downward hollow; the anterior and middle joint surface take on "∨"type and downward bulge; the two joints do not lie in the same plane. The axis of motion of the talus is from inferior-posterior-lateral to superior-anterior-medial, with a deviation angle of 20-30 degrees with the longitudinal axis of the foot passing between these condand third metatarsals, also a plantar declination angle of 5-8 degrees. Observation on the talocalcaneal trabecula in the sagital and transverse planes showed a highly unanimous arrangement along with transarticular stress. In the same section, the sparseness of trabecula is in inverse proportion with the size of joint surface. Talar trabecula is made up of two bundles: the anterior bundles accord with the axis of the talus from astragalotibial joint surface to talonavicular articulation surface; the trabecula of posterior bundles are almost vertical to talocalcaneal joint surface. These 4 bundles of calcaneus trabecula, the anterior, the median, the posterior and the circular trabeculae, take a form of a ball-like structure. (2) On neutral position and under a 600N load, the contact pressures in the subtalar joints were determined by means of the pressure sensitive film. Results showed that the average contact pressurewas 20.36kg/cm~2 in the joint; there was no significant difference among the pressures of the anterior, the medial and the posterior articulation (P<0.05); the area of posterior articulation is 3-4 times bigger than that of the anterior and the middle articulation. At the same time the image of the pressure sensitive film showed that the posterior articulation transmitted 46.8% of whole load, while the anterior and middle transmitted 20% (P<0.05). (3) On neutral position and under a 600N load, with an angle of rotation of 5°, 10°of the talar head with the talus axis after the talar neck fracture, the load and pressure that posterior articulation transmitted are smaller than the normal (P<0.05); while in medial circumflex 5°, the contact area is bigger than the normal (P<0.05); in lateral circumflex 5°, 10°, there was no significant difference in the contact area of the fracture with the normal(P>0.05). Conclusions: (1) The subtalar joint was made up by two compound saddle articulations that do not lie in the same plane and have the same axis of motion. There was a close dynamic relationship between their distribution; the density of the trabeculae was crossed and kitted, forming a truss st...