Abstract
The location of preparation margins may compromise the fixed prosthodontic restoration's resistance form.
This study evaluated changes of opposite wall margin position on rotational resistance form compared to the equal-height margin configuration.
The trigonometric computation of the minimal resistance form preparation taper, α
1, was determined using a previously described formula. The α
1-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 α
1-values represent resistance form with both opposing wall margins at the same vertical height position. The α
2-values were calculated with additional formulas to address uneven margin heights. Calculated α
2-values of 10 degrees or less were considered clinically significant due to the known level of difficulty for the clinician.
As a general trend, the α
2-values were reduced compared to the α
1-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.
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.