Which muscle is MOST likely to become fatigued in a patient whose prosthesis has an EXCESSIVE vertical dimension of occlusion?

Correct answer: A. Masseter

• A. CORRECT. The MASSETER is the most powerful ELEVATOR of the mandible and the muscle most affected when the vertical dimension is excessive. An over-opened VDO leaves inadequate freeway space (interocclusal rest space, normally 2–4 mm), so the elevator muscles are held in a state of sustained stretch and cannot fully relax at rest. The result is chronic muscle fatigue, soreness, and the classic complaints of clicking teeth, difficulty closing, a strained facial appearance and accelerated ridge resorption.
• B. The lateral pterygoid PROTRUDES the mandible and assists in opening — it is not the elevator strained by loss of freeway space.
• C. The buccinator is a muscle of FACIAL EXPRESSION that keeps the bolus positioned on the occlusal table. It does not elevate the mandible and is not fatigued by excessive VDO.
• D. The digastric is a mandibular DEPRESSOR, active in opening rather than in sustained closure.

Note: Excessive VDO signs: no freeway space, teeth clicking during speech, muscle fatigue and soreness, a strained or stretched facial appearance, difficulty swallowing, and rapid ridge resorption. INSUFFICIENT VDO produces the opposite — an aged appearance, angular cheilitis, and a chin positioned too close to the nose.