title: Constrained cups appear incapable of meeting the demands of revision THA.
abstract: BACKGROUND
  Failure rates of constrained cups for treating recurrent dislocation in revision THA range from 40% to 100%. Although constrained liners are intended to stabilize the hip by mechanically preventing dislocation, the resulting loss of range of motion may lead to impingement and, ultimately, implant failure.
  
  
  OBJECTIVE
  We therefore documented the mechanisms of failure of constrained acetabular cups in revision THA and determined the type and severity of damage (wear, fracture, and impingement) that occurs in situ.
  
  
  METHODS
  We retrieved 57 constrained components of four different designs at revision THA and examined for the presence of rim impingement, oxidation, cracks within the liner, backside wear, pitting, scratching, abrasion, burnishing, and the presence of embedded particles. Articular wear was calculated from the volume of the concave articular bearing surface, which was measured using the fluid displacement method.
  
  
  RESULTS
  Failure of the locking ring was responsible for 51% of failures, whereas 28% of revisions were the result of acetabular cup loosening, 6% backside wear, and 22% infection. Impingement damage of the rim of the polyethylene liner was seen in all retrievals with moderate or severe damage in 54%. The average volumetric wear rate of the articular surface was 95 mm(3)/year.
  
  
  CONCLUSIONS
  Failure of the locking liner ring and loosening of the acetabular cup are the primary causes of mechanical failure with constrained liners; polyethylene is an inadequate material for restricting motion of the hip to prevent instability. The durability of these devices is unlikely to improve unless the mechanical demands are modified through increased range of motion leading to less frequent rim impingement.
authors.affiliation: The Institute of Orthopedic Research and Education, Baylor College of Medicine, 6550 Fannin Street, Suite 2501, Houston, TX 77030, USA. pnoble@bcm.edu
authors.forename: Philip C
authors.initials: PC
authors.lastname: Noble
authors.forename: Salim K
authors.initials: SK
authors.lastname: Durrani
authors.forename: Molly M
authors.initials: MM
authors.lastname: Usrey
authors.forename: Kenneth B
authors.initials: KB
authors.lastname: Mathis
authors.forename: Nikolaos V
authors.initials: NV
authors.lastname: Bardakos
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000295:instrumentation*; D001696:Biomechanical Phenomena; D016009:Chi-Square Distribution; D020878:Device Removal; D005260:Female; D006617:Hip Dislocation / Q000209:etiology / Q000503:physiopathology / Q000601:surgery*; D006622:Hip Prosthesis; D006784:Hospitals, Teaching; D006801:Humans; D008297:Male; D008670:Metals; D008875:Middle Aged; D020959:Polyethylene; D011474:Prosthesis Design; D011475:Prosthesis Failure; D016059:Range of Motion, Articular; D012042:Registries; D012086:Reoperation; D013314:Stress, Mechanical; D013499:Surface Properties; D013781:Texas
chemical_list: D008670:Metals; D020959:Polyethylene