The Relationship between Dental Occlusion and “Prosthetic Occlusion” of Prosthetic Restorations Supported by Natural Teeth and Osseointegrated Dental Implants

Abstract

The concept of human dental occlusion represents much more than the mere physical contact of the biting surfaces of opposing teeth. It is not a static, unchanging, structural relationship, but rather a dynamic, real, physiological relationship between different tissue systems. It is best defined as the functional relationship between the components of the masticatory system, which includes the teeth, the periodontium, the neuromuscular system, the temporomandibular joints and the craniofacial skeleton. Biologically, occlusion represents a coordinated functional interaction between different cell populations of the masticatory tissue systems that differentiate, model, remodel, destroy and regenerate. When the functional balance of the masticatory system is disturbed or when occlusion is restored by various types of prosthetic restorations, specific goals of occlusal treatment become important, especially today with the rapid insertion of dental implants. The aim of this chapter is to highlight the characteristics of dental occlusion in relation to the characteristics and requirements of ‘prosthetic occlusion’ for different types of prosthetic restorations supported by natural teeth, gingiva, alveolar ridges and dental implants. A particular focus in writing the chapter is the analysis of the scientific literature on the interrelationship between the so-called occlusion concepts and the biomechanical aspects of different types of implant prosthetic restorations.

1. Introduction

There is no doubt that the human dentition must only be considered in the context of the human masticatory system and the whole organism. Knowledge of the characteristics of occlusion (tooth contacts at maximum intercuspation and movements of the mandible) and the interaction within the masticatory system are the basis for the development of so-called occlusal concepts, which have their diagnostic, therapeutic and clinical application in the daily practise of general dentists and specialists in dental prosthetics. The therapeutic capacity of dental occlusion is reflected in the fabrication of fillings made of different restorative materials, different types of prosthetic restorations supported by natural teeth, gingiva, alveolar bones of the maxilla and mandible, osseointegrated dental implants and other therapeutic devices such as different types of occlusal splints.

The term dental occlusion is often given the attribute “controversial” in professional and scientific literature. The contradiction is caused by different objective and subjective interpretations found in theories and empirical observations about occlusion, by the insufficient number and inadequate design of scientific studies (lower level of scientific evidence) about occlusion, up to the argument of certain authors that the characteristics of occlusion, occlusal schemes and concepts have a very low clinical significance. They justify this with the fact that in most cases the patient’s masticatory system can adapt to minor changes at the level of occlusion [1, 2, 3, 4, 5, 6].

However, common sense and daily clinical practise undoubtedly indicate that knowledge of the morphology and function of natural tooth occlusion cannot and should not be ignored, regardless of the current strength of scientific evidence. This knowledge, with certain modifications and specificities, certainly finds its application in the fabrication of prosthetic and implant prosthetic restorations.

2. Characteristics of dental occlusion and occlusal concepts for different types of prosthetic/implant prosthetic restorations

Mandibular positions and movements in static and dynamic occlusal relationships of the maxilla and mandible have their diagnostic and therapeutic applications, especially in prosthetic patients with partial and complete tooth loss and in patients with the clinical picture of temporomandibular disorder [7]. The joint and tooth position (retruded contact position, RCP) of centric relation (CR), maximum intercuspation tooth position (MIP) and physiological rest are diagnostic and therapeutic reference positions of the mandible used in dental prosthodontics for the fabrication of prosthetic restorations supported by natural teeth, gingiva, and alveolar bone ridges [8, 9, 10].

The impossibility of reaching a consensus on the definition of centric relation among experts (prosthodontists, orthodontists, periodontists, oral and maxillofacial surgeons) in the field of occlusion is particularly controversial. The most common conclusion of the studies [11, 12, 13, 14, 15] is that agreement on the definition that best or most accurately describes the position of centric relation has not been fully reached and that further and repeated research is needed. Zonnenberg et al. [16] suggest abandoning the term “centric relation” in scientific and academic communication. The authors noted that everyone has a unique relationship that cannot be described in a single term. In healthy patients, this relationship is determined by the maximum intercuspation of the teeth (MIP) and should therefore be considered biologically acceptable. However, they acknowledged that there are individual patients who present clinically with mandibular instability because they have an unstable MIP due to dental and/or skeletal injury or due to malocclusion requiring treatment. In these patients, establishing a new jaw relation is an important part of appropriate occlusal treatment procedures. There are three main groups of patients to whom this applies: edentulous (or partially edentulous) patients who require a construction of partial or full removable denture prostheses; patients who need full-mouth reconstruction, with or without implants; and patients who need full-mouth orthodontic and/or orthognathic therapy.

In addition, it is necessary to know the kinetics of the mandibular masticatory system in function with the masticatory muscles and temporomandibular joints, which are coordinated and controlled by the central nervous system. The evaluation of the occlusion is important in prosthodontics and restorative dentistry because the occlusal surfaces of the teeth to be restored must be functional units of the patient’s masticatory system. Specifically, the morphology of the cusps, fossae, grooves, and marginal ridges should support the mandible in the intercuspal position and where appropriate, during eccentric jaw movements and in functional activities such as mastication. Restored teeth should not interfere with mandibular function in mastication, speech, and swallowing nor should they transmit excessive force to the attachment apparatus or the temporomandibular joint either in the intercuspal or eccentric jaw positions or during movement [17].

