Biomechanics of the Healthy and Diseased Spine

4 Biomechanics of the Healthy and Diseased Spine

Hans-Joachim Wilke and Fabio Galbusera

Abstract

The intervertebral disc (IVD) is a pad of soft tissue that uniformly distributes the stresses due to spinal loads while ensuring the mobility of the motion segment. The disc consists of an inner nucleus pulposus (NP) with high water and proteoglycan content, and an outer annulus fibrosus (AF) mostly consisting of oriented lamellae of collagen fibers. Due to its structure and composition, the disc has the capability to imbibe water, and therefore develops an intradiscal pressure (IDP) that gives it a high capacity to sustain compressive loads and to act as a shock absorber.

Degenerative disorders of the IVD feature morphological changes (decreased disc height, end plate defects and sclerosis, osteophytes, and size reduction of the NP) as well as changes in the tissue composition (loss of water content, which causes the black disc appearance seen in magnetic resonance imaging [MRI]). These degenerative alterations have a direct impact on the biomechanics of the motion segment. The loss of water content induces a decrease of the IDP and the possible presence of stress peaks in the AF, which may be responsible for pain episodes.

Spinal fusion is considered the golden standard for the surgical treatment of symptomatic degenerative disorders of the IVD. Several authors have hypothesized that moderately degenerated discs may lead to spinal instability, that is, an abnormal motion response to physiological loads, which has paved the way for novel surgical treatments not aimed at spinal fusion, but rather at restoring the physiological spine stiffness. However, because numerous studies have contradicted the instability hypothesis, the issue is still under debate.

Keywords: compressive load, disc height, intradiscal pressure, spinal instability, stress distribution, swelling

4.1 Historical Perspective

Knowledge about the intervertebral disc (IVD) and its biomechanical functions dates back to the beginning of the scientific era. In 1680, Giovanni Alfonso Borelli published the first known essay about biomechanics, De Motu Animalium, in which the time-dependent mechanical response of the IVD was described.¹ The first modern extensive investigation of the structure of the disc as well as its changes due to aging and pathological processes was carried out in 1926 by Georg Schmorl, a pathologist who analyzed disc specimens gathered from human cadavers using laboratory techniques. For the first time, disc height loss, end plate defects, and annular tears were described; disc degeneration started to have an influence on the clinical and surgical management of pathologies such as back pain and sciatica.²

Spine fusion with autologous grafts and, beginning in the 1960s, with instrumentation such as pedicle screws and internal fixators remained the gold standards for the treatment of many degenerative spinal disorders for decades,³ until the concept of spinal instability emerged in the 1980s. Despite being first mentioned 30 years before,⁴ instability acquired a practical relevance after Kirkaldy-Willis and Farfan presented their theory about the degenerative cascade in which instability constitutes a key stage.⁵ Supported by the work by Pope and Panjabi,⁶ who defined instability as a pathological loss of spinal stiffness, i.e., hypermobility, the paper paved the way for novel surgical treatments not aimed at spinal fusion, but rather at restoring the physiological spine stiffness. Following the principle that restraining abnormal spinal motion to physiological levels may be an effective treatment for back pain, several motion-preserving, dynamic stabilization devices were introduced to the market and are still widely used.⁷ Success rates appear to be satisfactory in select patients, but whether these implants constitute an improvement with respect to instrumented fusion is not clear yet.⁸ Meanwhile, papers that refined the definition of spinal instability⁹ as well as biomechanical studies aimed at proving or disproving the validity of this concept have been published.¹⁰^,¹¹^,¹²

This chapter presents the changes occurring to IVDs due to aging and pathological processes and their implications in biomechanical terms. Special attention is given to the concept of spinal instability and to its relation to the radiological, morphological, and tissue composition changes of the degenerative IVD.

4.2 Biomechanics of the Healthy Intervertebral Disc

The IVD is a pad of soft tissue located between two adjacent vertebrae, the function of which is to uniformly distribute the stresses due to spinal loads while ensuring the mobility of the motion segment.¹³ The disc consists of an inner nucleus pulposus (NP) with high water and proteoglycan content,¹⁴ and an outer annulus fibrosus (AF) mostly consisting of oriented lamellae of collagen fibers. Proteoglycans are highly negatively charged molecules synthesized by the NP cells, which tend to attract cations in the interstitial fluid to achieve electrical equilibrium.¹⁵ Their presence gives the disc the capability to imbibe water, which creates an osmotic pressure gradient between the disc itself and the external environment, commonly named as intradiscal pressure (IDP). Due to its peculiar structure, the IVD has a high capacity for sustaining compressive loads and acting as a shock absorber.¹⁶ Pressure transducers implanted in living subjects have been employed to measure the IDP in different postures and during various daily activities. The technique was introduced by Nachemson and coworkers¹⁷^,¹⁸ and used in several later studies.¹⁹^,²⁰ Values ranging from 0.1 MPa during bed rest up to 2.3 MPa during weight lifting were recorded¹⁹ (▶ Fig. 4.1, ▶ Fig. 4.2, ▶ Table 4.1), thus demonstrating the capability of the disc to sustain high mechanical loads.

Fig. 4.1 Intradiscal pressure in a healthy L4–5 disc during relaxed sitting without backrest, relaxed standing, and lifting 20 kg, bent over with round back.¹⁹

Fig. 4.2 Measurements from in vitro tests in flexion-extension applying ± 7.5 Nm plus a follower load of 280 N. Intradiscal pressure changes strongly with increasing age (degeneration).

