The flexibility of the proximal thoracic curve (PTC) in Lenke 1 and 2 adolescent idiopathic scoliosis (AIS) is a crucial factor influencing surgical planning and postoperative shoulder balance. Extension of the PTC into the cervical spine is not uncommon and has important implications in flexibility assessment and upper instrumented vertebra (UIV) selection. This study aims primarily to determine the incidence of the PTC extending into the cervical spine in Lenke 1 and 2 AIS patients. The secondary objectives include assessing the flexibility of the PTC and evaluating shoulder discordance in these patients. A retrospective review of 661 AIS patients who underwent posterior spinal fusion (PSF) was conducted. Radiological parameters including flexibility of PTC, cervical axis, T1 tilt and clavicle angle were measured. A total of 139 patients (21%) had PTC extending into the cervical spine, with no significant difference between Lenke 1 and 2 (p = 0.110). These patients showed significantly higher cervical axis values compared to those without PTC extension to cervical spine (4.8° ± 3.4° vs 0.9° ± 3.8°, p < 0.001). The upper end vertebra (UEV) was most commonly C4 (26.6%) and C5 (30.2%). The revised PTC Cobb angle measurement with UEV (cervical) - LEV method demonstrated substantially higher PTC flexibility compared to the conventional T1-LEV measurement (91% vs 45.9%, p < 0.001). Consequently, only 24% (12/50) of original Lenke 2 patients remained in that group after UEV revision, with 76% (38/50) was reclassified to Lenke 1. 57.6% of patients in this study cohort have shoulder discordance which was higher in Lenke 2 group (60%). In conclusion, routine preoperative recognition of cervical extension of the PTC is critical to guide tailored surgical management as it influences UIV selection and postoperative shoulder balance.