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Dr. Harris' response to Dr. Cameron
Point-Counterpoint: Modularity in PrimaryTHA 337 It is, on the other hand, c o m m o n experience that the capsule surrounding a metal-polyethylene bearing is thick and stiff and that there is a p r o m i n e n t synovitis resembling that found in rheumatoid arthritis. Particles do play a role, therefore, but the role of polyethylene particles, as opposed to metal particles, is the production of a synovitis that leads to capsular scarring. The synovitis overproduces the fluid and the tight scarred capsule gives rise to high pressure. This means that polyethylene particles are important, but not in the way that has been thought previously. The treatment of osteolysis therefore has to be rethought. Perhaps a capsulectomy and synovectomy should be carried out w h e n osteolysis is first seen, and perhaps a bleeding valve should be sewn into the residual capsule. Polyethylene wear should, of course, be minimized. The thick necks of the original European 14/16 trunions have n o w been waisted to prevent impingement, and the t r u n i o n size in common use in the United States is 10/12, which is only liable to impinge if significant component malposition occurs at the time of surgery. Should metallic particles from t r u n i o n fretting enter the bearing, then theoretically third-body wear will occur. The evidence that this is important in the hip is scanty, as metallic particles are forced into the soft plastic. With 15 years of experience with modular heads, I have failed to notice any r u n a w a y wear in the plastic acetabular component, a Third-body wear may be a much greater problem in metal-metal bearings if the congruence is kept too tight to allow particle escape. With respect to the issue of t r u n i o n damage, having revised a large n u m b e r of hips, I have yet to see a t r u n i o n that had sustained sufficient damage that it was believed unsafe to replace a n e w head on an old trunion. In most ithe t r u n i o n appears normal or with very minimal damage to the naked eye. It is, of course, a personal choice, but it seems unreasonable that most surgeons should be asked to give up the proven benefits of modularity for the theoretical drawbacks. References
1. Schmalzried TP: Access disease: the pathophysiology of osteolysis. Adult Reconstructive Surgery Conference, Newport Beach, CA, October 1995 2. Cameron HU, Park YS: The long-term results of the cemented Minelmeier stem. J Orthop Rheumatol 8:20, 1995
Dr. Harris' Response to Dr. Cameron William H. Harris, MD Doctor Cameron's discussion, as I view it, appears to be focused specifically on the femoral side of primary total hip arthroplasty (THA) and primarily on cementless femoral components. The advantages described are
(I) the ability to combine different materials for the head and the stem, including ceramics, (2) the ability to allow equalization of leg lengths after the final stem has been inserted, and (3) the ability to reduce inventory and costs. First, the vast majority of femoral heads used worldwide are of chrome-cobalt. As chrome-cobalt is the preferred material for cemented femoral stems, there is no need to have different material for the head and the stem in a cemented femoral component. Because cemented femoral components, by far, are the most c o m m o n ones used throughout the world, limiting the discussion to only cementless devices overlooks a large part of the field. Second, the ability to allow equalization of leg lengths after the final stem has been inserted is an example of a narrowly restricted issue. In most cases, this problem relates only to cementless femoral components. With a cemented femoral component, you can determine the o p t i m u m leg length with the provisional components and then simply insert the cemented stem. Admittedly, this is not true for cementless femoral components and, consequently, is one of the negatives of using a cementless femoral component. Third, if one wishes to reduce inventory and costs, it would be appropriate to discontinue the use of ceramic femoral heads. There is, in the view of many, no compelling evidence that they provide a better long-term result, and they are substantially more expensive. In addition, a small percentage of them fracture. An additional advantage to femoral modularity is suggested in cases requiring isolated revision of the acetabular component. As I mentioned in my opening discussion, the femoral component is revised in 85% of all revisions in most large series. In fact, some series show 99% of the femoral components being revised. Thus, of all THAs, 85% are primaries. Of the remaining 15% (ie, the revisions), another 85% involve removal of the femoral component. Thus, the total n u m b e r of cases in which an isolated acetabular revision would be done constitutes about 2.5 % of all THAs. This is an u n c o m m o n operation. If instability is the problem requiring revision and a longer construct is needed, this can be achieved by using an offset polyethylene liner every bit as well as by lengthening the neck, and at a lower cost. Almost all cases requiring a high hip center concomitantly require femoral component revision. Thus, a calcar replacement can be used to provide the additional length. Stem extensions, mentioned by Dr. Cameron, are adverse biomechanically, because of the stress concentrations at the Morse taper associated with such stem extensions. Doctor Cameron acknowledges that fretting at the Morse taper is inevitable. He also agrees that the modular system requiring a skirted component should be avoided. A monoblock system automatically avoids these two problems as well as the possibility of mismatched components and fractures of the femoral neck secondary to corrosion. A n u m b e r of such fractures have already been published and obviously more have occurred that have not been published.
