The occlusal plane is an imaginary surface defined by the average alignment of the biting surfaces of the upper and lower teeth. This surface is not a flat, geometric plane but rather a conceptual representation of how the teeth meet. Establishing the correct orientation of this plane is a primary concern in dentistry, particularly for orthodontics, restorative work, and the fabrication of prosthetics like dentures. Its alignment is directly related to the stability of the bite, the efficiency of chewing, and the long-term health of the jaw joints.
Understanding the Geometry of the Occlusal Plane
The biting surfaces of the teeth are arranged in a series of curves, ensuring the jaw moves smoothly during function. The anteroposterior curvature, running from the front of the mouth toward the back, is known as the Curve of Spee.
The Curve of Spee follows the tips of the canine and posterior teeth, creating an upward curve in the lower jaw. This curvature allows for the disocclusion, or separation, of the posterior teeth when the jaw moves forward, preventing harmful interference.
The mediolateral curvature, visible when viewing the dental arch from the front, is called the Curve of Wilson. This curve is created because the cheek-side (buccal) cusps of the posterior teeth are typically positioned higher than the tongue-side (lingual) cusps in the lower jaw. The Curve of Wilson permits the sideways, or lateral, movements of the mandible without the teeth locking together.
Critical Role in Oral Function
An aligned plane ensures that the forces generated during mastication are directed axially, or straight down the long axis of each tooth. Directing forces vertically minimizes lateral stresses on the periodontal ligaments and the underlying jawbone.
The plane’s inclination is linked to the path the jaw follows during chewing. A correctly positioned plane facilitates a rhythmic and balanced chewing pattern, which contributes to the longevity of the teeth and supporting structures.
A properly established occlusal plane is required for the harmonized movement of the tongue and the cheek muscles. This balance helps keep food positioned effectively between the teeth and prevents the tongue or cheek tissues from being bitten. The correct plane also supports clear speech articulation and provides an aesthetic relationship with the lips when smiling.
Clinical Methods for Determining the Correct Plane
When natural teeth are missing, the dentist must re-establish the lost occlusal plane using external anatomical guides. This process relies on reference points that relate the jaw’s position to the skull.
One widely used guide for the frontal, horizontal orientation of the plane is the interpupillary line, an imaginary line connecting the centers of the two pupils. The plane is made parallel to this line to ensure the teeth appear level when the patient is viewed from the front.
For the sagittal, or front-to-back, orientation, clinicians use the ala-tragus line, which is an external measure that approximates the natural plane. This line, often referred to as Camper’s plane, runs from the inferior border of the ala (the wing of the nose) to a specific point on the tragus of the ear. The line serves as a reproducible reference to determine the posterior tilt of the plane.
These external measurements are transferred from the patient to a mechanical device called an articulator using a face bow instrument. The face bow records the relationship between the patient’s jaw and the hinge axis of the jaw joints, allowing the dental laboratory to create a model that mimics the patient’s actual jaw movements.
Impact of Alignment Discrepancies
When the occlusal plane is incorrectly angled or misaligned, a discrepancy changes the direction of the forces transmitted to the teeth and jaw, leading to a condition known as malocclusion. This improper force transmission can result in excessive wear on specific teeth surfaces, sometimes causing chipping or fracturing over time.
An incorrect inclination can alter the stress distribution within the Temporomandibular Joint (TMJ). For instance, a clockwise rotation of the plane, where the back of the arch is angled down, can increase compressive stress on the condyle, the rounded end of the jawbone that articulates with the skull. This altered loading pattern is a contributing factor to the development of painful temporomandibular disorders.
In patients with dentures, a faulty occlusal plane can compromise the stability of the prosthesis. If the plane is not correctly positioned, the tongue and cheek muscles may work against the denture base, causing it to rock or become dislodged during eating or speaking. This instability can lead to irritation of the gum tissue.