Effect of Potting Technique on the Measurement of Six Degree-of-Freedom Viscoelastic Properties of Human Lumbar Spine Segments

Abstract

Polymethyl methacrylate (PMMA) and Wood's Metal are fixation media for biomechanical testing; however, the effect of each potting medium on the measured six degree-of-freedom (DOF) mechanical properties of human lumbar intervertebral discs is unknown. The first aim of this study was to compare the measured 6DOF elastic and viscoelastic properties of the disc when embedded in PMMA compared to repotting in Wood's Metal. The second aim was to compare the surface temperature of the disc when potted with PMMA and Wood's Metal. Six human lumbar functional spinal units (FSUs) were first potted in PMMA, and subjected to overnight preload in a saline bath at 37 °C followed by five haversine loading cycles at 0.1 Hz in each of 6DOF loading directions (compression, left/right lateral bending, flexion, extension, left/right axial rotation, anterior/posterior, and lateral shear). Each specimen was then repotted in Wood's Metal and subjected to a 2-h re-equilibrating preload followed by repeating the same 6DOF tests. Outcome measures of stiffness and phase angle were calculated from the final loading cycle in each DOF and were expressed as normalized percentages relative to PMMA (100%). Disc surface temperatures (anterior, left/right lateral) were measured during potting. Paired t-tests (with alpha adjusted for multiple DOF) were conducted to compare the differences in each outcome parameter between PMMA and Wood's Metal. No significant differences in stiffness or phase angle were found between PMMA and Wood's Metal. On average, the largest trending differences were found in the shear DOFs for both stiffness (approximately 35% greater for Wood's Metal compared to PMMA) and phase angle (approximately 15% greater for Wood's Metal). A significant difference in disc temperature was found at the anterior surface after potting with Wood's Metal compared to PMMA, which did not exceed 26 °C. Wood's Metal is linear elastic, stiffer than PMMA and may reduce measurement artifact of potting medium, particularly in the shear directions. Furthermore, it is easier to remove than PMMA, reuseable, and cost effective.

Effect of degeneration on the six degree of freedom mechanical properties of human lumbar spine segments

“…It was found that the specimen viscoelastic properties did return to initial conditions before each loading direction, with percentage differences relative to the first reference test averaging 12%, and 9% for stiffness, and phase angle, respectively, over the testing sequence. These differences were within the range of systematic variations found in other studies and believed not to be of clinical significance …”

“…These differences were within the range of systematic variations found in other studies and believed not to be of clinical significance. 28 The comparison of the stiffness's and phase angles from the present study can be made to a similar study, which used a similar testing protocol and 6DOF hexapod robot. 25 However, that study tested isolated vertebra-disc-vertebra segments without the posterior elements and there were very similar magnitudes of stiffness for lateral and posterior shear, compression and lateral bending.…”

“…Axial and lateral radiographs were taken of all specimens to determine the instantaneous axis of rotation of the disc relative to the cups, which was used as the center of rotation for the rotational tests . The vertebral bodies were then embedded in fixation tooling using Wood's Metal after which the FSU underwent a comprehensive sequence of 6DOF mechanical testing (described below). The potting fixtures consisted of two 316 grade stainless steel cups for each vertebra.…”

“…The ranges over which stiffness was calculated for each DOF were: 0.4–0.58 mm for all shear tests, 2°–2.8° for flexion and lateral bending, 1.25°–0.85° for extension, 1.5°–1.8° for axial rotation, and 0.77–1 MPa for axial compression. Phase angles for each DOF were calculated between the input displacements and measured forces for all cycles using the cross spectral density estimate function (MATLAB: CSD.m) . Data from the combined flexion and extension test were not included in the study, since the individual flexion and extension tests were analyzed.…”

“…m). 25,28 Data from the combined flexion and extension test were not included in the study, since the individual flexion and extension tests were analyzed.…”