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Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression
Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression Hendrik Schmidt a,n, Aboulfazl Shirazi-Adl b, Christoph Schilling c, Marcel Dreischarf a a
Julius Wolff Institute Charité – Universitätsmedizin Berlin, Berlin, Germany École Polytechnique, Montréal, Canada c Research and Development, Biomechanical Research, Aesculap AG, Tuttlingen, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Accepted 6 May 2016
Disc hydration is controlled by fluid imbibition and exudation and hence by applied load magnitude and history, internal osmotic pressure and disc conditions. It affects both the internal load distribution and external load-bearing of a disc while variations therein give rise to the disc time-dependent characteristics. This study aimed to evaluate the effect of changes in compression preload magnitude on the disc axial cyclic compression stiffness under physiological loading. After 20 h of free hydration, effects of various preload magnitudes (no preload, 0.06 and 0.28 MPa, applied for eight hours) and disc-bone preparation conditions on disc height and axial stiffness were investigated using 36 disc-bone and 24 isolated disc (without bony endplates) bovine specimens. After preloading, specimens were subjected to ten loading/unloading cycles each of 7.5 min compression at 0.5 MPa followed by 7.5 min at 0.06 MPa. Under 0.06 MPa preload, the specimen height losses during high loading periods of cyclic loading were greater than corresponding height recoveries during low loading phases. This resulted in a progressive reduction in the specimen height and increase in its stiffness. Differences between disc height losses in high cyclic loads and between stiffness in both load increase and release phases were significant for 0 and 0.06 MPa vs. 0.28 MPa preload. Results highlight the significant role of disc preload magnitude/history and hence disc height and hydration on disc stiffness in loading/unloading and disc height loss in loading periods. Proper preconditioning and hence hydration level should be achieved if recovery in height loss similar to in vivo conditions is expected. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Disc stiffness Disc hydration Compressive loading Fluid flow Creep Recovery
1. Introduction The intervertebral disc, being made of viscoelastic tissues, exhibits a time-dependent response; for example, creep under constant compression force (Adams and Hutton, 1983; Kazarian, 1975; Keller et al., 1987; Koeller et al., 1984a,1984b; Markolf, 1972). The temporal response is believed to be due primarily to the fluid flow (i.e., loss–gain of water under loading–unloading, respectively) as well as the gradual rearrangement of collagen fibers and proteoglycans. The disc's water content can fluctuate by 15–20% during a diurnal cycle, resulting in an altered intradiscal pressure, magnetic resonance signal intensity, and internal load distribution (within the disc and adjacent vertebrae and between the disc, ligaments and facets) (Adams et al., 1987, 1990; Arun et al., 2009; n Correspondence to: Julius Wolff Institut Charité – Universitätsmedizin Berlin CVK, Institutsgebäude Süd/Südstraße 2, Augustenburger Platz 1, 13353 Berlin, Germany. Tel.: þ49 30 2093 46016; fax: þ49 30 2093 46001. E-mail address: [email protected] (H. Schmidt).
Botsford et al., 1994; Hutton et al., 2003; Ludescher et al., 2008; Masuoka et al., 2007; O'Connell et al., 2011a, 2011b; Reitmaier et al., 2013; Urban and McMullin, 1988; Wilke et al., 1999). An adequate understanding of the viscoelastic properties of the healthy non-degenerate intervertebral disc is important in the development of total and partial replacements by implants and tissue engineering with the aim to replicate as closely as possible the disc temporal response. The creep response of the intervertebral disc has previously been investigated in a number of in vitro studies (Adams and Hutton, 1983; Adams et al., 1996; Brown et al., 1957; Hirsch, 1955; Hirsch and Nachemson, 1994; Kazarian, 1975; Keller et al., 1987; Koeller et al., 1984a, 1984b; Kulak et al., 1975; Lin et al., 1978, 2009; Markolf, 1972; Virgin, 1951). These investigations highlighted the non-linear timedependent behavior of the disc showing rapid decreases in the disc height and axial compliance early after loading that levels off with time till equilibrium. A recent in vitro study on porcine discs reported the effect of load magnitude and history on the disc angular stiffness and damping response (Zondervan et al., 2016).
http://dx.doi.org/10.1016/j.jbiomech.2016.05.006 0021-9290/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎
As in any other soft tissue such as the articular cartilage, the intervertebral disc hydration strongly influences its biomechanical behavior. Under load and in short term, the trapped fluid supports a major portion of the applied compression while being contained by stretched networks of collagen fibrils but later with time and drop in fluid content and pressure, the load is gradually transferred to the tissue non-fibrillar matrix and remaining structures (e.g., facets) (Argoubi and Shirazi-Adl, 1996; Shirazi-Adl, 1992). The disc hydration level in experimental studies could alter by evaporation when specimens are exposed to air, osmotic swelling when specimens are left unloaded while immersed in a solution bath and/or external loading/unloading that causes fluid inflow/ outflow (Bezci et al., 2015; Costi et al., 2002; Ferguson et al., 2004; O'Connell et al., 2011a; Race et al., 2000; Vergroesen et al., 2014). The effects of preconditioning, preload or instantaneous fluid content on the disc axial stiffness under physiological cyclic loading regimes (high and low loading phases) either of the isolated intervertebral disc alone or combined with adjacent vertebrae have not comprehensively been investigated. This is important when attempting both to better understand the disc transient response as related to its water content and to adequately compare results of studies performed on discs at varying initial hydration levels. Therefore, the purpose of this study was to evaluate the effect of changes in compression preload (disc hydration) on the axial compression stiffness in simulated physiological loading regimes. For this purpose, we reanalyzed the extensive data recorded (but not all reported) in a previous in vitro study on bovine lumbar motion segments (Schmidt et al., 2015) under cyclic axial compressive loads. We hypothesize that the axial stiffness of intervertebral discs in compression is substantially influenced by the preload magnitude and the number of loading–unloading cycles. Many in vitro studies reported an incomplete fluid and height recovery within an appropriate time-scale similar to in vivo (e.g., 8 h recovery for 16 h of loading) (Lee et al., 2006; Lin et al., 2009; O'Connell et al., 2011a; Reitmaier et al., 2012a, 2012b; van der Veen et al., 2005, 2007, 2009). Therefore due to low fluid imbibition, we further
hypothesize that an incomplete recovery leads to a progressive increase in stiffness with each loading cycle.
