TO THE EDITOR:
In the article "Porous-Coated Acetabular Components with Screw Fixation. Five to Ten-Year Results" (78-A: 975—981, July 1996), Latimer and Lachiewicz emphasized that "the outer diameter of the acetabular component that was implanted was the same as the diameter of the final reamer that was used." In other words, they used line-to-line reaming. The authors, who criticized the technique of press-fitting into an underreamed acetabulum, believe that their technique does not create gaps at the polar dome and that the addition of multiple screws improved stability (although no data were given). They did state that "the last reamer was used only briefly at the acetabular rim."
A brief review of geometry shows that when the successive sizes of the reamers progressively increase by two millimeters, bone contact and reaming is accomplished only when the hemispherical reamer is within two millimeters of contact at the depth of the acetabulum. Any reaming before this boundary is due to wobbling caused by the inability of the surgeon to hold the reamer perfectly centered in the acetabulum, which is probably a common occurrence. When the final reamer is held briefly at the rim, a small amount of bone may be irregularly removed because of wobbling of the surgeon's hands. I believe that, unless the surgeon is very unsteady with this final reaming, he or she is in essence inserting the acetabular component into an acetabulum that was underreamed by slightly less than two millimeters. This probably approaches the optimum combination of fit and stability (one-millimeter press-fit) recommended by Kwong et al.1. Those authors did not find that the addition of screws under these conditions added stability.
Malcolm L. Ecker, M.D.: 8815 Germantown Avenue, Suite 12, Philadelphia, Pennsylvania 19118
Dr. Latimer and Dr. Lachiewicz reply:
One of the purposes of our paper was to contrast the results of this particular porous-coated acetabular component inserted with our method of preparation of the acetabulum and fixation with the results reported by others. In a study of eighty-three Harris-Galante porous-coated acetabular components3, there was little or no press-fit stability of the component with so-called line-to-line reaming and screws were necessary for stability. There was a high prevalence of peripheral gaps and progressive radiolucent lines. However, no component had been removed because of loosening at a minimum of five years postoperatively. Our technique of reaming briefly with the final reamer in a single direction was specifically devised so that some press-fit stability would be obtained with implantation of the same-sized component. However, there was no reliable intraoperative method to quantify the amount of press-fit stability obtained with this component (or any other). The press-fit stability of porous-coated acetabular components is dependent on numerous factors, including operative expertise (so-called wobbling), the manufacturers' tolerances for reamers and components, and the quality of the host bone. These factors are all difficult to quantify both preoperatively and intraoperatively.
The study by Kwong et al.1 was an in vitro study that has not been confirmed by an appropriate clinical study. Another in vitro study showed large gaps and decreased areas of bone contact with press-fit of oversized components2. With our technique of line-to-line reaming and screw fixation, a radiolucent line was seen in zone 2 in only three hips and there were no circumferential radiolucent lines. No component loosened. We remain concerned that press-fitting oversized components, which apparently is now very popular, will lead to acetabular fractures, gaps that will collect polyethylene debris, and a higher incidence of loosening. Although a one or two-millimeter press-fit without screws may be as successful as our technique for fixation of porous-coated components, this has not been shown in a clinical study of a large group of components with at least five years of follow-up.
Harrison A. Latimer, M.D.: Kinston Orthopaedic and Sports Medicine Center, Suite G, Kinston Clinic North, Kinston, North Carolina 28501
Paul F. Lachiewicz, M.D.: Department of Orthopaedics, University of North Carolina at Chapel Hill, 242 Burnett-Womack Building, Campus Box 7055, Chapel Hill, North Carolina 27599-7055