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© 1999 The Journal of Bone and Joint Surgery, Inc.
CorrespondenceTO THE EDITOR: In the article "The Natural History of Debonding of the Femoral Component from the Cement and Its Effect on Long-Term Survival of Charnley Total Hip Replacements" (80-A: 715–721, May 1998), Berry et al. defined debonding as "the loss of the bond between the metal femoral component and the acrylic cement." This definition is presumably based on the assumption that a bond does, in fact, exist between the smooth femoral component and the cement. I suggest that any such arrangement is merely an interference fit and employs the common engineering principle of male and female tapers engaging under load. For the system to become load-bearing, a slip must occur, preferably between the taper of the stem and the cement, or cement fixation will fail. Support of the distal end of the stem or engagement of the proximal part of the stem within or on the cement mantle can prevent or reduce this slip. Lateral separation of the stem from the cement occurs if the stem tilts into varus, producing the appearance of debonding. What is important is the consequence of the slip of the stem within the cement mantle and the preparation for such a possibility through the design of the stem and the operative technique. I believe that the term debonding should be reserved for situations when bonding of the stem to the cement is deliberate by design, as is the case when precoated components are used. To use "debonding" synonymously with "slip" is to confuse two fundamental principles and may even be an attempt to legitimize precoating and debonding as the natural event—that is, the slip of the taper.
B. M. Wroblewski, F.R.C.S.: The John Charnley Research Institute, Wrightington Hospital for Joint Disease, Hall Lane, Appley Bridge, Wigan, Lancashire WN6 9EP, United Kingdom
Dr. Berry, Mr. Harmsen, and Mr. Ilstrup reply: We thank Mr. Wroblewski for his insightful comments, and we agree that the nomenclature concerning the status of the prosthesis-cement interface is not ideal. Debonding has been the standard term to describe the lack of a bond between the cement and the femoral component. We agree that this term suggests that a bond was present to begin with, which may not always be the case. We chose to use the term debonding partly because of convention and partly on the basis of evidence that, even for smooth or polished metals, some bond can form between the metal and the acrylic cement1. Bundy and Penn postulated that this bond may be influenced more by atomic interactions for polished metal surfaces and by mechanical interactions for rough metal surfaces1. Nevertheless, our manuscript supports the concept, suggested by Mr. Wroblewski, that in many cases the smooth Charnley prosthesis rapidly becomes nonbonded, regardless of the strength of the initial bond between the metal and the cement, but maintains a mechanically stable interference fit in the cement and functions well clinically. We are reluctant to apply the term debonding selectively to implants designed to bond to the cement, for two reasons. First, many stem designs fall between the extremes of precoated stems, which are designed to bond to the cement, and polished tapered stems, which are not. Second, in the clinical setting, the strength of the initial bond between the implant and the cement cannot be predicted entirely on the basis of the implant design. Clinical conditions or operative technique may lead to a poor initial bond between the stem and the cement in some patients, even if the stem was designed to bond to the cement. From the practical viewpoint, we believe that the in vivo relationship between the femoral component and the surrounding cement can be characterized as (1) bonded, (2) nonbonded but mechanically stable, or (3) nonbonded and mechanically unstable. Thus, the clinical performance of a nonbonded implant appears to depend on mechanical stability (which is related in part to the geometry of the implant) and the surface finish of the implant. Implants with a smooth surface and the geometry of a Charnley implant often achieve a nonbonded but mechanically stable state and do well clinically. In distinction, some implants with a rough surface finish, when nonbonded and mechanically unstable, behave very differently and often lead to the development of osteolysis and clinical failure.
Daniel J. Berry, M.D.; William S. Harmsen, M.S.; Duane M. Ilstrup, M.S.: Department of Orthopedic Surgery, Mayo Clinic, 200 First Street S.W., Rochester, Minnesota 55905
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