From Reilly DT, Burstein AH: The elastic and ultimate properties of compact bone tissue, J Biomech 80:393–405, 1975.
Barbier and Schepers histologically evaluated implants loaded in the long axis versus off-axis loading in dogs.57 The long-axis–loaded implants had lamellar bone at the interface. Lamellar bone is mineralized and organized and is called load-bearing bone in orthopedics. The off-axis–loaded implants had woven bone at the interface. Woven bone is bone of repair. It is less mineralized, unorganized, and weaker than lamellar bone (Figure 31-19). Hence, the greater strains in the bone with off-axis loading may cause the bone to repair and places it at a higher risk of overload and resorption.
FIGURE 31-19A, A long-axis load to an implant found lamellar bone at the interface. B, An off-axis load to an implant found woven bone (bone of repair) at the interface, indicating higher strain conditions than ideal. (From Barbier L, Schepers E: Adaptive bone remodeling around oral implants under axial and nonaxial loading conditions in the dog mandible, Int J Oral Maxillofac Implants 12(2):215–223, 1997.)
In conclusion, the microstrain of the crestal bone is increased with an angled load and may shift from an axial load within physiologic limits to an angled load in the pathologic overload zone and, as a consequence, result in bone loss. The greater force, especially in shear, is generated to the entire implant system. The occlusal porcelain is weaker to shear and may fracture, the cement that retains the prosthesis is weakest to shear and may become unretained, the abutment screw more likely becomes loose with shear loads, the crestal bone region may resorb, and implant components fracture more often with higher shear loads. Therefore, when shear forces are increased with an angled load to the implant system, an attempt should be made to reduce the negative effect of angled loads.58
The primary component of the occlusal force therefore should be directed along the long axis of the implant body, not at an angle or following an angled abutment post (Figure 31-20). Angled abutments should be used only to improve the path of insertion of the prosthesis or improve the final esthetic result. The angled abutment, which is loaded along the abutment axis, transmits a significant moment load (i.e., tending to rotate or rock the implant) to the entire implant system.
FIGURE 31-20 An implant body ideally should be positioned perpendicular to the occlusal plane and along the primary occlusal contact. These maxillary posterior implants are placed over the opposing mandibular buccal cusps and are not vertical but perpendicular to the curves of Wilson and Spee.
Prosthetic Angled Loads
Greater crestal bone strains with angled forces have been confirmed with photoelastic and three-dimensional finite element analysis methods. Whether the occlusal load is applied to an angled implant body or an angled load (e.g., premature contact on an angled cusp) is applied to an implant body perpendicular to the occlusal plane, the results are similar (Figure 31-21). A biomechanical risk increases to the implant system.
FIGURE 31-21 When an angled load is placed on an implant body, the compressive stresses on the opposite side of the implant increase and the tensile and shear loads on the same side of the implant increase. Because bone is weaker to tensile and shear forces, the risks to the bone are increased for two reasons: (1) the amount of the stress increases, and (2) the type of stress is changed to more tensile and shear conditions. F, Force.
The implant surgeon may place the implant body ideally, perpendicular to the occlusal plane, yet the restoring dentist then may load the implant crown at an angle. Similar noxious forces are increased in shear, and a decrease in bone strength occurs to the crestal bone, an increase of shear loads on implant components, and the abutment screws. Hence, an angled implant body or an angled load on the implant crown increases the amount of crestal stresses on the implant system, transforms a greater percentage of the force to shear force, and reduces bone, porcelain, and cement strength. In contrast, the surrounding implant system stress magnitude is least, and the strength of bone, porcelain, and cement is greatest under a load axial to the implant body and perpendicular to the occlusal plane. All of these factors mandate the reduction of angled forces to the implant system (Figure 31-22).
FIGURE 31-22 The force applied to an implant body with an angled load or angled direction of force is increased in direct relation to the force angle. The major increase of force is a result of the offset angle of the load.
