[Figure 10-01] Canine guidance promotes disclusion of the posterior teeth on the working and non-working sides.

[Figure 10-02] Group function promotes disclusion of the posterior teeth on the non-working side by progressive contact of the canine, first premolar, second premolar, and mesiobuccal cusp of the maxillary first molar on the working side.

[Figure 10-03] Incisal guidance promotes disclusion of the posterior teeth in mandibular protrusive movements.

The envelope of function is an individual characteristic determined by hereditary and developmental patterns but is subject to changes from adaptive occlusal responses. Its understanding becomes relevant for the analysis and treatment planning of clinical cases requiring changes in the position or morphology of the anterior teeth [Figure 10-04].

[Figure 10-04] The patient has a three-dimensional (3D) envelope of extreme mandibular border movements (Posselt diagram^6–8) in the sagittal, frontal, and horizontal views. All mandibular movements of the patient are restricted to this area.

The reality of mandibular movements

One of the most overlooked aspects of functional guidance is that it is a “two-way street.” Historically, functional guidance has been established through excursive trajectories of the mandible, in a similar manner to the movements performed on an articulator during the analysis of study models, diagnostic wax-ups, and adjustments to prostheses. Such excursive movements of the mandible are helpful in daily life, as they facilitate the handling of the articulator and the intraoral adjustment of the restorations. However, they do not represent the reality of masticatory movements, which occur in the opposite direction; that is, they involve mandibular incursion^9–19.

Excursive functional guidance is performed through voluntary mandibular movements – requested from the patient-directed statement “from the inside to the outside.” In these movements, the incisors (in the case of incisal guidance), the canine, or other teeth responsible for the functional guidance on the working side remain in dynamic contact from the starting position (MIP) to the edge-to-edge position. This avoids the contact of posterior teeth on the working side that are not part of the guidance with those on the non-working side. Often, especially in young individuals, only the opposing canines maintain contact in the final part of this trajectory.

On the other hand, incursive functional guidance occurs from mandibular movements that are part of the patient’s natural masticatory cycle, in a direction from the outside to the inside. In these movements, the incisors (in the case of incisal guidance), the canine, or more teeth on the working side contact each other in the middle part of their fossae to crush food. The mandible is guided toward the end of the masticatory cycle by a combined, interdependent muscular action of the inclination of these fossae. This causes the guidance, from a functional point of view, to constitute the path of access of the cusps to their final position – MIP.

It is necessary to be clear that the functional guidance described does not differ only in terms of the direction of mandibular movements but mainly in terms of the magnitude of the occlusal forces resulting from the different muscular actions. In incursive movements, occlusal contacts occur in a more significant number and intensity due to the contraction of the mandibular elevator muscles^17–19, both during mastication and in parafunctional activities. In the bruxism rhythmic phase, there will still be a number or combination of multidirectional excursive and incursive mandibular movements that will be directly related to the risks of the treatment.

Muscle actions in different mandibular movements

Lateral movements

An asymmetric muscle activity¹⁷ occurs during lateral movements of the mandible. In mandibular excursive movements, contraction of the medial and posterior temporal bundles, deep fibers of the masseter and superior lateral pterygoid muscles of the working side occur, providing rotation stability for the condyle on this side. The condyle on the non-working side performs its translation by the action of the inferior lateral pterygoid muscle of that same side²⁰, with a small amount of action of the anterior temporal bundle and superficial fibers of the masseter muscle to stabilize the mandibular body in this movement [Figure 10-05].

[Figure 10-05] In the lateral movements of mandibular excursion, contraction of the posterior temporal, middle temporal, and deep masseter muscles on the working side occurs, providing rotation stability for the condyle on that side. The condyle on the non-working side is translated by the action of the inferior lateral pterygoid muscle of the same side²⁰. (Modified from Abjean et al¹⁷.)

The lateral movements of mandibular incursion on the working side are part of the masticatory cycle and involve more complex muscle actions. The jaw opening movement is initiated by the fast action of the inferior lateral pterygoid muscle on the working side, followed by the inferior lateral pterygoid muscle on the non-working side and the anterior digastric muscles. The amplitudes of these movements are contained in the envelope of function, according to the food and fossa pattern of the functional guidance.

The closing movement is initiated by the progressive action of the medial pterygoid muscle on the non-working side, which controls the return movement of the mandible. Near the MIP, predominantly the anterior temporal, superficial masseter, and medial pterygoid fibers on the working side rapidly increase their activity, along with the superior lateral pterygoid muscles, reaching their peak force about 30 milliseconds before occlusal contact. On the non-working side, the activity of the deep fibers of the masseter and posterior temporal bundle begins, followed by the movement to exit the masticatory cycle^20–24 [Figure 10-06].

