Occlusal Overload in Comparing Natural Teeth vs. Dental Implants
|Characteristic||Natural Teeth||Dental Implants|
|Interface||Periodontal membrane||Direct bone|
|Junctional epithelium||Hemidesmosomes and basal lamina (lamina lucida and lamina densa zones)||Hemidesmosomes and basal lamina (lamina lucida and lamina densa and sublamina lucida zones)|
|Connective tissue||12 groups: 6 insert perpendicular to tooth surfaces; ↓ collagen, ↑ fibroblasts||Only 2 groups: parallel and circular fibers; no attachment to implant surface; ↑ collagen, ↓ fibroblasts|
|Vascularity||Greater; supraperiosteal and PDL||Less; mainly periosteal|
|Biologic width||2.04–2.91 mm||3.08 mm|
|Pain||+/− (tooth may be hyperemic)||−|
|Attrition||+ wear facets, abfraction, fremitus||− (~ porcelain fracture, possible screw loosening)|
|Radiographic changes||+ increased radiopacity and thickness of cribriform plate||Crestal bone loss|
|Interference awareness||+ (proprioception)||− (osseoperception)|
|Nonvertical forces||Relatively tolerated||Results in bone loss|
|Force related movement||Primary: movement of PDL||Primary: osseous movement|
|Secondary: osseous movement|
|Lateral force||Apical of root surface||Crestal bone|
|Lateral movement||56–108 µm||10–50 µm|
|Apical movement||25–100 µm||3–5 µm|
|Signs of overloading||PDL thickening, fremitus, mobility, wear facets, pain||Screw loosening, screw fracture, abutment fracture, implant body fracture, bone loss|
Peripheral Feedback System
The control of the muscles of mastication is directly related to a peripheral feedback system that includes the enamel-dentin-pulp complex and mechanoreceptors in the dentate dentition. However, endosseous implants lack the periodontal proprioception feedback system, resulting in less fine motor control and alteration of the awareness of force.
Natural teeth have a unique sense of occlusal awareness in the control of mastication, force, and control of muscles during swallowing. The role of the enamel-dentin-pulp complex has been shown in the control of oral function via the peripheral feedback system.13 In combination with the periodontal mechanoreceptors (PMRs), afferent information is sent to the brain on the horizontal and vertical forces applied to teeth. The mechanoreceptors are very sensitive to low forces and allow for the recognition of occlusal force and the discrimination of load magnitude and direction. The sensitivity of teeth varies with anterior teeth being most sensitive to low forces (<1 N), whereas posterior teeth exhibit sensitivity to forces in the range of less than 4 N.
The number of mechanoreceptors have been shown to be more numerous around anterior teeth compared with posterior teeth. The anterior receptors relay information concerning forces in all directions because they have greater sensitivity to occlusal loading. These teeth contribute to the positioning of food in the oral cavity and manipulation of objects between the teeth.14
In comparing individual teeth, the cuspid plays a dominant role in the control of function. Anatomy (large crown and long root) and location in the dental arch (cornerstone position) allow for the cuspid to have an ideal physiologic feedback system from the mechanoreceptors. Additionally, the morphologic makeup of the cuspid, featuring a lingual contour that divides the mesial and distal via an axial ridge, allows for crown strength for anterolateral guidance. Because of the contour, arch location, and root length, this allows for the preferred mesial (not distal) canine guidance in mastication to confirm there is a distinct anterior component of the jaw and ipsilateral condylar movement.14
The posterior teeth in comparison have fewer mechanoreceptors, which results in a lower static and dynamic sensitivity allowing for posterior teeth to accommodate higher forces during mastication. The posterior teeth are strategically located with large clinical crowns and multiple roots (increased surface area) to sustain the increased posterior forces. Additionally, the first molars provide guidance during the eruption process and are functionally positioned in the center of the occlusal table.14
With dental implants, studies have shown that the lack of mechanoreceptors may influence jaw motor control because of a peripheral feedback system.15 Because of the absence of these mechanoreceptors, there is lack of afferent information to be relayed to the central nervous system (CNS) during biting and chewing with dental implants. The proprioception associated with dental implants is similar to natural teeth that are blocked by local anesthesia.16 However, the term “osseoperception” has been used to describe a different type of mechanical stimulation specific to dental implants. This type of sensation has been associated with mechanoreceptors in the orofacial tissues (e.g., most likely present in muscle, joint, mucosa, periosteal tissues).17 PMRs have been described to react to very low levels of force, and the detection of static force is approximately 10 times greater for patients with implants compared with dentate patients.18 The mechanoreceptors that are responsible for osseoperception have been shown to be located a distance from the actual implant and qualitatively remit different sensory signals to static and dynamic loads.
