TITLE: 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)
AUTHORS: Abdelrahman Hazem, Shaimaa Lotfy Mohamed, Sara Ibrahim Soliman Mohamed

[DESCRIPTION]
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.

[GUIDELINES]
During designing narrow areas of the model may be at risk of
fracture during functional load simulation


[STEPS]
SECTION: Abstract
1. 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.
SECTION: Abstract
2. 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 bone. (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 procedures.(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 abutments. (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 model. (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 screws. (6)  
Passive fit of implant
frameworks is crucial for achieving long-term success of osseointegration and
preventing future complications. (7-9) 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 misfit. (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 to a force. (11) They are efficient in quantifying
strain, which could give a direct indication of the stress exerted within the
structures. (12) Microscopes
of differing magnifying powers can be used in vitro to directly measure the misfit 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
multiunit abutments (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.
SECTION: Abstract
3.  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. 
SECTION: Abstract
4. 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. 
SECTION: Abstract
5. 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 abutments. (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 assessed. (20)

SECTION: Abstract
6. 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.