Introduction: Impingement is a factor of failure in total hip replacement (THR), causing instability and early wear. Its true frequency is not known; cup-retrieval series reported rates varying from 27 to 84%.
Hypothesis: The hypothesis was that a large continuous series of THR cup removals would help determine the frequency of component impingement.
Objectives: The hypothesis was tested on a continuous retrospective series of cups removed in a single center, with a secondary objective of identifying risk factors.
Material and methods: [corrected] Macroscopic examination looked for component impingement signs in 416 cups retrieved by a single operator between 1989 and 2004. Risk factors were investigated by uni- and multivariate analyses in the 311 cases for which there were complete demographic data. In these 311 cases, removal was for aseptic loosening (131 cases), infection (43 cases), instability (56 cases), osteolysis (28 cases) or unexplained pain (48 cases); impingement was explicitly implicated in only five cases (1.6%), always with hard-on-hard bearing components.
Results: Impingement was found in 214 of the 416 cups (51.4%) and was severe (notch>1mm) in 130 (31.3%). In the subpopulation of 311 cups, impingement was found in 184 cases (59.2%) and was severe in 109 (35%). Neither duration of implant use nor cup diameter or frontal orientation emerged as risk factors. On univariate analysis, impingement was more frequently associated with revision for instability, young patient age at THR, global hip range of motion >200° or use of an extended femoral head flange (or of an elevated antidislocation rim liner), and was more severe in case of head/neck ratio<2. On multivariate analysis, only use of an extended head flange (RR 3.2) and revision for instability (RR 4.2) remained as independent risk factors for impingement.
Discussion: Component impingement is frequently observed in cups after removal, but is rarely found as a direct indication for revision, except in case of hard-on-hard friction couples (polyethylene being the most impingement-tolerant material). Systematic use of extended head flanges and elevated antidislocation rims is not to be recommended, especially in case of excessive ROM. A good head/neck ratio should be sought, notably by increasing the head diameter in less impingement-tolerant hard-on-hard friction couples. Although not identified as a risk factor in the present study, implant orientation should be checked; computer-assisted surgery can be useful in this regard, for adaptation to the patient's individual range-of-motion cone.
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