Applicability and integration of plasma sprayed hydroxyapatite coated AO cortical bone screws in equine bone

To compare insertion temperatures and torques of Hydroxyapatite (HA) coated and uncoated 5.5 mm AO cortical screws in equine third metatarsal bones (MTIII) in vitro, and to compare insertion and extraction torques of HA coated and uncoated screws after 4, 8, 12 and 16 weeks of healing in equine third metacarpal bones (MCIII) in vivo. No significant temperature differences were recorded in cadaveric bones for AO and HA coated screws. Insertion torques were significantly higher for HA coated implants compared to uncoated screws. In vivo, the AO screws lost 50% of their initial stability within 4 weeks of healing and failed to gain stability over the next 12 weeks. The HA screws maintained stability (at 4 weeks), and roughly doubled (at 8 weeks) and tripled (at 12 weeks) their insertional torques over time. HA coated screws can safely be inserted in equine cortical bone. AO cortical stainless steel screws fail to maintain stability in equine cortical bone. The addition of HA coating to the screws enables active osseointegration over 3 months of healing, as indicated by significantly higher extractional torques after 8, 12 and 16 weeks respectively. Screw failure can occur under acute load and cyclic fatigue indicating the need for improved stability in the equine patient. HA coating, leading to active osseointegration, is commonly used in human implants for this purpose. Varying results in equine models led to guarded acceptance of implant coatings amongst equine surgeons. Our results support the osseointegrative properties of HA coated screws in horses

Implications of the principle of effective stress

While Terzaghi justified his principle of effective stress for water-saturated soil empirically, it can be derived by means of the neutrality of the mineral with respect to changes of the pore water pressure $_{}$. This principle works also with dilating shear bands arising beyond critical points of saturated grain fabrics, and with patterns of shear bands as relics of critical phenomena. The shear strength of over-consolidated clay is explained without effective cohesion, which results also from swelling up to decay, while rapid shearing of water-saturated clay can lead to a cavitation of pore water. The $_{}$-neutrality is also confirmed by triaxial tests with sandstone samples, while Biot’s relation with a reduction factor for $_{}$ is contestable. An effective stress tensor is heuristically legitimate also for soil and rock with relics of critical phenomena, particularly for critical points with a Mohr–Coulomb condition. Therein, the $_{}$-neutrality of the solid mineral determines the interaction of solid fabric and pore water, but numerical models are questionable due to fractal features