Seating load decisive for fit of long-span fixed dental prostheses

A new in vitro study has investigated how the force used during cementation affects the fit of a six-unit fixed dental prosthesis (FDP).

The study shows that a seating load below 50 N is not enough to achieve an acceptable fit, while increasing the load beyond 50 N does not markedly improve it further. The presence of shear loads during cementation was found to negatively affect the fit.

The study was carried out by researchers at Malmö University - Hanna Josefsson, Anna Fridell, and Anders Falk - together with Per Vult von Steyern, CEO of NIOM.

What is this about?

The fit of a fixed dental prosthesis affects its long-term prognosis. Poor marginal or internal fit can increase the risk of cement dissolution, plaque retention, and secondary caries, which may ultimately lead to loss of the restoration.

While previous studies have investigated seating loads for single crowns, there has been no consensus on which load is required for longer, more complex constructions. This study examined how different axial seating loads, as well as shear loads, affect the fit of a six-unit FDP made of titanium.

What did the researchers find out?

Using the replica technique, researchers tested seating loads of 20, 35, 50, and 70 N, as well as a 50 N shear load, on a six-unit FDP placed on a master cast.

A seating load of 20 N resulted in the poorest fit, with the largest and most variable cement gaps. At 35 N, results varied widely, suggesting the load needed for an acceptable fit lies somewhere between 35 and 50 N. At 50 N, the fit improved substantially and remained consistent, and increasing the load further to 70 N did not provide any additional improvement.

When shear loads were applied, the fit was negatively affected compared to axial loading at 50 and 70 N, particularly regarding the maximum gap values.

What are the key results?

The findings indicate that a seating load below 50 N is insufficient to achieve an acceptable fit for a six-unit FDP, while loads exceeding 50 N do not markedly improve the fit further.

Shear loads during cementation negatively affect the fit and should be avoided. The results also suggest that longer, more complex FDPs may require higher seating loads than single crowns.