Occlusal contacts that occur between the teeth of the upper and lower dental arches during mandibular movements under the influence of the function of the masticatory muscles and the anatomy of the temporomandibular joints in the natural dentition can take place through the so-called occlusal concepts or schemes. The “ideal occlusion” for eccentric movements can be classified by three schemes according to the tooth contact condition: mutually protected occlusion (canine guidance), group function, and balanced occlusion. The balanced occlusion concept is applied to complete denture patients while mutually protected occlusion and group function are applied for natural dentition and prosthetic restorations [18, 19, 20, 21].

The partial or complete loss of natural teeth and the various types of prosthetic restorations (complete and partial dentures) that compensate for this loss alter the normal regulation of oral functions such as movements and position of the lower jaw or functions such as biting or chewing. In a completely dentate individual, sensory organs (receptors in the dental pulp and periodontal mechanoreceptors) perform fine proprioceptive control of jaw function and influence the control of magnitude, direction and force of the bite (e.g. adaptation of masticatory muscle activity to the hardness of the food). In completely edentulous and partially edentulous individuals, the number of periodontal mechanoreceptors is completely and significantly reduced, so that an important source of tactile sensory input through the central nervous system is lost.

Wearers of complete and partial dentures often have impaired masticatory function because the remaining mechanoreceptors from the alveolar bone, gingiva and palatal mucosa cannot compensate for the loss of periodontal mechanoreceptors. This leads to a change in the basic forms of movement during mastication, i.e., an impairment of masticatory function in wearers of removable prostheses [22, 23, 24, 25, 26].

Fuentes et al. [27] come to a similar conclusion in their study. They claim that subjects with prosthetic restoration, regardless of the type of restoration, show a reduction in mandibular range of motion (border and functional) compared to the movements of a fully dentate subject. Rivera et al. [28] compared the characteristics of masticatory cycles in wearers of removable complete and partial dentures and overdentures retained with two dental implants. They concluded that wearers of mandibular overdentures supported by dental implants showed masticatory performance very similar to that described in the literature in younger, fully dentate subjects. They emphasize that this type of prosthetic restoration improves sensory perception and provides wearers with a greater sense of comfort, retention, and stability. They also point out that the wearers of removable implant prosthetic restoration (overdenture) showed increased chewing frequency and speed with smaller and faster chewing cycles.

Shiga et al. [29] studied the stability of mandibular movements during mastication in complete dentures, overdentures supported by dental implants and adult dentate subjects. The least differences in mandibular movements during mastication were found between adult dentate subjects and wearers of overdenture anchored by two implants. However, it was necessary to wear implant overdentures for 9 months to one year to adapt to the new chewing function.

These scientific findings gave rise to the term “osseoperception”, which is used to describe the sensations evoked by the mechanical stimulation of dental implants or a prosthetic restoration supported by dental implants. Osseoperception is defined as mechanoreception in the absence of a functional periodontal mechanoreceptive input but derived from temporomandibular joint (TMJ), muscle, cutaneous, mucosal, and/or periosteal mechanoreceptors, and which provides mechanosensory information for oral kinesthetic sensibility in relation to jaw function and artificial tooth contacts. Patients with implant supported prostheses have improved tactile discriminative capabilities and report improved motor function [23, 24, 30, 31].

It has been found that the masticatory efficiency with implant prosthetic restorations is very close to that of the natural dentition and that the maximum bite force with such a prosthetic restoration is equally high, if not higher [24]. However, in subjects with implant-retained prosthetic restorations, regulation of masticatory muscle activity was arranged in response to gradual changes in food consistency that occur during mastication [32].

In this context, there are differences between osseointegrated dental implants and natural teeth. Individuals with implant prosthetic restoration retain a good sense of dynamic loads (such as tapping on the tooth or contact between the tooth and a hard object) but an impaired sensitivity to static loads (e.g. spatial aspects) because the sensory signals underlying osseoperception are qualitatively different from those evoked when the load is directed at the natural tooth [24, 32].

2.1 Occlusal concepts in the fabrication of conventional complete dentures

Being a toothless person entails almost continuous morphological, functional, and behavioral changes in the tissues that support the complete dentures. This is particularly true of the oral mucosa and alveolar ridges of the edentulous maxilla and mandible, but the changes also affect the masticatory muscles, the temporomandibular joints and the lower third of the face. The above changes normally occur as part of the physiological aging process, but they can also be the result of inadequately performed therapy with complete dentures, where the changes can accelerate and aggravate the already difficult condition of edentulism. It should not be forgotten that chronologically, edentulous patients are mostly elderly people who are prone to polypharmacy due to their compromised health status, which translates into a decrease in their overall quality of life [33].

