ATM: OCCLUSAL ANATOMY AND PHYSIOLOGY

ATM: OCCLUSAL ANATOMY AND PHYSIOLOGY

PLAN 

1. Anatomical features of ATM
   1.  Definition 
   2.  Skeletal structures
      1. Temporal fossa 
      2. Mandibular condyle
      3. Articular disc
   3. Ligament insertions 
   4.  Muscles 
   5. Innervation and vascularization 
2. Functional characteristics of ATMs
3. Main concepts of occlusion in orthodontics

3.1 Historical occlusal objectives at the end of orthodontic treatment

Bibliography 

Goals 

• know the different anatomical components of the ATM
• know the different possible movements of the ATM 
• know the occlusion criteria from an orthodontic point of view 

1. Anatomical features of ATM

    1.1 Definition

The temporomandibular joints are paired, mobile, and symmetrical joints. Described as bicondylar diarthroses, they connect the mandible to the base of the skull, via two non-congruent articular surfaces.

 They comprise two distinct bony articular surfaces:

– At the level of the temporal bone, a fixed and concave articular surface, called the mandibular fossa, connected with,

– a mobile and convex mandibular part, the condylar process. 

Stable functioning of the joint is enabled by the presence of a biconcave articular disc, separating it into an upper, disco-temporal compartment, and a lower, condylo-discal compartment . 

These joints, the dental arches and the corresponding muscular system are part of the masticatory system and enable the functions of grasping, chewing, swallowing and speaking.

2. Skeletal structures

1.2.1 Temporal fossa

The articular surface of the temporal bone belongs to the squamous part of the temporal bone and is divided into two parts:

– In front is the articular tubercle of the temporal bone, also called the articular eminence. It is formed by the transverse process of the zygomatic bone, cylindrical in shape, convex from front to back and concave transversely. It is entirely covered with fibrocartilage.

– Further back is the glenoid fossa or mandibular fossa . It is part of the horizontal portion of the temporal bone squamous cell. It is hollowed out by an elliptical depression, and concave downwards. This fossa is limited behind by the external acoustic meatus, in front by the articular tubercle, outside by the longitudinal root of the zygoma and inside by the spine of the sphenoid bone. The mandibular fossa is divided into two zones by the petro-tympano-squamous fissure, formerly called Glaser’s fissure. The most anterior zone belongs to the squamous part of the temporal bone and is covered with fibrous articular tissue, the posterior tympanal portion is, for its part, non-articular.

       1.2.2 Mandibular condyle

The head of the condyle or condylar process represents an ovoid-shaped eminence of approximately 2 cm by 1 cm, and responds to the articular disc.

 It has a lateral pole and a medial pole, separated by a blunt crest. 

The anterior pole of this process is covered with cartilage; it is convex upwards and forwards, and faces the eminence of the temporal bone. 

The posterior slope is more vertical and constitutes the extension of the neck of the condyle. Devoid of cartilage, it does not participate in the function. 

The condylar process overhangs the neck and is oriented medially to it, forming an underlying notch, called the pterygoid fossa (Figure 1). It is at the level of this fossa that the inferior head of the lateral pterygoid muscle is inserted.

Figure 1: Mandibular bone and condylar process

1. Condylar process; 2. Coronoid process; 3. Mandibular notch; 4. Masseter insertion ridges and gonial angle; 5. Neck of condylar process; 6. Pterygoid fossa. (3)

       1.2.3 Articular disc 

The articular disc is biconcave, elliptical in shape in superior view. It is longer in the mediolateral direction (about 19 mm) than in the anteroposterior direction (about 13 mm) and follows the shape of the mandibular condyle. Three zones are described: the anterior zone, the intermediate zone and the posterior zone. In the sagittal plane, the posterior and anterior zones are thicker than the intermediate zone. This is the thinnest zone (figure 2). In its thinnest portion, the intermediate zone is around a millimeter thick, while the posterior zone can reach around 4 millimeters.

         Among its anatomical relationships, the disc is connected to the glenoid fossa and the condyle by a set of fibrous and richly vascularized tissues, which form the joint capsule. In front, the attachments of the disc form a tendinous zone called the prediscal lamina. This lamina includes a medial part, the majority, formed by the insertions of the lateral pterygoid muscle, and a less substantial lateral part, represented by insertions of the temporal and masseter muscles. Behind the disc, is the bilaminar zone. Divided into two fibrous laminae, this zone has on the one hand an upper lamina that connects the disc to the posterosuperior part of the mandibular fossa of the temporal bone, and on the other hand, a lower lamina that connects it to the lower part of the condylar head.

