J. BIOMED. MATER. RES. SYMPOSIUM
No. 6, pp. 99-103 (1975)
Dimensional Behavior of Curing Bone Cement Masses J. R . DE WIJN and F. C. M. DRIESSENS, Institute of Dental Materials Science and Technology, and T. J. J. H. SLOOFF, Clinic of Orthopaedic Surgery, University of Nijmegen, “Heyendael”, Erasmuslaan I , Nijmegen, the Netherlands
Summary The curing of bone cements is accompanied by release of polymerization heat and, hence, by a temperature rise of the curing cement mass. This temperature rise causes expansion of enclosed air bubbles and evaporation of the volatile monomer. An overall expansion of 3 to 5 vol % has been mentioned in the literature. It has often been stated that this expansion favours the fixation of metal endoprostheses in the marrow cavity of bone. T o check for the influence of this expansion on linear dimensions of the cured cement mass we filled stainless steel cylinders with a precision bore of 22,000 0,005 mm and a length of 120 mm with bone cement. After curing of the cement in a environment of 37°C the resulting cement rod was released from the cylinder and the diameter of the rod was measured at 37°C. The influence of the “foaming effect” on the transverse dimensions of the rods was studied by curing the cement at 37°C and 2 atm air pressure in a high-pressure-vessel. This method of curing eliminates porosity in the cement almost completely, so that curing shrinkage is to be expected rather than expansion of the cement mass. The results indicate that a volumetric expansion of the cement during curing of cylindrical rods in laboratory experiments, can be accompanied by a linear diametrical shrinkage of the cement mass. The explanation of this phenomenon is to be sought in the fact that the volumetric expansion takes place at a time when the cement is still plastic; by the formation of gas bubbles, the cement is forced in longitudinal direction into the cylinder and when the temperature of the mass has passed through a maximum, the cooling of the cement mass results in a thermal shrinkage of approximately 0.4% linearly. Extrapolating this laboratory result to a clinical situation one might doubt whether the overall expansion of bone cements during curing will result in a permanent positive pressure on the walls of marrow cavity and whether it will contribute to a better fixation of endoprostheses than in the case of a, still hypothetical, nonporous cement.
INTRODUCTION The fixation of endoprostheses in calcified tissue structures plays an important role in orthopedics, oral surgery, dental implantology and restorative dentistry. The fixation of appliances onto calcified tissues is applied in orthodontics and preventive dentistry. Calcified tissues are more or less rigid and they contain freely transportable water in their tissue 99
0 1975 by John Wiley & Sons, Inc.
DE WIJN, DRIESSENS, A N D SLOOFF
fluid as an essential component in vivo. If the aqueous surface layer is thin [ l ] the making of an adhesive bond is not a problem , as far as wetting by an inorganic or organic adhesive is concerned. Therefore, cementing is an obvious way t o attain fixation of prostheses to these tissues. Several practical difficulties are encountered in the clinical situation for obtaining a dry cavity surface and maintaining it dry until the prosthesis is inserted. Satisfactory techniques have been worked out by the clinicians t o solve these problems. The use of brittle inorganic cements prevails in restorative dentistry and orthodontics where the stiff tooth tissues are the substrate. However, tough organic cements are to be preferred, where the more flexible bone structures are the substrate. Several self-curing polymer systems have been proposed for the latter purpose. However, poly (methyl methacrylate) (PMMA) and copolymers seem t o be the least disadvantageous. Commercial bone cements are 2-component formulations of a P M M A powder and a M M A monomer whereby several compounds are added t o one of these components for several purposes (inhibitor, catalytic system for polymerization, radiopacifier, antibiotics, etc.). The polymerization is accompanied by a volume effect and a heat effect. The latter effect is dealt with in a separate paper . The present paper is confined to volume effects as far as they are related t o the chemistry of these materials and of importance to their application as a fairly bulky mass in surgery. Charnley  reported a n expansion of 3 to 5 vol % for a bone cement during setting. Other investigators have found values between 1.5 and 3%. Although serious objections can be made against the method of measurement used by these investigators [ 5 ] , the general conclusion seems inevitably an expansion of the bone cement. On the other hand, a shrinkage of 22 vol % should be expected for the conversion of M M A to P M M A . As the powderjliquid ratio in bone cements is about 2 t o 1, a shrinkage of about 7% should occur. The seeming contradiction disappears by viewing the microstructure of a bone cement. Micro- and even macropores develop in these materials, if they set under the conditions of orthopedic surgery . The amount of bone cement is so large that high temperature peaks occur in the inside. Air components dissolved in the monomer, air bubbles trapped in the mixture with powder and the monomer itself which has a boiling point near 100°C blow up the cement mass. If setting in a medullar cavity, the cement will force itself into that cavity. Fall of blood pressure and even fat embolism may result eventually. Due to the autocatalytic effect of heat in the polymerization, the peak temperature, and thus the porosity in a given mass of bone cement surrounded by a given structure, will be determined primarily by the
CURING BONE CEMENTS
polymerization rate and thus by the setting time. Therefore, the clinician must be advised to choose a material with a longer setting time in order to decrease the blood pressure fall or the chance of fat embolism. Simultaneously, he must avoid any abundance of bone cement and apply this material where it will contribute to the fixation of the prosthesis. I n addition he must use a cavity preparation technique and a prosthesis designed so that the amount of bone cement necessary for fixation can be limited to the extent to which an expansion is desirable for obtaining retention . I n the present study a working hypothesis was formulated about an additional volume effect occurring in curing bone cement masses. The hypothesis was based on the general consideration that the bone cement mass must cool down to body temperature after setting at or around the peak temperature so that a shrinkage after setting must result.