Many differences exist in comparing clinical aspects of force-related factors between natural teeth and dental implants.
Vertical Occlusal Loads
When a vertical occlusal load is applied to a natural tooth, a normal physiologic movement exists that is related to the surface area and root morphology. Therefore, the number, length, diameter, position, surrounding bone density, and health of the PDL has a primary influence on a tooth’s mobility. A healthy tooth under normal conditions exhibits zero clinical mobility in a vertical direction. Actual initial vertical tooth movement has been shown to be approximately 28 µm and is the same for anterior and posterior teeth (Fig. 17.2).19
Rigid fixation is a clinical term used to describe the absence of clinical mobility of an implant tested with vertical or horizontal forces less than 500 g. Osseointegration is a histologic term that is used to define bone in direct contact with an implant surface at the magnification of a light microscope (Fig. 17.3). Over the years, these two terms have been used interchangeably, and implant abutment support is most predictable with rigid fixation. Lack of implant mobility does not always coincide with a direct bone-implant interface.20 However, when observed clinically, rigid fixation usually means that at least a portion of the implant is in direct contact with bone, although the percentage of bone contact cannot be specified.21 A mobile implant usually indicates the presence of connective tissue between the implant and bone and soft tissue interface.
Lack of clinically observable movement does not mean the true absence of any movement. A healthy implant moves less than 73 µm; it appears as zero clinical mobility (rigid fixation). Sekine et al22 applied a gradually increasing load over a 2-second period to a tooth and an implant. The teeth moved immediately with a light load (primary tooth movement) and less with an additional load (secondary tooth movement). The implant, in contrast to the natural tooth, did not feature primary tooth movement. A heavier force caused the implant to move gradually, similar to the secondary tooth movement (Fig. 17.4).
Nonvertical Occlusal Loads (Horizontal)
When evaluating horizontal mobility on natural teeth, it is often difficult to determine the true movement. For example, a “nonmobile” posterior natural tooth actually moves horizontally 56–73 µm. The human eye does not perceive this movement. The anterior teeth, which often have slight clinically observable movement, actually can move approximately 0.1 mm.
With nonvertical loading, forces on natural teeth are better tolerated and will adapt to the force much more favorably than dental implants. Studies have shown that lateral forces on a healthy natural tooth are rapidly dissipated away from the bone crest toward the apex of the tooth. This is because natural teeth may move 56–108 µm with rotation around the apical one-third of the root. Muhlemann23 found that horizontal tooth movement may be divided into initial mobility and secondary movement. The initial mobility is observed when a light force is applied, will occur immediately, and is a consequence of the PDL. Initial horizontal tooth mobility is greater than initial vertical movement. A very light force (500 g) horizontally moves the tooth. The initial horizontal mobility of a healthy, “nonmobile” posterior tooth is less than that of an anterior tooth and ranges from 56–75 µm, which is two to nine times the vertical movement of the tooth. Initial horizontal mobility is even greater in anterior teeth and ranges from 70–108 µm in health.24
The secondary tooth movement described by Muhlemann occurs after the initial movement when greater forces are applied. When an additional force is applied to the tooth, a secondary movement is also observed, which is related directly to the amount of force. The secondary tooth movement is related to the viscoelasticity of the bone and can measure 40 µm under considerably greater force (Fig. 17.5).
