Fundamentals of Fixed Prosthodontics. James C. Kessler
2-16 A shallow protrusive condylar inclination requires short cusps (a), while a steeper path permits the cusps to be longer (b).
Effects of Anatomical Determinants
The anatomical determinants of mandibular movement (ie, condylar and anterior guidance) have a strong influence on the occlusal surface morphology of the teeth being restored. There is a relationship between the numerous factors, such as immediate lateral translation, condylar inclination, and even disc flexibility, on the cusp height, cusp location, and groove direction that are acceptable in the restoration. It is beyond the scope of this text to discuss all of the nearly 50 rules that have been written on the subject of determinants52; therefore, only those that have the greatest effect on morphology are considered.
Molar disocclusion
When subjects with normal occlusions perform repeated lateral mandibular movements, they will not trace the same path on electronic recordings, presumably because of the flexible nature of the articular disc. The measured deviation averages 0.2 mm in centric relation, 0.3 mm in working movements, and 0.8 mm in both protrusive and nonworking movements.53 To avoid occlusal interferences and nonaxially directed forces on molars during eccentric mandibular movements, molar disocclusion must equal or surpass these observed deviations in mandibular movement.
Healthy natural occlusions exhibit clearances that will accommodate these aberrations. Measurements of disocclusions from the mesiofacial cusp tips of mandibular first molars in asymptomatic test subjects with good occlusions showed separations averaging 0.5 mm in working, 1.0 mm in nonworking, and 1.1 mm in protrusive movements.54 Therefore, one of the treatment goals in placing occlusal restorations should be to produce a posterior occlusion with buffer space that equals or surpasses the deviations resulting from natural variations found in the TMJ.
Condylar guidance
Chief among those aspects of condylar guidance that will have an impact on the occlusal surface of posterior teeth are the protrusive condylar path inclination and mandibular lateral translation.
The inclination of the condylar path during protrusive movement can vary from steep to shallow in different patients. It forms an average angle of 30.4 degrees with the horizontal reference plane (43 mm above the maxillary central incisor edge).17,18 If the protrusive inclination is steep, the cusp height may be longer. However, if the inclination is shallow, the cusp height must be shorter (Fig 2-16).
Immediate mandibular lateral translation is the lateral shift during initial lateral movement. If immediate lateral translation is great, then the cusp height must be shorter or the fossa width wider (Fig 2-17). With minimal immediate translation, the cusp height may be made longer or the fossa may be narrower.
Ridge and groove directions are affected by the condylar path, particularly the lateral translation. The effects are observed on the occlusal surface of a mandibular molar and premolar with the paths traced by the palatal cusps of the respective opposing maxillary teeth. The working path is traced on the mandibular tooth in a lingual direction, and the nonworking path is in a distofacial direction. The nearer the tooth is to the working-side condyle anteroposteriorly, the smaller the angle between the working and nonworking paths (Fig 2-18). The farther the tooth is located from the working-side condyle, the greater the angle between the working and nonworking condyles. When immediate lateral translation is increased, the angle also becomes more oblique.
Fig 2-17 A pronounced immediate lateral translation requires that the cusps be short or the fossa wider (a), while a gradual lateral translation allows the cusps to be longer or the fossa narrower (b).
Fig 2-18 The angle between the working (W) and nonworking (NW) paths is greater on teeth located farther from the condyle.
Anterior guidance
During protrusive movement of the mandible, the incisal edges of mandibular anterior teeth move forward and downward along the palatal concavities of the maxillary anterior teeth. The track of the incisal edges from maximal intercuspation to edge-to-edge occlusion is termed the protrusive incisal path. The angle formed by the protrusive incisal path and the horizontal reference plane is the protrusive incisal path inclination, which ranges from 50 to 70 degrees.55,56 Although they are conventionally regarded as independent factors, there is evidence to suggest that condylar inclination and anterior guidance are linked, or dependent factors57,58 (Takayama H and Hobo S, unpublished data, 1989). In a healthy occlusion, the anterior guidance is approximately 5 to 10 degrees steeper than the condylar path in the sagittal plane. Therefore, when the mandible moves protrusively, the anterior teeth guide the mandible downward to create disocclusion, or separation, between the maxillary and mandibular posterior teeth. The same phenomenon should occur during lateral mandibular excursions.
Fig 2-19 (a) A pronounced vertical overlap of the anterior teeth permits posterior teeth to have longer cusps. (b) A minimum anterior vertical overlap requires shorter cusps.
Fig 2-20 (a) A pronounced horizontal overlap of the anterior teeth requires short cusps on the posterior teeth. (b) A minimum anterior horizontal overlap permits the posterior cusps to be longer
The palatal surface of a maxillary anterior tooth has both a concave aspect and a convexity, or cingulum. The mandibular incisal edges should contact the maxillary palatal surfaces at the transition from the concavity to the convexity in the centric relation position. The concavity represents a uniform shape in all subjects.59
Anterior guidance, which is linked to the combination of vertical and horizontal overlap of the anterior teeth, can affect occlusal surface morphology of the posterior teeth. The greater the vertical overlap of the anterior teeth, the longer the posterior cusp height may be. When the vertical overlap is less, the posterior cusp height must be shorter (Fig 2-19). The greater the horizontal overlap of the anterior teeth, the shorter the cusp height must be. With a decreased horizontal overlap, the posterior cusp height may be longer (Fig 2-20).
Fig 2-21 While a shallow protrusive path would require short cusps in the presence of minimal anterior guidance (a), the posterior cusps can be lengthened if the anterior guidance is increased (b).
Fig 2-22 (a) A pronounced immediate lateral translation would dictate short cusps where there is little anterior guidance. (b) However, the cusps can be lengthened if the anterior guidance is increased.
By increasing anterior guidance to compensate for inadequate condylar guidance, it is possible to increase the cusp height. If the protrusive condylar inclination is shallow, requiring short posterior cusps, the cusps may be lengthened by making the anterior guidance steeper (Fig