A Novel Planning Paradigm for Augmentation of Osteoporotic Femora

Summary

A modified planning paradigm has been created to reduce the injection volume for osteoporotic bone augmentation. The goal of this project is to validate this new planning approach through cadaveric experiments. In addition, we aim to create and validate a COMSOL Finite Element (FE) model to estimate the bone temperature after cement injection. Finally, we intend to introduce a methodology to reduce the cement’s curing temperature inside the bone.

  • Students: Mahsan Bakhtiarinejad, Amirhossein Farvardin
  • Mentor(s): Dr. Mehran Armand, Dr. Ryan Murphy

Background, Specific Aims, and Significance

The one-year mortality rate after osteoporotic hip fracture in elderly is 23% [1]. Current preventive measures commonly do not have a short-term (less than one year) effect. Moreover, the risk of a second hip fracture increases 6-10 times in elderly with osteoporosis [2]. Osteoporotic hip augmentation (femoroplasty) is a possible preventive approach for patients at the highest risk of fracture and who cannot tolerate other treatment modalities. Recent computational work and cadaveric studies have shown that osteoporotic hip augmentation with Polymethylmethacrylate (PMMA) can significantly improve yield load and fracture energy [3]. However, higher volumes of PMMA injection may introduce the risk of thermal necrosis. In this project, we validate a modified planning approach to lower the injection volume as compared to the previous work [3]. This will likely reduce the risk of thermal necrosis caused by exothermic polymerization of PMMA.

The modified planning paradigm involves three steps: 1) finite element (FE) optimization of the PMMA distribution, 2) geometric optimization for approximating the FE-optimized model geometry with spheroids, and 3) hydrodynamic simulation to predict the resulting PMMA distribution in the bone

Deliverables

  • Minimum: (Expected by 3/31/17)
    1. Pre-operative planning models for 4-5 osteoporotic femora
    2. Experimental post-operative results of osteoporotic femora
    3. Efficacy and statistical analysis of the new planning approach for femoroplasty
  • Expected: (Expected by 4/28/17)
    1. Temperature-rise measurements of the bone surface after the injection
    2. Heat transfer FE COMSOL model to predict the curing temperature of PMMA inside the bone
    3. Comparison of the experimental results with FE model
  • Maximum: (Expected by 5/5/17)
    1. A Methodology to reduce the curing temperature
    2. Experimental results and validation of the cooling system

Technical Approach

1. Planning workstation The modified planning paradigm involves three steps: 1) finite element (FE) optimization of the PMMA distribution, 2) geometric optimization for approximating the FE-optimized model geometry with spheroids, and 3) hydrodynamic simulation to predict the resulting PMMA distribution in the bone (Fig. 1). FE models of the femora were created using CT scans obtained from the specimens following the procedure described earlier in [4]. The boundary conditions simulated a fall to the side. For the first step of planning, three injection patterns were optimized utilizing the Bi-directional Evolutionary Optimization (BESO) method [5].

2. Surgical Execution and Tracking System The surgical execution and tracking system has been described in detail in [3]. Briefly, we remove the soft tissue from the femora that has been selected for augmentation. We then attach a tracking rigid body with reflective markers (NDI, Waterloo, ON, Canada) to the femur. Next, we utilize an in-house navigation system [6] to register the bone to its CT volume. For this purpose, we first identify three landmarks on the femur utilizing a tracking digitizer and perform a rigid transformation from the camera coordinates to the CT. We then digitize several surface points and perform a point cloud-to surface registration utilizing the iterative closest point (ICP) method. In this setup, we use a hand drill (DeWalt Inc., Baltimore, MD) with a custom attachment for a tracking rigid body to drill the desired injection path.

Cadaveric studies

4 pairs of osteoporotic femora were obtained from the Maryland State Anatomy Board. We then took computed tomography (CT) scan of each pair and keep them frozen at -20°C. We selected one femur from each pair randomly for augmentation and plan the injection per the architecture described above. One day prior to testing, femora were taken out of the freezer and left at the room temperature (25 °C). After execution of the injection plans, we performed a mechanical testing simulating a fall to the side on the greater trochanter. Effectiveness of the augmentation was assessed by performing paired t-tests on the mean differences in the fracture load and fracture energy between control and augmentation sets.

COMSOL Simulation Model

[To be Completed]

Bone Augmentation Cooling System

To assess the feasibility, we have conducted a controlled sawbone experiment utilizing a 3mm k-wire that is inserted through the injected cement. The metallic k-wire is attached to Iced-water bowl to lower the curing temperature.

In a pilot experiment, 15 cm3 was injected uniformly into a 130 mm x 45 mm x 40 mm block of an open cell block (7.5 PCF) resembling the human cancellous bone. Before injection, 30 cm3 of canola oil was added to the block mimicking the bone marrow. In this setup, temperature profile of the bone-cement interface was measured via k-type thermocouple at 3 different locations. Experiment was repeated with the cooling system for comparison.

Dependencies

Milestones and Status

  1. Milestone name: Conduct the Planning Approach and Evaluate the post-operative results of 4 Osteoporotic femora
    • Planned Date: 3/31/17
    • Expected Date: 3/31/17
    • Status: In progress
  2. Milestone name: Compare the simulation results with experimental data for temperature-rise of bone in cadaveric studies
    • Planned Date: 4/28/17
    • Expected Date: 4/28/17
  3. Milestone name: Evaluate the cooling system proposed
    • Planned Date: 5/5/17
    • Expected Date: 5/5/17

Reports and presentations

Project Bibliography

  1. Cummings, Steven R., Susan M. Rubin, and Dennis Black. “The future of hip fractures in the United States: numbers, costs, and potential effects of postmenopausal estrogen.” Clinical orthopaedics and related research 252 (1990): 163-166.
  2. Dinah, A. F. “Sequential hip fractures in elderly patients.” Injury 33, no. 5 (2002): 393-394.
  3. Basafa, Ehsan, Ryan J. Murphy, Yoshito Otake, Michael D. Kutzer, Stephen M. Belkoff, Simon C. Mears, and Mehran Armand. “Subject-specific planning of femoroplasty: An experimental verification study.” Journal of biomechanics 48, no. 1 (2015): 59-64.

Other Resources and Project Files

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courses/446/2017/446-2017-09/project.txt · Last modified: 2019/08/07 12:01 (external edit)




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