. The space formed between the two blades of the bilaminar zone is called the retrodiscal zone, a vascularized and innervated region. This space is fragile and is often the site of perforations. A synovial membrane lines the articular surfaces and lines the capsule.

Synovial fluid bathes the discotemporal and condylodiscal spaces (Figure 3). This fluid lubricates the surfaces, facilitates joint movements and helps transport metabolic substrates necessary for non-vascularized tissues.

The upper level has an average volume of 1.2 milliliters and the condylo-discal part has a volume of only 0.9 milliliters (6.7).

Figure 2   Sagittal section of the ATM

1. Upper lamina of the billionaire zone; 2. Lower lamina of the bilaminar zone; 3. Mandibular fossa; 4. Anterior and posterior parts of the articular disc; 5. Articular tubercle of the temporal bone; 6. Superior synovial cavity; 7. Prediscal lamina; 8. Lateral pterygoid muscle; 9. Articular capsule; 10. External acoustic meatus; 11. Fibrous membrane of the capsule; 12. Condylar head; 13. Inferior synovial cavity. The retrodiscal zone appears in yellow. 

       1.3 Ligamentous insertions

Ligaments are divided into intrinsic and extrinsic ligaments.

1. The intrinsic  ligaments are of three types: the collateral disc ligaments , the  lateral ligament , and the medial ligament . 

• The collateral disc ligaments: are not very extensible. 
• The lateral ligament  : is powerful and covers the joint capsule, limiting the movement of the mandible in lateral, retropulsion and lowering. It extends from the anterior zygomatic tubercle to the lateral part of the condylar process.
•  The medial ligament is less strong, weak and triangular. It extends from the medial part of the glenoid cavity, passing through the spine of the sphenoid and ending on the medial part of the condylar process.

b) the extrinsic  ligaments consist of the stylomandibular ligament , the sphenomandibular ligament , the pterygomandibular ligament or raphe , and the tympanomandibular ligament . They are also called accessory ligaments and play little role in the regulation of mandibular movements.

4. Muscles

The muscular system of the temporomandibular region is complex, and essentially includes 4 muscles, paired and symmetrical, the masseter, the temporal, the lateral pterygoid, and the medial pterygoid. The masseter and temporal muscles are located in the superficial plane. The pterygoid muscles are located, for their part, in deeper planes.

– The masseter muscle is an elevator of the mandible. It is parallel and extends from the zygomatic arch to the lower end of the mandibular ramus. It is the most powerful muscle in the human body in relation to its mass, hence its importance in joint dysfunctions. 

– The temporal muscle is not only an elevator but also a retropulsor of the mandible. It is wide, flat and radiated, and occupies, in a fan shape, the space of the temporal fossa.

– The medial pterygoid muscle is an elevator, propulsor, and diductor of the mandible. It is a powerful quadrilateral that extends from the medial blade of the pterygoid process to the internal aspect of the gonial angle.

– The lateral pterygoid muscle is a propulsor of the mandible when it contracts in concert with its contralateral muscle. Alone, it is a diductor. It is a short and thick muscle, formed of two distinct heads, stretched between the lateral blade of the pterygoid process on one side, and the prediscal blade and the neck of the condyle on the other side.

It is also important to note, more remotely, the existence of the mandibular depressor muscles. Depending on their position in relation to the hyoid bone, these eight muscles are classified into two groups: the suprahyoid muscles and the infrahyoid muscles.

1.5 Innervation and vascularization

– The sensory innervation of the ATM is dependent on the auriculotemporal nerve , a branch of the mandibular nerve (V3).

– Motor innervation is dependent on the branches of the mandibular nerve   which innervate:

• The lateral pterygoid muscle and the masseter muscle (masseteric nerve), and   
• the temporal muscle (anterior, middle and posterior deep temporal nerves).
• A common trunk from the mandibular nerve provides innervation  

• Medial pterygoid muscles,
•  tensor of the soft palate 
•  tensor tympani.

NB) The close relationships between the ATM and the ear, as well as their areas 

     innervation can explain that during an ATM dysfunction, a 

      earache may sometimes be noted.

The vascularization of the ATM is ensured by:

•  collateral arteries of the superficial and deep temporal arteries, 
•  the tympanic artery. 

NB) the articular disc has an avascular character; there is only vascular supply at its periphery, in the retro-discal zone (figure 4).