2. Methods The specimen preparation, testing apparatus and loading protocol were described in detail previously (Schmidt et al., 2015) and are only briefly summarized here for completion. Sixty C1–C2 and C2–C3 specimens (fifty four from the previous experiment) from skeletally mature bovine tails were used. After the removal of all surrounding muscles, soft tissues, facets and transverse processes, each vertebra was sawn approximately 5 mm away from the disc yielding disc-body units consisting of an intervertebral disc with parts of the upper and lower vertebral bodies. Twenty hours prior to testing, specimens were thawed initially for 18 h at 4 °C in phosphate-buffered saline (PBS, B. Braun Melsungen AG, Melsungen, Germany) to ensure uniform hydrated conditions (Fig. 1a). Specimens were then placed in a testing chamber filled with 39 °C (cow body temperature) PBS for additional two hours for temperature adjustment before tests start and during the entire tests allowing thus for a physiologically controlled environment. The loading protocol started with 8 h preload at 0.06 MPa (corresponding to 27.7–45.8 N compressive force depending on the measured disc cross-sectional areas) (Fig. 1a). Subsequently, specimens were subjected to 10 high/low loading cycles, followed finally by 2 h of low loading at 0.06 MPa. Each loading cycle consisted of 7.5 min axial compression at 0.5 MPa (corresponding to 211.6–373.4 N) and a low loading period of 7.5 min at 0.06 MPa (Fig. 1b). Load applications and releases were performed at 100 N/s. The compression tests were carried out with a servohydraulic material testing machine (858 Mini Bionix II, MTS, MN, USA). To investigate the influence of the preloading, two additional loading regimes were considered. In the first one, the preload magnitude was increased from 0.06 MPa to 0.28 MPa as the average of subsequent cyclic low and peak values of 0.06 MPa (low loading) and 0.5 MPa (high loading), respectively. Conversely in the second regime, the 8 h preload period at 0.06 MPa was removed altogether from the protocol thus proceeding directly to the 10 loading cycles, which circumvents the free fluid loss after prior 20 h of free swelling. The displacement was recorded at a frequency of 50 Hz. Each signal was dualpass filtered with a 2nd order low-pass digital Butterworth filter at 10 Hz cut-off frequency. A schematic of the displacement response with time is shown in Fig. 1c. The compressive stiffness (ΔN/Δmm) was calculated from the load–displacement curve in every phase of load increase (between 0.06 and 0.5 MPa) and release (between 0.5 and 0.06 MPa) as illustrated in Fig. 1d. The study, from which main part of the current data were extracted, aimed to examine the recovery capacity of the disc post mortem (Schmidt et al., 2015). For this purpose and with the intention to impede or enhance fluid flow into and out of the discs, various endplate preparation conditions were considered (i.e., rinsing with PBS, irrigation with an orthopedic debridement system or injection of PMMA at the exposed endplate surface). In addition, isolated discs were considered with
Fig. 1. (a) Time history of the compression test. Before testing, the specimens were immersed in a bath with phosphate buffered saline solution (PBS) for 18 h at 4 °C (blue bar). Subsequently, the specimens were placed for another 2 h in the testing chamber with PBS at 39 °C. The loading protocol consisted of 8 h preload at 0.06 MPa followed by 10 high loading (0.5 MPa) and low loading (0.06 MPa) cycles, each lasting 7.5 min (b). The effect of preload level was investigated by increasing the preload magnitude from 0.06 MPa to 0.28 MPa or by completely skipping the preload period going directly over to the 10 loading cycles. (c) Schematic of the temporal changes in the axial displacement as well as (d) the force–displacement curve of the first loading cycle. The compressive stiffness (ΔN/Δmm) is calculated for every loading (between 0.06 MPa and 0.5 MPa) and load release (between 0.5 MPa and 0.06 MPa) phase. (For interpretation of the references to color in this figure legends, the reader is referred to the web version of this article.)
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎ endplates removed at the cartilaginous level. In this latter isolated disc group, some specimens underwent superimposed sinusoidal load with an amplitude of 7 10 N at a frequency of 1 Hz (Fig. 1b). Since the displacement responses did not significantly alter with preparation conditions when comparing specimens with bony endplates and with added sinusoidal load in isolated disc specimens (Schmidt et al., 2015), we combined here all specimens into two principal groups of those with and those without endplates under 3 preload magnitudes as follows:
0.06 MPa preload: discs with and without endplates (12 specimens in each group)
0.28 MPa preload: discs with and without endplates (12 specimens in each group)
without preload: discs with endplates (12 specimens) All statistical analyses were carried out using SPSS software (SPSS Inc., Chicago, IL, USA, version: 21.0). The non-parametric Mann–Whitney U test and the Wilcoxon signed-rank test were used to compare two unrelated (e.g., discs with vs. discs without bony endplates) and related groups (e.g., height loss during high loading periods vs. height loss during low loading periods – results belong to the same specimen group), respectively. P-values less than 0.05 were considered as statistically significant.
3. Results
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3.2. Influence of preload magnitude Initial preload magnitude substantially influenced the displacement and axial compressive stiffness during cyclic loading. Differences between disc height gain and loss in high–low cyclic loads were significant for conditions without (0 MPa) (p¼ 0.002) and with a small (0.06 MPa) preload (pr 0.002) when compared to high (0.28 MPa) preload (Fig. 4). Under this highest preload, for the first time (in contrast to Figs. 2 and 4c), the recovery in height during low load periods matched or exceeded the earlier height losses during corresponding high load periods (Fig. 1a,b). In contrast to cases with no or 0.06 MPa preload, differences in axial stiffness between subsequent loading cycles disappeared in cases with 0.28 MPa preload, especially for the first few cycles (Fig. 5). No significant differences between stiffness values (while loading or releasing) were found in both discs with and without endplates. In specimens with endplates under 0.28 MPa preload, the axial stiffness under the first few loading cycles was approximately 780 N/mm being about 200 N/mm greater than that for the 0.06 MPa preload. In general, the lower the preload, the lower was the axial stiffness (Figs. 5 and 6). With time and load cycles, the extent of differences however decreased (Fig. 6).
3.1. 0.06 MPa preload 4. Discussion The specimen height loss during high loading periods of cyclic loading for discs with bony endplates continuously decreased from a cycle to the next while the disc height recovery during corresponding low loading phases remained significantly smaller (p r0.002) (Fig. 2a). This resulted in a progressive reduction in disc height with each loading cycle. Isolated disc specimens showed similar trends (p ¼0.002) but with significantly smaller displacements under loading and recovery phases compared to discs with endplates (Fig. 2b, p o0.001). Along with the progressive reduction in the disc height, the disc became stiffer from one cycle to another (Fig. 3). The higher stiffness values measured during load release indicate more fluid exudation in loading cycles than imbibition during corresponding release cycles (1st six loading cycles: 0.002r pr 0.028; last four loading cycles: 0.05 rp r0.071). With time and load cycles, the foregoing differences however disappeared indicating smaller fluid exchanges with time. In contrast to the displacement responses under 7.5 min high and low loading phases (Fig. 2), the stiffness responses did not significantly alter between specimens with and without bony endplates.
