Introduction:  Class II malocclusion is a common orthodontic discrepancy affecting one-third of the world’s population. Thus, functional appliances that cause forward posturing of the mandible can induce mandibular growth, and are therefore a popular approach for treatment of Class II malocclusion during the growth spurt period.
 This study aimed to compare the dental, skeletal, and soft tissue effects of the Twin Block (TB) and Guilan Functional Appliance (GFA) on Class II patients with mandibular deficiency.
Materials and Method: This retrospective study compared 30 patients treated with TB and 30 patients treated with GFA. All patients had ANB > 5 degrees, SNB < 77 degrees, and overjet > 5 mm. Preoperative lateral cephalograms of patients were compared with their lateral cephalograms obtained after completion of phase II of treatment. Comparisons were made by paired t-test (alpha=0.05).
Results: In the GFA group, SNA significantly decreased from 84.66 to 82.16 degrees (P<0.05) while SNB experienced an insignificant reduction from 75.83 to 74.63 degrees, and ANB experienced a significant reduction from 6.26 to 4.86 degrees (P=0.005). In the TB group, SNA significantly decreased from 82.26 to 81.96 (P<0.05) while SNB increased from 75.43 to 76.69 degrees (P=0.599), and ANB decreased from 6.3 to 5.67 degrees (P=0.049). both the TB and GFA decreased the Wits. This reduction was greater in the GFA group; however, the difference between the two groups did not reach statistical significance(P=0.931).
Conclusion: Despite different designs, both functional appliances were successful in treatment of patients with mandibular deficiency.