Rotational Resistance Form of 2-Dimensional Simulated-Fixed Partial Dentures

Objectives:  This study has been designed to evaluate the rotational resistance form of a simulated-fixed partial denture (FPD).  The maximum axial wall inclinations needed to resist rotation of the FPD around axes adjacent to the abutments were determined.

Methods:  Two simulated-FPD mandibular posterior abutment preparations were represented by 2-dimensional geometric figures (sagittal plane).  Each abutment preparation consisted of known parameters: vertical height (H=3, 4, 5mm), base widths (B_Premolar=6mm, B_Molar=9mm, B_FPD=24mm).  A simulated-1^stmolar-pontic was a rectangular bar-connector with the same horizontal dimension as the 2^ndmolar abutment. The maximal mesial- & distal-axial wall inclinations of each abutment (M_Premolar, D_Premolar, M_2ndMolar, D_2ndMolar in degrees) for resistance to rotation were the dependent variables. The maximal abutment inclinations (Alpha) were calculated with the following formula: Alpha = ˝ [Arc Sine (H/B)]; derivation previously published  Bowley et al, J Prosthet Dent 2004;92:547.  The analysis was performed for rotation around two separate axes at the mesial margin of the premolar and the distal margin of the 2^ndmolar (Axis-D_2ndMolar/-M_Premolar) in two rotational directions, Anterior-Posterior (A-P) and Posterior-Anterior (P-A).  The significance-level was established ≤6-degrees due to the high-level of clinical difficulty to accomplish this axial wall angulation.

Results: 

A-P&P-A Rotation,B-24mm  (Axis-MPremolar/-D2ndMolar)	Alpha (MPremolar&D2ndMolar, degrees)	Sig.
H-3mm	3.6	s
H-4mm	4.5	s
H-5mm	6.0	s
A-P Rotation,B-9mm (Axis-MPremolar)	(M2ndMolar, degrees)
H-3mm	9.6	ns ‡
H-4mm	13.2	ns ‡
H-5mm	16.0	ns ‡
P-A Rotation,B-6mm (Axis-MPremolar)	(DPremolar, degrees)
H-3mm	15.0	ns ‡
H-4mm	20.9	ns ‡
H-5mm	28.2	ns ‡

‡          Minimum wall inclination to provide resistance to rotation & assist/offset

            M_Premolar/ D_2ndMolar axial wall-inclination resistance.

Conclusions:  M_Premolar/ D_2ndMolar FPD abutment axial wall inclinations greater than 6-degrees would demonstrate a lack of rotational resistance at all abutment H-levels. Two other axial walls, M_2ndMolar & D_Premolar, provide rotational resistance to offset the other M_Premolar/ D_2ndMolar deficient walls, at greater inclinations, 9.6-28.2 degrees.