DOI: 10.25549/usctheses-c127-650531
title: Development, validation and testing of a new sensor array for intra-articular pressure measurement: in-vitro human lumbar spine intra-articular facet testing

description: There is a very high and increasing frequency of back pain in modern society with an estimate annual cost of $100 billion dollars just in the United States.  The precise etiology of low back pain lacks a general consensus.  However, the facet joints have been shown to be a significant source of spinal pain, and specifically low back pain.  Despite being a significant source of pain, very little accurate data is available on the loads within the facet joints. ❧ This study’s primary goal was to provide improved data, thus facilitating a better understanding of lumbar spine intra-articular facet loads during movement.  This was ultimately accomplished by the development, validation and implementation of a new method for direct measurement of intra-articular load.  It was initially thought that a better understanding of lumbar spine intra-articular facet loads during movement could be accomplished with the application of existing technology.  The preliminary phases of this study involved evaluating a large volume of literature related to previously published methods and techniques purporting to quantify intra-articular load, meeting and discussing design requirements with various manufacturers of potential technology, and finally bench top testing and evaluating the more plausible technologies.  Despite a substantial volume of published literature utilizing many of these technologies for this task and similar measurements in other articular joints (both human and non-human), none of the existing technologies were found to have the necessary geometry and/or were capable of taking accurate measurements when curved to physiologically prevalent radii.  These deficiencies in the existing technologies necessitated the development of an appropriate method to accurately measure intra-articular pressures. ❧ A new sensor array intended to accurately and directly measure both the spatial and temporal distributions of pressures within a highly curved intra-articular joint was developed and tested.  To evaluate performance of the new sensor array for application within intra-articular joints generally, and specifically to fit within the relatively restrictive space of the lumbar spine facet joint, geometric constraints of length, width, thickness and sensor spatial resolution were evaluated.  Additionally, the effects of sensor array curvature, frequency response, linearity, drift, hysteresis, repeatability, and total system cost were assessed. ❧ The new sensor array was approximately 0.6mm in thickness, scalable to below the nominal 12 mm wide by 15 mm high lumbar spine facet joint size, offered no inherent limitations on the number or spacing of the sensors with less than 1.7% cross talk with the sensors positioned immediately adjacent to one another. No difference was observed in sensor performance down to a radius of curvature of 7 mm and a 0.66±0.97% change in sensor sensitivity was observed at a radius of 5.5 mm.  The sensor array had less than 0.07 dB signal loss up to a loading frequency of 5.5 Hz, linearity was 0.58±0.13% full scale (FS), drift was less than 0.2% FS at 250 s and less than 0.6% FS at 700 s, and hysteresis was 0.78±0.18%.  Repeatability was excellent with a coefficient of variation less than 2% at pressures between 0 and 1.000 MPa.  Total system cost was relatively small as standard commercially available data acquisition systems could be utilized, with no specialized software, and individual sensors within an array could be replaced as needed.  The new sensor array had small and scalable geometry and very acceptable intrinsic performance including minimal to no alteration in performance at physiologically relevant ranges of joint curvature. ❧ The most limiting attribute of the sensor array is durability.  Although this was improved during the study with judicious selection of sensor orientation and modifications of the individual sensor construction via discussions with the manufacture, durability remained less than ideal.  This limitation was tempered by the fact that the componentized nature of the sensor array allowed easy replacement of the individual sensor elements within a given array if one or more sensors failed.  This characteristic is beneficial especially when compared to other commonly commercially available arrays such as those from Tekscan (FSR-T) and Novel (FSC-N) which are constructed by the manufacturer in fixed configurations and do not allow either redistribution or replacement of individual sensor elements within an array. ❧ Preliminary in vitro data is presented demonstrating the utility of the new sensor array in quantifying temporal and spatial distributions of pressure within the L4-5 facet joint.  Preliminary results are generally in line with singular peak pressure measurements from pressure sensitive film testing with peak pressures measured in the current study at between 1,210 kPa and 3,059 kPa.  Additionally, the distribution of pressure matched prior studies in that the measured facet pressure increased in extension, decreased in flexion, and the center of pressure migrated inferiorly and medially under increasing extension moments.  Initially, in vitro durability was problematic with very high initial sensor failure rates.  Manufacturing changes and orientation optimization of each individual sensor relative to load direction improved durability.  Durability was ultimately considered acceptable in light of the ease and relatively low cost of individual sensor replacement. ❧ The utility of the sensor in more accurately quantifying spatial and temporal changes in lumbar spine facet intra-articular pressure was demonstrated with testing six fresh-frozen human cadaveric lumbosacral specimens under pure moment bending (±10Nm).  The new sensor was inserted in the L4-5 facet joints.  L4-5 facet contact pressures were continuously measured at seven locations within the facet.  Center of pressure at various phases of loading was calculated.  The data demonstrated an increase in facet pressure with increasing extension moments and displacements.  Facet contact pressure increased relatively linearly in proportion to applied bending moment up to approximately 2-3 degrees of L4-5 extension and approximately 7-8 Nm of extension moment.  The highest average maximum pressures of 1,087 kPa were found in the midline sensor 2 mm medial of the midpoint and 973 kPa in the most inferior midline sensor.  The most superior midline sensor always had the lowest average peak pressures during extension.  The center of pressure started very near the anatomical center of the facet and migrated medially and inferior under increasing extension moments. ❧ The demonstrated functionality of the new sensors in the relatively small and sharply curved human lumbar spine facet joint should ensure viability and utility of the sensor array in other less geometrically demanding joints and surface interfaces such as the hip, knee and ankle joints.  The sensors could also be used as a source of tactile feedback in prosthetic designs or for external measurement of a portion of the body, such as the foot interacting with the ground or other objects.