Comparison of Condylar Position in Hyperdivergent and Hypodivergent Facial Skeletal Types

Orthodontists have long been interested in the multitude of differences in the diagnosis, treatment, and treatment response between hyperdivergent or dolichofacial facial types and hypodivergent or brachyfacial facial types. Moreover, in recent years, an increased number of orthodontists desire to achieve a treatment result wherein the upward and forward or a seated position of the condyle is maintained when the patient closes the mandible to the intercuspal position (ICP). Most orthodontists who look carefully at this joint-to-dentition relationship would agree that the larger the discrepancy between the seated condyle and tooth intercuspation pretreatment, the more difficult it is to achieve the desired result posttreatment. For the clinicians with this treatment perspective, it would be beneficial to know if hyperdivergent cases might present a greater condylar distraction. If this were so, hyperdivergent patients would present a greater problem in achieving a seated condylar position.

In many cases, it is possible to demonstrate that a small change in condylar position can have a profound effect at the level of the dentition (Figure 1). Due to the geometry of the skeletal pattern in general and the mandible in particular, this phenomenon may be more pronounced in the hyperdivergent skeletal type than the hypodivergent skeletal type.

There is substantial evidence indicating that, when dental influences are eliminated, a healthy elevating musculature will position the condyle in its most upward and forward position against the eminence.1–11 This seated condylar position has been referred to as centric relation. Centric relation has become a desirable goal for a growing number of orthodontists, primarily due to the influence of Roth.12,13 Moreover, there is agreement among numerous authors, clinicians, and teachers, recognized for their knowledge of gnathic function, that this seated condylar position is a desirable physiologic goal for the orthodontist.14–18 Okeson19 defined and described the importance of the musculoskeletally stable position of the temporomandibular joints. That definition is congruent with the seated condylar position identified above. Subsequently, Okeson19 said, “I believe the goal of every orthodontist should be to finalize the occlusion in harmony with the musculoskeletally stable position of the temporomandibular joints.” McNeil20 described what would be his treatment goal for the joints and dentition at the termination of orthodontic treatment as, “The condyles should be seated in the fossae, ideally in the anterior, superior position against the articulare disc. A mutually protected occlusion is ideal and desirable if possible.”

The purpose of this study was to determine if the centric relation to intercuspal position discrepancy was larger in patients with hyperdivergent facial type compared with patients with hypodivergent facial type.

It was hypothesized that a group of adolescent and young adult subjects with hyperdivergent facial-skeletal patterns would demonstrate greater condylar displacement from the upward and forward position to the intercuspal position than a matched group of subjects with hypodivergent facial-skeletal patterns.

A computer search of the literature reveals no published articles specifically studying the interrelationship between facial type and condylar position. Burke et al21 investigated a possible correlation between condylar characteristics and facial morphology in Class II adolescents. They used tomograms to measure joint spaces and found patients with vertical facial morphology (hyperdivergent) had decreased superior joint spaces while patients with horizontal facial morphology (hypodivergent) had increased superior joint spaces.

Stringert and Worms22 studied the relationship between skeletal pattern and internal derangement. They found a greater incidence of internal derangement in hyperdivergent skeletal patterns.

Subjects for the study were gathered randomly by a perusal of patient charts taken from an orthodontic practice treating adolescents and adults. Subjects were selected first by age and then by facial-skeletal characteristics, as measured cephalometrically, to generate 2 matched groups of 33 subjects each. Age was a criterion for subject selection since the intention was to study young adult subjects having completed or nearly completed growth. The final sample selected was composed of 10 males ranging in age from 16 to 30 years, with a mean age of 23.6 years, and 56 females ranging in age from 13 to 36 years, with a mean age of 24.2 years.

Facial-skeletal type was determined by using the Jarabak cephalometric analysis. Subjects were considered to be facially hyperdivergent if the posterior-anterior face height ratio (sella-gonion/nasion-menton) was 59% or less while the lower half of the gonial angle (nasion-gonion-menton) was 76 degrees or more. Subjects were considered to be hypodivergent if the posterior-anterior face height ratio was 65% or more while the lower gonial angle was 69 degrees or less (Figure 2A,B). For those not familiar with the Jarabak cephalometric analysis, the mean mandibular plane angle was 34 degrees for the hyperdivergent group and 16 degrees for the hypodivergent group.

In addition, subjects were excluded from the sample if there had been orthodontic treatment within the last 5 years, there were obvious signs of degenerative changes in the temporomandibular joints as determined by tomographic radiographs, or there was a history of significant trauma or surgery to the temporomandibular joints. It was felt these factors could significantly affect condylar length and/or the occlusion, which would in turn distort data gathered for the study.

