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Thermal Manipulation of Bone Cement

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Abstract

Many factors affect the rate of polymerization of polymethylmethacrylate (PMMA) and, therefore, the working time of bone cement. Surgeons may control some factors, but not all. A surgeon may change the temperature of the powder and the monomer, whereas the temperature and relative humidity of the operating room are more difficult to alter. The temperature of the mixing vessels may also be controlled. However, a surgeon has no control over the industrial mixture of the components of the bone cement, which can vary considerably from batch to batch. This article reviews the effect of temperature of cement mixtures on the working time of the cement, the interfacial strength of the cement-metal interface as well as the cement-bone interface, and tissue viability and cellular change at the bone-cement interface.

Many factors contribute to the long-term durability of a cement mantle in vivo. Although a surgeon cannot control all factors, numerous studies demonstrate the importance of creating a uniform cement mantle.^1-6

To achieve this mantle, a surgeon must deliver cement of appropriate consistency in a reproducible fashion. Surgeons may ask why thermal manipulation of bone cement is worth considering given the other important parts of cement technique. Three reasons are: to allow changes in the working time of the cement, to limit thermal injury to the bony interface, and to control porosity and degree of interfacial bond at the bone-cement interface.

Methods

Changing Working Time
Cement polymerization is often divided into three phases: the mixing phase in which monomer is mixed with powder and the result is a sticky, runny paste; the working phase in which the cement becomes dough-like and is not sticky; and the hardening phase which begins when a surgeon feels the cement has become hard. Changing the temperature of the PMMA components may affect the cement in two ways. Changing the temperature of the PMMA may lengthen the working time or dough phase to allow a surgeon to control the amount of time that is needed to install the cement in the femoral canal, to adequately pressurize the cement, and then to insert the femoral component safely into the cement. Changing the temperature may also decrease the viscosity of high viscosity cements so that cements may be introduced through a cement gun.

Debrunner et al⁷ showed that increasing the room temperature by 10° C (18° F) decreased the time to set by a factor of 1.5 to 2. Because it is often difficult for a surgeon to control the temperature of the operating room, the monomer, the powder, and the mixing vessels should be cooled.

A surgeon must remember that the published working times and set times of individual cements are based on standardized tests conducted in a laboratory, and that conditions in the operating room are often dramatically different. International Standard Organization (ISO) standard 5833 set a limit of 5 minutes (at 23° C, 73° F) for the mixing phase, and 3-15 minutes for the working time. The Table reviews the working time of common cements relative to changes in temperature. This demonstrates that a working time of only 2-3 minutes is common at 25° C (77° F), but that this increases to 4-7 minutes if the mixture is cooled to 17° C (63° F).

A surgeon must be cognizant of other factors that affect the working time of bone cement so that he or she may be prepared to alter the temperature of the cement ingredients in response to these factors. Relative humidity <40% can prolong working times for 1-3 minutes. Conversely, humidity >60% can shorten working times by similar amounts. Because relative humidity is generally increased in the summer months, and operating room temperatures are likely to be higher in the summer months, surgeons should be prepared for shorter working times during these months. Working times are likely to be longer in the winter months when operating room temperatures and humidity can be lower.

When commercially mixed by the cement producer, the addition of antibiotic powders is not associated with a change in the working curves of the same cement without antibiotics. However, if the anti-biotic is hand mixed by a surgeon, particularly at greater quantities than commercially available, the chance of inhomogeneous mixing is high. This may be associated with shortening of working time.

Because it is often difficult for a surgeon to control the room temperature, the temperature of the mixture can be controlled by changing the temperature of the cement powder, monomer, mixing vessels, or the prosthesis. If the monomer alone is cooled from room temperature (23° C) to refrigerated temperature (4° C), this will generally double the working time from 6-7 minutes to 12-14 minutes. It also increases the time that high-viscosity cements remain thin enough to be pushed through a cement gun.

This can be accomplished in the operating room in 2 ways. First, the monomer can be stored in the refrigerator, as is popular in parts of Europe and Scandinavia. The PMMA powder may also be stored in the refrigerator if it is packaged in foil packs. If packaged in polyethylene or paper packages, water can be absorbed, which may affect the mixing properties. Second, the monomer bottles can be placed in sterile ice (ie, pump ice used in cardiovascular surgery) (Table).

Table

Cement Working Times Cement Type & Manufacturer   25°C 23°C 17°C CMW 1 (homopolymer) Depuy Orthop. (Leeds, UK) Dough Set 1.5 min. 3.5 min. 1.7 min. 4.4 min. 2.8 min. 8 min. Endurance (copolymer) Depuy Orthop. (Leeds, UK) Dough Set 2.5 min. 4.5 min. 1.7 min. 4.4 min. 2.8 min. 8 min. Osteobond (copolymer) Zimmer Orthop. (warsaw, Ind) Dough Set 3.6 min. 6 min. 4.2 min. 7.2 min. 6.5 min. 12 min. Osteopal (copolymer) Merck Biomaterial (Darmstadt, Germany) Dough Set 2.5 min. 4.5 min. 3.0 min. 5.1 min. 6.2 min. 10 min. Palacos R (homopolymer) Heraeus Kulzer GmbH & Co. (Wehrheim, Germany) Dough Set 0.8 min. 4.4 min. 1.0 min. 5.0 min. 2.2 min. 6.5 min. Simplex P (copolymer) Stryker Osteonics Howmedica (Rutherford, NJ) Dough Set 2.2 min. 5.0 min. 2.7 min. 5.8 min. 5.2 min. 9 min. Dough = time from initial mix to cement no longer sticky Set = time to cement firm Adapted from working curves from Kuhn K. Bone Cements: Up-to-Date Comparison of Physical and Chemical Properties of Commercial Materials. Heidelburg, Springer-Verlag; 2000.

