The Vertical Dimension of Occlusion (VDO) defines the height of the lower third of the face when the dental arches are brought together in a biting position. Determining this measurement is challenging when fabricating complete dentures because the natural guides have been lost. An incorrect VDO can cause functional problems, discomfort, and poor aesthetics for the patient.
An incorrect VDO can be too high or too low. If the VDO is too high, the patient may experience muscle strain, jaw joint pain, difficulty closing their lips, and clicking sounds when speaking. If the VDO is too low, the face appears collapsed, the chin looks too close to the nose, and lip lesions may occur at the corners of the mouth. Restoring the proper vertical height is necessary for the long-term health and function of the jaw system.
Establishing the Vertical Dimension of Rest
Determining the correct VDO begins by establishing the Vertical Dimension of Rest (VDR). VDR is the height of the face when the jaw muscles are completely relaxed. This postural position of the mandible occurs when the patient is sitting comfortably upright and the jaw muscles are minimally active. The VDR is a reliable starting point because it is determined by the patient’s underlying muscle and bone structure, independent of teeth presence.
To record the VDR, the clinician places two arbitrary marks on the patient’s face, typically one on the tip of the nose and one on the chin. The patient is instructed to relax, often by licking their lips, swallowing, and then letting the jaw relax naturally. The distance between these two marked points is then measured using a ruler or specialized device.
The relationship between the VDR and the VDO is defined by the “Freeway Space,” also known as the interocclusal rest space. This is the small gap between the upper and lower arches when the jaw is at its resting position. The VDO is calculated by subtracting the freeway space from the measured VDR.
The freeway space is consistently found to be in the range of 2 to 4 millimeters in most individuals. This space acts as a buffer, ensuring the teeth do not contact during normal rest and allowing the jaw muscles to remain relaxed. For example, if a patient’s VDR measures 70 mm, the initial VDO goal would be 66 mm to 68 mm, allowing for the necessary 2 to 4 mm of freeway space.
Using Mechanical and Facial Reference Points
Dentists use external references and historical data to provide additional VDO estimates beyond the physiological rest position. Pre-extraction records offer a template of the patient’s original facial structure before tooth loss. These records might include old photographs showing the original lower face proportion, or diagnostic casts recording the original arch dimensions.
Previous dentures can also serve as a starting point for VDO determination. By assessing the freeway space with the old dentures in place, the clinician determines if the current VDO is deficient, which is common due to bone resorption. If the measured space is greater than 4 mm, the VDO is likely too small and needs to be increased for the new dentures.
Anthropometric measurements, based on average facial proportions, also guide the process. A classic technique uses a Willis gauge to compare the distance from the base of the nose to the chin with the distance between the pupils of the eyes. This method assumes these two facial segments should be roughly equal when the VDO is correct. While these external reference points provide a good estimate, they are not definitive because individual facial features vary significantly.
The information gathered is transferred to the trial denture base, usually as wax occlusion rims. These rims temporarily substitute for the teeth, allowing the clinician to adjust the vertical height until the distance between the arches matches the calculated VDO. This step establishes a preliminary vertical jaw relationship before the final artificial teeth are positioned.
Assessing Functional and Phonetic Requirements
Dynamic assessment methods focus on how the determined VDO functions during natural activities like speaking and swallowing. Phonetics provides a reliable way to fine-tune the VDO, particularly using sibilant sounds such as ‘S,’ ‘Ch,’ and ‘J’. These sounds require the jaw to move into a precise position where the upper and lower front arches come very close without touching.
The minimal distance between the incisal edges during pronunciation is called the “closest speaking space.” If the trial VDO is too high, the arches will audibly click or interfere when the patient speaks the ‘S’ sound. If the VDO is too low, the closest speaking space will be too large, resulting in muffled speech. The correct VDO is established approximately 1 to 2 millimeters below the closest speaking level.
Swallowing is another functional test that helps confirm the VDR measurement. When a patient swallows and relaxes, the mandible naturally returns to the physiological rest position. By asking the patient to swallow while wearing the trial denture bases, the clinician observes if the jaw returns to the measured VDR with the correct freeway space remaining between the rims. This technique ensures the jaw is not over-closed or over-opened during this natural action.
The correct VDO also provides proper support for the lips and cheeks, playing a large role in aesthetics. A VDO that is too low leads to a collapsed facial profile, giving the patient an aged or sunken appearance. When accurately restored, the VDO helps smooth the skin, restore the natural curvature of the lips, and ensure the lower third of the face appears proportionate.
Confirming the Final Vertical Dimension
Since no single method for VDO determination is foolproof, final acceptance depends on confirming results using multiple criteria during the trial stage. The ultimate test is the patient’s comfort and ability to function normally over a period of wear. The patient should wear the wax trial dentures for an extended period without reporting muscle soreness, joint tenderness, or facial strain.
A correct VDO is confirmed by the absence of clicking noises during speech and the presence of harmonious facial aesthetics. The final vertical height must allow the jaw muscles to function efficiently without being over-extended or compressed. This confirmation is important for the long-term success of the dentures, as an accurate VDO prevents excessive forces on the underlying gum and bone tissue.
The clinician must be satisfied that the calculated VDO balances muscle function, aesthetics, and phonetics. Accepting the final vertical dimension means the patient has a comfortable interocclusal rest space and the ability to close without effort. This confirmation process ensures the new dentures provide optimal function and a restored natural facial appearance.