Host response - Orthopaedic Biomaterials in Research and Practice - page 341

All implants are surrounded with a fibrous capsule. This is a con-
sequence of the classic foreign body response that attempts to degrade
all nonself material and, failing that, to sequester it. Such response is
to be expected for implanted materials. Direct tissue apposition to pure
titanium and tantalum and its alloys is possible, and it appears that the
early response to bioactive ceramics is also free of capsule formation.
However, these are isolated exceptions to the general rule.
The normal capsule arises through the process of matrix repair and
elaboration that accompanies normal wound healing. It is largely collag-
enous but may resemble fibrocartilage in areas of relative motion, as in
the acetabulum in contact with an endoprosthetic head. In areas around
loose prostheses, it may even appear synovial in nature. It is usually inte-
gral with normal bone or soft tissue and lies directly against the implant.
In its normal presentation, capsular tissue is relatively acellular, con-
taining only small numbers of fibroblasts with rare histiocytes and mac-
rophages. The major matrix protein is type I collagen; thus, the capsule
resembles granulation tissue rather than an early stage of fracture heal-
ing. However, the capsule is oriented with collagen fibers lying parallel
to the implant surface, and cells, when present, are more common in the
outer layers.
In occasional cases, there is a volume of serous fluid between the
capsule and the implant. Within articular joints, regenerating synovial
tissue produces a fluid similar to normal synovial fluid.
When the implant is a source of debris, either from articulating wear,
or fretting of multipart or modular devices, particles may be seen in
the matrix between the cells and within macrophages. Polymethyl meth-
acrylate (PMMA) is likely present in some instances; however, conven-
tional histologic preparation usually removes some or all of the PMMA
debris. Ceramic debris are rarely appreciated since they are generally
very small (<0.1 μm) and may be lost from the tissues during histologic
processing. Carbon-reinforced ultrahigh-molecular-weight polyethylene
(UHMWPE) articulating components shed submicron-sized carbon par-
ticles as well as short (10-30 μm), pulled-out sections of fiber, both of
which are easily recognized by their opacity, regularity of shape, and
black color. Metal wear particles are typically on the order of 1 to 2 μm
in size.
The capsular membrane is usually only 20-200 μm thick and thus
cannot be seen radiographically. It may be specifically thickened around
angles or protruberances of implants, such as at the knots in cerclage wire.
More generally, the capsule becomes thickened as a response to increased
chemical release from the implant, to stress-concentrating features, and
to relative tissue-implant motion. The 1 mm or wider radiolucent line,
which may be pathognomonic of incipient or actual loosening, is the
result of a more aggressive process, most frequently termed “osteolysis.”
In bony tissue, in the presence of acceptable implant stability (as dis-
cussed in Chapter 13), bone will grow into close apposition with the
implant surface. In PMMA-cemented devices, a zone of “dead” bone,
0.1-0.5 mm thick, is produced secondary to chemical toxicity of the
methacrylate monomer (MMA) and to the release of heat during PMMA