|Year : 2015 | Volume
| Issue : 3 | Page : 27-33
Principles of occlusion in implant dentistry
Mahesh Verma, Aditi Nanda, Abhinav Sood
Department of Prosthodontics, Maulana Azad Institute of Dental Sciences, Delhi, India
|Date of Web Publication||31-Dec-2015|
Maulana Azad Institute of Dental Sciences, Bhadur Shah Zafar Road, New Delhi - 110 002
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Dental implants require different biomechanical considerations from natural teeth. Also, with one of the criteria for long-term implant success being “occlusion,” it becomes imperative for the clinician to be well versed with the different concepts when rehabilitating with an implant prosthesis. All endeavors must be made to reduce the overload and noxious forces on implants during mandibular movements. The occlusal rehabilitation schemes for implant-supported prostheses are derivatives of the occlusal scheme for natural dentition. The implant-protected occlusion (IPO) scheme has been designed to ensure the longevity of both prosthesis and implant. The article reviews the concepts of IPO and their applicability in different clinical scenarios.
Keywords: Biomechanical, guidelines, implant, occlusal scheme, occlusion
|How to cite this article:|
Verma M, Nanda A, Sood A. Principles of occlusion in implant dentistry. J Int Clin Dent Res Organ 2015;7, Suppl S1:27-33
| Introduction|| |
Determining an occlusal scheme for the restoration of implants requires careful consideration. This stems from the fact that after osseointegration, mechanical stresses beyond the physical limits of hard tissues have been suggested as the primary cause of initial and long-term bone loss around implants.,,, Occlusal overload is often regarded as one of the main causes of peri-implant bone loss and implant prosthesis failure because it can cause crestal bone loss, thus increasing the anaerobic sulcus depth and peri-implant disease states., It can be rightly said that occlusion is a determining factor for implant success in the long run.,
The choice of occlusal scheme for implant-supported prosthesis is broad and often controversial. Almost all concepts are based on those developed with natural dentition and are transposed to implant support systems with a few modifications. The probable reason for this practise is the similarity (during mandibular movement) in the velocity, the pattern of movement and the operating muscles that are used by patients with implants and those with natural dentitions. Moreover, it has been established that the clinical success and longevity of implants can be achieved by biomechanically controlled occlusion. This implies that the occlusion provided must follow sound mechanical principles, direct forces predominantly along the long axis of the implant body, and minimize off-centered forces. The same should be aimed to impart and enhance biological stability.
However, there are a few innate differences between natural teeth and implants, which need to be considered when restoring implants. Natural teeth are associated with high occlusal awareness (proprioception) of about 20 µm. Occlusal proprioception is low in implants. For instance, between a tooth and an implant the proprioception is around 48 µm; between two implants it is around 64 µm; and between a tooth and an implant-supported overdenture it is around 108 µm.,,, The lack of proprioception and the absence of periodontal shock absorption are often associated with increased impact force with an implant-supported prosthesis than with a tooth-supported prosthesis.,,, Besides the proprioception, the presence of periodontal ligament as a shock absorber in a natural tooth brings about an apical intrusion by about 28 µm and lateral movement by around 50-108 µm. In the case of a similar load acting on an implant, no initial movement is seen and the delayed apical movement observed is around 10-50 µm. The same can be attributed to the viscoelastic properties of bone. Also, such a load acting on an implant is primarily concentrated on the crest of the implant.,
In case of occlusal trauma, mobility can develop in a tooth as well as in an implant. However, upon removal of the trauma, mobility can be reduced or controlled with a natural tooth, while no such response can be noted in an implant. In general the diameter of natural teeth is larger than the diameter of implants. Also, the cross-section of implants is rounded and the diameter is selected primarily according to bone available, not according to the load that it is anticipated to be subjected to. The cross-section of the root of a natural tooth, on the other hand, varies according to the force it has to withstand. For example, mandibular anterior teeth have wider diameters faciolingually, mainly to resist forces during protrusion. Likewise, the roots of canines are shaped to withstand lateral loads and those of molars to withstand axial loads.,,,
The issue of such differences between natural teeth and implants lead to the establishment of implant-protected occlusion (IPO), the credit for which goes to Dr. Carl Misch and Dr. MW Bidez. It is also called medially positioned lingulalized occlusion, and it stems from the change in relation of the edentulous maxillary ridge to the mandibular ridge due to resorption of edentulous ridges in a medial direction. As a result, a few unique concepts are associated with implant-supported prosthesis and these constitute the guidelines for IPO.,,
There are 14 considerations for following the IPO scheme that should be judiciously implemented before restoration. They are as follows:
Elimination of premature occlusal contacts
Premature contacts are defined as occlusal contacts that divert the mandible from a normal path of closure; interfere with normal smooth gliding mandibular movement; and/or deflect the position of the condyle, teeth, or prosthesis. It has been speculated that occlusal load from excessive lateral loads arising from premature contact may cause bone loss and implant failure. Prior to the evaluation of occlusion on implant reconstruction, the occlusion should be evaluated and all occlusal prematurities should be eliminated during maximum intercuspation and centric relation.,,,