The existence and definition of the concept of centric relation shares a similar confused and controversial fate as the selection of occlusal schemes in the fabrication of complete dentures in edentulous patients. There are various classifications of occlusal schemes for complete dentures mentioned in dental textbooks, professional and scientific journals. For this chapter, it seems appropriate to use the classification of balanced and non-balanced occlusal schemes/concepts in the fabrication of complete dentures. Regardless of the type of occlusal scheme (balanced and non-balanced) for complete dentures, the goals of the chosen scheme are generally the same: maximum retention/support and stability of complete dentures, adequate esthetics, adequate masticatory efficiency, sense of comfort and long-term preservation of oral tissues (gingiva and alveolar bone) due to optimal transfer of masticatory load to the loaded tissues [34, 35, 36, 37, 38]. In his 25-year study, Tallgren [39] found that the alveolar bone is continuously resorbed (resorption of the anterior mandibular ridge is four times higher than in the maxilla), which is the biggest problem for wearers of complete dentures.

In the latest edition (2017), “the Glossary of Prosthodontic Terms” [8] defines “bilateral balanced articulation” as: the bilateral simultaneous posterior occlusal contact of the teeth in maximum intercuspal position and eccentric positions. The concept of balanced or bilaterally balanced occlusion/articulation was created “under the pretext” of improving the stabilizing effect of complete dentures by achieving tooth contact on the non-working side of the dental arches during excursive movements of the mandible [40, 41]. In individuals with normal dentition (Angle Class I), occlusal contact on the non-working side of the dental arches represents premature/interferential tooth contact, which has a potential and pathological effect on the function of the masticatory system (association with temporomandibular disorder).

In contrast to the non-balanced occlusal schemes, whose representatives are the mutually protected occlusal schemes (canine guidance) and the group function of natural teeth [8, 42] and the concept of monoplane (neurocentric) occlusion without balance (with no compensating curves or posterior balancing ramp) [43, 44], the examples of balanced occlusal schemes are much more numerous. The confusion is compounded by the fact that different designs of the occlusal surfaces of artificial, predominantly acrylic posterior teeth (anatomical, semi-anatomical and non-anatomical (flat) posterior teeth) are used to realize balanced and non-balanced occlusal schemes for complete dentures. However, the most mentioned types of balanced occlusion schemes for complete dentures are bilateral balanced occlusion, lingualized occlusion, anatomical occlusion, monoplane occlusion with balance (with medio-lateral and antero-posterior compensating curves or posterior balancing ramp), buccalized occlusion and others [45, 46, 47, 48, 49, 50, 51, 52].

Scientific studies comparing balanced and non-balanced occlusion schemes of complete dentures regarding variables such as retention, stabilization, masticatory efficiency of complete dentures, comfort of wearing complete dentures, resorption of the alveolar ridge (load transfer) and quality of life of edentulous patients also come to contradictory results. Some studies [53, 54] favor non-balanced occlusal concepts (e.g., canine guidance) over balanced occlusions and vice versa [55], others emphasize the advantages of balanced occlusions per se and over non-balanced schemes [38, 47, 56, 57, 58, 59, 60, 61, 62, 63], and third studies record no differences between balanced and non-balanced occlusal concepts for complete dentures [64, 65, 66, 67, 68, 69] and consider them clinically equivalent in the context studied.

However, the general conclusion based on the current state of scientific evidence shows that edentulous patients function well with complete dentures regardless of the type of occlusal scheme chosen (it seems to the authors of this article that two occlusal schemes can be distinguished in the literature: lingualized occlusion (e.g. in clinical situations with severe resorption of the edentulous alveolar ridges [63]) and canine guidance (patients with good alveolar ridges without neuromuscular problems [46]), in which anatomical posterior teeth are used. Thus, the choice of occlusal scheme for complete dentures is mainly based on a well-determined vertical dimension of the occlusion, on the clinical assessment of the occlusal scheme and the experience of the clinician, and on the expected good relationship between patient and clinician [38, 46, 70]. Until the contrary is proved.

Figures 1–8 show the occlusion scheme (unbalanced occlusion—canine guidance/group function) for complete dentures used by the so-called “Zagreb School”, Croatia. The occlusion scheme is based on compliance with the lingual or neutral space; correctly determined vertical and horizontal relationships using the physiological rest position and centric relation; and Gerber’s reduced occlusion (the supporting cusps of the upper (palatal) and lower (buccal) artificial (anatomical) posterior teeth are projected to the centre of the edentulous alveolar ridges—the load generated by the function of such a posterior dentition is shifted to the centre of the edentulous ridge and lingually/palatinally, thus acting as a stabilizer in full dentures). One of the peculiarities of this scheme is that the upper and lower second molars in complete dentures are not placed at the expense of the lingual or neutral space.

Figure 1.

Setting teeth for complete dentures in the dental articulator: position of maximum intercuspation—“Zagreb School” (unbalanced occlusion—canine guidance/group function).

Figure 2.

Setting teeth for complete dentures in the dental articulator: protrusion movement—“Zagreb School” (unbalanced occlusion—canine guidance/group function).

Figure 3.

Setting teeth for complete dentures in the dental articulator: left laterotrusion movement—“Zagreb School” (unbalanced occlusion—canine guidance/group function).

Figure 4.

Setting teeth for complete dentures in the dental articulator: right laterotrusion movement—“Zagreb School” (unbalanced occlusion—canine guidance/group function).

Figure 5.

Marking of the occlusal contacts during maximum intercuspation (red articulation paper) in the dental articulator.

Figure 6.

Marking of the occlusal contacts during lateral and protrusive movements (blue articulation paper) in the dental articulator.