The records utilized included study casts mounted on an articulator in centric relation or the seated condylar position, tomograms of the temporomandibular joints, and lateral cephalometric radiographs. Radiographs were taken in centric occlusion or the intercuspal position of the mandible.

Study casts were poured in die stone and the maxillary cast was mounted in the articulator using the arbitrary facebow. The mandibular cast was mounted to the maxillary cast using a modified Roth power centric bite registration record as follows: Delar blue wax (Great Lakes Orthodontics, Buffalo, NY) was softened in a water bath at 130°F. The wax wafer was cut to 2 thicknesses posteriorly and 4 thicknesses anteriorly (in some cases of anterior open bite, the anterior section was thicker). After initial indexing of the wax wafer via repeated, gentle guided closures of the lower teeth into the wax (to prevent protrusion), the thicker anterior section of the wax was thoroughly chilled and hardened in ice water. Then the 2-thickness posterior section was further softened in a flame. The wax bite was subsequently repositioned onto the maxillary teeth via the anterior tooth indexing, and the mandible was again gently guided to the previously determined mandibular incisor indices. The patient was then instructed to bite firmly against the hard anterior wax segment until the lower posterior teeth registered cusp-tip indices approximately 1 to 1.5 mm deep in the soft wax of the posterior segment. Finally, the entire wax registration was chilled in ice water, repositioned in the mouth, and the mandible guided to open and close into the wax as a last check and means of reducing distortion.

Articulator-mounted casts enabled measuring movement of the condylar axis from the up/forward condylar position (centric relation) to the intercuspal position (centric occlusion) of the mandible. The MPI (SAM I*) instrument was used to measure and quantify differences between the 2 condylar axis positions. This instrumentation has been shown to be both accurate and reliable.^11,23–26 Horizontal and vertical measurements were recorded on 1-mm graph paper to the nearest 0.25 mm. Each MPI recording was measured and remeasured at least 3 times. The average of the 2 of 3 closest measurements was recorded.

Data gathered from the MIP measurements were tabulated and organized to compare in the horizontal and vertical dimensions the magnitude and direction of condylar axis movement from centric relation to centric occlusion between the hyperdivergent and hypodivergent groups.

Figure 3 shows the total of horizontal (x axis) condylar displacements for the 2 groups. The hyperdivergent group totaled 79.75 mm, which was 1.8 times greater than the corresponding 43.75 mm measured for the hypodivergent group. Mean values were also nearly twice as great for the hyperdivergent group (Table 1).

Figure 4 shows data comparing the total vertical (z axis) condylar displacements for the 2 groups. Note the hyperdivergent group totaled 112.50 mm, 1.4 times greater than the corresponding 79.0 mm measured for the hypodivergent group. Mean values are shown in Table 1. A statistical t-test was performed for the total X (horizontal axis) values for the hyper- and hypodivergent groups and the total Z (vertical axis) values for the hyper- and hypodivergent groups.

The t-test values for both the Z (vertical) and X (horizontal) dimensions were significant at the .01 confidence interval (Table 1). Thus, these findings are, by definition, statistically significant, ie, one can be 99% certain that the differences in these numerical values will not occur naturally or that the differences are not due to coincidence.

Displacement of the condyles in the horizontal (X axis) plane can be subdivided into forward (+X) and backward (−X) components. Figure 5 compares the forward and backward condylar displacements for the 2 groups. The forward displacement of condyles in the hyperdivergent group totaled 37.25 mm, which was over twice the 17.5 mm of forward displacement totaled for the hypodivergent group. A similar comparison of the backward component of the horizontal displacement had the hyperdivergent group totaling 42.5 mm, which was 1.6 times greater than the 26.25 mm of backward displacement totaled for the hypodivergent group. Thus, condyles in the hyperdivergent group were, on average, deflected a greater distance, both anteriorly and posteriorly, compared with the hypodivergent group.

When observing the considerable difference in skeletal form between the hyperdivergent and hypodivergent facial patterns, one might suspect that the anatomical constraints of the hyperdivergent pattern would dictate condylar function further out of the fossa. Logic suggests that the unfavorable posterior/anterior face height ratio dictates greater condylar distraction (especially in the vertical dimension) in order to bring anterior teeth to functional contact.

