Commentary & Perspective

Commentary & Perspective on
"Effects of Preheating of Hip Prostheses on the Stem-Cement Interface"
by Kazuho Iesaka, MD, et al.

Commentary & Perspective by
Eduardo A. Salvati, MD, and Timothy M. Wright, PhD*,
The Hospital for Special Surgery, New York, NY

In their report of an experimental model designed to duplicate in vivo stem insertion, Iesaka et al. present interesting data that support preheating the femoral stem to reduce interfacial porosity in cemented total hip replacement. They found that use of this technique improved metal-cement interfacial strength, as evidenced by the results of static and cyclic testing, and it reduced cement porosity. However, the authors raised the concern that the increase of 6°C in temperature that was observed at the bone-cement interface of the preheated stems may cause bone necrosis.

We have studied the surface temperature of the endosteal bone of the proximal part of the femur in vivo, during preparation for insertion of the femoral component, with use of an infrared thermometer (Cole-Parmer, Vernon Hills, Illinois). We observed that the temperature at the surface of the endosteal bone drops to approximately 32°C to 33°C at the time of cement injection because of exposure to the ambient temperature of the operating room and irrigation fluids. Thereafter, blood circulation further dissipates heat. Thus, we believe that bone necrosis is unlikely.

In addition, during the last decade, we have routinely preheated the polymer and stem to approximately 44°C, without observing any detrimental effect in the clinical and radiographic results of over 4000 total hip replacements performed at The Hospital for Special Surgery. We have also demonstrated that the heated metallic surface of the stem juxtaposed to the cement as it polymerizes does not affect the mechanical properties of the cement (including static properties, fracture toughness, and fatigue strength at the stem-cement interface) and it reduces cement porosity, as also shown by both Iesaka et al. and Bishop et al.^1,2

Bishop et al. introduced the concept of centrifugal versus centripetal polymerization of cement. Under normal conditions, the surface of the endosteal bone is warmer than the cement, inducing a centripetal polymerization, which starts on the surface of the cement that contacts the bone and progresses toward the stem. Conversely, when the stem is preheated, the polymerization occurs in a centrifugal fashion. The cement in contact with the stem polymerizes first and is less porous, more homogenous and free of defects, which optimizes the metal cement interface. The area of cement that polymerizes first has a better surface—homogeneous and free of defects. Thus, heating the stem optimizes the metal-cement interface, and the last surface to polymerize will be the bone-cement interface, which is already irregular due to its interdigitation with the endosteal bone. Because the in vivo temperature of the endosteal bone of the proximal part of the femur at the time of cement injection is lower than the temperature in the test model used by Iesaka et al. (who maintained the Sawbones cylinders [Pacific Research Laboratories, Vashon, Washington] at 37°C), the temperature gradient in the surgical setting is even higher when the stem is preheated.

The improvement in shear strength of the stem-cement interface, that was observed by Iesaka et al., is clinically important. The finding that the shear strength of the metal-cement interface increased more than could be explained by the reduction in porosity alone suggests that shrinkage and the creation of hoop stresses provide the additional benefit. Of course, shrinkage might also affect the bone-cement interface, a possibility not investigated by Iesaka et al., although again, our clinical results with use of preheated stems would suggest that any such effect, if present, has no clinical repercussion.

The decrease in polymerization time caused by preheating of the stem has other clinical benefits. In our experience, when the stem is inserted at operating room temperature, the polymerization time is approximately twelve to thirteen minutes. This time is reduced to six to seven minutes when the stem is preheated to approximately 44°C. Thus, the time needed for implantation of the femoral component is shortened by approximately six minutes, at a critical period in the procedure, when thrombogenesis is maximally activated, as the lower extremity is maintained in an extreme position with venous stasis caused by occlusion of the femoral vein. The local trauma to the vein with a longer period of venous stasis could also damage the endothelium, with resulting aggregation of platelets and leukocytes and their adherence to the exposed subendothelium, which further activates the clotting cascade³.