Most implant bodies inserted at an angle of greater than 12 degrees to the occlusal plane require an angled abutment. The surgeon and restoring dentist should understand that angled abutments are fabricated in two pieces and are weaker in design than a two-piece straight abutment without an angle. Because less metal flanks the abutment screw on one side of an angled abutment, it therefore is at more risk of fracture or is less able to be reduced in width for ideal crown contours. Furthermore, a larger transverse load component develops at the abutment screw and crest of the ridge as a result of angled loads and increases the risk of abutment screw loosening. In a study by Ha et al., the angled abutment was compared with a straight abutment for screw loosening in the anterior maxilla. The angled abutments showed more screw loosening with cyclic loading than the straight abutments.59
Solutions to Angled Loads
When lateral or angled loads cannot be eliminated, a reduction in the force magnitude or additional surface area of implant support is indicated to reduce the risk of biomechanical complications to the implant system. For example, if three adjacent implants are inserted with the first in the long axis to the load, the second at 15 degrees, and the third implant at 30 degrees, the surgeon may decrease the overall risk by (1) adding an additional implant in the edentulous space next to the most angled implant, (2) increasing the diameter of the angled implants, or (3) selecting an implant design with greater surface area. Of the three options, increasing the implant number is most effective to reduce overall stress to the system.60 In addition, a greater number of implants also has more retention for the restoration.
The restoring dentist may reduce the overload risk by (1) splinting the implants together, (2) reducing the occlusal load on the second implant and further reducing the load on the third implant, and (3) eliminating all lateral or horizontal loads from the most angled implant and completely eliminating them in all posterior regions.
The anterior mandible (with a force magnitude similar to the anterior maxilla) often has the implant body positioned perpendicular to the occlusal plane and restored with a straight abutment. In the anterior maxilla, even under ideal conditions, the implant should be angled away from the labial bone and results with the abutment toward the facial crown contour. An angled prosthetic abutment is required, and these implant bodies are more frequently loaded at an angle. In fact, maxillary anterior teeth are usually loaded at a 12- to 15-degree angle to the occlusal plane (Figure 31-23).
FIGURE 31-23 Maxillary anterior implants most often are placed at an angled load to the lower anterior teeth. As a result, the amount of the load should be reduced. Fortunately, the anterior teeth bite force is reduced when the posterior teeth do not occlude. However, clenching patients may develop a considerable anterior bite force. Occlusal contact reduction, larger-diameter implants, increasing implant number, splinting implants, and night guards are possible solutions.
The natural dentition reduces the increased stress to the maxilla by increasing the size of the roots compared with mandibular incisors and increasing the mobility of the tooth. Therefore, in the maxilla, a larger-diameter implant or a greater number of implants are indicated to minimize the crestal bone stress on each abutment, especially in patients exhibiting severe bruxism. Ridge augmentation may be necessary before implant placement to improve implant position or facilitate the use of a wider-diameter implant. IPO aims at reducing the force of occlusal contacts, increasing the implant number, or increasing the implant diameter for implants subjected to angled loads.
Posterior Crown Cusp Angle
The angle of force to the implant body may be influenced by the cusp inclination of the implant crown in similar fashion as an angled load to an implant body. The posterior natural dentition often has steep cuspal inclines, and 30-degree cusp angles have been designed in denture teeth and natural tooth prosthetic crowns (Figure 31-24). The greater cusp angles are often considered more esthetic and may even incise food more easily and efficiently.61 To negate the negative effect of an angle cusp contact, the opposing teeth need to occlude at the same time in two or more exact positions on the ipsilateral cusp angles of the crowns (Figure 31-25). This is not possible in a clinical setting.
FIGURE 31-24 Natural teeth often have cusp angles of 30 degrees. Therefore, if a premature contact occurs on a cuspal incline, the direction of load may be 30 degrees to the implant body if the implant crown duplicates a natural tooth cusp angle.
