Enhancement of bone growth into metal screws implanted in the medullary canal of the femur in rats.

The kinetics of growth of bone into control (nontreated) and heat-treated screws made of stainless steel (type 316L) and Ti-6Al-4V, implanted in the medullary canal of the femur in rats, were studied by mechanical, histological, and biochemical methods. A progressive and significant increase in the ingrowth of bone, as reflected by interfacial shear strengths of the screws, was measured with time after implantation. At all time intervals for as long as 35 days after implantation, the shear strength of the heat-treated Ti-6Al-4V and stainless-steel screws was significantly higher than (1.6-3.4 times) that of the control screws. The specific activity of alkaline phosphatase in extracts of tissue from around the implanted screws peaked 6 days after insertion, with significantly higher values at 5, 6, and 7 days postoperatively for the heat-treated screws than for the controls. The extent of calcification also was higher at all time intervals for the heat-treated screws than for the controls. The histological evaluation of formation of bone between the ridges of the implanted screw corroborated the mechanical and biochemical measurements. At each time interval, a more mature bone was noted around the heat-treated screws than around the controls. It was concluded that the heat treatment of metal implants before insertion can result in augmentation of osseous ingrowth 1.6-5.3 times that into control implants in an in vivo experimental model.