This work, exploiting data collected mainly in an earlier experimental in vitro study, was conducted to determine the effect of the magnitude of compression preload on the stiffness of bovine discs in compression in loading and unloading periods of cyclic loads. Preconditioning by various preloads influenced the disc hydration and height that acted as the primary factors governing the disc response under subsequent cyclic loads. The hypotheses were confirmed as the initial preload magnitude substantially affected the disc net height loss and axial stiffness during cyclic loading–unloading and that the stiffness increased with time and load cycles though only under lower preloads. The effect of preload magnitude on both disc height loss and disc stiffness as well as differences therein in loading and release (unloading) phases is governed primarily by the instantaneous fluid content and disc height of the specimens. After 20 h of initial free hydration and swelling, all specimens are set at their maximum level of fluid saturation and peak height. Under no preload period or alternatively with zero preload magnitude, the disc with high fluid content and height right at the beginning of cyclic
Fig. 2. Relative displacement changes during 7.5 min high loading and 7.5 min low loading phases for all ten loading cycles for disc specimens with (a) and without (b) adjacent bony endplates under 0.06 MPa preload. The red bars describe the net disc height losses in each cycle evaluated as the differences between the displacements during low loading and high loading phases. Values are also provided as Supplementary materials (Table 1). (For interpretation of the references to color in this figure legends, the reader is referred to the web version of this article.)
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 3. Axial compressive stiffness (ΔN/Δmm) for load increases (from 0.06 to 0.5 MPa) and load releases (from 0.5 to 0.06 MPa) for all 10 loading cycles for disc specimens with (a) and without (b) bony endplates under 0.06 MPa preload. Values are also provided as Supplementary materials (Table 2).
Fig. 4. Relative displacement changes during 7.5 min high loading and 7.5 min low loading phases for all ten loading cycles for (a) disc specimens with endplates (preload of 0.28 MPa), (b) isolated disc specimens without endplates (preload of 0.28 MPa) and (c) disc specimens with endplates (without any preload). The red bars describe the net disc height losses in each cycle evaluated as the differences between the displacements during low loading and high loading phases. Values are also provided as Supplementary materials (Table 1). (For interpretation of the references to color in this figure legends, the reader is referred to the web version of this article.)
loading shows maximum height losses and minimum stiffness that, respectively, gradually decrease and increase with the number of cycles (Figs. 4–6). Due to a constant recovery in height over low loading cycles, the net recovery in height loss over each full loading–unloading cycle markedly decreases with the number of cycles. These results point to a continuous loss of fluid content and disc height during load cycles from their peaks at the beginning. On the other extreme and after 8 h of high compression preload at 0.28 MPa, however, the specimens are left with minimum free fluid content and disc height right before cyclic loading–unloading starts. After this high preload and under cyclic loads, minimum height losses and maximum stiffness are found under loading
cycles with little differences with time from one cycle to the next. This results, especially at earlier cycles, even in net gain in disc height and hence in near full recovery of height losses in cycles of loading–unloading (Figs. 4–6). The preload magnitude of 0.06 MPa yields results in between those under foregoing extremes with high 0.28 MPa preload on the one hand and no preload at all on the other. Another interesting finding of this work (Fig. 6) is the observation of nearly no effect of preload magnitude and number of cycles on the recovery of height losses during unloading (release) periods of cyclic loading. In other words, despite the fact that the height losses during loading cycles are highest under no preload
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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Fig. 5. Axial compressive stiffness (ΔN/Δmm) for load increases (between 0.06 and 0.5 MPa) and releases (between 0.5 and 0.06 MPa) for all 10 loading cycles for (a) disc specimens with endplates (preload of 0.28 MPa), (b) isolated disc specimens without endplates (preload of 0.28 MPa) and (c) discs with endplates (without any preload). Values are also provided as Supplementary materials (Table 2).
Fig. 6. (a) Relative displacement changes and (b) axial compressive stiffness (ΔN/Δmm) for different preload conditions (without preload, 0.06 and 0.28 MPa preload).
and at early cycles, no such differences are found in height recoveries (gains) during release periods of cyclic loading. This confirms the fact that, in contrast to the disc height loss in loading
periods, the fluid content and height of a specimen have much less effects on the disc height recovery in release periods. It appears that the recovery in height is hence nearly independent on the
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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instantaneous mobile fluid content in the disc which indirectly supports our earlier findings (Schmidt et al., 2015) of near absence of fluid inflow during unloading (release) periods. This observation is in agreement with earlier in vitro investigations on human, ovine, bovine, porcine, caprine and rat discs (Lee et al., 2006; Lin et al., 2009; O'Connell et al., 2011a; Reitmaier et al., 2012a, 2012b; van der Veen et al., 2005, 2007, 2009). Instantaneous disc fluid content and height, though somewhat related, both influence the disc biomechanics. In short term at transient periods under relatively fast loading rate of 100 N/s considered in this study, the fluid plays a major compression loadbearing role while almost trapped within the tissues giving rise to high stiffness in this period. This transient stiffness is affected mainly by the disc height rather than the fluid content at this stage. This is the primary reason for the much greater stiffness found under 0.28 MPa preload compared to no or 0.06 MPa preload. The important role of disc height and changes therein on disc stiffness and response has also been recognized in earlier studies (Natarajan and Andersson, 1999; Niemeyer et al., 2012; Noailly et al., 2007). Post transient period and as time progresses, the fluid content and changes therein (outflow and inflow) play however a more prominent role in the recorded disc height losses during cyclic loading–unloading periods. The greater the initial fluid content is, the larger the height losses and alterations become in cyclic loads. In the present study, we note a marked increase in the axial compressive stiffness at lower fluid content and disc height associated with increasing preload magnitude. Similar results have been found by Bezci et al. (2015) who evaluated the effect of hydration on compressive stiffness through osmotic loading. The authors demonstrated that the compressive stiffness increased with hyperosmotic loading. This is because