Healing of large (2 mm) gaps around calcium phosphate-coated bone implants: a study in goats with a follow-up of 6 months.

Plasma-sprayed hydroxylapatite (HA) coatings are known for their ability to demonstrate osseointegration with bone. Recently it was found that the amount of bone apposition was strongly reduced 6 weeks after implantation in a goat model if gaps of two millimeters between bone and apatite coating existed. Stability of the apatite coatings examined did not influence the gap-healing ability. This study investigated whether a longer follow-up period of 24 weeks would be sufficient for the restoration of bone apposition on apatite coatings in an identical surgical model with 2 mm gaps, and whether bone apposition on the apatite coatings is influenced by the coating stability. Three coatings were investigated: 25-30% crystalline HA (aHA), 60-63% crystalline HA (cHA), and 85-90% crystalline fluorapatite (FA). Uncoated Ti-6A1-4V implants were used as controls. Implants were inserted in the femoral condyles of both femora of eight goats. Each goat received four implants. Histology revealed that bone formation on each of the apatite coatings remained low and did not increase with an extended follow-up period of 24 weeks. The coatings showed significantly (P < 0.01) more bone contact than the uncoated control implants. The three different coatings did not show significant differences in bone apposition. The aHA coating in most cases had disappeared completely after 24 weeks. Despite the disappearance of the aHA coating, bone contact was seen on the substrate surface without fibrous tissue interposition. The cHA coating showed minor signs of degradation while the FA coatings showed no visible degradation. It is concluded that non-press-fit implantation of apatite-coated implants leads to more bone apposition as compared to uncoated Ti-6A1-4V implants. However, it is suggested by these results that the upper limit of gaps around apatite implants is 2 millimeters in a non-weight-bearing model in goats. Bone apposition will not increase by extending the follow-up period more than six weeks, nor will it be altering the stability of the apatite coatings used.