|Year : 2017 | Volume
| Issue : 2 | Page : 29-34
The influence of facet joints on intervertebral disc pressures under complex loading
Celina A Pezowicz
Department of Biomedical Engineering, Mechatronics and Theory of Mechanisms, Wroclaw University of Science and Technology, ul. Łukasiewicza 7/9, 50-371 Wrocław, Poland
|Date of Web Publication||15-Sep-2017|
Celina A Pezowicz
Department of Biomedical Engineering, Mechatronics and Theory of Mechanisms, Wroclaw University of Science and Technology, ul. Łukasiewicza 7/9, 50.371 Wrocław
Source of Support: None, Conflict of Interest: None
Purpose: The purpose of the study was to determine the influence of the loading history on changes in the recorded pressure in the intervertebral disc (IVD) and the influence of changes in the support conditions as a result of the removal of facet joints on changes in pressure. Materials and Methods: The tests were carried out on motion segments of the thoracic spine for the physiological system (with articular processes preserved) and the pathological system (with articular processes removed). Our analysis assessed changes in intradiscal pressure for three consecutive loadings, i.e., cyclic compression loading, compression loading constant in time, and unloading. The tests were conducted while maintaining full hydration of the IVD. Results: During cyclic loading, we observed a progressive decrease in intradiscal pressure. Suppression of the pressure decrease is clearly visible in the system with the preserved complete articular triad, which should be regarded as characteristic of a properly functioning system. Constant compression loading caused a progressive decrease of intradiscal pressure in all tested systems. In general, greater decreases were recorded in the systems with articular processes removed. A decrease in intradiscal pressure leads to a simultaneous change in the stiffness of the analyzed systems. The process of relaxation was unstable and showed an upward trend throughout the unloading period. Conclusion: The obtained results clearly indicate that facet joints play an important role in the transmission of loads by the spinal column and that those structures play a protective role in the overload conditions of the IVD.
Keywords: Cyclic loading, facet joints, intervertebral disc, intradiscal pressure, recovery, spine
|How to cite this article:|
Pezowicz CA. The influence of facet joints on intervertebral disc pressures under complex loading. J Spinal Stud Surg 2017;1:29-34
| Introduction|| |
During normal physiological activities, the human body is subjected to constant cyclic variable loading, constant pressure loading, and periods of unloading (relaxation). All those types of loading, so different in terms of the amount and length of action, cause a specific response from the load-bearing structures of the spine, including the intervertebral disc (IVD).
Changes in the components of the functional spinal unit, which is the basic motion segments of the spine and in particular in the IVD, are the main source of changes in static and dynamic functioning of the spine. The processes occurring in the IVD are associated with mechanical as well as biomechanical factors. They result in, among others, a reduction of the water content of the disc  and, consequently, a loss of its primary functions, i.e., cushioning of the spine and ability to transmit loads in the spinal column. For this reason, an understanding of the distribution of intradiscal pressure is essential for a fuller understanding of the complex processes occurring in the spine.
Studies have so far indicated that the compression force resulting from the loads acting on the spine is transmitted mostly by the disc and, to a smaller extent, by articular processes, where the percentage share of the articular processes depends on the performed motor function (bending forward/backward, rotation). On the other hand, under action of the shear force, the IVD plays a relatively less significant role compared to articular processes.
Many in vitro experimental studies ,,,, confirm the observations that the compression load acting in the lumbar region is transmitted by both the articular processes and the IVD. In a system simulating long (approximately 3 h) standing in upright position, 16% of the acting load is transmitted through articular processes, while during short-term loading (approximately 5 min), it corresponds to just 4%.
Analyses of biomechanics of the spine in complex loading systems (used to simulate axial compression under simultaneous bending moment in the anteroposterior and lateral planes) also indicate a very important role of facet joints in active transmission of loads. Operations of the complex system of forces result in transmission by articular processes of between 10% and 40% of the applied load.,, At the same time, the force acting on articular processes during backward and forward bending increases progressively with time.,,
Similar relationships are observed in analyses using numerical simulations.,,, Undamaged, properly functioning articular processes transmit between 3% and 25% of the acting compression load in the physiological system, and for the simulated joint inflammation, the load on articular processes increases up to 47%. Additional backward bending moment leads to a significantly increased loading of articular process in the range of 10%–30%. These results clearly show that the articular processes transmit a substantial part of the applied load during bending and that additional impact of the compression and shearing forces leads to a further increase in the load acting on these structural elements.
