TO THE EDITOR:
The article "The Role of Antibiotic-Loaded Cement in the Treatment of an Infection after a Hip Replacement" (76-A: 1742—1751, Nov. 1994), by Duncan and Masri, presented the history and basic science of antibiotic-loaded bone cement in a comprehensive way. The authors should be commended for describing antibiotic-loaded bone cement as an efficacious and viable option in the treatment of infection after hip arthroplasty.
In describing the mechanisms of elution of antibiotic-loaded bone cement, the authors said that "the release of antibiotics occurs only from the surface, from voids and cracks in the bone cement" and cited the study by Baker and Greenham1 to support this observation. The conclusion that "bone cement with greater porosity would be expected to allow more antibiotic release than one with less porosity" was attributed to Baker and Greenham. Given this, Duncan and Masri observed that "it is not surprising, then, that Palacos bone cement, with greater porosity than other cements, has the most rapid elution characteristics."
The release of antibiotics from polymethylmethacrylate-based bone cement is a complex process that is dependent on several physicochemical variables related to both the particular antibiotic and the composition of the bone cement4,6. For instance, it has been shown that different antibiotics possess widely varying elution properties from a given brand of bone cement. In addition, small differences in the chemical structure of polymethylmethacrylate may dramatically affect the amount of eluted antibiotic7. In light of this, the description of the pharmacokinetics of antibiotic release in bone cement as a simple function of porosity is an oversimplification. The role of porosity in the elution properties of a given cement is not clear, and the conclusion of Baker and Greenham (and, hence, Duncan and Masri) may be challenged with reference to some recent work.
Wahlig and Dingeldein8 examined the effects of mixing techniques on the elution characteristics of gentamicin-loaded Palacos and Osteopal bone cements. In that study, the in vitro elution performance of hand-mixed samples was compared with that of samples fabricated with centrifugation, vacuum-mixing, or pressurization. For the Palacos samples, the hand-mixed samples released the lowest amount of gentamicin (after ten days), compared with the samples fabricated with the other mixing techniques. The Osteopal samples exhibited the opposite behavior; the hand-mixed samples released the greatest amount of gentamicin. While the porosity of the cement may have played a role in this behavior, the maxim that greater porosity leads to greater elution obviously does not hold true in every instance and for every brand of cement.
My colleagues and I have extended the work of Wahlig and Dingeldein8 into the clinical arena by studying the release of gentamicin from Palacos bone cement mixed with one of two different techniques in patients who were having bilateral hip arthroplasty. The prosthesis on one side was fixed with hand-mixed Refobacin Palacos cement (Palacos cement with gentamicin) while that on the other side was fixed with vacuum-mixed Refobacin Palacos cement. By performing a two-stage operation, we were able to segregate the elution response during each stage. Gentamicin was measured in serum, urine, and drainage fluid as a function of time since the implantation. We observed no significant differences in the release of gentamicin between the hand-mixed and vacuum-mixed Refobacin Palacos cement mantles.
The observation that Palacos cement possesses "greater porosity than other cements" is noticeably unsubstantiated. Yet, this observation was later used to question the efficacy of a cement that has enjoyed success for more than two decades in operating rooms throughout Europe and the world. Duncan and Masri wrote that "the higher porosity [of Palacos cement] that is responsible for its improved elution characteristics makes it less amenable to modern cementing techniques ... and may produce a deleterious effect on the mechanics and long-term durability of the hip reconstruction." Such speculation is unfortunate, especially without the benefit of supporting evidence. Unconfirmed opinions should not be disguised as statements of fact.
Gerhart Waertel, M.D.: Orthopädische Klinik, BRK-Rheumazentrum, 93074 Bad Abbach, Germany
Dr. Duncan and Dr. Masri reply:
We agree that the process of antibiotic elution from acrylic is potentially more complex than that suggested by the work of Baker and Greenham1. However, the opinion that Palacos cement possesses greater porosity than other cements is further supported by a surface electron microscopic study performed by Marks et al.5, who found larger pores in Palacos cement than in Simplex cement.
Additionally, our concerns regarding the use of Palacos cement with the modern cementing techniques widely employed in North America are based not only on the handling characteristics that we have experienced in our unit but also on the published work of Davies et al.3. In that study, centrifigation increased the cycles to fatigue failure of Simplex cement from 11,790 to 35,769, compared with an insignificant decrease from 13,866 to 9236 with Palacos cement. The reduction after the addition of gentamicin to the Palacos cement was significant (it fell to 5020 cycles). The authors remarked that "Palacos R and Palacos R with gentamycin are not improved by centrifugation because of their high viscosity"—an observation that matches our experience regarding handling characteristics.
Furthermore, it serves no purpose to confuse the issue more by reference to the unpublished work of Waertel et al. in the clinical setting. Studies of in vivo elution characteristics have been notoriously contradictory of previous information gathered in the more controlled laboratory setting. Brien et al. did not observe a significant difference between elutions from Simplex and Palacos cement in wound suction drainage2. The experience of Marks et al. was similar5. Perhaps this is why Waertel, according to his own account, failed to demonstrate a difference between hand-mixed and vacuum-mixed Refobacin Palacos cement, thereby contradicting the earlier work by Wahlig and Dingeldein8, on which he based much of his criticism of our conclusions.
We are indebted to our colleagues from many parts of Europe for their contributions to this subject, and we welcome opportunities such as this for an open exchange of ideas.
Clive P. Duncan, M.D., F.R.C.S.(C); Bassam A. Masri, M.D., F.R.C.S.(C): Department of Orthopaedics, Vancouver Hospital and Health Sciences Center, Third Floor, Laurel Street Pavilion, 910 West 10th Avenue, Vancouver, British Columbia V5Z 4E3, Canada