Summary Objectives: The purpose of this study was to investigate the effect of mandibular advancement on the rat TMJ using a fixed functional appliance during treatment and after retention. Materials and methods: Sample: 54 male Sprague-Dawley rats 4 weeks of age (100-120gm) were used in this study. 30 rats were used as an experimental group, while 24 rats were used as a control group. The experimental group was further subdivided into 3 equal groups (d=10, e=10 andf=10). The comtrol group was also subdivided into three equal subgroups (a=8, b=8, andc=8). All groups were given a soft diet and kept under the same environment in sterilized cages during the whole procedure that was approved by Beirut Arab University Institutional Animal Care and Committee (IRB). The thirty rat's in the experimental group were subjected to an orthodontic inclined bite plane cemented to the incisors (fixed functional appliance) to advance the mandible, while the others acted as a control group. One month later, sub-group d (full-time wearing appliance) and sub-group a were sacrificed. Gross morphological anaylsis was manually done using digital vernier on specific selected landmarks of the mandible to obtain 2 lines. The first one represents mandibular length, and the 2nd represents mandibular base. Specimens of hemi-cranium were cut and dissected. Slices at 7pm thickness were cutted by microtome and spread on slides which were then histopathological stained (H-E stain) and immunohistochemical stained using different markers (MMP-1,8 and 13) in addition to Collagen type 1,11 and type III and examined under light microscope. The sub-groupe (half-day-wearing appliance, retention period) and the sub-group were sacrificed, and slides were examined under light microscope. The sub-group (without appliance, post-retention period) and the sub-group c were sacrificed one month later and examined as usual. Body weight monitoring assessment was weekly conducted for all groups at each step of the experiment. Results: The body weight in both groups increased progressively, and significant differences (p< 0.05) were observed between weeks 5 to 10. At the end of the experiment, no significant differences were observed. However, gross-morphological changes were clear in the rats'skulls between the experimental group, downward forward mandibular advancement and the control groups, a very high significant increase in the length of mandible (p< 0.001) was noticed. The significant increase in the length of the mandible was also noticed between the experimental sub-group e and sub-group d in addition to the sub-group e and sub-group b of the control group. The difference is also significant (P<0.05) between sub-group e (retention period) and sub-group f (post retention group) of the experimental group and between d and eof the base mandibular experimental group. Then a high significant difference (p<0.01) decrease in the base mandibular length compared with sub-group a'of the control group and d' (active period) of the experimental group. Moreover, high significant increase in the mandibular base was (P<0.01) compared with sub-group 6 (active period) and sub-group e' (retention period) of the experimental group. Besides, increase significant difference (P<0.05) between sub-group d' (active period) and sub-group f' (post retention period) of the experimental group. Abundant cellular proliferation expressed at posterior border of Condylar cartilage in the experimental group than in control group. The effect of the jumping-bite appliance in young rats showed great substantial histologic changes in TMJ region. In the first month, histological examination of articular condyle for experimental sub-group (d) revealed a significant increase in proliferative zone (p<0.05) in comparison with the control sub-group (a) with no significant changes in the other two zones. In the second month, the sub-group (e) that partially worn the appliance, showed a well developed fibrotic zone which had a significant increase (p<0.05) in comparison with the control sub-group (b). On other hand, both the proliferative and the hypertrophic zones decreased significantly (p<0.05) in comparison with sub-group (d). Regarding the third month post-retention sub-group (f), no significant changes in fibrous and proliferative zones were noticed with the comparison previous two months and the control. However, the hypertrophic zone revealed significant decrease (p<0.001) in comparison with the sub-group (e) and the control sub-group(c). Cellular proliferation, without proper ossification, was noticed on the condyles of young rats that belong to experimental group. However, rats revealed a significant condylar ossification and higher osteoblastic activity towards the last week of treatment and retention period, where the mandibular condylar growth was accelerated by the bite-jumping appliance treatment and remained stable within limits after certain period of retention. An expression of MMP1- in both, control and experiment groups, positive in the cellular matrix area in the proliferation layer and differentiation at approximately the same level with no statistical difference between all groups. An expression of MMP-8 positive in the cellular matrix of the inferior region of hperatrophic layer and statistically no significant difference between all groups and an expression of MMP-13 has a significant positive (P<0.05) in both, layer of differentiation and hyperatrophic layers of an experimental group during the first month (active period) with positive an expression during period of retention of the experimental group. An expression of collagen type 1 and III was positive in the proliferative extracellular matrix and differentiating layers of experimental group specimens, but statistically there were no significant differences between all groups. In contrast, when compared the control group in the active and retention periods that took place during the first and second months of the experiment, an intensive positive decrease expression in the matrix of proliferation, differentiation and hyperatrophic layers occurred with an expression of collagen type II in the experimental group. 6.2. Conclusion In the present study, mandibular Condylar changes were accelerated using fixed functional appliances.These appliances increased the osteogenesis during active mandibular advancement by using methylmetha acrylate bite plate cemented to the upper anterior teeth during pubertal period. These changes, even for a long period of retention, revealed stability of treatment which is confirmed through the post—retention period due to endocondral ossification pattern improvement. Based on our findings, the following conclusions may be drawn: 1- The mandible demonstrated forward and downward growth because of that attachment with obvious increase in length of the mandible due to remodeling of posterior aspect of the mandibular condyle. 2- MMP-1, -8, and -13 were reproduced by fibrocartilage overload where collagen degradation was facilitated. 3- There is a strong relationship between MMP-13 expression and Collagen type II degradation as a result of mandibular repositioning by using fixed functional appliances. 4- The expression of collagen II during active and retention periods of appliance group was noticed. Intensive expression of MMP-13 in active period of appliance group was increased, and the shape of the condyle was improved. This may be due to secondary effect and slight role of MMP-8 and collagen type I and III during orthodontic advancing mandibular procedure through pubertal period. 5- Collagens and MMPs play an important role in the recovery of articular condyle after functional orthodontic treatment of Cl II malocclusion. Without the knowledge of gene activity and its signaling transduction pathway, the mechanism that controls mandibular condylar cartilage development or remodeling woud be impossible. Keywords: Metaioproteinases, Mandibular condylar cartilage, Morphological changes, Mandibular advancement.