To the authors' knowledge and until the advent of CBCT as an analysis, the condylar volume and surface have not been routinely calculated, although a recent study in rats shows enlargement of the left mandibular condyle compared to the rights due to the animal closing to one side, [12
] thus suggesting that volumetric data can be useful in understanding the TMJ function.
Using semiquantitative reverse-transcription polymerase chain reaction, a study examined the expression of insulin-like growth factor-1, fibroblast growth factor-2, and their receptors in the mandibular condylar cartilage of 28 day-old rats at 3, 7, and 14 days after placing intraoral appliances designed to produce a lateral functional shift of the mandible [13
]. This shift resulted in a transverse rotation of the mandible so that the condyle on the side away from the shift was distracted anteriorly (ie, protruded) from the glenoid fossa, while the contralateral condyle remained in place or moved slightly posteriorly (ie, nonprotruded). Gene expression for genes resulted significantly different between the protruded and the nonprotruded condylar sides. More specifically, mRNA expression on the protruded side compared with age-matched controls was altered in the opposite direction from the nonprotruded side, generating alterations in proliferative activity of cartilage.
According to the view that growth of the mandibular condylar cartilage adapts to its local functional-biomechanical environment, cartilage thickness was also investigated, [14
] and it resulted on the protruded side significantly greater than that in the controls at each time interval, with the difference increasing slightly with time. Trends on the nonprotruded side were generally opposite, culminating in reduced thickness and proliferation after 14 days, suggesting that changes in mandibular condylar cartilage thickness and proliferative activity might accompany a lateral functional shift of the mandible also in growing subjects, so generating a different condylar volume.
A certain role of the trigeminal motor nucleus in the postnatal ontogeny of the mammalian craniofacial skeleton was also demonstrated in a study where a group of 42 adult rats underwent stereotaxic surgery: [15
] among them, 21 received electrolytic lesions to their left-side nucleus (lesioned group) while 21 received a stimulation with no actual electrolytic lesion (sham group). The masticatory muscle weight, and osteological growth were compared: lesioned animals showed significant differences from sham animals, but, interestingly, sham animals also demonstrated significant between-side differences for medial pterygoid muscle weight, mandibular height and length data, suggesting that even relatively slight damage to trigeminal motor nucleus can create morphological changes within the craniofacial complex, mostly with the mandible.
Studies on the condylar development [16
] stated that the mandibular condyle is convex along the surface that receives the force, wider in the medio-lateral dimension, and has an oval shape antero-posteriorly. This observation and others, such as the relation of the articular disc with the condyle and the temporal bone, muscle attachments, and occlusion, requires a 3-D approach to analyze the TMJ complex [16
]. Matrix and cell surface proteoglycans (PGs) may play important roles in the control of cellular actions of heparan-binding growth factors such as fibroblast growth factor (FGF) during chondrogenesis and osteogenesis, as the inhibition of PG synthesis resulted in reduced incorporation of chondroitin sulfate into cartilage and bone matrix, that was associated with a 75% reduction in cell growth in condyle (equivalent to a reduced volume), determined by DNA synthesis, and in collagenous matrix synthesis in condyle, evaluated by immunohistochemistry. Morphological and quantitative autoradiographic analyses also showed that inhibition of PG synthesis blocks bone matrix formation by perichondral progenitor cells in condyles.
The purpose of our study has not been to test the accuracy of linear measurements calculated with CBCT scans, but to utilize the proven accuracy [17
] for measuring condylar volume and surface in a selected Caucasian young adult population with malocclusion and no pain or dysfunction of TMJ, to improve the knowledge of condylar shape in subjects with malocclusion. Although condylar volume and condylar surface are not commonly and presently evaluated in clinics, 3D diagnostic tools should become more prevalent in the future.
The study on method errors indicates that CBCT allows for accurate volumetric and surface evaluation of mandibular condyle. No significant difference was found between the two measurements made by the same operator.
In this study, the data of only young adult subjects were included. Older subjects were excluded because they are expected to suffer more frequently from progressive degenerative bone changes owing to the development of TMJ osteoarthritis than do the younger patients [3
Furthermore, Schluetera et al. (2008) [19
] developed a reliable and reproducible method for cutting the condylar area and calculating its volume. However, they used different software and cut the condyle at the base of its neck, while the authors believe that the data on the isolated condylar head can be better associated with the growing mandibular process.
The mean difference between the means of condylar volume in males and females is 21.61 mm3, which is about the 3.3% of the mean condylar volume in the whole sample. This difference was statistically significant (Table ).
The difference between the means of condylar surface in males and females is 11.25 mm2, which is about 2.8% the condylar surface of the whole sample), but not statistically significant (Table ).
These differences expressed as percentages respect to the means found in the whole sample, are in accordance with those of a recent study that investigated the female-to-male proportions in head and facial linear dimensions, and found a mean difference of 3-5% in the frontal and lateral views in young and adult patients, between males and females [20
The differences in volume or surface suggest high variability among the subjects. But, it is not clear the clinical relevance, as no subject included in this report had any pain or dysfunction of TMJ, but only malocclusions.
For the variable MI that indicates the ratio between volume and surface, the difference between females and males is 0.018, which corresponds to about 1% of the mean MI in the whole sample. There is not a statistically significant difference between males and females in the MI values (Table ).
These data confirm that sexual dimorphism concerns mostly with condylar volume, rather than with condylar surface or the Morphological Index, almost in subjects with malocclusions.
Difference between the right and left sides
The mean difference between the means of condylar volume in the right and the left TMJ is 25.39 mm3, which is about the 3.9% of the mean condylar volume in the whole sample (p < 0.01)
The difference between the means of condylar surface in the right and the left TMJ is 17.15 mm2, which is about 5.45% the condylar surface of the whole sample (p < 0.01).
