Monday, January 31, 2022

TEMPOROMANDIBULAR JOINT (TMJ)

 Overview and Topographic Anatomy 

  • The temporomandibular joint (TMJ) is the articulation between the squamous portion of the temporal bone and the the condyle of the mandible 
  • It is a ginglymoarthrodial joint because it has both a hinge and a gliding action 

STRUCTURAL COMPONENTS 

  • The TMJ comprises 2 types of synovial joints-hinge and sliding-and consists of the following:
  1. Squamous portion of the temporal bone
  2. Articular disc (contained within the TMJ)
  3. Condyle of the mandible
  4. Ligaments (also serve as boundaries)

TMJ SYSFUNCTION 

  • Affects 33% of the population and may be severe 
  • Causes include arthritis, trauma, infection, bruxism, and disc displacement 
  • More common in females






OSSEOUS STRUCTURES 

Squamous Portion of the Temporal Bone 

  • The TMJ articulation is located on the squamous portion of the temporal bone 
  • Has an avascular articular surface composed primarily of fibrous connective tissue and some fibrocartilage instead of  hyaline cartilage  
  • The main load-bearing areas are on the lateral aspect of the squamous portion, condyle, and articular disc 
  • The dense fibrous connective tissue is thickest in the load-bearing areas 
  • :Relations of the squamous portion of the temporal bone 
  1. Anterior-articular eminence 
  2. Intermediate-glenoid fossa 
  3. Posterior-tympanic plate tapering to the postglenoid tubercle 

structure 

comments 

Articular eminence 

The strong bony prominence on the base of the zygomatic process 

Articular tubercle  

The lateral part of the articular eminence is referred to as articular tubercle and provides attachment for the capsule and lateral temporomandibular ligament  

Glenoid fossa 

The depression into which the condyle is located Superior to this thin plate of bone is the middle cranial fossa 

The anterior boundary of the glenoid fossa is the articular eminence 

The posterior boundary of the glenoid fossa is the tympanic plate  

The glenoid fossa can be divided into 2 parts by the squamotympanic fissure (lateral) and the petrotympanic fissure (medial): 

  • Anterior articular area-squamosal part of the temporal bone (this is where the articulation occurs) 

  • Posterior nonarticular area-tympanic portion ( parotid gland may extend into this area) 

Tympanic plate 

The vertical plate located anterior to the external auditory meatus 

Postglenoid tubercle 

An inferior extension of the squamous portion of the temporal bone  

Makes the posterior sapect of the glenoid fossa 

Provides attachment for the capsule retrodiscal pad 

 

Mandibular Condyles 

Articulate with the articular disc 
Ovoid in shape: 
  1. Mediolateral- 20mm 
  2. Anteroposterior- 10mm  
Articular surface is avascular fibrous connective tissue instead of hyaline cartilage  
The main load-bearing areas are on the lateral aspect 

 


ARTICULAR DISC 

  • Composed of dense fibrous connective tissue 
  • Located between the squamous portion of the temporal bone and the condyle 
  • Is avascular and aneural in its central part but is vascular and innervated in the peripheral areas, where load-bearing is minimal  
  • The main load-bearing areas are located on the lateral aspect; this is an area of potential perforation  
  • Merges around its periphery, attaching to the capsule 
  • Divided into 3 bands: 
  1. Anterior-this thick band lies just anterior to the condyle with the mouth closed 
  2. Intermediate-this band, the thinnest part, is located along the articular eminence with the mouth closed 
  3. Posterior-this thick band is located superior to the disc with the mouth closed  
  • Additional attachments: 
  1. Medial/lateral-strong medial and lateral collateral ligaments anchor the disc to the condyle 
  2. Anterior-the disc is attached to the capsule and the superior head of the lateral pterygoid, but not the condyle, allowing the disc to rotate over the condyle in an anteroposterior direction 
  3. Posterior-the disc is contiguous with the bilaminar zone that blends with the capsule 
 

 

BILAMINAR ZONE (Posterior Attachment Complex) 

  1. A bilaminar structure located posterior to the articular disc 
  2. Highly distortable, especially on opening the mouth 
  3. Composed of: 
  • Superior lamina (stratum)-contains elastic fibers and anchors the superior aspect of the posterior portion of the disc to the capsule and bone at the postglenoid tubercle and tympanic plate  
  • Retrodiscal pad-the highly vascular and neural portion of the TMJ, made of collagen, elastic fibers, fat, nerves, and blood vessels (a large venous plexus fills with blood when the condyle moves anteriorly) 
  • Inferior lamina (stratum)-contains mainly collagen fibers and anchors the inferioraspect of the posterior portion of the disc to the condyle 



