Pergamon .I Bmnrchhon~cs, Vol. 28. No. 3. 269 279. 1995 pp. Copyright 0 1994 Elsev~er Science Ltd Printed in Great Britain. All rights reserved 0021~ 9290/95 S9.50 + .OO 0021-9290(94)00076-X CONTACT AREAS AND PRESSURE DISTRIBUTIONS IN THE SUBTALAR JOINT Chung-Li Wang,* Cheng-KungCheng,? Chao-Wang Chen,f Chung-Ming Lu.~ Yi-Shiong Hang* and Tang-Kue Liu* * Department of Orthopedic Surgery; t Center for Biomedical Engineering; and S’Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan Abstract-We investigated how foot position and ankle arthrodesis affect the contact characteristics of the subtalarjoint. Nine fresh-frozen specimens of amputated lower legs were used. Pressure-sensitive films were inserted into the anterior and posterior articulation of the subtalar joint. The contact areas and pressure for various foot positions and under axial loads of 600, 1200, and 1800 N were determined based on the gray level of the digitized film. In neutral position and under a 600 N load, the maximum contact pressure in the subtalarjoint was 5.13+ 1.16 MPa. Thecontact area(1.18k0.35 cm’) was only 12.7% of the whole subtalar articulation area (9.31 iO.66 cm’), and the total force (348.5k41.7 N) transmitted via this contact area was about 58% of the applied load (600 N). Dorsiflexion of the foot increased the contact area and the force transmitted, but decreased the average contact pressure in the subtalar joint, while the reverse occurred in plantar flexion. Eversion increased the subtalar contact stress, whereas inversion up to 10” decreased it. Ankle joint arthrodesis shifted the contact areas in the subtalar joint posteriorly in all inversion/eversion positions, Moreover, total force transmitted through the subtalar joint as well as the contact pressure increased. Keywords: Subtalar joint, contact area, contact pressure. INTRODUCTION The subtalar joint isone of the mostcomplexweight- bearingjoints. Minor biomechanical or anatomical details often may have considerable clinical signifi- cance(Jahss, 1991). The subtalarjoint is responsible for the conversionof the rotatory forces of the lower extremities(Donatelli, 1990) and dictates the move- mentsof the midtarsaljoints and the forefoot (Elf- tmann, 1960). How the contact characteristics of the subtalar joint are influenced by positionalchange has never been clearly documented. Kyne and Mankin (1965) stated that the subtalar pressure would increase during ever&on. Mann (1986)and Jahss (1991)also pointed out that ankle fusion would increasethe subtalar contact stress. However, there has beenno direct measurement of data to validate their clinical observation. It has long been assumed that increased pressure leads to increased wear and break-down of the articu- lar cartilage (Beaudoin et al., 1991; Jahss, 1991; Walker and Nicol, 1991). Sincethe subtalar joint is a weight-bearing joint, why is spontaneous degener- ation of the ankle and subtalar joints so uncommon, while that of the hip or knee joint so common‘? Frankel and Nordin (1980) pointed out that the large Received in final fbrm 12 May 1994. Address correspondence to: Cheng-Kung Cheng, Ph.D. Associate Professor, Orthopedic Biomechanics Lab., Center for Biomedical Engineering, College of Medicine, National Taiwan University, No. 1, Sec. 1, Jen-Ai Road, Taipei, Taiwan. weight-bearingsurfaceof the ankle joint resultsin Lower stresses across it than in the knee or hip joint. Perry (1983)postulated that the main deterrent to spontaneous osteoarthrosis of the ankle joint is a normal subtalar motion, which decreases the rotatory forces on the ankle. How is this weight-bearing trans- mission madein the subtalar joint? Since the motion in the subtalar joint isintimately related to the motion in the anklejoint, what would happen to the contact stress in the subtaiar joint oncethe anklejoint lost its motion after an ankle arthrodesis? This in vitro study was designed primarily to evalu- ate the contact areas and pressure distributionsin the subtalarjoint under various load magnitudeand in differentfoot positions. Attempts were made to answer the following questions: What are the contact charac- teristicsin the subtalar joint and how are they influ- encedby different positional situations? Does ankle arthrodesis increase the subtalarcontact stress ashas long been believed clinically?Suchinformation regar- ding the subtalar joint could provide clues to the pathogenesis of the degeneration process, the mech- anism for calcaneal fractures, and the prosthetic design. MATERIALS AND METHODS Nine amputated specimens of the lower leg were obtained from the operating room at the National Taiwan University Hospital. The soft tissues of the foot and ankle wereintact and the lengths of the tibia and fibula werelongerthan 15 cm. Roentgenographic examinations were made to rule out any bony changes 269