hyperosmotic loading decrease the amount of water imbibed by the disc prior to testing resulting thus in an increase in the compressive stiffness. Race et al. (2000) investigated the effect of disc hydration on the compressive properties of bovine intervertebral discs. They found a rapid increase in the stiffness initially over the first half hour of creep induced dehydration from a fully hydrated state that slowed down in the remaining 7 h. This is in agreement with the rapid changes in stiffness during early cycles found in the current study under no preload (Fig. 5d). Costi et al. (2002) reported that the stiffness of ovine disc-bone and isolated disc specimens were significantly different when tested in a saline bath environment compared to air alone. This difference was noted also in other modes of loading, particularly torsion, flexion and left bending (Costi et al., 2002) where the specimens were stiffer in air and more compliant in a saline environment. Recent in vitro studies on porcine discs also report similar marked effects of load history by recording higher sagittal angular stiffness and damping in specimens exposed to earlier loads (Zondervan et al., 2016). There are some limitations in this study that should be emphasized. Clearly, care should be exercised when extrapolating findings obtained using an animal model to the human disc. Although the overall anatomic characteristics of the bovine and human discs are generally similar, there are significant differences in the ratio of height to anterior–posterior diameter and the relative heights and thicknesses of the anterior and posterior aspects of the annular wall. The bovine endplate is distinctly convex toward its vertebral body, whereas the human lumbar endplates are nearly flat to slightly concave. Nevertheless, bovine discs have a relatively large size in comparison to other animal specimens (O'Connell et al., 2007) and are assumed to show similarities to human discs (Demers et al., 2004; Oshima et al., 1993). They are easily available from the local abattoir and have been thoroughly characterized biomechanically and in organ culture studies (Chan and Gantenbein-Ritter, 2012; Haglund et al.,
2011; Michalek and Iatridis, 2012). Load applications and releases were performed at a constant loading rate of 100 N/s. It has been shown that the loading rate has a substantial influence on the disc compressive mechanical properties (Race et al., 2000). For the current investigations and due to the lack of statistical difference (Schmidt et al., 2015), we combined specimens with different endplate preparations or additional sinusoidal loading into two principal groups of those with and those without endplates, respectively. For example, results of specimens with exposed endplates prepared by rinsing with PBS, irrigation with an orthopedic debridement system or injection of PMMA were combined together. In summary, results highlight the significant role of disc preload magnitude and hence disc hydration and height on disc stiffness and height loss/recovery during cyclic loading periods in healthy intervertebral discs when employing an animal model. It highlights the consideration of preconditioning phases when analyzing or comparing results of various studies. Finally, to replicate in vivo conditions that demonstrate full recovery in disc height upon load release, reasonable preload magnitudes are recommended to avoid excessive initial hydration.
Conflict of Interest There is no conflict of interest.
Acknowledgments This work is funded by the German Research Foundation (SCHM 2572/3-1).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jbiomech.2016.05.006.
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Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Demers, C.N., Antoniou, J., Mwale, F., 2004. Value and limitations of using the bovine tail as a model for the human lumbar spine. Spine (Philadelphia 1976) 29, 2793–2799. Ferguson, S.J., Ito, K., Nolte, L.P., 2004. Fluid flow and convective transport of solutes within the intervertebral disc. J. Biomech. 37, 213–221. Haglund, L., Moir, J., Beckman, L., Mulligan, K.R., Jim, B., Ouellet, J.A., Roughley, P., Steffen, T., 2011. Development of a bioreactor for axially loaded intervertebral disc organ culture. Tissue Eng. Part C: Methods 17, 1011–1019. Hirsch, C., 1955. The reaction of intervertebral discs to compression forces. J. Bone Jt. Surg. Am. 37, 1188–1196. Hirsch, C., Nachemson, A., 1994. New observations on the mechanical behavior of lumbar discs. Acta Orthop. Scand. 23, 254–283. Hutton, W.C., Malko, J.A., Fajman, W.A., 2003. Lumbar disc volume measured by MRI: effects of bed rest, horizontal exercise, and vertical loading. Aviat. Space Environ. Med. 74, 73–78. Kazarian, L.E., 1975. Creep characteristics of the human spinal column. Orthop. Clin. N. Am. 6, 3–18. Keller, T.S., Spengler, D.M., Hansson, T.H., 1987. Mechanical behavior of the human lumbar spine. I. Creep analysis during static compressive loading. J. Orthop. Res. 5, 467–478. Koeller, W., Funke, F., Hartmann, F., 1984a. Biomechanical behavior of human intervertebral discs subjected to long lasting axial loading. Biorheology 21, 675–686. Koeller, W., Meier, W., Hartmann, F., 1984b. Biomechanical properties of human intervertebral discs subjected to axial dynamic compression. A comparison of lumbar and thoracic discs. Spine (Philadelphia 1976) 9, 725–733. Kulak, R.F., Schultz, A.B., Belytschko, T., Galante, J., 1975. Biomechanical characteristics of vertebral motion segments and intervertebral disks. Orthop. Clin. N. Am. 6, 121–133. Lee, C.R., Iatridis, J.C., Poveda, L., Alini, M., 2006. In vitro organ culture of the bovine intervertebral disc: effects of vertebral endplate and potential for mechanobiology studies. Spine (Philadelphia 1976) 31, 515–522. Lin, H.S., Liu, Y.K., Ray, G., Nikravesh, P., 1978. Systems identification for material properties of the intervertebral joint. J. Biomech. 11, 1–14. Lin, L.C., Hedman, T.P., Wang, S.J., Huoh, M., Chang, S.Y., 2009. The analysis of axisymmetric viscoelasticity, time-dependent recovery, and hydration in rat tail intervertebral discs by radial compression test. J. Appl. Biomech. 25, 133–139. Ludescher, B., Effelsberg, J., Martirosian, P., Steidle, G., Markert, B., Claussen, C., Schick, F., 2008. T2- and diffusion-maps reveal diurnal changes of intervertebral disc composition: an in vivo MRI study at 1.5 T. J. Magn. Reson. Imaging 28, 252–257. Markolf, K.L., 1972. Deformation of the thoracolumbar intervertebral joints in response to external loads: a biomechanical study using autopsy material. J. Bone Jt. Surg. Am. 54, 511–533. Masuoka, K., Michalek, A.J., MacLean, J.J., Stokes, I.A., Iatridis, J.C., 2007. Different effects of static versus cyclic compressive loading on rat intervertebral disc height and water loss in vitro. Spine (Philadelphia 1976) 32, 1974–1979. Michalek, A.J., Iatridis, J.C., 2012. Height and torsional stiffness are most sensitive to annular injury in large animal intervertebral discs. Spine J. 12, 425–432. Natarajan, R.N., Andersson, G.B., 1999. The influence of lumbar disc height and cross-sectional area on the mechanical response of the disc to physiologic loading. Spine 24, 1873–1881. Niemeyer, F., Wilke, H.J., Schmidt, H., 2012. Geometry strongly influences the response of numerical models of the lumbar spine – a probabilistic finite element analysis. J. Biomech. 45, 1414–1423.
