Plasma-sprayed coating of hydroxyapatite are biocompatible and, because
of their osteoconductive properties, may contribute to the early fixation
of total joint prostheses. To evaluate this interface, we histologically
analyzed five hydroxyapatite-coated femoral stems which, along with the
surrounding bone, were retrieved from three humans at autopsy. The five
femoral components had been in situ for a mean duration of twelve months
(range, almost five to twenty-five months) and had been inserted for
osteonecrosis (two), osteoarthrosis (two), and as an uncermented revision
for failure of a cemented stem. The three patients had had a good or
excellent clinical result and had died of causes unrelated to the joint
arthroplasty. A coating of hydroxyapatite was identified on each stem.
There was a variable amount of apposition of bone (32 to 78 per cent of
available surface per section). The deposition of bone was most prominent
on the surface of the prosthesis that was close to the endosteal surface of
the bone, especially in areas that are predicted by Wolff's law (anterior
and medial aspects of the implant, and at lateral-oblique corners). There
were occasional foci of bone-remodeling around the implant, including
osteoclast-mediated removal of the coating of hydroxyapatite along with
adjacent bone. Occasional particles of ceramic were present within
macrophages in the adjacent bone marrow. Other areas showed formation of
new bone with a few areas of bone directly against the metal substrate. The
over-all histological features suggest mechanically stable implants with
bone-remodeling at the surface of the bone-implant interface.