Objectives: To develop a simplified MRI-based shorthand assessment method, referred to as the Sagittal Tibial Epi-Physis (STEP) Shorthand, for skeletal age assessment in skeletally immature patients with anterior cruciate ligament (ACL) injuries. This study aimed to elaborate a single-plane MRI-based skeletal age estimation tool and to explore its feasibility and inter-rater reliability in comparison with existing MRI-based shorthands. 

Methods: This prospective study included 130 knee MRIs (79% males) from 97 skeletally immature patients (overall average age of 14.0 ± 2.1 years) with ACL injuries treated between February 2022 and January 2025. A new shorthand assessment method was developed based on sagittal T1-weighted MRI evaluation of the proximal tibial epiphysis. A validation cohort of 74 MRIs was independently evaluated by four raters with different levels of expertise using the STEP, Meza, and Politzer shorthand atlases. Inter-rater reliability (ICC), intra-rater agreement (Cohen’s kappa), and association with chronological age (Spearman rho) were calculated. 

Results: The STEP Shorthand tool demonstrated a strong association with chronological age (rho = 0.890, p < 0.001) with consistent associations across sex subgroups. Inter-rater reliability was high and comparable to established MRI-based shorthands. The use of a focused sagittal T1-weighted evaluation allowed for a simplified and reproducible assessment across raters with varying experience levels. 

Conclusions: The STEP Shorthand represents a pragmatic and reliable tool for MRI-based skeletal age assessment in pediatric and adolescent patients with ACL injuries. The STEP Shorthand can support timely decision-making in surgical planning and enhance standardization across different levels of clinical expertise.