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Objectives and questions: The Coronal Plane Alignment of the Knee classification (CPAK) describes nine knee phenotypes by defining arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO). In this study, the CPAK classification was used to analyse the development of lower limb alignment in children and adolescents, providing reference values and allowing insights into the morphogenesis of the knee joint during skeletal maturation.

Material and methods: Angular measurements of 655 knees taken on CT scans of 333 children aged between 4 and 18 years from a publicly available radiologic database were retrospectively analysed, applying the CPAK classification by calculating the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO).

Results: The aHKA averaged 3.4° ± 3.1°. Joint line obliquity became more pronounced with age, decreasing from an average of 176.8° ± 4.2° at the age of 4 to 172.8° ± 3.1° at the age of 18. The most common CPAK type under the age of 18 was CPAK III (58.2%), the second most common type was CPAK II (24.7%) and the third most prevalent type was CPAK IV (10.1%).

Discussion and conclusions: This is the first study analysing lower limb alignment in children and adolescents using the CPAK classification. By simultaneously developing a distal femoral valgus and a counteracting proximal tibial varus morphology, overall constitutional alignment as represented by the aHKA remains almost constant between the age of 4 and 18. Conversely, joint line obliquity changes significantly with age towards an apex distal phenotype. We provide a comprehensive explanation for the physiological development of lower limb alignment during skeletal growth.