Dentistry is a comprehensive and practical medical discipline. It is rich in content and can be broadly divided into three major fields, including basic dentistry, preventive dentistry and clinical dentistry. Among them, the four clinical disciplines (1) are oral and maxillofacial surgery, orthodontics, prosthodontics and internal stomatology, which are closely related to clinical treatment. The connection between the secondary disciplines in dentistry is close and complex.
However, the starting and ending points of all the secondary clinical disciplines in dentistry are the same, that is, to restore better physiological functions of the oral and maxillofacial systems for patients, and to restore the harmonious and beautiful anatomical structures as much as possible. The dentition and jaw position are the main anatomical and physiological structures for the function of the oromandibular system, and this paper is a review of their internal functions, interrelationships and clinical significance.
1. The anatomy and physiology of the dentition and jaw position
Occlusion or occlusion refers to the phenomenon that the maxillary and mandibular dentition are in contact with each other. The contact between the upper and lower dentition can be static or dynamic. The former refers to the contact relationship between the upper and lower jaw teeth reaching the most extensive and uniform, tightest and stable cusp-fossa interlocking contact, which can be referred to as dentition at this time; the latter refers to the contact relationship between the upper and lower teeth during the chewing movement, which can be referred to as occlusion at this time. Therefore, there is still a difference between the internal letters of dentition and occlusion: the upper and lower contact sites of dentition do not change within a certain period of time, while the sites of the upper and lower teeth that come into contact during occlusion are constantly changing.
The functions of the oral and maxillofacial system are: mastication, swallowing, speech and maintenance of normal jaw morphology, etc. The most basic and primary function to maintain human survival is, of course, mastication. The realization of mastication inevitably involves the movement of the teeth and the jaw, and the movement of the jaw will produce a change in the position of the mandible in relation to the maxilla or the skull, which is the change in jaw position.
Therefore, the study of the interrelationship and clinical significance between dentition and jaw position as well as the most basic repeatable jaw positions can organically link the various secondary clinical disciplines of dentistry, thus facilitating the cooperation among various disciplines in clinical practice, and finally achieving the unity of the physiological function and morphological structure of the treated oromandibular system in terms of harmony, comfort and aesthetic stability.
2. Interrelationship between dentition and jaw position and the four basic jaw positions
2.1 Interrelationship between dentition and jaw position
The dentition and jaw position are two sides of the same issue regarding the physiological process of mastication. The dentition emphasizes the contact relationship between upper and lower teeth, which is the tooth contact relationship in a specific jaw position; while the jaw position focuses on the position of the mandible, and a specific jaw position must have a corresponding dentition, both of which are unified in the instantaneous masticatory movement type of an individual.
People can chew without thinking, and can even be distracted from other things while eating; patients with missing teeth can gradually adapt to the restoration and use it as a substitute to regain chewing function. This physiological phenomenon precisely illustrates the masticatory motor pattern formed by the unification of the dentition and jaw position in the central nervous system.
(i) It is a cortical kinetic stereotype.
② is a programmed movement. In addition, the “dual mechanism” of the neural control of functional jaw movements, i.e., randomness and reflexes, also indicates that dentition and jaw position are two aspects of the same problem of masticatory movements, and that they are unified. The reflex allows the masticatory system to acquire highly complex and delicate functions, such as speech and expression, while the reflex allows a rapid response to external situations at will to avoid injurious stimuli.
2.2 Interrelationship of ICP, RCP, MCP, MPP
The position of the lower jaw, i.e. jaw position, is theoretically infinite, and it is not only impossible to study all its positional changes, but also has no medical significance. The jaw positions in normal physiological function are worth studying only the few and far between that refer to good repeatability, namely the cusp interposition (ICP), the receding contact position (RCP), the muscle contact position (MCP) and the mandibular postural pull (MPP), and the anatomical relationship between the four is very subtle and complex (2).
The ICP is determined by the cusp intercuspation (ICO) and is therefore also known as the dental position (3); the RCP is determined by the retractability of the ligaments and is also known as the ligament position, which is a physical position; the MCP is closely related to the contraction of the ascending jaw muscles and is also known as the muscle position (MP); the MPP is the mandibular position without active function of the mandible and the perimandibular muscles and is therefore also known as the resting jaw position, which relies on the tension of the perimandibular muscles and the It is maintained by the tension of the perimandibular muscles and the weight of the mandible.
Under normal circumstances, there is a free median domain from RCP to ICP, i.e., from RCP to ICP, the posterior teeth are in symmetrical and uniform contact, with no horizontal deflection or deflection not more than 0.5-1.0L, no unilateral occlusal contact, and the relationship between the two is about 1L in a predominantly anterior-posterior position, which is called “two positions”.
ICP and MCP are normally consistent, meaning that the mandible is closed by MPP along the muscular closure tract until the upper and lower teeth are in contact, and the position of the mandible is exactly ICP. and there is no contact of individual teeth or few teeth (early contact), which is called: the coordination of tooth position and muscular position. And MPP is the contact with ICP or MCP through the normal muscular closure tract (4).