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Noailly, J., Wilke, H.J., Planell, J.A., Lacroix, D., 2007. How does the geometry affect the internal biomechanics of a lumbar spine bi-segment finite element model? Consequences on the validation process. J. Biomech. 40, 2414–2425. O'Connell, G.D., Vresilovic, E.J., Elliott, D.M., 2007. Comparison of animals used in disc research to human lumbar disc geometry. Spine (Philadelphia 1976) 32, 328–333. O'Connell, G.D., Jacobs, N.T., Sen, S., Vresilovic, E.J., Elliott, D.M., 2011a. Axial creep loading and unloaded recovery of the human intervertebral disc and the effect of degeneration. J. Mech. Behav. Biomed. Mater. 4, 933–942. O'Connell, G.D., Vresilovic, E.J., Elliott, D.M., 2011b. Human intervertebral disc internal strain in compression: the effect of disc region, loading position, and degeneration. J. Orthop. Res. 29, 547–555. Oshima, H., Ishihara, H., Urban, J.P., Tsuji, H., 1993. The use of coccygeal discs to study intervertebral disc metabolism. J. Orthop. Res. 11, 332–338. Race, A., Broom, N.D., Robertson, P., 2000. Effect of loading rate and hydration on the mechanical properties of the disc. Spine 25, 662–669. Reitmaier, S., Shirazi-Adl, A., Bashkuev, M., Wilke, H.J., Gloria, A., Schmidt, H., 2012a. In vitro and in silico investigations of disc nucleus replacement. J. R. Soc. Interface 9, 1869–1879. Reitmaier, S., Wolfram, U., Ignatius, A., Wilke, H.J., Gloria, A., Martin-Martinez, J.M., Silva-Correia, J., Miguel Oliveira, J., Luis Reis, R., Schmidt, H., 2012b. Hydrogels for nucleus replacement-facing the biomechanical challenge. J. Mech. Behav. Biomed. Mater., 14C; b, pp. 67–77. Reitmaier, S., Schmidt, H., Ihler, R., Kocak, T., Graf, N., Ignatius, A., Wilke, H.J., 2013. Preliminary investigations on intradiscal pressures during daily activities: an in vivo study using the merino sheep. PLoS One, 8. Schmidt, H., Schilling, C., Reyna, A.L., Shirazi-Adl, A., Dreischarf, M., 2015. Fluid-flow dependent response of intervertebral discs under cyclic loading: on the role of specimen preparation and preconditioning. J. Biomech. Shirazi-Adl, A., 1992. Finite-element simulation of changes in the fluid content of human lumbar discs. Mechanical and clinical implications. Spine (Philadelphia 1976) 17, 206–212. Urban, J.P., McMullin, J.F., 1988. Swelling pressure of the lumbar intervertebral discs: influence of age, spinal level, composition, and degeneration. Spine 13, 179–187. van der Veen, A.J., Mullender, M., Smit, T.H., Kingma, I., van Dieen, J.H., 2005. Flowrelated mechanics of the intervertebral disc: the validity of an in vitro model. Spine (Philadelphia 1976) 30, E534–E539. van der Veen, A.J., van Dieen, J.H., Nadort, A., Stam, B., Smit, T.H., 2007. Intervertebral disc recovery after dynamic or static loading in vitro: is there a role for the endplate? J. Biomech. 40, 2230–2235. van der Veen, A.J., 2009. Mechanical Behaviour of the Intervertebral Disc Under Sustained Compressive Loading. Vrije Universiteit, Amsterdam, p. 160. Vergroesen, P.P.A., van der Veen, A.J., van Royen, B.J., Kingma, I., Smit, T.H., 2014. Intradiscal pressure depends on recent loading and correlates with disc height and compressive stiffness. Eur. Spine J. 23, 2359–2368. Virgin, W.J., 1951. Experimental investigations into the physical properties of the intervertebral disc. J. Bone Jt. Surg. Br. 33, 607–611. Wilke, H.J., Neef, P., Caimi, M., Hoogland, T., Claes, L.E., 1999. New in vivo measurements of pressures in the intervertebral disc in daily life. Spine 24, 755–762. Zondervan, R.L., Popovich, J.M., Radcliffe, C.J., Pathak, P.K., Reeves, N.P., 2016. Sagittal rotational stiffness and damping increase in a porcine lumbar spine with increased or prolonged loading. J. Biomech. 49, 624–627.
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression Hendrik Schmidt a,n, Aboulfazl Shirazi-Adl b, Christoph Schilling c, Marcel Dreischarf a a
Julius Wolff Institute Charité – Universitätsmedizin Berlin, Berlin, Germany École Polytechnique, Montréal, Canada c Research and Development, Biomechanical Research, Aesculap AG, Tuttlingen, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Accepted 6 May 2016
Disc hydration is controlled by fluid imbibition and exudation and hence by applied load magnitude and history, internal osmotic pressure and disc conditions. It affects both the internal load distribution and external load-bearing of a disc while variations therein give rise to the disc time-dependent characteristics. This study aimed to evaluate the effect of changes in compression preload magnitude on the disc axial cyclic compression stiffness under physiological loading. After 20 h of free hydration, effects of various preload magnitudes (no preload, 0.06 and 0.28 MPa, applied for eight hours) and disc-bone preparation conditions on disc height and axial stiffness were investigated using 36 disc-bone and 24 isolated disc (without bony endplates) bovine specimens. After preloading, specimens were subjected to ten loading/unloading cycles each of 7.5 min compression at 0.5 MPa followed by 7.5 min at 0.06 MPa. Under 0.06 MPa preload, the specimen height losses during high loading periods of cyclic loading were greater than corresponding height recoveries during low loading phases. This resulted in a progressive reduction in the specimen height and increase in its stiffness. Differences between disc height losses in high cyclic loads and between stiffness in both load increase and release phases were significant for 0 and 0.06 MPa vs. 0.28 MPa preload. Results highlight the significant role of disc preload magnitude/history and hence disc height and hydration on disc stiffness in loading/unloading and disc height loss in loading periods. Proper preconditioning and hence hydration level should be achieved if recovery in height loss similar to in vivo conditions is expected. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Disc stiffness Disc hydration Compressive loading Fluid flow Creep Recovery
1. Introduction The intervertebral disc, being made of viscoelastic tissues, exhibits a time-dependent response; for example, creep under constant compression force (Adams and Hutton, 1983; Kazarian, 1975; Keller et al., 1987; Koeller et al., 1984a,1984b; Markolf, 1972). The temporal response is believed to be due primarily to the fluid flow (i.e., loss–gain of water under loading–unloading, respectively) as well as the gradual rearrangement of collagen fibers and proteoglycans. The disc's water content can fluctuate by 15–20% during a diurnal cycle, resulting in an altered intradiscal pressure, magnetic resonance signal intensity, and internal load distribution (within the disc and adjacent vertebrae and between the disc, ligaments and facets) (Adams et al., 1987, 1990; Arun et al., 2009; n Correspondence to: Julius Wolff Institut Charité – Universitätsmedizin Berlin CVK, Institutsgebäude Süd/Südstraße 2, Augustenburger Platz 1, 13353 Berlin, Germany. Tel.: þ49 30 2093 46016; fax: þ49 30 2093 46001. E-mail address: [email protected] (H. Schmidt).
