EDITOR'S NOTE: The authors were invited to respond to the letter, but to date, have not done so.
To The Editor:
The article “Dislocation Rate after Conversion from Hip
Hemiarthroplasty to Total Hip Arthroplasty”(1) has methods and conclusions which are not scientifically
complete.
When reporting a study on dislocation, there are certain data (and certain
radiographic techniques) currently required from the investigators which were not
included in this paper. Impingement is accepted as the most common cause of
dislocation and it is incumbent on investigators to eliminate it as a
cause.
1. The radiographic evaluation must include computed tomography scans
for component position of both the acetabular and femoral components, at
least of the hips that have dislocated. In this study(1), which was
retrospective, there are some recurrent dislocators who were not
revised and who could have been studied. Radiographic methods from 1982 are
outdated and the precision is not acceptable when a better method is
available(2).
2. The authors admit in the manuscript that the acetabular component
position alone has not been found to correlate to dislocation. Pierchon,
et al.(3) reported this with computer tomography scans which is not cited
by these authors. Komeno, et al. correlated combined anteversion of the cup
and stem to dislocation(4). Since the cup itself does not relate to
dislocation, and the authors knew that, why didn’t they study the femoral
component? Did they conduct the study having already implicated the soft
tissues?
3. There are no data on hip length and offset. This is basic data for a
study of dislocation. Errors in either can cause component-to-component
or bone-to-bone impingement. This is the most commonly associated
radiographic finding to “soft tissue imbalance”.
4. The authors do not describe or footnote the method of soft tissue
repair they used, or list in how many hips it was not possible.
The conclusion that failure of soft tissue capsular healing was the
cause of dislocation has no data to support it. To validate this conclusion the authors
should have done magnetic resonance image examinations to document failure
of healing. In fact, there are data to dispute this conclusion:
1. The conversion group had a larger mean femoral head size (30.6 vs.
29.5 mm), yet a smaller mean cup size (55.7 vs. 58.1 mm) which means a
smaller dead space, yet they had a higher dislocation rate.
2. The head size/cup size ratio is meaningless as averages of the group.
The authors needed to do a patient-to-patient comparison of head size, cup
size, combined anteversion, offset and hip length with a multivariate
regression analysis to make a judgment on head size. The authors admit a
“large head did not insure hip stability.”
3. There is no data on muscle strength which is highly correlated to
recurrent dislocations(2). Dynamic soft tissue balance (related to
resting muscle length and number of previous operations) may be more a
determinant of dislocation from soft tissue stability than static capsular
healing.
The only conclusion which can be drawn from this study is that the
authors had a high dislocation rate after conversion arthroplasty for
reasons unexplained.
The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated.
References:
1. Sah AP, Estok II DM. Dislocation rate after conversion from hip hemiarthroplasty to total hip arthroplsty. J Bone Joint Surg Am. 2008;90:506-516.
2. Dorr LD, Wan Z. Causes of and treatment protocol for instability
of total hip replacement. Clin Orthop Relat Res. 1998;Oct(355):144-151.
3. Pierchon, F, Pasquier G, Cotten A, Fontaine C,
Clarisse J, and Duquennoy A. Causes of dislocation of total hip
arthroplasty. CT study of component alignment. J Bone Joint Surg Br. 1994;76(1):45-8.
4. Komeno M, Hasegawa M, Sudo A, and Uchida A.
Computed tomographic evaluation of component position on dislocation after
total hip arthroplasty. Orthopedics. 2006;29(12):1104-8.
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