Effect of auxiliary grooves on molar crown preparations lacking resistance form: a laboratory study.
OBJECTIVE
To investigate the effect of auxiliary grooves on resistance to dislodgment of crowns on compromised molar preparations lacking resistance form.


METHODS
Thirty human molar teeth were randomly assigned to three groups of ten, and prepared to a height-to-width ratio of 0.3 with a total convergence of 50 degrees , and 1-mm shoulder margin. Base metal alloy copings were constructed with a 45 degrees loading platform and cemented with zinc phosphate cement under a 50 N load. Initially, resistance testing was conducted using a Universal Testing Machine (Instron) at 1 mm/min for all 30 specimens. Following crown dislodgment, Group 1 copings were recemented and retested, Group 2 had one axial groove added, and Group 3 had two axial grooves added. New copings for Groups 2 and 3 were made, cemented, and again tested for resistance. Standardized radiographs were taken prior to initial cementation and scanned into digital images. The percentage of area occupied by the pulpal chamber above the acrylic mounting (PS), and the closest distance to pulp from the preparation surface (CD) were measured.


RESULTS
Recementation or the addition of one groove did not affect the dislodgment values (p > 0.05), but addition of two grooves caused a highly significant increase in resistance (p < 0.001). Regression analysis showed an inverse relationship between initial resistance values and pulpal space area. Lower resistance values were observed when the pulpal space area was large (p= 0.004).


CONCLUSIONS
Crowns can be recemented without affecting resistance to dislodgment. Two grooves should be incorporated into compromised molar crown preparations to increase resistance form. Teeth with large pulps and therefore less coronal dentine have poorer resistance form, and therefore would benefit from placement of auxiliary grooves.
Lu
Po-Ching
PC
Prosthodontist, Private Practice, Sydney, AustraliaAssociate Professor in Restorative Dentistry, School of Dental Science, The University of Melbourne, Melbourne, Australia.
Wilson
Peter
P
Prosthodontist, Private Practice, Sydney, AustraliaAssociate Professor in Restorative Dentistry, School of Dental Science, The University of Melbourne, Melbourne, Australia.
D002484:Cementation; D002858:Chromium Alloys / Q000737:chemistry; D003442:Crowns; D003722:Dental Alloys / Q000737:chemistry; D017267:Dental Prosthesis Design; D017268:Dental Prosthesis Retention / Q000379:methods*; D003782:Dental Pulp / Q000000981:diagnostic imaging; D003786:Dental Pulp Cavity / Q000000981:diagnostic imaging; D003804:Dentin / Q000000981:diagnostic imaging; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008422:Materials Testing; D008963:Molar / Q000000981:diagnostic imaging; D011859:Radiography; D013314:Stress, Mechanical; D013499:Surface Properties; D019231:Tooth Preparation, Prosthodontic / Q000379:methods*; D015036:Zinc Phosphate Cement / Q000737:chemistry
D002858:Chromium Alloys; D003722:Dental Alloys; D015036:Zinc Phosphate Cement