Aim and objective: In deformity correction around the knee, the mechanical lateral distal femoral angle (mLDFA) and medial proximal tibial angle (MPTA) are used in surgical planning routinely. While plain radiographs are generally adequate, some surgeons utilise intraoperative arthrograms to visualise the articular contours and assess a younger child\'s true joint alignment, often with findings that these are discrepant from that measured just using bone alignment. The age cutoff for a discrepancy between the two is not defined. Materials and methods: We queried our picture archiving and communication systems (PACS) database for MRIs with a radiological read of “normal” for patients between the ages of 4 and 16 years at the time of the study. Anatomic axes were used to determine the anatomic LDFA (aLDFA) and MPTA angles using end-cartilage and end-bone landmarks independently. Results: We reviewed 116 MRIs, 56% male, with approximately 9 studies per year of age. There were no significant overall differences between aLDFA and MPTA when measured at the bone vs cartilage surfaces (p = 0.42 and p = 0.53, respectively). In the 4- to 6-year age range, there was a significant difference between bony and cartilaginous aLDFA (p = 0.02) but not MPTA (p = 0.88). Conclusion: In children 6 years of age and younger, intraoperative arthrogram should be considered while treating knee deformity, as plain films may not fully represent the true deformity of the distal femur in particular. Clinical significance: Supports the need for advanced imaging or intraoperative arthrogram for joint corrective surgery in young patients. Level of evidence: Level 3 diagnostic.