The strength of multiple major connector designs under simulated functional loading.

Statement Of Problem: The design of a removable dental prosthesis (RDP) must balance functional strength, comfort to the patient, and the health of the tissue. While research has been conducted to enhance the strength of major connectors, little has been done to determine if the dimensions of major connectors can be reduced in order to enhance patient comfort and tissue health.

Purpose: The purpose of this study was to determine the minimum major connector dimensions of 1 mandibular and 2 maxillary major connectors that would provide adequate functional strength.

Material And Methods: Sixty chromium-cobalt alloy (Vitallium) RDP frameworks were fabricated. The major connector designs were: a mandibular lingual bar, a maxillary palatal strap, and a maxillary anterior-posterior (A-P) palatal strap. Four groups of 5 frameworks with diminishing dimensions were fabricated for each major connector design. The lingual bar was tested at 4, 3, 2.5, and 2 mm in height, occlusogingivally, and 1.6 mm in thickness; the palatal strap at 8, 6, 4, and 2 mm, anteroposteriorly; and the A-P palatal strap at 10 x 6, 8 x 4, 6 x 2.5, and 4 x 2 mm, anteroposteriorly. All maxillary frameworks were 0.65 mm in thickness. The frameworks were of a Kennedy Class II Mod I design with 3 widely separated vertical reference points to measure deformation. Two tests were conducted to evaluate the functional strength for each framework. The first test was masticatory simulation, or torsional force. The second test was a drop test from a height of 3 feet. Permanent deformation was then determined after each test. The Cochran-Armitage test (alpha=.05) was used for both the torsion test and the drop test.

Results: A statistically significant difference in permanent deformation was found for the palatal strap design among the 4 different dimensions for the compressive test (P=.015) and the drop test (P=.044).

Conclusion: It is safe to reduce the dimensions of some major connectors under normal loads. The reduced size of the connectors places the removable partial denture at increased risk for deformation when dropped from a height.