Figure 7.

Distribution of static (red) and dynamic (blue) occlusal contacts in lower occlusal rim/full denture.

Figure 8.

Distribution of static (red) and dynamic (blue) occlusal contacts in upper occlusal rim/full denture.

2.2 Occlusal concepts in the fabrication of conventional fixed (single crowns, fixed bridges) restorations and removable partial dentures

Historically, occlusion and articulation are concepts/schemes that are constantly evolving. Bilaterally balanced occlusion, unilaterally balanced occlusion (group function) and mutually protected occlusion (canine guidance) are basic concepts of occlusion used daily in clinical practise. In the fabrication of fixed prosthetic restorations and removable partial dentures, but generally in restorative dentistry, the concept of mutually protected occlusion (alternatively group function) is recommended by the education of doctors and various specialists in dental medicine. As restorative treatment requirements may vary, the clinician should be aware of the combinations of occlusion schemes, their advantages and disadvantages and indications [71].

Otherwise, during the anamnesis and clinical examination of a patient who is to be treated with fixed restorations (single crowns, bridges), removable partial dentures (cast metal) or so called., combined removable-fixed prosthetic restorations (e.g., surveyed crowns + various types of dental attachments + removable partial dentures), an analysis of the state of the existing occlusion is inevitable. Those types of prosthetic restorations usually involve a partially edentulous patient in whom the vertical dimension of the occlusion (the height of the lower third of the face) may or may not be preserved. Regardless of the clinical situation, the static and dynamic occlusal relationships between the existing upper and lower dental arches (occlusogram) should be considered and recorded within the normal function (masticatory muscles and temporomandibular joints) of the masticatory system [24, 72]. Static occlusal contacts are the position and distribution of contacts/dots/marks in the tooth position of maximum intercuspation on the occlusal surfaces of the dental arches, and dynamic occlusal contacts are the position of the path line that occurs during excursion movements of the mandible (in the canine guidance concept, the line is on the palatal surface of the upper canine and the labial surface of the lower canine on the working side). Static and dynamic analysis of occlusion may reveal the presence of premature occlusal contacts in the centric (shift between the retruded contact position and maximum intercuspation) and on the non-working sides of the dental arches (most commonly the maxillary and mandibular second molars) during laterotrusion and protrusion movements of the mandible. In the literature, the causal relationship between premature occlusal contacts and the prevalence of clinical signs and symptoms of TMJ disorders is controversial, but in any case, they should be excluded when performing any form of prosthetic rehabilitation [7, 24, 71].

Occlusal analysis can be performed in the patient’s mouth and on study plaster models in an adjustable dental articulator (with facebow transfer). As part of the occlusal analysis of the study models in the adjustable dental articulator, the vertical dimension of the occlusion (which may be adequate or reduced) can be assessed as part of the diagnostic procedure; resulting changes in the horizontal jaw relation (joint position of the centric relation); the degree of wear of the tooth structure on the remaining natural teeth as a result of parafunctional activity of the masticatory system in the form of bruxism; the change in the alignment of the occlusal/prosthetic plane; the presence of centric and eccentric premature occlusal contacts; the position of the teeth of the potential abutments of the prosthetic restorations (which may be inclined in all directions; in supraocclusion; in infraocclusion), etc. The above parameters may also influence the prosthetic treatment plan (selection of the type of prosthetic restoration) and the choice of occlusal scheme. The clinician has the option of choosing all three types, from canine guidance to group function to bilateral balanced articulation (e.g. when one jaw is edentulous and a complete prosthesis needs to be made and there is partial tooth loss in the opposing jaw that can be treated with a fixed restoration or removable partial denture) [24].

Static and dynamic occlusal contacts can be marked on the biting surfaces with thin articulating papers (in different colors) or with a shimstock foil. The resulting marks on the tooth surfaces provide information about their position, distribution, and intensity. When articulating paper is used to identify and verify static and dynamic occlusal contacts in the patient’s mouth, the procedure is not entirely reliable in terms of leaving an adequate trace of each mark on the biting surface of the teeth due to the moist environment in the oral cavity. Even if articulating papers with a thickness of 200 μm are used, they leave a trace of a larger and unclear surface, which makes them additionally “positively false”. Therefore, for testing and marking occlusal contacts, shimstock thickness (8–10 μm) or articulating paper thickness (12–40 μm) are more reliable, provided that a “dry working field” has been achieved on the occlusal surfaces [73, 74, 75]. Kernstein and Radke [76] also claim that the use of articulating paper in occlusal analysis is influenced by the clinician’s subjective and inaccurate assessment, as it is not possible to distinguish how the differences between heavy and light loads/bites on the articulating paper affect the intensity of occlusal contact marks. They propose a more objective method of occlusal analysis where it is possible to determine the relative occlusal force and duration of static and dynamic occlusal contacts. Devices (e.g., the T-scan device) for so-called computer-assisted occlusal analysis [77, 78] are capable of this type of occlusal analysis. It should be noted that these devices are currently used more in scientific research and are slowly finding their way into clinical practise.