While restoring an implant, a thin, articulating paper is used (<25 µm) for the initial implant occlusion adjustment in centric occlusion under light tapping forces. The implant prosthesis should barely make contact, and the surrounding teeth in the arch should exhibit greater initial contact. The implant crown should exhibit light axial contact. This is because a natural tooth exhibits greater vertical movement than an implant. Once equilibration under light occlusal force is completed, the occlusion is refined under heavy occlusal contact. A tooth may not return to its original position for several hours after the application of heavy occlusal force. As a consequence, light occlusal forces on the adjacent natural teeth are equilibrated first. The occlusal contact should remain axial over the implant body and may be of similar intensity on the implant crown and adjacent teeth when under greater bite force. This implies that all elements react similarly to heavy occlusal loads. The harmonization under light occlusal loads is followed by adjustment under heavy occlusal load. The heavy occlusal load positions the natural teeth closer to the depressed position of the implant, thereby permitting equal sharing of the load between the implant and the natural teeth. However, an important part of the philosophy behind IPO is the regular evaluation of occlusal contacts at regularly scheduled hygiene appointments so that minor variations occurring during long-term functioning help in preventing porcelain fracture and other stress-related complications.,,
Provision of adequate surface area to sustain load transmitted to the prosthesis
Increased load can be compensated for by increasing the implant width; reducing crown height; ridge augmentation if necessary; increasing the number of implants; or splinting the prosthesis.,
Controlling the occlusal table width
The width of the occlusal table is directly related to the width of the implant body., The wider the occlusal table, the greater the force developed to penetrate a bolus of food. However, a restoration mimicking the occlusal anatomy of natural teeth often results in offset load (increased stress), increased risk of porcelain fracture, and difficulties in home care (due to horizontal buccolingual offset/cantilever).,, As a result, in the nonaesthetic regions the width of the occlusal table must be reduced in comparison to a natural tooth.
Mutually protected articulation
This implies that during excursion the posterior teeth are protected by the anterior guidance, whereas during centric occlusion the anterior teeth have only light contact and are protected by the posterior teeth. It must be kept in mind that the anterior guidance of the implant prosthesis with anterior implants should be as shallow as practicable. The steeper the anterior guidance, the greater are the anticipated forces on anterior implants. In case of a single tooth implant replacing a canine, no occlusal contact is recommended on the implant crown during excursion to the opposite side. The rationale of mutually protected occlusion is that the forces are distributed to segments of the jaws with an overall decrease in force magnitudes. It must also be kept in mind that if anterior implants must disocclude the posterior teeth, two or more implants splinted together should help dissipate lateral forces whenever possible.
Implant body orientation and influence of load direction
Whether the occlusal load is applied to an angled implant body or an angled load is applied to an implant body perpendicular to occlusal plane, the biomechanical risk increases. This is attributed to the anisotropic nature of the bone, resulting in separation of the load to compressive, shear, and tensile stresses. Anisotropy refers to the character of bone whereby the mechanical properties depend on the direction in which the bone is loaded. The greater the angle of the load, the greater is the shear component of the load. It must be borne in mind that cortical bone is the strongest and most able to withstand compressive forces. Its ability to withstand tensile and shear forces is 30% and 65% less, respectively, than its ability to withstand compressive forces.,,
Additionally, a force at a 30-degree angle decreases the bone strength limit by 10% under compression and by 25% under tension. The increase in the shear component of stresses is by almost three times, which predisposes the bone to increased crestal bone loss and impairs successful bone growth. During loading, the primary component of occlusal forces should be directed along the long axis of the implant body. The three conditions where one can anticipate angled loads are: Angled abutments, angled implant bodies, and premature occlusal contact. Angled abutments are used to improve the path of insertion of the prosthesis or to improve the final aesthetic results. The implant body should be placed perpendicular to the occlusal plane and along the primary occlusal contact. Premature occlusal contacts result in the localized lateral loading of opposing contacting crowns. Because the surface area of a premature contact is small, the magnitude of stress in bone increases. Also, the contact is most often on an inclined plane; therefore, it increases the horizontal component of load and increases the tensile crestal stress. In general, whenever lateral/angled loads cannot be eliminated, a reduction in force magnitude or additional surface area of the implant surface is indicated to reduce the risk of bone loss or of implant component fracture. Such measures include increasing the diameter of angled implants, selecting implant design with greater surface area, adding an additional implant next to the most angled implant, and splinting of implants.,,
Crown cusp angle
