Mandibular Flexure

November 02, 2024

The mandible or the lower jaw bone is the only movable bone in the facial region. The movement of this bone allows us to chew and speak unhindered. It houses the lower teeth and is also an attachment site for various muscles. These muscles exhibit a pull on the mandible when the mouth is opened or moved forward. The pull causes the mandible to flex minimally. This is called mandibular flexure.

Since the amount of mandibular flexure is minimal, it was initially considered insignificant. However, studies have now shown that understanding mandibular flexure is helpful in successful treatment that involves fabricating dental prosthesis and implants.

The muscles that take part in the flexure of mandible are lateral pterygoid, medial pterygoid, platysma and superior constrictor of pharynx. The mandible is a U-shaped bone which flexes inwards by the force of the four mentioned muscles. The flexure of mandible can be of four types.

a) Symphyseal bending

The symphysis is the area where the two halves of mandible fuse with each other in the region of the chin. The mandible bends or flexes from this region due to the pull of the lateral pterygoid muscle.

b) Dorsoventral shear

In the dorsoventral shear type of mandible flexure, both the lateral and medial pterygoid muscles pull the mandible vertically which caused it to flex.

c) Corporal Rotation

This type of flexure occurs during chewing when the jaw exhibits a thrust from down to up. This results in decreased dimension of the lower arch.

d) Anteroposterior shear

The anteroposterior shear occurs when the lower jaw moves upwards during chewing. The direction of bending is from the back to the front region.

It is not necessary for the mandible to be in motion for flexure to occur. Bruxism is a condition where patients have a bit of clenching their teeth abnormally. The excessive pressure caused by this clenching can transmit the force to adjacent muscles which can lead to flexion of the mandible.

By now we understand that the force exhibited by the musculature is directly proportional to the mandible flexure. Some researchers suggest that patients with a shorter facial height exhibit a stronger vertical force and thus a greater mandibular flexure compared to those with a longer facial height.

Another factor that determines the intensity of mandibular flexure is the strength of the bone on which the muscles are attached to. A stronger bony architecture will resist the mandibular flexure better. This means that the possibility of having a greater mandible flexure is more in aged patients since their bone density reduces over time. It is also imperative that patients who are suffering from bone diseases like osteoporosis are prone to a greater mandibular flexure.

The height of the bony part of the mandible in relation to the chin (symphysis) and the total length of the mandible also contributes to mandibular flexure. A shorter symphysis and longer mandible flexes more easily.

The mandibular flexure causes a reduction of the arch dimension between 0.3 mm to 1 mm. Such minute measurements are calculated by studying study models of patients at various stages of mouth opening or closing. It can also be calculated by callipers, gauges and transducers that can be used directly in the patient's mouth. More advanced techniques includes the use of digital three-dimensional scans for accurate calculation.

The preliminary steps while fabricating a prosthesis is to take an impression of the arch to make a replica. However, this procedure requires the mouth to be open, and mandibular flexure varies when the mouth is open or closed. The result of this would be that the said prosthesis would be fabricated in a changed dimension. Once the patient wears this prosthesis, it increases the chances of undue forces being exerted on the mandible. Certain prosthesis may even damage the tissue surrounding the tooth, thereby making the tooth mobile. Hence, in patients with mandible flexion, the mouth should be kept as closed as possible during impression taking.

Mandible flexion should always be considered during implant placement. This is because placement of implant during a flexed position of mandible can cause undue strain on the surrounding bone and improper distribution of forces in the muscles. Moreover, since the implant is screwed within the mandible, there is a greater chance of more flexion of that region if it is not positioned in the right area and direction.

Modern technologies like Computer Assisted Design and Computer Assisted Manufacturing along with materialistic advances have made it possible for making prosthetic designs for the asymmetric jaw. Mandible flexion is thus an important aspect to consider by oral surgeons during treatment planning.

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