ABSTRACT
Adolescent idiopathic scoliosis (AIS) is one of the most common deformities. Distal adding-on phenomenon, as one of the postoperative complications of AIS, results in unsatisfactory radiological and clinical outcomes. Moreover, it is more common in patients with selective thoracic fusion. It appears as an extension of the primary curve to the unfused distal vertebra. The most common used clinical definition of distal adding-on phenomenon is the distal migration of the lower end vertebra and progressive increase in the number of included distal vertebrae within the primary curve combined with either an increase of more than 5 mm in deviation of the first vertebra below the lowest instrumented vertebra from the center sacral vertical line or an increase of more than 5° in the angulation of the first disc below the lowest instrumented vertebra. The risk factors for distal adding-on phenomenon have become to a hotspot of research. Scholars have conducted lots of studies on the choice of the lowest instrumented vertebra, which is regarded as the most important risk factor. The choice of the lowest instrumented vertebra varies based on surgeons' decision due to the different criteria and still in controversy. However, the occurrence of distal adding-on phenomenon is not directly caused by a single risk factor. Some studies identified other risk factors, including skeletal maturity, distance between the lowest instrumented vertebra and the center sacral vertical line, L 4 tilt score, shoulder balance, coronal balance, lumbar flexibility, main thoracic curve correction rate, the lowest instrumented vertebra rotation and so on. The development of distal adding-on phenomenon has negative effects on patients, such as brace treatment and even surgical revision. However, there is a limited number of literatures about the development of distal adding-on phenomenon and the indications of surgical revision. The present study reviews the current research status of the risk factors, prevention and treatment of postoperative distal adding-on phenomenon in AIS.