Students frequently ask the question, “Why put all those grooves on the occlusal surface? Why not put simple inclined planes on a flat table?” To answer the question, it is necessary first to analyze the features of the occlusal surface of the tooth. This surface is comprised of positive and negative features: the cusp tips and ridges make up the positive features, while the grooves and fossae are the negative. The ridges, when properly formed, are convex in all directions, and contacts with opposing teeth are numerous (Fig 1).
Fig 1 There are numerous occlusal contacts (arrows) on an occlusal surface that is correctly waxed. The lingual surface of the mandibular first molar has been cut away to show the occlusion with the maxillary first molar.
The convex surfaces of ridges produce a distributed contact pattern with small contacts. In this way, occlusal forces are distributed over a wide area of the occlusal surface, minimizing stress and wear. Contact areas that are small and bordered by marked depressions (ie, grooves) allow for complete and rapid disclusion in excursive movements (Fig 2).
Fig 2 Properly formed occlusal morphology allows for small centric contacts (A) and complete disclusion in excursions (B).
On the other hand, occluding surfaces that are flat are more likely to produce large contact areas (Fig 3). Broad, flat contacting areas are more likely to remain in apposition without disclusion when mandibular excursions occur. The friction resulting from teeth continuing to touch during excursions will often cause greater wear.
Fig 3 Occlusal morphology made with only inclined planes produces large contacts in centric occlusion (A) with a likelihood that there will be continued contact in excursions (B).
Because of the reduced contact surface area resulting from an occlusal surface comprised of convex ridges bordered by grooves, the restoration is more efficient. Ridge against ridge produces a shearing action that is more effective than the pounding or crushing action produced by flat surface against flat surface. The difference is real: patients who have restorations with flat surfaces replaced by those with ridges and grooves will often comment on the reduced effort required for chewing food.
The most important features on an occlusal surface are the ridges. The seams along which the ridges meet form the grooves and the fossae of the occlusal surface. Many novice waxers attempt to create occlusal morphology by cutting V-shaped furrows into flat inclined planes on the occlusal surface of a wax pattern (Fig 4). The results are rarely, if ever, satisfactory.
Fig 4 Well-formed ridges and grooves will be more convex and functional (A) than will the occlusal surface in which grooves have been carved into a flat plane (B).
The triangular ridge is the major ridge or lobe of every cusp. It extends from the cusp tip to the central groove (Fig 5A). It is essentially triangular: narrow at the cusp tip and wide at its base in the central groove. The grooves to the mesial and distal of it converge toward the cusp tip (Fig 5B).6
Fig 5 The triangular ridge is triangular (A) and is bounded by converging grooves (B) (after Burch6).
When viewed in either a faciolingual section or a mesiodistal section, the triangular ridge is convex (Fig 6). If the triangular ridges are placed correctly, the correct groove pattern will occur as a natural by-product.
Fig 6 The triangular ridge is convex in faciolingual section (A) or in mesiodistal section (B) (after Burch6).
The grooves themselves perform an important function in occlusal restoration. If they are adequate in width and depth, and if they transverse the occlusal surface in the proper directions, they will permit the unimpeded passage of opposing cusps in excursive movements. The grooves in a wax pattern are refined by moving the articulated casts through the various excursions in order to detect any interferences which should be removed from the walls of the grooves (ie, the sides of the ridges.
The Effect of Anterior Guidance
The incisors and canines are the most anterior teeth in the dental arches. They comprise the anterior determinant of mandibular movement and help to guide the mandible in its excursions forward (protrusive movements) and to the left and right (lateral movements). Dentists have direct control over this determinant through restoration, equilibration, or orthodontic movement. The other determinants of occlusion, the temporomandibular joints and the neuromuscular system, are not to be discounted in their effect on mandibular movement; however, they are not under the direct control of the dentist.
The contact of the anterior teeth in excursions affects the movements of the mandible. This in turn affects cusp placement and height, and groove direction and depth, which will be tolerable in the posterior restoration. Restoration of anterior teeth must be undertaken carefully, since changes in the morphological features of these teeth can change the anterior guidance, with possible deleterious effect on the posterior occlusion.
The greater the vertical overlap of the anterior teeth, the longer the posterior cusps may be (Fig 7). Conversely, the less the vertical overlap, the shorter the cusps of posterior teeth must be. The greater the horizontal overlap of the anterior teeth, the shorter the cusps of posterior teeth must be (Fig 8). The cusps may be longer when the horizontal overlap is small.
Fig 7 An increased vertical overlap of anterior teeth allows longer posterior cusp length (A). Decreased vertical overlap requires a decrease in posterior cusp length (B).
Fig 8 An increased horizontal overlap of anterior teeth requires a decrease in posterior cusp length (A). Decreased horizontal overlap allows longer posterior cusp length (B).