TMJ COMPARTMENTS 

  • The articular disc divides the TMJ into superior and inferior compartments  

  • The internal surface of both compartments contains specialized endothelial cells that form a synovial lining that produces synovial fluid, making the TMJ a synovial joint 

  • Synovial fluid acts as: 

  1. A lubricant 

  1. A medium for providing the metabolic requirements to the articular surfaces of the TMJ   

Superior compartment  

Between the squamous poetion of the temporal bone and the articular disc  

Volume = 1.2 ml 

Provides for the translational movement of the TMJ 

Inferior compartment 

Between the articular disc and the condyle  

Volume = 0.9 ml  

Provides for the rotational movement of the TMJ  

 

CAPSULE AND LIGAMENTS 

Capsule  

  • Completely encloses the articular surface of the temporal bone and the condyle  

  • Composed of fibrous connective tissue 

  • Toughened along the medial and lateral aspects by ligaments  

  • Lined by a highly vascular synovial membrane 

  • Has various sensory receptors including nociceptors 

  • Attachments: 

  1. Superior-along the rim of the temporal articular surfaces 

  1. Inferior-along the condylar neck 

  1. Medial-blends along the medial collateral ligament 

  1. Lateral-blends along the lateral collateral ligament 

  1. Anterior-blends with the superior head of the lateral pterygoid m. 

  1. Posterior-along the retrodiscal pad 

Ligaments 

Collateral ligaments 

  • Composed of 2 ligaments: 

Medial collateral ligament-connects the medial aspect of the articular disc to the medial pole of the condyle 

Lateral collateral ligament-connects the lateral aspect of the articular disc to the lateral pole of the condyle  

  • Frequently called the discal ligaments 

Composed of collagenous connective tissue; thus, they are not designed to stretch 

Temporomandibular (lateral) ligaments 

The thickened ligament on the lateral aspect of the capsule 

Prevents posterior displacement of the condyle 

Composed of 2 separate bands: 

  1. Outer oblique part-largest portion; attached to the articular tubercle; travels posteroinferiorly to attach immediately inferior to the condyle; this limits the opening of the mandible 

  1. Inner horizontal part-smaller band attached to the articular tubercle running horizontally to attach to the lateral part of the condyle and disc; this limits posterior movement of the articular disc and the condyle 

Stylomandibular ligaments 

Composed of a thickening of deep cervical fascia  

Extends from the styloid process to the posterior margin of the angle and the ramus of the mandible 

Helps limit anterior protrusion of the mandible 

Sphenomandibular ligament 

Remnant of Meckels cartilage  

Extends from the spine of the sphenoid to the lingula of the mandible Some authors suggest it may help act as a pivot on the mandible by maintaining the some amount of tension during both opening and closing of the mouth  

Some authors suggest it may help limit anterior protrusion of the mandible  

Is the ligament most frequently damaged in an inferior alveolar nerve block 

 



  • Up to 33%of adults have a TMJ-related problem
  • Perforations of the articular disc typically occur in the later stages of TMJ dysfunction
  • Women have a higher prevalence of disc perforations than men 
  • Many factors may contribute to changes in the disc:
  1. Bruxism
  2. Trauma
  3. Lateral pterygoid muscle abnormal activity 
  4. Overloading
  • Most disc perforations occur in the lateral or posterior portions of the disc and vary in size
  • Crepitus and clicking sounds on opening the mouth are common clinical manifestations
  • Anterior disc displacement also is common
  • Mandibular dislocation ( or subluxation of the TMJ ) occurs when the condyle moves anterior to the articular eminence
  1. With dislocation, the mouth  appears "wide open"
  2. Because the condyle is displaced anterior to the articular eminence, a depression can be palpated posterior to the condyle 
  • Spontaneous dislocations can occur from a variety of actions ranging from a simple yawn to an extended dental treatment
  • Because the mandible is dislocated, the patien has a great deal of difficulty verbalizing his or her predicament
  • Relocation involves repositioning the condyle posterior to the articular eminence
  • Opening the mandible involves a complex series of movements 
  • Initial movement is rotational, which occurs in the lower TMJ compartment:
  1. Lateral pterygoid ( inferior head) initiates the opening of the jaw (the superior head of the lateral pterygoid is described as being active during elevation of the mandible in a "power stroke")
  2. As the mandible is depressed, the medial and collateral ligaments tightly attach the condyle to the articular disc, thereby allowing for only rotational movement
  3. Once the TMJ becomes taut, no further rotation of the condyle can occur 
  4. Normally, rotational movement continues until the upper and the lower teeth are about 20 mm away from each other
  • For additional/further opening of the mandible, translational movement must occur:
  1. A translational movement occurs in the upper TMJ compartment and provides for most of the mandible's ability to open
  2. In this movement, the articular disc and the condyle complex slide inferiorly on the articular eminences, allowing for maximum depression of the mandible

