Abstract
Introduction
The prosthetic impingement occurs if the range of motion of the hip exceeds implant mobility or in case of component malorientation. This retrieval study was designed to assess the frequency and the risk factors of this phenomenon.
Material and Methods
The frequency and the severity of the impingement were calculated from a continuous series of 311 cups retrievals collected between 1989 and 2004 by a single surgeon. The reason for retrieval was loosening (131 cases), infection (43 cases), instability (56 cases), osteolysis (28 cases), unexplained pain (48 cases) and prosthetic impingent (5 cases all with hard bearings). The notching at the cup rim was assessed twice by two examiners with optic magnification. The risk factors were analyzed from clinical charts by univariate and cox multihazard.
Results
Among the 311 cups explants, the frequency of impingement was 59.2% (it was severe in 11%, the notching exceeding 3 millimeters). The impingement was the reason for removal in only 1.6%, meaning that it was mainly unexpected (98.4 percent). The impingement was more frequent when revisions were performed because of instability (notching 80%), when the sum of hip motion exceeded 200 degrees (sum of motion in the 6 degrees of freedom of the hip) (notching 66%). The other risk factors were: use of heads with skirts, liner with an elevated rim, and head-neck ratio below 2, younger age at surgery. The multivariate analysis identified only two independent factors: 1) the use of skirted heads (Odd ratio 3.2 (1.2–15.3)), 2) and revision because of instability (Odd ratio 4.2 (1.1–16.2)). In contrast different classical factors were not correlated with impingement on retrieval: cup inclination, the duration before retrieval, the indication for primary prosthetic replacement.
Discussion and Conclusion
This study underlines the impingement is common when assessing cup retrievals (over 50 percent). One should be aware of impingement when performing hip replacement in patients having a high range of motion. This situation may require prostheses with a high head-neck ratio, as well as use of computer-assisted surgery. One should avoid liners with elevated rim as well as heads with skirts to prevent dislocation, particularly when other risk factors are detected.
