Controls for in vivo investigations of tissue compatibility can include (1) contralateral intact tissues as anatomic controls; (2) sham-operated controls (e.g., surgical incision only); (3) unfilled surgical implant sites; and (4) material and device controls. The nature of the controls employed will depend on the endpoint (i.e., outcome variables) of the experiment. Because of the important effect of surgical trauma on wound healing and tissue response to implants, unfilled surgical implant site controls can be particularly valuable. For example, if a surgical site is prepared in a particular tissue and no implant is inserted, the amount of scar formed could be a valuable guide in assessing the significance of a fibrous capsule found around an implant at the test site. A sham-operated limb can display the effects of altered load bearing on the recipient tissue, and thus serve as control for the effects of altered load bearing on the limb in which the implant has been placed.
Material and device controls must be assessed for their chemistry, mechanical properties, and topography, relative to the experimental specimens, prior to their use as positive or negative controls. In some cases the electrical characteristics of the materials might also have to be considered. For functional devices (e.g., femoral stems of total hip replacement prostheses), the test and control devices should be of identical shape and size.
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