The adding-on phenomenon in Lenke type I adolescent idiopathic scoliosis

Objective. To analyze the incidence of adding-on phenomenon in the surgery of Lenke type 1 idiopathic scoliosis. Material and Methods. The study included prospective analysis of radiographs of 89 patients (82 females and 7 males) with idiopathic scoliosis who met the criteria for inclusion. The age of patients at the time of surgery ranged from 12 to 25 years (mean: 16.3 ± 4.4 years). The average follow-up period was 2.3 ± 0.4 years. Scoliotic deformity corresponded to grade III according to V.D. Chaklin’s classification in 24 patients, and to grade IV in 65. The magnitude of the primary thoracic curve varied from 30° to 103° of Cobb angle (mean: 61.1° ± 17.1°). In all cases, segmental third generation instrumentation (hybrid or laminar) was used in combination with intraoperative skeletal traction with an afford equal to 50 % of the patient’s body weight. Laminar fixation was used in 6 patients, and hybrid fixation with different extent of screw installation in 83. Results. The following parameters showed significant increase: the magnitude of thoracic curve according to Cobb – 16.0° ± 4.3°, the distance from the center of the vertebra located one level distal to the lowest instrumented vertebra, to the central sacral vertical line (LIV+1- CSVL) – 3.6 ± 2.5 mm, the tilt of the lower instrumented vertebra in the coronal plane (LIV tilt angle) – 3.6° ± 2.8°, the tilt of the ver- tebra located one level distal to the lowest instrumented vertebra (LIV+1 tilt angle) – 4.6° ± 2.5°, and the distance from the center of the apical vertebra of the primary curve to the central sacral vertical line (AV-CSVL) – 17.2 ± 12.0 mm. A significant correlation was found between postoperative magnitude of the scoliotic curve and AV-CSVL distance, postoperative LIV+1-CSVL distance and postoperative LIV+1 tilt angle. An increase in LIV+1 tilt angle in combination with an increase in LIV+1-CSVL distance by more than 4 mm (or degrees) could be a parameter for measuring the adding-on phenomenon. Conclusion. The indication for reoperation may probably be a significant tilt of the vertebra located below the lowest instrumented vertebra, accompanied by pain syndrome and coronal imbalance.