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The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics
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Journal of Biomechanics 41 (2008) 2104–2111 www.elsevier.com/locate/jbiomech www.JBiomech.com
The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics Marco Cannellaa, Amy Arthurc, Shanee Allena, Michael Keanec, Abhijeet Joshia, Edward Vresilovicb, Michele Marcolongoa, a
Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA b Department of Orthopaedic Surgery, Pennsylvania State University, Hershey, PA, USA c Synthes Spine, West Chester, PA, USA Accepted 30 April 2008
Abstract To study the effect of denucleation on the mechanical behavior of the human lumbar intervertebral disc through a 2 mm incision, two groups of six human cadaver lumbar spinal units were tested in axial compression, axial rotation, lateral bending and flexion/extension after incremental steps of ‘‘partial’’ denucleation. Neutral zone, range of motion, stiffness, intradiscal pressure and energy dissipation were measured; the results showed that the contribution of the nucleus pulposus to the mechanical behavior of the intervertebral disc was more dominant through the neutral zone than at the farther limits of applied loads and moments. r 2008 Published by Elsevier Ltd. Keywords: Lumbar spine; Intervertebral disc; Nucleus pulposus; Denucleation; Anterior column unit; Neutral zone
1. Introduction Many investigators have studied the dependency of disc mechanical behavior on alteration of the nucleus pulposus (NP). These studies include examination of changes with aging and degeneration (Nachemson, 1960; Miller et al., 1988; Panjabi et al., 1988; Urban and McMullin, 1988; Thompson et al., 1990; Iatridis et al., 1997), pressurization (Andersson and Schultz, 1979), and partial or total nucleotomy (Hirsch and Nachemson, 1954; Markolf and Morris, 1974; Kulak et al., 1976; Panjabi et al., 1984; Shirazi-Adl et al., 1984; Brinckmann and Horst, 1985; Goel et al., 1985, 1986; Seroussi et al., 1989; Shea et al., 1994). Compressive loading after removal of the NP increases disc deformation and radial bulging, and decreases disc height and intradiscal pressure when compared with the intact disc (Panjabi et al., 1984; Brinckmann and Horst, 1985). These changes are dependent on the quantity of NP removed (Goel et al., 1985; Brinckmann and Grootenboer, Corresponding author. Tel.: +1 215 895 2329; fax: +1 215 895 6760.
E-mail address: [email protected] (M. Marcolongo). 0021-9290/$ - see front matter r 2008 Published by Elsevier Ltd. doi:10.1016/j.jbiomech.2008.04.037
1991; Shea et al., 1994). Under compressive loading and partial removal of the NP, inner annulus layers bulge inward suggesting an increased radial stress within the annulus (Seroussi et al., 1989; Meakin and Hukins, 2000; Meakin et al., 2001). However, none of the previously cited studies directly investigated the change of the intervertebral disc stiffness at different loads after discectomy. A prior investigation (Shea et al., 1994) showed no statistical changes in disc stiffness at only 800 N in axial compression between intact and percutaneous or conventional discectomy, leaving a lack of understanding of disc stiffness changes for lower loads. In this work we investigated the effect of denucleation on compressive, tensile, bending and torsional behavior of the intervertebral disc as examined through human lumbar anterior column unit (ACUs). We hypothesized that the contribution of the nucleus pulposus to the mechanical behavior of the ACU was more predominant through the neutral zone than at the further limits of the applied loads and moments. A percutaneus approach was chosen for this study because it is less disruptive of the annulus wall than conventional discectomy.
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2. Materials and methods Twelve human (3M/3F) cadaver ACUs, were prepared by cutting through the two adjacent discs and removing the posterior elements. Removal of the zygapophyseal joints changes the intervertebral joint biomechanics under torsion and bending loads (Thompson et al., 2003), enabling the study of isolated annulus and nucleus biomechanics (Spenciner et al., 2006) by removing the effects of load sharing with the posterior elements. The absence of bridging osteophytes, fractures or severe disc space narrowing (o5 mm) was confirmed and intervertebral disc height (DH) was measured by a calibrated X-ray imaging system (Fluoroscan Imaging Systems, Northbrook, IL). The ACUs were potted using Smooth-Cast 300 (Smooth-On, Inc., Easton, PA) and kept moist throughout testing with phosphate-buffered saline. The specimens were divided into two data sets (DS1 and DS2). Progressive denucleation and nucleus pressure measurements were performed on DS1 in axial loading (AL). DS2 was tested in AL, axial rotation (AR), lateral bending (LB) and flexion/extension (FE) after maximum denucleation. The combination of multiaxial testing at each denucleation step would have resulted in a complicated test protocol with chances of altering the disc mechanics (Johannessen et al., 2004). Mechanical tests were conducted on a servohydraulic dynamic test system (Model 8874, Instron, Corporation, Norwood, MA) and the nucleotomy was performed with a 2 mm diameter automated vacuum tissue removal system (Nucleotome, Model 22500, Clarus Medical, LLC, Minneapolis, MN) using a posterolateral approach. After testing, each specimen was sectioned in the transverse plane, annular dimensions were measured (Table 1) and absence of severe disc degeneration was confirmed.
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Under a 50 N compressive load, the PT was retracted into the annular wall and a portion of the nucleus was removed. Tissue removal was performed in four 5 min intervals. At each step the removed tissue was dried and weighed. The PT was reinserted back into the NP and the disc was tested with the same loading protocol as the intact disc. Change in DH was evaluated by comparing the Instron actuator position at 50 N before and after the denucleation step.
2.2. Maximum denucleation study (DS2) A custom jig modeled as reported by Spenciner et al. (2006) was used to multiaxially test the ACUs. The test specimen and a six-degree-of-freedom load cell (model MC3A, AMTI, Inc., Watertown, MA) were positioned above an XY table (Fig. 1). Prior to the test, jig stiffness was evaluated. Jig stiffness was 1700 N/mm in compression, 30 N m/deg in torsion and 10 N m/deg in bending. Due to the jig setup, preconditioning was run in force control within the load range of DS1 (50 cycles, 50–150 N compression, havertriangle waveform, 1 Hz).
2.1. Partial denucleation study (DS1) Preconditioning was performed in displacement control (3% of DH, 50 cycles, havertriangle waveform, 1 Hz). The intact disc was then tested in load control from 150 N tension to 1500 N compression (5 cycles, havertriangle waveform, 0.1 Hz). A pressure transducer (Model 060S, Precision Measurement Company, Ann Arbor, MI) embedded in a 14 gauge needle (Hamilton, Reno, NV) was inserted inside the NP through the anterior annulus fibrosus (AF) and placed at half of the minor axis length from the outer annular wall. The disc was tested to ensure that its mechanical properties were not affected by the insertion of the pressure transducer (PT).
Fig. 1. Test setup for axial loading and axial rotation (left) and for lateral bending and flexion/extension (right): (A) anterior column unit, (B) specimen fixtures, (C) 6DOF load cell, (D) XY table, (E) vertical and rotational control of the actuator and (F) locking device used for axial compression and rotation tests.
