Ultrastructure of the fibrous tissue surrounding internal fixation devices

The possible bursal nature of the fibrous tissue that develops like a capsule over internal fixation devices was studied ultrastructurally in 13 human and four canine cases. In both species, the basic structural pattern was similar. The capsule had two distinct zones. The superficial compact zone appeared as a pseudomembrane, and consisted of elongated fibrocytes arranged transversely in a collagenous and microfilamentous matrix. In the deep zone, the cytoplasm and its constituents of connective tissue cells varied from meager to abundant. The structural organization of the capsule as well as its cellular characteristics did not conform to the specificities of an adventitious bursa. The capsular tissue did not appear to be affected by such host factors as age; implant alloy composition; the type of the underlying bone or its abnormalities; duration of implantation, or superficial or deep location of the metal device. Inflammation rarely occurred in the capsular soft tissue, including those cases where pain developed over the implants following an uneventful bone union. Because metal implants do not induce formation of adventitious bursa, pain over an uncomplicated implant is not likely to be due to a bursitis.