Method and Instrumented Fixture for Femoral Fracture Testing in a Sideways Fall-on-the-Hip Position

Summary

In this manuscript, we present a protocol to fracture test cadaveric proximal femora in a sideways fall on the hip configuration using instrumented fixtures mounted on a standard servo hydraulic frame. Nine digitized signals comprising forces, moments, and displacement along with two high speed video streams are acquired during testing.

Abstract

Mechanical testing of femora brings valuable insights into understanding the contribution of clinically-measureable variables such as bone mineral density distribution and geometry on the femoral mechanical properties. Currently, there is no standard protocol for mechanical testing of such geometrically complex bones to measure strength, and stiffness. To address this gap we have developed a protocol to test cadaveric femora to fracture and to measure their biomechanical parameters. This protocol describes a set of adaptable fixtures to accommodate the various load magnitudes and directions accounting for possible bone orientations in a fall on the hip configuration, test speed, bone size, and left leg-right leg variations. The femora were prepared for testing by cleaning, cutting, scanning, and potting the distal end and greater trochanter contact surfaces in poly(methyl methacrylate) (PMMA) as presented in a different protocol. The prepared specimens were placed in the testing fixture in a position mimicking a sideways fall on the hip and loaded to fracture. During testing, two load cells measured vertical forces applied to the femoral head and greater trochanter, a six-axis load cell measured forces and moments at the distal femoral shaft, and a displacement sensor measured differential displacement between the femoral head and trochanter contact supports. High speed video cameras were used to synchronously record the sequence of fracture events during testing. The reduction of this data allowed us to characterize the strength, stiffness, and fracture energy for nearly 200 osteoporotic, osteopenic, and normal cadaveric femora for further development of engineering-based diagnostic tools for osteoporosis research.

Introduction

Development of novel methods for femoral fracture risk assessment and fracture prevention for a fall on the hip require a comprehensive understanding of the biomechanical processes involved during fracture. Cadaveric proximal femur strength testing has proven to be effective in determining the relation between femoral strength and factors affecting the structural capacity of the femur providing important insights in this process^1,2,3. Experimentally measured femoral strength is also used for validation of Quantitative Computed Tomography-based Finite Element Analysis (QCT/FEA....

Protocol

1. Large Fixture Attachment

1. Remove standard fixtures from the machine.
2. Move crosshead apart to accommodate the in-house fixture.
3. Place the aluminum block (part No. 1 in Figure 1A) on the machine and securely fasten on the machine using two bolts; the hole in the center accommodates the machine load cell.
4. Place the main fixture structure (part No. 2 in Figure 1B) on the aluminum block and securely attach it to the block using 4 bolts.
5. Place a four-ton jack under the part of fixture that does not rest on the aluminum block to support the fixture (

Results

In-house fixtures are mounted after the standard fixtures are removed from the testing machine. First, the bottom heavy fixture is mounted and secured (Figure 1). This includes an extended arm to hold the 6-channel load cell which also enables the femoral shaft to be aligned at a desired adduction angle. Next, the crosshead fixture including two frictionless slide bearings is mounted to accommodate the application of load and femoral head movement during frac.......

Discussion

We proposed a protocol to fracture test proximal cadaveric femora in a fall on the hip configuration with which we have successfully tested about 200 samples. The protocol includes several in-house designed fixtures for femoral strength testing under different loading conditions. The fixture allows for testing of both right and left femora at different testing speeds and bone orientations. After mounting the fixture and the measuring instruments, a fiberglass femur is tested to fracture to assure that all the hardware an.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CT scanner	Siemens	Somatom Definition scanner (Siemens, Malvern, PA)	CT scanning equipment
Quantitative CT Phantom	Midways Inc, San Francisco, CA	Model 3 CT calibration Phantom	Used for obtaining BMD values from Hounsfield units in the CT image
Hygenic Orthodontic Resin (PMMA)	Patterson Dental Supply	H02252	Controlled substance and can be purchased with proper approval
Freezer	Kenmore	N/A	This is a -20oC storage for bones
X-ray scanner	General Electric	46-270615P1	X-ray imaging equipment.
X-ray films	Kodak	N/A	Used to display x-ray images
X-ray developer	Kodak X-Omatic	M35A X-OMAT	Used for developing X-ray images
X-ray Cassette	Kodak X-Omatic	N/A	Used for holding x-ray films
Physiologic Saline (0.9% Sodium Chloride)	Baxter	NDC 0338-0048-04	Used for keeping samples hydrated
Scalpels and scrapers	Bard-Parker	N/A	Used to clean the bone from soft tissue
Fume Hood	Hamilton	70532	Used for ventilation when preparing PMMA for potting of specimens
Single axis load cell	Transducer Techniques, Temecula, CA, USA	LPU-3K; S/N 219627	Capacity 3000 LBS
Six channel load cell	JR3,Woodland, CA	45E15A4	Mechanical load rating 1000N
Linear potentiometer	Novotechnik, Southborough, MA, USA		Used to acquire linear displacements during testing
Slide ball bearing	Schneeberger	Type NK	Part of the testing fixture
Mechanical testing machine	MTS, Minneapolis, MN	858 Mini Bionix II	Used for compression of femur
Lighting unit	ARRI		Needed for high speed video recordings
high-speed video camera	Photron Inc., San Diego, CA, USA	Photron Fastcam APX-RS	Used to capture the high speed video recordings of the fracture events
Photron FASTCAM Viewer	Photron Inc., San Diego, CA, USA	Ver.3392(x64)	Used to view the high speed video recordings
Camera lens	Zeiss	Zeiss Planar L4/50 ZF Lens	Needed to high image resolution
Signal conditioner board (DAQ)	National Instruments		Input/output signal connector
Signal Express	National Instruments	N/A	Data acquisition software
Laptop Computer	Dell	N/A	Used to monitor and acquire all signals from the testing procedure