Factors affecting intradiscal pressure measurement during in vitro biomechanical tests

Abstract

ObjectivesTo assess the reliability of intradiscal pressure measurement during in vitro biomechanical testing. In particular, the variability of measurements will be assessed for repeated measures by considering the effect of specimens and of freezing/thawing cycles.MethodsThirty-six functional units from 8 porcine spines (S1: T7-T8, S2: T9-T10, S3: T12-T11, S4: T14-T13, S5: L1-L2 and S6: L3-L4) have been used. The intervertebral discs were measured to obtain the frontal and sagittal dimensions. These measurements helped locate the center of the disc where a modified catheter was positioned. A fiber optic pressure sensor (measuring range: -0.1 to 17 bar) (360HP, SAMBA Sensors, Sweden) was then inserted into the catheter. The specimens were divided into 3 groups: 1) fresh (F), 2) after one freeze/thaw cycle (C1) and 3) after 2 freeze/thaw cycles (C2). These groups were divided in two, depending on whether specimens were subjected to 400 N axial loading or not. Ten measurements (insertion of the sensor for a period of one minute, then removal) were taken for each case. Statistical analyses evaluated the influence of porcine specimen and the vertebral level using a MANOVA. The effect of repeated measurements was evaluated with ANOVA. The difference between freeze/thaw cycles were analysed with U Mann-Whitney test (P≤0.05).ResultsWithout axial loading, the F group showed 365 mbar intradiscal pressure, 473 mbar for the C1 group, and 391 mbar for the C2 group. With 400N axial load, the F group showed intradiscal pressure of 10610 mbar, the C1 group 10132 mbar, the C2 group 12074 mbar. The statistical analysis shows a significant influence of the porcine specimen (p<0.001), with or without axial loading and of the vertebral level with (p=0.048) and without load (p<0.001). The results were also significantly different between the freeze/thaw cycles, with (p<0.001) and without load (p=0.033). Repeated measurement (without load p = 0.82 and with p = 0.56) did not show significant influence.ConclusionsThe results tend to support that freezing/thawing cycles can affect intradiscal pressure measurement with significant inter-specimen variability. The use of the same specimen as its own control during in vitro biomechanical testing could be recommended.

Biomechanical evaluation of cervical disc replacement with a novel prosthesis based on the physiological curvature of endplate

“…First, there is no doubt that the fresh cadaveric specimens are the optimal choice for in vitro biomechanical testing. Second, freezing may affect the IDP measurement, especially multiple freeze/thaw cycles, as IDP is based on the hydrostatic behaviour of the nucleus pulposus [ 30 ]. Again, repeated measurements (up to ten) on a single specimen at different testing conditions do not significantly affect the IDP [ 30 ].…”

“…Second, freezing may affect the IDP measurement, especially multiple freeze/thaw cycles, as IDP is based on the hydrostatic behaviour of the nucleus pulposus [ 30 ]. Again, repeated measurements (up to ten) on a single specimen at different testing conditions do not significantly affect the IDP [ 30 ]. However, factors affecting IDP measurement during in vitro biomechanical testing are complex and diverse; further investigations are still needed.…”

Single-Level Rigid Fixation Combined with Coflex: A Biomechanical Study

“…For these tests, an apparatus was used in which a system of cables and pulleys imparts nondestructive, nonconstraining torque in conjunction with a standard biomechanical test system (Zwick/Roell, Germany, provided by the Mechanics Laboratory of Shanghai University). An axial load of 500 N was applied to the specimen using a Zwick/Roell universal testing machine [ 11 ] in a rostral-to-caudal direction. A torque of ±8 Nm was applied in all directions (flexion-extension, lateral bending, and axial rotation) at a speed of 0.5°/s.…”