MATERIALS AND METHODS Two commercial bone cements were investigated. Their characteristics  are known. The femoral cavity was simulated by a hollow steel cylinder having an internal precision diameter of 22.000 & 0.005 mm and a height of 10 cm. A glass rod having a diameter of 10 mm simulated the prosthesis. The cylinder was heated to 37"C, filled with cement and kept at 37°C. After polymerization and cooling down to 37°C the cement lump containing the glass rod was removed from the cylinder and its diameter was measured with a micrometer at 6 heights, while it was kept at 37°C. Then the lump was placed back into its original position within the cylinder and stored at 37°C for 24 hr in water. Finally, the measurements of the diameter were repeated.
RESULTS AND CONCLUSIONS At a certain height the standard deviation in the diameter found in different radial directions was about f 3 p . The standard deviation between the means at different heights of a specimen was about f 5 p . However, the occurrence of a significant diametral shrinkage of the cement lumps containing the glass rod could be assessed (Table 1). The overall mean of the shrinkage of the specimens right after cooling down to body temperature was 81 f 8 p , which amounts to 65 & 7 p/cm for the bone cement material. As the average peak temperature of the curing bone cement is about 100°C and as the coefficient of linear thermal expansion of PMMA is 80 x this shrinkage is equal to the amount of the expected thermal shrinkage within the limits of error. The fact that there is no significant difference between the 2 brands, is consistent with this con-
DE WIJN, DRIESSENS, A N D SLOOFF
t3 L 11 7
After immersion for 2 4 hr in water at 3 7 ’ ~ (urn) 74
4 9 5
clusion. I n the long run there might be a slight compensation effect by water sorption. Immersion in water at 37°C for 24 hr was too short for the detection of such an effect. The apparent contradiction of volumetric expansions accompanied by a linear diametral shrinkage can be explained by the fact that the expansion takes place largely as the cement is still plastic, whereas the thermal shrinkage takes place after curing of the cement. The diametral shrinkage, found in this study, leaves a space between the bone and the cement mass. It offers an explanation for the observations that loosening occurs at that interface and that, i n spite of the volumetric expansion, the cement does not generate strains in the material in which it is confined . Further it may offer an alternative explanation for the occurrence of fibrous tissue between cement and bone after complete healing. This tissue might be formed not by foreign-body reaction alone, but more likely just to fill the gap between bone and cement. The authors are indebted to Mr. van Kesteren for carrying out the experiments.
References [ I ] A . Salomon, in Dental Tissues and Materials, F. C . M. Driessens, Ed., University of Nijmegen. Nijmegen, the Netherlands, 1971, p. I I I .  F. C. M . Driessens and P. M . Hoppenbrouwers, “The wetting of dentinal cavity walls by cold-curing poly (methyl methacrylate)”, paper presented at the 5th Annu. Biomater. Symp., Clemson University, Clemson, S. Carolina, April 14-1 8, 1973. [ 3 ] J . R . de Wijn, J . Biomed. Maler. Res., 8, 421 (1974).  J . Charnley, Acry‘lir Cemenl in Orthopaedic Surgery. E. and S . Livingstone, Edinburgh, 1970, p. 29.  P. S. Walker and M . Bienenstock, Rev. Hosp. Spec. Surg., I , 27 (1971).  J . R . de Wijn, T. J . J . H. Slooff, and F. C . M. Driessens, “Mechanical properties of
C U R I N G BONE CEMENTS
bone cements in vivo and in vitro,” paper presented at the Inter. Ort. Congress on the Knee Joint, Rotterdam, September 13-15, 1973.  J . R. de Wijn, T. J . J. H. Slooff, and F. C. M. Driessens, “Characterization of bone cements,” in press.  T. J. J . H. Slooff, in Dental Tissues and Materials, F. C. M. Driessens, Ed., University of Nijmegen, Nijmegen, the Netherlands, 1971, p. 244.