The biomechanical Nature of nucleus Pulposus in a fissured disc revealed by quantitative MRI performed under dynamic conditions

Keywords:

• Intervertebral disc
• nucleus biomechanics
• quantitative MRI
• radial fissure
• mechanical diagnosis and therapy

1. Introduction

Physiotherapy is a health care discipline based on movement assessment and treatment. Mechanical Diagnosis and Therapy (MDT) is a reliable and efficient approach using repeated end-range movements to reverse symptoms among patients presenting with musculoskeletal disorders (May et al. 2018). The rationale behind this approach in regard to low back pain is based on the dynamic nature of the pathological intervertebral disc biomechanics. When the Annulus Fibrosus (AF) is centrifugally fissured (radial fissure), the Nucleus Pulposus (NP) could spread into the fissure under patient movement influence. For example, an extension movement can push the NP into an anterior fissure and increase patient pain while a flexion movement would purge the fissure of the NP and reduce patient pain.

The mechanical behaviour of NP in a healthy disc has already been fully described. However, evidence for disc pathological condition remains conflicting, inconsistent and suffers from methodological weaknesses due to imaging technical limitations (Kolber and Hanney 2009).

MRI technologies have significantly improved over the last decades, increasing the accuracy of data acquisition and processing. For example, quantitative MRI with T₁ and T₂ mapping returns relaxation times pixel-wise, allowing objective characterization of the disc (Galley et al. 2017). Nevertheless, such techniques have never been used to assess the nucleus dynamic properties.

We aimed to characterize the mechanical behavior of intervertebral disc by using MRI relaxometry (T₁/T₂ mapping) as a quantitative marker to assess, quantify and compare the NP displacement and strain of a cadaveric lamb vertebral segment with intact and fissured disc according to sagittal loads.

2. Methods

2.1. Specimen preparation

We used a fresh lamb spine segment comprising 3 vertebrae and 2 intervertebral discs, wrapped in a saline soak gauge and placed under creeped load 1.5 h before the experiments with an axial compressive load. The same disc was characterized before and after inducing an anterior tear.

2.2. MRI acquisition and processing

We used a 3.0-Tesla imager (Siemens, Munich, Germany) to run 3 different types of sequences for each mechanical state:

• multi-slice 2D T₂-weighted Turbo Spin Echo

• multi-echo spin echo sequence for T₂ mapping

• Inversion Recovery TSE for T₁ mapping

Images were processed using scripts and functions developed with MATLAB (MathWorks, Natick, MA). The maps were obtained after normalizing the data and fitting them pixel wise according to: $$\begin{array}{c}{S}_1=a·(1-2·e^{\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$T_1$}\right.})\\ S_2=b·e^{-\raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$T_2$}\right.}\end{array}$$ S is the normalized signal intensity in arbitrary units, t the time obtained from the echo train or inversion times, and a/b constants used for fitting purposes.

2.3. Mechanical loading

We controlled the boundary conditions by fixing the lower vertebra and by using two different elastic systems to impose displacements with an arm level on the upper vertebra providing an angular variation and combined axial/bending loads (Figure 1-a). We imposed 3 loading states (neutral, flexion and extension) on both disc conditions: healthy vs fissured.

Figure 1. Measurement of the nucleus sagittal boundary displacement (w), the nucleus pulposus (NP) migration (m) within the annulus fibrosus (AF) and nucleus sagittal strain according to compression (F_c) and bending (F_b) force: (a) setup; (b) reference mechanical loading state (neutral, non-fissured disc); (c) mechanical state with partial migration of the nucleus.

2.4. Measurements

We used a mark tracking system to measure the bending angle and the axial strain. We measured the global nucleus displacement by evaluating: 1- the centroid position of NP relative to the centroid position of the spinal canal, 2- the displacement on sagittal boundaries of the NP (w-w₀), and 3- the NP strain in the sagittal direction ((w – w₀)/w₀). We also estimated the degree of nucleus migration into the fissure by calculating the ratio of the nucleus displacement (m) and the thickness of the annulus (Figure 1).

3. Results and discussion

We observed a comparable biomechanical behavior of the disc as was previously published (Kolber and Hanney 2009), showing an anterior migration of the NP in extension and a posterior migration in flexion.

The radial fissure was visible using all MRI sequences. On T₂-weighted images, its morphology changed depending on the bending load with a grade 2 fissure on the Dallas classification when the disc is in extension and a grade 1 when it is in flexion (Sachs et al. 1987).

The quantitative mapping confirmed the NP migration into the fissure with respect to the applied movement. In extension, the T₁ relaxation time of the tissue inside the fissure is superior to 800 ms, similar to the one measured in the nucleus. This characteristic disappeared in flexion (see Figure 2). Our results show that the nucleus elongated by 21.1 mm in extension while it contracted by 3.1 mm in flexion. The anterior boundary of the nucleus moved forward by 5.0 mm in extension while it moved backward by 0.6 mm in flexion (see Table 1).

Figure 2. T₁ map of the fissured disc positioned in extension (left) and flexion (right).

Table 1. Mechanical measurement of the disc angle variation, NP strain and displacement (mean values).

M^al states	Angle (°)	NP strain (%)	NP global (mm)	NP boun^ary (mm)	NP mig^o (%)
neutral	4.7	–	–	–	–
flexion	−8.8	−5.6	−3.65	−2.20	0
extension	10.4	−6.1	−0.95	2.22	0
FISSURE
extension	11.1	21.1	3.70	5.0	100
flexion	−8.9	−3.1	−1.60	−0.6	21
neutral	8.7	20.8	1.55	4.5	100

The AF fissure modified the biomechanics of the NP by increasing its strain and its displacement in all our measurements (see Table 1).

These results revealed a relationship between disc abnormality and displacement possibility with a pattern similar to the one hypothesized by MDT therapists.

These results are encouraging but need to be validated in larger sample sizes, with human specimen at different stages of disc degeneration and by using different fissure orientations.

4. Conclusions

Quantitative MRI mapping is an effective way of assessing disc radial fissure mechanical behaviour. The NP moves in a consistent pattern according to bending load with either healthy or fissured disc. However, the fissure increases the displacement preference which appears consistent with the clinical hypothesis raised by MDT practitioners.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

MY, NS, and MS are supported by the Swiss National Science Foundation (PP00P2_170575).