Determining the correct facial height is a fundamental step in prosthodontics for individuals who have lost all their natural teeth, a condition known as complete edentulism. Fabricating dentures requires restoring not only the appearance of the smile but also the proper relationship between the upper and lower jaws to ensure comfortable function. The absence of natural teeth removes the body’s primary reference points, making the process of establishing the correct Vertical Dimension of Occlusion (VDO) a challenging, multi-faceted procedure. This measurement directly impacts the patient’s ability to speak, chew, and maintain a natural facial profile.
Defining the Vertical Dimensions
The process of finding VDO relies on two distinct measurements of the lower facial height. The first concept is the Vertical Dimension of Rest (VDR), which is the distance between two arbitrary reference points when the jaw is in its physiological rest position. The VDR is the postural position of the mandible when the muscles are relaxed. It is considered relatively stable and independent of the presence of teeth, making it the baseline for calculations.
The second measurement is the Vertical Dimension of Occlusion (VDO), which is the distance between those same two reference points when the upper and lower biting surfaces are in contact. For edentulous patients, this contact is provided by temporary wax rims or the final dentures. The difference between these two measurements, VDR and VDO, defines the third concept, known as the Freeway Space or Interocclusal Distance.
The Freeway Space is the natural gap between the upper and lower biting surfaces when the jaw is at rest. This space typically measures between 2 to 4 millimeters and is necessary for the muscles to remain relaxed. The ultimate goal is to establish the VDO by measuring the VDR and then subtracting the patient’s individualized Freeway Space.
Primary Methods for Determining VDO
Since no single method is perfect, clinicians rely on a combination of techniques, categorized as physiological, anthropometric, or mechanical. The initial step for nearly all methods is establishing the VDR. This involves selecting two facial reference points (e.g., on the nose and chin) and measuring the distance between them. The patient is seated upright and asked to relax their jaw muscles, often by swallowing or repeating the letter “M,” before the measurement is taken.
Anthropometric Methods
Anthropometric methods use established facial proportions as a guide. The Willis Gauge method, for instance, suggests that the distance from the outer corner of the eye to the corner of the mouth should be approximately equal to the distance from the base of the nose to the chin when the jaw is at the correct VDO. Although this method provides a quick starting point, it is based on averages and may not be accurate for every individual due to facial asymmetry and varying bone loss patterns.
Physiological Methods
Physiological methods utilize natural muscle function and speech patterns. The swallowing technique involves asking the patient to repeatedly swallow while wearing temporary wax rims. The theory is that the biting surfaces momentarily approach the correct VDO during the final phase of swallowing. The wax rims are adjusted until the patient can swallow comfortably without straining the facial muscles.
Phonetic Methods
The phonetic method, particularly the “S” sound technique, determines the closest speaking space, which guides VDO. The patient pronounces sibilant sounds, such as counting from 60 to 66, while the distance between the wax rims is observed. The smallest gap between the biting surfaces during this speech is the closest speaking space. The correct VDO must be slightly greater than this space, typically 1 to 2 millimeters less than the VDR, ensuring the teeth do not click during conversation.
Mechanical Methods
Mechanical methods, such as the parallelism of the ridges, rely on anatomical relationships. The principle suggests that the residual upper and lower toothless ridges should appear parallel when the jaw is positioned at the correct VDO. This technique is often used with a measuring instrument like a Fox plane to ensure the upper wax rim is parallel to both the interpupillary line and the ala-tragus line, establishing the proper occlusal plane before the final VDO height is set.
Verification and Clinical Adjustment
Once an initial VDO measurement is established, the value is transferred to temporary devices called wax rims or trial bases. This stage verifies the accuracy and comfort of the proposed VDO before the final dentures are processed. The patient wears the trial bases while the clinician performs functional and aesthetic checks.
Phonetic verification is crucial, as an incorrect VDO immediately affects speech. If the VDO is too high, the patient may exhibit a lisp or a clicking sound because the closest speaking space is eliminated. Conversely, if the VDO is too low, the patient’s speech may be muffled due to insufficient space for the tongue’s movement.
Esthetic verification assesses the patient’s facial appearance, ensuring the VDO provides adequate lip support and a natural profile. An excessive VDO causes the lips to strain, creating a “stuffed” appearance. Conversely, a deficient VDO makes the chin appear too close to the nose, resulting in a collapsed or prematurely aged look. The wax rims are adjusted until facial contours and lip position appear natural and symmetrical.
Patient comfort and muscle tolerance are the final checks. An excessive VDO leads to constant muscle strain, potential temporomandibular joint pain, and rapid bone resorption. A VDO that is too low results in a loss of chewing efficiency and lack of proper support. The final determination of VDO integrates objective measurements with subjective feedback and functional performance. The wax rims are incrementally adjusted until a harmonious balance of aesthetics, comfort, and function is achieved.