Botsford et al., 1994; Hutton et al., 2003; Ludescher et al., 2008; Masuoka et al., 2007; O'Connell et al., 2011a, 2011b; Reitmaier et al., 2013; Urban and McMullin, 1988; Wilke et al., 1999). An adequate understanding of the viscoelastic properties of the healthy non-degenerate intervertebral disc is important in the development of total and partial replacements by implants and tissue engineering with the aim to replicate as closely as possible the disc temporal response. The creep response of the intervertebral disc has previously been investigated in a number of in vitro studies (Adams and Hutton, 1983; Adams et al., 1996; Brown et al., 1957; Hirsch, 1955; Hirsch and Nachemson, 1994; Kazarian, 1975; Keller et al., 1987; Koeller et al., 1984a, 1984b; Kulak et al., 1975; Lin et al., 1978, 2009; Markolf, 1972; Virgin, 1951). These investigations highlighted the non-linear timedependent behavior of the disc showing rapid decreases in the disc height and axial compliance early after loading that levels off with time till equilibrium. A recent in vitro study on porcine discs reported the effect of load magnitude and history on the disc angular stiffness and damping response (Zondervan et al., 2016).
http://dx.doi.org/10.1016/j.jbiomech.2016.05.006 0021-9290/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎
As in any other soft tissue such as the articular cartilage, the intervertebral disc hydration strongly influences its biomechanical behavior. Under load and in short term, the trapped fluid supports a major portion of the applied compression while being contained by stretched networks of collagen fibrils but later with time and drop in fluid content and pressure, the load is gradually transferred to the tissue non-fibrillar matrix and remaining structures (e.g., facets) (Argoubi and Shirazi-Adl, 1996; Shirazi-Adl, 1992). The disc hydration level in experimental studies could alter by evaporation when specimens are exposed to air, osmotic swelling when specimens are left unloaded while immersed in a solution bath and/or external loading/unloading that causes fluid inflow/ outflow (Bezci et al., 2015; Costi et al., 2002; Ferguson et al., 2004; O'Connell et al., 2011a; Race et al., 2000; Vergroesen et al., 2014). The effects of preconditioning, preload or instantaneous fluid content on the disc axial stiffness under physiological cyclic loading regimes (high and low loading phases) either of the isolated intervertebral disc alone or combined with adjacent vertebrae have not comprehensively been investigated. This is important when attempting both to better understand the disc transient response as related to its water content and to adequately compare results of studies performed on discs at varying initial hydration levels. Therefore, the purpose of this study was to evaluate the effect of changes in compression preload (disc hydration) on the axial compression stiffness in simulated physiological loading regimes. For this purpose, we reanalyzed the extensive data recorded (but not all reported) in a previous in vitro study on bovine lumbar motion segments (Schmidt et al., 2015) under cyclic axial compressive loads. We hypothesize that the axial stiffness of intervertebral discs in compression is substantially influenced by the preload magnitude and the number of loading–unloading cycles. Many in vitro studies reported an incomplete fluid and height recovery within an appropriate time-scale similar to in vivo (e.g., 8 h recovery for 16 h of loading) (Lee et al., 2006; Lin et al., 2009; O'Connell et al., 2011a; Reitmaier et al., 2012a, 2012b; van der Veen et al., 2005, 2007, 2009). Therefore due to low fluid imbibition, we further
hypothesize that an incomplete recovery leads to a progressive increase in stiffness with each loading cycle.
2. Methods The specimen preparation, testing apparatus and loading protocol were described in detail previously (Schmidt et al., 2015) and are only briefly summarized here for completion. Sixty C1–C2 and C2–C3 specimens (fifty four from the previous experiment) from skeletally mature bovine tails were used. After the removal of all surrounding muscles, soft tissues, facets and transverse processes, each vertebra was sawn approximately 5 mm away from the disc yielding disc-body units consisting of an intervertebral disc with parts of the upper and lower vertebral bodies. Twenty hours prior to testing, specimens were thawed initially for 18 h at 4 °C in phosphate-buffered saline (PBS, B. Braun Melsungen AG, Melsungen, Germany) to ensure uniform hydrated conditions (Fig. 1a). Specimens were then placed in a testing chamber filled with 39 °C (cow body temperature) PBS for additional two hours for temperature adjustment before tests start and during the entire tests allowing thus for a physiologically controlled environment. The loading protocol started with 8 h preload at 0.06 MPa (corresponding to 27.7–45.8 N compressive force depending on the measured disc cross-sectional areas) (Fig. 1a). Subsequently, specimens were subjected to 10 high/low loading cycles, followed finally by 2 h of low loading at 0.06 MPa. Each loading cycle consisted of 7.5 min axial compression at 0.5 MPa (corresponding to 211.6–373.4 N) and a low loading period of 7.5 min at 0.06 MPa (Fig. 1b). Load applications and releases were performed at 100 N/s. The compression tests were carried out with a servohydraulic material testing machine (858 Mini Bionix II, MTS, MN, USA). To investigate the influence of the preloading, two additional loading regimes were considered. In the first one, the preload magnitude was increased from 0.06 MPa to 0.28 MPa as the average of subsequent cyclic low and peak values of 0.06 MPa (low loading) and 0.5 MPa (high loading), respectively. Conversely in the second regime, the 8 h preload period at 0.06 MPa was removed altogether from the protocol thus proceeding directly to the 10 loading cycles, which circumvents the free fluid loss after prior 20 h of free swelling. The displacement was recorded at a frequency of 50 Hz. Each signal was dualpass filtered with a 2nd order low-pass digital Butterworth filter at 10 Hz cut-off frequency. A schematic of the displacement response with time is shown in Fig. 1c. The compressive stiffness (ΔN/Δmm) was calculated from the load–displacement curve in every phase of load increase (between 0.06 and 0.5 MPa) and release (between 0.5 and 0.06 MPa) as illustrated in Fig. 1d. The study, from which main part of the current data were extracted, aimed to examine the recovery capacity of the disc post mortem (Schmidt et al., 2015). For this purpose and with the intention to impede or enhance fluid flow into and out of the discs, various endplate preparation conditions were considered (i.e., rinsing with PBS, irrigation with an orthopedic debridement system or injection of PMMA at the exposed endplate surface). In addition, isolated discs were considered with
Fig. 1. (a) Time history of the compression test. Before testing, the specimens were immersed in a bath with phosphate buffered saline solution (PBS) for 18 h at 4 °C (blue bar). Subsequently, the specimens were placed for another 2 h in the testing chamber with PBS at 39 °C. The loading protocol consisted of 8 h preload at 0.06 MPa followed by 10 high loading (0.5 MPa) and low loading (0.06 MPa) cycles, each lasting 7.5 min (b). The effect of preload level was investigated by increasing the preload magnitude from 0.06 MPa to 0.28 MPa or by completely skipping the preload period going directly over to the 10 loading cycles. (c) Schematic of the temporal changes in the axial displacement as well as (d) the force–displacement curve of the first loading cycle. The compressive stiffness (ΔN/Δmm) is calculated for every loading (between 0.06 MPa and 0.5 MPa) and load release (between 0.5 MPa and 0.06 MPa) phase. (For interpretation of the references to color in this figure legends, the reader is referred to the web version of this article.)
