tooth - implant support . . . with or against ? ! ! !

 

Various treatment modalities are available to replace missing teeth depending on number, condition of remaining teeth, space available, adequacy of bone support, cost, and patient desires. Implant supported fixed prosthesis (ISP) has been proven as an efficient treatment modality. Implant is connected to remaining natural teeth sometimes when there is an anatomic limitation of space for implants or failure of an implant to osseointegrate.

 

The advantages of tooth-implant supported prosthesis (TISP), include

1. increased mechanoreception and additional support for load transfer .
2. such connection broadens treatment modalities for the prosthodontist for completing the treatment plan
3. reduces the cost for teeth replacement . 
4. avoids the use of cantilevers. 

The disadvantages include :

• higher need for maintenance and repair with such connections. The problem with this connection arises from the fact that the tooth & the osseointegrated implants have dissimilar mobility patterns, and this may subject the implant to excessive stresses. This led to the controversy of whether such connection is a viable option. Various complications have been reported in the literature, including intrusion of the teeth , mechanical failure of the prosthesis , caries & loss of occlusal contacts . In addition, there is no clear guideline on when and how implant to natural teeth connection should be achieved.

 

Various reasons support tooth-implant connection: 

The most common reasons are local & systemic conditions preclude placement of additional implants, such as; failed implant(s) with some remaining implants, financial constraints for additional implant placement and bone augmentation procedures and anatomic limitations in the posterior areas where inadequate bone is present.  Other reasons include need for additional support where implants have to provide occlusal guidance and there is a need to share the load with the natural teeth to prevent over loading of the implant. Also, in periodontally-compromised teeth, implants can provide additional support to the remaining natural teeth., Finally, restoring aesthetics in implants is more challenging than the natural teeth. Whenever possible it is better to preserve the natural teeth and connect it to the implant.

 Mismatch in the tooth and implant movement: 

Natural teeth are attached to the surrounding alveolar bone by means of periodontal ligament fibers, while osseointegrated implants are rigidly anchored to the bone. This difference creates a potential biomechanical mismatch of the supporting units. The tooth exhibits normal physiological movement in vertical, horizontal & rotational direction. The primary factors influencing this movement include the health of periodontium, number, length, diameter, shape and position of the roots.

Tooth movement may be divided into two phase pattern. The first phase consists of rapid movement when the light force is applied and occurs as the periodontal ligament is compressed or stretched. The second phase consists of more linear movement that occurs as the alveolar socket is elastically deformed. When a secondary force is applied, secondary movement occurs and is directly proportional to the amount of force. This measures up to 40 μ under considerably greater forces

On the other side, osseointegrated implants show only linear movement during the entire loading cycle in proportion to the applied load without initial rapid movement due to lack of periodontal ligament. This linear movement is because of the viscoelastic nature of the bone. A healthy natural tooth can move 200 μ in response to a 0.1 N force while an implant can be displaced 10 μ or less.  The ratio of the amount of movement of the tooth in a healthy periodontium to that of an implant has been estimated to be 10:1 and 100:1. 

Because of rapid initial movement, structure of the periodontium will allow a tooth to intrude about 50 μ under a force of less than 20 N, while an implant under a force of 20 N will intrude only about 2 μ due to lack of initial movement. 

see this video to understand the biomechanical difference between natural tooth and implants , , ,

example of tooth intrusion 

The above observations indicate; 

1- Both implants & teeth share the occlusal loads.

i.e.; not all the forces are transferred to the implant alone. Resiliency of implant component, cushioning effect of cement layer, and force deflection in superstructure  may contribute to this phenomenon. However, the implant would receive higher amount of loads in function and could lead to potential complications.

 2- The physiologic movement of the natural tooth causes the prosthesis to act as a cantilever, generating maximum resultant load up to two times the applied load on the implant. When the applied load is concerned, both magnitude & duration of the force have significant effect on the stress transferred to the bone around a tooth. From a clinical standpoint this may be significant in patients with bruxism.

 Types of Connection:

there are 3 types of tooth implant connections 

1- Rigid connection (RC):

The tooth is rigidly connected to the implant with a fixed prosthesis. 

2- Non-rigid connection (NRC):

The tooth is non-rigidly connected to the implant by means of precision attachments, non-precision attachments and telescopic restoration. It acts as a stress breaking element.

3- Resilient connection:

It incorporates a flexible component (intra-mobile element “IME”) that simulates the periodontal ligament. It acts as a stress absorbing element.

Type of connection in tooth-implant supported prostheses is very controversial; The use of RCs has been considered as an acceptable procedure by many authors in order to potentially gain support from the tooth, to preserve the tooth or to provide stability to rotational forces directed at the screw joint of the implant supported part of the prosthesis. 

They reported decreased rate of mechanical failure of the prosthesis. The use of NRCs & Resilient Connectors was advocated because of the assumption that RCs add more strain on the implant because of the mismatch in the mobility patterns of tooth and implant.  As regard to Resilient Connectors, the effect of IME on providing vertical & rotational flexibility of tooth-implant supported prostheses is contradictory.  Some authors claim that IME provides enough flexibility of the prosthesis on the implant to compensate for the prosthesis movement on the natural tooth, while others concluded that IME does not compensate for the mismatch of the mobility pattern of implant and natural tooth. Furthermore, bending forces were transmitted to the retaining screw of the implant.