FIGURE 31-25 When opposing crowns occlude, the three cuspal inclines must hit at the same time to result in a long-axis load.
The occlusal contact along only one of the angled cusps result in an angled load to the implant system even when it is not premature to other occlusal contacts (Figure 31-26). The magnitude of the force is minimized when the angled occlusal contact is not a premature contact but instead is a uniform load over several teeth or implants. However, the angled cusp load does increase the resultant tensile and shear stress with no observable benefit. Hence, no advantage is gained, but the biomechanical risk is increased (e.g., increased abutment screw loosening, porcelain fracture, and unretained restoration).
FIGURE 31-26 The mandibular buccal cusp incline is occluding with the lingual incline of the maxillary cusp. An occlusal contact on a cusp angle transmits an angled load to the implant body.
The occlusal contact over an implant crown therefore should be ideally on a flat surface perpendicular to the implant body. This occlusal contact position usually is accomplished by increasing the width of the central fossa to 2 to 3 mm in posterior implant crowns, which is positioned over the middle of the implant abutment. The opposing cusp is recontoured to occlude the central fossa of the implant crown directly over the implant body (Figure 31-27). In other words, the laboratory technician should identify the middle of the implant body and then make a central fossa 2 to 3 mm wide over this position parallel to the curves of Wilson and Spee (Figure 31-28). The buccal and lingual contour of the crown may then be established (reduced on the buccal for the posterior mandible and the lingual for the posterior maxilla). The opposing tooth may require recontouring of an opposing cusp to help direct the occlusal force along the long axis of the implant body.
FIGURE 31-27 A posterior implant crown should have a wider central fossa perpendicular to the implant body and parallel to the occlusal plane. The opposing tooth occluding cusp should be modified to occlude with the widened central fossa.
FIGURE 31-28 The laboratory technician usually will place the implant abutment under the central fossa of the implant crown.
Mutually Protected Articulation
Anterior, compared with posterior, bite force measurements and electromyographic studies provide evidence that the stomatognathic system elicits significantly less force when the posterior segments are not in contact when the anterior teeth occlude.62,63 For example, the maximum bite force in the posterior regions of the mouth (with no anterior occlusal contact) is 200 to 250 psi. The maximum bite force in the anterior region (with no posterior occlusal contact) is 25 to 50 psi. This difference results from a biological response and a mechanical condition when the posterior teeth do not contact. Almost two thirds of the temporalis and masseter muscles do not contract when posterior teeth do not occlude.62 In addition, the TMJ and teeth complex form a class 3 lever condition (i.e., the nutcracker).25 As a result, the closer the object is placed toward the hinge (TMJ), the greater the force on the object. In addition, the greater lateral mobility of the anterior teeth compared with the posterior teeth (108 microns vs. 56 microns) also decreases the consequences of the lateral forces during excursions.
Many occlusal schemes for natural teeth opposing each other suggest the use of anterior teeth to disocclude the posterior teeth during excursions (i.e., incisal guidance steeper than the condylar disc assembly).63–69 This has been called mutually protected occlusion because the posterior teeth protect the anterior teeth in CO, and the anterior teeth protect the posterior teeth in mandibular excursions. This occlusal design is based on the concept of using the maxillary canine as the key of this occlusion scheme to avoid lateral forces on the posterior teeth.69 In CO, the anterior teeth contacts are shared and protected by the occlusal contacts of the posterior teeth. When the canine separates the posterior teeth in right or left lateral excursions, the term canine or cuspid protected occlusion may be used.
If healthy anterior teeth or natural canines are present, the mutually protected occlusion scheme allows those teeth to distribute horizontal (lateral) loads during excursions while the posterior teeth disocclude during excursions (e.g., canine guidance or mutually protected articulation) (Figure 31-29). The posterior teeth are protected from lateral forces by the anterior guidance during excursions, and the anterior teeth have lighter forces in excursions because the posterior teeth do not contact. In other words, when lateral or angled forces are applied to the anterior teeth, the magnitude of the stress is increased. However, when mutually protected occlusal philosophies are applied, the consequences of the lateral forces are reduced.
