Background: The mechanisms leading to aseptic loosening of a total
hip replacement are not fully understood. A fibrous tissue interface can be
present around the implant. Hypothetically, component micromovements can
compress this interface and cause increased fluid pressure according to
biphasic models. We tested the hypothesis that compression of a fibrous
membrane with or without the presence of high-density polyethylene particles
leads to bone degradation.
Methods: A titanium implant was inserted in forty-five rabbit
tibiae, and, after osseous integration was achieved, a fibrous tissue
interface was generated. The animals were randomized to undergo a sham
operation, treatment with compression of the fibrous membrane, treatment with
high-density polyethylene particles, or treatment with both compression and
particles. Morphometric analysis of the surrounding bone was performed on
cryostat sections after Giemsa staining and staining of tartrate-resistant
acid phosphatase activity.
Results: Forty specimens were available for analysis; five tibiae
with an infection were excluded. After nine weeks, the controls showed vital
bone, whereas the specimens treated with compression showed necrosis of bone
and replacement of bone by cartilage in a discontinuous layer (p < 0.05 for
both) but not fibrous tissue. Treatment with high-density polyethylene
particles caused replacement of bone by fibrous tissue (p < 0.05) but not
necrosis or cartilage formation. Compression combined with the presence of
high-density polyethylene particles caused bone necrosis and loss of bone with
replacement by cartilage and fibrous tissue (p < 0.05).
Conclusions: In this in vivo study in rabbits, fibrous membrane
compression led to bone necrosis and cartilage formation, possibly because of
fluid pressure or fluid flow, whereas the presence of high-density
polyethylene particles led to the loss of bone with replacement of bone by
fibrous tissue. Cartilage formation may be a protective response to fluid
pressure and/or fluid flow. Fibrous membrane compression may play an important
role in the early stages of loosening of a total hip replacement.
Clinical Relevance: The findings of this study suggest that
implantation techniques that prevent the formation of a fibrous tissue
interface (which may act as source of fluid pressure and/or fluid flow) may be
beneficial in reducing implant loosening.