牙阻生（impaction of teeth）是指超过了应该正常完全萌出的时间，牙仍在颌骨内未萌出或者仅部分萌出。阻生可以是一颗牙或者数颗牙受累，常对称性发生。牙阻生常见于恒牙列，最常累及第三磨牙、下颌前磨牙和上颌尖牙，乳牙列罕见［1-4］。在乳牙阻生的报道中，多为下颌第二乳磨牙阻生,上颌第二乳磨牙少见［2-7］。上颌第一乳磨牙及其继承恒牙易位埋伏阻生更为罕见，本研究报道1例上颌第一乳磨牙与第一前磨牙异位埋伏阻生的患者，对该病的发病机制和治疗进行探讨。

Tooth eruption is defined as the movement of a tooth from its site of development within the alveolar process to its functional position in the oral cavity. The process of tooth eruption can be divided into different phases: pre-eruptive bone stage, alveolar bone stage, mucosal stage, preocclusal stage, occlusal stage and maturation stage. Any disturbance in these phases can lead to eruptive anomalies. The incidence of unerupted teeth is usually higher among permanent teeth than among deciduous ones. Of the primary teeth reported as unerupted, second deciduous molars are the teeth most frequently involved, followed by primary central incisors. At present almost no coverage is seen about the impaction of the first deciduous molar. In this case, a 4-year-old boy who presented with an impacted left maxillary first deci-duous molar came to the Department of Pediatric Dentistry, Peking University School and Hospital of Stomatology. This tooth, located higher to the left maxillary first premolar, was well near to the maxillary sinus. The family and medical histories were noncontributory and his physical findings were within normal limits. The mother was reported as having experienced no illness or other complications and taken no medications during pregnancy. His clinical extraoral examination was noncontributory. His clinical intra-oral examination revealed that the maxillary left first primary molar was not present. No enlargement of the area was apparent visually or on palpation. The remaining primary dentition was well aligned and in good condition. His oral hygiene was good, although there were incipient occlusal carious lesions in the mandibular second primary molars. There was no history or evidence of dental trauma. A diagnosis of a left maxillary first deciduous molar was made on the basis of the clinical and radiographic evidence. Numerous local etiologic factors have been described for impacted teeth. These include anomalous teeth, malposition, fusion with adjacent or supernumerary teeth, odontoma, dentigerous cysts, tumors, underdevelopment of the jaws, keratinized epithelial lining, hereditary conditions, and trauma. In this case, the reason for impaction was not clear. After the comprehensive clinical evaluation, treatment consisted of placement of a space maintainer, the periodic examination was indicated for the follow-up, so that early interventions, such as subsequent surgical intervention and orthodontic traction could be recommended timely to manage orofacial disfigurement and to avoid consequent problems with resultant proper functioning and good periodontal health.