To The Editor:
I read with interest the article "Relationship Between Polyethylene
Wear and Osteolysis in Hips with a Second-Generation Porous-Coated Cementless
Cup After Seven Years of Follow-up" (2003;85:10959), by Orishimo et al.
This study looked at the relationship of linear and volumetric wear to the
development of osteolysis in a series of fifty-six arthroplasties from an
original cohort of 102. A 28-mm head was used with a second-generation
porous-coated cementless acetabular component, with use of either a Hylamer or
an Enduron polyethylene liner. The mean duration of follow-up was 7.7 ±
0.8 years.
Despite the relatively short follow-up period, this was the fourth paper,
and third implant design, in which it was determined that an average annual
wear rate of 0.2 mm/year was a "critical threshold" for the
subsequent development of osteolysis1-3.
In the Discussion section, when considering the results of an earlier
study1, the authors incorrectly stated that "only 30% of the
total hip replacements in that study survived more than twenty years."
The earlier study assessed the results of 235 Charnley low-friction
arthroplasties performed on 163 patients with an average follow-up of 19.5
years (range, six to thirty years). In that study, it was determined that, for
every additional millimeter of wear, the risk of both acetabular and femoral
component revision or failure increased significantly (p < 0.019).
Survivorship analysis, with use of revision or radiographic failure of either
component as the end point, revealed that the survival rate at twenty years
was 24% (95% confidence interval, 6% to 42%) for implants with an average
annual wear rate of 0.2 to 0.24 mm/yr and only 7% (95% confidence interval, 0%
to 18%) for implants with an average wear rate of >0.25 mm/yr. However, the
twenty-year survival rate for both of the original components for the entire
series was 67% (95% confidence interval, 60% to 73%) and the twenty-five-year
survival rate was 54% (95% confidence interval, 45% to 63%). The complete
results of the survivorship analysis, with revision or failure as the end
point, were as follows:
We greatly appreciate Dr. Sochart's interest in our study. As suggested in
his 1999 study1, we
were able to confirm the existence of a critical threshold of linear wear rate
of 0.2 mm/year in our analysis for the development of osteolysis.
His study was based on the clinical results and survivorship analysis for
235 Charnley low-friction arthroplasties with a mean follow-up of 19.5 years,
and we do apologize that the sentence referencing his study in the Discussion
of our paper unintentionally left room for potential misinterpretation of his
study results.
The sentence that read, "Furthermore, only 30% of the total hip
replacements in that study survived more than twenty years," was
intended to be understood by the reader in the context of the previous
sentence that read, "In a study of cemented all-polyethylene cups that
articulated with cobalt-chromium femoral heads, Sochart found that no total
hip replacement with a linear wear rate of >0.2 mm/year survived
twenty-five years when revision and component loosening due to osteolysis were
used as end points," and not to the entire study population.
A clearer wording of our statement to avoid misinterpretation would be:
"Furthermore, <30% of the components with a linear wear rate of
>0.2 mm/year survived twenty years."
Sochart DH. Relationship of
acetabular wear to osteolysis and loosening in total hip arthroplasty.
Clin Orthop.1999;363:
135-50.363135
1999
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K. Factors affecting aseptic failure of fixation after primary Charnley
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1618-27.791618
1997
Dowd JE, Sychterz CJ, Young AM, Engh
CA. Characterization of long-term femoral-head-penetration rates.
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1102-7.821102
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