Scaling Mechanical Knee Joints for Pediatric Transfemoral Prostheses: Does a Linear Geometric Factor Work?

Introduction: Pediatric prosthetic knee joints must be appropriately scaled from adult designs to ensure proper gait biomechanics. However, direct dimensional scaling without considering biomechanical implications may lead to functional discrepancies. This study aims to evaluate whether a linear scaling factor can effectively adapt the knee for pediatric use. The study assesses whether such an approach yields a viable pediatric prosthetic knee joint by applying a fixed scaling factor and analyzing the resultant knee geometry. Methods: The linear scaling factor was determined based on the pylon tube diameter, a key constraint in compact pediatric knee design. Given a pediatric pylon diameter of 22 mm, the length of tibial link was set to 22 mm, yielding a scaling factor of 0.6875 when compared to the adult-sized knee. This scaling factor was used to identify the dimensions of the pediatric four-bar (scaled) knee joint. Static geometric analysis was conducted using GeoGebra® to model the lower body segment lengths. The knee joint performance was evaluated based on stance and swing phase parameters. These metrics were compared between the scaled knee and a commercial pediatric knee. Results: The geometric analysis revealed that while the linear scaling factor maintained proportional relationships, certain biomechanical parameters deviated from expected pediatric norms. The scaled knee maintained negative x/y ratios at heel contact, showing significant stability in push-off moments, while the stance flexion angle remained within an acceptable range. The scaled knee exhibited slightly lower toe clearance compared to the commercial pediatric knee, which may impact swing phase efficiency. Conclusion: A linear scaling factor provides a straightforward method for adapting adult prosthetic knee designs to pediatric use. However, deviations in key biomechanical parameters indicate that further experimental study may be required to validate the applicability of the scaled knee joint for pediatric users. Future work should explore dynamic simulations and experimental validations to refine the design further and ensure optimal gait performance.