When fabricating fixed crowns and bridges supported by natural teeth, the occlusal settings are adapted to the static and dynamic occlusal contacts of the remaining natural teeth. If no natural canines are present (due to extraction or, for example, in individuals with Angle class anomalies II and III, where canine guidance is not realistic), the group function concept can be used for the above types of prosthetic restorations. In this case, with the group function concept, the central and lateral incisors take over the anterior guidance instead of the canines and “protect” the posterior teeth from overloading. In addition, a detailed occlusal analysis is important as it shows all tooth contacts and anterior guidance that should be maintained when preparing the teeth. It also allows clinicians to decide whether to maintain the existing occlusal scheme (conformative approach) or to modify the existing occlusal scheme (reorganized approach) [79].

After analyzing the occlusion of the existing condition of the masticatory system according to the same principles, the selection of an occlusal scheme (canine guidance and group function) for a removable partial denture can be recommended. Therefore, in a partially edentulous patient in whom, the fabrication of a removable (metal) partial denture (RPD) or a combined removable fixed prosthetic restoration (metal RPD + surveyed crowns + dental attachments) is indicated, the objectives of establishing an appropriate occlusal scheme should be combined in such a way that the supporting teeth, gingiva and alveolar bone are functionally appropriately loaded (within the adaptability of the aforementioned tissues) and that they are harmonized with the morphology and function of the masticatory muscles and temporomandibular joints [80].

3. Occlusal concepts in the fabrication of implant prosthetic restorations

In the recent literature, numerous factors are mentioned (e.g. bone quality and density; type, surface and design of dental implant; type of prosthetic restoration and its passive fit on dental implants; skills and experience of the clinician; health status of the patient (influence of systemic diseases); oral parafunctions; smoking, oral hygiene and others) that can affect the long-term clinical success of implant prosthetic therapy. Among other factors, the biomechanical environment in which implant prosthetic restoration’s function is cited as a major cause (along with infectious factors) of initial and long-term bone loss around dental implants. Biomechanical loading refers to the way in which occlusal/masticatory forces are transferred to dental implants by different types of prosthetic restoration, which occurs during static and dynamic occlusal contacts, i.e. mandibular movements [5, 6, 81, 82]. Scientific studies [83, 84, 85] have shown that mandibular movements and function (by form and velocity), chewing efficiency (habitual chewing) are very similar in natural dentition and implant prosthetic restorations compared to wearers of removable conventional complete dentures thanks to the phenomenon of osseoperception.

In general and historically, the parameters of a therapeutically functionally optimal occlusion to ensure longevity (test of time) and success of prosthetic restoration are as follows: (a) simultaneous bilateral occlusal contacts and even distribution of occlusal forces; (b) absence of premature occlusal contacts in the centric; (c) smooth, even, symmetrical lateral and protrusive movements of the mandible without premature occlusal contacts on the non-working sides of the dental arches [1, 4, 86, 87, 88]. In other words, the concepts of occlusion in natural teeth were transferred, with certain modifications, to the concepts of occlusion in implant prosthetic restoration. The clinician’s responsibility in selecting an occlusal scheme for implant prosthetic restoration is to minimize occlusal overload at the interface between the alveolar bone and the surface of the dental implant. In addition, an accurate diagnosis and treatment plan provides for an appropriate number and placement of dental implants inserted for removable and fixed implant prosthetic restoration, a passive fit of the prosthetic restoration supported by the implants, and progressive loading to increase the amount and density of bone around the implant to reduce the risk of loading beyond physiological limits [89].

Following the same principle, Misch [81, 89] introduced an occlusal scheme for implant prosthetic restoration at the end of the last century, called implant-protective occlusion (IPO) or medially positioned lingualized occlusion. The IPO concept considers several conditions to reduce the stress on the implant-bone interface. These include: the timing of occlusal contacts, the influence of the implant surface, the mutually protected articulation, the angle of the implant or crown body to the occlusal load, the cusp angle of the crowns, the cantilever (offset) distances, the crown height, the crown contour, the protection of the weakest component and the occlusal material of the implant crowns. The following modifications must be considered when creating the “IPO scheme” for implant prosthetic restoration: The occlusal morphology of the prosthetic restoration must direct the occlusal load in the axial direction, use a narrower occlusal plane, reduce the inclination of the tooth cusps, reduce the length of the cantilever in the mesiodistal and buccolingual directions and use crossbite occlusion in certain situations. In short, the principles of the “IPO scheme” are bilateral stability in the central (habitual) occlusion, even distribution of occlusal contact and load, no premature contacts between the centric relation and central occlusion, realization of the “freedom in centric” (relation between functional/supportive cusps and central fossae of the posterior teeth based on a flat plateau of 1–1.5 mm), anterior guidance whenever possible; and free lateral and protrusive excursion of the mandible without non-working premature occlusal contacts [89].

