Effect of Reduced Diameter Neck Stem on Incidence of Radiographic Cup Loosening and Revisions in Charnley Low-Frictional Torque Arthroplasty

Previous studies of the Charnley low-frictional torque arthroplasty have established an exponential correlation between the depth of cup penetration and the incidence of ultrahigh molecular weight polyethylene cemented cup migration. Impingement of the neck of the stem on the rim of the cup was considered to be the cause. We compared the incidence of radiographic loosening and revision of the cup in 2 groups of patients: those with 12.5-mm-diameter neck stem (972 hips) and those with 10-mm-diameter neck stem (261 hips) over a 20-year period, at comparable depths of cup penetration. The benefit of the 10-mm-diameter neck could be expressed as delaying radiographic loosening and revision of the cup by approximately 2 mm of cup penetration because loosening occurred at 2 mm penetration with a 12.5-mm-diameter neck but not until 4-mm penetration with a 10-mm-diameter neck. When a particular depth of cup penetration is reached will clearly depend on factors affecting wear.