The purpose of this study was to test the hypothesis that, on average, there is a greater degree of condylar distraction from centric relation to centric occlusion in hyperdivergent (dolicho) facial types than in hypodivergent (brachy) facial types. Movements of the condylar axis in the horizontal and vertical dimensions represented the amount and direction of the condylar displacement. The magnitude and direction of displacement for the hyper- and hypodivergent facial patterns was compiled and compared. The data clearly showed that the hyperdivergent subjects had greater displacement of the condyle in both the horizontal and vertical dimensions. Statistical significance was high at the .01 level.

Notable from the horizontal, or X-axis, data (Figure 5), the forward shift (+X) for the hyper group totaled 37.25 mm, or over twice the 17.5 mm recorded for the hypo group. Of all the dimensions totaled and compared, this one showed the greatest difference between the 2 groups. Thus, one might suspect the amount of forward condylar displacement for the hyperdivergent patient, on average, will be about twice that of the hypodivergent patient. For nearly every dimension recorded, the hyperdivergent group was from 1.4 to 1.8 times greater than the hypodivergent group.

When the 2 groups were compared concerning anterior or posterior deflection of the condyle, it was found that the greatest movement occurred to the posterior in the hyperdivergent group (Figure 5). This supports Roth's concept of a molar fulcrum and may be important since posterior displacement of the condyle away from the eminence would theoretically compromise joint stability and/or function.

It has been hypothesized that displacement of the condyle away from the eminence may be detrimental to joint health and/or stability since there is subsequent loss of juxtaposition between the condyle, disc, and eminence.27–29 In orthopedics, a border joint position is generally thought to be more stable than a nonborder position.30 The increased intra-articular space may predispose to internal derangement, either through mechanical posterior displacement of the condyle31–34 as demonstrated in monkeys by Isberg and Isacsson35 and/or hyperactivity of the superior head of the lateral pterygoid muscle.36 Some studies indicate the superior head of the lateral pterygoid muscle inserts into the articular disc;37–42 posterior condylar distraction in conjunction with activity of this muscle could hypothetically put the joint at risk for complication such as internal derangement. In cases where the condyle is distracted away from the eminence, the temporomandibular ligament may be elongated in order to accommodate this mandibular position.43,44 Hylander45 has shown that the horizontal fibers of the temporomandibular ligament insert at the lateral pole of the condyle along with the collateral ligament that secures the disc to the condyle. Tanaka46 has stated there is evidence that disc displacements are most frequently initiated at the lateral pole of the condyle.

Stringert and Worms22 found hyperdivergent facial types to have a greater frequency of internal derangement than hypodivergent facial types. They suggested this might be a consequence of degenerative changes within the temporomandibular joints or, as they said, “… for some reason, persons with hyperdivergent characteristics are more prone to internal derangements.” It should be noted that all subjects in this study had temporomandibular joint tomograms as part of their records, and a criteria for case selection was no obvious degenerative changes as seen on tomograms.

The comparative differences in condylar shift between the 2 facial types became even more conspicuous when measuring the number of condyles that were displaced to the extreme (2 mm or more). The differences were evident regardless of the direction in which the extreme measurements were compared (Table 2). For example, the number of hyperdivergent joints shifting 2 mm or more in the vertical plane (Z axis) numbered 19, almost 4 times greater than the 5 condyles that were recorded in the hypodivergent group.

Some cases in the hyperdivergent group presented with anterior open bite and minimal condylar distraction. It is valid to conclude such cases would have registered a greater condylar distraction had the mandible been closed over the molar teeth to incisal contact. In other words, the anterior open bite likely prevailed instead of significant condylar shift. Anterior open bite may be associated with compromised health and/or stability of the gnathic system.47,48 

For the orthodontist desiring to treat to the upward and forward or seated condylar position, this study is helpful because it shows most pretreatment patients will have a centric relation to centric occlusion discrepancy. The information gathered from mounted casts can have a profound affect on treatment planning.49 The data gleaned from this study is particularly valuable because it indicates the clinician can generally assume condylar distractions will be much greater in hyperdivergent facial patterns than in hypodivergent ones. Certainly each case must be evaluated separately, but the clinician is better prepared for diagnosis with this knowledge.

The amount of condylar movement from the upward and forward position (joint dominated) to the intercuspal position (tooth dominated) was measured and compared for 2 matched samples of subjects with hyperdivergent and hypodivergent facial types.

It was hypothesized that the hyperdivergent group would exhibit greater condylar displacement than the hypodivergent group. The findings of this study demonstrated significantly greater condylar distraction for the hyperdivergent group in all 3 dimensions.