OCCLUSAL CONSIDERATION IN IMPLANT PROSTHODONTICS.

OCCLUSAL CONSIDERATION IN IMPLANT PROSTHODONTICS.
Dental News
February 2005
Y.H.Ismail, DMD, MS, PHD*. Nidal Yacoub, DMD
INTRODUCTION:
When addressing the subject of oral implantology, one must consider not only the surgical phase of placing the implant, but the entire treatment sequence, specifically the prosthodontic rehabilitative procedures. Proper knowledge and applications of sound prosthodontic principles in the selection, placement, and restorative phases, are pre-requisites for successful implant treatment. Predictability and long-term success (Branemark and Zarb, 1985) are based on several factors, which include proper selection of patient and implant, aseptic and atraumatic surgical procedures, adequate no-loading healing period, correct prosthodontic reconstruction, and proper follow-up care.
In an early published Swedish GovernmentÇƒÙs evaluation (Dergman, 1983) the prosthodontic phase was cited as the most critical aspect for implantÇƒÙs long-term success.
Other authors (English, 1988; Ismail, 1987; Rasmussen, 1987,and Adell, et al, 1981) have emphasized the importance of prosthetic principles in all phases of implant treatment, and indicated that most implant failures following the restorative phase are due to prosthetic failure (functional and mechanical).
Implant prosthodontics does not only deal with the technical aspects of fabricating an implant-supported prosthesis, but it also deals with the proper application of occlusal principles for implant selection, implant placement, and the prosthetic phase of the treatment.
Two primary aspect of occlusion must be considered, occlusal design and materials, and occlusal forces and their transmission to supporting tissues. According to English (1988), "Accurate occlusion is essential to the long-term success of implant treatment. Implants cannot bail out our faulty occlusionÇƒ?.
OCCLUSAL DESIGN AND MATERIALS:
(A) Occlusal design:
According to the Dentists' Desk Reference (ADA, 1983),many implant failures can be attributed to improper occlusal design. Poor occlusal design can result in concentrated stresses in the bone, which can lead to rapid bone resorption. Certain rules have been recommended in the DDR (1983) to establish proper occlusal design in implant prosthodontics. Among these rules are:
(1) Cusp designs and crown alignment should be made so that stresses are directed along the long axis of the implant. Lateral stresses should be avoided or at least minimized.
(2) The width of the occlusal table of the implant crown should be minimal. Whenever possible, the occlusal table shouldnÇƒÙt be wider than the width of the implant fixture.
When these rules are closely examined, one realises that they are not being followed in many implant treatments.
Many clinical articles and presentations, by the "experts" show wide occlusal tables and a wide range of cusp heights or cusp inclinations. Several manufacturers' presentations and/or advertisements illustrate numerous cases where the crowns and fixtures are not aligned to direct stresses along the long axis of the implant roots. Even some manufacturers present techniques and prosthetic components that position crowns at 20-40 degrees from the implant fixture. Other manufacturers present bendable and castable prosthetic abutments to facilitate prosthetic reconstruction of improperly positioned implant fixtures.
These approaches and techniques should be discouraged from routine implant procedures, and reserved only for the most uncommon circumstances when no alternatives are available for achieving proper placement of implant fixtures and proper alignment of crowns. The alignment of the crown with the implant fixture is a definite prosthodontic requirement in order to direct occlusal loads along the long axis of the implant.
Studies of generated stresses in a variety of implants and their supporting tissues (Michel, 1984; Garrana, 1986; Ismail, et al, 1985; Ismail, et al, 1986; and Hadeed, et al 1988) have indicated the harmful effect of lateral loading of implant fixtures.
The transmitted stresses to the supporting bone in these studies were excessive compared to the stresses generated as a result of vertical loading. In many instances, lateral loads generated high levels of stresses in the crestal alveolar supporting osseous structures that exceeded the tolerance limit of bone, and accordingly, bone resorption at the cervical region around implants.
It is recommended that the occlusal table shouldnÇƒÙt be wider than the implant fixture and that permits optimal transmission of stresses along the implant fixture.
Even natural posterior teeth exhibit an occlusal table width that is approximately the same width as the roots of these teeth or less.(Sicher and DuBrul, 1970).
