Increased Bone Ingrowth on a Biomimetic Nanocrystalline Apatite Surface

Abstract

Typical plasma-sprayed hydroxyapatite coatings work well on non-porous substrates but do not coat the inner surfaces of open-porous substrates. Solution deposition can produce consistent bioceramic coats of precise thickness on porous surfaces. The resultant “biomimetic” surface more closely mimics the trabecular pattern and biochemistry at the bone interface. This report compares bone response to porous surfaces with biomimetic hydroxyapatite coatings. Implants were manufactured as Ti6Al4V cylinders (5-mm diameter, 41-mm long) coated with c.p-Ti PorocoatÒ porous layer with a thickness of 750 (± 250 µm). Implants were divided into three groups based on surface treatments. The porous surfaces of control group implants did not receive any treatment. The porous surfaces of HA group implants were plasma sprayed with hydroxyapatite. The porous surfaces of BAp group implants were coated with a biomimetic apatite (BAp) coating using a lowtemperature solution-based process that mimics bone mineralization. BAp coating is pure apatite coating of uniform structure and composition, with a thickness of approximately 15 µm on the outer beads. Because of the reduced thickness, the BAp coating does not block the pores or alter the porous structure. Bilateral femurs in thirty-six rabbits were implanted with one of the above groups. Twelve rabbits each were euthanized at 2, 4, and 12 weeks. Osseointegration was measured by automated computerized histomorphometry of scanning electron microscopy images of sections taken through the implant. Bone ingrowth on the Control surface was 45 % at 2 weeks and 47% at 12 weeks. Bone ingrowth on the PS surface increased from 51% at 2 weeks to 67% at 12 weeks. Bone ingrowth on the BAp surface increased from 45 % at 2 weeks to 71% at 12 weeks. At both time points mean bone ingrowth on PS and BAp coated implants was significantly higher than the control uncoated implants (p < 0.01). By 12 weeks the PS hydroxyapatite coat began showing evidence of fragmentation and debris production on SEM. This was not evident in the BAp coat. This study supports the hypothesis that apatite coating benefits osseointegration. A biomimetic coat of solution deposited apatite may not show the disadvantages of coating delamination and particle generation. Biomimetic apatite coatings may be attractive alternatives for noncemented total hip arthroplasty.