TO THE EDITOR:
Ash et al. are to be congratulated for their interesting article "Revision Total Hip Arthroplasty with Cement after Cup Arthroplasty. Long-Term Follow-up" (78-A: 87—93, Jan. 1996), in which they observed that the rate of revision due to loosening of the socket is unexpectedly low after cup arthroplasty, compared with that in a series of primary Charnley low-friction arthroplasties3.
They suggested that the inferomedial repositioning of the socket confers biomechanical benefit. Have they considered the possibility that the increased area of the bone-cement interface after cup arthroplasty increases the resistance to torque (the low-frictional-torque principle of Charnley), thereby reducing the possibility of loosening of the socket?
James R. Loudon, F.R.C.S.: Wrightington Hospital for Joint Disease, Hall Lane, Appley Bridge, Wigan, Lancashire WN6 9EP, England
Dr. Ash, Dr. Callaghan, and Dr. Johnston reply:
We appreciate Mr. Loudon's observations. Studies from our laboratory1 as well as the clinical experience of others2 have demonstrated the potential mechanical benefits of inferomedial placement of the cup in terms of lowered joint-reaction forces. We agree that the increased resistance to torque (the low-frictional-torque principle of Charnley) could also be a contributing factor. As with many clinical observations, it is difficult to determine which factor or factors contribute to the observation.
Stephen A. Ash, M.D.; John J. Callaghan, M.D.: Department of Orthopaedic Surgery, University of Iowa College of Medicine, Iowa City, Iowa 52242
Richard C. Johnston, M.D.: Des Moines Orthopaedic Surgeons, 6001 Westown Parkway, West Des Moines, Iowa 50266