By Moby Parsons, MD

Biological fixation refers to the process where bone grows into microscopic pores on a roughened metal surface. The interdigitation of bone into the metal locks the two together firmly and theoretically, this fixation can be permanent. Joint replacement implants, which are usually made of titanium, have taken advantage of biological fixation for many years and its success in total hip replacement is very well established. As long as the implant is stable when first inserted and there is no micromotion between the prosthesis and bone, ingrowth of bone into the metal is quite reliable and generally solid within 6 weeks.

The success of bone ingrowth in total hip replacement has spurred recent interest in its use in total knee replacement. While this concept is not new, there has been resurgent interest given improvements in surface porosity and coatings that promote bone healing like hydroxyapatite. When implants are inserted without the use of bone cement, they are referred to as press-fit. This means that the original fit is so snug and flush that the implants will not move under stress.

Press-fit knee implants have several theoretical advantages over cemented implants. First, cement can cause heat damage to the bone. Second, like any grout material, cement can slowly debond over time causing loosening of the implants. Third, cement takes time to harden adding about 10-15 minutes to each case. Press-fitting the components saves time which can reduce cost and complications like blood loss and infection.

Conceptually there are not major disadvantages to press-fit components except the theoretical potential of failure of bone ingrowth. This could occur if the initial press-fit was not snug leading to micromotion between the implant and bone. Early results of this new technology look favorable and its established use in hip replacement has been so successful that press-fit hip implants are used in well over 90% of cases. While the majority of total knees nation-wide are still cemented, it is likely that press-fit knee implants will gain increasing popularity moving forward and may become the standard in the near future.

At this time, press-fit implants are used mostly for younger patients with very good bone stock. Patients with weak bone due to osteopenia, osteoporosis or vitamin D deficiency may be better off with cemented implants for fear of lack of adequate support to provide initial solid fixation. As experience grows with the use of these new implants, the indications and contraindications will become better defined.