Abutment selection in Fixed partial denture

Describes how to do Abutment selection in Fixed partial denture

1. Abutment Selection in Fixed Partial Dentures

2. Specific learning objectives… Define an abutment Types of abutments Criteria for abutment selection Clinical evaluation of abutment Evaluation of abutments on mounted casts Radiographic evaluation of abutment

3. Contents: • Introduction • Definition • Assessment of abutment teeth • Criteria of selection • Factors governing abutment selection Crown Crown to root ratio Root configuration Root surface area • Biomechanical Consideration – Length of span – Height of connector – Double abutment  

4. Types of Abutment – Pier Abutments – Cantilever Fixed Dental Prosthesis – Tilted Molar Abutments – Canine replacement FDP Conclusion References 

5. introduction Every restoration must be able to withstand the constant occlusal forces to which it is subjected. This is of particular significance when designing and fabricating a fixed partial denture, since the forces that would normally be absorbed by the missing tooth are transmitted, through the pontic , connectors, and retainers, to the abutment teeth.

6. Abutment teeth are therefore called upon to with- stand the forces normally directed to the missing teeth, in addition to those usually applied to the abutments.

7. definitions Fixed Partial Denture – A partial denture that is luted or otherwise securely retained to natural teeth, tooth roots, and/or dental implant abutments that furnish the primary support for the prosthesis. -GPT8 ABUTMENT – That part of a structure that directly receives thrust or pressure; an anchorage . Or A tooth, a portion of a tooth, or that portion of a dental implant that serves to support and/or retain a prosthesis . -GPT8

8. ASSESSMENT OF ABUTMENT TEETH • Considerable time and expense are spared, and loss of a patient’s confidence can be avoided, by thoroughly investigating each abutment tooth before proceeding with tooth preparation. • Radiographs are made, & pulpal health is assessed by evaluating the response to thermal & electrical stimulation.

9. The primary purpose of roentgenograms is to disclose hidden areas and structures such as – Root length and morphology – Pulp outline – Periodontal ligament space – Alveolar bone – Infrabony defects – residual roots – impacted or supernumerary teeth – The extent of present or past caries – Evaluate root canal filling Assessment of Abutment Teeth 

10. • Existing restorations, cavity liners, & residual caries are removed & a careful check is made for possible pulpal exposure. • In younger adults generally the pulp horns are high & broad, but as age progresses there is receding of the pulp horns & also sometimes calcification occurs.

11. SELECTION OF ABUTMENT TOOTH • A tooth can be used as an abutment if it is; 1. VITAL 2. ENDODONTICALLY TREATED(and asymptomatic)

12. Abutment teeth should not be 1. A teeth that is pulp capped in the process of preparing tooth should not be used before they are endodontically treated. 2. A mobile tooth.

13. • The periodontal tissues should be healthy and free from inflammation. • The roots and the supporting structures should be evaluated for 3 factors; A. crown-root ratio B. root configuration C. periodontal ligament area CROWN-ROOT

14. CROWN TO ROOT RATIO: This ratio is the measure of the length of tooth occlusal to the alveolar crest of the bone, to the length of the root embedded in the bone. • If the ratio is high, it is less likely that the tooth will be able to withstand additional occlusal forces. The problem is even greater when nonaxial ( faciolingual ) forces act on the prosthesis.

15. As the Level of the alveolar bone moves apically , the lever arm of the portion out of 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

16. ROOT CONFIGURATION • Labiolingually conical root – more stable compared to circular. • Apically divergent > apical convergence

17. Abutment teeth should possess adequate root anchorage in the bone to effectively resist and transmit the occlusal load. The length of the abutment root is directly proportional to the stability and strength of the prosthesis

18. Periodontal ligament area Larger teeth have a greater surface area and are better able to bear added stress. the areas of root surfaces of the various teeth have been reported by Jepsen and are shown below;

19. Ante’s law states that ‘the combined pericemental area of the abutment teeth should be equal to or greater than the pericemental area of the tooth or teeth to be replaced’.