It is the repeatable anatomical relationship between ICP, RCP, MCP, and MPP that ensures thousands of changes in jaw position during mastication, while comfortably performing functions without causing functional or structural disorders of the entire oromandibular system (5). Therefore, it is crucial to maintain or correct the anatomical relationship between ICP, RCP, MCP, and MPP during treatment, restoration, orthodontics, and surgery in all clinical departments, otherwise, it will lead to various functional or even structural disorders of the oromandibular system of medical origin.
3. Clinical significance of dentition and the four basic jaw positions – ICP, RCP, MCP, MPP
3.1 Clinical significance of dentition
The dentition is the most special anatomical and physiological structure of the human body, and it can even be considered as a special joint, a continuation of the temporomandibular joint (TMJ). Although the dentition does not have joint auxiliary structures such as joint disc, joint capsule and ligament, it has the function of joint movement.
Its special characteristics are mainly manifested in.
(1) Large individual differences.
(2) Limited ability to repair itself.
(3) Lifelong continuous changes.
This suggests that the clinician should establish the concept of functional adaptation of the dentition when treating occlusal disease, and that the so-called “ideal dentition” need not and cannot be the standard of treatment. At the same time, attention should be paid to the impact of the occlusal factors on the entire oromandibular system, especially on the TMJ, as perhaps a seemingly trivial adjustment of the dentition can cure TMJ disorders. In addition, attention should be paid to the effect of muscle function on the dentition. In cases of abnormal muscle function, the dentition should not be adjusted blindly, otherwise a new dentition disorder will occur when the muscle function returns to normal and induce TMD. All clinical treatment should also pay attention to the restoration and maintenance of the stability of the dentition, so as to promote the health of the whole masticatory system with the dentition as the attributing point.
3.2 Clinical significance of ICP
A normal ICP can ensure the closest and most extensive contact between the maxillary and mandibular cusps and fossae, thus increasing the masticatory area of the dentition and facilitating the improvement of masticatory efficiency; it is also conducive to the dispersion of dental forces and avoiding periodontal trauma to individual teeth or a few teeth; in addition, due to the presence of ICP, even if individual teeth are missing, it does not cause complete loss of function of the opposing teeth, and it can prevent the adjacent teeth and the opposing teeth from tilting and shifting within a short period of time. In addition, due to the presence of ICP, even if an individual tooth is missing, it does not cause complete loss of function of the opposing tooth. This provides the clinical assurance of timely restoration.
3.3 Clinical significance of RCP
When total or most of the teeth are missing, the ICP determined by the ICO (interdental cusp) will also be difficult to determine or disappear. This is not the case with the RCP determined by the ligament, which can still be accurately obtained in the case of total tooth loss, which is irreplaceable and necessary in the restoration of complete dentures. This makes it possible to reconstruct a functional and aesthetic prosthetic occlusion for patients with total edentulism.
3.4 Clinical significance of MCP
The consistency of the MCP with the ICP is directly related to the proper functioning of the oromandibular system. It is because of the presence of the MCP that we can determine possible disorders of the natural dentition or denture reconstruction, such as early contact and interfering dentition, and remove them accurately. At the same time, we can also infer whether the cause of the disorder is due to a disorder of the dentition or to an abnormality of the muscle function by referring to the abnormality of the ICO.
3.5 Clinical significance of MPP
MPP is determined by the gravitational force of the mandible itself and the detrusor reflex of the ascending jaw muscle. The normal MPP can make the facial expression natural and beautiful, and can make the functional state may cause minor trauma within the oromandibular system that does not yet have clinical symptoms (such as intra-articular) have enough rest period for repair, in fact, the human dentition contact in about 24h and itself. In addition, MPP is also an important reference factor for determining the vertical distance (VD) in complete denture restorations.
4.Summary
The core theoretical point of view is that the dentition has a mechanical control on the jaw movement (i.e., changing jaw position), while the dentition can also provide feedback regulation on the change of jaw position (i.e., jaw movement) through the neuromuscular. Mandibular movements are controlled by two types of factors, anatomical and physiological, to define the boundary conditions of the dentition and jaw position and to provide neural feedback regulation of jaw position. The result is a masticatory system that functions with minimal risk of trauma, minimal energy consumption and maximum efficiency and natural comfort.
The interrelationship between dentition and jaw position and with the four basic jaw positions ICP, RCP, MCP and MPP is complex and crucial. It is the coordinated alternation between them throughout the jaw movement that allows the oromandibular system to function comfortably and naturally without the occurrence of various disorders (6). Therefore, the purpose of treatment in all clinical specialties of dentistry should be to restore the anatomical relationship between the damaged dentition and jaw position and the four basic jaw positions and to maintain stability. It is very important for clinicians to correctly understand the anatomy and physiology of dentition and jaw position, ICP, RCP, MCP and MPP, as well as their interrelationship and clinical significance to guide clinical practice.