Teeth with associated periodontium and surrounding bone are inherently biomechanically designed to absorb excessive occlusal/masticatory forces, which is not the case with osseointegrated dental implants without periodontium, which are therefore more sensitive to occlusal overload. The mobility of a natural tooth under axial masticatory loading is in the range of 25–100 μm, that of a dental implant is 3–5 μm. Under axial loading, the periodontium of a natural tooth allows the tooth to move according to the indicated range and the functional adaptation of the alveolar bone (“shock or stress absorber”). The dental implant “reacts” to the axial load with a linear deflection to the extent allowed by the elastic deformation of the alveolar bone. Therefore, the compressibility and deformability of the periodontium of a natural tooth under axial loading may make a difference in adapting to static and dynamic occlusal forces compared to osseointegrated implants. A natural tooth subjected to transverse or lateral loading can move rapidly by 56–108 μm, rotating in the apical third of the root, and the periodontium of the tooth tends to immediately reduce this direction of force from the crestal bone towards the root. On the other hand, the movement of the implant under the same lateral load is gradual, reaching a displacement of 10–50 μm, and the concentration of the force is greater at the level of the crestal bone around the shoulder of the implant, without the possibility of implant rotation. In addition to direction, occlusal forces may show differences in duration and intensity during normal or parafunctional activity of the masticatory system. Conditions of increased muscle activity due to bruxism, unbalanced static and dynamic occlusal contacts on different types of implant prosthetic restorations, lack of passive fit between prosthetic components (mesostructure), prosthetic restorations and dental implants are potential factors for prolonged and more intense occlusal overload, which consequently increases the occurrence of biological and technical complications in implant prosthetic restorations [82, 89, 90, 91, 92, 93].

It must be said, however, that there are studies and opinions of authors [94, 95] who believe that the role of occlusal overload as a causative factor in the occurrence of peri-implant bone loss has not yet been scientifically proven and that this should be done by future studies. The relationship between mechanical loading and biological consequences (increase in peri-implantitis rate) on bone response has been established, but specific thresholds have not been correlated with prosthetic design (crown-implant length ratio, cantilever implant prostheses and splinting) and occlusal scheme guidelines.

3.1 Occlusal concepts in the fabrication of removable implant prosthetic restorations (overdentures)

The fabrication of removable implant overdentures in edentulous maxillae and mandibles represents a higher standard of treatment for completely edentulous patients. According to the ITI classification, there are three loading protocols for dental implants [96]: conventional, early and immediate loading. When fabricating lower overdentures in the edentulous mandible, the standard protocols are the conventional and immediate loading protocols (2–4 dental implants placed) and for upper overdentures the conventional protocol (4 dental implants placed, mainly due to the lower quality and density of the upper edentulous alveolar ridge). From a biomechanical point of view, the choice of an occlusal scheme for removable implant prosthetic restoration has the effect of reducing the magnitude of the load and the mechanical stress/strain force, especially in the crestal bone area around osseointegrated dental implants. As in the fabrication of conventional full dentures, occlusal schemes of bilateral balance occlusion and its variants are most frequently mentioned in the published papers. The lingualized occlusion is recommended in clinical situations when the edentulous alveolar ridges are preserved, while the so-called monoplane (flat plane) occlusion is recommended in extremely atrophic ridges [97, 98, 99, 100]. On the other hand, Aarts et al. [101] compared patient satisfaction with the physiological occlusion (in terms of features most similar to the concept of canine guidance (unbalanced occlusion)—used for complete dentures with anatomical posterior acrylic teeth [8]) and lingualized occlusion over a 3-year period of wearing a lower overdenture supported by two implants and an upper conventional complete denture. Most patients rated prosthetic restorations with physiological occlusion better, which justifies the authors in concluding that, in addition to the expected balanced occlusion schemes, unbalanced occlusion concepts can also be used for implant-supported overdentures.

There are no clinical scientific studies that favor balanced occlusal schemes over nonbalanced ones and vice versa. The reasons for this are that there are not many published studies comparing different occlusal schemes for overdentures supported by dental implants. The most common are comparisons of occlusal schemes between the lower overdenture supported by two dental implants and the upper conventional complete denture (no comparison in relation to the natural dentition, different types of fixed, removable prosthetic or implant prosthetic restorations and possible combinations); the small number of subjects participating in this type of research (shortcomings in the study design); and the objective finding that this type of clinical research on this topic is very difficult to conduct for ethical, financial and other reasons.

3.2 Occlusal concepts in the fabrication of fixed implant prosthetic restorations (single crowns, fixed bridges and full arch implant-supported restorations)

When fabricating fixed implant prosthetic restoration such as single crowns or bridges with a shorter span (maximum five to six units), the application of the occlusal scheme is mainly determined by the loading protocol (conventional, early and immediate) of the dental implants and the status of the remaining natural occlusion. In such situations, the existing status of the natural occlusion (static and dynamic occlusal contacts) should be checked and recorded before starting implant-prosthetic therapy. If there is an indication for conventional loading, the occlusal characteristics registered on the natural dentition are applied after a phase of osseointegration. This would mean that the static occlusal contacts on the crown or bridge in the position of maximum intercuspation should be spatially distributed as close as possible to the axial axis of the inserted dental implant (reduced inclination of the tooth cusps in this type of prosthetic restoration to reduce the generation of damaging lateral forces due to leverage). Then, the distance between the single crown or fixed bridge in relation to the antagonist tooth/teeth is 30 μm (due to the lack of tooth periodontium), which avoids subjecting the prosthetic restoration to excessive forces during higher loading (e.g. bruxism). Adequate clearance between the centric relation position and the maximum intercuspation (1–1.5 mm) provides more favorable vertical load lines, reducing the possibility of premature contacts in the centric during function. The anterior and lateral guidance of the mandible (dynamic occlusal contacts) is provided by the patient’s natural teeth (canine guidance or group function) and there are no premature working and non-working contacts during excursion movements of the mandible. In summary, the occlusal parameters mentioned are parameters that describe the occlusal scheme of implant-protective occlusion [5, 81, 89, 98, 102, 103].

