Abstract
Introduction Dynamically identifying the distribution of pressure between any two given surfaces such as articulating joints is of fundamental importance in understanding their interaction. The purpose of this laboratory study was to assess the potential of a dynamic pressure measurement system, Tekscan. ( I-Scan 5076, Tekscan Inc., MA, USA) via a study which observed the changes in the load profile through the vertebral body of harvested ovine lumbar functional spinal units (FSU’s) with a created defect in the intervertebral disc.
The system was used to determine pressure distributions in isolated vertebral bodies inferior to the disc, during axial compression of normal and injured discs of an ovine functional spinal unit.
Methods Four ovine lumbar segments L1-L3 were harvested The superior vertebral body (VB) remained complete, whilst the inferior VB was sectioned 2mm from the endplate and the surface smoothed using emery paper in order to achieve maximum contact area. The neutral axis of bending for each specimen was identified and marked. In accordance with the manufacturer guidelines, the sensor was conditioned and calibrated between 20-200N of load. Testing was carried out in a materials testing machine (Instron 8511, Instron, High Wycombe, UK), where 200N of axial load was applied through the FSU and a snapshot of the instantaneous pressure distribution was taken. A 12 x 2 mm gap defect was created in the right ventro-lateral (2 specimens) and the right lateral (2 specimens) aspect of the IVD. The specimens were returned to the Instron and 200N of load was applied axially through the NAB. A recorded image of the pressure footprint was taken.
Results Comparing the recorded colour-coded images together with their centroids of force of the pre- and post-injury pressure distributions of the vertebral bodies, it was clearly evident that there was a major shift of the load through the IVD. As predicted and as seen in the pressure footprint, the pressure shifted in the opposing direction of the injury in order to maintain a balanced system. A pressure reading validation was also carried out with the use of the Instron, where the experimental pressure of the sensor was within 3% of the NATA calibrated load cell.
Discussion The system was used to sample pressure in real time and display it as a 3D colour-coded map, allowing for visualisation of normal pressure distributions. The associated software has numerous aids and functions, allowing real-time visualisation of the dynamic forces and the balance of forces across two interacting surfaces, making the system an invaluable analytical tool.
The Tekscan system will be used to observe the effect of disc injury on the pressure distribution of the adjacent vertebral body. The relationship between the pressure distribution across the vertebral body and bone architecture will also be studied
This study illustrated that this system is a valid tool for qualitatively and quantitatively assessing dynamic pressure distributions.
The abstracts were prepared by Professor Bruce McPhee. Correspondence should be addressed to him at Orthopaedics Division, The University of Queensland, Clinical Sciences Building, Royal Brisbane & Women’s Hospital, Herston, Qld, Australia
