Paper #32 The Effect of Polyethylene Thickness on the Wear of Highly Cross-Linked Acetabular Liners in a “Near-Impingement” Hip Simulator Model Natalie Kelly BS, Amar Rajadhyaksha, MD, Suzanne Maher, PhD, Joseph Lipman, MS, Timothy M. Wright, PhD, Geoffrey H. Westrich, MD Introduction: Concern for stability and increased range of motion in THA has increased popularity of large femoral heads, while thin polyethylene liners have allowed acetabular component thickness to remain constant. Highly cross-linked polyethylene improves wear, but is accompanied by reduced toughness. Decreasing thickness may increase wear or cause gross fracture, especially at extremes of motion when contact between head and acetabulum nears the liner edge. We determined the effect of liner thickness on wear with large diameter heads in a near impingement model. Methods: A hip simulator was used with 54 mm OD liners in 3 groups (n=4 per group): (1) conventional polyethylene–36mm head (liner thickness=7.9mm), (2) highly cross-linked polyethylene–36 mm head (thickness=7.9mm), (3) highly cross-linked polyethylene–44 mm head (thickness=3.8 mm). A Paul-type load profile was used at 1 Hz for 5 million cycles. The mounting angle of the liners resulted in “near impingement” conditions between the liner and femoral neck at the maximum load for each cycle. Every 0.5 million cycles, liners were weighed and corrected with load-soaked controls. Liners were also examined for damage. Results: Weight losses were: Group (1) 123.71 + 26.33mg, (2) 6.05 + 10.53mg, and (3) 3.30 + 10.98mg; wear rates (mg/million cycles) were: Group (1) 25.22 + 7.33, (2) 1.77 + 7.19, (3) 1.84 + 10.60. Thus, conventional polyethylene showed >20 times the wear of both highly cross-linked groups. Although the test was “near impingement”, 2 liners in group 1 and 3 liners in group 2 showed evidence of impingement and experienced more wear. No cracking or gross failure was noted in any sample. Discussion: Highly cross-linked polyethylene showed negligible wear compared to conventional polyethylene, even under severe conditions. The wear difference between the cross-linked groups was minimal, suggesting that large heads may be used safely without increased wear or fracture. A stracts b e35

The Effect of Polyethylene Thickness on the Wear of Highly Cross-Linked Acetabular Liners in a “Near-Impingement” Hip Simulator Model

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Page 1: The Effect of Polyethylene Thickness on the Wear of Highly Cross-Linked Acetabular Liners in a “Near-Impingement” Hip Simulator Model

Paper #32The Effect of Polyethylene Thickness on the Wear of Highly Cross-Linked AcetabularLiners in a “Near-Impingement” Hip Simulator Model

Natalie Kelly BS, Amar Rajadhyaksha, MD, Suzanne Maher, PhD, Joseph Lipman, MS,Timothy M. Wright, PhD, Geoffrey H. Westrich, MD

Introduction: Concern for stability and increased range of motion in THA has increased popularity of largefemoral heads, while thin polyethylene liners have allowed acetabular component thickness to remain constant.Highly cross-linked polyethylene improves wear, but is accompanied by reduced toughness. Decreasingthickness may increase wear or cause gross fracture, especially at extremes of motion when contact betweenhead and acetabulum nears the liner edge. We determined the effect of liner thickness on wear with largediameter heads in a near impingement model.

Methods: A hip simulator was used with 54 mm OD liners in 3 groups (n=4 per group): (1) conventionalpolyethylene–36mm head (liner thickness=7.9mm), (2) highly cross-linked polyethylene–36 mm head(thickness=7.9mm), (3) highly cross-linked polyethylene–44 mm head (thickness=3.8 mm). A Paul-type loadprofile was used at 1 Hz for 5 million cycles. The mounting angle of the liners resulted in “near impingement”conditions between the liner and femoral neck at the maximum load for each cycle. Every 0.5 million cycles,liners were weighed and corrected with load-soaked controls. Liners were also examined for damage.

Results: Weight losses were: Group (1) 123.71 + 26.33mg, (2) 6.05 + 10.53mg, and (3) 3.30 + 10.98mg; wearrates (mg/million cycles) were: Group (1) 25.22 + 7.33, (2) 1.77 + 7.19, (3) 1.84 + 10.60. Thus, conventionalpolyethylene showed >20 times the wear of both highly cross-linked groups. Although the test was “nearimpingement”, 2 liners in group 1 and 3 liners in group 2 showed evidence of impingement and experiencedmore wear. No cracking or gross failure was noted in any sample.

Discussion: Highly cross-linked polyethylene showed negligible wear compared to conventional polyethylene,even under severe conditions. The wear difference between the cross-linked groups was minimal, suggestingthat large heads may be used safely without increased wear or fracture.

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