Table 1 Specimens were divided into two DSs for the experiments described Disc height (mm)
Major axis (mm)
Minor axis (mm)
Age
Level
Gender
Data Set 1 (DS1) ACU #1 ACU #2 ACU #3 ACU #4 ACU #5 ACU #6 Mean7SD
8.0 10.6 10.9 10.5 5.4 9.2 9.172.1
51 53 58 50 40 44 4976
38 38 37 36 37 36 3771
64 64 64 30 30 30 47719
T12-L1 L2-L3 L4-L5 L4-L5 T12-L1 L2-L3
M M M F F F
Data Set 2 (DS2) ACU #1 ACU #2 ACU #3 ACU #4 ACU #5 ACU #6 Mean7SD
9.5 7.4 6.6 6.2 6.3 7.4 7.271.3
54 49 51 47 50 57 5174
38 34 35 34 34 42 3673
53 53 47 47 26 65 49713
L3-L4 L1-L2 L4-L5 L2-L3 L4-L5 L4-L5
M M F F F M
DS1 was used in the partial denucleation study of axial loading (compressive and tensile) while DS2 was used in the fully denucleated conditions for compression, bending and torsion.
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Five cycles of AL (150 to 1000 N, sinusoidal waveform, 0.1 Hz), AR, LB and FE (77.5 N m, sinusoidal waveform, 0.1 Hz) were applied separately to each intact ACU. A sinusoidal waveform was used to reduce jig inertia during the inversion of force; maximum compression was limited to 1000 N to prevent possible damage of the specimen brought on by multiaxial testing. Denucleation was performed under 50 N axial load for 20 min and the removed nucleus dry mass was weighed. The denucleated ACU was tested in AL, AR, LB and FE with the same protocol as the intact disc.
(Graphpad Software, San Diego, CA). Student’s t-tests (po0.05) were used to analyze differences between the two Data Set populations. Paired t-tests (po0.05) were performed to determine if the PT produced significant differences in disc mechanical behavior and to examine statistical differences between the intact and the maximum denucleated states of DS2. Repeated-measures one-way ANOVA (po0.05) with posthoc Tukey’s test were performed on DS1.
2.3. Data collection and analysis
3.1. Intact disc
Neutral zone (NZ), range of motion (ROM), stiffness and energy dissipation (HYS) were calculated (Fig. 2) for the intact and denucleated states of each ACU using the fifth loading cycle. ACU displacement and rotation were evaluated by subtracting the neutral position from the Instron axial position and angle. The neutral position was calculated as the mid-point between the positions (angles) at zero load (torque). For DS2 the displacement and rotation were corrected to take test apparatus stiffness into account. NZ was calculated (Wilke et al., 1998) as the difference in displacement (rotation) at zero load (torque) between the loading and unloading curves. ROM, defined as the maximum displacement (rotation) in one direction of motion was calculated: compressive and tensile ROM (cROM and tROM) from the AL test, flexion and extension ROM (fROM and eROM) from the FE test, and right and left ROM (rROM and lROM) from each one of the AR and LB tests. Stiffness in AL was calculated as the slope of the load-displacement curve during the loading cycle at 100, 0, 50, 100, 200, 400, 800 and 1400 N, respectively (Fig. 2(a)). Stiffness in AR, LB and FE were was calculated as the slope of the torque-rotation curve during the loading cycle (Wilke et al., 1997) at 0, 70.5, 72, 74 and 77 N m (Fig. 2(b)). The viscoelastic behavior of the specimen was evaluated with the area (HYS) enclosed by each loading cycle (Fig. 2(a) and (b)). Intradiscal pressure was calculated as the average between the loading and unloading curve at 100, 0, 50, 100, 200, 400, 800 and 1400 N. Since intradiscal pressure increases linearly with compressive load (Nachemson, 1960), a linear regression was performed and the ratio between intradiscal pressure and the axial applied stress (Nachemson, 1981) was calculated for each denucleation step. The values for the denucleated specimens were normalized to their intact values to reduce specimen variability associated with human biological tissue. Statistical analyses were performed using Prizm
No statistical difference (Student’s t-test) was observed in dimensions (DH, major and minor axes) and ages of the two DSs used in this study (Table 1). ROMs and NZ of the intact specimen for each different loading condition are reported in Table 2. The asymmetry of the disc with respect to the sagittal plane causes a difference of about 11 between fROM and eROM. Fig. 3(a) shows the mean axial stiffness at different loads for DS1 (with and without PT) and DS2. The NZ, tROM, cROM and stiffness values at each load in axial compression for DS1 were not statistically different (non-paired t-test) from the ones of DS2, however, a direct comparison between these two groups may be confounded by the different experimental setups and preconditioning protocols used for each of the two data sets. Fig. 3(b) shows the mean stiffness in AR, LB and FE at different moments for DS2. No statistical difference (paired t-test) was observed on the mechanical behavior of the human lumbar intervertebral disc by the insertion of the PT. Pressure in the nucleus increased with an average rate of 1.170.2 MPa/kN; it was 1.5870.14 times the axial applied stress. The intradiscal pressure for the intact disc at 50 N axial compression was 152730 kPa, in keeping with the values measured in vivo for lying prone (Quinnell and Stockdale, 1983; Wilke et al., 1999).
3. Results
Fig. 2. Typical loading curves for a lumbar intervertebral disc under (a) axial loading and (b) bending. The figures describe the methods used to calculate (a) neutral zone (NZ), compressive range of motion (cROM), tensile range of motion (tROM) and stiffness for axial loading, and to calculate (b) NZ, ROM in each direction of motion and stiffness for axial rotation, lateral bending and flexion/extension. The area within each curve represents the energy dissipation (HYS).
ARTICLE IN PRESS M. Cannella et al. / Journal of Biomechanics 41 (2008) 2104–2111 Table 2 Mechanical behavior of the intact specimens of DS1 and DS2 for ranges of motion (ROM) and neutral zones (NZ) Data Set 1 (DS1)
Data Set 2 (DS2)
Intact
Intact (pt)
Axial compression NZ (mm) tROM (mm) cROM (mm)
0.1270.05 0.3470.15 0.9870.17
0.1370.05 0.3370.19 0.9870.16
0.3070.26 0.6170.37 1.0470.38
Axial rotation NZ (1) rROM (1) lROM (1)
– – –
– – –
1.2770.62 4.5771.78 4.1871.13
Lateral bending NZ (1) rROM (1) lROM (1)
– – –
– – –
2.3171.33 6.4871.31 6.2971.39
Flexion/Extension NZ (1) From (1) eROM (1)
– – –
– – –
4.1072.55 8.9072.44 9.8473.88
Fig. 3. Intact stiffness in (a) axial loading (Data Sets 1 and 2) and (b) axial rotation, lateral bending and flexion/extension (Data Set 2).
