title: Deformation characteristics and eigenfrequencies of press-fit acetabular cups.
abstract: BACKGROUND
  elastic deformation of press-fitted acetabular cups during implantation provides primary stability. Excessive deformation can lead to chipping or improper seating of ceramic inlays and is dictated by cup stiffness, which also affects its vibrational characteristics. Purpose was to investigate the influence of cup design on deformation during press-fitting and on vibration properties.
  
  
  METHODS
  deformation of ten acetabular cups (with and without ceramic inlay) was tested for radial loads clinically occurring during press-fitting (0-2000N). Eigenfrequencies were measured using experimental modal analysis and related to mass and stiffness.
  
  
  RESULTS
  the first eigenfrequency of the shells varied greatly (4-9kHz); insertion of inlays caused an increase (16-33 kHz). The range of shell stiffness was high (2.7-48.4kN/mm), increasing due to inlay insertion (124.7-376.2kN/mm). Stiffness and mass were sufficient predictors for eigenfrequencies (p<0.001,R²=0.94).
  
  
  CONCLUSIONS
  the cups investigated represent a large stiffness range. Lower cup stiffness can increase primary stability but jeopardize inlay seating, and a suitable balance must be achieved by the designer. Eigenfrequencies also decrease with decreasing stiffness but were all found to lie considerably above clinically observed squeaking frequencies, indicating that these cup designs play no predominant role in the squeaking phenomenon. The observed relation between eigenfrequencies and the quotient of stiffness and mass might be used in the development of new thin walled cup designs so that their contribution to system vibrations is prevented. Presently, surgeons should be aware of the deformation characteristics of cups in order to select a suitable press-fit magnitude.
authors.affiliation: TUHH, Hamburg University of Technology, Biomechanics Section, Denickestrasse 15, 21073 Hamburg, Germany. hothan@tuhh.de
authors.forename: Arne
authors.initials: A
authors.lastname: Hothan
authors.forename: Gerd
authors.initials: G
authors.lastname: Huber
authors.forename: Cornelius
authors.initials: C
authors.lastname: Weiss
authors.forename: Norbert
authors.initials: N
authors.lastname: Hoffmann
authors.forename: Michael
authors.initials: M
authors.lastname: Morlock
mesh_terms: D000077:Acetabulum / Q000503:physiopathology*; D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation* / Q000379:methods; D001696:Biomechanical Phenomena; D002516:Ceramics; D004548:Elasticity; D006622:Hip Prosthesis; D007834:Lasers; D009991:Oscillometry; D011336:Probability; D011474:Prosthesis Design; D013314:Stress, Mechanical; D014732:Vibration