Everything You Must Know About Vertical Dimension, Freeway, and Speaking Space

Introduction

Establishing the correct vertical dimension of occlusion (VDO) is one of the most critical and challenging aspects of restorative dentistry, complete denture fabrication, and full-mouth rehabilitation. An incorrect VDO can lead to a host of problems, including muscle fatigue, temporomandibular joint disorders, poor denture stability, and accelerated alveolar bone resorption. To navigate this complex process, clinicians rely on two fundamental physiological concepts: the Freeway Space and the Closest Speaking Space. Understanding the distinction between these two parameters is not merely academic; it is essential for achieving functional and biological success in prosthetic and restorative treatments.

This article delves into the definitions, physiological basis, clinical significance, and practical implications of these two key spaces, providing a clear guide for the practicing dentist.

1. Freeway Space (Interocclusal Rest Space)

The Freeway Space (FWS), also known as the Interocclusal Rest Space, is a foundational concept in prosthodontics. It is defined as the vertical distance between the occlusal surfaces of the maxillary and mandibular teeth when the mandible is in its physiological rest position.

• Physiological Basis: The rest position is a postural state maintained by the innate tonicity of the mandibular elevators and depressors muscles, independent of tooth contact. It is a static, involuntary position. As described by foundational texts, this space is a reflection of the innate neuromuscular balance of the stomatognathic system at rest.
• Clinical Measurement: The classic teaching, as cited by authorities like Zarb, Bolender, and Carlsson in “Boucher’s Prosthodontic Treatment for Edentulous Patients”, establishes that the average freeway space is between 2 to 4 mm measured in the first premolar region. This measurement is not directly taken but is the result of calculating the difference between the rest vertical dimension (RVD) and the occlusal vertical dimension (OVD).
• Clinical Significance: The FWS is the “safety margin” of the masticatory system. It allows the oral tissues to rest and permits a slight separation of the teeth during swallowing. A common clinical error is over-closing the VDO, which leads to a diminished or non-existent freeway space. This can cause constant clenching, muscle hyperactivity, and pain. Conversely, opening the VDO too much results in an excessive freeway space, forcing the patient to close too far to achieve occlusal contact, which can compromise denture stability and lead to inefficient mastication.

2. Closest Speaking Space (CSS)

While the Freeway Space represents a state of rest, the Closest Speaking Space represents a state of function. First described in detail by Silverman, the CSS is the closest consistent relationship of the mandible to the maxilla during rapid speech.

• Physiological Basis: The CSS is a dynamic, functional position. It is established when the mandible and the associated muscles of speech are in motion. Specific sibilant sounds, such as “s” (as in “sixty”) or “j” (as in “judge”), require the teeth to be very close together to allow for the precise escape of air. This creates a reproducible, measurable vertical dimension.
• Clinical Measurement: During the pronunciation of these sounds, the space between the upper and lower anterior teeth (or the ridges in edentulous patients) is observed. This space is typically smaller than the freeway space, averaging 1 to 2 mm. This provides a real-time, functional check of the established OVD.
• Clinical Significance: The CSS is an invaluable phonetic verification tool. After setting a tentative VDO (e.g., with wax rims on a denture trial base), the clinician asks the patient to repeat words containing “s” sounds. If the teeth contact or click during speech, the VDO is likely too small, leaving insufficient space for the tongue’s function. If the space is excessively large, producing a lisp, the VDO has been opened too much. The CSS acts as a immediate and patient-specific biofeedback mechanism.

Comparative Analysis: A Clinical Synthesis

The table below synthesizes the key differences:

Feature	Freeway Space (FWS)	Closest Speaking Space (CSS)
Described by	Thompson & Miswonger	Silverman
State	Static (Rest Position)	Dynamic (Functional Speech)
Definition	Vertical space at physiological rest.	Vertical space during pronunciation of sibilant sounds (“s”, “j”).
Average Value	2-4 mm	1-2 mm
Primary Use	To establish and verify the Occlusal Vertical Dimension (OVD).	To functionally verify the OVD via speech.

The Interrelationship in Clinical Practice

These two spaces are not independent; they are part of a continuum. A correctly established OVD will result in both an adequate FWS (~3mm) and a clear CSS (~1.5mm). When the VDO is altered, both spaces are affected predictably:

• Reduced VDO: Both the FWS and the CSS will increase. The patient will have an excessive freeway space at rest and their teeth will click together during speech due to an insufficient CSS.
• Excessive VDO: Both the FWS and the CSS will decrease or be eliminated. The patient will feel “overclosed” at rest and may develop a lisp because the teeth are too far apart to articulate “s” sounds properly.

Conclusion

For the modern dentist, the Freeway Space and Closest Speaking Space are not relics of dental curriculum but are vital, functional guides. The FWS, derived from the rest position, provides the foundational framework for setting the VDO. The CSS, derived from speech, provides a dynamic, real-world test of that framework. By mastering the application of both concepts—using the FWS to build the correct vertical height and the CSS to refine it—clinicians can move beyond mechanical reconstruction to a truly physiological and patient-centered restoration. This ensures not only the structural longevity of the prosthesis but also the comfort, function, and long-term health of the patient’s entire masticatory system.

References:

1. Zarb, G. A., Bolender, C. L., & Carlsson, G. E. (2013). Boucher’s Prosthodontic Treatment for Edentulous Patients (13th ed.). Elsevier Mosby.
2. Sharry, J. J. (1974). Complete Denture Prosthodontics (3rd ed.). McGraw-Hill. (For foundational concepts from Thompson & Miswonger).
3. Silverman, M. M. (1963). The Closest Speaking Space in Denture Prosthetics. The Journal of Prosthetic Dentistry, 13(4), 625-630. (Seminal original work).
4. Okeson, J. P. (2019). Management of Temporomandibular Disorders and Occlusion (8th ed.). Elsevier. (For the neuromuscular perspective on rest position).