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3.2. Partial denucleation (DS1) and maximum denucleation (DS2) in AL The dry weight of the NP removed after each denucleation step for DS1 were interpolated with a one-phase exponential association with a time constant of 18 min (Fig. 4). For DS2, after 20 min of denucleation, 0.15 g of dry NP was removed which was not statistically different (Student t-test) from the amount of tissue removed in DS1 after 20 min. In Fig. 5(a) the relative changes in DH, NZ, cROM and tROM during each denucleation step (DS1) and for maximum denucleation (DS2) are depicted with respect to the equivalent value for the intact disc. After 20 min of denucleation, there is a more than 15% reduction in DH, more than 300% increase in NZ, about 50% increase in cROM and about 300% increase in tROM, with respect to the intact values. The ANOVA test showed that DH (po0.001), cROM (po0.05) and tROM (po0.05) were significantly different from the intact values at each denucleation step, while NZ (po0.01) was significantly different from the intact values only after 15 min of denucleation. A paired t-test on DS2 showed that denucleated DH (p ¼ 0.004), NZ (p ¼ 0.041), cROM (p ¼ 0.043) and tROM (p ¼ 0.027) were significantly different from the intact values. Fig. 5(b) and (c) show the relative changes in axial stiffness and intradiscal pressure at different loads with respect to the intact disc after each denucleation step. The stiffness of the disc in tension (100 N) decreased to about 55% of the intact value after 5 min of denucleation. No additional significant changes were observed beyond 5 min. In the compressive range, the stiffness of the disc decreases with denucleation particularly in the lower range of loads (0–400 N). Compared with the intact state, at 50 N the stiffness reduction was on the order of 50–75%, while at 400 N the reduction was only 3–12% for different denucleation time. At 800 N, both the Tukey’s test (DS1) and the paired t-test (DS2) showed no statistical difference
Fig. 4. The specimens of Data Set 1 were denucleated using the Nucleotome for 5, 10, 15 and 20 min. There is a non-linear removal of tissue using the Nucleotome with an 18 min time constant.
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Fig. 5. Partially denucleated specimens were mechanically tested in AL. (a) Normalized DH, NZ, cROM and tROM for partially denucleated specimens. There is a marked loss of disc height from the intact condition which increases with increasing nucleus removal, while the ROM in tension and compression and the NZ increase well beyond that of the intact condition. (b) Normalized stiffness for partially denucleated specimens show decreasing stiffness with increased removal of tissue for the 50 and 200 N load levels. By 400 N the stiffness approaches that of the intact condition. (c) Normalized pressure for partially denucleated specimens at different loads shows a reduction in intradiscal pressure of the denucleated condition. (d) The same reduction is presented as the decrease of the ratio between pressure and applied force over disc area. (*statistically different from intact, **statistically different from intact and 5 min of denucleation, ***statistically different from intact, 5 and 10 min of denucleation).
in stiffness from intact values. For higher loads (1400 N) the stiffness slightly increased with respect to the intact value. In only one case (15 min) there was a statistical difference in the Tukey’s post-hoc test (po0.05). A student’s t-test showed no statistical difference in DH, NZ, cROM, tROM and stiffness values between DS1 and DS2 after 20 min of denucleation. As with the intact data for these groups, a direct comparison between may be
confounded by the different experimental setups and different preconditioning protocols used. In tension (100 N), intradiscal pressure decreased by more than 50% after 5 min of denucleation. It decreases with further denucleation, remaining statistically different from intact only (po0.001). Between 0 and 1400 N, the pressure values were always statistically different from the intact values. The ratio between pressure and force per disc
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area decreased by 13% and 33% after 5 and 20 min of denucleation, respectively. 3.3. Maximum denucleation (DS2) in AR, LB and FE NZ increased by about 150% (p ¼ 0.002), 165% (p ¼ 0.017) and 140% (p ¼ 0.01) for AR, LB and FE, respectively; ROMs increased by about 57–68% for AR (p ¼ 0.021, 0.011), 33–26% for LB (p ¼ 0.024, 0.046) and 37% for FE (p ¼ 0.021, 0.024) (Fig. 6(a)). Denucleated stiffness in LB and FE were reduced in the lower levels of applied moment (Fig. 6(b)) with respect to the intact values. In LB there was a statistical difference from the intact state at 0 N m (62% decrease, po0.001), 0.5 N m (24% decrease, p ¼ 0.046), 4N m (17% increase, p ¼ 0.023) and 7 N m (13% increase, p ¼ 0.003), whereas in FE statistical differences were at 7 N m (12% increase,
Fig. 7. Hysteresis values for each loading condition. For AL, there is an increase in HYS to approximately 56% over that of the intact disc. For the maximum denucleated (20 min) disc, AR had 46% increase in HYS, while LB and FE increased approximately 25% over the intact condition (*statistically different from intact, ***statistically different from intact, 5 and 10 min of denucleation).
po0.001), 4 N m (16% decrease, p ¼ 0.026) and 0 N m (58% decrease, p ¼ 0.002). Stiffness in AR was reduced from the intact state under all applied moments (Fig. 6(b)). The values were always statistically different from the intact values with the exception of 4 N m. The most significant difference (po0.01) from the intact condition was between 2 and 2 N m, in particular at 0 N m where stiffness was reduced by 64% (po0.001). Fig. 7 shows the increase in HYS values due to further denucleation steps for the four different loading conditions. All HYS values after 20 min of denucleation were statistically different from the intact values (po0.001), showing the importance of the nucleus in limiting the energy absorbed by the AF. 4. Discussion
Fig. 6. Mechanical behavior of denucleated specimens in AR, LB and FE. (a) Normalized NZ and ROMs for fully denucleated specimens shows the typical increase in neutral zone and range of motion for all loading conditions. The NZ measurements are particularly high indicating an increase in instability through this loading region in particular. (b) Normalized stiffness in AR, LB and FE for totally denucleated specimens shows again, a reduction in stiffness in the lower regions of applied moment for LB and FE, however, AR does not achieve the level of the intact disc through the applied moments tested here. (*statistically different from intact).