With dental implants, the nonvertical stress will most likely result in trauma to the supporting bone. An implant will gradually move and may move 10–50 µm.19 An implant will have greater forces around the crest of the bone because an implant does not pivot as much as a tooth does, and all the forces are generated at the crest, which usually will result in bone loss.
Sekine et al25 evaluated the movement of endosteal implants with rigid fixation and found a range of 12–66 µm of movement in the labiolingual direction. Komiyama26 reported 40–115 µm of implant movement in the mesiodistal direction under a force of 2000 g (≈4.5 psi) and a labiolingual range of 11–66 µm. The greater implant movement in the mesiodistal dimension corresponds to the lack of cortical bone between the implants in this direction compared with the thicker lateral cortical plates present in the labiolingual dimension. Rangert et al27 suggested that part of this implant movement may also be due to component flexure of the implant abutment and screw. The mobility of implants varies in direct proportion to the load applied and the bone density, reflecting the elastic deformation of bone tissue.
Excessive Contacts/Occlusal Overloading
When premature contacts or excessive contacts occur on a natural tooth, the tooth will often become mobile, become hyperemic, or fracture. The associated proprioception will allow the neuromuscular system and peripheral feedback system to control the occlusal forces during function. This will minimize the possibilities of prematurities and interferences. In comparison, when a premature contact is associated with a dental implant, the patient usually is asymptomatic, and the implant will have no mobility. Because of the lack of PMRs, there is no feedback system present. This will most likely result in crestal bone loss or mechanical failure will result because there is no feedback system to make the patient aware of a prematurity.
Prematurities on natural teeth are less common and may be uneventful for years. Natural teeth may orthodontically reposition themselves or become symptomatic. However, excessive contacts on dental implants are more damaging because they will usually result in a force overload with resultant crestal bone loss (Fig. 17.6).
With a natural tooth, the specialized PMRs are responsible for providing neural information to the muscles during mastication. Several hundred of these specialized receptors are present, which are used for fine motor control and conscious perception of tactile forces when applied to the teeth.17
The compression of the collagen fibers in the PDL will send nerve signals to the CNS. These receptors surrounding teeth have different sensitivities to the force applied, and each individual receptor is stimulated independently depending on the position within the PDL. The resultant force vectors have varying numbers depending on the anatomic location within the oral cavity. Usually, the number of force vectors decreases from anterior to posterior teeth. The anterior natural teeth may perform very delicate tasks, whereas the posterior teeth are less sensitive. This is why the anterior teeth can split food very precisely, whereas the posterior teeth are mainly used to grind food.28
Many studies have evaluated the masticatory efficiency between teeth and implants. Svensson et al29 completed studies on the mastication effectiveness of food between (1) natural teeth, (2) natural teeth with full coverage restorations, and (3) implant-supported restorations. Results showed that natural teeth were far superior in the refined action of splitting or incising food. Natural teeth were shown to be significantly better able to position the food boluses between the teeth and be able to fine-tune the direction of the bite force. This is most likely the result of the signal of the PMRs to the CNS on the spatial location and direction of forces needed to chew the food. However, with splinted fixed crowns and implant prostheses, the results showed inconsistent and poor incising of food. With tooth-supported prostheses, even though PMRs are present, the tooth-supported prosthesis (splinted crowns) dissipates the force, and the PMRs are not activated. With an implant prosthesis, because no PMRs are present, no peripheral feedback system exists to allow for the refined motor movement (Fig. 17.7). These findings are consistent with studies by Trulsson and Gunne,30 in which prostheses supported by the oral mucosa (removable complete dentures) or dental implants (fixed prostheses) had difficulty in holding and splitting food, similar to patients with natural teeth and profound anesthesia. The afferent information is blocked by the local anesthesia (Fig. 17.8).
Natural teeth are associated with greater masticatory efficiency, and less trauma to the dental complex will be present. Patients are able to refine their masticatory movements because of the mechanoreceptors that are present. However, with dental implant prostheses (also tooth-supported prostheses), the masticatory refinement is much poorer, leading to the greater possibility of angled- or force-related complications to the implant or prosthesis.
