It has been proposed that the amount of energy transferred to the bone during a high velocity projectile injury determines the extent of bony injury. We studied the validity of this theory Fresh rear skeletally mature deer femurs were subjected to progressively increasing velocity projectile injuries within a pneumatic ballistic chamber with non-deforming steel spheres capturing the energy transferred. Analysis of fracture severity was performed including micro computer tomography analysis of micro-fractures. The effect of projectile caliber size was then analyzed.

Characteristic fractures patterns were observed with fracture lines extending radially from the impact site, often propagating longitudinally along the sample. It was found that a greater energy transfer resulted in more severe fracture for a given projectile. However, fractures of differing severity were produced by different projectiles for similar energy transfer. Neither specific energy transfer nor energy density could explain this phenomenon.

Although energy transfer plays a role in ballistic fracture, it is not the sole determinant. Other factors such as contact surface area, projectile mass and angle of impact may need to be considered.

Measuring strain in biological specimens has always been inherently difficult due to their shape and surface properties. Traditional methods such as strain gauges require contact and therefore have reinforcing effects, also the surface preparation can be time consuming and if proper fixation is not achieved the results will be inaccurate. Using a non contact method to measure strain such as photogrammetry has several advantages. The strain over the whole surface of a specimen can be mapped, depending on the field of view of the camera used. It has a large dynamic range, from microns to millimetres which can be decided upon at the post processing stage. Specimens can be tested to destruction without damaging any measurement equipment. Also there is considerably less set up time involved between testing different specimens once the system is in place. We aimed to test speckle photogrammetry, a method used in industry and fluid dynamics as a tool for assessing proximal femur fracture stability and repair techniques. A Zwick Roell materials testing machine was used to axially apply a staircase loading pattern to sawbones femora, simulating the load experienced by the femur when standing. Firstly an intact bone was tested then a set of three identical fractures of each of three common fracture configurations were produced by osteotomy. The first femur of each configuration was loaded un-repaired to failure; the remaining two were repaired using common techniques for that particular fracture type then also loaded to failure. The bone and fixation device were covered with stochastic, high contrast paint speckle prior to testing. This speckle pattern was recorded at regular load intervals by a digital camera which was attached to the materials testing machine via a rigid frame to eliminate any camera movement. These images were then transferred to a computer where they were converted to 8 bit bitmap images. Matlab was used to process the data from subsequent images to produce vector and colour maps of the displacements and strains over the entire visible surface of the proximal femur and to show the comparative displacements and strains experienced by the individual bone fragment and the fixation devices. Non contact optical strain measurement has proved itself to be a useful tool in assessing the stability of fractures and the repair techniques of these fractures. Additionally it can also be used to validate finite element models to compare theoretical and experimental results due to the similar data and graphic visualisation outputs which are produced by both techniques.

Since cementless stem fixation in hip arthroplasty is becoming more and more common, the overall incidence of intraoperative femoral fractures has risen considerably. Depending on primary or revision arthroplasty, literature reports fracture rates between a few percent up to one third of the cases.

In this study, methods commonly applied in the field of structural testing were customized for this specified interference fit situation. A cementless hip system (ABG II, Stryker) was used on animal bones and biomechanical bones.

Transient excitation in the form of regular hammer strokes and sinusoidal excitation using a shaker served as an input. The output of the system under test was measured on the greater trochanter using a piezoelectric accelerometer.

The signals were digitized with a high-speed data acquisition system and analyzed in real-time with spectrum analysis software.

Analysis included threshold detection in the time domain to determine the time delay between the input and output transducer. Spectrum analysis in the frequency domain included FFT analysis and frequency response function analysis to identify shifts of fundamental frequencies and harmonics to describe the vibrational changes with increasing stability.

A digital imaging system was set up to take pictures of the metal-bone site to measure inducible displacement with each hammer impact and correlate it with the vibrometry results. Furthermore a strain gauge circularly mounted around the proximal femur monitored accurately any hairline fracture.

This study shows that changes of the vibrational spectrum are directly related to implant fit. The range of interest is well in the sonic range, which apparently is the reason for many surgeons to listen and ‘feel’ carefully during advancing the broach or the final implant into the femur.

The study is trying to extract critical vibrational parameters correlated with stability and femoral integrity. Due to the different dimensions of the tested animal bones and lack of soft tissue damping, further experiments on cadavers need to be carried out.

Vibrational spectrum analysis could prove to be a useful tool to readily assess implant stability and femoral integrity. It seems to be most beneficial in revision surgery or minimally invasive hip replacement, where the risk of femoral fractures is increased or fissures could easily be missed.