The Effect of Radiation on the Shear Strength of Acrylic Bone Cement JOHNP. SCULLIN, M.D., A. SETHGREENWALD, D. PHIL. (OXON), ALANH. WILDE,M.D. AND R. DAVID BECK,M.D.
Bone loss due to secondary neoplastic disease of bone has frequently presented the orthopedic surgeon with difficult, or insoluble problems of surgical management. Since 1971, the use of polymethylmethacrylate cement in conjunction with internal fixation has provided an excellent means of solving the mechanical problem of bone loss. However, residual tumor cells remain a biological problem with the potential of recreating the mechanical problem. Local radiation is the most effective and widely used form of therapy for this type of metastatic disease, and is almost invariably administered within a few weeks of surgery. The general assumption has been that clinical levels of radiation have no significant effect on the mechanical properties of methylmethacrylate. One study has reported that surface clinical doses cause discoloration of bone cement and that 45-160 million rads cause it to fail with hand pressure.2 Another study utilized specimens of bone cement which had been machined in lathes and
milled prior to irradiation and mechanical testing. They found no statistically significant difference in mechanical properties of irradiated and non-irradiated bone cement.' There has been n o direct study of the effect of irradiation, on the mechanical properties of bone cement as it is used by the orthopedic surgeon. T h e purpose of this experiment was to determine the effect of clinical levels of radiation o n the shear strength of hand molded polymethylmethacry late. MATERIALS A N D METHODS Using a previously described technique.3 Surgical Simplex P Bone Cement was injected in cylindrical molds measuring 0.5 inches in diameter by 1.5 inches in length, and packed by finger pressure. Sixty specimens of plain methylmethacrylate, and 65 specimens of Radiopaque methylmethacylate containing 10% barium sulfate, U.S.P., were so obtained. The specimens were then divided into a control group, and groups to be exposed to, respectively, 2500 rads, 3000 rads, 5000 rads,10,000 rads, and 20,000 rads; each group consisting of plain and radiopaque specimens. After a 10-14 day period, simulating the clinically observed delay from the time of operation to the time of initiation of radiotherapy, the specimens were immersed in a pan with a 3 cm level of water. to obtain a near equilibrium distribution of radiation. The specimens were then exposed to radiation at a
From the Cleveland Clinic Foundation Cleveland, Ohio. Reprint requests: A. Seth Greenwald, D. Phil, Cleveland Clinic, 9500 Evelid Avenue, Cleveland, Ohio, 44106. Received: April 12, 1977. 20 I
Clinical Orthopaedicr and Related Research
Scullin, et al.
ated in the final determinations. The results were then subjected to statistical analysis by means of the student “t” test.
rate of 400 rads per minute as generated by a 10 mev linear accelerator. The groups were successively removed from radiation once having obtained the assigned total dosage for that particular group. Mean diameter of each specimen was determined from 3 micrometer measurements of each individual specimen. Within 24 hours, the specimens were subjected to single shear testing on an Instron Testing Machine according to a previously described protocol (Fig. l ) . 3 The mean shear strength for each group was determined from the following expression: Shear Force at Failure Mean Shear Strength= N&=, Cross Sectional Area i where N is the number of specimens studied. Any specimen with grossly evident bubble or other defects at the shear interface was elimin-
A n initial comparison between the shear strength of plain and radiopaque specimens within each group revealed n o significant variation in shear strengths. When the irradiated plain specimens were compared against the plain control specimens, there was n o statistically significant difference in shear strength between controls and specimens at any level of exposure tested (Table 1 ) . Table 2 shows similar data of comparison of the radiopaque specimens also revealing n o consistently significant diminution of shear strength at the various levels of exposure.
DISCUSSION The objective of this study was to simulate clinical conditions as closely as possible, while defining a physical parameter of the bone cement. Standard sized specimens, however, are necessary to obtain meaningful statistical results. Eftekhar and Thurston’
FIG.2. Bar graph indicating the effect of irradiation on the shear strength of Surgical Simplex P.
I A D I A l I O N D O I I G E (rods)
Number 129 November-December, 1977
Bone Cement Strength
TABLE 1. The Effect of Irradiation on the Shear Strength of Surgical Simplex P Radiation Dosage
Control 2500 r 3000 r
Number of Specimens
Mean Shear Strength (Ibs/in*)
Student t Value
- 2.50008 - 2.06928
Level of Significance
ns* ns ns ns p < .025 ns
* ns -non-significant. (To express values of shear strength in kg/cm', multiply by 0.07).
used lathing and milling processes to obtain appropriately sized specimens. While both their control and experimental specimens were treated in similar manner, the heat and stress of milling and lathing are likely to produce different ultimate results as compared to unmachined specimens. We felt that a molding technique utilizing finger pressure would more closely simulate actual surgical conditions. The clinical dosage of radiation for malignant disease of bone at the Cleveland Clinic is 3000 rads delivered to the site. This is divided into 300 rad doses administered over a 10-14 day interval. Within the range of
radiation dosages tested, there was no demonstrable deleterious effect on the shear strength of methylmethacrylate. Indeed, levels of radiation over 6 times the usual clinical dosage did not significantly alter the shear strength of bone cement, with or without barium sulfate (Fig. 2). This is in general agreement with the results of Eftekhar and Thurston.' They measured flexion, torsion, and compression in Simplex P and CMW Bone Cements following irradiation with 10,000 rads. Though they did not measure shear strength, they too found no significant effeot of radiation on the bone cement.
TABLE 2. The Effect of Irradiation on the Shear Strength of Surgical Simplex P with Barium Sulfate ~
Number of Specimens
Mean Shear Strength (lbs/inP)
Student t Value ~
Control 2500 r 3000 r 5000 r 10,OOO r 20,000 r
- 1.70668 - 2.14S3.6 - 0.98552
* ns -non-significant. (To express values of shear strength in kg/cmP, multiply by 0.07).
Level of Significance
p < .025
ns* ns p < .05
Scullin, et al.
SUM'MARY Bone loss due to secondary neoplastic disease of bone has frequently presented the orthopedic surgeon with difficult or insoluble problems of surgical management. Local radiation is the most effective, and widely used form of therapy for this type of metastatic disease. Levels of radiation in vitro comparable to therapeutic doses, as well as levels 6 times that commonly used in vivo on patients, demonstrate no significant effect on the mechanical properties of acrylic bone cement.
Clinical Orthopardicr and Rdatrd Rrrrarch
ACKNOWLEDGMENTS Grateful acknowledgement is made to Mr. Edward Chernak, Radiation Physicist, who kindly conducted the irradiation of the specimens. REFERENCES 1. Eftekhar, N. S. and Thurston, C. W.: Effect
of irradiation on acrylic cement with special reference to fixation of pathological fractures. J. Biomech. 8:53, 1975. 2. Murray, J. A., Bruels, M. C. and Lindberg, R. D.: Irradiation of polymethylmethacrylate, J. Bone Joint Surg. 56A:311, 1974. 3. Wilde, A. H. and Greenwald. A. S.: Shear strength of self-curing acrylic cement, Clin. Orthop. 106:126, 1975.