This is the first study that has evaluated, by means of CBCT, mandibular condylar cortical bone thickness of adult patients, comparing 3 groups with different vertical skeletal growth pattern. Other studies evaluated cortical bone of different mandibular regions of patients with different vertical patterns.
Evidence showed that patients with different vertical skeletal growth pattern produce different forces during swallowing, chewing and maximum bite force [3,24–27].
Specifically, long-face patients showed reduced occlusal forces and short-face patients showed increased occlusal force during maximum bite. This evidence confirmed that muscular function and facial morphology are related to some extend.
Different clinical studies [16,17,28], performed with the aim to evaluate the relationship between different facial vertical skeletal growth types and thickness of mandibular cortical bone, demonstrate that vertical skeletal growth pattern can affect specific mandibular characteristics. These studies evaluated cortical bone thickness by mandibular cross-section scans obtained from CBCT exams in patients with different vertical facial dimensions. The results of these studies showed that long-face patients present thinner cortical mandibular bone compared to short face patient [16,17,28,29].
Experimental studies seem to indicate that the cortical bone characteristics of patients with different skeletal pattern could be related to the specific muscular function . Tsai and coworkers injected into the left masseter muscle of rats Botulinum neurotoxin type A (BTX-A) to reduce masticatory muscle function . Ninety days after the rats were sacrificed, and the authors noticed next to the botulin injection a significant decrease of cortical thickness, bone mineral content and ramus height. Moreover, it was noticed on the experimental side the increase of gonial angle and crown height of the posterior teeth. Tsai and coworkers reached the conclusion that muscular function is able to affect both the morphology of the overall mandible (gonial angle dimension, ramus height) as well as bone characteristics (cortical thickness and bone mineral content).
The results of our study seemed to show that patients with increased vertical skeletal growth pattern showed a significantly greater cortical condylar bone thickness compared to patients with normal and reduced vertical skeletal growth pattern. In order to explain this finding we analyzed forces geometries produced by the masticatory muscles in patients with different vertical growth patterns (Figure 6).
In figure 6 we represented with the red color the forces produced by the masseter muscle on mandibular skeleton and teeth. We know that masticatory forces are released on the occlusion and on temporomandibular joints, so we decomposed the masticatory forces of the masseter into two forces directed towards the anatomical structure (TMJ and teeth) on which masticatory forces are directed.
Analyzing the geometry of the vectors of masticatory force and its components, it is possible to observe that the masticatory forces of high angle patients are closer to the temporomandibular joint compared to the low angle patients, consequently the component of the force acting on the condyle of high angle patients is higher in module compared to the force module acting on the condyle of low angle patients (figure 6). Moreover, for the same geometric reasons the force acting on teeth of high angle patients are decreased compared to those acting on low angle patients.
For these geometric considerations, it is possible to hypothesize, that the load on the condyle tend to be higher in patients with hi angle mandibular plane.
This hypothesis is indirectly confirmed by some evidence. Ramos and co-workers analyzed load transfer in the fossa component based on two numerical models of total temporomandibular joint implants at both 5 and 15mm of mouth opening . They found that the load on the TMJ at 15mm of mouth opening was increased compared to the 5mm mouth opening.
Moreover, Sim et al. evaluated cartilage remodeling of the mandibular condyle in young adult monkeys after an increase in vertical dimension of the midface by the use of 5, 10 and 15mm fixed bite-splints for 48 weeks . These authors found that in the experimental animals the prechondroblastic layer (absent in control animals) was very distinctive (30–75 microns); and there was a 62% thickness augmentation in the of the chondroblastic layer. The results of this study seemed to show that the mouth opening leads to a higher ossification of condyle, this ossification could be related to the muscular function alteration consequently to the induced mandibular postural changes and to the increased TMJ load related to the biomechanical consideration previously reported.
This is the first study in literature that evaluates the condyle cortical bone characteristics of patients with different skeletal growth patterns. The results of this study can provide new evidence about specific skeletal characteristics of patients with different muscular activity and consequently can help us understand the mechanisms that can contribute to develop skeletal malocclusions and the possible adverse effects of surgical treatments of skeletal malocclusion.
It is known that condylar resorption can occur as an unfortunate sequela of orthognathic surgery.
Kerstens et al.  observed radiographic evidence of condylar atrophy in 12 patients out of 206 that underwent surgical orthodontic treatment. All 12 patients had the same dentofacial deformity: high-angle mandibular retrognathia. They considered the dentofacial deformity to be the main reason for condylar resorption and hypothesized that orthognathic surgery acts as a factor that stimulates the progress of the disease by increasing the load on TMJs.
The results of this study and the explanation provided by its authors can help explain the cortical differences observed among patients with different vertical skeletal pattern, and they could help orthognathic surgeons and orthodontists to evaluate the role of masticatory function in patients with different skeletal characteristics.
Different aspects must be taken into consideration when evaluating the results of this investigation including its limitations. This study did not evaluate the possible differences between male and female subjects. However, in a recent study by Farnsworth et al., no sex differences in cortical bone thickness in the mandible were found between both sexes .
Limitations of CBCT imaging should also be considered in terms of spatial resolution and in terms of bone density evaluation . CBCT images do not have consistent Hounsfield values and consequently it does not provide an accurate bone density evaluation . Further research will be necessary in order to directly correlate cortical condylar bone and masticatory function.