Deformation and Recovery of Thin Metal-Backed, Two-Piece Polyethylene Acetabular Resurfacing Components

Acetabular underreaming is a bone preservation and fixation strategy in hip resurfacing. Prior reports of acetabular component deformation have been cadaveric studies. In this study, I sought to determine deformation of thin shells and very thin highly cross-linked polyethylene in patients, the effects of increasing underreaming and liner-head clearance, and whether deformation accelerates wear. Overall, 32 two-piece metal-backed polyethylene resurfacing acetabular components placed with 3-4 mm of underreaming were retrieved postmortem or during revision after a mean of 9 years of patient use. Shells were 54-64 mm in external diameter. The components were 2-mm-thick porous-coated shells and 4-mm highly cross-linked polyethylene liners. Deformation was measured during insertion, 30 minutes after implantation, after reducing the hip and testing range of motion, and at retrieval. Standard liner-head clearance was 0.3 mm, and clearances up to 2.8 mm were also used. Acetabular shells deformed a mean of 0.58 mm on insertion and deformed 0.23 mm at retrieval. Initial liner deformation of 0.29 mm decreased to 0.15 mm on retrieval. With liner-head clearance of 2.8 mm, the mean shell and liner deformations at retrieval were 0.36 mm and 0.29 mm, respectively. The calculated insertional force was 367 Nm. No acetabular fractures, shell or liner failures, or adverse clinical consequences were observed. Mean linear wear was 0.005 mm/yr. Thin resurfacing acetabular shells and liners deform when placed with 3-4 mm of underreaming, which lessens with loading, bone relaxation, and clinical use. Bone preservation by underreaming is beneficial during hip resurfacing surgery.