[A study on the mechanical properties of bone cement (methylmethacrylate) and its strength alteration in vivo (author's transl)].
Bone cement (Methylmethacrylate) is now extensively used for the fixation of various endoprostheses in orthopaedic surgery. Although considerable number of works have been accumulated on the use of this material, relatively little is known about its basic characteristics such as inherent physical properties and the strength alteration in vivo. In recent years, loosening of the prosthesis has become one of the major problems, jeopardizing the use of the prosthesis in conjunction with the bone cement. The aim of this study is to investigate the mechanical properties of the commonly available bone cements and, particularly, to see any chronological deterioration of the material in vivo.


METHODS
1. For the study of the mechanical properties, dumbbell-shaped test pieces were made of three different types of commercially available bone cement (Surgical Simplex, CMW and Palacos R) and tested under tension load using Shimazu Autograph in a temperature and moisture controlled room (Experiment I). 2. The same test pieces (Surgical Simplex and CMW) were imbedded in the paravertebral muscle of rabbits for one, six and sixteen months and, then, tested in the same fashion as in Experiment I (Experiment II). 3. The fracture surface of the test piece after the tension load testing was examined under a scanning electron-microscope to find any changes before and after the imbedding (Experiment III).


RESULTS
1. The bone cement showed a viscoelastic behavior under the tension load in vitro. 2. The modulus of elasticity was the smallest in Palacos R which showed the greatest amount of viscoelastic deformation in vitro. 3. The longer was the imbedded period, the weaker was the tensile strength. The amount of viscoelastic deformation tended to decrease with time, which resulted in the decrease of the total amount of deformation. The modulus of elasticity was the smallest in the sixteen week group in both Surgical Simplex and CMW. 4. The scanning electromicroscopic studies failed to reveal any significant changes on the fracture surface after the imbedding.


CONCLUSIONS
In view of this considerable strength alteration of the bone cement in vivo, the author considers this would be an important factor leading the prosthesis into loosening other than the hitherto proposed various factors such as chemical damage of the bone by the monomer, thermal bone necrosis by the polymerization heat and fatigue fracture of the bone adjacent to the bone cement. While the improved technique in the use of the bone cement is mandatory, the author stressed the importance of further research into the better material or method for the interface between the bone and the prosthesis.
D000818:Animals; D001843:Bone Cements; D004548:Elasticity; D006245:Hardness Tests / Q000295:instrumentation; D007595:Joint Prosthesis; D008768:Methylmethacrylates; D011817:Rabbits; D013997:Time Factors; D014783:Viscosity
D001843:Bone Cements; D008768:Methylmethacrylates