It is important to control this, as the angle of force to the implant body may be influenced by cusp inclination, which in turn will increase crestal bone stress. The occlusal contact over an implant crown should, therefore, ideally be on a flat surface perpendicular to the implant body. This positioning is accomplished by increasing the width of the central groove to 2-3 mm in posterior implant crowns, which are positioned over the center of the implant abutment. It may be necessary to recontour the opposing cusp to occlude in the central fossa over the implant body. If the implant crown mimics the natural cusp angle, the premature contact will occur on a cuspal incline and the resulting direction of load may be 30 degrees to the implant body.,,
Cantilevers and IPO
Cantilevers are class-1 levers, which increase the amount of stress on implants. Twice the load applied at the cantilever will act on the abutment farthest from the cantilever, and the load on the abutment closest to cantilever is the sum of the other two components. Cantilevers also add to noxious stresses (force on a cantilever is compressive, while force on a distal abutment is tensile).,, The force and the length of the cantilever are directly proportional to the force on the implant. For a system with 4-6 implants, the following cantilever lengths are recommended: Maxillary anteriors-10 mm; maxillary posteriors-15 mm; mandibular posteriors-20 mm. In general the goal should be to reduce the length and hence the force on the cantilever. In addition, a gradient type of occlusal contact force along the length of cantilever may be beneficial.,,,
Crown height and IPO
An increased crown height acts as a vertical cantilever, magnifying the stress at the implant-bone interface. It also leads to angled load with a greater lateral component of force. It is important to note that crown height is determined at the time of diagnosis and that all methods of either reducing the load or reducing the crown-implant ratio should be applied before restoration.
Occlusal contact position
The ideal occlusal contact is over the implant body. This contact leads to the axial loading of implants. A posterior implant is hence placed under the central fossa of the implant crown. A buccal cusp contact is an offset or cantilever load. A marginal ridge contact is also a cantilever load, as the marginal ridge may also be several millimeters away from the implant body. In fact, the marginal ridge contact may be more damaging than the buccal offset, as the mesio-distal dimension of the crown often exceeds the buccolingual dimension. Moreover, the moment of force on the marginal ridge may contribute to forces that increase abutment screw loosening. Thus, the ideal primary occlusal contact should reside within the diameter of the implant within the central fossa. The secondary occlusal contact should remain within 1 mm of the periphery of the implants to decrease the moment loads. The marginal ridge contact is not an offset load when located between implants splinted to one another, and is acceptable only under such circumstances. Moreover, adjacent crowns should preferably be splinted in order to decrease occlusal stresses to crestal bone and to reduce screw loosening.
Implant crown contour
Due to ridge resorption, the direction of the remaining ridge shifts lingually and the implant body is most often not under the buccal cusp tip position of natural teeth. In fact, it may be either under or near the central fossa or more lingual under the lingual cusp of a natural tooth, depending on the resulting position of the remaining ridge due to resorption. Hence, making the buccal contour the same as the original, natural tooth will lead to buccal offset load to the implant. All attempts should be made to provide a narrow occlusal table with reduced buccal contour, facilitating daily home care, improving axial loading, and reducing the risk of porcelain fracture. Crown contour in Division A bone has been described in the respective figures [Figure 1], [Figure 2], [Figure 3]. In Division B-Division D bone, the implant position is often lingual to the position of the natural tooth. Care has to be taken in case of mandibular posterior implants regarding the limitation imposed by the submandibular fossa. In case of excessive medial positioning of the implant, it may be necessary to use angulated abutment and a straight lingual profile. Maxillary posterior implants in division B-D bones may often require restoration in crossbite [Figure 4]. In case of Division C and D bone, all attempts must be made to perform a bone augmentation procedure and create a condition as close as possible to Division B bone.,,
|Figure 1: maxillary natural tooth vs mandibular implant-supported prosthesis in division a bone,|
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|Figure 2: maxillary implant-supported prosthesis vs mandibular natural tooth in division a bone,|
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|Figure 3: maxillary implant-supported prosthesis vs mandibular implant-supported prosthesis in division a bone,|
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|Figure 4: maxillary implant-supported prosthesis vs mandibular natural tooth in division b-d bone, might require cross-arch relation of teeth,|
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Design of the prosthesis should favor the weakest arch
Usually the maxilla is the weaker of the two arches, predominantly due to less dense bone. From a biomechanical perspective, an implant-restored premaxilla is often the weakest section compared with the other regions of the mouth. Compromised anatomical conditions include narrow ridges and the need for narrow implants, the use of facial cantilevers, oblique centric contacts, lateral forces in excursion, reduced bone density, the absence of a thick cortical plate at the crest, and accelerated bone loss in the incisor region often resulting in instability when placing central and lateral incisor implants without substantial augmentation procedures. In the anterior premaxilla, 15% higher maximum bone strain for a straight abutment has been predicted compared to an angled abutment. It has been suggested that, when restoring implants in the anterior maxilla, the use of an angled abutment, compared to a straight abutment, may decrease the strain on the bone. In fact, it has been recommended to increase the number and the diameter of implants and provide splinting when force factors are great.