In this work, we examined a minimally invasive approach to denucleate the intervertebral disc through a 2 mm external diameter incision. The Nucleotome, aside from the annular insult, can create a continuous cavity, while other more mechanical approaches of denucleation can lead to pockets of removed tissue with the possibility of re-swelling as the tissue rehydrates, as suggested by Brinckmann and Grootenboer (1991). Compressive stiffness values for intact human motion segments reported in literature vary widely in a range of 700–3200 N/mm (White and Panjabi, 1990; Langrana et al., 1996), depending on differences with respect to age, degeneration stage, disc level, gender and testing protocol. Nonetheless, the compressive stiffness values for intact motion segments reported here fall within this range. Previous observations, as well as measurements reported here for the denucleated condition, showed that removal of nucleus tissue decreased DH and segment stiffness, and
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increased NZ and ROM (Hirsch and Nachemson, 1954; Markolf and Morris, 1974; Kulak et al., 1976; Panjabi et al., 1984; Shirazi-Adl et al., 1984; Brinckmann and Horst, 1985; Goel et al., 1985, 1986; Seroussi et al., 1989; Shea et al., 1994). A relatively linear relation between reduction in DH and mass of tissue removed (0.8 mm/1.0 g wet wt) was noted by Brinckmann and Grootenboer (1991). In this study, we observed a reduction in DH with a rate of about 0.4 mm decrease in DH for each 100 mg of dry weight tissue removed. Clearly, the depressurization associated with denucleation (Fig. 5(c)) plays a significant role in the disc dimensional changes. Decreased pressure requires either increased load supported by the AF and/or increased disc deformation. The increase in the NZ and tROM during the denucleation procedure, and the different reduction of compressive stiffness with denucleation as the load level increases, lead to the following observations. First, the mechanical behavior in AL for the denucleated disc is more unstable than in the intact condition, as previously reported (Wilke et al., 2006), and this instability increases with additional removal of nucleus tissue. Second, the role of the NP is more pronounced in the lower load regions. The initial loading of the disc, up to about 400 N, relies on the nucleus to create an intradiscal pressure that tensions the AF. With this tensioning, the disc can operate in the normal, intact condition. By the time the disc is loaded to a functional 800 N or beyond, the reduction in stiffness is minimized or reduced completely. Shea et al. (1994) also reported no difference in compressive stiffness between intact and denucleated conditions at 800 N of loading. In this region, the AF likely dominates the mechanical behavior of the disc, and while the stress state in the disc is different from the intact condition and the intradiscal pressure remains about 25% lower than the intact level, the global measurement of stiffness is closer to the intact values. Similar observations can be made for AR, LB and FE with a reduction of stiffness in the lower moment regions and an increased NZ. For LB and FE the stiffness values at higher levels of applied moment become equal to those of the intact condition, while in AR the stiffness values are below the level of the intact disc. It may be that in FE and LB the role of the nucleus is diminished compared to the role that it plays in AR. These results suggest that in FE and LB, the annulus can be put into a tensile state through two mechanisms: (1) internal pressurization from the NP and (2) traction from the tensile region of the bending condition. The tensioning of the AF in bending is not as dependent on intradiscal pressure as it is in AL. Even if the nucleus provides enough pressurization to the disc, shown by a 2.4-fold increase in NZ when the nucleus is removed, it also shifts towards the portion of the AF tensioned by the applied moment. Stiffnesses in AR were more affected by denucleation than LB and FE. In this loading condition, the nucleus plays an important role in the pressurization of the annulus
for maintaining resistance against torsion via the ‘‘screwdown’’ motion the disc takes in rotation. Further, the coupling between the compression and torsion stiffness matrices of the annulus could be highly affected by the uniform tensioning of the AF (Broberg, 1983). Without tension (for the case of the denucleated disc) the fibers are slack and need to undergo significantly more deformation to obtain the same degree of resistance to the externally applied moments. This study is limited, in part, because although every effort was made to limit or eliminate off-axis loading in the ‘‘pure’’ load conditions, data from the 6 DOF load cell indicated that there was approximately 5% off-axis loading, which may have some influence on the mechanical behavior of the disc. Even though the specimens were carefully centered in the potting fixtures to avoid misalignments between the geometrical axis of the specimen and the fixture axis, the off-axis loads and moments were mainly due to the offset between the actuator axis and the specimen axis of rotation that may move during the load application. Although removal of the zygapophyseal joints can significantly change the biomechanics of the disc under torsion and bending loads (Thompson et al., 2003), compressive stiffness at loads under 500 N is not altered (Gardner-Morse and Stokes, 2004) and there is little effect on intradiscal pressure up to 800 N (Reuber et al., 1982). Posterior elements were removed in this study to thoroughly understand isolated changes within the disc, rather than in the entire triple joint complex. In addition, with the minimally invasive nucleus removal system, some NP can remain adjacent to the insertion site where it is difficult to direct the cannula of the removal system. This tissue may have also affected some of the mechanical behavior that we observed. Further, it is difficult to extend these data to the clinical situation precisely because of the ‘‘pure’’ loading conditions. Nonetheless, these types of loading conditions have been used by others in the laboratory to understand disc behavior (Goel et al., 1985; Spenciner et al., 2003; Spenciner et al., 2006).
5. Conclusions From this work and the work of others (Goel et al., 1985; Brinckmann and Grootenboer, 1991; Shea et al., 1994), we see that the NP is important in disc mechanics. As shown in this work, through a lower load and moment region, where there is insufficient loading to engage the AF into a tensile or optimal load bearing state, the NP assumes a critical role in mechanical behavior. While discectomy is used as a method to alleviate pain of the disc, the long-term effects of an unstable disc are not well-understood. The relationship, if any, between mechanical instability of the disc and a clinically painful disc are worthy of continued study as is the role that the herniated/degenerated nucleus plays in creating pain and accelerated degeneration.
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Conflict of interest I, Michele Marcolongo, declare that I have no proprietary, financial, professional or other personal interest of any nature or kind in any product, service and/or company that could be constructed as influencing the position presented in this paper. Acknowledgments The authors wish to thank Jeff Stein for contributions to this project with respect to the statistical analysis. Nucleotome instrument consoles and ancillary access instruments were provided by the Clarus Medical and Synthes Spine Co. L.P., respectively. Financial support for this study was kindly provided by the Synthes Spine, West Chester, PA, USA. References Andersson, G.B., Schultz, A.B., 1979. Effects of fluid injection on mechanical properties of intervertebral discs. Journal of Biomechanics 12 (6), 453–458. Brinckmann, P., Grootenboer, H., 1991. Change of disc height, radial disc bulge, and intradiscal pressure from discectomy. An in vitro investigation on human lumbar discs. Spine 16 (6), 641–646. Brinckmann, P., Horst, M., 1985. The influence of vertebral body fracture, intradiscal injection, and partial discectomy on the radial bulge and height of human lumbar discs. Spine 10 (2), 138–145. Broberg, K.B., 1983. On the mechanical behaviour of intervertebral discs. Spine 8 (2), 151–165. Gardner-Morse, M.G., Stokes, I.A.F., 2004. Structural behavior of human lumbar spinal motion segments. Journal of Biomechanics 37 (2), 205–212. Goel, V.K., Goyal, S., et al., 1985. Kinematics of the whole lumbar spine. Effect of discectomy. Spine 10 (6), 543–554. Goel, V.K., Nishiyama, K., et al., 1986. Mechanical properties of lumbar spinal motion segments as affected by partial disc removal. Spine 11 (10), 1008–1012. Hirsch, C., Nachemson, A., 1954. New observations on the mechanical behavior of lumbar discs. Acta Orthopaedica Scandinavica 23 (4), 254–283. Iatridis, J.C., Setton, L.A., et al., 1997. Alterations in the mechanical behavior of the human lumbar nucleus pulposus with degeneration and aging. Journal of Orthopaedic Research 15 (2), 318–322. Johannessen, W., Vresilovic, E.J., et al., 2004. Intervertebral disc mechanics are restored following cyclic loading and unloaded recovery. Annals of Biomedical Engineering 32 (1), 70–76. Kulak, R.F., Belytschko, T.B., et al., 1976. Nonlinear behavior of the human intervertebral disc under axial load. Journal of Biomechanics 9 (6), 377–386. Langrana, N., Edwards, W., et al. 1996. In: Wiesel, S., Weinstein, J.N., Herkowitz, H.N., Dvorak, J., Bell, G. (Eds.), The Lumbar spine, Biomechanical Analyses of Loads on the Lumbar Spine, W.B. Saunders Company, Washington, DC, pp. 163–171. Markolf, K.L., Morris, J.M., 1974. The structural components of the intervertebral disc. A study of their contributions to the ability of the disc to withstand compressive forces. The Journal of Bone and Joint Surgery—American Volume 56 (4), 675–687. Meakin, J.R., Hukins, D.W., 2000. Effect of removing the nucleus pulposus on the deformation of the annulus fibrosus during compres-