The clinical relevance of this high variability in the mean differences is yet to be understood, considering that no subject included in the sample had any pain or dysfunction of TMD, but only malocclusions. Consequently, the observed anatomical differences between the two condyles cannot be related to the subject's predisposition to develop any specific pathology on one side, rather than on the other. However, these types of associations are better evaluated through longitudinal studies.
In conclusion, it is hard to confirm whether the dimensions measured on the images correspond to the real dimensions of the structures, because the data obtained here cannot be compared with the anatomical truth. Nevertheless, it is attempted to interpret these data on the basis of the asymmetry that is normal to all human-body structures.
A comparison between the results of this study and those of a recent study (based on 30 subjects with class I malocclusion [aged 15-30 years]), reveals differences, though not statistically significant, in the size of the mandibular condyle between the right and left sides: [21
] mean of medio-lateral condylar diameter is higher on the right side than that on the left side. However, the contrary is true for the antero-posterior condylar diameter.
Besides, other variables concerning TMJ morphology also show significant difference between the two sides. The depths of the mandibular fossae and the mean anterior and posterior joint spaces are higher for the right TMJs than those of the left TMJs; however, the contrary is true for the mean superior joint space. As regards the posterior joint space, the difference between the two sides is significant. These observations corroborate the present finding of a physiologically significant asymmetry of condylar size, in almost all subjects with malocclusion.
Also, in a study conducted on eight cadaveric heads (16 dissected mandibular condyles) to investigate the position of auriculo-temporal nerve in relation to the mandibular condyle, the nerve was observed to have a closed anatomic relationship with the condyle; however, although it was in direct contact with the condylar neck in all the cases, there was no correlation between the positions of the nerves in the right and left sides, suggesting marked asymmetry in the right and left condylar anatomy [22
The asymmetry observed in the present study between the right and left sides can also be related to the presence of a preference side for mastication in subjects with malocclusion.
According to literature, the most significant morphologic alterations and positioning asymmetries of TMJ structures are related to the absence of teeth, dental abrasion, premature occlusal contact points, functional mandibular deviations, unilateral posterior crossbites, and dentoskeletal asymmetries. Specifically, the rule of articular cartilage - a growth center - has been demonstrated to respond to the degenerative changes and nonphysiological strain in the joint areas (application of soft diet or extractions), through changes in the thicknesses of single cartilage layers and total layer thickness, causing a change in the vertical dimensions and width, which is manifested by changes in the maturation processes of centrally unloaded cartilage sections in rats [23
]. With regard to the condylar volume, an earlier study demonstrated that hyperplasia of mandibular condyle is histologically characterized by the presence of an uninterrupted layer of undifferentiated germinative mesenchymal cells, a layer of hypertrophic cartilage, and the presence of islands of chondrocytes in the subchondral trabecular bone [24
]. The present study suggests that about 4-6% of difference in volume and surface can be observed between the two sides of subjects with malocclusion and without pain or dysfunction in TMJs, along with a wide range of percentages in physiological asymmetry. It has also been shown that the shape and linear dimensions of the mandibular condyle are highly variable [25
Thus, in the light of the wide range of percentages of asymmetry between the right and left sides, these clinical variables do not seem to be accurate or clinically useful to determine the existence of an abnormal condition in a subject with malocclusion and without pain of dysfunction in TMJs.
Instead, MI could be a better indicator, because no statistically significant difference is found in the MI values between the right and the left sides, or between males and females, in young adult Caucasian subjects with malocclusion and without pain or dysfunction in TMJs.
In young adult Caucasian subjects with malocclusion and without pain or dysfunction in TMJs, its range of values seem to be (both in males and females; both in the right and left sides) 1.72 ± 0.17 mm, with a total range from 1.15 to 2.15 mm.
One can interpret the data on volume and surface of condyle from a clinical point of view. But, before doing so the clinician must know that (i) the functional loads applied to the TMJ might influence TMJ's morphology, (ii) the shape and function are intimately related, although this concept is given due importance only in studies on class II and class III skeletal patterns, [26
] and (iii) both the volume and the surface of a condyle differ between the right and left sides, in subjects with malocclusion and without pain or dysfunction in TMJs.
With the advent of 3-D CBCT scan, the clinician can request the radiologist to directly evaluate or calculate condylar volume and surface, as also the MI, using dedicated software.
If the standards of MI are established, then the clinician can use them to directly establish the presence or absence of an abnormal shape.
It would also be useful to evaluate the abnormality in the TMJ morphology of a specific patient, based mostly on the fact that no difference exists in the MI score between males and females or between right and left TMJs of almost all Caucasian young adult subjects, suggesting that it must be between 1.15 and 2.15 in subjects with malocclusion and TMJ without pain or dysfunction.
Limits of the study
Numerous factors should be considered in applying the results of this investigation to clinical situations. The 3-D volumetric depiction depends on the appropriateness of segmentation, the thresholding of bone pixel values, and the accurate suppression of the surrounding tissue values to enhance the structure of interest. The depiction is dependent on the software algorithm, the spatial and contrast resolution of the scan, the thickness and degree of calcification or cortication of bone structure, and the technical skill of the operator. The Mimics software used in this study enables semi-manual segmentation by interaction of the operator with the data to produce a visually acceptable 3-D rendering. According to Periago (2008), [27
] these limitations cause deficiencies or voids in the surface of the image, which occur in regions represented by few voxels or have gray values still representing the bone, but outside the threshold. These areas include the cortical bone of the mandibular condyle, and thus may lead to greater identification error (e.g., for condylar contours) and consequently to measurement error. However, no significant difference is found between the intra-observer method errors, thus suggesting that an accurate procedure of segmentation can occurr.