Little is known about the long-term histological features of allograft
bone incorporation following revision total hip arthroplasty. Some
authors have reported evidence of incorporation of morselized graft
following impaction grafting12-15.
Most of the histological data about massive allografts pertain to
specimens retrieved from patients who underwent reconstruction following en
bloc resection of a tumor16,17.
We are aware of only one report on incorporation of structural allografts
following revision of the femoral component; in that study, specimens
were obtained from five irradiated bone allograft struts that had
been implanted two to twenty-seven months previously18. The authors noted a time-dependent
incorporation process, with the most marked osteoclastic resorption
seen in the samples that had been obtained soon after implantation.
Remodeling and new-bone formation were most marked in the specimens
that had been in situ for longer periods. The extent
of revascularization into the allograft was not indicated18.
In our patient, at the ten-year follow-up evaluation, the massive
allograft was partly revascularized where it was in contact with
host bone. The incorporation process had been very slow, as newly
formed bone was not observed beyond a depth of 5 mm into the graft.
The allograft’s density and relatively small surface area
may explain its slow and incomplete revascularization. The portions
of the allograft that were not in contact with host bone were mostly
resorbed.
The presence of microfractures in the unremodeled area of the allograft
cannot be attributed to artifacts as microfractures were not present
in remodeled areas or in host bone. It is also unlikely that they
were related to the sterilization process19,20.
We believe that they were of mechanical origin, as hypothesized
by Gouin et al.17. Irradiation
at a standard dose does not alter the elasticity of the material,
but it has been proven to diminish the bone’s capacity
to withstand load21,22. This results
in a loss of the plastic behavior of the graft (embrittlement),
with a reduction in the hoop strength of the structure, perhaps
explaining the formation of microfractures through a fatigue mechanism.
While this is a single case report, our findings suggest that, when
a structural femoral graft is utilized during revision hip arthroplasty,
the host femur should be retained in order to enhance allograft
incorporation.
Note: The authors are very grateful to Miguel E. Cabanela, MD,
and Daniel J. Berry, MD, of the Mayo Clinic, Rochester, Minnesota,
for their comments.