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sion of the intervertebral disc. Journal of Biomechanics 33 (5), 575–580. Meakin, J.R., Redpath, T.W., et al., 2001. The effect of partial removal of the nucleus pulposus from the intervertebral disc on the response of the human annulus fibrosus to compression. Clinical Biomechanics (Bristol, Avon) 16 (2), 121–128. Miller, J.A., Schmatz, C., et al., 1988. Lumbar disc degeneration: correlation with age, sex, and spine level in 600 autopsy specimens. Spine 13 (2), 173–178. Nachemson, A., 1960. Lumbar intradiscal pressure. Experimental studies on post-mortem material. Acta Orthopaedica Scandinavica (Suppl. 43), 1–104. Nachemson, A.L., 1981. Disc pressure measurements. Spine 6 (1), 93–97. Panjabi, M., Brown, M., et al., 1988. Intrinsic disc pressure as a measure of integrity of the lumbar spine. Spine 13 (8), 913–917. Panjabi, M.M., Krag, M.H., et al., 1984. Effects of disc injury on mechanical behavior of the human spine. Spine 9 (7), 707–713. Quinnell, R.C., Stockdale, H.R., 1983. The use of in vivo lumbar discography to assess the clinical significance of the position of the intercrestal line. Spine 8 (3), 305–307. Reuber, M., Schultz, A., et al., 1982. Bulging of lumbar intervertebral disks. Journal of Biomechanical Engineering—Transactions of the ASME 104 (3), 187–192. Seroussi, R.E., Krag, M.H., et al., 1989. Internal deformations of intact and denucleated human lumbar discs subjected to compression, flexion, and extension loads. Journal of Orthopaedic Research 7 (1), 122–131. Shea, M., Takeuchi, T.Y., et al., 1994. A comparison of the effects of automated percutaneous diskectomy and conventional diskectomy on intradiscal pressure, disk geometry, and stiffness. Journal of Spinal Disorders 7 (4), 317–325. Shirazi-Adl, S.A., Shrivastava, S.C., et al., 1984. Stress analysis of the lumbar disc-body unit in compression. A three-dimensional nonlinear finite element study. Spine 9 (2), 120–134. Spenciner, D., Greene, D., et al., 2006. The multidirectional bending properties of the human lumbar intervertebral disc. The Spine Journal 6 (3), 248–257. Spenciner, D., Paiva, J., et al. 2003. Testing of human cadaveric functional spinal units to the ASTM draft standard, ‘‘Standard test methods for static and dynamic characterization of spinal artificial discs’’. In: Melkerson, M., Griffith, S., Kirkpatrick, J. (Eds.), Spinal implants: are we evaluating them appropriately? ASTM STP 1431, ASTM International, West Conshohocken, PA, pp. 114–126. Thompson, J.P., Pearce, R.H., et al., 1990. Preliminary evaluation of a scheme for grading the gross morphology of the human intervertebral disc. Spine 15 (5), 411–415. Thompson, R.E., Barker, T.M., et al., 2003. Defining the neutral zone of sheep intervertebral joints during dynamic motions: an in vitro study. Clinical Biomechanics 18 (2), 89–98. Urban, J.P., McMullin, J.F., 1988. Swelling pressure of the lumbar intervertebral discs: influence of age, spinal level, composition, and degeneration. Spine 13 (2), 179–187. White, A.A., Panjabi, M.M., 1990. Clinical Biomechanics of the Spine. Lippincott Williams & Wilkins, Philadelphia. Wilke, H.J., Kettler, A., et al., 1997. Are sheep spines a valid biomechanical model for human spines? Spine 22 (20), 2365–2374. Wilke, H.J., Wenger, K., et al., 1998. Testing criteria for spinal implants: recommendations for the standardization of in vitro stability testing of spinal implants. European Spine Journal 7 (2), 148–154. Wilke, H.J., Neef, P., et al., 1999. New in vivo measurements of pressures in the intervertebral disc in daily life. Spine 24 (8), 755–762. Wilke, H.J., Heuer, F., et al., 2006. Is a collagen scaffold for a tissue engineered nucleus replacement capable of restoring disc height and stability in an animal model? European Spine Journal 15 (Suppl 3), S433–S438.
Journal of Biomechanics 41 (2008) 2104–2111 www.elsevier.com/locate/jbiomech www.JBiomech.com
The role of the nucleus pulposus in neutral zone human lumbar intervertebral disc mechanics Marco Cannellaa, Amy Arthurc, Shanee Allena, Michael Keanec, Abhijeet Joshia, Edward Vresilovicb, Michele Marcolongoa, a
Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA b Department of Orthopaedic Surgery, Pennsylvania State University, Hershey, PA, USA c Synthes Spine, West Chester, PA, USA Accepted 30 April 2008
Abstract To study the effect of denucleation on the mechanical behavior of the human lumbar intervertebral disc through a 2 mm incision, two groups of six human cadaver lumbar spinal units were tested in axial compression, axial rotation, lateral bending and flexion/extension after incremental steps of ‘‘partial’’ denucleation. Neutral zone, range of motion, stiffness, intradiscal pressure and energy dissipation were measured; the results showed that the contribution of the nucleus pulposus to the mechanical behavior of the intervertebral disc was more dominant through the neutral zone than at the farther limits of applied loads and moments. r 2008 Published by Elsevier Ltd. Keywords: Lumbar spine; Intervertebral disc; Nucleus pulposus; Denucleation; Anterior column unit; Neutral zone
1. Introduction Many investigators have studied the dependency of disc mechanical behavior on alteration of the nucleus pulposus (NP). These studies include examination of changes with aging and degeneration (Nachemson, 1960; Miller et al., 1988; Panjabi et al., 1988; Urban and McMullin, 1988; Thompson et al., 1990; Iatridis et al., 1997), pressurization (Andersson and Schultz, 1979), and partial or total nucleotomy (Hirsch and Nachemson, 1954; Markolf and Morris, 1974; Kulak et al., 1976; Panjabi et al., 1984; Shirazi-Adl et al., 1984; Brinckmann and Horst, 1985; Goel et al., 1985, 1986; Seroussi et al., 1989; Shea et al., 1994). Compressive loading after removal of the NP increases disc deformation and radial bulging, and decreases disc height and intradiscal pressure when compared with the intact disc (Panjabi et al., 1984; Brinckmann and Horst, 1985). These changes are dependent on the quantity of NP removed (Goel et al., 1985; Brinckmann and Grootenboer, Corresponding author. Tel.: +1 215 895 2329; fax: +1 215 895 6760.
E-mail address: [email protected] (M. Marcolongo). 0021-9290/$ - see front matter r 2008 Published by Elsevier Ltd. doi:10.1016/j.jbiomech.2008.04.037
1991; Shea et al., 1994). Under compressive loading and partial removal of the NP, inner annulus layers bulge inward suggesting an increased radial stress within the annulus (Seroussi et al., 1989; Meakin and Hukins, 2000; Meakin et al., 2001). However, none of the previously cited studies directly investigated the change of the intervertebral disc stiffness at different loads after discectomy. A prior investigation (Shea et al., 1994) showed no statistical changes in disc stiffness at only 800 N in axial compression between intact and percutaneous or conventional discectomy, leaving a lack of understanding of disc stiffness changes for lower loads. In this work we investigated the effect of denucleation on compressive, tensile, bending and torsional behavior of the intervertebral disc as examined through human lumbar anterior column unit (ACUs). We hypothesized that the contribution of the nucleus pulposus to the mechanical behavior of the ACU was more predominant through the neutral zone than at the further limits of the applied loads and moments. A percutaneus approach was chosen for this study because it is less disruptive of the annulus wall than conventional discectomy.
