What distinguishes Class II occlusion from Class I?

Class II occlusion, commonly referred to as retrognathism, is primarily distinguished by the relationship of the maxillary first molar to the mandibular first molar. In a Class II occlusion, the first molar relationship is characterized by the positioning of the maxillary first molar being more towards the front of the dental arch compared to the mandibular first molar. This positioning indicates a discrepancy in the anteroposterior positioning of the dental arches.

In contrast, Class I occlusion features the maxillary first molar positioned directly above the mandibular first molar, which serves as the key marker for a normal occlusion where the relationship between these molars is as expected.

Other factors such as the relationship of the incisors or the positioning of canines and archwire alignment are important in orthodontics but do not specifically define the general distinction between Class I and Class II occlusions. Therefore, the critical point that distinguishes Class II from Class I is indeed the relationship of the maxillary first molar to the mandibular first molar.