Biomechnical considerations and Configurations of FPDs

Abutment Evaluation

Why do we need to evaluate abutment when we plan for FPD / RPD ?The idea is it support the prostheses then it will carry the hole load .It's all retained load + the new load of the missing teeth . when extract the teeth and we do replacement , the abutment teeth carry the load of itself + the missing teeth . so we need to evaluate the abutment before fabricate our prostheses .

Abutment evaluation mainly 2 :

1- pulpal assessment of the abutment .2- the supporting tissue :♥ crown – root ratio .♥ Root configuration .♥ periodontal ligament area .

■ Pulpal assessment :

In fixed prosthodontic always vital teeth are preferable . it could be nonvital , if it's non vital and it's endo treated it must be properly evaluated . so either vital or proper endo treated teeth , this endo treated teeth should not support more than 3 unit bridge . if we have upper right 6 pulpal treated and we have missing 5 and 4 then the 6 is a pulpate abutment to carry the load from the canine to 6 you can't use it to carry 4 unit bridge . so endo treated teeth the guideline to use it when we have 3 unit bridge OK . 3 unit bridge is the max extension of the bridge to use it if have endo treated teeth as abutment . in between either we have vital or endo treated teeth some time

you could have pulpal – capping . this pulp-capped teeth are not not advised to be used as abutment it's contraindication . the reason is 8 % of sound abutment after preparation within 10 years they will be pulpal involved ( periapical infection , pulpitis , whatever ) this mean pulp-capped teeth have high risk … clear

Dr said if you don't understand come to me in clinic could be do this procedure .

■ Crown – Root Ratio :

The definition is the amount of tooth structure above the alveolar crest and the amount of structure below the alveolar crest . this is the ratio part of the tooth which is outside the alveolar crest and part of tooth which is inside . this is crown-root ratio . ideally it's 2 : 3 when we use it for fixed prosthodontic abutment ideally should be 2 to 3 .In the exam if I said a tooth 40 mm length then the ideal crown-root ratio is ?16 mm is the best crown-root ratio .Now the worst thing is to have 1 : 1 ratio , so when you have 50 % out of the alveolar crest , the resistant area of the tooth to the load is less… right , and the resistant area of the periodontal ligament area is less . the such scenario we could accepted depending on the opposing occlusion . in another word the natural teeth when bite the load per abutment about 70 – 75 kg when you have opposing partial denture the load is about 30 kg , when you have bridge the load about 26 kg .so the opposing occlusion will determined the amount of load will be retained by the bridge … clear If I have 2 to 3 ratio this is the best scenario in all cases , but if I have 1 to 1 then I looking for more acceptable occlusion which is if I have opposing acrylic partial denture or CD or I have bridge because the load will be less than natural teeth .

imagine I said in the exam you have crown-root ratio 1 : 1.5 , this is the best option for ?

○ acrylic partial denture .○ cobalt partial denture .○ fixed partial denture .○ implant prostheses .○ natural teeth .

The more load from the crown-root ratio the less load you need , the need load with acrylic prostheses more than cobalt crom and the reason the cobalt crom supported by teeth .

You have to concentrate on two thing :- the ratio itself .- the opposing occlusion .

■ Root configuration :

- Triangular root better than round ones .The more irregular structure inside alveolar bone the better the resistance because more irregularity it's more surface area more periodontal ligament area more resistance .- Multi-rooted are better than single-rooted .- Widely spaced roots are preferable .If you have upper 6 the root separate from each other better from fused root because the surface area become bigger . - irregular roots are better .that's mean curved lateral incisor better than typical lateral incisor .

■ periodontal ligament area :

- The greater the area is better .

As we said before the greater area you will have more resistance , it's a relative values . 4ex : mine lower 6 have surface area it's 25 mm and sharif has lower 6 it's 40 mm surface area this not matter , the important thing is 40 mm the 7 and 5 adjacent of it . ya3ni 7 and 5 for sharif bigger or same mine 7 and 5 . so the relative value is the important thing not the absolute figure .