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2. Materials and methods Twelve human (3M/3F) cadaver ACUs, were prepared by cutting through the two adjacent discs and removing the posterior elements. Removal of the zygapophyseal joints changes the intervertebral joint biomechanics under torsion and bending loads (Thompson et al., 2003), enabling the study of isolated annulus and nucleus biomechanics (Spenciner et al., 2006) by removing the effects of load sharing with the posterior elements. The absence of bridging osteophytes, fractures or severe disc space narrowing (o5 mm) was confirmed and intervertebral disc height (DH) was measured by a calibrated X-ray imaging system (Fluoroscan Imaging Systems, Northbrook, IL). The ACUs were potted using Smooth-Cast 300 (Smooth-On, Inc., Easton, PA) and kept moist throughout testing with phosphate-buffered saline. The specimens were divided into two data sets (DS1 and DS2). Progressive denucleation and nucleus pressure measurements were performed on DS1 in axial loading (AL). DS2 was tested in AL, axial rotation (AR), lateral bending (LB) and flexion/extension (FE) after maximum denucleation. The combination of multiaxial testing at each denucleation step would have resulted in a complicated test protocol with chances of altering the disc mechanics (Johannessen et al., 2004). Mechanical tests were conducted on a servohydraulic dynamic test system (Model 8874, Instron, Corporation, Norwood, MA) and the nucleotomy was performed with a 2 mm diameter automated vacuum tissue removal system (Nucleotome, Model 22500, Clarus Medical, LLC, Minneapolis, MN) using a posterolateral approach. After testing, each specimen was sectioned in the transverse plane, annular dimensions were measured (Table 1) and absence of severe disc degeneration was confirmed.
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Under a 50 N compressive load, the PT was retracted into the annular wall and a portion of the nucleus was removed. Tissue removal was performed in four 5 min intervals. At each step the removed tissue was dried and weighed. The PT was reinserted back into the NP and the disc was tested with the same loading protocol as the intact disc. Change in DH was evaluated by comparing the Instron actuator position at 50 N before and after the denucleation step.
2.2. Maximum denucleation study (DS2) A custom jig modeled as reported by Spenciner et al. (2006) was used to multiaxially test the ACUs. The test specimen and a six-degree-of-freedom load cell (model MC3A, AMTI, Inc., Watertown, MA) were positioned above an XY table (Fig. 1). Prior to the test, jig stiffness was evaluated. Jig stiffness was 1700 N/mm in compression, 30 N m/deg in torsion and 10 N m/deg in bending. Due to the jig setup, preconditioning was run in force control within the load range of DS1 (50 cycles, 50–150 N compression, havertriangle waveform, 1 Hz).
2.1. Partial denucleation study (DS1) Preconditioning was performed in displacement control (3% of DH, 50 cycles, havertriangle waveform, 1 Hz). The intact disc was then tested in load control from 150 N tension to 1500 N compression (5 cycles, havertriangle waveform, 0.1 Hz). A pressure transducer (Model 060S, Precision Measurement Company, Ann Arbor, MI) embedded in a 14 gauge needle (Hamilton, Reno, NV) was inserted inside the NP through the anterior annulus fibrosus (AF) and placed at half of the minor axis length from the outer annular wall. The disc was tested to ensure that its mechanical properties were not affected by the insertion of the pressure transducer (PT).
Fig. 1. Test setup for axial loading and axial rotation (left) and for lateral bending and flexion/extension (right): (A) anterior column unit, (B) specimen fixtures, (C) 6DOF load cell, (D) XY table, (E) vertical and rotational control of the actuator and (F) locking device used for axial compression and rotation tests.
Table 1 Specimens were divided into two DSs for the experiments described Disc height (mm)
Major axis (mm)
Minor axis (mm)
Age
Level
Gender
Data Set 1 (DS1) ACU #1 ACU #2 ACU #3 ACU #4 ACU #5 ACU #6 Mean7SD
8.0 10.6 10.9 10.5 5.4 9.2 9.172.1
51 53 58 50 40 44 4976
38 38 37 36 37 36 3771
64 64 64 30 30 30 47719
T12-L1 L2-L3 L4-L5 L4-L5 T12-L1 L2-L3
M M M F F F
Data Set 2 (DS2) ACU #1 ACU #2 ACU #3 ACU #4 ACU #5 ACU #6 Mean7SD
9.5 7.4 6.6 6.2 6.3 7.4 7.271.3
54 49 51 47 50 57 5174
38 34 35 34 34 42 3673
53 53 47 47 26 65 49713
L3-L4 L1-L2 L4-L5 L2-L3 L4-L5 L4-L5
M M F F F M
DS1 was used in the partial denucleation study of axial loading (compressive and tensile) while DS2 was used in the fully denucleated conditions for compression, bending and torsion.
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Five cycles of AL (150 to 1000 N, sinusoidal waveform, 0.1 Hz), AR, LB and FE (77.5 N m, sinusoidal waveform, 0.1 Hz) were applied separately to each intact ACU. A sinusoidal waveform was used to reduce jig inertia during the inversion of force; maximum compression was limited to 1000 N to prevent possible damage of the specimen brought on by multiaxial testing. Denucleation was performed under 50 N axial load for 20 min and the removed nucleus dry mass was weighed. The denucleated ACU was tested in AL, AR, LB and FE with the same protocol as the intact disc.
(Graphpad Software, San Diego, CA). Student’s t-tests (po0.05) were used to analyze differences between the two Data Set populations. Paired t-tests (po0.05) were performed to determine if the PT produced significant differences in disc mechanical behavior and to examine statistical differences between the intact and the maximum denucleated states of DS2. Repeated-measures one-way ANOVA (po0.05) with posthoc Tukey’s test were performed on DS1.
2.3. Data collection and analysis
3.1. Intact disc
Neutral zone (NZ), range of motion (ROM), stiffness and energy dissipation (HYS) were calculated (Fig. 2) for the intact and denucleated states of each ACU using the fifth loading cycle. ACU displacement and rotation were evaluated by subtracting the neutral position from the Instron axial position and angle. The neutral position was calculated as the mid-point between the positions (angles) at zero load (torque). For DS2 the displacement and rotation were corrected to take test apparatus stiffness into account. NZ was calculated (Wilke et al., 1998) as the difference in displacement (rotation) at zero load (torque) between the loading and unloading curves. ROM, defined as the maximum displacement (rotation) in one direction of motion was calculated: compressive and tensile ROM (cROM and tROM) from the AL test, flexion and extension ROM (fROM and eROM) from the FE test, and right and left ROM (rROM and lROM) from each one of the AR and LB tests. Stiffness in AL was calculated as the slope of the load-displacement curve during the loading cycle at 100, 0, 50, 100, 200, 400, 800 and 1400 N, respectively (Fig. 2(a)). Stiffness in AR, LB and FE were was calculated as the slope of the torque-rotation curve during the loading cycle (Wilke et al., 1997) at 0, 70.5, 72, 74 and 77 N m (Fig. 2(b)). The viscoelastic behavior of the specimen was evaluated with the area (HYS) enclosed by each loading cycle (Fig. 2(a) and (b)). Intradiscal pressure was calculated as the average between the loading and unloading curve at 100, 0, 50, 100, 200, 400, 800 and 1400 N. Since intradiscal pressure increases linearly with compressive load (Nachemson, 1960), a linear regression was performed and the ratio between intradiscal pressure and the axial applied stress (Nachemson, 1981) was calculated for each denucleation step. The values for the denucleated specimens were normalized to their intact values to reduce specimen variability associated with human biological tissue. Statistical analyses were performed using Prizm
No statistical difference (Student’s t-test) was observed in dimensions (DH, major and minor axes) and ages of the two DSs used in this study (Table 1). ROMs and NZ of the intact specimen for each different loading condition are reported in Table 2. The asymmetry of the disc with respect to the sagittal plane causes a difference of about 11 between fROM and eROM. Fig. 3(a) shows the mean axial stiffness at different loads for DS1 (with and without PT) and DS2. The NZ, tROM, cROM and stiffness values at each load in axial compression for DS1 were not statistically different (non-paired t-test) from the ones of DS2, however, a direct comparison between these two groups may be confounded by the different experimental setups and preconditioning protocols used for each of the two data sets. Fig. 3(b) shows the mean stiffness in AR, LB and FE at different moments for DS2. No statistical difference (paired t-test) was observed on the mechanical behavior of the human lumbar intervertebral disc by the insertion of the PT. Pressure in the nucleus increased with an average rate of 1.170.2 MPa/kN; it was 1.5870.14 times the axial applied stress. The intradiscal pressure for the intact disc at 50 N axial compression was 152730 kPa, in keeping with the values measured in vivo for lying prone (Quinnell and Stockdale, 1983; Wilke et al., 1999).