ARTHRITIS

  • Arthritis is the most common cause of pathologic changes in the TMJ
  • When rheumatoid arthritis occurs, usually both TMJs are affected, and other joints tend to be affected before the TMJ
  • Radiologic images in the initial disease stages show decreased joint space without osseous changes
  • Radiologic images in the late disease stage show decreased joint space with osseous changes, possibly including ankylosis 
  • In osteoarthritis, causes include normal wear, trauma, and bruxism, and clinical manifestations may range from mild to severe

ANKLYOSIS

  • Anklyosis is an obliteration of the TMJ space with abnormal osseous morphologic features, which often occurs as a result of trauma or infection
  • Classified as either true (intracapsular) or false (extracapsular, usually associated with an abnormally large coronoid process or zygomatic arch) ankylosis 
  • Treatment varies in accordance with the cause but may include a prosthetic replacement or condylectomy


Saturday, January 8, 2022

PRINCIPLES OF OCCLUSION

Most restorative procedures affect the shape of the occlusal surfaces. Proper  dental care ensures that functional occlusal contact relationships are restored in harmony with both dynamic and static conditions. Maxillary and mandibular teeth should contact uniformly onclosing to allow optimal function, minimize trauma to the supporting structures, and allow for uniform load distribution throughout the dentition. Positional stability of well aligned teeth is crucial if arch integrity and proper fuunction are to be maintained over time.
Most dentitions deviate from optimal alignment and occlusion. Many patients adapt well to less than optimal occlusion, but malocclusion may be associated with undesirable changes to the teeth, the musculature, the temporomandibular joints (TMJs), or the periodontium. As an aid to the diagnosis of occlusal dysfunction, it is helpful to evaluate the condition of specific anatomic featurres and functional aspects of a patients occlusion with reference to a concept of (optimum) or (ideal) occlusion.
Deviation from this concept can then be measured objectively and may prove to be a useful guide during treatment planning and active treatment phases.
Over time, many concepts of (ideal) occlusion have been proposed. In the literature, the concepts of what is (ideal,) (acceptable,) and (harmful) continue to evolve. 

ANATOMY

Temporomandibular Joints

The major components of the TMJs are the cranial base, the mandible, and the muscles of mastication with their innervation and vascular supply. The TMJs are ginglymoarthrodial, meaning that they are capable of both a hinging and a gliding articulation. An articular disk separates the mandibular fossa and the articular tubercle of the temporal bone from the condylar process of the mandible.
The articulating surfaces of the condylar processes and fossae are covered with avascular fibrous tissue (in contrast to most other joints, which have hyaline cartilage). The articular disk consists of dense connective tissue; it also is avascular and devoid of nerves in the area where articulation normally occurs. Posteriorly , it is attached to loose highly vascularized and innervated connective tissue: the retrodiscal pad or bilaminar  zone (called bilaminar because it consists of two layers: an elastic superior layer and a collagenous inelastic inferior layer). The retrodiscal pad connects to the posterior wall of the articular capsule surrounding the joint. Medially and laterally, the articular disk is attached firmly to the poles of the condylar process. Anteriorly, it fuses with the capsule and with the superior lateral pterygoid muscle. Superior and inferior to the articular disk are two spaces: the superior and inferior synovial cavities. These are bordered peripherally by the capsule and the synovial membranes and are filled with synovial fluid. Because of its firm attachment to the poles of each condylar process, the articular disk follows condylar movement during both hinging and translation, which is made possible by the loose attachment of the posterior connective tissues.

