The antegonial notch is a functional one created due to the action of the elevators (masseter and medial pterygoid) which are creating an upward traction and the primary and secondary depressors of the mandible - the lateral pterygoid, digastric and strap muscles which indirectly pull the mandible down.

As the mandible is unable to swing down, it gets modified to have a prominent antegonial notch by constant pull of the muscles. If the mandible were able to open normally, the forces acting would be lesser and the antegonial notch is just just a gentle curve in normal mandibles. In cases of resricted mobility, the forces are more and also the mandible is unable to swing down. These create a bend with the fulcrum somewhere at the anterior point of masseter as posterior to that the area is kind of splinted between masseter and medial pterygoid. So the antegnonial notch then occurs at a point just anterior or along the anterior border of masseter muscle as there is no more muscle support there to counter the downloadly acting secondary depressors of mandible.

I hope this is clear.

Regards,

Dr. Akilesh. R

I accept to what Dr.Akilesh has mentioned above.

In a TMJ ankylosis patient with limited or nil mouth opening, there is a possibility of elongation of coronoid process and a prominent antegonial notching. This is because there is every tendency for the patient to open the mandible even though there is limitation or no movement in the mandible while feeding. During this process, the muscles of mastication, especially the temporalis which is attached to the coronoid process and the pterygomassetric sling which lies on the antegonial notch, present just anterior to the angle at the lower border of mandible gets activated during mild movement of the mandible or the muscles goes for spasm when there is absolutely no movement of the mandible. In both the situations, there is elongation of coronoid process as well as the antegonial notch becomes more prominent as the disease gets chronic. Apart from this the secondary muscles of mastication like the digastric, strap muscles which acts in opposite direction to the muscles mentioned before makes the coronoid process and antegonial notch more elongated and prominent respectively.

It is a fallacy that the action of mouth opening creates downward forces which cause deepening of antegonial notch and elongation of coronoid process in ankylosis cases.

These patients may not be able to open mouth, but nothing stops them from "clenching" their teeth together. This isometric contraction (in an attempt to chew) creates forces in the elevator muscles e.g. temporalis and masseter which in turn develop according to "Force-length relationship". Force-length relationship, also called the length-tension curve, relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with greatest active force when close to an ideal length (often their resting length). In order to generate maximum forces of contraction, masseter and temporalis develop to attain an ideal length, causing the mandible to grow in proportion (Functional matrix theory). In ankylotic patients,since the mandible is prevented from responding normally to these forces (because of lack of function and growth center destruction), the attempt of the muscles to attain an ideal length causes the bone to remodel at their point of attachment. Thus we see that the gonion increases in length as does the coronoid process. The downward bending of the mandible at the antegonion is probably caused by depressors trying to open mouth, but their contribution to deepening of the notch is minimal.

Due to the constant action of the depressor and elevator muscles of the mandible against each other

I think that the prominent angle due to continuous force on this bony part by the action of medial pterygoid muscle leading to bone formation and remodeling.Also the coronoid process by the action of temporalis.

I think it's the action of Lasseter and temporalis which is unaffected in spite of no antero postetior growth of the mandible. The pull if these muscles is responsible for the prominence.

Action of the elevator and depressor muscles that pull the jaw upward and downward during growth that causes the distinctive antegonial notching in the mandible.

Chakranarayan A, Jeyaraj P. Coronoid hyperplasia in chronic progressive trismus. Med Hypotheses. 2011 Nov;77(5):863-8. doi: 10.1016/j.mehy.2011.07.058. Epub 2011 Aug 23.

Please find a copy of the article attached. Hope it helps!!