Growth Plate Measurement Device Team Members: Andrea Schnelle Barry Bass Rafael Connemara Adam Graf Client: Ellen Leiferman Norman Wilsman Advisor: Paul Thompson 12/13/02Abstract For this project we looked at methods to measure the differential elongation of the growth plate to quantify the timing pattern for a given rate of bone elongation in a lamb’s tibia. Past methods of measurement involved the use of fluorescent dyes and a Contact DVRT. These methods became inadequate in trying to precisely examine differential growth. For accurate measurements a device is needed that does not impede natural movement in its host or span the growth plate, measures up to 10 mm, produces a time intensive sampling rate every 1 to 5 minutes, and has an accuracy within 10-20µm per measurement. We have created a prototype consisting of a pair of inductors that are implanted on either side of the growth plate. One inductor has a controlled voltage passing through it that is detected by the second inductor. Displacement can be measured by the strength of the signal received by the second inductor.Design Problem Create a device to measure rate of growth across single growth plate in a young, actively growing lamb by time intensive sampling. Summary of the Design Information The first step in this project was to study the prior methods of bone growth to build upon existing concepts. The first method studied was the fluorescent dying. This consisted of precisely timed injections of oxytetracycline, a chemical that adheres to cells only in the growth plate (Figure 1). This chemical would be left behind as the bone grows so a second injection would show a displacement between the two bands of oxytetracycline in the bone. Dividing the displacement by the time between the two injections provides a growth rate. Dr. Norman Wilsman and Ellen Leiferman used this method of measurement. They found that this method had drawbacks, as it was not very precise. It only gave one measurement and the lamb would have to be killed in order to get the measurement. Figure 1: This picture is a close up of the growth plate. The image on the right shows the two lines of fluorescent dye. These lines are then measured to get a growth rate (Wilsman et al., 2002). The second method used to measure growth was the Contact DVRT built by Microstrain (www.microstrain.com). This device had an element spanning the growth plate in a lamb tibia and was attached by bone screws (Figure 2). It would send a signal of differential reluctance through leads to a computer outside the lamb. This could be translated into a growth rate. This system worked, but only for a maximum of a 3-day span. After that time the device would become over-grown with immune system cells and lose its function. It was also prone to lose its signal while the lamb was on its feet or moving around. This caused the data gathered during that time to be disregarded for study purposes. A plastic sleeve was slipped over the device in an attempt to stop the build up of scar tissue, but this did not work. Immune system cells still broke down the material and adhered to the device.Figure 2: This is a diagram of the Microstrain Contact DVRT. It is attached by two bone screws and has an element spanning the growth plate that is capable of measuring displacement by detecting the coils differential reluctance (Wilsman et al., 2002). Since the second method of measurement worked well except for a few drawbacks we decided to use that idea and improve upon the design. We did not want an element spanning the growth plate that could be disrupted by fibrous tissue, so we used magnetic wire to create coils. When a current is passed through the coil a magnetic field is produced that can be detected by a second coil. Therefore if we had two coils separated by a known initial distance a current could be passed through one coil and the resulting voltage could be sensed by the second coil. This provides a method to measure displacement in the growth plate of an elongating bone. Specifications While thinking of ways to approach the design of our device, our group thought a good place to start would be to make a list of all of the important specifications we needed to follow. One of the most important specifications that the device must follow is that it should not have any effect on the natural movements of the lamb. Currently a telemetry unit hovers above the lamb as it moves around its pen. There are no wires or leads that would cause the lamb to trip. The next specification that our device must follow is that it should have the ability to accurately measure displacement up to 10 millimeters. There are no other current non-contact devices that we found in our research that can make accurate measures of displacement at 10 millimeters. The accuracy of the device we create is also an important factor. In current contact devices, used in measuringdisplacement, average resolutions are between 10-20 micrometers per measurement. The accuracy of our device should be within this range. Another important aspect of the device is that it has the ability to make rapid measurement rates. The optimal rate of measurement would be about one measurement every two minutes. Rapid sample rates are very important for the application of this device because the purpose of the device is to show that growth of the tibia occurs in rapid spurts. The final important specification for the device is that it should not span the growth plate. Current contact devices have a limited period utility due to the fact that after short periods of time being exposed to physiological type environments fibrous scar tissue begins to encapsulate the device. To solve this problem, our group plans to implement a non-contact type device that eliminates this problem. Design Options Three alternative solutions were further analyzed beyond the brainstorming phase. These three alternatives were modifying the current design (contact DVRT), use a non-contact DVRT, or design a new non-contact device. Ideally the current device would be modified because it currently fits most of the specifications and this would also keep the cost down. As mentioned previously the current design does not operate as well after implantation because of tissue growth on the device, among other factors. The DVRT contains a ferrite core which has a tendency to shift within the body of the device. After