Articulation and spatial planes
In dentistry, articulation refers to absolutely all types of movements that occur in the temporomandibular joint (TMJ). This comprehensive concept defines any possible spatial positions of the mandible relative to the maxilla.
To accurately describe biomechanics and movement trajectories, dentists examine jaw movements strictly within three mutually perpendicular planes. These include:
- Frontal (or vertical) plane.
- Sagittal plane.
- Transversal (or horizontal) plane.
It is within this three-dimensional coordinate system that all functional processes of the masticatory apparatus take place.
Occlusion: when teeth meet
The concept of occlusion is narrower and represents only a specific type of articulation. The main characteristic of occlusion is the mandatory contact of the upper and lower teeth. Such contact can occur during various movements of the mandible in the joint.
To analyze occlusal contacts, clinicians use two important geometric landmarks:
- Occlusal plane. This is an imaginary line that originates from the incisal edge of the central lower incisor. It then runs posteriorly and ends either at the tip of the distobuccal cusp of the second (sometimes third) molar or reaches the middle of the retromolar pad.
- Occlusal surface of the dental arches. Unlike the plane, it passes directly through all the chewing surfaces of the molars and premolars, and also encompasses the incisal edges of the anterior teeth.
Occlusal curves and the curve of Spee
The anatomy of the dental arches is not perfectly flat. In the region of the posterior teeth, the occlusal surface forms a specific curvature. A characteristic feature of this curvature is that its convexity is always directed strictly downward.
Biomechanics uses the following terms to describe this geometry:
- Sagittal occlusal curve is a general, collective name for this curvature in the posterior teeth region.
- Curve of Spee (also known as the sagittal compensatory curve). This is a more precise line drawn sequentially along the incisal edges of the anterior teeth and then continuing along the buccal cusps of the posterior teeth. When viewed in the sagittal projection, this curve forms a segment of a circle facing downward with its convex part.
Briefly: answers to topical questions
What types of occlusion are distinguished in propaedeutic dentistry?
Depending on the position and direction of mandibular displacement, the following states are distinguished:
- Central occlusion — multiple fissure-cusp contacts of the dental arches with the central position of the TMJ condyles in the articular fossae.
- Anterior occlusions — characterized by the forward and downward displacement of the mandible.
- Lateral occlusions — represent the right and left displacement of the mandible.
- Distal contact position of the mandible.
Through which landmarks does the transversal occlusal curve (Wilson's curve) pass?
The transversal occlusal curve passes through the masticatory surfaces of the right and left premolars and molars in a transverse direction. For the lower dental arch, the reference point for this curve is the first premolar. Overall, it illustrates the contact of antagonist teeth in the transversal plane at the level of the premolars (teeth 1.4 and 2.4; 1.5 and 2.5) and molars (teeth 1.6 and 2.6; 1.7 and 2.7).
Which muscles protrude the mandible forward?
Mandibular protrusion is carried out by the bilateral contraction of the lateral pterygoid muscles. This movement belongs to anterior occlusions, which are sagittal movements of the mandible. During this biomechanical act, the mandible is displaced forward and downward, while the incisal edges of the anterior teeth meet in an edge-to-edge relationship.
Specifically, what mandibular movements occur in the sagittal plane?
Movements corresponding to anterior occlusions occur in the sagittal plane, namely the forward and downward displacement of the mandible. The following pathways of movement are executed:
- Sagittal condylar pathway — downward displacement of the articular condyles and forward movement of the jaw along the slopes of the articular tubercles.
- Sagittal incisal pathway — movement of the lower incisors forward along the palatal surfaces of the upper incisors.
What is the sagittal condylar pathway?
The sagittal condylar pathway is the trajectory formed when the articular condyles displace downward and the mandible moves forward along the slopes of the articular tubercles. The line of this pathway forms the sagittal condylar angle (Gysi angle) with the occlusal plane. The individual norm of this angle ranges within 20–40°, and its average value according to Gysi is 33°.
Articulation is a CAR in motion (absolutely any jaw movements), while Occlusion is a STOP bumper-to-bumper (physical contact of the teeth is mandatory).
Class and exam questions
What is the fundamental difference between articulation and occlusion?
Articulation encompasses absolutely any movements of the mandible in the TMJ. Occlusion, on the other hand, is merely a specific case of articulation characterized by the mandatory physical contact of the teeth of the upper and lower jaws.
Through which anatomical landmarks does the occlusal plane pass?
It begins from the incisal edge of the central lower incisor and goes to the tip of the distobuccal cusp of the second or third molar. It can also end at the middle of the retromolar pad.
Where is the convexity of the curve of Spee directed?
The curve of Spee (or sagittal compensatory curve) forms a segment of a circle whose convexity is always directed downward.
What else to study on this topic
- Detailed analysis of mandibular movements in the frontal (vertical) plane
- Features of jaw movements in the sagittal plane
- Specifics of transversal (horizontal) mandibular movements
- The role of the temporomandibular joint (TMJ) in articulation biomechanics
- Functional significance of compensatory occlusal curves
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Other topics in "Chapter 5. Biomechanics of the Masticatory Apparatus"
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