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PURPOSE: The purpose of this study is to compare the resistance to fracture of teeth prepared with occlusal, MOD and mesial/distal slot preparations against unprepared control teeth.

MATERIALS & METHODS: 32 non-carious, non-restored maxillary premolar teeth were divided into 3 experimental groups and a control group of 8 teeth in each.  After preparation, teeth were loaded to fracture on an Instron machine using a 5mm loading rod resting on the buccal and lingual cusps at 10mm/min. 

GROUP 1:  Occlusal preparations were prepared with a # 245 bur 1mm wide and 2mm deep. 

GROUP 2:  MOD was prepared with a #245 bur also, with retention grooves prepared with a #169 bur.

GROUP 3:  mesial/distal slot prepared similar to MOD, but with no occlusal extension

GROUP 4:  control group of unprepared teeth

RESULTS: There was no significant difference in fracture resistance among the experimental groups; however, all were less resistant to fracture than the unprepared control group.   The unprepared control group exhibited the highest fracture values.  The MOD and occlusal groups tended more toward to a vertical fracture @ 75%.  The mesial/distal slot preparations demonstrated single cuspal fracture @ 60%.  The unprepared control group tended more to fracture a single cusp @ a 75% rate.

DISCUSSION: Conservative preparation design may limit fracture patterns.  Also, the depth of preparations is more important than the width in determining the fracture resistance of as tooth.  Loss of occlusal integrity was primarily responsible for the weakening of teeth.  The importance of maintaining occlusal dentine led to the development of the ‘slot’ preparation. Cuspal fractures generally originate at the internal line angle on the pulpal floor and exits at the CEJ.  This suggests that the bulk of interaxial dentin allows for a more physiological distribution of forces.  The results were similar to one from a study conducted by El-Mowafy in 1993. 

SUMMARY/COMMENTS: Any preparation tends to decrease a tooth’s resistance to fracture.  It may be wise to combine MO/DO slot preparations with pit and fissure sealants if as a conservative alternative to an MOD restoration when the occlusal surface is caries free.  When possible, the clinician should elect to conserve as much healthy tooth structure as possible as well as design preparations so as to be less susceptible to fracture or failure.