However, in the indication for immediate loading of dental implants and the fabrication of single crowns and fixed bridges with shorter spans with preserved natural dentition, there are some changes regarding occlusion. First, this form of implant prosthetic therapy consists of two prosthetic phases. In the first phase, a provisional or temporary prosthetic restoration is fabricated on newly placed dental implants due to one or more lost teeth, using a “softer” prosthetic material such as polymethyl methacrylic (PMMA), composite, metal-acrylic or metal-composite. The aim of this approach with the use of “softer prosthetic material” is to protect newly placed dental implants from excessive functional and parafunctional forces (e.g. bruxism, severe clenching) in a partially edentulous patient [104, 105]. Second, for the same reasons, two occlusal loading protocols for dental implants have been proposed and applied: the immediate functional loading protocol and the immediate non-functional loading protocol. Immediate functional loading assumes that, in this case, a temporary single crown or a fixed bridge supported by a newly placed dental implant is in occlusal contact with the opposite part of the dental arch. The protocol of immediate non-functional loading implies that there is no occlusal contact (neither static nor dynamic contacts) between the provisional prosthetic restoration and the opposing dental arch [106]. The published scientific studies on this subject provide contradictory results and interpretations. There are studies [107, 108] that report lower survival rates of dental implants after immediate functional loading than after immediate non-functional and even delayed conventional loading. Other authors observe no differences between immediate functional and immediate non-functional loading in terms of implant survival, peri-implant bone loss or soft tissue healing, especially in the mandible, at short or medium follow-up periods [109, 110, 111, 112, 113]. In contrast to the mandible, the immediate loading protocol for single crowns and fixed bridges is carried out more cautiously in the maxilla. Zarrabi et al. [114], in their randomized clinical trial comparing a non-functional immediate protocol and a conventional protocol in the posterior maxilla, found no differences in the follow-up results (1 year) of clinical and radiological parameters evaluating implant prosthetic therapy. Once the period of osseointegration has elapsed (3–6 months), provisional fixed implant prosthetic restorations can be replaced by definitive single crowns and fixed bridges made of different prosthetic materials, using occlusal schemes based on the principles described in the first paragraph of this chapter.

In modern digital dentistry, the so-called “digital workflow” allows the virtual design of different types of prosthetic restorations supported by dental implants in accordance with the principles of the chosen occlusal scheme. Figures 9–17 show the possibilities of virtual occlusal design in the fabrication of single crowns supported by dental implants in the partially edentulous mandible. In the specialized software, there is the possibility to perform mandibular movements in the virtual articulator and to adjust static and dynamic occlusal contacts to definitive prosthetic/implant prosthetic restorations. This reduces the need to grind the occlusal contacts in the patient’s mouth and compromise the mechanical integrity of the finished prosthetic restoration.

Figure 9.

Fabrication of fixed implant prosthetic restorations—virtual design of single crowns supported by dental implants.

Figure 10.

Fabrication of fixed implant prosthetic restorations—virtual design of single crowns supported by dental implants. Display of modeling in the position of maximum intercuspation in the virtual articulator.

Figure 11.

Movement of the mandible in the virtual articulator (right laterotrusion).

Figure 12.

Movement of the mandible in the virtual articulator (left laterotrusion).

Figure 13.

Movement of the mandible in the virtual articulator (protrusion).

Figure 14.

Point occlusal contacts on the supporting cusps of modeled single implant crowns to achieve optimal load transfer along the longitudinal axes of the inserted dental implants (right lateral view).

Figure 15.

Point occlusal contacts on the supporting cusps of modeled single implant crowns to achieve optimal load transfer along the longitudinal axes of the inserted dental implants (left lateral view).

Figure 16.

Point occlusal contacts on the supporting cusps of modeled single implant crowns to achieve optimal load transfer along the longitudinal axes of the inserted dental implants (occlusal view).

Figure 17.

Final appearance of single implant crowns (screw retained) in the mandible (0cclusal view).

The fabrication of full-arch fixed implant prosthetic restorations in completely edentulous patients also has its own peculiarities and occlusal characteristics, especially since they can be made of different prosthetic materials (e.g. metal-acrylic, metal-ceramic and all-ceramic prostheses). Between biological and technical complications in implant prosthetic restoration, technical or prosthetic complications are more common, some of which can lead to occlusal instability and occlusal overload. This type of implant prosthetic restoration is most often applicable through a conventional and immediate protocol (e.g. All-on-4 concept) of loading dental implants with a high survival rate (95–100%) at a follow-up of at least five years or more [115, 116, 117].

For conventional loading, the characteristics of the occlusion scheme for definitive full-arch fixed implant prosthetic restoration are closest to those of implant-protective occlusion [81, 89], which combines the characteristics of natural dentition and lingualized occlusion.