- Ante's law is not valid always .the periodontal ligament area of the abutment should be equal or more than periodontal ligament area of the missing teeth . if it's not then you can't fabricate a bridge .in another word you have a missing 6 the periodontal ligament area of the 7 and 5 should be equal or more than periodontal ligament area of the 6. this is called Ante's law it's biomechanical because you are not consider periodontal status of your abutment, you have a good periodontal ligament area but it's infected so it's biomechanical law , it's not always correct .- Bridge with more than 2 posterior teeth have high risk .- Teeth with poor periodontal support could be used if you

have experienced clinician and well motivated patients .The percentage in the table 3-1 is important for us as Dr said .The first molar have the highest surface area

Slide 3-page 2Again this is Ante's law if you follow Ante's law first ex it's OK , second picture OK we are within the limit because the first premolar almost the same size of the second premolar and the second molar less then first molar we are a broaching the border . the same case no way to fabricate the bridge according to Ante's law , but if you have opposing partial denture if you have good crown-root ratio if you have good periodontal health then I can discharge the Ante's law and fabricate my prostheses .

Biomechanical considerations

- bending and deflection of bridges .- Dislodging forces on FPD .- Primary and secondary abutment .- Arch curvature and torque movements .

Slide5-page 2Beam theory is the deflection of the structure under load .If you have a solid structure mounted between two points and apply a load in the middle of that structure this structure , the longer structure the more deflection , the thicker structure the less deflection . the relation is cubic .

4ex if you have mesiodistal root is deep with apply force it will deflect in certain amount . visa versa on thickness , the thicker structure the less bending .

The movement in fixed partial denture imagine 4 unit bridge extending from premolars to molars , there is o way to move buccolingually it will move mesiodistal the reason if you look at the load this load it will go to the center of prostheses there will be a deflection forces on premolars and molars . when you apply the force on the bridge it will be deflect downward , now what happen in the borders ?It will go upward , now this movement we need to resist it .The premolar move backward and the molar move forward we need something to prevent this movement . we have something called grove . we do the groves on the buccal and lingual surfaces in preparation . now part of the metal crown stick out , this projection inter inside this groves and prevent backward forward movements this form called retentive form .

Slide 6-page 2 :- Double retainer We have 6 unit bridge from 7 to canine replacement 6 and 5 there is a concept called double retainer . the first retainer the 7 called primary retainer and the retainer on the 8 called secondary retainer and this system called double abutment because we have two abutment at one side and two in the other side . why do we think we need double retainer sometimes ? if you follow Ante's law you will find 7 and 4 they have insufficient periodontal ligament area to replace 6 and 5 . so you need to add farther surface area this farther surface area you get it from secondary abutment . The problem in this system you need comparable length and crown-root ratio at primary and secondary abutment . the reason if you have a load and where is the weakest link here ? the 8 and the canine . when apply a load the deflection will happen in the most terminal abutment the canine and 8.If you have poor abutment then the long run you will loss them because the load on them and there will be recurrent caries on 8 and canine then somebody can tell me you can add resistant groves on 8 and canine …. Clear .Your primary and secondary abutment should be comparable in crow-root ratio , periodontal ligament space , resistant form , retentive form we will talking about it in the next lecture .Now in all scenario the pontic is usually in the inter abutment line . draw straight line between the two abutment and the bridge must be on this line . in the upper anterior segment it's no way come on this line because you are replacing the original position of teeth to get the better aesthetic for your patient if the pontic outward from this line , meeting load on the canine will be favorable or not ?

وياه : آنا علينا مثال قال الدكتور

نقدر راح موازي بشكل الدكتور وبين بيني صندوق حطينا لولنا . موازي وغير بالجنب الصندوق كان لو من أسهل نرفعه

Now if we have v-shaped arch , meeting the load is favorable or unfavorable ?Favorable , if you have more lever arm then you need more resistant arm .