Ligaments

The body of the mandible is attached to the base of the skull by muscles and three paired ligaments: the temporomandibular (also called the lateral), the sphenomandibular, and the stylomandibular ligaments.
Ligaments cannot be stretched significantly, and thus joint movement is limited. The temporomandinular ligaments restrict rotation of the mandible, limit border movements, and protect the structures of the joint. The sphenomandibular and stylomandibular ligaments limit separation between the condylar process and the articular disk; the stylomandibular ligaments also limit forward (protrusive) movement of the mandible.

Musculature

Several muscles are responsible for mandibular movements. These can be grouped as the muscles of mastication and the suprahyoid muscles. The former include the temporal, masseter, and medial and lateral pterygoid muscles; the latter are the geniohyoid, mylohyoid, and digastric muscles. Their respective origins, insertions, innervation, and vascular supply.

Muscular Function

The functions of the mandibular muscles are well coordinated and complex. Three paired muscles of mastication provide elevation and lateral movement of the mandible: the temporal, masseter, and medial pterygoid muscles. The lateral pterygoid muscles eaach have two bellies that function as two separate muscles, which contrct in the horizontal plane during opening and closing; the inferior belly (inferior lateral pterygoid muscle) is active during protrusion, depression, and lateral movement of the mandible; the superior belly (superior lateral pterygoid muscle) is active during closure. Because the superior belly has been shown to attach to the articular disk and the neck of the condyle, it is thought to assist in maintaining the integrity of the condyle-articula disk assembly by pulling the condylar process firmly against the articular disk.
The suprahyoid muscles have a dual function: They can elevate the hyoid bone or depress the mandible. The movement that results when they contract depends on the state of contraction of the other muscles of the neck and mandibular region. When the muscles of mastication are in a state of contraction, the suprahyoid muscles elevate the hyoid bone. However, if the infrahyoid muscles (which anchor the hyoid bone to the sternum and clavicle) are contracted, the suprahyoid muscles depress and retract the mandible. The geniohyoid and mylohyoid muscles initiate the opening movements, and the anterior belly of the digastric muscle completes mandibular depression. Although the stylohyoid muscle (which also belongs to the suprahyoid group) may contribute indirectly to mandibular movement through fixation of the hyoid bone, it does not play a significant role in mandibular movement.

Dentition 

The relative positions of the maxillary and mandibular teeth influence mandibular movement. Many (ideal) occlusions have been described. In most of these, the maxillary and mandibular teeth contact simultaneously when the condylar processes are fully seated in the mandibular fossae, and the teeth do not interfere with harmonious movement of the mandible during function. Ideally, in the fully bilateral seated position of the condyle-articular disk assemblies, the maxillary and mandibular teeth exhibit maximum intercuspation. This means that the maxillary lingual and mandibular buccal cusps of the posterior teeth are evenly distruted and in stable contact with the opposing occlusal fossae. These functional cusps can then act as stops for vertical closure without excessively loading any one tooth, while left and right TMJs concurrently are in an unstrained position.
Howevver, in many patients, maximal intercuspal contact occurs with the condyles in a slightly translated position. This position is referred to as maximum intercuspation, which is defined as the complete intercuspation of the opposing teeth, independent of condylar position; this is sometimes considered the best fit of the teeth regardless of condylar position.
If the mesiobuccal cusp of the maxillary first molar is aligned with the buccal groove of the mandibular first molar, the orthodontic relationship is considered Angle classI; this is considered normal occlusion. In such a relationship, the anterior teeth overlap both horizontally and vertically. This position is defined as the dental relationship in which the anteroposterior relationship of the jaws is normal, as indicated by correct intercuspation of maxillary and mandibular molars, Orthodontic textbooks have traditionally described an arbitrary 2-mm horizontal overlap and 2-mm vertical overlap as being ideal. For most patients, however, greater vertical overlap of the anterior teeth is desirable  for preventing undesirable posterior tooth contact. Mandibular flexing during mastication also may contribute to such undesirable contact. Empirically, dentitions with greater vertical overlap of the anterior teeth appear to have a better long-term prognosis than do dentition with minimal vertical overlap.

