Validation and optimization of Patient-specific orthopedic plate design for proximal femur fixation in slipped capital femoral epiphysis patients

Abstract

Slipped capital femoral epiphysis (SCFE) is a condition in adolescents affecting the proximal femur. Increased mechanical forces result in fracture of the growth plate. Factors increasing the mechanical forces include for example obesity and endocrine disorders. As a result of the fracture a shift of the bone shaft towards the femur head occurs, causing mobility issues and pain. A treatment option for these patients is an intertrochanteric osteotomy with plate fixation. Restoring range of motion and pain reduction are the main aims of the surgery.

This intertrochanteric osteotomy alters the femur anatomy, resulting in a mismatch between bone and conventional plate fixation. These plates do not fit to the reconstructed bone, resulting in high chances of plate failure. The best option for these patients is to use patient-specific designed bone plates for femur fixation. Optimal mechanical properties of the patient-specific plate are important in limiting plate failure. These properties are dependent on design parameters, which can be adapted dependent on patient criteria and anatomy. Investigations towards patient-specific bone plates and the optimal design parameters for efficient and stable fracture fixation is lacking.

The focus of this study is on a specific SCFE patient case treated with corrective osteotomy using a patient-specific Ti-6Al-4V plate and screws for bone fixation and stability. A valid numerical model (i.e., finite element analysis (FEA)) is created, with physiological loading corresponding to two leg stance and walking, for an initial designed (ID) patient-specific bone plate to analyse plate performance. Screw configuration analysis and topology optimization of the ID plate were performed to design an alternative topology optimized (TO) plate. Biomechanical experiments with digital image correlation (DIC) techniques were performed for mechanical strength analysis and validation of the finite element model (FEM).

Both ID and TO plate were able to withstand two leg stance loading conditions according to both FEA and experimental testing. For walking loading conditions the ID plate showed instability, which could result in higher probability of failure in the plating construct. Quasi-static compression experiments showed failure in the most lateral proximal screw in all tests. The failure in the screws was caused by high bending moments. Optimization of the ID plate resulted in an alternative plate design in SCFE treatment. The TO plate consisted of six screws, decreasing plate length by 18.1%. The TO plate had a variable thickness, where thickness was increased around the lateral side of the plate compared to the ID plate.

The steps followed in this study to design a patient-specific implant and evaluation plate mechanical behaviour with FEM and additional experimental testing have shown to be adequate in investigating plate performance and can be used in the evaluation of newly designed patient-specific bone plates in SCFE patients.