2. Functional characteristics of ATMs
3. Joint kinematics

2.1. 1 Reference positions

    2.1.1- 1 Rest

At rest, the disc is “molded” on the condylar process and responds to the glenoid cavity and the articular tubercle. All muscles are inactive except for their tone. There is no dento-dental contact, and a muscular balance is observed. A free inter-occlusal space, called “free inocclusion space”, persists, from 1 to 3 millimeters . Clinically, this position is studied by referring either to the dental occlusion or to the condylar position.

1. – 2 Maximum intercusp occlusion

Maximum intercuspation occlusion (MIO) is defined by the most numerous occlusal dento-dental contacts, with a balanced distribution of the applied forces. It is constantly being modified during dentitions but also due to wear phenomena, pathologies or dental care.

   2.1.1 – 3     Centered relation

This is the ” highest reference condylar position , achieving a simultaneous and transversely stabilized bilateral condylo-disco-temporal coaptation , suggested and obtained by non-forced control, reiterative in a given time and for a given body posture and recordable from a mandibular rotation movement without dental contact .”

It appears that this is the highest position of the condyles, and that this position is considered reproducible by the practitioner’s guidance.

2.   Movements

2.1.2 1 Basic movements

The ATMs allow two types of elementary movements: a rotational movement or a translational movement. During the rotational movement, the articular condyle will rotate against the lower surface of the articular disc. It occurs in the lower, condylo-discal compartment. During the translational movement, the articular condyle accompanied by the articular disc will slide against the glenoid fossa up to the articular eminence. It is performed in the upper, disco-temporal compartment.

2.1.2 2   Fundamental movements

1. Opening

Facilitated by gravity, the mouth opening movement will be carried out by the action of the lowering muscles of the mandible, and two articular phases are distinguished.

Within the disco-temporal compartment, a pure rotational movement of the condylar process occurs around its transverse axis , over about twenty millimeters, limited by the lateral collateral ligament. In a second phase, the condylar process continues its rotation while performing a translational movement . The condylar head slides about 12 mm, forward and downward, projecting in front of the articular tubercle of the temporal bone. This translation takes place in the lower, disco-mandibular compartment. The opening path is physiologically straight and without deflection.

2. Closing

During closing, the opposite movements to those described above occur. Closure is ensured by the action of the mandibular elevator muscles.

3. Propulsion and retropulsion

The propulsion movement drives the mandible forward, and requires a minimum of mouth opening, in order to disengage the dento-dental complex. It is made possible by the simultaneous contraction of the lateral pterygoid muscles. The condylo-discal complex will proceed with a translational movement until it projects slightly in front of the articular process of the temporal bone.

Similar to propulsion, the retropulsion movement requires disengagement of the cuspid teeth and minimal oral opening. This movement is possible in particular thanks to the action of the posterior bundle of the temporal muscle, but it is limited because the condylar processes quickly abut backward against the anterior wall of the external acoustic meatus.

4. Diduction and laterality

Diduction corresponds to a movement during which the mandible is projected transversely to the right or left. It requires a minimum of mouth opening. On the working side, a rotation of the mandibular condyle occurs, permitted by the contraction of the posterior fibers of the temporalis and the digastric muscle. On the non-working side, a translation of the condyle downwards and forwards and inwards is carried out by the action of the lateral and medial pterygoid muscles, but also of the anterior fibers of the homolateral temporalis muscle. This translation is carried out in a direction forming with the sagittal plane an angle called the Bennett angle. 

3. Main concepts of occlusion in orthodontics

The definition of ideal occlusion is based on different criteria that vary according to the authors. Respecting these criteria, as guides, would ensure better results over time.

 However, a “perfect” occlusion alone would not be sufficient to guarantee stability. 

It is essential to keep in mind that the objective of our treatments is to obtain a functional occlusion, specific to each person, in a balanced muscular environment. 

       3.1 Historical occlusal objectives at the end of orthodontic treatment

Lawrence F. Andrews

Are based on 06 important characteristics called “Andrews’ six keys”

• 1st Key: based on the Molar relationship:

• The mesio-vestibular cusp of the upper first molar is received in the vestibular groove of the lower first molar, between the mesial and medial cusps. Angle’s work was therefore validated.      
•  The distal surface of the maxillary first permanent molar’s distobuccal cusp contacts the mesial surface of the mandibular second molar’s mesiobuccal cusp. Therefore, the adequacy of the normal molar relationship described by Angle must be questioned. The closer the distal surface of the maxillary first permanent molar’s distobuccal cusp is to the mesial surface of the mandibular second molar’s mesiobuccal cusp, the closer one gets to optimal wedging and long-term stability (Figure 1).