Special problemsSlide 3-page 3

1-Pier Abutments ( الوسيط )It's a lone freestanding tooth with edentulous spaces on both sides .If you have pier abutments and you have missing teeth on both side and you have replacement , who is know how much the physiological movement of teeth in the socket ?The max 108 micron could move buccolingual . this movement is not fixed on each tooth , the anterior move more than posterior . in this bridge the 7 will move less than canine and the 5 will be in between if you joint deferent structure together and they move in deferent level .

ربط : فيها المسألة لو حتى األمثلة نكتب الزمفينا واحد وكل الشيخ وحسين والدكتور آنا ربطتونا لو يعني

؟ بيصير شو ، تختلف حركتهأصحاب . بنبطل

There will be unfavorable load distribution . providing the movement are at angle , ya3ni the canine will move more or less labialy . the 7 will be more mainly buccaly and the 5 will be in between . if you have deferent movement rate and you have deferent movement direction that's mean you will have unfavorable deferent load distribution because a certain points they will be a lever action . so we have high stress on the 7 and canine during loading . now to avoid this we need create something to prevent them to moving together this

called fixed movable joint this joint will break the stress at the distal side of 5 for this reason in the abutment we used fixed movable design and n this design the 7 will move separately .

Slide 5- page 3

2-Tilted molar abutment :The most common scenario when you loss the lower 6 the 7 usually move forward . you have to have a common path of insertion parallel . this is can corrected during preparation . the problem where in the mesial structure of the 7 , and you need to prepare the mesial side this is could be an option .If the tilted molar not too much u can corrected during preparation , if it's too much what you can do ?

♥ ortho it's golden option .♥ partial crowns♥ telescopic crown .♥ non-rigid connector .

Slide 6 – page 3Now if we have tilted molar the first option is prepare molar if we can't then we go for ortho then partial crown .Partial crown in the lower molar should be cover mesial , distal , lingual , occlusal . we call it three quarter crown .

Note :

♣ three quarter crown don't cover the buccal surface .♣ 90 degree rotated partial crown don't cover the distal surface .

Another option we do fixed movable and we have two pieces bridge male-female attachment on the distal surface . the third option is telescopic crowns this more common in implant . imagine we have this pontic ( go back to slide ) and every

tooth in deferent direction what we do now ? we prepare the teeth then the technician fabricate metal crown used surveyor to make them parallel . ya3ni there are two crowns , telescopic crown then take impression and fabricate new crown then bond them together .

3-Canine – replacement FPD :It's complicated case and we don't deal with because - it's a curvature area the load unfavorable .- weak adjacent abutments .- outward stresses in the maxilla , inward in the mandible .- no more then one additional missing tooth .

4-Cantilever FPD :- Abutment at one end with a lever effect When you have bridge replacing the lateral incisor if we apply the load on the lateral incisor buccal or lingual or palatal the whole bridge will moved . so cantilever bridge is direct lever action or unfavorable load on the abutment , for that reason we always use double retainer .the common ex is mesial cantilever that replacing lateral incisor and we use a canine as abutment . cantilever replacing first premolar and we use second premolar as abutment this called mesial cantilever and this preferable cantilever we don't like distal cantilever . SO

- 2 abutment are generally required .- Replacing one tooth only with sturdy abutment .- Controlled occlusal load .- Mesial cantilever is preferable and mainly to replace max.

lateral incisor and first premolar .- Distal cantilever are more destructive and should be used

judiciously with modifications. You have to reduce the occlusal table to reduce the load .

5-limited inter-arch space :- no treatment .- Extraction .- Occlusal adjustment .- Dahl appliance : used mainly to intrude over erupted

teeth .

6-third molar abutments :- upright- long separate roots .- fully erupted .- health cuff of attached gingivae like operculum .

the 8 is not suitable abutment for fixed prosthodontic .if you don't have keratinized attach gingivae surround any abutment this abutment is high risk if we use it as abutment ……OK

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College : Ali Althani

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