Wear and deformation of ceramic-on-polyethylene total hip replacements with joint laxity and swing phase microseparation

Abstract

Wear of polyethylene and the resulting wear debris-induced osteolysis remains a major cause of long-term failure in artificial hip joints. There is interest in understanding engineering and clinical conditions that influence wear rates. Fluoroscopic studies have shown separation of the head and the cup during the swing phase of walking due to joint laxity. In ceramic-on-ceramic hips, joint laxity and microseparation, which leads to contact of the head on the superior rim of the cup, has led to localized damage and increased wear in vivo and in vitro. The aim of this study was to investigate the influence of joint laxity and microseparation on the wear of ceramic on polyethylene artificial hip joints in an in vitro simulator. Microseparation during the swing phase of the walking cycle produced contact of the ceramic head on the rim of the polyethylene acetabular cup that deformed the softer polyethylene cup. No damage to the alumina ceramic femoral head was found. Under standard simulator conditions the volume change of the moderately crosslinked polyethylene cups was 25.6 +/- 5.3 mm3/million cycles and this reduced to 5.6 +/- 4.2 mm3/million cycles under microseparation conditions. Testing under microseparation conditions caused the rim of the polyethylene cup to deform locally, possibly due to creep, and the volume change of the polyethylene cup when the head relocated was substantially reduced, possibly due to improved lubrication. Joint laxity may be caused by poor soft tissue tension or migration and subsidence of components. In ceramic-on-polyethylene acetabular cups wear was decreased with a small degree of joint laxity, while in contrast in hard-on-hard alumina bearings, microseparation accelerated wear. These findings may have significant implications for the choice of fixation systems to be used for different types of bearing couples.

Edge Loading Has a Paradoxical Effect on Wear in Metal-on-Polyethylene Total Hip Arthroplasties

“…From experimental studies of microseparation in the hip simulator, Fisher's group [26,27,31] showed the microseparation form of edge loading used in their experimental model accelerated wear of the overall articulation with both MOM and COC articulations. Importantly, the results of similar studies using MOP articulations were exactly the opposite.…”

“…These studies showed the wear at the articulation of the MOP joints was actually decreased compared to similar studies without the addition of microseparation. Furthermore, studies from that same group also compared microseparation versus no microseparation in the hip simulator using a ceramic femoral head against conventional polyethylene material [31]. In this comparison, they found reduced wear of the polyethylene articulation under conditions of microseparation compared to identical studies without the microseparation.…”

“…In contrast, for MOP THA under the same experimental conditions, deformation occurred at the edge of the polyethylene acetabular component (plastic deformation and edge wear), but the overall wear of the articulation was not accelerated. Rather, it was decreased [31].…”

“…The relationship between microseparation, edge loading, and wear across the joint was evaluated experimentally by Fisher's group [26,27,31] using a model system that simulated microseparation in a modified hip simulator. Their model showed microseparation produced both edge wear and accelerated general wear in COC articulations, as well as in MOM joints using the same type of model [14].…”

Effect of microseparation on contact mechanics in metal‐on‐metal hip replacements—A finite element analysis

“…Additionally, translational malpositioning can produce a more severe contact condition and edge loading 15. A typical condition resulting from translational malpositioning is dynamic microseparation16, 17 of hip joints or subluxation, for which a translation of the head relative to the cup occurs leading to contact at the superior lateral rim of the cup 18, 19. There are many contributing factors for microseparation including head offset deficiency, medialized cup, stem subsidence, impingement and laxity of the soft tissues 11.…”