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Assessment of Biological Properties of Implants before Their Clinical Use

gested, and the idea has gained almost universal support, that so-called 'undifferentiated connective tissue cells' or 'primitive mesenchymal cells', can in certain circumstances be induced to become osteoblasts by a process akin to that which takes place during embryonic development. The most thorough investigations, utilizing bone treated in various ways and transplanted into soft tissues, have suggested that the hypothetical 'inducer' is a part of the organic component of preformed bone (Urist et al. 1967, Burwell 1966). The spontaneous growth of bone inside an implant of polyHEMA sponge, in the skin, by a process of intramembranous ossification, demonstrates conclusively that in this case the initiation of heterotopic ossification does not depend on the presence of preformed organic or inorganic components of the skeleton, or its cells. This interesting phenomenon requires further analysis, but meanwhile it may be pointed out that in the sponge the deposition of calcium phosphate mineral very clearly precedes ossification. The same is true of endochondral ossification and of bone formation in the ischwmic kidney and some other examples of heterotopic bone formation. As a working hypothesis, therefore, it is postulated that insoluble calcium salt is the moderator which determines that the newly formed connective tissue cells, originating most likely from mononuclear cells brought to the site in the blood vessels, shall proceed to manufacture bone. It is probable that other local factors, including mechanical ones, are important in determining the kind of connective tissue that will be formed.