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎ endplates removed at the cartilaginous level. In this latter isolated disc group, some specimens underwent superimposed sinusoidal load with an amplitude of 7 10 N at a frequency of 1 Hz (Fig. 1b). Since the displacement responses did not significantly alter with preparation conditions when comparing specimens with bony endplates and with added sinusoidal load in isolated disc specimens (Schmidt et al., 2015), we combined here all specimens into two principal groups of those with and those without endplates under 3 preload magnitudes as follows:
0.06 MPa preload: discs with and without endplates (12 specimens in each group)
0.28 MPa preload: discs with and without endplates (12 specimens in each group)
without preload: discs with endplates (12 specimens) All statistical analyses were carried out using SPSS software (SPSS Inc., Chicago, IL, USA, version: 21.0). The non-parametric Mann–Whitney U test and the Wilcoxon signed-rank test were used to compare two unrelated (e.g., discs with vs. discs without bony endplates) and related groups (e.g., height loss during high loading periods vs. height loss during low loading periods – results belong to the same specimen group), respectively. P-values less than 0.05 were considered as statistically significant.
3. Results
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3.2. Influence of preload magnitude Initial preload magnitude substantially influenced the displacement and axial compressive stiffness during cyclic loading. Differences between disc height gain and loss in high–low cyclic loads were significant for conditions without (0 MPa) (p¼ 0.002) and with a small (0.06 MPa) preload (pr 0.002) when compared to high (0.28 MPa) preload (Fig. 4). Under this highest preload, for the first time (in contrast to Figs. 2 and 4c), the recovery in height during low load periods matched or exceeded the earlier height losses during corresponding high load periods (Fig. 1a,b). In contrast to cases with no or 0.06 MPa preload, differences in axial stiffness between subsequent loading cycles disappeared in cases with 0.28 MPa preload, especially for the first few cycles (Fig. 5). No significant differences between stiffness values (while loading or releasing) were found in both discs with and without endplates. In specimens with endplates under 0.28 MPa preload, the axial stiffness under the first few loading cycles was approximately 780 N/mm being about 200 N/mm greater than that for the 0.06 MPa preload. In general, the lower the preload, the lower was the axial stiffness (Figs. 5 and 6). With time and load cycles, the extent of differences however decreased (Fig. 6).
3.1. 0.06 MPa preload 4. Discussion The specimen height loss during high loading periods of cyclic loading for discs with bony endplates continuously decreased from a cycle to the next while the disc height recovery during corresponding low loading phases remained significantly smaller (p r0.002) (Fig. 2a). This resulted in a progressive reduction in disc height with each loading cycle. Isolated disc specimens showed similar trends (p ¼0.002) but with significantly smaller displacements under loading and recovery phases compared to discs with endplates (Fig. 2b, p o0.001). Along with the progressive reduction in the disc height, the disc became stiffer from one cycle to another (Fig. 3). The higher stiffness values measured during load release indicate more fluid exudation in loading cycles than imbibition during corresponding release cycles (1st six loading cycles: 0.002r pr 0.028; last four loading cycles: 0.05 rp r0.071). With time and load cycles, the foregoing differences however disappeared indicating smaller fluid exchanges with time. In contrast to the displacement responses under 7.5 min high and low loading phases (Fig. 2), the stiffness responses did not significantly alter between specimens with and without bony endplates.
This work, exploiting data collected mainly in an earlier experimental in vitro study, was conducted to determine the effect of the magnitude of compression preload on the stiffness of bovine discs in compression in loading and unloading periods of cyclic loads. Preconditioning by various preloads influenced the disc hydration and height that acted as the primary factors governing the disc response under subsequent cyclic loads. The hypotheses were confirmed as the initial preload magnitude substantially affected the disc net height loss and axial stiffness during cyclic loading–unloading and that the stiffness increased with time and load cycles though only under lower preloads. The effect of preload magnitude on both disc height loss and disc stiffness as well as differences therein in loading and release (unloading) phases is governed primarily by the instantaneous fluid content and disc height of the specimens. After 20 h of initial free hydration and swelling, all specimens are set at their maximum level of fluid saturation and peak height. Under no preload period or alternatively with zero preload magnitude, the disc with high fluid content and height right at the beginning of cyclic
Fig. 2. Relative displacement changes during 7.5 min high loading and 7.5 min low loading phases for all ten loading cycles for disc specimens with (a) and without (b) adjacent bony endplates under 0.06 MPa preload. The red bars describe the net disc height losses in each cycle evaluated as the differences between the displacements during low loading and high loading phases. Values are also provided as Supplementary materials (Table 1). (For interpretation of the references to color in this figure legends, the reader is referred to the web version of this article.)
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 3. Axial compressive stiffness (ΔN/Δmm) for load increases (from 0.06 to 0.5 MPa) and load releases (from 0.5 to 0.06 MPa) for all 10 loading cycles for disc specimens with (a) and without (b) bony endplates under 0.06 MPa preload. Values are also provided as Supplementary materials (Table 2).