3. Results
Fig. 2. Typical loading curves for a lumbar intervertebral disc under (a) axial loading and (b) bending. The figures describe the methods used to calculate (a) neutral zone (NZ), compressive range of motion (cROM), tensile range of motion (tROM) and stiffness for axial loading, and to calculate (b) NZ, ROM in each direction of motion and stiffness for axial rotation, lateral bending and flexion/extension. The area within each curve represents the energy dissipation (HYS).
ARTICLE IN PRESS M. Cannella et al. / Journal of Biomechanics 41 (2008) 2104–2111 Table 2 Mechanical behavior of the intact specimens of DS1 and DS2 for ranges of motion (ROM) and neutral zones (NZ) Data Set 1 (DS1)
Data Set 2 (DS2)
Intact
Intact (pt)
Axial compression NZ (mm) tROM (mm) cROM (mm)
0.1270.05 0.3470.15 0.9870.17
0.1370.05 0.3370.19 0.9870.16
0.3070.26 0.6170.37 1.0470.38
Axial rotation NZ (1) rROM (1) lROM (1)
– – –
– – –
1.2770.62 4.5771.78 4.1871.13
Lateral bending NZ (1) rROM (1) lROM (1)
– – –
– – –
2.3171.33 6.4871.31 6.2971.39
Flexion/Extension NZ (1) From (1) eROM (1)
– – –
– – –
4.1072.55 8.9072.44 9.8473.88
Fig. 3. Intact stiffness in (a) axial loading (Data Sets 1 and 2) and (b) axial rotation, lateral bending and flexion/extension (Data Set 2).
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3.2. Partial denucleation (DS1) and maximum denucleation (DS2) in AL The dry weight of the NP removed after each denucleation step for DS1 were interpolated with a one-phase exponential association with a time constant of 18 min (Fig. 4). For DS2, after 20 min of denucleation, 0.15 g of dry NP was removed which was not statistically different (Student t-test) from the amount of tissue removed in DS1 after 20 min. In Fig. 5(a) the relative changes in DH, NZ, cROM and tROM during each denucleation step (DS1) and for maximum denucleation (DS2) are depicted with respect to the equivalent value for the intact disc. After 20 min of denucleation, there is a more than 15% reduction in DH, more than 300% increase in NZ, about 50% increase in cROM and about 300% increase in tROM, with respect to the intact values. The ANOVA test showed that DH (po0.001), cROM (po0.05) and tROM (po0.05) were significantly different from the intact values at each denucleation step, while NZ (po0.01) was significantly different from the intact values only after 15 min of denucleation. A paired t-test on DS2 showed that denucleated DH (p ¼ 0.004), NZ (p ¼ 0.041), cROM (p ¼ 0.043) and tROM (p ¼ 0.027) were significantly different from the intact values. Fig. 5(b) and (c) show the relative changes in axial stiffness and intradiscal pressure at different loads with respect to the intact disc after each denucleation step. The stiffness of the disc in tension (100 N) decreased to about 55% of the intact value after 5 min of denucleation. No additional significant changes were observed beyond 5 min. In the compressive range, the stiffness of the disc decreases with denucleation particularly in the lower range of loads (0–400 N). Compared with the intact state, at 50 N the stiffness reduction was on the order of 50–75%, while at 400 N the reduction was only 3–12% for different denucleation time. At 800 N, both the Tukey’s test (DS1) and the paired t-test (DS2) showed no statistical difference
Fig. 4. The specimens of Data Set 1 were denucleated using the Nucleotome for 5, 10, 15 and 20 min. There is a non-linear removal of tissue using the Nucleotome with an 18 min time constant.
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Fig. 5. Partially denucleated specimens were mechanically tested in AL. (a) Normalized DH, NZ, cROM and tROM for partially denucleated specimens. There is a marked loss of disc height from the intact condition which increases with increasing nucleus removal, while the ROM in tension and compression and the NZ increase well beyond that of the intact condition. (b) Normalized stiffness for partially denucleated specimens show decreasing stiffness with increased removal of tissue for the 50 and 200 N load levels. By 400 N the stiffness approaches that of the intact condition. (c) Normalized pressure for partially denucleated specimens at different loads shows a reduction in intradiscal pressure of the denucleated condition. (d) The same reduction is presented as the decrease of the ratio between pressure and applied force over disc area. (*statistically different from intact, **statistically different from intact and 5 min of denucleation, ***statistically different from intact, 5 and 10 min of denucleation).
in stiffness from intact values. For higher loads (1400 N) the stiffness slightly increased with respect to the intact value. In only one case (15 min) there was a statistical difference in the Tukey’s post-hoc test (po0.05). A student’s t-test showed no statistical difference in DH, NZ, cROM, tROM and stiffness values between DS1 and DS2 after 20 min of denucleation. As with the intact data for these groups, a direct comparison between may be
confounded by the different experimental setups and different preconditioning protocols used. In tension (100 N), intradiscal pressure decreased by more than 50% after 5 min of denucleation. It decreases with further denucleation, remaining statistically different from intact only (po0.001). Between 0 and 1400 N, the pressure values were always statistically different from the intact values. The ratio between pressure and force per disc
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area decreased by 13% and 33% after 5 and 20 min of denucleation, respectively. 3.3. Maximum denucleation (DS2) in AR, LB and FE NZ increased by about 150% (p ¼ 0.002), 165% (p ¼ 0.017) and 140% (p ¼ 0.01) for AR, LB and FE, respectively; ROMs increased by about 57–68% for AR (p ¼ 0.021, 0.011), 33–26% for LB (p ¼ 0.024, 0.046) and 37% for FE (p ¼ 0.021, 0.024) (Fig. 6(a)). Denucleated stiffness in LB and FE were reduced in the lower levels of applied moment (Fig. 6(b)) with respect to the intact values. In LB there was a statistical difference from the intact state at 0 N m (62% decrease, po0.001), 0.5 N m (24% decrease, p ¼ 0.046), 4N m (17% increase, p ¼ 0.023) and 7 N m (13% increase, p ¼ 0.003), whereas in FE statistical differences were at 7 N m (12% increase,
Fig. 7. Hysteresis values for each loading condition. For AL, there is an increase in HYS to approximately 56% over that of the intact disc. For the maximum denucleated (20 min) disc, AR had 46% increase in HYS, while LB and FE increased approximately 25% over the intact condition (*statistically different from intact, ***statistically different from intact, 5 and 10 min of denucleation).