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The Journal of Arthroplasty Vol. 11 No. 3 April 1996
I agree with Dr. Cameron w h e n he acknowledges that cemented stems are not required to accurately reproduce the geometry of the femoral canal. This is one of their major advantages. Doctor Cameron points out that another potential advantage to proximal modularity is its ability to version change. Cement, of course, provides the same capability. He suggests that use of a modular noncemented stem is indicated in cases in "which the use of cement is undesirable." The 15-year record for primary THAs in older patients, the 15-year record for primary THAs in younger patients, and the 15-year record for femoral revisions using contemporary cementing techniques support the use of cement for each of these categories. Those three categories constitute nearly all THAs. Doctor Cameron also acknowledges that the micromotion at modular junctions will result in the liberation of metallic debris. This is already a problem, including, specifically, chromium 3-orthophosphate. And, importantly, experience with these junctions is still very short, ranging at most from 5 to 10 years. We must anticipate that the corrosion and fretting problems will only increase with time, in both severity and Irequency. All of
us are aiming for 20-year or longer longevity for the vast majority of THAs. If we are already encountering corrosion and fretting difficulties at 5 to i 0 years, it surely will be worse with increasing time. Doctor Cameron's clinical results with his modular stem confirm some of the disadvantages of cementless femoral components, including a 7.4% incidence of femoral osteolysis, with a m e a n follow-up period of perhaps 5 years, and one fracture through an area of osteolysis. All studies of osteolysis have shown progressive increase, in both extent and incidence, with the passage of time. In addition, he reports one case of osteolysis below the stem/sleeve junction, This occurred in the patient who fractured through that area of osteolysis. The modularity did not prevent osteolysis from occurring distally. In short, the arguments in favor of a monoblock stem remain intact and substantially compelling. The arguments in favor of a modular acetabular component are also compelling, specifically for revision surgery and for the younger age group. Further followup evaluation is required to determine which is preferable for primary THA in the older age group.