CENTRIC  RELATION

Centric relation is defined as the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective articular disks with the complex in the anterosuperior position against the shapes of the articular eminences. This position is independent of tooth contact. It is also clinically discernible when the mandible is directed superior and anterior and is restricted to a purely rotary movement about the transverse horizontal axis.
Centric relation is considered a reliable and reproducible reference (and treatment) position. If maximum intercuspation coincides with the centric relation position, restorative treatment is often straightforward. When maximum intercuspation dose not coincide with centric relation, it is necessary to determine whether corrective occlusal therapy is needed before restorative treatment is initiated.

MANDIBULAR  MOVEMENT 

As any other movement in space, complex three-dimensional mandibular movement can be divided into two basic components: translation, in which all points within a body have identical motion, and rotation, in which the body is turning about an axis. Every possible three-dimensional movement can be described in terms of these two components. It is easier to understand mandibular movement when the components are described as projections in three perpendicular planes: sagittal, horizontal, and frontal.

Reference Planes

Sagittal plane

In the sagittal plane, the mandible is capable of a purely rotational movement, as well as translation. Rotation occurs around the terminal hinge axis, an imaginary horizontal line through the rotational centers of the left and right condylar processes. The rotational movement is limited to abbout 12 mm of incisor separation before the temporomandibular ligaments and structures anterior to the mastoid process force the mandible to translate. The initial rotation or hinging motion occurs between the condylar process and the articular disk. During translation, the inferior lateral pterygoid muscle contracts and moves the condyle-articular disk assembly forward along the posterior incline of the tubercle. Condylar movement is similar during protrusive mandibular movement.

Horizontal Plane

In the horizontal plane, the mandible is capable of rotation around several vertical axes. For example, lateral movement consists of rotation around an axis situatef in the working (laterotrusive) condylar process with relatively little concurrent translation. A slight lateral translation of the condyle on the working side in the horizontal plane-known as laterotrusion, Bennett movement, or mandibular side shift  is frequently present. This may be in a slightly forward  direction (lateroprotrusion) or slightly backward direction (lateroretrusion). The orbiting (nonworking) condyle travels forward and medially as limited by the medial aspect of the mandibular fossa and the temporomandibular ligament. In addition, the mandible can make a straight protrusive (anterior) movement. 

Frontal plane

In a lateral movement in the frontal plane, the nonworking (mediotrusive) condyle moves down and medially, whereas the working (laterotrusive) condyle rotates around the sagittal axis perpendicular to this plane. Again, as determined by the anatomy of the medial wall of the mandibular fossa on the mediotrusive side, transtrusion may be observed; as determined by the anatomy of the medial wall of the mandibular fossa on the laterotrusive side, this movement may be lateral and upward (laterosurtrusion) or lateral and downward (laterodetrusion). A straight protrusive movement observed in the frontal plane, with both condylar processes moving downward as they slide along the tubercular eminences. 

Tuesday, January 4, 2022

MUSCLES OF MASTICATION

 Muscles of Mastication

  • Mastication is the process of chewing food in preparation for deglutition (swallowing) and digestion.
  • All muscles of mastication originate on the skull and insert on the mandible. 
  • All muscles of mastication are innervated by the mandibular division of the trigeminal nerve. 
  •  All muscle of mastication are derivatives of the 1st pharyngeal arch.
  • Movement of the mandible are classified as:  

      1. Elevation 
      2. Depression
      3. Protrusion
      4. Retrusion
      5. Side-to-side (lateral) excursion 

  • Mastication prepares food by chwing for deglutition and digestion.
  • It is the 1st step in the breakdown of food by:

      1.  Making smaller pieces from larger pieces( thus increasing the surface area for digestive breakdown).
      2. Helping soften and lubricate the food with saliva.

BONES INVOLVED

  • Base of the skull and the mandible.
  • They articulate at the temporomandibular joint (between the squamous portion of the temporal bone {skull} and the condyle of the mandible)
  • MUSCLES INVOLVED

    • 4 muscles of mastication:

      1. Masseter
      2. Temporalis
      3. Medial pterygoid 
      4. Lateral pterygoid
    • All muscles of mastication are innervated by the mandibular division of the trigeminal nerve (nerve of the 1st pharyngeal arch).
    • Mastication involves using the 4 muscles in different combinations to move the mandible in 1 of 3 planes in an antagonistic fashion:
      1. Elevation - depression
      2. Protrusion - retrusion
      3. Side - to -side excursion
    • Although the buccinator is not a muscle of mastication, it aids in keeping the bolus of food against the teeth to help in mastication.


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