Intraprosthetic dislocation: a specific complication of the dual-mobility system.
BACKGROUND
The dual-mobility concept was proposed as an alternative to prevent postoperative dislocation events. However, intraprosthetic dislocation (IPD) is a troublesome and specific complication induced by the loss of the polyethylene retentive rim and escape of the femoral head from the polyethylene liner. The factors associated with IPD are unknown as only isolated cases have been reported and do not provide a clear understanding of the mechanisms of failure.


OBJECTIVE
We therefore (1) identified features related to different types of IPD and (2) determined factors related to the timing of IPD.


METHODS
We identified 81 cases (80 patients) with IPD from among 1960 primary THAs performed between January 1985 and December 1998. To classify the types of IPD we considered perioperative (presence of arthrofibrosis, cup loosening, and type of liner wear) and radiographic (radiographic cup loosening or migration, and ossification) features.


RESULTS
We identified three types of IPD with the following causal mechanisms: Type 1 was pure IPD without arthrofibrosis and without cup loosening (n = 26), Type 2 was IPD secondary to blocking of the liner (n = 41), and Type 3 was IPD associated with a cup loosening (n = 14). The mean times of onset were, 11, 8, and 9 years after THA, respectively. We found no difference according to the stem design regarding timing of the IPD.


CONCLUSIONS
This new IPD classification allows clinicians to anticipate the possible conditions they will encounter with revision surgery and plan surgery (cup removal, liner exchange, synovectomy). The implant characteristics and this new classification accounted for the differences in the timing of occurrence.


METHODS
Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
Department of Orthopaedic Surgery, University Hospital Center of Saint Etienne, Saint Étienne Cedex 2 42055, France. remi.philippot@chu-st-etienne.fr
Remi
R
Philippot
Bertrand
B
Boyer
Frederic
F
Farizon
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000295:instrumentation*; D001696:Biomechanical Phenomena; D005260:Female; D006617:Hip Dislocation / Q000000981:diagnostic imaging / Q000209:etiology*; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D011474:Prosthesis Design; D011475:Prosthesis Failure; D011859:Radiography; D013997:Time Factors; D017211:Treatment Failure; D055815:Young Adult