THE EFFECT OF OPERATIVE FIT AND HYDROXYAPATITE COATING ON THE MECHANICAL AND BIOLOGICAL RESPONSE TO POROUS IMPLANTS

Femoral intramedullary implants were constructed by threading 4.0-mill imeter-thick disks with a titanium-alloy (Ti-6Al-4V) porous bead coati ng onto a two-millimeter diameter threaded rod. Each porous-coated dis k, which was 6.0, 8.0, 9.0, or 10.0 millimeters in diameter, was separ ated by a two-millimeter-thick acrylic disk with a diameter of ten mil limeters. Implants with and without a hydroxyapatife coating of twenty -five micrometers were inserted into fifteen skeletally mature adult m ongrel dogs. The femoral canal was sequentially reamed bilaterally to a ten-millimeter diameter, resulting in uniform initial implant-bone i nterface gaps of 0.0, 0.5, 1.0, and 2.0 millimeters. Each animal recei ved paired hydroxyapatite-coated and uncoated implants. Three animals each were killed at four, eight, twelve, twenty-four, and fifty-two we eks after the implantation. The harvested femora were sectioned throug h the acrylic spacers, transverse to the long a;bs, to produce individ ual push-out test specimens for mechanical testing. Characteristics of interface attachment were determined with test fixtures that supporte d the surrounding bone to within 150 micrometers of the interface. His tological sections were prepared, and the amount of bone within the po rous structure and the amount of the original gap that was filled with new bone were quantified with a computerized video image-analysis sys tem. Mechanical attachment strength and bone ingrowth were found to in crease with the time after implantation and with a decrease in the siz e of the gap. Placement of the implant in proximal (cancellous) compar ed with distal (cortical) locations had no significant effect on the s trength of attachment, bone ingrowth, or gap-filling. However, implant s with a large initial gap (1.0 or 2.0 millimeters) demonstrated great er attachment strength in cancellous bone than in cortical bone. With a few exceptions, hydroxyapatite-coated implants with an initial gap o f 1.0 millimeter or less demonstrated significantly increased mechanic al attachment strength and bone ingrowth at all time-periods. Interfac e attachment strengths were positively correlated with bone ingrowth, the time after implantation, the use of a hydroxyapatite coating, and decreasing initial gap size. CLINICAL RELEVANCE: Initial implant-bone apposition is thought to be a prerequisite for good biological fixatio n. This apposition is often not achieved because of the design of the implant or instruments and the operative technique. Poor initial fit d uring the operation may decrease the longevity of the implant. The res ults of the present study indicate that attachment strength and bone i ngrowth are significantly affected by gaps in the interface, particula rly those of more than 1.0 millimeter. In addition, the use of a hydro xyapatite coating on the implants was shown to have a positive effect on fixation of the implant, especially when the initial gap was small.