The occlusal plane represents the imaginary surface where the chewing surfaces of the upper and lower teeth meet. This plane dictates how biting forces are distributed across the dental arches. Unlike a flat surface, the human occlusal plane possesses a distinct, natural curvature fundamental to jaw mechanics. This three-dimensional shape ensures efficient chewing and proper alignment.
Identifying the Arc: The Curve of Spee
The specific curvature observed when viewing the dental arches from the side is called the Curve of Spee. This arc runs in an anterior-posterior (front-to-back) direction along the line of occlusion. It is a defining feature of the human jaw structure, influencing how the teeth engage during forward and backward movements.
The Curve of Spee begins at the tip of the lower canine tooth. It follows the buccal (cheek-side) cusp tips of the premolars and molars toward the back of the mouth. This upward-sweeping curve can be visualized as part of the circumference of a large circle.
The arc is typically measured by its radius, which is commonly cited as being around 4 inches, or approximately 10 centimeters. The concept was first described in 1890 by German anatomist Ferdinand Graf Spee, who recognized this consistent pattern in human dentition. Reproducing this specific radius is important in modern dentistry for creating balanced restorations.
This forward-sweeping curve is a functional requirement for mandibular movement. It helps guide the lower jaw smoothly as it slides forward.
The Three-Dimensional Nature of Occlusion
While the Curve of Spee accounts for the front-to-back arc, the occlusal plane is further defined by a secondary curvature. This complementary arc is known as the Curve of Wilson, which addresses the medial-lateral (side-to-side) relationship of the teeth. It describes the inward inclination of the molar and premolar teeth across the dental arch.
The Curve of Wilson ensures that the chewing surfaces of the teeth are aligned relative to the tongue and cheek muscles. When viewed from the front, the lower posterior teeth display a slight concave (cupping-in) curve, while the upper posterior teeth show a slight convex (bulging-out) curve. This side-to-side orientation helps to centralize the forces of mastication over the roots of the teeth.
The combination of the anterior-posterior Curve of Spee and the medial-lateral Curve of Wilson creates a complex, three-dimensional geometry of the occlusal surface. These two curves work together to establish a stable and functional biting relationship.
This combined curvature led to the spherical theory of occlusion, often associated with the work of George Monson. This theory suggests that the cusps and incisal edges of all teeth ideally lie on the surface of a sphere. This hypothetical sphere encapsulates the entire occlusal arrangement, illustrating the interconnectedness of all chewing surfaces. Understanding this precise three-dimensional relationship directly impacts the stability of the jaw joint and the ability to chew food effectively.
Importance in Dental Function and Restoration
The precise geometry of the Curves of Spee and Wilson generates “occlusal clearance.” This clearance is the small gap that forms between the opposing upper and lower teeth during non-centric jaw movements. Without this space, the teeth would collide and interfere during chewing.
During lateral movements, the Curve of Wilson helps disengage the teeth on the non-working side of the mouth. Similarly, the Curve of Spee helps guide the teeth during protrusive movements (sliding the jaw forward) to prevent the back molars from striking too hard. This protective mechanism prevents undue wear and potential fracture of the chewing surfaces.
Accurately recreating these natural curves is fundamental in restorative and prosthetic dentistry. When a dentist fabricates a crown, bridge, or complete denture, the curve must be incorporated to ensure the restoration functions harmoniously. Failure to reproduce the correct arc can lead to instability, premature wear, or discomfort.
In orthodontics, correcting malocclusion often involves adjusting the depth of the Curve of Spee to achieve a flatter, more stable occlusal plane. A deep or exaggerated curve can place excessive strain on the temporomandibular joints and chewing muscles. Maintaining the proper arc is therefore directly linked to the long-term health and stability of the entire masticatory system.