P Adam1, F Farizon2, M-H Fessy2. 1. Centre d'orthopédie et traumatologie, hôpital Bellevue, CHU de Saint-Étienne, 42055 Saint-Étienne cedex 2, France. Electronic address: rcot@sofcot.fr. 2. Centre d'orthopédie et traumatologie, hôpital Bellevue, CHU de Saint-Étienne, 42055 Saint-Étienne cedex 2, France.
Abstract
PURPOSE OF THE STUDY: Dual mobility systems with retentive acetabular liners have been used in clinical practice for total hip arthroplasty since 1976. The dual mobility system preserves a wide range of motion while providing greater stability. This study measured wear on the concave and convex surfaces of 40 retrieved polyethylene liners, to evaluate the advantages of this system in relation to wear. MATERIAL AND METHODS: Forty polyethylene inserts that had been removed due to infection or mechanical failure after a mean 8 years were analyzed. The mean age of patients at arthroplasty was 46 years old. Macroscopic analysis was followed by surface analysis with direct measurement of changes in the curvature radii. The internal concave surface was measured in three dimensions using a 4-mm stylus (BNH 706). External convexity was measured by lateral projection. The estimated error was ± 5 μm for both measurement methods. Manufacturers' tolerance for these implants was approximately 50 μm. Linear wear and wear volume was determined by comparing the measured dimensions with the theoretical dimensions of new liners. RESULTS: Macroscopically, all of the pieces studied had lost the initial machined grooves on the convex surface; 40% of the pieces showed visible wear of the retentive collar. Mean annual convex surface wear was 9 μm (SD 9μm) and 73 μm (SD 69 μm) for the concave surface. Mean total wear, which was the sum of the wear on the convex and concave surfaces was 82 μm (SD 72 μm). Wear volume was 28.9 mm(3)/yr for the convex surface (SD 27.6) and 25.5 for the concave surface (SD 23.2) with a mean annual total wear volume of 54.3 mm(3)/yr (SD 39.6). DISCUSSION: Total wear in the 40 dual mobility liners that had functioned in vivo was similar to that reported in metal-polyethylene bearings with 22.2mm femoral heads. The results of wear in both the convex and concave surfaces show that wear with the dual mobility system was not increased compared to conventional metal-polyethylene bearings, while providing better retention and greater stability. CONCLUSION: The use of dual mobility acetabular liners is an attractive solution when a metal-polyethylene bearing is needed. The increased joint stability is not associated with increased wear.
PURPOSE OF THE STUDY: Dual mobility systems with retentive acetabular liners have been used in clinical practice for total hip arthroplasty since 1976. The dual mobility system preserves a wide range of motion while providing greater stability. This study measured wear on the concave and convex surfaces of 40 retrieved polyethylene liners, to evaluate the advantages of this system in relation to wear. MATERIAL AND METHODS: Forty polyethylene inserts that had been removed due to infection or mechanical failure after a mean 8 years were analyzed. The mean age of patients at arthroplasty was 46 years old. Macroscopic analysis was followed by surface analysis with direct measurement of changes in the curvature radii. The internal concave surface was measured in three dimensions using a 4-mm stylus (BNH 706). External convexity was measured by lateral projection. The estimated error was ± 5 μm for both measurement methods. Manufacturers' tolerance for these implants was approximately 50 μm. Linear wear and wear volume was determined by comparing the measured dimensions with the theoretical dimensions of new liners. RESULTS: Macroscopically, all of the pieces studied had lost the initial machined grooves on the convex surface; 40% of the pieces showed visible wear of the retentive collar. Mean annual convex surface wear was 9 μm (SD 9μm) and 73 μm (SD 69 μm) for the concave surface. Mean total wear, which was the sum of the wear on the convex and concave surfaces was 82 μm (SD 72 μm). Wear volume was 28.9 mm(3)/yr for the convex surface (SD 27.6) and 25.5 for the concave surface (SD 23.2) with a mean annual total wear volume of 54.3 mm(3)/yr (SD 39.6). DISCUSSION: Total wear in the 40 dual mobility liners that had functioned in vivo was similar to that reported in metal-polyethylene bearings with 22.2mm femoral heads. The results of wear in both the convex and concave surfaces show that wear with the dual mobility system was not increased compared to conventional metal-polyethylene bearings, while providing better retention and greater stability. CONCLUSION: The use of dual mobility acetabular liners is an attractive solution when a metal-polyethylene bearing is needed. The increased joint stability is not associated with increased wear.
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