FIGURE 31-29 In all mandibular excursions, the anterior teeth should disocclude the posterior teeth.
The mutually protected articulation concept is used in IPO. In protrusive mandibular movements, the central and lateral incisors disocclude the posterior teeth. In lateral excursions, the canine (and lateral incisor when possible) disocclude the posterior teeth. In CO, the posterior and canine teeth occlude. When the centrals and lateral incisors are natural, they may also occlude in CO (or MI). When the anterior teeth are implants, they may not occlude in centric, especially when the opposing dentition is also implant supported.
Group function (or unilateral balance) has been suggested with periodontal bone loss on the remaining teeth. The concept was to share the lateral loads during excursions with more teeth. For example, in this philosophy, a mandibular excursion to the right contacts as many anterior and posterior teeth on the right as possible. This is not indicated in IPO. The lateral posterior forces increase the moment loads to posterior implants. The posterior contacts during excursions also have greater forces to the posterior implants because more muscle mass contracts and the occlusal contacts are closer to the TMJ (class 3 lever). In addition, the posterior lateral loads increase the force to the anterior teeth or implants during the excursions. As a result, both the anterior and posterior implant components receive a greater force (Figure 31-30).
FIGURE 31-30A, This patient was restored in group function. B, The posterior maxillary right two implants fractured. C, The posterior mandibular right two implants fractured. D, The maxillary anterior implants lost integration.
In a study by Jemt et al., when implant-supported restorations were used in the maxilla opposing natural dentition, the velocity of the mandible during excursions was greater with group function than when incisal guidance was present.15 Hence, the force to the implant system was greater with group function. It is interesting to note that Kinsel and Lin reported that group function in patients with implant-supported prostheses had a porcelain fracture rate of 16.1% and occurred in 51.9% of implant patients.30 When anterior disclusion was the occlusal scheme in excursions, the fracture rate on implant crowns was 5.3%, and this complication affected 15.9% of patients (more than a threefold difference).
The steeper the incisal guidance, the greater the force on the anterior teeth or implants. Therefore, the anterior guidance of an implant prosthesis with anterior implants should be as shallow as practical. According to Weinberg and Kruger, for every 10-degree change on the angle of disclusion, there is a 30% difference in load70 (Figure 31-31). A 10-degree force on the anterior implants with a 68-psi load will increase to 100 psi when the incisal guidance is 20 degrees and will further increase to 132 psi if the incisal guidance is 30 degrees. As a consequence, the impression by these authors is the incisal guidance should be less than 20 degrees. However because the condylar disc assembly is usually 20 to 22 degrees, the incisal guidance should be greater than this amount to separate the posterior teeth.25 When the incisal guidance is less than the angle of the eminentia of the TMJ, the posterior teeth will still contact in excursions. Hence, in most patients, an incisal guidance of at least 23 to 25 degrees is suggested in IPO.
FIGURE 31-31 For every 10-degree change on the angle of disclusion, there is a 30% difference in load. (From Weinberg LA, Kruger G: A comparison of implant/prosthesis loading for clinical variables, Int J Prosthodont 8:421–433, 1995.)
The increase in load that occurs from the incisal guidance angle is further multiplied by the crown height above the initial occlusal contact (the vertical overbite) because it acts as a lever while the mandible slides down the incline plane (Figure 31-32). An ideal vertical overbite in prosthetics has been reported to be 5 mm and often is more, especially in Angle's class II, division II patients. However, especially in parafunctional patients, incisal guidance should be as shallow as possible in implant prostheses (23–25 degrees) and the vertical overbite reduced to less than 4 mm, yet the posterior teeth should disocclude in the excursions (Figures 31-33 and 31-34).