Speed of Mastication
Patients with natural teeth take longer to masticate food compared with tooth-supported and implant prostheses. This is most likely due to natural teeth requiring time for the PMRs to signal the CNS. This slow and delayed response is the result of the timing required to collect and process the spatial information and stimulate the appropriate motor program and efferent output to manipulate the muscles to fire with respect to the direction and amount of bite forces. The processing of PMRs requires time for the central processing, resulting in a longer contact phase of mastication. These results are consistent with other studies that have shown patients with natural teeth will initially hold the food under low force directly after contact and then will apply greater biting forces. There is a delay between the contact of the food and the splitting of the food.
Patients with a tooth-supported fixed prosthesis and implant prosthesis have altered spatial information. Although patients with tooth-supported fixed partial dentures (FPDs) have PMRs, because of the rigid connection, minimal signals to the CNS exist to initiate motor function. In patients with an implant-supported prosthesis, osseoperception must occur. This involves signaling sensory information concerning contact forces (dynamic loading) to the artificial teeth in an osseointegrated dental implant fixed prosthesis from remote receptors in other tissues activated by vibrations transmitted via the jawbone.31 The sensory information of PMRs is significant with respect to masticatory efficiency and protection of the dentition. Impairment (tooth-supported FPD) or absence (implant-supported prosthesis) results in poorer performance and altered motor activity. Patients with tooth-supported and implant-supported prostheses also have a compromised protective mechanism in which they masticate their food with higher force, less delay (i.e., may not be able to comprehend that the food is too hard), and less precision. With increased speed of mastication, the greater possibility of damage to the prosthetic system exists.
PMRs present in natural teeth provide proprioception and early detection of occlusal forces and interferences. Bite forces on natural teeth during mastication and parafunction are not as strong because of the fine motor control of the mandible. Trulsson and Johansson16 showed that the lack of proprioception leads to a heavier bite in patients with implants compared with natural dentition. Mericske-Stern et al32 measured the oral tactile sensibility with test steel foil and showed that minimal pressure was significantly higher with implants than with natural teeth (3.2 vs. 2.6 steel foil sheets). Jacobs and van Steenberghe33 evaluated occlusal awareness and found that interference perceptions of natural teeth, implants with opposing teeth, and implants opposing implants were approximately 20 µm, 48 µm, and 64 µm. Hämmerle et al34 also concluded the mean threshold value of tactile perception for implants (100.6 g) was ninefold higher than that of natural teeth (11.5 g).
Biting forces are significantly higher with an implant-supported prosthesis compared with natural teeth, and this may lead to excessive forces on the implant system because of lack of awareness. The sensory feedback system present in teeth cannot be modulated the same around dental implants. The increased biting force may lead to abnormal forces on the dental implant system, which may lead to crestal bone loss, screw loosening, or component fracture (Fig. 17.9).
Because of the biomechanical differences between teeth and implants, modifications must be made in the development of occlusal schemes for prosthetic rehabilitation. An ideal occlusal scheme is a primary requisite for long-term implant prosthetic survival, especially when parafunction or a marginal foundation is present. A poor occlusal scheme increases the magnitude of loads and intensifies mechanical stresses (and strain) to the implant system. These factors increase the frequency of complications of the prosthesis and bone support. Crestal bone loss most likely leads to an increase in peri-implantitis. The conditions may also cause tissue shrinkage and loss of interdental papillae and poor esthetic results. All these complications may be initiated by biomechanical stress as a result of excessive occlusal loads (functional or parafunctional).
The concept of implant-protected occlusion (IPO) was developed by Misch. It is unique and specifically designed for the prosthetic rehabilitation of dental implants. This protocol provides for an environment to reduce the biomechanical stress to the implant and prosthesis. Minor modifications from conventional prosthodontic occlusal concepts have been established with the sole purpose of reducing stress on the prosthesis.
