Coronal Plane Alignment of the Knee (CPAK) in children and adolescents : an analysis of 655 knees according to the CPAK classification.

Aims: 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 analyze 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. Methods: Angular measurements of 655 knees taken on CT scans of 333 children aged between four and 18 years from a publicly available radiological database were analyzed, applying the CPAK classification by calculating the aHKA and JLO. Results: The mean aHKA was 3.4° (SD 3.1°). JLO became more pronounced with age, decreasing from a mean of 176.8° (SD 4.2°) at the age of four years to 172.8° (SD 3.1°) at the age of 18 years. The most common CPAK type between four and 18 years 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%). Conclusion: This is the first study analyzing 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 four and 18 years. 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.