FIGURE 31-32 The anterior load during excursions is increased from the centric occlusal contact (far right) to the incisal edge (far left).
FIGURE 31-33 The vertical overbite in implant prosthesis should be reduced to 4 mm or less. When implants oppose each other, there is no occlusal contact between the canines in centric occlusion.
FIGURE 31-34A, A full-arch maxillary and mandibular implant prosthesis with a vertical overbite of 3 mm. B, The incisal guidance is 25 degrees, so the posterior teeth separate in any mandibular excursion.
A clinical condition that sometimes causes confusion is the occlusal scheme for a single-tooth implant replacing a maxillary canine. A missing maxillary canine is indicated for a single-tooth implant crown. The lateral incisor is the weakest anterior tooth, and the first premolar is often the weakest posterior tooth. Hence, these abutments are not great candidates for a three-unit prosthesis, especially because lateral forces would be placed on the premolar.
The proprioceptive mechanism of the natural canine in excursions blocks approximately two thirds of the activity of the masseter and temporalis muscles and decreases the bite force when posterior teeth disocclude.62 An anesthetized canine has more muscle mass that contracts in both clenching and in the lateral excursion compared with the same patient before anesthesia.71 Hence, the natural canine periodontal ligament nerve complex helps decrease the force in excursions.
The anesthetized natural canine has been compared with the proprioawareness of an implant.72 There is a proprioawareness transmitted through the bone from an implant but a reduced amount compared with a natural tooth. A mutually protected occlusion is still a benefit when a single-tooth canine implant is restored. In other words, a greater decrease in lateral forces occurs when a natural anterior tooth root is involved in the excursion compared with an implant crown, but an implant crown also can decrease the force and is better than a pontic in the canine position. In addition, the class 3 lever mechanism of the canine position still is able to reduce the force in excursions when the posterior teeth do not contact.
No occlusal contact occurs on the single-tooth canine implant crown during mandibular excursions to the opposite side. During protrusion, no contact on the canine implant crown is ideal. If a contact is necessary, it is adjusted so a light bite force has no occlusal contact on the implant crown. Under a heavy bite force in protrusive movements, the canine implant crown may contact.
The occlusion during the working excursion toward the canine implant crown is of particular concern. The dentist should make an attempt to include a natural tooth in the lateral excursion because teeth have greater proprioception than implants. To create a mutually protected articulation scheme that includes a lateral incisor is preferable because this tooth is farther from the TMJ. Hence, with a light working lateral excursion, the lateral incisor occludes first and moves 97 microns (when in health), and then the canine implant crown engages and helps disocclude the posterior teeth. During a heavy bite force excursion, the lateral incisor and implant crown contact with similar magnitude (Figure 31-35). However, in Angle's skeletal class II, division 1 patients, the first premolar may need to be included in the excursion process, rather than the lateral incisor, because the horizontal overjet may be excessive.
FIGURE 31-35A, A maxillary right canine is replaced with an implant. This patient is an Angle's class II, division 2 patient and therefore has a deep vertical overbite. A steep incisal guidance places greater force than a shallow incisal guidance, which may be why the canine tooth fractured after endodontic therapy. B, The right mandibular excursion is evaluated first with a light and then a heavy bite force. Ideally, the lateral incisor should contact first and then the canine. Therefore, the first premolar contact should be eliminated to decrease the force to the implant.
In summary, all lateral excursions in IPO opposing fixed prostheses or natural teeth use anterior teeth or implants whenever possible to disocclude the posterior components. The resulting lateral forces are distributed to the anterior segments of the jaws, with an overall decrease in force magnitude. This occlusal scheme should be followed whether or not anterior implants are in the arch. However, if anterior implants must disocclude posterior teeth, natural teeth (whenever possible) are first used during the initial primary tooth movement. When multiple anterior teeth are missing, two or more implants splinted together (when possible) should help dissipate the lateral forces.