Subject-specific planning of femoroplasty: a combined evolutionary optimization and particle diffusion model approach

Abstract

A potential effective treatment for prevention of osteoporotic hip fractures is augmentation of the mechanical properties of the femur by injecting it with agents such as (PMMA) bone cement - femoroplasty. The operation, however, is only in research stage and can benefit substantially from computer planning and optimization. We report the results of computational planning and optimization of the procedure for biomechanical evaluation. An evolutionary optimization method was used to optimally place the cement in finite element (FE) models of seven osteoporotic bone specimens. The optimization, with some inter-specimen variations, suggested that areas close to the cortex in the superior and inferior of the neck and supero-lateral aspect of the greater trochanter will benefit from augmentation. We then used a particle-based model for bone cement diffusion simulation to match the optimized pattern, taking into account the limitations of the actual surgery, including limited volume of injection to prevent thermal necrosis. Simulations showed that the yield load can be significantly increased by more than 30%, using only 9 ml of bone cement. This increase is comparable to previous literature reports where gross filling of the bone was employed instead, using more than 40 ml of cement. These findings, along with the differences in the optimized plans between specimens, emphasize the need for subject-specific models for effective planning of femoral augmentation.

Keywords: Femoroplasty; Finite element; Optimization; PMMA cement; Planning.

Figure 1

Trial points (blue) for injection. The red dots represent head and neck center points.

Figure 2

Sample cross section of fixed particle (cyan) and fluid particles (yellow) arrangements for SPH simulations. The red line represents the cannula path.

Figure 3

From left to right: evolution of the cement placement (green elements) in a representative model.

Figure 4

Correlation between the intact yield load of the augmented models and the volume required to increase the load by 100%.

Figure 5

Cross sections of optimized models for specimens #2 (A), #4 (B) and #6 (C). Only the cemented and the outer most bone elements are shown for clarity.

Figure 6

Evolution of cement volume (A) and predicted yield load (B) for a sample model. The dashed line in (B) indicates the target yield load.

Figure 7

Planned paths of injection and placed cement elements for specimen #2 (A) and #4 (B). The dashed lines represent the paths of injection.

Figure 8

Degree of augmentation in the augmented group vs. relative volume of injection.