Host response - Orthopaedic Biomaterials in Research and Practice - page 340

normal response to implantation is that of wound healing, although with
some significant differences.
Figure 14.1 shows the cellular progression that is expected in heal-
ing of soft tissues (above) and the development of stiffness and strength
of the healing tissues (below). The cell progression noted here should
be familiar, producing the events of cleanup, repair, and reconstruction
of the damaged tissues. Note that stiffness is observed to return before
strength, in both soft and hard tissues. This is reasonable if we consider
that normal mechanical function requires “all” of stiffness, whereas nor-
mal stresses usually do not exceed 10%-20% of ultimate strength. The
balance of strength is required to protect against rare overload events;
this capability returns late in the healing process.
Even in the absence of an implant, many factors may affect the devel-
opment shown here. In particular, hard tissue matures more slowly than
soft tissue, whereas functional use, promoting remodeling, probably
accelerates the later stages of healing of both.
Introduction of an implant assures that events will progress differently.
Early protein interactions with the implant surface may activate comple-
ment and will produce hapten complexes, which may activate other por-
tions of the immune system. Macrophages will be unable fully to clean up
the dead space in the wound, owing to the presence of a physical implant.
Three particular cellular events, not usually seen in healing wounds,
occur to one degree or another in the presence of all implants:

1. Macrophages may remain in the vicinity of the implant.

2. Significant numbers of lymphocytes (and even plasma cells) may
be seen near the implant.

3. At later times, a population of multinucleated giant cells, called
foreign body giant cells, may appear and remain for long periods
or chronically.

Fibrin,
platelets

PMN leukocytes
Macrophages
Lymphocytes

Bone
mineral

Collagen

Fibroblasts

?

FB “giant” cells

?

Stiffness

Strength

Implant

Injury/
operation

7 14 21 28

Days post-trauma

FIGUre 14.1 Cell populations and mechanical properties of a healing

wound.