This finite element model (FEM) study revealed the unique adaptation mechanism of the body after a fractured condyle. As expected, results showed a change in load not only in the fractured condyle but also in the non-fractured condyle. It was hypothesized that after a fracture, the load in the fractured condyle during open and closing movements would decrease, while the load on the non-fractured condyle would increase. The results confirmed this after shortening of the ramus height. There was an expected inversely proportional relationship between the amount of ramus shortening and the load on the fractured side. The proportional relationship was expected to be positive between the amount of ramus shortening and the load on the non-fractured side during open and closing movements. This was also seen in the comparison between the non-fractured model and the models with shortening of the ramus due to fracture on the right side. The amount of difference in load is not easy to test or quantify. The decrease in load in the fractured condyle after a shortening of 16 mm both in open and closed mouth is evident, but the differences in load on the non-fractured side during closed mouth are harder to define. Possibly these fluctuations can be attributed to inaccuracies in the model, hence for the interpretation of the load, attention was given to numbers that differed more than 10%. Therefore, the sudden drop in load of more than 50% in the fractured condyle during closed mouth between a shortening of 6 mm to 8 mm is considered significant.

This study showed that a fracture that induces shortening of the ramus drastically decreases the load in the fractured condyle during the entire open-close movement. More shortening led to a further decrease in load. In line with the results on the fractured side, the load on the non-fractured condyle increased with shortening of the ramus during the mouth-opening phase. The amount of decrease on the fractured side was not directly compensated by the non-fractured condyle in case of 16 mm shortening. While load increased on the left side by 27%, the decrease on the right side was 3.3%. Possibly, part of this difference is compensated by other structures in the masticatory system, such as muscles.

The sudden drop in load of more than 50% on the fractured condyle during closed mouth is the most significant change in load throughout all results. Whereas all other results show a steady trend with an occasional outlier, the pattern changes significantly between 6 and 8 mm of shortening. Although this does not cause a significant load increase on the other, non-fractured, condyle, it does lead to a sudden large load difference between the condyles and, therefore, a more asymmetric balance. As this sudden increase in difference between condyles is only visible during closed mouth, the role of the dentition should be taken into account. Possibly the stabilization of the dentition has an increasing effect on load of the fractured condyle and a decreasing effect on the load of the non-fractured condyle. However, this cannot entirely explain the sudden drop between 6 and 8 mm. It seems that the two condyles keep working together in distributing the asymmetry in load up to 6 mm. This cut-off point could implicate limitations of recovery of the body if the ramus length asymmetry rises above 6 mm.

In contrast to the load increase in the non-fractured condyle during the mouth-opening phase, the load in this condyle remained stable during the phase of closed mouth. Even during the sudden drop of load in the fractured condyle between the runs with 6 mm and 8 mm shortening, no major change is visible on the non-fractured side. This difference is likely caused by the support of the dentition during the phase of closed mouth. In this FEM periodontal structures have not been taken into account; the teeth are placed in the jaws in an ankylotic form. Literature suggests, however, that tooth, bone, and the periodontal ligament in between react to mastication [11]. Therefore, these structures may have an even greater damping effect on the load in the entire masticatory system, during closed mouth.

The differences in load between the TMJs before and after shortening of the ramus height are substantial. Although the literature shows that the disc plays a major role in the absorption of load in the mandibular TMJ [12, 13], possibly these large load differences can cause clinical sequelae such as pain or impaired function. Remarkably, however, shortening of ramus height due to condylectomy does not show these complications, even with a mean reduction of 8 mm ramus height [14]. Two conclusions can be drawn from this. If a difference in ramus height between the right and left side has a slow onset instead of the rapid change after a fracture, the masticatory system can adapt. Also, the change in ramus height after condylectomy can be compared to the change in ramus height after condylar fracture. In case of condylectomy, the sudden change in ramus height creates a more symmetrical masticatory system, instead of making the system less symmetrical as with unilateral condylar fractures. This implies that not the sudden change in ramus height after condylar fracture is causing complications, but rather the sudden asymmetry in load between the condyles, as is made visible in this study.