Fig. 4. Relative displacement changes during 7.5 min high loading and 7.5 min low loading phases for all ten loading cycles for (a) disc specimens with endplates (preload of 0.28 MPa), (b) isolated disc specimens without endplates (preload of 0.28 MPa) and (c) disc specimens with endplates (without any preload). The red bars describe the net disc height losses in each cycle evaluated as the differences between the displacements during low loading and high loading phases. Values are also provided as Supplementary materials (Table 1). (For interpretation of the references to color in this figure legends, the reader is referred to the web version of this article.)
loading shows maximum height losses and minimum stiffness that, respectively, gradually decrease and increase with the number of cycles (Figs. 4–6). Due to a constant recovery in height over low loading cycles, the net recovery in height loss over each full loading–unloading cycle markedly decreases with the number of cycles. These results point to a continuous loss of fluid content and disc height during load cycles from their peaks at the beginning. On the other extreme and after 8 h of high compression preload at 0.28 MPa, however, the specimens are left with minimum free fluid content and disc height right before cyclic loading–unloading starts. After this high preload and under cyclic loads, minimum height losses and maximum stiffness are found under loading
cycles with little differences with time from one cycle to the next. This results, especially at earlier cycles, even in net gain in disc height and hence in near full recovery of height losses in cycles of loading–unloading (Figs. 4–6). The preload magnitude of 0.06 MPa yields results in between those under foregoing extremes with high 0.28 MPa preload on the one hand and no preload at all on the other. Another interesting finding of this work (Fig. 6) is the observation of nearly no effect of preload magnitude and number of cycles on the recovery of height losses during unloading (release) periods of cyclic loading. In other words, despite the fact that the height losses during loading cycles are highest under no preload
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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Fig. 5. Axial compressive stiffness (ΔN/Δmm) for load increases (between 0.06 and 0.5 MPa) and releases (between 0.5 and 0.06 MPa) for all 10 loading cycles for (a) disc specimens with endplates (preload of 0.28 MPa), (b) isolated disc specimens without endplates (preload of 0.28 MPa) and (c) discs with endplates (without any preload). Values are also provided as Supplementary materials (Table 2).
Fig. 6. (a) Relative displacement changes and (b) axial compressive stiffness (ΔN/Δmm) for different preload conditions (without preload, 0.06 and 0.28 MPa preload).
and at early cycles, no such differences are found in height recoveries (gains) during release periods of cyclic loading. This confirms the fact that, in contrast to the disc height loss in loading
periods, the fluid content and height of a specimen have much less effects on the disc height recovery in release periods. It appears that the recovery in height is hence nearly independent on the
Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i
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H. Schmidt et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎
instantaneous mobile fluid content in the disc which indirectly supports our earlier findings (Schmidt et al., 2015) of near absence of fluid inflow during unloading (release) periods. This observation is in agreement with earlier in vitro investigations on human, ovine, bovine, porcine, caprine and rat discs (Lee et al., 2006; Lin et al., 2009; O'Connell et al., 2011a; Reitmaier et al., 2012a, 2012b; van der Veen et al., 2005, 2007, 2009). Instantaneous disc fluid content and height, though somewhat related, both influence the disc biomechanics. In short term at transient periods under relatively fast loading rate of 100 N/s considered in this study, the fluid plays a major compression loadbearing role while almost trapped within the tissues giving rise to high stiffness in this period. This transient stiffness is affected mainly by the disc height rather than the fluid content at this stage. This is the primary reason for the much greater stiffness found under 0.28 MPa preload compared to no or 0.06 MPa preload. The important role of disc height and changes therein on disc stiffness and response has also been recognized in earlier studies (Natarajan and Andersson, 1999; Niemeyer et al., 2012; Noailly et al., 2007). Post transient period and as time progresses, the fluid content and changes therein (outflow and inflow) play however a more prominent role in the recorded disc height losses during cyclic loading–unloading periods. The greater the initial fluid content is, the larger the height losses and alterations become in cyclic loads. In the present study, we note a marked increase in the axial compressive stiffness at lower fluid content and disc height associated with increasing preload magnitude. Similar results have been found by Bezci et al. (2015) who evaluated the effect of hydration on compressive stiffness through osmotic loading. The authors demonstrated that the compressive stiffness increased with hyperosmotic loading. This is because hyperosmotic loading decrease the amount of water imbibed by the disc prior to testing resulting thus in an increase in the compressive stiffness. Race et al. (2000) investigated the effect of disc hydration on the compressive properties of bovine intervertebral discs. They found a rapid increase in the stiffness initially over the first half hour of creep induced dehydration from a fully hydrated state that slowed down in the remaining 7 h. This is in agreement with the rapid changes in stiffness during early cycles found in the current study under no preload (Fig. 5d). Costi et al. (2002) reported that the stiffness of ovine disc-bone and isolated disc specimens were significantly different when tested in a saline bath environment compared to air alone. This difference was noted also in other modes of loading, particularly torsion, flexion and left bending (Costi et al., 2002) where the specimens were stiffer in air and more compliant in a saline environment. Recent in vitro studies on porcine discs also report similar marked effects of load history by recording higher sagittal angular stiffness and damping in specimens exposed to earlier loads (Zondervan et al., 2016). There are some limitations in this study that should be emphasized. Clearly, care should be exercised when extrapolating findings obtained using an animal model to the human disc. Although the overall anatomic characteristics of the bovine and human discs are generally similar, there are significant differences in the ratio of height to anterior–posterior diameter and the relative heights and thicknesses of the anterior and posterior aspects of the annular wall. The bovine endplate is distinctly convex toward its vertebral body, whereas the human lumbar endplates are nearly flat to slightly concave. Nevertheless, bovine discs have a relatively large size in comparison to other animal specimens (O'Connell et al., 2007) and are assumed to show similarities to human discs (Demers et al., 2004; Oshima et al., 1993). They are easily available from the local abattoir and have been thoroughly characterized biomechanically and in organ culture studies (Chan and Gantenbein-Ritter, 2012; Haglund et al.,
2011; Michalek and Iatridis, 2012). Load applications and releases were performed at a constant loading rate of 100 N/s. It has been shown that the loading rate has a substantial influence on the disc compressive mechanical properties (Race et al., 2000). For the current investigations and due to the lack of statistical difference (Schmidt et al., 2015), we combined specimens with different endplate preparations or additional sinusoidal loading into two principal groups of those with and those without endplates, respectively. For example, results of specimens with exposed endplates prepared by rinsing with PBS, irrigation with an orthopedic debridement system or injection of PMMA were combined together. In summary, results highlight the significant role of disc preload magnitude and hence disc hydration and height on disc stiffness and height loss/recovery during cyclic loading periods in healthy intervertebral discs when employing an animal model. It highlights the consideration of preconditioning phases when analyzing or comparing results of various studies. Finally, to replicate in vivo conditions that demonstrate full recovery in disc height upon load release, reasonable preload magnitudes are recommended to avoid excessive initial hydration.
Conflict of Interest There is no conflict of interest.
Acknowledgments This work is funded by the German Research Foundation (SCHM 2572/3-1).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jbiomech.2016.05.006.
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Please cite this article as: Schmidt, H., et al., Preload substantially influences the intervertebral disc stiffness in loading–unloading cycles of compression. Journal of Biomechanics (2016), http://dx.doi.org/10.1016/j.jbiomech.2016.05.006i