The selection of occlusal materials depends on the opposing dentition, the remaining dentition, and the quadrant to be restored. The selection is usually made from among porcelain, zirconia, metal, and resin-based materials.,
Many studies have reported that parafunctional activities and improper occlusal designs are correlated with implant bone loss and failures. Further, it has been proposed that the numbers and distribution of occlusal contacts had major influences on the distribution of force. Naert et al., reported that overloading from parafunctional habits such as clenching or bruxism seemed to be the most probable cause of implant failure and marginal bone loss. According to them, shorter cantilevers, proper location of the fixtures along the arch, a maximum fixture length, and night-guard protection should be prerequisites to avoid parafunctional habits or the overloading of implants in these patients.
Timing of loading
Implant loading can be either delayed (submerged), progressive bone loading or immediate bone loading. Bone density is the key determinant in deciding the amount of time between implant placement and prosthesis restoration.,, Progressive bone loading is specifically indicated for less dense bones. Progressive bone loading allows a “development time” for load-bearing bone and allows bone adaptability to loading via the gradual increase in loading. The concept is based on incorporating time intervals (3-6 months), diet (avoiding chewing with a soft diet, then progressing to harder food), occlusion (gradually intensifying the occlusal contacts during prosthesis fabrication), prosthesis design, and occlusal materials (from resin to metal to porcelain) for poor bone quality conditions.
Occlusal guidelines for different clinical situations
In case of a full-arch fixed prosthesis, if the opposing arch is a complete denture, balanced occlusion is recommended. Group function or mutually protected occlusion with shallow anterior guidance is recommended when opposing natural dentition or a full-arch fixed prosthesis. There should be no working side and balancing contact on the cantilever.,,,,, The infraocclusion of the cantilever segment should be by 100 µm , and freedom in centric should be 1-1.5 mm. In case of overdentures, bilateral balanced occlusion with lingualized occlusion should be used. In case of severely resorbed ridges, monoplane occlusion should be used.,
If the posterior arch is rehabilitated with a fixed prosthesis, contacts should be centered over the implant body, and narrow occlusal tables, flat cusps with minimized cantilever should be employed. Where necessary, the posterior occlusion must be placed in crossbite. Anterior guidance should be with the natural dentition, and group function occlusion should be employed with compromised canines.,
Guidelines for choice of reconstruction and occlusal concept when rehabilitating the edentulous mandible with oral implants have been suggested by Quirynen M et al. In case of the fully edentulous maxilla, whether the mandibular rehabilitation is done on an overdenture supported on two implants or on a mucosal-implant-supported overdenture (four implants with a bar attachment), a balanced occlusal scheme (bilateral/lingualized/monoplane) is recommended. In conditions where a Kennedy class I partially edentulous condition is present in the maxillary arch and mandibular mucosa-implant supported (four implants with a bar attachment) or an implant-supported prosthesis is planned for the mandibular arch, balanced occlusion is recommended. In case of a maxillary arch presenting with Kennedy class II condition, if a mucosal-implant-supported prosthesis is planned for the mandibular arch, balanced occlusion is recommended. If an implant-supported prosthesis is advised for the mandibular arch, group function or mutually protected occlusion is advised. In case of Kennedy's class I in maxillary arch that has been restored with fixed denture prosthesis (FDP) or with implants, and a mandibular implant-supported prosthesis is advised, it is recommended to follow group function or mutually protected occlusion. In cases where the maxillary arch presents with Kennedy's class III and IV and implant-supported prosthesis is advised for the mandible, group function or mutually protected occlusion is recommended. Lastly, in case of the fully dentate maxilla and implant-supported prosthesis, group function or mutually protected occlusion is recommended.
| Conclusion|| |
A poor selection of occlusal scheme can lead to biological and mechanical complications.,, The various consequences that can be encountered are implant failure, early crestal bone loss, screw loosening, uncemented restorations, component failure, porcelain fracture, prosthesis fracture, and peri-implant disease.,. An IPO scheme addresses several conditions to minimize overload on bone/implant interfaces and implant prostheses, thus restricting implant loads within physiological limits. The guidelines need to be implemented in specific conditions to decrease stresses and develop an occlusal scheme to allow the restoration to function in harmony with the rest of the stomatognathic system and to maximize the longevity of the implants and prosthesis.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]