The present study investigates load in the condylar TMJ through contact and equivalent stress. The latter is a combined number of different stress-components according to the Von Mises criterion [15]. This represents predominantly shear stress [15]. Since shear does not represent volumetric changes, cartilage handles this type of stress very well [16]. However, results on this type of stress still represent a valuable overview of tensions and deformations that make the cartilaginous structures vulnerable to damage.

Overall, this study showed a fair amount of change in load in the TMJs after shortening of the ramus. The results of this finite element study are in line with the observation that the non-fractured condyle is loaded more heavily during mastication as compared to the fractured condyle [17]. As the results of this FEM study are computed, they cannot be used to draw firm conclusions on clinical outcomes such as pain and remodelling. However, results on changes in load do suggest a possible clinical effect on the surrounding tissue. "Wolff's law" states that mechanical forces guide changes in structure and shape of the bone [18]. Literature has endorsed that disuse of bone will lead to bone loss, whereas mechanical stimulation will promote bone formation [19, 20]. Forces with cyclic impact, like mastication, have a more significant effect on bone formation than a steady high force [21]. If these principles are applied to the results of this study, it would mean that the fractured, shortened side is loaded less, leading to bone loss.

Additionally, the non-fractured side is loaded more, and that would lead to bone formation. However, recent research found that high-intensity mechanical loading induces degradation of bone, instead of formation [22]. Moreover, a higher and longer load leads to more degradation of the bone [23]. The increase of load found in this FEM would lead to degradation of bone on the non-fractured side. Clinical evidence is available that indeed shows a decrease in volume of this non-fractured condylar area [24].

Interestingly, this decrease in volume is not only visible after conservatively treated condylar fractures, but also after surgical treatment [24]. In case of surgery, remodelling could also contribute to the restoration of balance. Another possible explanation for the regain of balance is that soft tissues play a role [24]. A more extreme change in the height of the ramus might not be possible without damage to soft tissues like muscles and tendons, as the damage to the soft tissues is in proportion to the severity of the condylar injury [25]. Also, the cartilage layer has a damping effect on the changes in load, it also shows signs of degradation after high-intensity loading [26].

In conclusion, an increase in load on the non-fractured side could have consequences for the shape of the condyle, in order to regain a balanced distribution of load between the right and left side. As shortening of the ramus due to unilateral condylar fracture causes an increase of the load on the non-fractured side, one would expect more remodelling on the non-fractured side with more shortening. The change of load in the TMJs after shortening is expected to have more effect on the non-fractured side, as this TMJ is loaded up to 27% more after shortening of 16 mm. The change in load through most steps of shortening was gradual. The only cut-off point is the sudden drop in load on the fractured side during closed mouth between 6 and 8 mm shortening. And although the increase in the non-fractured condyle remains proportional, the difference in load between the condyles enlarges significantly. So, this cut-off point may have a significant effect on the remodelling, due to the large difference in load between the condyles. It is also possible that this sudden difference is too large for the body to compensate for, clinical studies could clarify this subject further.

The findings of this study are based on a FEM. Therefore, all results of this study should be interpreted within the boundaries of a FEM. Although the model mimics the masticatory system to a great extent, it should be noted that parts of the model are based on human cadavers. These data may be less representative for mimicking the masticatory system of a healthy adult. Also, the maximal mouth opening of the model was only 30 mm, where in clinical cases a healthy maximal mouth opening for females is around 46 mm and for males around 54 mm [27]. The shortening applied in the fractured condyle is not an imitation of a clinical case, as in patients most shortenings coincide with angulation of the fractured part. By placing a focus on just the shortening, its effect was visible. Before applying these results to clinical cases, it would also be valuable to compare to results with angulation of the fractured condylar part. Apart from the disc, no special attention was given to the soft tissues surrounding the TMJ. These tissues could have a damping effect on contact and/or internal forces of the TMJ. The assumption was that contribution of these soft tissues could be minor, as literature on zygomatic fractures states that soft tissue acts as a temporal buffer only [28]. Future studies could look into the role of soft tissues in condylar fractures. Finally, the implications set in this study by the findings on the cut-off point provide an opportunity for future studies and clinical studies to investigate if an asymmetry between condyles of more than 6 mm leads to more difficulties in clinical situations. And, if so, this could help decisions on treatment of condylar fractures in the future.