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Current Concepts Review:
Aamer Malik, Aditya Maheshwari, and Lawrence D. Dorr
Impingement with Total Hip Replacement
J Bone Joint Surg Am 2007; 89: 1832-1842 [Abstract] [Full text] [PDF]
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[Read Letter to the Editor] Dr. Dorr et al. respond to Mr. Nijhof.
Lawrence D. Dorr, M.D., Aamer Malik, M.D., Aditya Maheshwari, M.D.   (27 November 2007)
[Read Letter to the Editor] Impingement with Total Hip Replacement
Marc W Nijhof   (5 November 2007)

Dr. Dorr et al. respond to Mr. Nijhof. 27 November 2007
Previous Letter to the Editor Top
Lawrence D. Dorr, M.D.
The Arthritis Institute, Inglewood, CA,
Aamer Malik, M.D., Aditya Maheshwari, M.D.

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Re: Dr. Dorr et al. respond to Mr. Nijhof.

patriciajpaul{at}yahoo.com Lawrence D. Dorr, M.D., et al.

We thank Mr. Nijhof for his letter. We wrote this Current Concepts article with expectations that the reader would understand that the terms medialization and lateralization mean a change in the acetabular center of rotation. Perhaps Mr. Nijhof did not equate these terms to a change in center of rotation, because he did not discuss this concept in his letter.

Because the bony acetabulum in arthritic patients averages 55 degrees of inclination(1), the center of rotation must be moved superiorly or medially to obtain coverage of the cup with inclination of 45 degrees or less. Inclination should not exceed 45 degrees for limiting wear of the articulation(2). We never implied that changing the center of rotation medially or superiorly automatically changes inclination. The angle of inclination remains the surgeon’s choice, but changing the center of rotation affords the surgeon the opportunity to get it correct.

Mr. Nijhof suggests that it is hard to understand how medialization reduces component impingement. We would respond by noting that the metal neck of the stem cannot impinge on the edge of the cup when the edge is covered by bone. Furthermore, his comment about lower edge loading is not clear to us. We have conducted numerous wear studies, including one that analyzes the vector directions of wear, and except for measurement errors with computer generated wear measurements, inferior loading does not occur(3).

If the center of rotation is lateralized with an identical inclination to a medialized cup, the metal edge of the cup is at risk of impinging with the metal neck of the stem. Correct coverage eliminates that. The elimination of bony impingement is a function of offset and leg length.

The figures used in the article are illustrations to represent a principle. The principles illustrated with these figures are that correct inclination and coverage of the cup occurs with change of the center of rotation as needed to achieve this, and correct offset and leg length will avoid bony impingement.

Mr. Nijhof is correct about references 2 and 3 which were used to confirm the benefit of not lateralizing the center of rotation. As he states, these references confirm the benefit.

The avoidance of impingement is critical for the most comfortable and most durable total hip replacements. The concept of correctly changing the center of rotation to avoid impingement is one that surgeons must understand and that is why it was emphasized in this article.

References:

1. Dorr LD: Hip arthroplasty. Minimally invasive techniques and computer navigation, 1st edition. Philadelphia, PA, Saunders Elsevier, 2006.

2. Patil S, Bergula A, Chen PC, Cowell CW, Jr., D’Lima DD: Polyethylene wear and acetabular component orientation. J Bone Joint Surg Am. 2003;85-A Suppl 4:56-63.

3. Wan Z, Boutary M, Dorr LD: Precision and limitation of measuring two-dimensional wear on clinical radiographs. Clin Orthop Relat Res. 2006 Aug;449:267-74.
Impingement with Total Hip Replacement 5 November 2007
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Marc W Nijhof,
Fellow Orthopaedic Surgeon
Endo-Klinik, Hamburg, GERMANY

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Re: Impingement with Total Hip Replacement

m.w.nijhof{at}umcutrecht.nl Marc W Nijhof

To The Editor:

With interest we read the paper of Malik et al.(1). The statements on the effect of medialization/lateralization of the cup on component impingement are confusing. It is stated that medialization of the cup can avoid component impingement and that the surgeon increases the risk of impingement by placing the cup in a lateralized horizontal position. The authors also state that appropriate coverage of the cup (45 degrees or less of abduction) can be achieved by moving the cup medially and/or superiorly.

What is confusing to us is that the authors seem to imply that medialization/lateralization per se automatically results in a subsequent change in inclination of the cup.

It is also hard to understand why a medialized cup with reduced inclination (with subsequent lower edge loading) would give less component impingement than a lateralized cup with the same inclination.

The figures presented in this paper show that lateralization and uncoverage of the cup can result from an abducted cup, especially when the acetabulum is steep and shallow. However, in contrast to what the authors state, it is the effect of the reduced inclination, and not the mere lateralization or uncoverage that causes component-to-component impingement. Furthermore, one could also imagine that a surgeon, who places the cup too lateral, would be tempted to place the cup more vertically (to increase coverage), thereby reducing the change of metal neck-on-cup impingement.

Finally, two studies that are referenced (2,3) did not study the effect of (lateralized horizontal) cup placement on impingement. What was shown in these studies was that contact forces are lower when the cup is not lateralized with a possible beneficial effect on preventing prosthetic loosening.

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. Malik A, Maheshwari A, Dorr D. Current Concepts Review: Impingement with Total Hip Replacement. J Bone Joint Surg Am 2007; 89: 1832-1842

2. Johnston RC, Brand RA, Crowninshield RD. Reconstruction of the hip. A mathematical approach to determine optimum geometric relationships. J Bone Joint Surg Am. 1979;61:639-52.

3. Yoder SA, Brand RA, Pedersen DR, O’Gorman TW. Total hip acetabular component position affects component loosening rates. Clin Orthop Relat Res. 1988;228:79-87.