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Scientific Articles   |    
Dislocation Rate After Conversion from Hip Hemiarthroplasty to Total Hip Arthroplasty
Alexander P. Sah, MD1; Daniel M. EstokII, MD2
1 Department of Orthopaedics, Massachusetts General Hospital, 55 Fruit Street, WHT 535, Boston, MA 02114-3117. E-mail address: asah07@gmail.com
2 Department of Orthopedic Surgery, Brigham and Women's Hospital, 75 Francis Street, Boston, MA 02115
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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families 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 authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at Brigham and Women's Hospital, Boston, Massachusetts

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2008 Mar 01;90(3):506-516. doi: 10.2106/JBJS.G.00479
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Abstract

Background: Revision hip arthroplasty is associated with a dislocation rate that is three to five times greater than the rate following primary hip replacement. Conversion of a hip hemiarthroplasty to a total hip replacement is a revision arthroplasty, but it differs from revisions of total hip arthroplasties because a native acetabulum is replaced and the subsequent prosthetic femoral head is smaller. It was our purpose to determine whether the risk of dislocation following conversion surgery is the same as or greater than that following revision total hip replacement.

Methods: From 1994 to 2005, eighty-nine hemiarthroplasties were converted to a total hip arthroplasty in seventy-seven patients, and the results were compared with those of 115 first-time revision total hip replacements following a primary total hip replacement in 111 patients. A retrospective chart review was performed, and radiographic measurements were obtained. The patient demographics were similar between the two groups. The percentages of patients who had undergone revision of only the acetabular component as compared with both components as well as the percentages of those who had received a modular femoral stem as compared with a nonmodular stem were also similar between the two groups.

Results: Postoperatively, the femoral head size and the positioning of the acetabular component were similar between the two groups. The acetabular components were significantly larger (p < 0.001) in the group in which a total hip arthroplasty had been revised because they required additional acetabular reaming for placement of a new component. There were significantly more dislocations after the conversion procedures (22%) than after the revisions of the total hip arthroplasties (10%) (p < 0.018). Within both groups, the size of the acetabular component, the intraoperative range of motion, and the positioning of the acetabular component were similar between the hips that dislocated and those that did not. However, smaller femoral head components were at greater risk for dislocation after conversion surgery than after revision of a total hip arthroplasty.

Conclusions: A substantial reduction of the size of the prosthetic femoral head is unique to conversion arthroplasty and appears to play a role in instability after the revision surgery. While the smallest heads dislocated in the conversion group, a larger femoral head did not ensure stability. The increased dislocation risk with conversion surgery requires emphasis on soft-tissue balance and avoidance of excessive downsizing of the femoral head in an attempt to maximize hip stability.

Level of Evidence: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.

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    References

    Accreditation Statement
    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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    Lawrence D Dorr, M.D.
    Posted on April 04, 2008
    Lack of Data to Support Conclusions
    The Arthritis Institute, 637 S. Lucas Avenue, Los Angeles, CA 90017

    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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