1. Schmalzried TP: Access disease: the pathophysiology of osteolysis. Adult Reconstructive Surgery Conference, Newport Beach, CA, October 1995 2. Cameron HU, Park YS: The long-term results of the cemented Minelmeier stem. J Orthop Rheumatol 8:20, 1995
Dr. Harris' Response to Dr. Cameron William H. Harris, MD Doctor Cameron's discussion, as I view it, appears to be focused specifically on the femoral side of primary total hip arthroplasty (THA) and primarily on cementless femoral components. The advantages described are
(I) the ability to combine different materials for the head and the stem, including ceramics, (2) the ability to allow equalization of leg lengths after the final stem has been inserted, and (3) the ability to reduce inventory and costs. First, the vast majority of femoral heads used worldwide are of chrome-cobalt. As chrome-cobalt is the preferred material for cemented femoral stems, there is no need to have different material for the head and the stem in a cemented femoral component. Because cemented femoral components, by far, are the most c o m m o n ones used throughout the world, limiting the discussion to only cementless devices overlooks a large part of the field. Second, the ability to allow equalization of leg lengths after the final stem has been inserted is an example of a narrowly restricted issue. In most cases, this problem relates only to cementless femoral components. With a cemented femoral component, you can determine the o p t i m u m leg length with the provisional components and then simply insert the cemented stem. Admittedly, this is not true for cementless femoral components and, consequently, is one of the negatives of using a cementless femoral component. Third, if one wishes to reduce inventory and costs, it would be appropriate to discontinue the use of ceramic femoral heads. There is, in the view of many, no compelling evidence that they provide a better long-term result, and they are substantially more expensive. In addition, a small percentage of them fracture. An additional advantage to femoral modularity is suggested in cases requiring isolated revision of the acetabular component. As I mentioned in my opening discussion, the femoral component is revised in 85% of all revisions in most large series. In fact, some series show 99% of the femoral components being revised. Thus, of all THAs, 85% are primaries. Of the remaining 15% (ie, the revisions), another 85% involve removal of the femoral component. Thus, the total n u m b e r of cases in which an isolated acetabular revision would be done constitutes about 2.5 % of all THAs. This is an u n c o m m o n operation. If instability is the problem requiring revision and a longer construct is needed, this can be achieved by using an offset polyethylene liner every bit as well as by lengthening the neck, and at a lower cost. Almost all cases requiring a high hip center concomitantly require femoral component revision. Thus, a calcar replacement can be used to provide the additional length. Stem extensions, mentioned by Dr. Cameron, are adverse biomechanically, because of the stress concentrations at the Morse taper associated with such stem extensions. Doctor Cameron acknowledges that fretting at the Morse taper is inevitable. He also agrees that the modular system requiring a skirted component should be avoided. A monoblock system automatically avoids these two problems as well as the possibility of mismatched components and fractures of the femoral neck secondary to corrosion. A n u m b e r of such fractures have already been published and obviously more have occurred that have not been published.
338
The Journal of Arthroplasty Vol. 11 No. 3 April 1996
I agree with Dr. Cameron w h e n he acknowledges that cemented stems are not required to accurately reproduce the geometry of the femoral canal. This is one of their major advantages. Doctor Cameron points out that another potential advantage to proximal modularity is its ability to version change. Cement, of course, provides the same capability. He suggests that use of a modular noncemented stem is indicated in cases in "which the use of cement is undesirable." The 15-year record for primary THAs in older patients, the 15-year record for primary THAs in younger patients, and the 15-year record for femoral revisions using contemporary cementing techniques support the use of cement for each of these categories. Those three categories constitute nearly all THAs. Doctor Cameron also acknowledges that the micromotion at modular junctions will result in the liberation of metallic debris. This is already a problem, including, specifically, chromium 3-orthophosphate. And, importantly, experience with these junctions is still very short, ranging at most from 5 to 10 years. We must anticipate that the corrosion and fretting problems will only increase with time, in both severity and Irequency. All of
us are aiming for 20-year or longer longevity for the vast majority of THAs. If we are already encountering corrosion and fretting difficulties at 5 to i 0 years, it surely will be worse with increasing time. Doctor Cameron's clinical results with his modular stem confirm some of the disadvantages of cementless femoral components, including a 7.4% incidence of femoral osteolysis, with a m e a n follow-up period of perhaps 5 years, and one fracture through an area of osteolysis. All studies of osteolysis have shown progressive increase, in both extent and incidence, with the passage of time. In addition, he reports one case of osteolysis below the stem/sleeve junction, This occurred in the patient who fractured through that area of osteolysis. The modularity did not prevent osteolysis from occurring distally. In short, the arguments in favor of a monoblock stem remain intact and substantially compelling. The arguments in favor of a modular acetabular component are also compelling, specifically for revision surgery and for the younger age group. Further followup evaluation is required to determine which is preferable for primary THA in the older age group.