The clinical use of plasma-sprayed hydroxyapatite coatings (HACs) on metal implants has been widely adopted. Faster and stronger fixation and more bone growth have been revealed. Owing to resorption or degradability of HAC in biological environment, the long-term stability of HACs has been doubted for clinical use. Previous study showed that heat treatment in vacuum after spraying could improve the bonding strength, impurity phase content, and crystallinity of HACs. The present study investigated the in vitro and in vivo biological responses of vacuum heating HAC. Culture of UMR106 osteoblast-like cell on two HACs showed that vacuum heating HAC generated lower cell number compared to plasma-sprayed HAC, but there was no significant difference in cell growth on both coatings during 10-day culture. The histological responses of two HACs were investigated in a goat transcortical femoral model after 6 and 12 weeks of implantation. The capacity for osteoconductivity and osseointegration of implants were histomorphometrically analyzed from back-scattered electron images (BEIs). Throughout all implant periods, vacuum heating HAC conducted new bone formation and revealed direct bone-to-coating contact. Owing to some particles having dissociated from the plasma-sprayed HAC, this group showed a significantly lower extent of new bone
apposition than vacuum heating HAC at 12 weeks. Consequently, the vacuum heating HAC could achieve the fast bone healing and apposition for initial fixation and stability of implants in clinical use. Also, vacuum post-heat treatment could be used to improve the mechanical strength, crystallinity, and phase composition of HAC, which are the major factors for both implant fixation and its long-term stability.