|Year : 2013 | Volume
| Issue : 2 | Page : 153-156
Occlusion in implant prosthodontics
Radhika B Parekh, Omkar Shetty, Rubina Tabassum
Department of Prosthodontics, Dr. Dnyandeo Yashwantrao Patil Dental College and Hospital, Nerul, Navi Mumbai, India
|Date of Web Publication||25-Sep-2013|
Radhika B Parekh
20 Kailas, 50 Pedder Road, Mumbai 400 026
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Implant prosthodontics is a vast and varied field. The most crucial stage in the replacement of a missing tooth using an implant supported prosthesis is the occlusal loading of the fixture. The longevity and the success of the restoration are primarily determined by the direction and amount forces in occlusion. The quality of bone, type of implant, type of prosthesis and patient factors all play important roles in the selection of an occlusal scheme. Each patient must be treated with an individualistic approach. The guidelines for the choice of restoration or type of occlusal scheme must be customized to allow for longevity of the restoration in harmony with the health of the surrounding dentoalveolar structures. This review article encapsulates the different factors to be considered while planning implant restorations and establishing occlusal and prosthetic schemes to protect and preserve the associated oral structures.
Keywords: Implant prosthodontics, implants, occlusion
|How to cite this article:|
Parekh RB, Shetty O, Tabassum R. Occlusion in implant prosthodontics. J Dent Implant 2013;3:153-6
| Introduction|| |
The ultimate success and longevity of any restoration in the oral cavity is dependent on the forces which act on it and the ability of the underlying structures to absorb or react to these forces. Occlusion is critical for implant longevity because of the nature of the attachment of the bone to the titanium surfaced implant. Occlusal overload and its relationship to implant overload and failure is a well-accepted phenomenon.
Most occlusal concepts and ideas have been designed for natural teeth and are applied to implants without any modification and as osseointegrated implants lack specific defence mechanisms, poorly restored occlusion on osseointegrated implants can result in deleterious effects to the prosthesis and supporting alveolar bone.
The outcome of treatment is often quite successful in spite of the fact that dentists use different concepts of occlusion. Individuals with an ideal occlusion are seldom and most of our patients deviate in one or more ways from the ideal norm of occlusion but may still function well. They are physiologically acceptable and do not need any intervention. ,,
A therapeutic occlusion has been defined as one modified by various therapeutic measures so that it falls within the parameters of a physiological occlusion. Among the many varying recommendations for therapeutic occlusions presented in prosthodontic textbooks, a concept of a functionally optimal occlusion originally presented in the 1950s by Beyron , has gained much support over the years and has been considered to have stood the test of time.  Some of the occlusal goals for implant supported restorations  are (i) bilateral simultaneous contacts and equal distribution of occlusal forces  (ii) no occlusal prematurities  (iii) Smooth, even lateral excursive movements with no non-working interferences  (iv) No soft tissue impingement during occlusal contact.
As compared to endosteal implants, natural teeth are designed to reduce the force that it receives by various mechanisms, the periodontal membrane which acts a visco-elastic shock absorber, biomechanical design, elastic modulus of supporting tissues, occlusal material and type of surrounding bone all aid in decreasing the occlusal forces that the tooth can sustain.
Conversely the rigid fixation between the implant and the surrounding bone does not allow for dissipation of the occlusal forces that it receives, thus often translating into problems with the prosthesis, the screws that holding the implants and abutments or the frameworks together often fracture, and in cases of cement retained restoration shear stresses at the cement interface cause de-cementation. The precursor signs of premature contact or occlusal overload on natural teeth are usually hyperemia, sensitivity or in severe cases, mobility whereas implants rarely show any clinical signs other than fatigue fracture.
| Premature Occlusal Contacts|| |
Initial tooth movement ranges from 8 μ to 28 μ in a vertical direction under a 3 lb-5 lb load depending on factors such as tooth shape, position, geometry of roots, and time elapsed since last load application. , The secondary tooth movement is similar to the movement seen in implants (3 μ-5 μ)  and is reflective of the property of the surrounding bone. When teeth come in contact, the combined intrusive movement is about 56 μ (28 μ +28 μ) but when an implant opposes a natural tooth, only 28 μ of movement occurs, hence though the occlusal design may be ideal, premature occlusal contacts on the implant may still occur due to the difference in the vertical movement of the teeth and the implants in the same arch. The implant prosthesis should just barely contact and the surrounding teeth in the arch should exhibit greater initial contacts. Hence, in cases of implants restorations opposed by natural teeth, the dentist should use a heavy bite force followed by a light bite force to differentiate between the occlusal contacts. Heavy bite forces causes depression of the natural teeth and positions them closer to the implant and thus permits equal sharing of the load.  In situations where implant restorations oppose each other in two posterior quadrants the heavy bite force must account for the 56-μ difference in vertical movement of the teeth in the contra-lateral quadrants. Anteriorly teeth exhibit greater apical and lateral movement then the implants hence in clinical situations of anterior implants, first a light biting force with a thin articulating paper is used (20 μm) to ensure that no implant crown comes in contact during the initial movement, followed by a heavy biting force in centric occlusion so as to develop simultaneous contacts on the implants and crowns. Isidor et al., , reported that excessive occlusal overloading can cause severe crestal bone resorption and loss of osseointegration.