The above data show that the posterior column of the spine in the form of articular processes significantly supports and unloads the IVD during transmission of time-varying loads.
A parameter directly characterizing the behavior of the IVD is the pressure inside it. Analysis of intradiscal pressure enables accurate determination of the pressure distribution profile in its cross-section or determination of the impact of degenerative changes on the change to this parameter.,
The information from analysis of intradiscal pressure can also be used to determine the extent to which changes in support of the posterior column (on articular processes) can influence the amount and distribution of pressure.
Despite many studies that undertake analyses of the impact of various factors on changes in the IVD, there is lack of explanation of the role of posterior spinal joints in the biomechanics of the IVD.
The purpose of this study was to determine the impact of changes under the conditions of support of the spinal column and the loading history (in the form of simulated walking, constant loading, and relaxation) on changes in pressure in the IVD. An important part of the research was to clarify the role of interarticular joints in changes generated in the IVD, including its relaxation ability (restoration of the initial conditions to hydration, after the long-term compression loading comes to an end) in variable support systems.
| Materials and Methods|| |
The researchers utilized postmortem specimens of the human thoracic spine (aged 20–52 years).
All specimens for tests were subjected to X-ray analysis (X-ray images) to identify and eliminate specimens showing degenerative or pathological changes. Radiological documentation of each spinal specimen was performed immediately after sampling of the research material. If X-ray analysis did not reveal any destruction of the bone structures of the sampled spinal region, the specimen was admitted for biomechanical tests.
The specimens were stripped of soft tissue in the form of muscles, leaving only vertebrae, IVDs, interarticular joints, and ligaments. The material was then divided into individual motion segments consisting of two adjacent vertebrae with the IVD between them. The tests were conducted on 12 specimens of the thoracic spine [Table 1].
The prepared research material was stored until the test day in double plastic packaging at a temperature of − 20°C. Defrosting took place at room temperature several hours before the start of tests.
The tests were conducted for two systems:
- Segments with facet joints preserved (physiological system – intact)
- Segments with facet joints removed (pathological system – damaged).
The first system represented a physiological method of transmitting loads by articular processes and the IVD, the so-called three-joint-complex. In the second system, loads were transmitted only through the IVD without support on articular processes.
During the experimental analysis, the examined motion segments were loaded with variable forces acting in the axis of the spine. The main objective was to simulate several basic loads acting in the course of normal human physiological functions, i.e., walking, relaxation, and prolonged constant loading in the axis of the spine. This task was carried out by the application of consecutive loads (loading protocol):
- Cyclic loading (3360 cycles) with a frequency of 1 Hz (to simulate normal walking) and an amplitude of 1 mm
- Constant compression loading period (1 h) – simulating loads characteristic of load bearing
- Recovery period (1 h) – simulating relaxation.
Before execution of the tests, each specimen was loaded with initial compression strength equal to 60 N for 30 min in an environment of full hydration. This action was intended to create reproducible conditions for all examined specimens, stabilize and supplement the specimens, and restore of the high level of hydration in IVDs.
The main analyzed parameter was changes in pressure occurring in the IVD during the entire cycle of motion segment loading. Measurements of IVD pressure utilized miniature strain gauges. The gauge was 1.5 mm in diameter, 0.3 mm in depth, and its measurement range was 3.5 MPa.,
The time span of the test for a single motion segment was >3 h. Therefore, to approximate the test set-up to physiological conditions, where a highly hydrated structure of the IVD can continuously replenish tissue fluid during both work and relaxation, we created a test set-up that ensured full hydration of the working environment.
The test set-up consisted of a cuvette and two clamps: upper and lower, which were fastened to the material testing machine MTS 858 Mini Bionix. The test specimens were immersed in 15% saline solution.
Statistical analyses were performed on the ultimate-dependent variables such as force, stiffness, and intradiscal pressure using analysis of variance (ANOVA) and correlation analysis. Based on the ANOVA results, tests using the t-test were performed to compare individual differences between means for datasets varying by age, rib geometry, and rib level. Significance was determined by a P value of 0.05 or less.
This study was approved by the Human Research Ethical Committee at the University of Medical Sciences in Wroclaw.
| Results|| |
Changes in IVD pressure were analyzed quantitatively with respect to the tested system: with either intact or removal facet joints.
An analysis of changes in pressure distribution during cyclic loading was grouped into five series (consisting of an average of 200 cycles each).
