Analysis of the general features of the soft tissue response to some metals and ceramics using quantitative histomorphometry.

This article analyzes the general features of the tissue response to orthopedic biomaterials when implanted in the paraspinal muscles in rats for periods between 1 week and 1 year. Using a computer-assisted cell counting method, it was shown that the membrane thickness was a stable parameter that poorly correlated with the other histomorphometric parameters except fibrocyte distance parameters. Macrophages were significant in number only in the short term and were always located at the implant-tissue interface. At 4 weeks, the period corresponding to which macrophages disappeared, fibrocytes were found nearer the interface than at earlier periods and did not migrate for up to 52 weeks, although their densities decreased to 50% of their initial values. Therefore, it can be histomorphometrically confirmed that 1) the presence of an implant induced a delay in the healing process; 2) the immune system was not usually involved in the response to bulk metals; and 3) at early time periods, the presence of PMN cells was correlated to the long-term number of macrophages. On the contrary, the histomorphometric findings suggested that around the tested materials, a large number of macrophages did not induce an immediate or a delayed fibrocyte proliferation. If one considers that a low fibrocyte surface density is related to activated cells surrounded by a matrix high in collagen content, then the large number of macrophages induced the stimulation of collagen synthesis by the fibrocytes already included in the membrane. Accordingly, in the short-term, a high fibrocyte density and few macrophages may be a criterion for testing biocompatibility.