An important aspect of occlusal adjustment for implant restorations is the periodic recall and re-evaluation of the occlusal contacts. This ensures the longevity and success of the restoration.
| Relation of Surface Area to Stress Distribution|| |
The forces that are delivered to the implant must be able to be capably sustained with minimum effect on the surrounding crestal bone. Studies have shown that when implants of decreased surface area are subjected to angled loads, the magnified stress and strain magnitudes in the interfacial tissues can be minimized by placing an additional implant in the region of concern. ,,, In cases where forces are increased in magnitude, direction, or duration (parafunction) ridge augmentation, reduction in crown height or increase in implant width or number may be useful in compensating for the increased stresses. The type of prosthesis may also be modified from a fixed prosthesis to a removable prosthesis while incorporation of modifications such as increased soft tissue support to relieve undue stresses.
| Occlusal Scheme for Implant Restorations|| |
The most ideal occlusal concept advocated for implant supported restorations is that of mutually protected articulation. The posterior and anterior groups of teeth mutually protect each other. In protrusion, only the anterior teeth are controlled by the incisal guidance  and there is uniform disocclusion seen in the posterior region whereas in centric occlusion there is intercuspation of the posterior teeth and the anterior teeth are free of any contact. In cases where a healthy canine is present, only the canine disoccludes the rest of the posterior teeth in lateral excursions. ,,
In implant prosthodontics, the incisal guidance should be as shallow as possible, Weinberg and Kruger  noted that for every 10-degree change in the angle of disocclusion, there was a 30% difference in the load. Hence all lateral excursions opposing fixed prostheses or natural teeth must disocclude all posterior components.
| Direction of Occlusal Loading|| |
The primary component of the occlusal force should be directed along the long axis of the implant, not at an angle. Angled abutments should only be used to improve esthetics or allow for a favorable path of withdrawal.
| Cantilevers|| |
When cantilevers are used, the maximum length of the cantilever should never exceed 20 mm and it is best kept less than 15 mm.  The shorter the length minimizes the torque to the posterior implant abutment thereby reducing crestal bone loss on the distal implant. Duyck et al.,  reported that when a biting force was applied to a distal cantilever, the highest axial forces and bending movements recorded on the distal implants were more pronounced in prostheses with only three implants compared those with five or six implants. Eliminating prematurities in RCP and IP, reduction in the occlusal table and reducing the occlusal contacts also aid in the reduction in the occlusal contacts, no lateral loads are applied to the cantilever portion of the restoration. 
| Crown Modifications|| |
The greater the crown height, the greater the resulting crestal moment with any lateral component of force including those forces that develop because of an angled load.  The central fossa of an implant crown should be 2-3-mm wide in the posterior teeth and parallel to the occlusal plane. Secondary contacts should remain within 1 mm of the periphery of the implant to decrease the moment loads and marginal ridge contacts should be avoided. Splinted crowns also decrease occlusal forces to the crestal bone and reduce abutment screw loosening; hence adjacent implant crowns should be splinted. The center of the implant most often is placed in the center of the edentulous ridge, as the ridge shifts lingually with resorption, the implant body is most often not under the buccal cusp tips but rather near the central fossa or sometimes under the lingual cusp of the natural tooth. A buccal or lingual cantilever is called an offset load, which acts as a class 1 lever. Greater the offset, greater the compressive tensile and shear forces at the implant crest. Hence, reduction in buccolingual dimension of the crown helps minimize these loads. 
| Progressive Bone Loading|| |
The concept of progressive loading considers the role of Wolffs law  where bone mass increases in response to controlled stresses. Gradually increasing the load applied to implants in poor quality bone to increase the mass and density by gradually increasing the function. The elements of progressive loading include the time interval, diet, occlusal material, occlusal contacts, and prosthesis design.  The time interval between the two surgical appointments depends on the type of bone (D1-D4) in which the implant is placed. The diet of the patient is controlled from soft to semi-soft to hard in order to be able to control the amount of force being delivered to the implant. The occlusal material is initially acrylic or composite resin which is later replaced by metal or ceramometal. The prosthesis is initially kept out of function and slowly occlusal contacts are increased though cantilevers are always kept of occlusion.
| Conclusions|| |
The success of an implant is based on its long-term prosthetic efficiency, careful treatment planning and sound decision play a vital role in its success. Careful consideration should be made in identifying the weakest link in the overall restoration and establishing occlusal and prosthetic schemes to protect that component of the structure. Each patient must be treated with an individualistic approach. The guidelines for the choice of restoration or type of occlusal scheme [Table 1] must be customized to allow for longevity of the restoration in harmony with the health of the surrounding dentoalveolar structures.
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