Considering non rigid connections 

Different types are described in TISP, with the most common; key & key way .

 The placement of the key way on the natural teeth seems to be beneficial as it would allow for physiological tooth movement under function.

Studies evaluating bone loss around implants in TISP using long-term radiographic follow up, reveal less bone loss with NRCs compared with RCs. 

On the other hand, biomechanical studies reveal that a shift of force distribution from the superstructure to the supporting teeth occurs when NRCs are used and tooth intrusion was considered as potential complication of NRCs.  However, reports of incidence of intrusion with both RCs and NRCs  do exist in the literature.

 Most clinical observations reveal non-significant differences in function between RCs & NRCs in TISP.  And, laboratory studies reveal non-significant differences in the magnitude & pattern of stress generation with RCs & NRCs.  

Moreover, some authors have advocated the use of RCs because both implant & prosthesis possess an internal flexibility to compensate for the mismatch in the movement pattern.  McGlumpy et al., reported that bending of the titanium superstructure screw provides the required flexibility and thereby ensures even load distribution. 

 Rangert et al., reported that flexibility in the screw joint of the implant matches the mobility of the periodontal ligament of a tooth and allows sharing of occlusal loads between the implant & the tooth, and as the load increases the tooth is increasingly involved. 

For this to occur, the implant components & their retaining screws must show some degree of flexibility, the periodontal support of the natural tooth must be adequate and constant and the extent of movement of the prosthesis must be minimal. This extent of movement will also affect the magnitude of the force to the screw joint on the implant and must be less than the preload of the retaining screw in order to prevent screw loosening. Constant bending of the screw could lead to metal fatigue and failure of the implant components.

therefore the choice of NRCS will be limited to the purpose of implant part retrievability 

Tooth to implant connection causes a large portion of the load to be transferred to the implant as it is rigidly fixed to the bone. This stress concentration is proportional to the natural tooth mobility and length of the prosthesis. The degree of overloading depends on the occlusal factors, tooth mobility and the existing number of implants. In other words, in a case with mobile teeth number of implants necessary to enable the implant to support the forces is important than the teeth. As there is little risk to the tooth, overloading of the implant can lead to screw loosening, screw fracture, implant fracture or loss of implant through bone resorption.

Complications associated with TISP:

in order to avoid all these complications , authors suggested guidelines for implementing TISP  . . . 

these Guidelines include 

Guideline 1: Splint implants to natural teeth only when the teeth need support, teeth do not stabilize implants: 

Proper case selection & treatment planning are essential to manage the biomechanical challenge associated with TISP. Natural teeth must have adequate periodontal support with good long-term prognosis. Decreased periodontal support increases forces on the remaining bone and on the implant.  However, when a decision is made to include weakened natural teeth, the method & placement of attachment for the natural abutment to the implant supported abutments must be carefully decided.

 Becker et al., suggested to splint implant to two teeth when NRCs are considered.  When a key & keyway NRC is considered, there should be a minimum of two natural teeth connected (double abutted). 

The process of double abutting the natural teeth greatly reduces the chance for intrusion but may not totally eliminate the occurrence. As regards the placement of the attachment, the key is using a stress breaker with the female part on the implant crown and the male end on the pontic to eliminate the risk of intrusion and allow the tooth to move without overloading the implant. The one problem with this arrangement is loss of retrievability.

Guideline 2: Do not end the fixed prosthesis on the weakest splinted abutment:

 Natural abutments should not show clinical mobility or poor retentive form. A weak tooth doesn’t offer additional support & further burdens healthier abutments.

Guideline 3: Regardless of the connection, teeth must be cemented using definitive cement: 

Tooth pushes 28 um, but rebounds only to about 8 um. The fixed prosthesis rebounds and pulls-on the tooth. The cement eventually breaks, causing a space to develop. The prosthesis acts as an orthodontic appliance and pushes in the vertical direction. When a natural tooth acts as a pier abutment it must be considered as a pontic as it does not contribute to the support of the prosthetic load.

Guideline 4: For a natural pier abutment between two implants a stress breaker (NRC) is not indicated, but a RC: 

A living pontic (natural pier abutment) decreases the impact of occlusal loads. This is due to the proprioceptive aspect of the periodontal complex. 

Guideline 5: Avoid telescopic attachments whenever possible: 

Guideline 6: Design of the prosthesis should allow minimal movement in a buccolingual direction.

 Selective grinding procedures must be employed to reduce the cantilever effect and redistribute stress in maximum intercuspation or lateral working position for a TISP. 

here is a clinical case using RC with hybrid cemented -screw retained to achieve retrievability and stress cushion effect at the same time . . . 

 Conclusion:

 It is evident from the above review that connecting implant to natural teeth is accompanied by various adverse sequelae. Nevertheless, when the situation indicates, the implantologist can and should consider the option of connecting natural teeth to implant. It is essential to formulate a treatment plan for predictable treatment outcome. A risk-benefit analysis and expected complications should be presented to the patient and appropriate consent obtained before the treatment plan is finalized. The main focus should be to reduce the risk of intrusion of the tooth and of overloading the implant.

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