100 R. F. SOGNNAES 



A most interesting pattern of erosion-like defects has recently 

 been reported by Rost and Brodie (1961), whose observations sug- 

 gest that mechanical friction without any chemical action had 

 caused localized destruction, otherwise more typical of erosion. For 

 example, in several cases where amalgam or gold fillings had been 

 placed for the express purpose of repairing such lesions, the eroded 

 pattern was subsequently found on the fillings themselves. In one 

 case the erosion lines were found within 8 months on the surface of 

 a plastic (methyl methacrylate ) filling which had been placed in 

 the center of the labial surface of an upper central incisor for the 

 repair of a small congenital defect. In other cases delicate lines 

 could be seen in the plastic base of dentures. The authors suggested 

 that a possible cause of these lines might be some "hyperactivity" 

 of the soft tissue environment of the structures involved, whether 

 natural or artificial, and that such environmental abrasion might be 

 one of the causes of the lesions observed. Since the plastic material 

 would be resistant to any chemical action encountered in the mouth, 

 one must conclude — in the opinion of this writer — that here are 

 cases in which physical forces have been at work, possibly a twitch- 

 ing of soft tissue elements rubbing against the plastic material or 

 perhaps even streams of saliva continuously running like miniature 

 rivers in confined areas. 



There can be no question that the hard tissues of the mouth are 

 susceptible to destructive actions of mechanical or chemical nature 

 or both. Hence one must expect to encounter mechanical abrasion 

 and chemical erosion as well as combined erosion-abrasion of dual 

 etiology. Calcium phosphate is obviously soluble in calcium-binding 

 chemical agents, whether these be of exogenous or endogenous 

 origin, as long as the agents are present sufficiently long, in adequate 

 quantities, and in susceptible locations. Thus it is not surprising, as 

 noted above, that chemical erosion of rats' teeth occurs after pro- 

 longed ingestion of what in human equivalents would be enormous 

 quantities of acidic beverages. Similarly, it is to be expected that 

 frictional forces will wear away the dental hard tissues. Perhaps it 

 is more surprising that the dental organs can in fact last the pro- 

 longed lifetime of modern man without being either dissolved or 