[Figure 10-06] The lateral movements of mandibular incursion on the working side involve complex muscule actions. The jaw opening movement is initiated by the fast action of the inferior lateral pterygoid muscle on the working side, followed by the inferior lateral pterygoid muscle on the non-working side and the anterior digastric muscles. The closing movement is initiated by the progressive action of the medial pterygoid muscle on the non-working side, which controls the return movement of the mandible. Close to the MIP, the temporal and masseter muscles on the working side rapidly initiate and increase their activity, together with the medial pterygoid muscle on this same side and the superior lateral pterygoid muscle. (Modified from Abjean et al¹⁷.)

Anteroposterior movements

Mandibular protrusive movements are performed by the simultaneous and symmetric action of the inferior lateral pterygoid muscles, with minor participation of the superficial fibers of the masseters, medial pterygoid, and anterior bundles of the temporal muscles, which stabilize the mandible and maintain anterior tooth contact.

During food incision, the main muscle actions derive from the deep fibers of the masseters, middle, and posterior bundles of the temporals, superior lateral pterygoid, and posterior belly of the digastric muscles. These muscles generate a vigorous anteroposterior movement, in which posterior occlusal contacts not evident in the protrusive movements can be observed²⁰ [Figures 10-07 and 10-08].

[Figure 10-07] Mandibular protrusive movements are symmetrically performed by the inferior lateral pterygoid muscles, with a small amount of participation of the superficial masseters, medial pterygoid, and anterior temporal muscles, which have the function of stabilizing the jaw and maintaining anterior tooth contact.

[Figure 10-08] During food incision, the main muscles recruited are the deep masseters, middle temporal, posterior temporal, superior lateral pterygoid, and posterior belly of the digastric muscles. These muscles generate a vigorous anteroposterior movement, in which posterior occlusal contacts not evident in the protrusive movements can be observed²⁰. (Modified from Abjean et al¹⁷.)

Functional guidance: canine guidance or group function

Mandibular movements are affected by the interrelation of factors such as TMJ anatomy, the neuromuscular system, functional guidance, and head posture. Of these, functional guidance is the only one that the dentist can predictably control, and it constitutes one of the critical aspects for the stability of the planned occlusal scheme.

A historical debate about the most appropriate way to establish functional guidance persists, resulting in controversies capable of confusing most dentists. There are supporters of canine guidance^5,24–28 [Figure 10-09] and those of group function^11,29–32 [Figure 10-10]. To date, there is no scientific evidence regarding the ideal occlusal scheme for all patients, both standards of functional guidance being clinically acceptable and compatible with the health of the stomatognathic system^33–36.

[Figure 10-09] The great advantage of canine guidance is its prosthetic convenience due to the simplicity of elaboration and laboratory or clinical adjustment. In addition, canines are favorable teeth to guide mandibular movements due to their anatomy, position in the dental arch, and proprioceptive capacity.

[Figure 10-10] Group function provides a restorative alternative for patients with structurally fragile canines or reduced periodontal support. It can also be recommended when a dental implant has replaced the canine. The absence of the periodontal ligament in this tooth reduces its proprioceptive ability to guide mandibular movements compared with the natural canine.

Some studies on the prevalence among the types of functional guidance have shown a higher percentage of young individuals with canine guidance. An increased prevalence of group function can be observed with aging that could be related to the physiologic or pathologic wear of the canines^5,37. Nevertheless, canine guidance has a moderate prevalence in Angle Class I patients, a high prevalence in Class II patients, and a minimal one in Class III patients³⁶.

The advantage of canine guidance is its prosthetic convenience due to its ease of reproducibility and simple laboratory or clinical adjustment. Canines are favorable teeth to guide mandibular movements because of their anatomy, position in the dental arch, and proprioceptive capacity. These teeth are structurally adequate to receive occlusal forces because they have a crown structure with accentuated palatal ridges and long and wide roots located in compact and dense bone^38–40, in addition to having a sensitive proprioceptive system^4,41–45. Canine guidance is indicated when these teeth are in good structural condition, have adequate periodontal support, and are well positioned in the dental arch to receive lateral forces.

Group function provides an alternative for patients with structurally fragile canines or reduced periodontal support. It can also be recommended when a dental implant replaces the canine. The absence of the periodontal ligament in this tooth reduces its proprioceptive ability to guide mandibular movements compared with the natural canine^46,47. These differences in proprioception between teeth and implants are considered risk factors for the failure of implants or the prostheses over them⁴⁸. Proprioceptive thresholds vary according to the antagonist teeth, being around 20 micrometers (μm) for a natural tooth opposing a natural tooth, 50 μm for an implant opposing a tooth, and 70 μm for an implant opposing an implant⁴⁷.

Group function can be described according to the number of participating teeth. In addition to the canine, it can incorporate the first premolar, the second premolar, and the mesiobuccal cusp of the maxillary first molar on the working side. Including the second molar is not recommended due to the increased incidence of forces when approaching the point of force application of the jaw elevator muscles²⁴. This functional guidance pattern can distribute the lateral forces to many teeth. However, it demands an optimized relationship with the condylar trajectory, which is necessary to develop the appropriate anatomy of these teeth during the wax-up, preferably in a fully adjustable articulator, and a meticulous “progressive” occlusal adjustment. In general, the greater the number of teeth involved in group function, the greater the complexity of the adjustments in these multiple teeth, and the greater the activity of the jaw elevator muscles^1–4.