ATM: OCCLUSAL ANATOMY AND PHYSIOLOGY

Fig 1: Incorrect molar relationship. 2: Improved molar relationship. 3: Further improved molar relationship. 4: Correct molar relationship

•  2nd ^Key  : Mesio-distal coronal angulation

Corresponds to the mesio-distal angulation. The term coronal angulation refers, not to the angulation of the dental axes including the roots but to the angulation of the axis of the crowns . In other words, the crowns all have a mesial coronal version, called “positive angulation” relative to the perpendicular to the occlusal plane.

Fig. “Positive” angulation of dental crowns

       3rd ^Key : Vestibulo-lingual coronal inclination

        The optimal occlusion is the vestibulolingual inclination of the crown. This is obtained by comparing the axis of the crown with the perpendicular to the occlusal plane.

At the anterior level , the upper incisors have a positive inclination (corono-vestibular), the mandibular incisors have a slightly negative inclination (corono-lingual). The inclination of the upper and lower incisors conditions the posterior coverage and occlusion.

At the posterior level, the inclination of the lateral sectors is negative. In the maxillary arch, it is identical for the canines and premolars, and increased for the molars. This is explained by the fact that the measurement of the inclination of the molars is made at the level of the groove of the vestibular face and not on the cuspidal ridge like that of the canines and premolars. In the mandibular arch, the inclination of the mandibular crowns is progressive: increasingly negative from the incisors to the second molars.

Fig. Correct torque of the anterior crowns leading to distalization of the contact points of the lateral sectors: optimal occlusion

4th Key: Absence of ^rotations

Absence of tooth rotation because it would increase the mesio-distal space required for a cusped tooth

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Fig. Absence of dental rotation

5th ^Key : Interproximal contact points

Represents the continuity of interproximal contact points . In the absence of dento-dental disharmony (DDD), the contact points between each tooth must be clear. Default DDDs must be corrected by additive coronoplasties and not by space closure at the expense of good occlusion.

6th ^Key   Curve of Spee leveled

Represents the leveling of the curve of Spee. In his study, the latter varied from flat to slightly concave upwards. Andrews, however, sets a complete leveling of the curve of Spee as a treatment goal. He considers this to be a form of overcorrection. Indeed, this curve tends to deepen again over time after treatment (especially in patients with downward and forward mandibular growth). In addition, intercuspation and dental axes are better when the curve of Spee is flat

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Fig: A. steep curve of spee, B. flat curve of spee: better intercuspation

BIBLIOGRAPHY 

1. Gaudy J.-F. et al. Clinical Anatomy, 2nd edition. CdP Wolters Kluwers France Editions, 2007

2.Vacher C. Clinical anatomy of the temporomandibular joint. Actual Odonto-Stomatol 2009;246:129-133.

3. Aoun M. Development and validation of a temporomandibular joint model by finite elements. Doctoral thesis in mechanics and engineering. University of Bordeaux I, 2010.

4.POSSELT TU. Physiology of occlusion and rehabilitation. Paris: J Prélat; 1969. 363 p.

5.SELINGMAN DA. and PULLINGER AG. The role of intercuspal occlusal relationships in temporomandibular disorders: a review. J Craniomandib Disord Facial Oral Pain 1991, 5(2): 96-105.

6. RAMFJORD SP. and ASH MM. Occlusion. Paris: J Prélat; 1975. 414 p.

7.PERDRIX G., LAMENDIN H. and GINISTRY J. . Posture and dental occlusion. In: MESURE, S. and LAMENDIN, H. Posture, sports practice and rehabilitation. Paris: Editions Masson; 2001

8.ORTHLIEB JD., RE JP. and PEREZ C. Anterior occlusal stop. Inf Dent. 2007, 32(1):1913-14.

9.ORTHLIEB JD., LAURENT M. and LAPLANCHE O. Cephalometric estimation of Occlusal Vertical Dimension. J Oral Rehabil 2000, 27(9): 802-7.

10.ORTHLIEB JD., CHOSSEGROS C., CHEYNET F., GIRAUDEAU, MANTOUT B., PEREZ C., et al. Diagnostic framework of masticatory system dysfunctions (MSD). Inf Dent 2004, 19(1):1196-203; 39(1):2626-32.

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  Wisdom teeth can cause infections if not removed.
Dental crowns restore the function and appearance of damaged teeth.
Swollen gums are often a sign of periodontal disease.
Orthodontic treatments can be performed at any age.
Composite fillings are discreet and durable.
Composite fillings are discreet and durable.
Interdental brushes effectively clean tight spaces.
Visiting the dentist every six months prevents dental problems.
 

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