Lateral pterygoid: opening the mouth and jaw deviation
The lateral pterygoid is the muscle of mastication (the chewing muscles) that pulls the condyle (the rounded head of the mandible) forwards and so helps to open the mouth.
Remember
Among the four muscles of mastication, only the lateral pterygoid contributes to opening the jaw, and it does this through pulling the condyle forwards; the other three close it. If the mandibular nerve is injured on one side, the jaw deviates (swings) towards the weak side on opening, because the intact lateral pterygoid on the other side pulls without opposition.
Memory aid
Lateral pterygoid Lowers the jaw, while the other three Close it
The matching first letters tie the lateral pterygoid to lowering the jaw, while the other three muscles of mastication close it.
The four muscles of mastication are the chewing muscles, and they all act on the temporomandibular joint, the jaw joint just in front of the ear. Three of them mainly raise (elevate) the mandible and so close the mouth. These are the masseter (the strongest jaw-closing muscle, lying over the side of the face), the temporalis (a fan-shaped muscle over the side of the skull) and the medial pterygoid (which lies on the inner surface of the mandible).
The lateral pterygoid is the odd one out. It sits deep in the infratemporal fossa (a space at the skull base, deep to the masseter), and its fibres run almost horizontally to the neck of the mandible, just below the condyle (the rounded head of the mandible). As it contracts, it pulls the condyle forwards, and this movement is called protrusion. This forward glide contributes to depression of the mandible (lowering of the lower jaw), so the mouth opens.
Opening is a team effort. Gravity does much of the work, and the suprahyoid muscles help too, when the hyoid bone is fixed. These are small muscles above the hyoid bone, in the neck. Digastric is one of them: it helps to lower the jaw, especially against resistance, but it is not a muscle of mastication.
The mandibular nerve, a branch of the trigeminal nerve, supplies all four muscles of mastication. Normally the two lateral pterygoids pull the two condyles forwards equally, and the jaw opens in a straight line. If the nerve is injured on one side, the lateral pterygoid on that side is paralysed. The healthy muscle on the other side then pushes its condyle forwards without opposition, so the jaw turns around the weak side. The chin deviates (swings) towards the side of the injury, which is the weak side.
Key facts
- Opening muscle
- the lateral pterygoid pulls the condyle forwards, which contributes to lowering the mandible.
- Closing muscles
- the masseter, temporalis and medial pterygoid raise the mandible and so close the mouth.
- Other help
- gravity and the suprahyoid muscles take part in opening the mouth too.
- Not mastication
- digastric also helps to lower the jaw, but it is a suprahyoid muscle, not one of the four muscles of mastication.
- Examination
- the medial pterygoid can be felt from inside the mouth, but the lateral pterygoid cannot be felt clinically.
- Deviation rule
- with a mandibular nerve injury on one side, the jaw deviates on opening towards the side of the injury, which is the weak side, because the intact lateral pterygoid on the other side acts without opposition.
Exam traps
- The medial pterygoid lies just below the lateral pterygoid and is the classic confusion with it, but it does not open the mouth, because it elevates the mandible, which closes the mouth.
- The masseter is the most powerful of the chewing muscles, but that does not make it a jaw-opening muscle, because it closes the mouth by elevating the mandible.
- Temporalis does not open the mouth, because it elevates the mandible to close it, and its rear fibres also pull the jaw backwards.
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