In practical terms, partial group function, including a premolar to help share forces with the canine or allow some degree of proprioception when an implant has replaced the root of a canine, is clinically favorable and provides an easy fit. According to this author, this approach also reduces mechanical complications, such as fractures in the canine region, in patients with intense parafunctional activity and in those who have hybrid implant-supported prostheses.

In temporomandibular disorders (TMDs), group function can provide mandibular support and reduce forces on the TMJ on the working side by sharing the occlusal forces on this side with the posterior teeth throughout the trajectory of jaw movement⁴⁹. A careful adjustment is necessary to this group function, so that the contacts are uniformly balanced, and none of the teeth is overloaded during the movement. Spear⁴⁹ recommends testing the disclusion pattern of these patients with TMDs with an interocclusal splint, starting with canine guidance due to its simplicity. If pain or discomfort persists, guidance ridges should be incorporated into the first premolar through the addition of acrylic resin or adjustments, to reduce the slope of the canine guidance. After an evaluation period, if the symptoms do not regress, the guidance contact should be added to the second premolar or up to the first molar successively, and the patient’s responses observed.

In summary, from the aspects discussed above, the selection of functional guidance between the options of canine guidance and group function should be based on individual clinical determinants such as the health of the TMJs, inter- and intraarch aspects, and the structural status of the teeth, in addition to the signs and symptoms of parafunctional activity.

Interarch aspects refer to the Angle skeletal relationship, overbite, and overjet. Angle Class I patients have an excellent maxillomandibular relationship for both patterns. Canine guidance, located on the mesial ridge of the lingual surface of the maxillary canines, is favorable from a biomechanical point of view, as it avoids the incidence of forces with distalizing vectors in the TMJ on the working side⁵⁰. Angle Class II patients usually present canine guidance determined by the distal ridges of this maxillary canine and with greater inclination due to the significant overbite. These characteristics can generate a component of distalizing force of the condyle on the working side, which can compress the retrodiscal zone and cause discomfort or even joint clicking. Angle Class III patients, on the other hand, have a predominance of group function due to absent or reduced overjet in the guidance [Figure 10-11A–C].

[Figure 10-11A–C] The interarch relationship must be considered when planning the pattern of functional guidance for rehabilitation treatment. Angle Class I patients favor the establishment of canine guidance on the mesial ridge of the maxillary canines. On the other hand, Angle Class II patients usually present the canine guidance determined by the distal ridge of the maxillary canines, with greater inclination due to the enlarged overbite. Angle Class III patients have poor functional guidance or predominance of group function due to absent or reduced overjet in the guidance.

Regarding the intraarch aspects, the development of functional guidance can be hindered by tooth misalignments, twists, and distribution. For example, in the presence of proclination or rotation of the canines, the arches must be orthodontically aligned to avoid compensatory tooth preparations or restorations with abnormal morphologies in order to provide guidance.

It is still necessary to carefully analyze the structural state of the teeth to guide mandibular movements. Endodontically treated teeth, with little remaining structure and the absence of a ferrule, are at a higher risk of failure, along with those that are periodontally compromised.

Patients with parafunctional activity warrant extra attention. Canine guidance has been indicated for having the ability to reduce the activity of the mandibular elevator muscles during excursive movements and thus would be favorable for these patients^1–4. However, for some patients, the incidence of substantial forces on a single tooth may overload it or lead to fatigue failure of the restorative materials. Also, canine guidance transmits more force to the TMJ on the working side than group function⁴⁹, causing symptoms for patients with a predisposition to TMDs.

Functional incisal guidance

Functional incisal guidance should allow the incisal edges of the mandibular anterior teeth to slide along the marginal ridges of the lingual surfaces with a smooth and uninterrupted movement. This anatomy should be adjusted to distribute loads evenly over the incisors, allowing symmetric muscle work and accommodating functional movements with minimal friction and the necessary freedom^11,31,51,52. In the edge-to-edge position, the maxillary incisal edges should ideally be parallel to the mandibular ones for better dissipation of forces in this region during functional and parafunctional activities [Figure 10-12A–F].

[Figure 10-12A–F] Functional incisal guidance should allow the incisal edges of the mandibular anterior teeth to slide, preferably along the marginal ridges of the palatal surfaces, with a smooth and uninterrupted movement. Incisal guidance promotes the disclusion of the posterior teeth in mandibular protrusive movements through the maxillary central incisors [A], sometimes accompanied by the lateral incisors [B], and, in the minority of situations, the canines [C]. Functional incisal guidance should allow the incisal edges of the mandibular anterior teeth to slide along the marginal ridges of the lingual surfaces with a smooth and uninterrupted movement [D]. In the edge-to-edge position, the maxillary incisal edges should ideally be parallel to the mandibular ones for better dissipation of forces in this region during functional and parafunctional activities [E,F].

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