Spine CT Scan
The following section highlights the steps taken towards achieving the experimental aspects of the project.
4.1Specimen procurement and preparation
A porcine spine was obtained from Animal Organs for Research which is based in Brentwood, Essex (UK). They “supply a wide range of porcine & bovine organs for biomedical research and teaching purposes. The tissues originate from healthy animals culled for the meat industry. Organs are removed within a few minutes of slaughter and placed immediately on ice or into a wide range of physiological solutions for preservation.”[22] The spine obtained for this project was from a healthy adult pig weighing approximately 180kg. The spine was packaged in ice and transported to the university biomechanics lab within an hour.
Before placing into a freezer, it was necessary to remove part of the spine so that it may be placed into the freezer without bending it into an awkward position. It was known at the time that the sacrum region of the spine will not be needed and so it was removed. The spine was then wrapped in doubled polyethylene bags and placed into the freezer for animal organs at −20 °C as mentioned in the literature.[23]
The spine was removed from the freezer for the duration of performing the CT scan which lasted approximately two hours (section 5.1). Subsequently the spine was removed twice more to reduce the spinal column to the L1-L4 region and to reduce the soft tissue surrounding the spine, keeping the vertebrae and the three intervertebral discs in between, the facet capsules and the ligaments in the region in tact. This information is relevant as it is not exactly known what the effects of freezing and defrosting cycles have on the material properties of the spine, and thus it is important to reduce the number of cycles as much as possible.
The specimen was prepared in the biomechanics lab while observing the safety conduct required at all times. Extreme care was taken when using the sharp surgical instruments such as scalpels and saws in particular, and all instruments used were washed with an alcohol solution after use and left to dry. Lab coat and gloves were worn at all times within the biomechanics lab. A complete document outlining the working practices of the biomechanics laboratory provided by the head of section was read prior to entering the lab to ensure safe practice.
4.2Designed and manufactured fixtures for experiment
In order to affix the specimen into the Instron loading machine, end plates had to be designed in which the specimen would be cemented. The joints that are placed into the Instron machine have been manufactured previously and fit exactly and therefore will be re-used. The other end of the plates would attach to these fixtures. Several conceptual designs were contemplated. In order to achieve bending, moments arms also had to be manufactured. Therefore, the final design was such that the end plates would attach to the moment arms by screws that can be placed in several holes so that the position of the end plates can be varied to achieve compression only and bending only. The moment length can also be varied along the moment arm. The end plates could attach to the moment arms in two perpendicular directions so that both flexion/extension and right/left lateral bending could be achieved. The end plates and moment arms were manufactured from steel.
The engineering drawings of the end plates and moment arms are in Appendix A.
The moment arms were made by sawing out the shape from a steel block. A mill was used to reduce the dimensions and hence the weight of the arms. The holes were drilled through at the required locations using a vertical drill.
The end plates were manufactures by drilling out the bulk of material by using a drill but on the lathe and the remainder turned down and faced-off using a cutter. Coolant was used throughout these procedures and the appropriate safety procedures observed.
Spacers were also manufactured to provide the sufficient constraints required to prevent rotation and axial motion along the rod joining it joint.
4.3Cementing the bone into the end plates
The specimen was removed from the freezer the day before the experiment and was cemented into the end plate cups. Vaseline was used on the inside surface of the cups to allow easier removal of the cement if required. The cement was prepared from a white powder of polymethylmethacrylate (PMMA) and liquid methylmethacrylate (MMA). When mixed they yield a dough-like cement that gradually hardens through an exothermic reaction.
The substances were mixed in the corner of a polyethylene bag squeezed tight around the finger. The mixture was mixed till it was felt to begin hardening. The corner of the bag was cut open and the mixture was allowed to pour into the plate with the spinal segment held in place. The MMA liquid has a very strong smell and inhalation close to it was avoided as it is a possible carcinogen even though is used to affix implants in patients. The spinal segment was held so that the caudal end of L4 was embedded into the cement, the disc between L2 and L3 was aligned horizontally as outline in the literature [24] and the orientation of the spine corresponds with the holes of the plate for the flexion/extension and lateral bending. The spine was held in position for a further five minutes to ensure the cement sets. The cement doesn't so much bond to the surfaces like glue but rather acts as grout in filling the space between the surface of the cup and the surface of the bone and thus preventing motion. The temperature of the cement increased to approximately 70 °C as the reaction took place. This is unfortunate as it damages the bone in that area and affects the tissue properties. To try and cool the spine it was sprayed with cold water throughout. The same procedure was repeated for the cranial end of L1. Care was taken to not stress the spinal segment due to weight of the plate on the L4 end as it was being cemented at the L1 end. Prior to testing the specimen is usually thawed at 8 °C for 16 hours.[25] Therefore after the cementing was completed, a damp paper towel was wrapped around the spine before placing it into the fridge the day before the experiment.
4.4Experimental procedure
The experiment was initially planned to take place on the hydraulic Instron machine in the biomechanics lab. However, when it was realised that the Tekscan sensors available for this project are incapable of measuring the pressure at the facets due to the geometry, it was decided that it was no longer necessary to carry out the torsion test for which the hydraulic machine was required. Thus the screw-driven Instron machine was used instead throughout the experiment. Before assembling the parts together and fixing them into the Instron machine, incisions were made at the three discs into which the Tekscan sensors fit.
The Tekscan sensors provided for this project are thin, flexible, tactile grid-based sensors with a sensing area of 15mm x 15mm. Each “sensor consists of two thin, flexible polyester sheets which have electrically conductive electrodes deposited in varying patterns”, one of which forms rows and the other columns forming a grid. The spacing between each column and row is approximately 1mm. A thin, semi-conductive coating between the two films changes in resistance as force is applied at each of the intersecting points. This change in the current flow through the conductive leads is measured and displayed on the computer using the I-scan software.[26] As the pressure increases, the resistance decreases. The sensors are capable of measuring pressures significantly higher than expected for this experiment. Four sensors could be used at any one time as only one handle that connects the sensors to the data logger was available. As the pressures at the facets were no longer to be measured, three sensors were used at each of the intervertebral disc positions.
The assembly was then attached to the Instron machine for testing. The Instron machine was pre-programmed for each of the tests. Since pressures at the facets was no longer achievable, the torsion test was not carried out and the extension case was also not carried out as this will only relieve the pressure sensors located at the discs. Therefore, only four of the tests could be carried out, that is, compression, flexion and right/left lateral bending. The inputs for these four were as follows:
Pre-condition: Compressive Load: Increase to 500 N incrementally [27]
Load Rate: 2N/s
Hold at 250 N for 5mins. Hold at 500 N for 5 mins. Unload
Compression:Load up to 1 kN in 100 N increments
Loading rate: 10 N/s
Hold at every 100 N for 30 seconds. Unload
Repeat further two times
Bending (flexion, right/left lateral bending):
Apply pure moments ± 1, ± 2.5, ± 5, ± 7.5 and ± 10 Nm [28]
Loading rate: 1.0o/s (to avoid viscoelastic superposition of the motion response)
Hold at every increment for 30 seconds. Unload
Repeat further two times
For the flexibility testing, specimens undergo 2.5 loading cycles whereas the first 1.5 serve as precycles in order to minimise the effect of viscoelastic response. The last cycle is used for data evaluation.[29]
The data logger recorded the pressure distribution as movie files. Also, at the intervals at which the load was held image captures were taken.
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