Statement of the problem: The location of preparation margins may compromise the fixed prosthodontic restoration's resistance form. Purpose This study evaluated changes of opposite wall margin position on rotational resistance form compared to the equal-height margin configuration.
Material and methods: The trigonometric computation of the minimal resistance form preparation taper, alpha1 , was determined using a previously described formula. The alpha1-values for different tooth sizes with variation of base widths (range 4 to 10 mm) and vertical wall heights (range 4 to 9 mm) were calculated. The alpha1-values represent resistance form with both opposing wall margins at the same vertical height position. The alpha2-values were calculated with additional formulas to address uneven margin heights. Calculated alpha2-values of 10 degrees or less were considered clinically significant due to the known level of difficulty for the clinician.
Results: As a general trend, the alpha2-values were reduced compared to the alpha1-values with shortening opposing vertical wall heights in all tooth-size categories. Clinically significant changes in the resistance form taper were shown in all tooth size categories except the smallest 4-mm tooth base size.
Conclusion: Fixed prosthodontic restoration resistance form is negatively affected by uneven vertical margin placement. This phenomenon reduces the resistance form of the restoration compared to equal-height margin placement.