Using a statistical shape model to estimate the knee landmarks for aligning femurs for femoral finite element models

The BOne Strength (BOS) score is a CT-based tool to assess fracture risk for patients with femoral bone metastases using finite element (FE) models. Until now, the knee joint center (KJC) and centers of the condyles (CoCs) were needed to create the FE model, hence BOS scores of incompletely scanned femurs could not be calculated. In this study, a statistical shape model (SSM) was used to align FE models of femurs with a removed knee anatomy. The aim was to determine the effect of using an SSM with different proximal femur fractions on KJC and CoC locations, and on the BOS score.

QCT scans of 117 femurs were used to generate patient-specific FE models of the proximal femur. These models were aligned using the knee joint center (KJC), center of condyles (CoC) and femoral head center. The femurs were artificially shortened by removing 30 %, 50 % or 70 % of the femur. A recently developed SSM was used to reconstruct the distal femur. For each of the femur fractions, the difference between the original and SSM-reconstructed KJC and CoC were determined and the BOS scores were calculated.

Although the individual differences between the original and SSM-reconstructed KJC and CoC location could be large, the effect on the individual BOS scores was limited. The SSM-reconstructed BOS scores were highly correlated to the original BOS scores.

Using SSM to align femurs with a removed knee anatomy resulted in varying estimation of knee anatomy between patients but relatively accurate BOS scores.