Envelope of Mandibular Motion
The movements of the mandibular condyles in the temporomandibular joints (TMJs) can be divided into: (1) rotational and (2) sliding along the "S"-shaped articular surface of the temporal bone – from the mandibular fossa to the articular tubercle and back. The combined nature of the joint implies that the movement of both condyles occurs simultaneously, but not necessarily symmetrically. The freedom of combinations of rotational and sliding, bilaterally symmetrical and asymmetrical movement makes this joint very difficult for standard anatomical description (range of motion, axes of rotation, etc.). This problem was addressed through the work of Ulf Posselt, who proposed using the concept of the envelope of motion to describe the movements of the lower jaw1, 2. The projection of the boundaries of this envelope onto the sagittal, frontal, and horizontal planes is referred to as "Posselt's diagrams," or "Posselt's envelopes of motion." Simply put, Posselt's diagrams describe the trajectory of the movement of the incisors of the lower jaw along the maximum possible boundaries. The resulting trajectory has a characteristic shape, passing through nodal points, the positions of which can be easily reproduced3.
Below are the standard positions of the mandible at the reproducible points of Posselt's diagram. These points are numbered and may be represented in multiple projections. For example, the point of maximum mandibular depression (point #4) is present in both the sagittal and frontal diagrams.
All the provided images are interactive — by clicking on them, you can observe the shift of the lower jaw from the current to the initial position (central occlusion; point #1)
Posselt's Sagittal Diagram
The initial position of the lower jaw with maximum intercuspation, as well as the most uniform, balanced bilateral occlusal load. The upper incisors overlap the lower incisors (overjet) by 2–4 mm1.
Ideally, the position of central occlusion coincides with the 'centric relation,' in which the mandibular condyle is under the arch of the mandibular fossa, and the articular disc is in an anterior-superior position, contacting the mandibular condyle with its thinnest avascular part2.
This position is characterized by contact between the incisors along their cutting edges (margo incisalis). The path from central occlusion – point #1 – to point #2 is defined by the sliding of the lower incisors along the palatal surfaces of the upper incisors and is called the 'sagittal incisal path' (anterior guidance). The angle at which this path occurs depends on the configuration and height of the cusps of the central incisors and is approximately 55°1, 2.
As the mandibular condyle moves from central occlusion to point #2, it slides along with the disc along the anterior wall of the mandibular fossa, and the angle of this slide relative to the Frankfurt plane is between 47-57°1, 2. As a result, the mandible moves diagonally downwards and forwards with minimal rotation. This movement automatically disengages the tight contact between the molars.
The active protrusion of the mandible is limited by the positioning of the masticatory muscles and the stretch limit of the TMJ capsule. The curved line on Posselt's diagram, connecting point #2 to point #3, is shaped by the unique spatial relationships between the teeth along the chewing surfaces where this movement occurs.
As the mandibular condyle approaches point #3, it, along with the disc, is guided into the lowest position by the articular eminence of the temporal bone. The movement of the disc along this path appears to be passive and occurs automatically. The muscles attached to the disco-capsular system of the TMJ are active only during the reverse movement – retrusion1,2. During this, the disc normally moves in such a way that the maximum compression exerted by the mandibular condyle is applied to its thinnest avascular part3.
The position at point #4 is characterized by an almost complete lack of freedom for mandibular movement in the horizontal plane (protrusion, retrusion, lateral excursion). This is due to the muscular fixation of the mandibular condyles and the stabilizing role of the articular disc. As at point #3, the discs are positioned on the articular eminences of the temporal bones and are pressed by the mandibular condyles. In this position, the pressure on the surface of the discs is more evenly distributed, rather than being concentrated on their central part1. This is due to the varying curvature of the articular surface of the condyles: during the rotation of the mandible, the flatter part of the condyle faces the disc. Upon reaching the point of maximum lowering, the mandible becomes locked, and release from this position can only occur through upward movement2.
The movement of the mandible around a stable frontal axis is called hinge movement, and the maximum mouth opening with rotation around this stable axis is called the 'terminal hinge position.' The distance between the incisors at point #5 rarely exceeds 20 mm, and further increase in this distance leads to the inevitable sliding of the mandibular condyle along with the disc along the anterior wall of the mandibular fossa towards the articular eminence of the temporal bone*. In cases of TMJ disc displacement, such sliding may be limited or impossible due to displacement of the articular disc. In such cases, chewing movements are often limited to the mandible moving between points #5, 6, and 1.
The retruded contact position is limited by the bones – specifically by the mandibular condyles being pressed against the posterior wall of the mandibular fossa of the temporal bone. Point #6 is always slightly lower than point #1. This is caused by the sliding of the cusps of the lower molars along the inclined occlusal surfaces of the upper molars. As a result, the mandible experiences a slight downward movement and separation of contact between the incisors and canines*. The position of the articular disc during this movement remains almost unchanged, but the pressure exerted on it by the mandibular condyles significantly decreases.
Posselt's Frontal Diagram
The frontal view shows clearly that the teeth of the upper jaw overlap the teeth of the lower jaw along the entire perimeter of the dental arch. This occlusal relationship is examined in more detail in the section on centric occlusion.
When transitioning from the position of centric occlusion to the incisors' tête-à-tête position, the aforementioned diagonal downward shift of the mandible is clearly noticeable, with almost no rotation.
The interactive image demonstrates that, compared to the position of centric occlusion (point #1), the shift of the lower incisors is close to horizontal, while the mandibular condyles, sliding onto the articular eminence of the temporal bone, move downward.
In the frontal and sagittal projections, point #4 is located in the lowest depression of Posselt's diagrams, excluding the combination of this position with lateral excursion, as well as with protrusion or retrusion.
During the laterotrusion (lateral excursion) of the mandible from the position of central occlusion (point #1), there is initially an exit from maximal intercuspation with a gradual separation of tooth contacts. This movement cannot occur without a slight initial lowering of the mandible. In Posselt's diagram, this lowering is represented by a slight 'droop' of the segment between points #1 and #7.
Note the slight caudal displacement of the mandibular condyle on the side opposite to the direction of the lateral excursion. The mechanism of this effect is addressed below.
Posselt's Horizontal Diagram
Posselt's diagram in this projection is somewhat more complex to interpret because the lower incisors, which define the boundaries of the diagram, are hidden. Nevertheless, the illustrated diagram is presented to scale and corresponds to the range of mandibular movement in the 3D model of the Anatomy Standard.
The inferior projection demonstrates that during symmetrical protrusion with contact, the mandibular condyles simultaneously exit the articular fossae and move toward the articular eminences of the temporal bones.
A view from below of the mandible in maximum contact retrusion demonstrates the primary limiting factor of retrusion – the posterior wall of the mandibular fossa of the temporal bone, against which the mandibular condyles abut.
The essence of the mechanism of lateral excursion (laterotrusion) of the mandible is its rotation around the so-called 'working' condyle. This term refers to the condyle that remains in the mandibular fossa of the temporal bone and is located ipsilateral to the direction of the lateral excursion. The working condyle may experience slight movement in the horizontal plane (Bennett movement1). Simultaneously, the opposite, or 'non-working,' condyle undergoes significant movement, slipping along with the articular disc out of the mandibular fossa and onto the surface of the articular eminence of the temporal bone.
During laterotrusion, there is an automatic separation of contacts between the molars. The mechanism of this separation involves, firstly, the diagonal downward shift of the non-working condyle as it exits the mandibular fossa (condylar guidance) and, secondly, the sliding of the lower canine along the lingual surface of the upper canine (canine guidance), which also moves diagonally downward2.