Biomechanical Study Of Subtalar Joint After The Talar Neck Fracture

Objectives:(1) To study the pressure distribution in the subtalar joint and its clinical significance.(2) To study the pressure distribution and the clinical significance of subtalar with a shortage of0.0mm,1.0mm,2.0mm or3.0mm of the talar neck after the talar neck fracture.Methods:(1) Anatomical observation of the subtalar joint were carried out on a dozen talus; exclude the history of bone diseases, trauma and metabolic diseases.(2) On neutral position and under a650N load,12fresh 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 a650N load; The types of the talar neck fracture included:shortage of0.0mm,1.0mm,2.0mm or3.0mm of the talar neck and fixation with two4.0mm half thread screws. 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 about3-4times bigger than that of the anterior and the middle. The posterior joint surface takes downward hollow; the anterior and middle joint surface take downward bulge; the two joints do not lie in the same plane. 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.(2) On neutral position and under a600N load, the contact pressures in the subtalar joints were determined by means of the pressure sensitive film. Results showed that the average contact pressure was18.36kg/cm2in 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 is3-4times 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 transmitted47.8%of the whole load, while the anterior and middle transmitted about20%of the whole load(P<0.05).(3) On neutral position and under a650N load, with shortage of0mm of the talar neck after the talar neck fracture, the load and pressure that anterior, middle and posterior articulation transmitted are similar with the normal (P>0.05); while with shortage of1mm,2mm or3mm, the contact area is smaller than 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.(2) On neutral position and under a650N load, the contact pressures in the posterior subtalar joints transmitted about47.8%of the whole load, which has an important clinical significance.(3) All of the talar neck fractures with an shortage of more than lmm will notably influence the subtalar articulation biology mechanics; with an shortage of more than2mm will leads to the sequelae of posttraumatic degenerative arthritis of the subtalar joint. So maintain the length of the talar neck is an important principle in treating the fracture of talar neck.