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Saturday, March 18, 2000

Impingement can reduce longevity of THA

Impingement of the femoral neck on the acetabular liner can reduce the longevity of total hip arthroplasty (THA) by inducing subluxation/dislocation, increasing the generation polyethylene debris, and transmitting impingement forces to the fixation interface, said researchers in a study presented in poster exhibit 44.

Modular skirted femoral heads help to maintain offset and leg length, but also increase head:neck ratio, thereby, increasing the risk of impingement damage, rthe researchers said. The purpose of this study was to examine the significance of skirted femoral heads in reducing arc of motion (AOM) and increasing impingement damage in retrieved THA components.

Fifteen retrieved, skirted femoral heads were matched to 15 retrieved unskirted controls based on head size and hood geometry. Components were matched for manufacturer and model where possible. Analysis included mechanical AOM and impingement score. AOM was assessed from digital images, based on motion through a plane perpendicular to the acetabular opening, running through the center of the cup. Impingement was scored based on severity, and percent of circumferential polyethylene rim damage.

The study found that head size, the presence of a skirted head or liner hood appear to independently influence AOM and impingement damage in THA components. Impingement score was inversely related to AOM in both the skirted and control groups (r = -0.722; r = -0.62). Skirted components with a 26 mm femoral head and lipped liner exhibited the least AOM and highest impingement score. Conversely, unskirted, unhooded components with 32 mm heads displayed the highest AOM and lowest incidence of impingement damage. AOM was significantly lower (p<0.01) in 32 mm skirted components when compared to unskirted controls. Impingement score was significantly higher in both 26 mm (p<0.05) and 32 mm (p<0.05) skirted components when compared to the unskirted controls.

The researchers said the study shows that modular implants with skirted femoral heads exhibited decreased mechanical AOM and increased damage around the acetabular rim. The addition of a hooded polyethylene liner further decreases AOM and increases impingement damage. The use of modular skirted heads and hooded polyethylene liners are important options in optimizing implant mechanics in THA. However, careful consideration must be given to the cumulative reduction in mechanical AOM and resultant potential for impingement damage.

Co-authors of the study, all of London Health Sciences Centre, London, Ontario, are Shannon K.T. Puloski, MD, BSc; Kevin Skarratt, BSc; Richard W. McCalden, MD; Robert B. Bourne, MD, professor; and Cecil H. Rorabeck, MD, professor, chair, division of orthopaedic surgery.

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