In a physiologically normal system (with articular processes preserved), cyclic compression loading caused a large drop in pressure after the first loading series [Figure 1] (left)]. The difference between the first and second series was 15% of the initial pressure with an average pressure of the first series equal to 0.20 ± 0.06 MPa. In the subsequent loading series, there was a slight decrease in pressure from 1.7% to 0.6%. The pressure recorded in the last cyclic loading series amounted to 0.17 ± 0.07 MPa.
|Figure 1: Mean intradiscal pressure during cyclic loading: left - in the successive series of cycles (error bars show standard deviations of the 200 measurements for each 790 cycles), right - for both study groups; (*statistically significant differences P < 0.05).|
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As shown in [Figure 1] (right), removal of articular processes causes a significant increase in intradiscal pressure. The average pressure for the entire cyclic loading was 0.18 ± 0.03 MPa for the system with preserved articular processes and 0.31 ± 0.04 MPa after removal of articular processes (a statistically significant difference, P < 0.001).
Removal of support on articular processes resulted in a uniform decrease of pressure in the successive series of cyclic loading. After the initial 14% drop, occurring after the first series, we recorded approximately 3% decrease after each subsequent loading series.
A decrease in intradiscal pressure leads to a change in the stiffness of the analyzed systems [Figure 2].
|Figure 2: Stiffness changes as a function of the number of load cycles for the functional spinal unit with and without connection in the facet joint. Error bars show standard deviations of 200 measurements for each 790 cycles.|
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Higher stiffness values are found in the physiological system, which maintains support on articular processes: 205.1 ± 47.3 kNm and 248.1 ± 55.4 kNm (for, respectively, the first and the last loading series; the statistically significant differences, P < 0.001). Removal of the processes causes on average of 23% difference in stiffness in comparison to the intact system: 151.2 ± 32.4 kNm and 187.5 ± 44.5 kNm (for, respectively, the first and the last loading series; the statistically significant differences, P < 0.001).
Constant compression loading
After completion of cyclic loading, the tested systems were subjected to constant compression loading. [Figure 3] shows a sample creep curve for the system with articular processes preserved.
|Figure 3: Example of relaxation curve after cyclic loading for intact spinal unit.|
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In both tested systems, the recorded pressure was decreasing throughout constant loading, with a characteristic large drop in the initial creep stage (10–15 min). The average rate of pressure decrease in the first few minutes was approximately 0.1 kPa/min for the intact specimen and 1 kPa/min for the system with articular processes removed.
For the system with articular processes preserved, the average pressure decrease during the 60-min creep was 0.14 ± 0.03 MPa. For the system with articular processes removed, the corresponding average pressure decrease was 0.21 ± 0.07MPa [Figure 4].
|Figure 4: Average pressure decrease during constant compression loading depending on the tested system (*statistically significant differences P < 0.05).|
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During the 60-min unloading, we observed a continuous, progressive increase in IVD pressure. The largest increase in pressure occurred in the first 10 min of unloading. For the system with processes, it was in the range of 0.03–0.06 MPa.
[Figure 5] shows average increases in pressure during the 60-min relaxation process. The increase in pressure amounted to 0.09 ± 0.05 MPa for the system with articular processes and 0.16 ± 0.09 MPa for the system without articular processes.
|Figure 5: The average increase in pressure during relaxation in relation to the tested system.|
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| Discussion|| |
The completed tests demonstrated the importance of facet joints in transmission of loads through the spine.
The presented results explain the phenomena associated with maintenance and loss of stability of the spinal column depending on the existing loading conditions. The results show that interarticular joints play an important role in maintaining equilibrium in the distribution of load-induced forces and pressures and have a significant role in the processes taking place in the IVD. The lack of support on facet joints not only results in the loss of an important fulcrum of spinal column but also influences indirectly the IVD, in particular the process of fluid flow. This phenomenon is important for tissue metabolism but also determines self-stabilization of the disc. The loads transmitted from stiff vertebrae to IVD structures cause an increase in intradiscal pressure and a simultaneous pressure of the nucleus pulposus on annuli fibrosi, which become deformed. The correct ratio of the water content to the acting load maintains a stable arrangement between the IVD and the adjacent vertebrae.