po0.001), 4 N m (16% decrease, p ¼ 0.026) and 0 N m (58% decrease, p ¼ 0.002). Stiffness in AR was reduced from the intact state under all applied moments (Fig. 6(b)). The values were always statistically different from the intact values with the exception of 4 N m. The most significant difference (po0.01) from the intact condition was between 2 and 2 N m, in particular at 0 N m where stiffness was reduced by 64% (po0.001). Fig. 7 shows the increase in HYS values due to further denucleation steps for the four different loading conditions. All HYS values after 20 min of denucleation were statistically different from the intact values (po0.001), showing the importance of the nucleus in limiting the energy absorbed by the AF. 4. Discussion
Fig. 6. Mechanical behavior of denucleated specimens in AR, LB and FE. (a) Normalized NZ and ROMs for fully denucleated specimens shows the typical increase in neutral zone and range of motion for all loading conditions. The NZ measurements are particularly high indicating an increase in instability through this loading region in particular. (b) Normalized stiffness in AR, LB and FE for totally denucleated specimens shows again, a reduction in stiffness in the lower regions of applied moment for LB and FE, however, AR does not achieve the level of the intact disc through the applied moments tested here. (*statistically different from intact).
In this work, we examined a minimally invasive approach to denucleate the intervertebral disc through a 2 mm external diameter incision. The Nucleotome, aside from the annular insult, can create a continuous cavity, while other more mechanical approaches of denucleation can lead to pockets of removed tissue with the possibility of re-swelling as the tissue rehydrates, as suggested by Brinckmann and Grootenboer (1991). Compressive stiffness values for intact human motion segments reported in literature vary widely in a range of 700–3200 N/mm (White and Panjabi, 1990; Langrana et al., 1996), depending on differences with respect to age, degeneration stage, disc level, gender and testing protocol. Nonetheless, the compressive stiffness values for intact motion segments reported here fall within this range. Previous observations, as well as measurements reported here for the denucleated condition, showed that removal of nucleus tissue decreased DH and segment stiffness, and
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increased NZ and ROM (Hirsch and Nachemson, 1954; Markolf and Morris, 1974; Kulak et al., 1976; Panjabi et al., 1984; Shirazi-Adl et al., 1984; Brinckmann and Horst, 1985; Goel et al., 1985, 1986; Seroussi et al., 1989; Shea et al., 1994). A relatively linear relation between reduction in DH and mass of tissue removed (0.8 mm/1.0 g wet wt) was noted by Brinckmann and Grootenboer (1991). In this study, we observed a reduction in DH with a rate of about 0.4 mm decrease in DH for each 100 mg of dry weight tissue removed. Clearly, the depressurization associated with denucleation (Fig. 5(c)) plays a significant role in the disc dimensional changes. Decreased pressure requires either increased load supported by the AF and/or increased disc deformation. The increase in the NZ and tROM during the denucleation procedure, and the different reduction of compressive stiffness with denucleation as the load level increases, lead to the following observations. First, the mechanical behavior in AL for the denucleated disc is more unstable than in the intact condition, as previously reported (Wilke et al., 2006), and this instability increases with additional removal of nucleus tissue. Second, the role of the NP is more pronounced in the lower load regions. The initial loading of the disc, up to about 400 N, relies on the nucleus to create an intradiscal pressure that tensions the AF. With this tensioning, the disc can operate in the normal, intact condition. By the time the disc is loaded to a functional 800 N or beyond, the reduction in stiffness is minimized or reduced completely. Shea et al. (1994) also reported no difference in compressive stiffness between intact and denucleated conditions at 800 N of loading. In this region, the AF likely dominates the mechanical behavior of the disc, and while the stress state in the disc is different from the intact condition and the intradiscal pressure remains about 25% lower than the intact level, the global measurement of stiffness is closer to the intact values. Similar observations can be made for AR, LB and FE with a reduction of stiffness in the lower moment regions and an increased NZ. For LB and FE the stiffness values at higher levels of applied moment become equal to those of the intact condition, while in AR the stiffness values are below the level of the intact disc. It may be that in FE and LB the role of the nucleus is diminished compared to the role that it plays in AR. These results suggest that in FE and LB, the annulus can be put into a tensile state through two mechanisms: (1) internal pressurization from the NP and (2) traction from the tensile region of the bending condition. The tensioning of the AF in bending is not as dependent on intradiscal pressure as it is in AL. Even if the nucleus provides enough pressurization to the disc, shown by a 2.4-fold increase in NZ when the nucleus is removed, it also shifts towards the portion of the AF tensioned by the applied moment. Stiffnesses in AR were more affected by denucleation than LB and FE. In this loading condition, the nucleus plays an important role in the pressurization of the annulus
for maintaining resistance against torsion via the ‘‘screwdown’’ motion the disc takes in rotation. Further, the coupling between the compression and torsion stiffness matrices of the annulus could be highly affected by the uniform tensioning of the AF (Broberg, 1983). Without tension (for the case of the denucleated disc) the fibers are slack and need to undergo significantly more deformation to obtain the same degree of resistance to the externally applied moments. This study is limited, in part, because although every effort was made to limit or eliminate off-axis loading in the ‘‘pure’’ load conditions, data from the 6 DOF load cell indicated that there was approximately 5% off-axis loading, which may have some influence on the mechanical behavior of the disc. Even though the specimens were carefully centered in the potting fixtures to avoid misalignments between the geometrical axis of the specimen and the fixture axis, the off-axis loads and moments were mainly due to the offset between the actuator axis and the specimen axis of rotation that may move during the load application. Although removal of the zygapophyseal joints can significantly change the biomechanics of the disc under torsion and bending loads (Thompson et al., 2003), compressive stiffness at loads under 500 N is not altered (Gardner-Morse and Stokes, 2004) and there is little effect on intradiscal pressure up to 800 N (Reuber et al., 1982). Posterior elements were removed in this study to thoroughly understand isolated changes within the disc, rather than in the entire triple joint complex. In addition, with the minimally invasive nucleus removal system, some NP can remain adjacent to the insertion site where it is difficult to direct the cannula of the removal system. This tissue may have also affected some of the mechanical behavior that we observed. Further, it is difficult to extend these data to the clinical situation precisely because of the ‘‘pure’’ loading conditions. Nonetheless, these types of loading conditions have been used by others in the laboratory to understand disc behavior (Goel et al., 1985; Spenciner et al., 2003; Spenciner et al., 2006).
5. Conclusions From this work and the work of others (Goel et al., 1985; Brinckmann and Grootenboer, 1991; Shea et al., 1994), we see that the NP is important in disc mechanics. As shown in this work, through a lower load and moment region, where there is insufficient loading to engage the AF into a tensile or optimal load bearing state, the NP assumes a critical role in mechanical behavior. While discectomy is used as a method to alleviate pain of the disc, the long-term effects of an unstable disc are not well-understood. The relationship, if any, between mechanical instability of the disc and a clinically painful disc are worthy of continued study as is the role that the herniated/degenerated nucleus plays in creating pain and accelerated degeneration.
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