Reducing polymerization time also decreases the risk of inadvertently moving the stem while the cement is polymerizing. Also, with the cost of operating-room time estimated to be approximately $20 to $25 per minute, the cost savings due to preheating the stem would be approximately $120 to $150 per procedure, an obvious benefit. Over the course of 100 total hip replacements, ten hours of operating-room time that would be spent in waiting for the cement to harden can be saved.

Surgeons should proceed cautiously in implementing this technology. We recommend preheating the stem only when the surgery is performed by an experienced surgeon working with an efficient and skilled surgical team. In the early use of this technique, the stem should be preheated to only 35° to 40°C, and once the surgeon becomes comfortable with the shorter setting time, the temperature can be progressively increased to 44°C in later procedures.

Additional studies of fracture mechanics may increase our understanding of the direct impact of preheating the stem on interfacial strength. Such studies have shown that even under simple loading conditions, such as the axial loading employed by Iesaka et al., the mechanics of debonding are complex.⁴ Debonding occurs under mixed-mode loading; shear stresses are the major stresses that cause debonding, but other types of stress, such as hoop stress, also influence mechanical debonding. Studies of the fracture mechanics of the stem-cement interface have been limited mainly, however, to flat, two-dimensional experimental models, which do not capture hoop stresses.

A number of other factors could influence the strength of the stem-cement interface around preheated stems, including the type of cement, its viscosity at the time of stem insertion, and the surface finish of the component. For example, in our study of the effects of a heated metallic surface placed next to the cement as it polymerizes, we found that changes in the mechanical properties and the porosity of the cement differed somewhat among the four types of commercial cement that we tested¹.

Roughening of stem surfaces has been advocated as a means of increasing interfacial strength, although the clinical experience has demonstrated a higher prevalence of early osteolysis secondary to the increased debris generated by micromotion at roughened interfaces, once debonding occurs. Furthermore, surface roughness also affects the mechanics of interfacial adhesion, with a grit-blasted surface having nearly fourfold the area over which a gap can form during polymerization of the cement in comparison with that area seen in components with a polished finish⁵.

Clinical experience has demonstrated in several studies^6,7 the better long-term success of properly designed cemented stems with polished/smooth surfaces, and on the basis of the findings of Iesaka el al., preheating the stem can improve the strength of the stem-cement interface,even when the stem is smooth or polished.

*The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

References

1. Parks ML, Walsh HA, Salvati EA, Li S. Effect of increasing temperature on the properties of four bone cements. Clin Orthop. 1998;355:238-48.
2. Bishop NE, Ferguson S, Tepic S. Porosity reduction in bone cement at the cement-stem interface. J Bone Joint Surg Br. 1996;78:349-56.
3. Salvati EA, Pellegrini VD Jr, Sharrock NE, Lotke PA, Murray DW, Potter H, Westrich GH. Recent advances in venous thromboembolic prophylaxis during and after total hip replacement. J Bone Joint Surg Am. 2000;82:252-70.
4. Mann KA, Bhashyam S. Mixed-mode fracture toughness of the cobalt-chromium alloy/polymethylmethacrylate cement interface. J Orthop Res. 1999;17:321-8.
5. Race A, Miller MA, Ayers DC, Cleary RJ, Mann KA. The influence of surface roughness on stem-cement gaps. J Bone Joint Surg Br. 2002;84:1199-204.
6. Collis DK, Mohler CG. Loosening rates and bone lysis with rough finished and polished stems. Clin Orthop. 1998;355:113-22.
7. Sporer SM, Callaghan JJ, Olejniczak JP, Goetz DD, Johnston RC. The effects of surface roughness and polymethylmethacrylate precoating on the radiographic and clinical results of the Iowa hip prosthesis. A study of patients less than fifty years old. J Bone Joint Surg Am. 1999;81:481-92.
8. Ong A, Wong KL, Lai M, Garino JP, Steinberg ME. Early failure of precoated femoral components in primary total hip arthroplasty. J Bone Joint Surg Am. 2002;84:786-92.