Mechanical Validation of Composite Femurs for Testing Patient Specific Implants

The study I am working on is a sister study to one published last year:

Little JP, Horn TJ, Marcellin-Little DJ, et al. Development and validation of a canine radius
replica for mechanical testing of orthopedic implants. Am J Vet Res 2012;53:27-33.

The focus of the study that I am involved in is the construction of canine femur replicas to
be used for the mechanical testing of orthopedic implants. This study is relevant to current
veterinary medicine because the development of improved orthopedic implants is dependent on the methods available for accurate testing of those devices prior to use in patients. Because researchers wish to simulate in vivo conditions of orthopedic implants during testing, cadaveric bones are often used. However, cadaveric bones pose several problems. An extremely large sample size is required to overcome inter-sample variability, yet access to such a large number of samples is often difficult. Additionally, cadaveric bones will begin to degrade during testing.

A replica whose mechanical properties matched that of bone and that could be manufactured with low variability would allow for greater progress in the development of orthopedic implants.

I am mechanically testing both cadaveric canine femurs and composite replicas. The replica
femurs consist of a polyurethane foam core which imitates the cancellous interior of a bone,
and a fiberglass reinforced epoxy resin exterior that simulates the cortical layer of bone. There are six cadaver samples of the left femur that correspond to dogs within a specific weight range.  These animals were euthanized for reasons unrelated to this study.  The tests being performed include cranio-caudal four-point bending, medio-lateral four-point bending, torsional rotation, and axial compression. In order to test the samples during torsional rotation and axial compression, the ends of each bone were potted to be held in a consistent manner within the testing fixture. The composite femurs were potted using a silicone rubber mold which sealed to the contour of each bone along the diaphysis and opened into a cylinder at each epiphysis. These cylinders were then filled with Bondo, and the assembly was removed from the mold after curing. The potting process of the cadaveric femurs proved to be more challenging. Since each cadaveric bone has its own unique geometry, a single mold could not be used to match the entire group and hold each in alignment while being potted. Instead, a fixture was designed to suspend the distal end of the bone over a cylindrical mold which was then filled Bondo. Once cured, the potting was inserted into a fixture which ensured concentricity and alignment while potting the proximal end.

Mechanical-Validation-of-Composite-Femurs-for-Testing-Patient-Specific-Implants-1
Mechanical-Validation-of-Composite-Femurs-for-Testing-Patient-Specific-Implants-2

Thirteen pairs of left and right cadaver femurs underwent preliminary destructive testing to provide a baseline for the six cadaver femurs that were to be tested nondestructively. These femurs were taken from dogs within the same weight range as the six that were tested nondestructively.

dog femurs placed under pressure
Destructive medio-lateral 4-point bend of cadaver femur

After reviewing the data for the destructive tests, maximum load values were determined that would be used during nondestructive testing. Cadaver femurs were wrapped in saline gauze and kept frozen between testing, and were allowed to thaw completely before tests began. Each bone underwent a total of twelve tests which consisted of three loading events per test type, performed in a random order. Immediately prior to every loading event, each bone was conditioned by a preloading event, which allowed it to settle within the fixture before experiencing the test loading event.

Nondestructive torsional rotation of cadaver femur
Nondestructive torsional rotation of cadaver femur
Spiral fracture on composite femur due to destructive torsional rotation
Spiral fracture on composite femur due to destructive torsional rotation
Nondestructive axial compression of composite femur
Nondestructive axial compression of composite femur

Destructive tests of designated composite femurs were also performed prior to nondestructive testing. Testing procedures and fixtures for composite bones are the same as those used during cadaver testing.