Thermal Injury to Interface Tissue
Although most studies demonstrate that current cement techniques are well tolerated by tissue at the bone cement interface, some concern exists about attempts by surgeons to accelerate the polymerization process by heating of the cement ingredients. Mjoberg et al⁸ speculated that thermal injury at the bone- cement interface was responsible for the radiolucent lines noted around cemented femoral prostheses and suggested that they may play a part in the pathogenesis of loosening. They also suggested that metallic femoral stems acted as a heat sink, which decreased the amount of thermal injury to the bone interface. This information should be considered when surgeons heat the femoral component to achieve a better interfacial bond, particularly in light of the above information about the local environmental conditions in the operating room.

Iesaka et al⁹ noted that the polymerization temperatures of the cement mantle increased an average of 6° C when the prosthesis was heated to body temperature (37° C). Li et al¹⁰ noted, however, that heating of the prosthesis reversed the direction of polymerization from the bone-cement interface toward the prosthesis. They noted that with preheating the prosthesis, the polymerization starts at the prosthesis and progresses toward the interface, and that may reduce the degree of thermal necrosis at the interface. Berman et al¹¹ noted in their rabbit model that bone necrosis occurred consistently when polymerization temperatures reached 70° C.

Using ISO standard 5833, Kuhn¹² found in vitro polymerization temperatures >70° C for many popular cements including CMW 1 (Depuy, Leeds, UK), Simplex P (Howmedica, Rutherford, NJ), and Palacos (Heraeus Kulzer, Wehrheim, Germany). Hansen and Jensen⁵ found lower polymerization temperatures using the standard ISO 5833, but close to the 70° C threshold.

Sato’s¹³ experiments suggested that the surgical trauma of reaming and broaching the femoral canal may have more to do with the necrosis seen at the bone interface than the thermotoxicity of the bone cement.

A final consideration regarding heat of polymerization is to avoid heat inactivation of antibiotics added to the cements. In vitro tests have shown polymerization heats of >80° C for several common antibiotic cements (Palacos G, CMW G, and Antibiotic Simplex). These temperatures are not associated with inactivation of antibiotics commonly commercially mixed in cement such as gentamicin and tobramycin, but should be a consideration for a surgeon when hand mixing other antibiotics. Mixing cement components that have been stored at 4° C (39° F) in a refrigerator decreases the in vitro polymerization heat by 10° C, as well as prolongs the working time of the cement, and is recommended.

Interfacial Bond and Porosity of the Cement Mantle
Shepard et al¹⁴ studied the effect of degree of polymerization of cement related to the interfacial bonding strength of cement to metal in an in vitro experiment. They concluded that prostheses with roughened surfaces developed greater tensile and shear strength of the cement implant bond when the prosthesis was introduced into the cement at an early stage of polymerization (before the dough stage), but that the degree of polymerization did not have an effect on the bonding strength of polished stems. They concluded that surgeons cementing roughened femoral components may want to slow down the rate of polymerization of the cement by lowering the temperature of the monomer.

In another in vitro experiment, Keller et al¹⁵ found improved bonding strength for constructs created before, or at the onset of, the dough stage of cement polymerization. This concept has been challenged by Mann et al¹⁶ who believed that increases in interfacial porosity at the interface negated any advantages in bonding strength related to early cementation. Iesaka et al⁹ found that heating the femoral stem to 37° C decreased interfacial porosity and increased the cement bond strength but that it decreased setting times by 12% and increased the heat of polymerization by 6° C on average.

In another study, Iesaka et al¹⁷ found that heating or cooling the monomer did not have a significant effect on the bonding strength of the cement prosthesis construct. Therefore, the aggregate data suggest that surgeons cementing rough or smooth femoral stems should install cement in the dough phase for the advantages of improved cement intrusion, improved stability during insertion, and decreased interfacial porosity. Heating the femoral stem to 37° C will not cause significant additional toxicity at the interface but will accelerate polymerization after the stem is installed and may decrease interfacial porosity.

Conclusion
Clinical situations exist in which a surgeon may want to consider thermal manipulation of bone cement. These include environmental conditions such as increased humidity or temperature of the operating room, which will shorten the expected working time of the bone cement and require a surgeon to take steps to counteract this. Secondly, certain clinical situations may dictate the need for a longer working time for the cement, such as cementing a longer stem, or cementing in a large or deformed femoral canal. Similarly, situations occur in which there is minimal deformity so that less working time is needed. The clinician should also be aware that thermal manipulation affects cements differently and it is necessary to learn these differences.

The easiest and safest way for a surgeon to increase the working time of the cement is to cool the cement components. Cooling may be performed the night before surgery by placing the cement components in a refrigerator or, if more rapid cooling is necessary, by placing the monomer in an ice bath made from pump ice.

The easiest and safest way to decrease the working time of the cement is to heat the prosthetic component to body temperature (37° C) in an operating room warmer. The cement can be installed in an unhurried fashion, and the acceleration of the polymerization phase does not start until the prosthesis is placed into the cement.

If a surgeon elects to increase the temperature of the prosthesis or the cement components, he or she should consider that this will increase the final heat of polymerization of the cement, and that the degree of thermal toxicity may increase significantly at a heat of polymerization >70° C.

References

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Author

From the Department of Orthopaedic Surgery, Virginia Commonwealth University Health System, Richmond, Va.