Effect of OT-Bridge System Versus Multiunit Abutment on the Passive Fit and Stresses Induced by CAD/CAM Fabricated All-on-Four Screw-Retained Prostheses (In Vitro Study)

The hazardous effect of stresses

induced by lack of passivity in screw-retained prostheses necessitates the

selection of a suitable intermediate component between the implant fixture and

the prosthetic framework. Thus, this study aims to evaluate passivity and

stresses induced by CAD/CAM screw-retained prostheses fabricated using the OT

Bridge system compared to the multi-unit abutment anchoring system. Two

All-on-4 digital implant models will be fabricated, one for each group. Group 1;

nine CAD/CAM frameworks will be fabricated employing the OT Bridge system

abutments, while Group 2; nine CAD/CAM frameworks will be fabricated employing

the multi-unit abutments. The passive fit will be assessed through microscopic

measurement of misfit gaps at one terminal abutment, and micro-strain analysis

will be conducted using strain gauges after functional load simulation.

Introduction

The “All-on-4^®” treatmentconcept played a great role in solvingmany 

problems associated with the placement of implants in

anatomically unsuitable ridges. The concept developed by

Paulo-Malo is based on using straight and angled multi-unit abutments (MUA) to provide patientswith an immediatelyloaded full-arch restoration with only four implants. This method advocates tilting distal implants

to enable; longer implant placement, improved prosthetic

support, shorter cantilever arms, increased

inter-implant distance, and improved anchorage in ^(1,2)(1,2) The idea of the OT Bridge fixed prosthetic system (Rhein83,

Bologna, Italy) was born from the need to overcomethe disadvantages of angled multi-unit abutments and to greatly simplifyprosthetic proc⁽³⁾res.(3) This systemis based on the use of the low-profile OT Equator that is foremostused to provide retention for implant-retained overdentures. The morphology of this abutment allows a betterdistribution of the load to the surrounding tissues and it has greater fracture resistance compared to

multiunit abu⁽⁴⁾nts. (4)

The innovation of the OT Bridge systemlies in the use of an “extragrade” titanium abutment and a

seeger system that guarantees the connection stability between the abutmentand OT Equator and passivation

in the presence of serious disparallelism, the unique design of the “extragrade” abutment can overcome high implant divergence even in extreme casesover 80 degrees. The tolerance between the

“extragrade” abutment and the OT Equator

has been designed to compensate for minor misalignments that can be produced duringthe impression and the pouring of the cast mod⁽⁵⁾ (5)  Another important aspect is relatedto seeger's retentive force. The tightness of the “extragrade” abutmenton the equator is not linked so much to the presenceof the connecting

screw but to the mechanical retention given by the Seeger. The clinician will

be able to use the abutmentsin "blind mode" without any connection screw, entrusting theconnection only to the seeger. Thus, it is possible

to realize a fixed full-arch prosthesis by avoiding

unaesthetic holes for the connection ⁽⁶⁾ews. (6)  

Passive fit of implant

frameworks is crucial for achieving long-term success of osseointegration and

preventing future complications^(7-9)9) To assess framework

fit, there are two in vitro approaches: modeling and dimensional measurement

techniques. The modeling techniques (photoelastic analysis, strain gauge

analysis &finite element analysis) are useful to assess the effect of the

inaccuracy of fit of the prosthesis on the implant-bone complex. The

dimensional measurements (stereomicroscopes, optical microscopes &

micro-CT) are done mainly to measure the gap between the prosthesis and the

implant as an indicator for framework⁽¹⁰⁾fit. (10) Strain gauge analysis is a technique for measuring

micro-strains. It is based on the principle that certain materials change their

electrical resistivity when subjected t⁽¹¹⁾force. (11) They are efficient in quantifying

strain, which could give a direct indication of the stress exerted within the ⁽¹²⁾uctures. (12) Microscopes

of differing magnifying powers can be used in vitro to directly measure th⁽¹⁰⁾sfit gap. (10)Microscopy has been used to evaluate the fit of partial or complete arch

implant-supported fixed prostheses connected to external connection implants or

mult^{(13-15,16,17,18} (13-15,16,17,18)

The fixed OT Bridge

prosthesis is reported to have numerous advantages versus multiunit abutments,

however, to the best of our knowledge, no studies of comparison for

misfit-induced stresses between MUA and OT-Bridge are today present in

literature. Thus, the

present study will be conducted in an attempt to investigate one of the

advertised benefits of the new OT bridge system by assessing and comparing the

passive fit of screw-retained CAD/CAM frameworks fabricated employing the

modern OT Bridge system versus the traditional use of MUA as intermediate

abutments in all-on-4 full arch rehabilitations.

Sample Grouping:

This study will comprise two equal groups based on

the type of intermediate abutments employed for the fabrication of the

screw-retained all-on-four implant frameworks, as follows:

Group I: nine CAD/CAM frameworks will be fabricated

on the first model utilizing the new OT Bridge system.

Group II: nine CAD/CAM frameworks will be fabricated

on the second model utilizing the multiunit abutment system.

Laboratory Steps:

A. CAD/CAM fabrication of Digital Implant Models (DIMs):

Two digital implant

models with soft tissue crests will be fabricated for installation of four digital

implant analogs; two straight equidistant analogs anteriorly and two 30-degree

angled analogs posteriorly. During designing, the model creator software will

be used to create the slots for the strain gauges buccal and lingual to each

implant and parallel to their long axes. The CAM files will be sent to a

three-dimensional printer. Two models will be printed, one for each group. The

digital implant analogs will be inserted into the models after their manufacturing.

Four OT equator abutments will be installed to the first model, and four

multiunit

abutments will be installed to the second model.

B. Capturing geometry:

            For each model, the corresponding

scan abutments will be installed. Using the laser scanning technique each model

will be projected & scanned with an intraoral scanner.

C. Digital designing of frameworks:

For each model, the

design of the corresponding frameworks will be virtually accomplished based on

the computed position of the implants and using the virtual abutments in the

digital library available for each of the two systems (Extragrade abutments and

MUA retentive abutments), creating STL files for CAM production.

D. CAM production of implant frameworks: :

Eighteen Chrome-cobalt

frameworks for a hybrid prosthesis design will be fabricated using a 5–axis

milling machine to allow milling of the connection features and the screw

channels.

Methods of Evaluation:

A. Frameworks passive fit:

The passivity of fit will

be evaluated by direct measurement of the vertical gap at one terminus of each framework

under the stereomicroscope; one side of the framework will be screwed to its

respective abutment in the working model using a screwdriver and torque wrench

under torque specified by manufacturer, then retightened after 10 minutes to

avoid preload screw loosening and discrepancies will be observed at the other

unscrewed side. For each specimen, two stereomicrographs at the buccal and

distal surfaces of the unscrewed side will be captured by a digital camera. Images

will be then transferred to the computer software system for analysis and measurement

of the gaps between the margins of the framework and corresponding a⁽¹⁹⁾nts. (19)

B. Micro-strains induced around the implants:  

For micro-strain

analysis, strain gauges will be positioned & bonded in their places in each

model. After functional simulation and load application the micro-strains

induced around each implant will be assess⁽²⁰⁾20)

Data analysis:

The data will first be

recorded manually and then transferred into digital form. The obtained data will be recorded,

tabulated, and statistically analyzed using the appropriate statistical tests.