title: The influence of head diameter and wall thickness on deformations of metallic acetabular press-fit cups and UHMWPE liners: a finite element analysis.
abstract: BACKGROUND
  To increase the range of motion of total hip endoprostheses, prosthetic heads need to be enlarged, which implies that the cup and/or liner thickness must decrease. This may have negative effects on the wear rate, because the acetabular cups and liners could deform during press-fit implantation and hip joint loading. We compared the metal cup and polyethylene liner deformations that occurred when different wall thicknesses were used in order to evaluate the resulting changes in the clearance of the articulating region.
  
  
  METHODS
  A parametric finite element model utilized three cup and liner wall thicknesses to analyze cup and liner deformations after press-fit implantation into the pelvic bone. The resultant hip joint force during heel strike was applied while the femur was fixed, accounting for physiological muscle forces. The deformation behavior of the liner under joint loading was therefore assessed as a function of the head diameter and the resulting clearance.
  
  
  RESULTS
  Press-fit implantation showed diametral cup deformations of 0.096, 0.034, and 0.014 mm for cup wall thicknesses of 3, 5, and 7 mm, respectively. The largest deformations (average 0.084 ± 0.003 mm) of liners with thicknesses of 4, 6, and 8 mm occurred with the smallest cup wall thickness (3 mm). The smallest liner deformation (0.011 mm) was obtained with largest cup and liner wall thicknesses. Under joint loading, liner deformations in thin-walled acetabular cups (3 mm) reduced the initial clearance by about 50 %.
  
  
  CONCLUSIONS
  Acetabular press-fit cups with wall thicknesses of ≤5 mm should only be used in combination with polyethylene liners >6 mm thick in order to minimize the reduction in clearance.
authors: Biomechanics and Implant Technology Research Laboratory, Department of Orthopaedics, University Medicine, Doberaner Str. 142, 18057 Rostock, Germany. paul.goebel@med.uni-rostock.de
authors: Paul
authors: P
authors: Goebel
authors: Daniel
authors: D
authors: Kluess
authors: Jan
authors: J
authors: Wieding
authors: Robert
authors: R
authors: Souffrant
authors: Horst
authors: H
authors: Heyer
authors: Manuela
authors: M
authors: Sander
authors: Rainer
authors: R
authors: Bader
mesh_terms: D000077:Acetabulum / Q000503:physiopathology* / Q000601:surgery*; D019644:Arthroplasty, Replacement, Hip; D017076:Computer-Aided Design; D020342:Finite Element Analysis; D006622:Hip Prosthesis; D006801:Humans; D008422:Materials Testing; D011095:Polyethylenes; D011474:Prosthesis Design; D011475:Prosthesis Failure; D016059:Range of Motion, Articular / Q000502:physiology; D013499:Surface Properties
chemical_list: D011095:Polyethylenes; C111601:ultra-high molecular weight polyethylene