The following presentation is taken of normal joints during normal motion. The structures are able to maintain their structural integrity because the forces are generated in a physiologic manner and do not exceed the adaptive capacity to withstand loading. The first slide demonstrates the joint in a fully closed relationship where the mandibular teeth are fully occluding with the maxillary teeth.
The teeth and supporting structures withstand the loading generated during closure and a slight force is applied to the joint structures during this loading phase.
During opening, the anterior dental teeth still are touching and this helps decrease much of the elevator muscle activity, thus decreasing the stresses on the associated structures. It is important to note what happens to the mandibular centerline in this opening. The lower jaw has moved slightly to the patient’s left upon opening from the fully occluded position in the fully occluded picture. This would cause the joints to be moved in a slightly non-symmetrical manner.The angulation of the maxillary anterior teeth should coincide with the angulation of the slope of the eminence within a matter of a few degrees. As the mouth is closed into a fully occluded position, the condyles will fully seat against the relatively avascular central region of the disc against the anterior slope of the eminences. The forces applied to the teeth due to the dental angulations should coincide with the anatomy of the joint for these structures to function in a harmonious relationship.
In these two photographs of the same patient the angulation of the lingual surface of the maxillary incisors is depicted to a true horizontal and vertical reference. The lingual surface of the maxillary teeth guides of the movement of the mandible on opening and closing. The second slide demonstrates the anterior slope of the eminence which is the posterior guide for mandibular movement on front to back movements.
Clearly there is a asymmetry in the anterior and posterior guidance of this patient who has undergone orthodontic treatment. This lack of harmony of structures is further investigated in the following link regarding occlusion and joint function.
When the anterior slope of the eminence and the anterior guidance are in harmony, a symmetrical guide for movement occurs. When this relationship is not harmonious, compromises exist in the joint function, the occlusal guidance (wear) and the associated muscle function.
With the mandible 1/3 open the guidance for movement is now determined by the muscles of mastication and the proprioception of the joint and upper quadrant structures. The forces directed by the condyle are against the relatively avascular central region of the disc against the anterior slope of the eminence.
When the mouth is ½ to two thirds open, the entire guidance of mandibular movement is the condyles and muscles and proprioception of the upper quadrant. The forces on the condyle, disc and eminence are minimal as the muscles that open the vertical are flexing the mandible open, but some forces are exerted from the condyle to the central region of the disc against the tip of the eminence.
It is important to note there are no contractile elements (muscles) in the posterior aspect of the joint. The elastic connective tissue exerts very little force on the posterior of the condyle. The disc is maintained in this functional relationship by the anatomy and physiology of force application to the joint structures. Upon full opening the mandible is flexed from the maxilla between 48 to 52mm (normal adult opening) and the condyle travels anterior to the tip of the eminence and is separated from the eminence by the central region of the disc. The elevator muscles have a minimal muscle tone, while the depressor (opening) muscles have maximal muscle activity. The muscle activity begins to reverse upon closure where the elevator muscles exert the forces on the mandible and the depressor muscles relax.
When the mandible opens, the head has a tendency to raise and when the mandible closes, the head has a force that would flex it towards the floor. The posterior cervical muscles are directly involved with all opening and closing movements by keeping the relationship of the head stable in space during all mandibular movements. The cervical region is directly involved with all jaw and joint movements.
During mandibular closure the elevator muscle begin to move the mandible posterior and the condyle exerts a compressive force on the central region of the disc against the anterior aspect and tip of the eminence. Note there is no contractile apparatus that moves the disc with the condyle. The posterior elastic tissue begins to respond to the movement of the disc posterior as it begins to fold as the elastic fibers resume the normal “non-stretched” physiologic anatomy. The posterior bilaminar zone of the disc is the anatomic aspect that keeps the disc in the proper physiologic relationship between the condyle and eminence. The forces are still applied from the condyle to the central region of the disc to the tip of the eminence.
The joint structures are loaded during closure as the elevator muscles move the mandible posterior and the condyle begins to translate up the anterior slope of the eminence. The proprioception of the muscles and joints determine the relationship of the mandible to the maxilla. The posterior elastic connective tissue of the disc is compressed by the movement of the disc posterior and superior.The mandible continues to approximate guided in the movements by the proprioception of the joints, muscles and upper quadrant structures. The joint structures are loaded by the elevator muscles and the forces are directed from the condyle to the central region of the disc to the anterior slope of the eminence.
The disc moves posterior and superior because the condyle exerts a force against the relatively avascular central region of the disc and the posterior bilaminar zone. This force causes the disc to move posterior and superior as the condyle is directed in this motion by the muscles of mastication and the joint proprioception with secondary input by the cervical structures.
The mandible continues to close until the first dental contact is acquired in the anterior region. The wear facets of the anterior teeth coincide at this relationship and the pressure within the joint is directed anterior and superior from the condyle against the relatively avascular central region of the disc against the eminence. The disc position is maintained as the posterior bilaminar zone and the pressure exerted by the condyle. There are no contractile fibers within the joint to move the disc in this direction. This occurs because of the anatomy of the structures and the physiologic response.
When the teeth contact the proprioception of the teeth begins to determine mandibular movements. In a symmetrical relationship the guidance of the anterior teeth is in harmony with the anatomy and physiology of the joint structures and the vector of motion coincides as depicted in this slide. The tip of the mandibular tooth is guided against the lingual surface of the maxillary incisors in the same way that the condyle is guided by the angulation of the anterior slope of the eminence.
The dental relationships guide the mandible in a harmonious relationship with the structures within the joint. The forces of closure are directed to the teeth and the joint does not withstand excessive compressive loading.
Maximal loading is exerted on the dentition with little alterations in the condyle disc fossa relationship as the predominant forces are directed anterior and superior. The posterior bilaminar zone of the disc serves to keep the disc in the proper relationship between the condyle and eminence in normal joint movements.In normal healthy function the teeth and support structures receive the primary loading of the complex and the joint functions properly if the forces exerted by the condyle are directed anterior and superior. This is the normal joint and occlusal associations.