SHEAR AND TENSILE-STRENGTH OF HYDROXYAPATITE COATING UNDER LOADING CONDITIONS - AN EXPERIMENTAL-STUDY IN DOGS

The shear and tensile strength of a hydroxyapatite (HA) coating on a f emoral component was studied after physiological loading conditions in 8 German Shepherds. A proximal macrostructure on the stem was used to protect this region from shear stresses. Another four implantations w ith uncoated components were used as controls. In vitro testing of the HA layer demonstrated excellent tensile strength and stability to sur face deformation. The loaded implants were tested at 6, 12, and 24 wee ks. At 6 weeks the HA-coated components could easily be removed by axi al loading, whereas the HA layer remained undamaged on the metal. Howe ver, pull out tests of implants older than 12 weeks showed complete de bonding of the HA layer from the non-macrostructured surface due to sh ear forces in all cases. Debonding of the HA layer was also observed w ith microradiography. The macrostructured surface prevented dislodging of the component from this area at pull out test by distributing shea r forces. Unlike in uncoated implants, considerable amounts of bone re mained attached onto the HA macrostructure when the surrounding femur was pulled out. Shear forces cause debonding of the HA layer, while te nsile stress affects failure within the bone. Physiological loading pa rtially produces gaps at the interface so direct transmission of tensi le forces onto the bone is lost, and the coating-metal interface becom es the weak point in the system.