Immediate loading is a little different. Temporary fixed implant prosthetic restorations are made of a “softer” prosthetic material (e.g. PMMA or metal acrylic) and the occlusion requirements fall under the protocol for immediate loading. This means that the temporary fixed implant prosthetic restoration has established stable centric static occlusal contacts with the opposing dental arch. If cantilevers are present on fixed prosthetic restorations, the cantilevers should be left out of contact (10–30 μm). As this is a one-piece fixed prosthetic construction (in other words, the dental implants are connected or splinted by a prosthetic restoration), the distribution of static and dynamic occlusal contacts is within the area (polygon) created by the connection of the installed dental implants. Splinting of dental implants with fixed restorations, passive fit of a fixed structure supported by implants, harmonized static and dynamic occlusal contacts (without premature interference) ensure successful completion of osseointegration of dental implants. After six months of osseointegration of dental implants, the provisional prosthetic restorations can be replaced and final fixed implant prosthetic restorations with different prosthetic materials can be started and completed.

Recently, Yoon et al. [118] in their review suggested occlusal considerations or guidelines based on the prosthetic material chosen when performing a full-arch fixed implant prosthetic restoration in both edentulous jaws. They presented five combinations of prosthetic materials and natural dentition, focusing on centric and eccentric occlusal relationships. In the situation where a fixed metal acrylic implant prosthetic restoration is fabricated in a centric relationship compared to a metal acrylic restoration, the following should be achieved: simultaneous bilateral contacts; “freedom in centric” (1–1.5 mm) and on the cantilevers and anteriors a distance of 10 μm (thickness of the Schimstock foil); and during excursion movements: in lateratrusion (group function); in protrusion (shallow anterior guidance) and without occlusal contacts on the cantilevers. Combination of full-arch fixed metal-acrylic bridge and natural dentition in centric relationship: simultaneous bilateral contacts; “freedom in centric” (1–1.5 mm) and on the cantilevers and anteriors 10 μm; in excursion movements: in laterotrusion canine guidance if canines are preserved/group function if canines are compromised; in protrusion shallow anterior guidance and no occlusal contacts on the cantilevers. The third combination of prosthetic materials shows centric and eccentric occlusal relationships when fixed implant prosthetic restorations are made of all-ceramic systems such as zirconia ceramics (zirconia framework design: fully contoured monolithic and veneering ceramics limited to facial/buccal). In centric relationship, simultaneous bilateral contacts are present; 10 μm distance at cantilevers; equal intensity of contact at anterior and posterior teeth; and “freedom in centric”. In case of excursion movements of the mandible, i.e. laterotrusion, group function; in case of protrusion, shallow anterior guidance and no occlusal contact at the cantilever. Combination of all-ceramics and fixed metal-acrylic prosthetics in centric: simultaneous bilateral contacts; 10 μm spacing on cantilevers and anterior teeth; and “freedom in centric”. In lateratrusion, group function; narrow anterior guidance of protrusions and without the presence of occlusal contacts on the cantilever. The last combination shows an all-ceramic and natural dentition. In a centric relationship: simultaneous bilateral contacts; 10 μm spacing on cantilevers and anterior teeth; and “freedom in centric”. In an eccentric relationship: in laterotrusion, canine guidance when canines are preserved/group function when canines are compromised; in protrusion, shallow anterior guidance, and no occlusal contacts on cantilevers. Türker et al. [119] compared three occlusal schemes (canine guidance, group function and lingualized occlusion) using finite element analysis (in vitro study) in full-arch All-on-4 restorations. The stress/strain forces transmitted from the occlusal contacts to the prosthetic restorations, the abutments, and the screws during maximum intercuspation and excursion movements were measured. The most even distribution of forces on the mentioned prosthetic components was recorded with the group function. Different prosthetic materials have different mechanical properties in function and should be considered when designing an occlusal scheme for full-arch fixed implant prosthetic restoration. The article is transparent and very useful for clinicians working on this topic, although the authors themselves point out that the recommendations are “experience-based” and not “evidence-based”.

4. Conclusion

The selection of an occlusal scheme for prosthetic restoration supported by natural teeth or osseointegrated dental implants in fully and partially edentulous patients must be adapted to each individual patient, with the aim of ensuring the longevity of the prosthetic restoration in accordance with the health and function of all components of the masticatory system. The characteristics of the natural dentition in relation to the characteristics of the “prosthetic/implant prosthetic occlusion” in the fabrication of fixed and removable prosthetic or implant prosthetic restorations are presented and compared. The design of an artificial prosthetic occlusion or occlusal scheme aims to ensure optimal load transfer to supports such as natural teeth or dental implants, mucosa, and alveolar bone and to reduce the occurrence of biological and mechanical complications.

Unfortunately, it is almost traditional that when this topic is written about, the most common conclusion of published papers is that occlusion is an important and contradictory factor in prosthetic therapy, but that there is no significant scientific evidence recommending one type of occlusal scheme over another [95]. The main argument for this is that it is difficult to design and conduct well-controlled long-term studies with large sample sizes to compare different occlusal schemes. Clinicians are on their own and must apply their knowledge of occlusion acquired in training, through experience and practise, and their common sense in their daily work. The good thing is that with this knowledge and skills about occlusion, the prosthetic restorations will work in the patient’s mouth. It will stay that way until future research proves otherwise.