title: Ceramic-on-ceramic bearing decreases the cumulative long-term risk of dislocation.
abstract: BACKGROUND
  It is unclear whether late THA dislocations are related to mechanical impingement or to a biological mechanism that decreases the stability provided by the capsule (eg, inflammation secondary to osteolysis). It is also unknown if alumina-on-alumina bearing couples decrease the risk of late dislocation as a result of the absence of wear and osteolysis.
  
  
  OBJECTIVE
  We asked (1) whether the cumulative number of dislocations differed with alumina-on-alumina (AL/AL) or alumina-on-polyethylene bearings (AL/PE); (2) whether patient factors (age, sex, and diseases) affect risk of late dislocation; (3) whether mechanical factors (component malposition, penetration resulting from creep and wear) or (4) biologic hip factors at revision (thickness of the capsule, volume of joint fluid removed at surgery, histology) differed with the two bearing couples.
  
  
  METHODS
  One hundred twenty-six patients (252 hips) with bilateral THA (one AL/AL and the contralateral AL/PE) received the same cemented implants except for the cup PE cup or an AL cup. The cumulative risk of dislocation (first-time and recurrent dislocation) was calculated at a minimum of 27 years. We measured cup position, creep and wear, and capsular thickness in the hips that had revision.
  
  
  RESULTS
  AL/PE and AL/AL hips differed by the cumulative number of dislocation (31 with AL/PE versus four with AL/AL) and by the number of late dislocations (none with AL/AL, 28 with AL/PE). Cause of osteonecrosis, age, and sex affected the number of dislocations. The frequency of component malposition did not differ between the two bearing couples. The risk of late dislocation appeared less in AL/AL hips with increased capsular thickness (mean, 4.5 mm; range, 3-7 mm) compared with the thinnest (mean, 1.2 mm; range, 0.2-2 mm) capsule of AL/PE hips.
  
  
  CONCLUSIONS
  AL/AL bearing couples decreased the cumulative risk of dislocation as compared with AL/PE bearing couples.
authors: Department of Orthopaedic Surgery, University Paris East (UPEC), Hôpital Henri Mondor, 94010, Creteil, France, philippe.hernigou@wanadoo.fr.
authors: Philippe
authors: P
authors: Hernigou
authors: Yasuhiro
authors: Y
authors: Homma
authors: Olivier
authors: O
authors: Pidet
authors: Isaac
authors: I
authors: Guissou
authors: Jacques
authors: J
authors: Hernigou
mesh_terms: D000328:Adult; D000367:Age Factors; D019644:Arthroplasty, Replacement, Hip; D002516:Ceramics; D005260:Female; D006617:Hip Dislocation / Q000209:etiology*; D006621:Hip Joint / Q000601:surgery; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D008875:Middle Aged; D010014:Osteolysis / Q000601:surgery; D011379:Prognosis; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012306:Risk; D012737:Sex Factors