To The Editor:
I read with interest "Characterization of Long-Term
Femoral-Head-Penetration Rates. Association with and Prediction of
Osteolysis" (82-A: 1102-1107, Aug. 2000), by Dowd et al.
In this study of temporal femoral-head-penetration patterns, the
authors examined forty-eight hips with a minimum follow-up of ten years.
The implants had a large-diameter femoral head (32 mm) and a thin polyethylene
liner in a porous-coated acetabular component, a combination that
is likely to lead to high rates of wear. The authors demonstrated
that true wear rates tended to be constant and that increased wear
was significantly (p < 0.001) associated with the development
of osteolysis at ten years. They concluded that the measurement of
early true wear rates might enable orthopaedic surgeons to predict
which patients are at risk for the development of osteolysis and
to stratify patients for follow-up accordingly.
These findings and conclusions are similar to those of a previously
published study with an average follow-up of 19.5 years1; it is of particular interest that
this earlier study also demonstrated that the critical wear rate
for development of osteolysis was 0.2 mm/yr. Furthermore, this
and another study with similar findings in regard to the critical
wear rate reported the long-term results of a cemented all-polyethylene
acetabular component with a small-diameter femoral head (22.25 mm)1,2.
The fact that an identical critical wear rate was found in association
with two very different implant designs lends further support to
the authors’ suggestion that there may be a critical number of
microscopic wear particles produced, above which the periprosthetic tissues
are overwhelmed, leading to the eventual development of osteolysis.
J.E. Dowd, C.J. Sychterz, A.M. Young, and C.A. Engh reply:
We appreciate Dr. Sochart’s interest in our article,
and we thank him for pointing out relevant articles that provide useful
information regarding the development of osteolysis around cemented total
hip arthroplasty components. Like Dr. Sochart, we also find it interesting that
there appears to be a "critical" wear rate of
0.2 mm/yr, above which patients are at greater risk for
wear-related complications, regardless of the type of implant. In
our study of porous-coated cementless components, the predominant
wear-related complication was osteolysis. More than 80% of
patients with rates greater than 0.2 mm/yr developed large
lytic lesions. In Dr. Sochart’s study of cemented Charnley
components1, wear rates above
this critical rate were associated with a substantially greater risk
of loosening and revision.
Taken together, the findings of our study and those cited by
Dr. Sochart emphasize the clinical impact that polyethylene particle
debris has on the outcome of both cemented and cementless total
hip arthroplasty. They also underscore the importance of continued research
regarding methods of decreasing particle burdens in situ.