4.1 Introduction
Previous chapters have introduced the concepts relevant to this investigation. The various methods of measurement both for mechanical properties and ultrasonic parameters have been of a broad nature. This chapter will describe in greater detail the specific methods applied. The following parameters were investigated;
• Mechanical parameters: Young's modulus (E), strength(c) and apprent density (p) • Ultrasonic properties: velocity (V) and broadband ultrasonic attenuation (BUA) • Structural properties: permeability (k)
The experimental set-up to acquire these parameters will be described with special emphasis on the precautions taken to achieve accurate and reproducible readings. The in vitro measurements were performed in the following order:
1. Samples prepared. 2. Samples degassed.
3. Ultrasonic measurements (BUA and Velocity). 4. Samples defatted and degassed.
5. Ultrasonic measurements. 6. Permeability measured.
7. Mechanical properties (Young's modulus and strength) measured 8. Apparent density measured
4.2 Specimen
For the in vitro studies, proximal and distal bovine femi, human calcaneus and human vertebrae were investigated.
4.2.1 Preparation
The bovine samples were obtained fresh from a local butcher. No information on the sex, age and weight of the bovine samples were provided. The femi were cleaned and soft tissue removed. Large blocks of cancellous bone were then cut out using a band saw. These blocks were then cut down into 20 mm cubes under constant irrigation using a 220 grit diamond wafering blade, with fine smoothing on fine carborundum grit if necessary. These specimen were cut out at different locations from both the proximal and distal ends of the femi. The edges of the cubes were approximately in line with the longitudinal ( proximal-distal PD), anteroposterior (AP) and mediolateral(ML) axes of the bones from which they were obtained. The orientation with respect to the bone axis was noted and the sample's physical dimension measured.
The vertebrae specimens were obtained dried. No information of age, sex nor pathology were available. Cubes of variable sizes were cut out as described in the preceding paragraph. They were hydrated before any measurements were carried out. The calcaneus samples were obtained post mortem from 20 different cadavers(ten males and ten females) with age range of 59 to 90 years. No pathological information on the specimens was available.
• After removal of soft tissue, a coring drill with an internal diameter of 21 mm was used to remove samples in the mediolateral direction. This is approximately the area where clinical(m vivo) measurements are carried out.
• The cortical end surfaces were removed to leave a cylinder of trabecular bone using 220 grit diamond wafering blade under constant irrigation.
• The fat was then removed from the samples. This involves subjecting the specimen to high speed jet of water, and then compressed air. This process is repeated until no fat is visible. The specimen is then tumbled overnight in excess of 2:1 chloroform-methanol mixture.
For all samples, care was taken to produce smooth parallel surfaces. This is crucial for accurate determination of Young's modulus [Currey, 1970]. The specimens were kept
frozen ( -20°C) until required for testing, then they were thawed and allowed to attain room temperature before measurement.
During preparation, all specimen were kept wet and cool. All measurements were carried out at room temperature.
4.2.2 Specimen Degassing
Any slight introduction of air bubbles would increase the attenuation substantially since an air interface effectively acts as a total reflector. In the clinical situation ( in vivo), no air bubbles would be present. So, to ensure that no air bubbles are left inside the samples, they were thoroughly degassed under water using a water powered vacuum pump connected to a dessicator. The samples were left degassing overnight at a pressure of 30 inches of mercury (1 bar) on bowden gauge. The specimens were transferred to the water bath without exposure to air. A wetting agent (tepol) was applied to the water during degassing to enhance gas removal.
4.3 Ultrasonic Methods
Conventional ultrasonic technique are based on the pulse-echo technique which utilises a single transducer both to transmit and to receive the ultrasonic signals. An ultrasonic pulse generated by the transducer travels through the sample and is reflected from the far side interface, back into the transducer where it is detected. The pulse therefore travels twice through the measured sample. Due to the highly attenuating nature of cancellous bone, the reflected signal is very weak and thus difficult to detect. Therefore the pulse-echo technique is not applicable to cancellous bone and a transmission technique must be adopted. The transmission technique uses two transducers placed at opposite ends of the samples, one as the transmitter and the other as the receiver.
This technique has been incorporated into the Contact Ultrasonic Bone Analyser (CUBA) system [Langton et al., 1990b] which was used throughout this investigation. CUBA consisted of a portable PC interfaced to a spike generator (transmitter ) and digital receiver (Thurlby DSA 524) with dedicated menu driven software (Figure 4.1).
Two 1 MHz (nominal frequency), broadband ultrasonic transducers were mounted on a hand-held sliding calliper. One transducer acts as the transmitter and the other as the receiver. Two pairs of broadband ultrasonic transducers were used during the investigation; a 19 mm and 13 mm diameters. The former were used for phantom studies while the later for in vitro studies. The calliper was attached to a digital vernier gauge to measure specimen thickness. The transmitting transducer was excited by a fast rising 600V pulse (width 1 ps), producing a short RF pulse of broadband ultrasound (Figure E.6). This signal after propagating through the sample (submerged in water) is received by the other transducer positioned on the other side of the specimen.
Coupling is an important consideration when performing ultrasound measurement since an air interface effectively acts as a total reflector. Water provided the coupling in this case. The samples were submerged in a water tank and all the measurement carried out under water. Immersion contact technique was used in the in vitro studies reported in this thesis. Tap water was used with tepol detergent to improve sample wetting and to increase the cavitational threshold [Njeh , 1990]
At the start of the measurements, the system is switched on after setting up and initially allowed 30 minutes before commencing measurements. This allows the water to settle, air bubbles to disperse and the electronics to warm up. Samples were also allowed a setting time of 5 minutes in the water. This stabilising period is to allow air bubbles introduced by placing the sample to escape.