A wide occlusal table on a narrow implant fixture transmits high level of stresses through the implant fixture and to the supporting osseous structures, which in turn may lead to inflammatory response and bone resorption.
Cusp height is another factor in establishing proper occlusion in implant prosthodontics. Cusp height, or cusp angle, is closely related to the width of the occlusal table. The guidelines by the DDR (1983) stated that cusp height should be minimized to provide only centric function, thus decreasing lateral stresses that may be applied to the implant fixtures and their supporting osseous tissues. In natural dentition and in fixed prosthodontics (Tylmann, 1978, Lundern, 1971; and D'Amico, 1965) proper cusp height and inclinations are determined by the proper contact of cusps to fossae (triangular cuspal elevations of cusps to triangular marginal ridges of central grooves). As the cusp height and angle are reduced, wider occlusal contact areas are created as centric stops, and lesser lateral guidance and stress are produced.
Since osseointegrated implant function differently from natural teeth (presence of periodontal ligament), the dentist must design the implant occlusal surfaces with minimum cusp heights and minimum occlusal table width.
(B) Occlusal materials:
Several statements have been made in the literature regarding the use of resins as the occlusal material of choice in implant prosthodontics (skalak, 1985, Adell et al, 1981,Haraldson, 1971). These authors suggested the use of resins because of their function as a shock absorber, and thus, less stresses on the implant fixtures and their supporting osseous structures. skalak (1983) and Skalak (1985), explained that, "a stiff prosthesis is preferable over a flexible one in the super structure which is supported by osseointegrated implants, and will distribute loads more effectively to the supporting abutments. The use of a shock-absorbing material, such as acrylic resin in the form of artificial teeth on the surface of the denture, can provide adequate shock protection to the stiff and close connection of an osseointegrated implant to the supporting boneÇƒ?.
Skalak (1985) added, Çƒ?large impact forces can be developed during chewing, if a hard object is inadvertently encountered when the moving mandible and muscles are suddenly stopped by impacting on the obstruction.
An osseointegrated implant supporting a metallic bridge with no softer materials interspersed can result in a sufficiently large peak force to cause fracture of the supporting bone. To reduce this peak force, it should be spread out in time by placing a layer of softer material in the path of the force transmission. Resin, in the form of plastic teeth, has a much lower modulus of elasticity than metals, and provides internal damping; thus, reducing the impact forcesÇƒ?.
Although, theoretically, skalak's explanation is correct, there hasbeen no correlation to biting forces, chewing efficiency, and the specific teeth functions of cutting, tearing, reducing bolus size and grinding.
In the natural dentate state, shock absorbing is achieved through the periodontal ligaments while the occluding surfaces are either enamel or a relatively comparable restorative material in hardness or stiffness.
Reversing this arrangement may result in less chewing efficiency and increased biting forces in order to cut, tear, reduce, and grind food with resin opposing surfaces.
The results of some publications (lundgren and Laurell, 1986; Lundgren, et al, 1987) on the biting forces of patients with osseointegrated implant supported prostheses indicated a significant increase in the biting forces of these patients as compared to patients with complete dentures, twice as large as those with cantilever fixed partial dentures and comparable to subjects with natural dentition although with greater chewing frequency.
Rather than interpreting these results in light of the preceding discussion, Lundgren (1987) emphasized the lack of material failure only.
In the dentulous as well as in the edentulous states, the limiting factor for the biting forces is the innate self protective character of the individual tissue or tissues; somatic sensory mechanism, specifically the proprioceptive mechanism. Conceptually, proprioception includes periosteal and periodontal sensibility to position, movement, and resistance to force (Ruch & Patton, 1979; Ness, 1954; Ramfjord & Ash, 1983). Periodontal afferent nerves and nerve endings (mechanoreceptors) are sensitive to mechanical stimuli and are able to discriminate between forces of different magnitudes and directions (Pfaffmann, 1939; and Scott,1969). Biting forces must be within the capacity of the supporting structures to tolerate them.
Biting forces increase with periodontal anesthesia (Carlsson, 1974), thus it is rational to assume that the periodontal membrane sensibility (in dentulous subjects) acts as the limiting factor for occlusal forces. In the edentulous state in the absence of periodontal sensibility, a comparable limiting factor for biting forces transmitted through complete dentures would be the tolerance of the mucosa to these forces, thus the r