20. To make this a mathematical calculation, average values for root surface area of permanent teeth were given by Jepson (1963)

21. According to this law one missing tooth can be successfully replaced by taking two abutments for support. If two teeth are missing, they can be replaced taking support of two abutments, but the limit is being reached. It is unacceptable to replace three teeth with two abutments

22. Biomechanical considerations As the length of the edentulous span (number of teeth being replaced) increases, there is an increased load on the abutments and the FPD also flexes more. The flexure is directly proportional to the cube of length and inversely proportional to the cube of occlusogingival thickness of pontic .

23. That is, an FPD with three pontics will flex 27 times (3 × 3 × 3) times more than an FPD with one pontic . Also if the thickness of the pontic is halved, the FPD flexure will be eight times greater

24. Double abutments can be used to distribute the load by reducing leverage forces in long spans . To reduce flexure in long-span FPDs, pontics could be chosen with greater occlusogingival dimension. Alloys such as nickel-chromium which have high rigidity also reduce the flexure.

25. 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.

26. • sufficient crown length and space between adjacent abutments to prevent impingement on the gingiva under the connector .

27. 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.

28. Arch form • When pontics lie outside the interabutment axis line - act as a lever arm, which can produce a torquing movement. • Mainly if paced 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.

29. Special problems 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 arches.

30. In this situation, if a five-unit rigid FPD is planned by using all the natural teeth adjacent to the edentulous areas as abutments, forces are transmitted to terminal retainers as a result of the intermediate abutment (pier) acting as fulcrum, causing failure (loosening) of a weaker retainer (anterior).

31. tension between the terminal retainers and their respective abutments rather than a pier fulcrum caused intrusion of the weaker abutment which lead to failure (loosening) of a weaker retainer (anterior). These loosened castings will cause marginal leakage and secondary caries.

32. The use of a nonrigid connector has been advocated to overcome this problem. It is a broken stress mechanical union of retainer and pontic , the movement in it is enough to prevent the transfer of stress from the segment being loaded to the rest of FPD. It consists of a T shaped key attached to the pontic , and a dovetail key way placed within a retainer.

33. The location of this device is important. It is usually placed on the middle abutment, since placement on the terminal abutments could result in pontic acting as a lever arm. The key way should be placed within the distal contour of pier abutment and key, on the mesial side of distal pontic .

34. This seats the key more solidly into the keyway as forces usually are mesially directed due to the mesial inclination of posterior teeth. Another option is to cantilever the premolar pontic provided the periodontal support of the abutments is adequate

35. Tilted molar abutments Early loss of a mandibular first molar with mesial tilting 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 mesially tilted 3rd molar will not allow seating of prosthesis.

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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.

42. 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

43. 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 .

44. 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

45. Canine replacement fixed partial dentures • Replacing canine with a fixed partial denture is often difficult as the canine lies outside the interabutment axis and as described earlier, the fulcrum line is labial to the arch circumference. Hence, the abutments are subjected to increased stresses.

46. The prospective abutments are the lateral incisor, weakest tooth in the arch and the first premolar, weakest posterior tooth. No FPD replacing a canine should replace more than one additional tooth.

47. The forces acting on the maxillary canine act outward, labially , subjecting it to a greater stress, as compared to the mandibular canine where the forces act lingually .

48. Questionable Abutment • Definition – A common dilemma especially when the abutment is periodontally involved, partially fractured, polycarious or in poor arch position. 

49.  Disturbances In Size of Teeth – Microdontia – Macrodontia Disturbances in Number of Teeth – Anodontia (Complete or Partial) – Supernumerary Teeth 

50. conclusion Success of a prosthesis depends on many foundational steps taken to prepare it. Proper handling of the abutment teeth is one of the most important foundational steps that either enhances or detracts from the eventual value of the prosthesis.

51. when conditions are proper like crown contour ,retention and criteria of good preparation techniques and design are met ,sound abutment considerations will be strong link in the success of the prosthesis. selecting a suitable abutment forms the preliminary treatment planning for FDP,whose proper selection and preparation aids in long term durability of the restoration