Existing C2 - T1 finite element (FE) models can produce boundary condition artefacts at the cervicothoracic junction (CTJ) when analysing adjacent segment biomechanics after cervical laminectomy and posterior instrumented fusion (CLPIF) extending to C7 or T1. We aim to assess whether extending the model to T2 reflects the actual postoperative adjacent-segment mechanics. Validated, anatomically detailed C2–T1 and C2–T2 FE CLPIF models were evaluated under 2 Nm flexion/extension. Segmental motion, intradiscal pressure (IDP), and capsular ligament strain (CLS) were evaluated from C2-C3 through C7-T1, and all outcomes were normalized to intact. There was no significant difference in the intact global kinematics (≤1.5% in flexion and ≤2.4% in extension) between the models. However, an increase in adjacent-level ROM (C3–C4: +6.6% flexion, +10.76% extension; C7–T1: +2.85% flexion, +11.68% extension), IDP (C3–C4: +13.76% in flexion, +17.12% in extension; C7–T1: +23.19% in flexion, +29.38% in extension), and CLS (C3–C4: +14.75% in flexion; C7–T1: +14.95% in flexion) was observed in C2-T2 CLPIF model. Addition of a caudal functional spine unit (T1-T2) preserved intact global kinematics while reducing boundary-condition artefacts at the cervicothoracic junction (CTJ), thereby yielding a more physiologic representation of postoperative mechanics. It exposes the actual biomechanical alterations and higher postoperative demands at the adjacent segments following a CLPIF. Hence, inclusion of T2 is advisable when evaluating adjacent segment mechanics after posterior instrumented cervical fusions ending at or adjacent to the CTJ.