The ideal occlusion for an implant prosthesis is to control the stress on the implant system, provide a prosthetic and biologically acceptable implant interface, and maintain long-term stability of the marginal bone and prosthesis. The occlusal scheme should maintain the occlusal load that has been transferred to the implant system within the physiologic and biomechanical limits of each patient. However, these principles are not identical for all patients or restorations. The forces generated by a patient are influenced by ranges of parafunction, masticatory dynamics, implant arch position and location, arch form, crown height, and crown morphology. The treatment planning philosophy for dental implants varies greatly and depends on these several parameters. The implant dentist can address these force factors best by selecting the most ideal implant position, number, and size; using the progressive bone loading concept in poorer bone densities; and selecting the appropriate occlusal scheme using stress-relieving design elements.
The following guidelines and principles have been established to restore fixed and removable implant-supported prostheses. The IPO principles for fixed and removable prostheses address several conditions to decrease stress to the implant system, including existing occlusion, implant body angle to occlusal load, cusp angle of implant crowns, mutually protected articulation, cantilever or offset loads, crown height, crown contour, occlusal contact position, timing of occlusal contacts, and protection of the weakest component (Box 17.1).
Preimplant Occlusal Principles
The first step in the process of treatment planning for dental implants is the fabrication of accurate diagnostic casts for the evaluation of the patient’s existing occlusion. The value of diagnostic casts or study models is crucial in all phases of dentistry, especially in oral implantology. When edentulous sites exist, the combination of continued bone loss and dentition changes related to missing teeth greatly increases the number of factors that must be considered for oral rehabilitation compared with traditional prosthodontic treatment. The implant dentist must determine the type of implant prosthesis initially (i.e., FP-1, FP-2, FP-3, RP-4, RP-5), followed by the number and location of ideal and optional abutment sites and the final occlusal scheme.
Diagnostic casts must be accurate reproductions of the maxillary and mandibular arches with complete representation of the edentulous areas (Fig. 17.10). Diagnostic casts mounted on an articulator allow for an initial evaluation for implant site selection, angulation requirements, prosthesis selection, existing occlusion, and fabrication of a surgical template. In addition, these study casts allow for a preoperative assessment of treatment options that may be discussed with other practitioners and laboratory technicians and during patient consultations.
To accurately assess the maxillomandibular relationship of the implant patient, proper mounting of the study casts must be completed using an articulator. An articulator is defined as a “mechanical instrument that represents the temporomandibular joints (TMJs) and jaws, to which maxillary and mandibular casts may be attached to simulate some or all mandibular movements.”35 Today, the use and indications for the various types of articulators employed in prosthetic dentistry are very controversial. At the present time, wide arrays of articulator types are available, with multiple ranges of movements and adjustments, making classification and nomenclature very confusing. In the dental literature, many different classifications exist; however, today the most simplistic and most often used classification parallels the “Glossary of Prosthodontic Terms.” Articulators maybe categorized into four groups according to the adjustability of the articulators. This classification is based on the ability of the articulator to accept the five most common patient records: (1) facebow transfer, (2) centric jaw record, (3) protrusive record, (4) lateral records (Bennett movement), and (5) intercondylar distance (Table 17.2 and Fig. 17.11).36
|Hinge Axis (Facebow)||CR Record||Protrusive Movement||Lateral Excursion||Bennett Movement|
|Nonadjustable (Simple and Average)||No||Yes||No||No||No|
|Semiadjustable||Approximate (arbitrary)||Yes||Yes (straight line)||Yes (straight line)||Approximate|
|Fully Adjustable||Yes (kinematic)||Yes||Yes (curved)||Yes (curved)||Yes|
To evaluate the patient’s occlusion, an accurate mounting of diagnostic casts with an open-bite registration and facebow transfer allows for the static and dynamic (semi or fully adjustable articulators) relationships of the teeth and edentulous ridges, without interference from protective neuromuscular reflexes. By evaluating the articulated study casts, abnormalities or interferences that are not easily detectable intraorally can be determined along with comprehensive information that is paramount in dental implant treatment planning (Box 17.2).37,38