Abstract: BACKGROUND: Previous studies have shown that there are significant differences in occlusal and maxillofacial changes in patients with skeletal class III malocclusion of different ages after maxillary protraction treatment. However, the difference of temporomandibular joint changes in patients with different ages remains unknown. 
OBJECTIVE: To study the influence of age on the changes of the temporomandibular joint after maxillary protraction treatment of skeletal class III malocclusion. 
METHODS: Fifty-eight children with skeletal class III malocclusion aged 8-14 years were divided into two groups. Twenty-nine children aged 8-11 years were diagnosed with late mixed dentitions. Twenty-nine children aged 12-14 years were diagnosed with early permanent dentitions. X-ray lateral cephalograms were taken pre- and post-treatment, and cephalometric analysis of occlusal and maxillofacial regions and quantitative analysis of temporomandibular joint position in the coordinate system were carried out. Paired t-test was used for intragroup comparison. Independent t-test was used for intergroup comparison. Pearson correlation analysis was used to analyze the correlation between occlusal and maxillofacial changes and temporomandibular joint changes. 
RESULTS AND CONCLUSION: The occlusal and maxillofacial structure and the temporomandibular joint changed significantly in the two groups after treatment. In the sagittal direction of the occlusal and maxillofacial structure, subspinale-nasion-supramental angle (ANB) in the 8-11-year-old group increased by 4.76°, and nasion-subspinale line and pogonion-subspinale line angle (NA-PA) increased by 9.02°, which was significantly larger than those in the 12-14-year-old group (3.97° and 6.88°) (P < 0.05). In the vertical direction, the mandible plane and sella-nasion line angle (MP-SN) and Frankle plane and sella-nasion line angle (FH-MP) in the 8-11-year-old group increased by 1.92° and 3.02° respectively, which was significantly higher than those in the 12-14-year-old group (1.02° and 0.78°) (P < 0.05). There were significant changes in the temporomandibular joint between the two age groups. Fossa summit on the X axis (S-Fsx) in the 8-11-year-old group moved backward more significantly than that in the 12-14-year-old group. Fossa posterior on the X axis (S-Fpx) in the 8-11-year-old group moved backward, while that in the 12-14-year-old group moved forward. Condyle anterior (S-Cix) and condyle tangent on the X axis (S-T4x) in the two groups both moved backward, which were more significant in the 8-11-year-old group than the 12-14-year-old group. The changes of the temporomandibular joint were correlated with the changes of the occlusal and maxillofacial structure. In the 8-11-year-old group, there was a negative correlation between S-Fpx and sella-nasion-supramental angle (SNB), nasion-pogonion line and Frankle plane angle (NP-FH) (P < 0.05, r=-0.489; P < 0.05, r=-0.424). In the 12-14-year-old group, S-Fpx was positively correlated with the distance from the upper incisor to the nasion-subspinale line (U1-NAmm) (P < 0.01, r=0.439). In the 8-11-year-old group, S-Cix, S-T4x and the condyle superior on the X axis (S-Cox) were negatively correlated with SNB, NP-FH and MP-SN. To conclude, age has obvious effects on the changes of the temporomandibular joint in skeletal class III malocclusion following maxillary protraction. In the 8-11-year-old group, articular fossa and condyle moved backward obviously, fossa anterior moved up, and the posterior joint space increased. In the 12-14-year-old group, the fossa posterior moved forward and the condyle anterior moved down, but the joint space did not change. There are different occlusal and maxillofacial factors affecting temporomandibular joint changes at different ages.

Key words: maxillary protraction, skeletal class III malocclusion, temporomandibular joint, age, maxillofacial