As demonstrated by the results of numerous studies,,, the impact of constant compression loading and long-term cycling loading leads to water loss from disc structures, reducing their height,, and increasing the disc circumference (outward bulging). These changes are caused by “squeezing out” of fluid and deformation resulting from creep of annulus fibrosus.,, Constant loading leads to many changes, which under physiological conditions revert to the preloading state during relaxation of tissues (during unloading in recumbent position), probably due to rehydration of disc tissues.,
While the biomechanics of the IVD under load is discussed relatively frequently, analyses of the phenomena taking place during unloading are rarely performed inin vitro studies. It is not known if mechanisms restoring preload values of the disc appear directly after unloading and if viscoelastic properties of the disc can be estimated based on cyclic loads and relaxation inin vitro studies. The test results presented above characterize to some extent changes in hydration that occur during unloading through an increase in the intradiscal pressure. It was further found that different loading lengths affect the dynamics of IVD hydration through a reduction of the water content. It is also important that during 1-h unloading the process of hydration was not stabilized, showing the characteristics of a progressive process. As shown by Johannessen, full restoration of the initial state occurred only after several hours of unloading.
An important factor determining the correct process of water flow and hydration of the IVD during unloading is the maintenance of full support on articular processes. It is important for stability and gradual, progressive increase in intradiscal pressure. The conducted tests showed that after removal of articular processes, the increase in pressure during relaxation was uneven, even erratic, which indicates that the system supporting the spinal column is unstable. Removal of support on articular processes significantly interferes with the hydration process (through reduced fluid flow and reduced height of the IVD), which in the course of further loading and incomplete unloading (insufficient to obtain the initial state of disc hydration) will get worse, leading to a loss of the basic load-bearing and cushioning functions.
This indicates a very important role of articular processes not only in transmission of loads but also in the process of IVD hydration.
Test results of cyclic loading and constant compression loading show that articular processes transmit as much as 40% of the load acting on the motion segment.
The presented values are greater than the values obtained in other studies, according to which articular processes transmit between 10% and 40% of the compression load acting on the motion segment of the spine.,,These differences are due to, among others, the loading method and the testing conditions. In this study, the tests were carried out for a high-cyclic compression loading and constant compression loading, which may simulate fatigue processes, while in the cited studies, the data relate to short-term static compression loading (a single load-unload loop), or constant compression loading. The working environment of the test object, i.e., the state of complete hydration (as in the presented study) or just maintenance of relative humidity of the tissues (spraying the specimens during the tests), may also be an important factor affecting differences in the obtained results.
Furthermore, the changes in stiffness during cyclic loading and constant compression loading indicate a significant role of the facet joint in the process of transmission of loads. Preservation of the complete articular triad generates higher stiffness compared to a damaged system. The increase in stiffness observed during cyclic loading is consistent with the results of other studies.,,, A decrease in stiffness in damaged systems with a simultaneous pressure decrease may consequently lead to instability of the spinal column. Similar dependencies related to the influence of posterior vertebral elements on stiffness of the tested systems were also observed in a number of other studies.,
As mentioned above, pressure changes generated by variable loading conditions determine the flow of fluid between the IVD and the surrounding tissues. The IVD is an avascular structure; therefore, its feeding mechanism is based on flow of fluid between the vertebrae and discs.
Studies by Rajasekaran et al. and Ogata and Whiteside  show that the most important part of the fluid flow is the flow between the IVD and the vertebrae located above and below it. Of less importance is the flow between disc components, i.e., the nucleus and annulus fibrosus.
The presented results clearly indicate the importance of facet joints in transmission of loads by the spinal column and the protective role of those structures in the overload conditions of the IVD.
| Conclusion|| |
The completed tests demonstrated the importance of the basic elements of the triad in transmission of loads and the mechanism by which the loading history affects changes occurring in the IVD.
The presented results explain the phenomena associated with maintenance and loss of stability of the spinal column depending on the existing loading conditions. The results show that interarticular joints play an important role in maintaining equilibrium in the distribution of load-induced forces and pressures and have a significant role in the processes taking place in the IVD. Lack of support on facet joints not only results in the loss of an important fulcrum of spinal column but also influences indirectly the IVD, in particular the process of fluid flow. This phenomenon is important for tissue metabolism but also determines self-stabilization of the disc. The loads transmitted from stiff vertebrae to IVD structures cause an increase in intradiscal pressure and a simultaneous pressure of the nucleus pulposus on annuli fibrosi, which become deformed. The correct ratio of the water content to the acting load maintains a stable arrangement between the IVD and the adjacent vertebrae.
The removal of articular processes and long-term variable loading affect the loss of stability of the IVD and thus the loss of its functions.
In addition, identification of the role of the IVD and articular processes in a kinematic system, such as the spine, defines the role of these interarticular joints in the performance of physiological functions of the locomotor system. Cooperation of the two joints determines proper distribution of load-induced forces and pressures.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]