Fractures of clasp-retained partial dentures.

Fractures of cast clasps are practically irreparable and impaire both thera­ peutic and prophylactic value of partial dentures. On purpose to explain causes of this rather frequently occuring damage, examination of fracture surface with optical and electronic microscopes and assessment of stress in the retainers were carried out. Microscopic examinationes comprised fracture surfaces of 30 cast clasps divided into 2 groups: Group I (control group): cast clasps formed and cast 'under controlled laboratory conditions; Group II (test group): fractured cast clasps. It was found that surfaces of cast clasp fractures had a complex structure containing developed dendrites and separated carbide phases. In contrast to the control group, fracture surface from the test group showed signs of alloy fatigue, increased porosity and presence of new phases. On base of the analysis of partial denture movement it was assessed that stress in cast clasps approached both proportionality and fatigue level. In the conclusion it is pointed to two main factors causing cast clasp fractures: plastic deformation and material porosity.