Selection of abutment teeth

SELECTION OF ABUTMENT TEETH

Deepak K Gupta

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Factors for selecting abutment teeth

1. Crown-root ratio2. Root configuration3. Periodontal ligament area4. Biomechanical consideration

a) Span lengthb) Direction of forcec) Secondary abutmentsd) Arch Curvature

5. Health of periodontium6. Endodontic consideration


Crown-root ratio• level of the alveolar bone moves apically , the lever

arm of the portion outof bone increases, and the chance for harmful lateral forces increases

• Optimum ratio – 2:3, acceptable – 1:1

• If the opposing tooth is artifical then a lower ratio can be acceptable


Root configuration

• Labiolinguallyconical root –more stable compared to circular.

• Apically divergent > apical convergence


Periodontal ligament area

Ante’s Law: for the observation that the combined pericemental area of all abutment teeth supporting a fixed dental prosthesis should be equal to or greater in pericemental area than the tooth or teeth to be replaced


Periodontal ligament area

• Nyman and Ericsson doubted on the validity of this law.

• Clinically lower surface area of abutment teeth has been equally supportive provided a good periodontal health has been maintained.


Span length

• Relative deflection

– directly proportional (span length)3

– Inversely proportional (Occlusogingivalthickness) 3


Direction of force• The dislodging forces on a FPD retainer tend to act in a

mesiodistal direction, as opposed to the more common faciolingual direction of forces on a single restoration.

• Preparations should be modified accordingly to produce greater resistance and structural durability


Secondary Abutment

• Overcomes several problems– unfavorable crown-root ratios – long spans

• Secondary Abutment should be comparable to primary in terms of :• root surface area • favorable a crown-root ratio• Retainers

• Because when pontic flexes, tensile forces will be applied to the retainers on the secondary abutments

• sufficient crown length and space between adjacent abutments to prevent impingement on the gingivaunder the connector


Secondary Abutment


Arch curvature• When pontics lie outside the

interabutment axis line - act as a lever arm, which can produce a torquingmovement.

• Mainly if pointed in the anterior

• Secondary retention (R) must extend a distance from the primary inter-abutment axis equal to the distance that the pontic lever arm (P) extends in the opposite direction.


Health of periodontium

• Healthy periodontal tissues are a prerequisite for all fixed restorations

• Adequate crown to bone ratio – inadequate periodontal health turns prognosis to poor in long run where turns to be worst in case inadequate crown to bone ratio

• Fixed prosthesis has been successful even in the presence of inadequate crown to root ratio after periodontal tissues have been returned to excellent health and long-term maintenance has been ensured


Endodontic consideration

• Teeth in which pulpal health is slightly doubtful also should be endodonticallytreated before the initiation of fixed prosthesis

• If unhealthy pulp turns to be deleterious it will jeopardize the prognosis of the give fixed prosthesis

• Endodontically treated teeth are more favourable for a giving a fixed prosthesis


Pier abutments

• An edentulous space can occur on both sides of a tooth, creating a lone, freestanding pier abutment.

• There is different faciolingual and intrusive force on different teeth in different archs


Pier abutments• If a long span fixed

prosthesis is given on this, it will create huge stress on the terminal abutment and pier abutment will acts as a fulcrum and failure of prosthesis

• Different views regarding:– Rocking of retainer – Bending of retainer– Tension between

abutment and retainer– Intrusion of retainer on

abutmentfacebook.com/notesdental

Pier abutments• Two alternatives are

there to minimizes the stress i.e.

a. Non rigid connector: broken stress mechanical union of retainer (dovetail keyway) and pontics (T -shaped key).

b. Cantilever (1st premolar pontic): adequate periodontal support


Tilted molar abutments• Early loss of a mandibular

first molar with mesialtilting and drifting of the second and third molars.

• Impossible to achieve common path of insertion.

• In an attempt to do excessive preparation has to be done or mesiallytilted 3rd molar will not allow seating of prosthesis


Treatment modality

• Extract the third molar and upright the tilted 2nd molar orthodontically

• fixed appliance– premolars and the canine

are banded and tied to a passive stabilizing wire

– A helical uprighting spring is inserted into a tube on the banded molar

– activated by hooking it over the wire on the anterior segment


If orthodontic correction is impossible

• Modified preparation design.

– Proximal half crown : ¾ crown.

– Non-rigid connector on the distal aspect of the premolar retainer compensates for the inclination of the tilted molar


Telescopic crown

• used as a retainer on the distalabutment

• A full crown preparation with heavy reduction is made to follow the long axis of the tilted molar

• inner coping is made to fit the tooth preparation

• the proximal half crown that will serve as the retainer for the fixed partial denture is fitted over the coping


Canine-replacement fixed partial dentures

• Difficult - it often lies outside the interabutmentaxis

• Maxillary more difficult than mandible due to labially and lingually acting force respectively,

• No fixed partial denture replacing a canine should replace more than one additional tooth


Cantilever fixed partial dentures

• FDPs in which only one side of the pontic is attached to a retainer,

• long-term prognosis of the single abutment cantilever is poor.

• Vertical – tipping

• Horizontal forces –rotation of abutment teeth



• 3 unit FPD, resist forces much better since the teeth have to be moved bodily rather than merely rotated or tipped



• Essential requirement for abutment teeth.

– lengthy roots with a favourable configuration,

– Long clinical crowns,

– good crown-root ratios,

– healthy periodontium


Maxillary Lateral Incisor – Cantilever FPDs

• no occlusal contact on the pontic in either centric or lateral excursions

• canine must be used as an abutment - root configuration of a central incisor makes it an undesirable cantilever abutment.

• solo abutment – only if it has long root and good bone support

• Metallic rest on the distal of the central incisor to prevent rotation of the pontic and abutment facebook.com/notesdental

first premolar - Cantilever FPDs

• best if occlusalcontact is limited to the distal fossa

• Full veneer retainers are required on both the second premolar and first molar

• excellent bone support


Molars• When there is no distal

abutment present

• Pontics prequisite– possess maximum

occlusogingival height to ensure a rigid prosthesis

– Light occlusal contact with absolutely no contact in any excursion


Molars

• When the Pontics loaded occlusally, the adjacent abutment tends to act as a fulcrum, with a lifting tendency on the farthest retainer

• Minimize the leverage effect,– the pontic should be kept

as small as possible– more nearly representing

a premolar than a molar


References

• Contemporary Fixed Prosthodontics, 4th Edition - ROSENSTIEL

• Fundamentals of Fixed Prosthodontic 4th edition - Herbert T. Shillingburg


Health & Medicine

Selection of abutment teeth