Table 4.1 Absolute values of intradiscal pressure for different postures and exercises, normalized to relaxed standing (chosen arbitrarily as 100%) (after Wilke et al¹⁹)

Position	Pressure (MPa)	%
Lying supine	0.1	20
Side-lying	0.12	24
Lying prone	0.11	22
Lying prone, extended back, supporting elbows	0.25	50
Laughing heartily, lying laterally	0.15	30
Sneezing, lying laterally	0.38	76
Peaks by turning around	0.7–0.8	140–160
Relaxed standing	0.5	100
Standing, performing Valsalva maneuver	0.92	184
Standing, bent forward	1.1	220
Sitting relaxed, without backrest	0.46	92
Sitting actively straightening the back	0.55	110
Sitting with maximum flexion	0.83	166
Sitting bent forward with thigh supporting the elbows	0.43	86
Sitting slouched into the chair	0.27	54
Standing up from the chair	1.1	220
Walking barefoot	0.53–0.65	106–130
Walking with tennis shoes	0.53–0.65	106–130
Jogging with hard street shoes	0.35–0.95	70–190
Jogging with tennis shoes	0.35–0.95	70–190
Climbing stairs, one at a time	0.5–0.7	100–140
Climbing stairs, two at a time	0.3–1.2	60–240
Walking down stairs, one at a time	0.38–0.6	76–120
Walking down stairs, two at a time	0.3–0.9	60–180
Lifting 20 kg, bent over with round back	2.3	460
Lifting 20 kg as taught in back school	1.7	340
Holding 20 kg close to the body	1.1	220
Holding 20 kg, 60cm away from the chest	1.8	360
Carrying 20 kg (in the left or in the right hand)	1.0	200
Carrying 40 kg (20 kg left and 20 kg right)	0.9	180
Pressure increase during the night rest (over a period of 7 h)	0.1–0.24	20–48

The AF has a fundamental role in determining the strength of the IVD. Under loading, the NP is pressurized and creates a tensile stress in the collagen fibers of the AF, which bulges outward.²¹^,²² The optimized organization of the fibers allows them to be loaded mostly in tension under loading conditions typical of daily life such as bending, thus acting similarly to the fiber reinforcement of a car tire. The IVD is connected to the vertebral bone through osseous and cartilaginous layers called end plates, which contribute in uniformly distributing the spinal stresses and in damping the response to fast loads.²³

4.3 Radiological Changes with Potential Biomechanical Relevance

The assessment of the degenerative changes of the spine with biomedical imaging techniques has a fundamental importance in the clinical management of spinal disorders, because imaging constitutes a substantial part of the information available to the clinician in diagnostic and therapeutic decision making. Establishing a link between the degenerative changes that can be highlighted with imaging methods and their biomechanical relevance is therefore of critical importance to improving the clinical management of disc degeneration.

Planar radiography allows for an easy and direct visualization of some changes related to the degenerative disease. A degenerated motion segment commonly appears as having low intervertebral space due to disc height loss and reduced segmental lordosis, as well as increased bone density in the end plates and adjacent bony tissue (end plate sclerosis).²⁴ In frequent cases, osteophytes, that is, heterotopic bone formations commonly located near the anterior and posterior longitudinal ligaments, are also visible.²⁵ End plate anomalies and defects (Schmorl’s nodes, notches, shape irregularities), either related to the degenerative processes or not, as well as foraminal stenosis can in some cases be detected. Subtle abnormalities and lesions of the IVD can be diagnosed by means of provocative discography, that is, injection of radiopaque contrast medium in the disc aimed at highlighting small tears and provoking pain in symptomatic discs. This method is however currently under heated debate due to its high false-positive rate and the risk of degeneration in the long term due to the needle puncture.²⁶^,²⁷

Sagittal radiographic projections in flexion-extension are commonly used to diagnose the so-called radiological instability of the lumbar spine, for which various definitions have been provided (e.g., sagittal translation >3 mm,²⁸ 10–15% of the vertebral width, ²⁹ or segmental range of motion > 20 degrees at L4–5³⁰). However, the clinical relevance of radiological instability as well as its correlation with back pain remains unclear.

Magnetic resonance imaging (MRI) allows for a high contrast within soft tissues such as the IVD and ligaments.³¹ As a matter of fact, the most commonly employed grading system for disc degeneration (the Pfirrmann scale) is based on MRI scans.³² T2-weighted imaging of the NP shows a progressive reduction in the MRI signal with degeneration, which is related to a loss of water content and fibrotization of the tissue. A dehydrated, T2-hypointense disc is usually referred to as a black disc and is commonly considered one of the most important signs of disc degeneration. MRI also allows for an excellent characterization of the disc height and morphology; it is the method of choice for the detection of Schmorl’s nodes and end plate defects, as well as for Modic changes.³¹ Recent studies showed the potential of ultra high field MRI (>7 T) for the detection of small disc tears that are currently not discernible with conventional clinical scanners, thus also highlighting the possible use of MRI as an alternative to provocative discography.³³

Computed tomography (CT) is seldom used in cases of degenerative disc disease (DDD) for diagnostic purposes; nevertheless, it can be useful for assessing spinal stenosis and for planning decompression surgeries.³⁴ CT is also commonly employed in degenerative processes involving the facet joints, due to its capability to show reductions in the articular space and local calcifications.³⁵

4.4 Morphological and Tissue Composition Changes in the Intervertebral Disc

The radiological signs mentioned above outline a degenerative disorder of the IVD featuring morphological changes (decreased disc height, end plate defects and sclerosis, osteophytes, and size reduction of the NP), as well as changes in the tissue composition (e.g., loss of water content, which results in the black disc appearance). Macroscopic analysis of cadaveric disc specimens with different degrees of degeneration showed that the radiographic signs correspond well, despite some exceptions, to the actual changes occurring in the motion segment²⁴ (▶ Fig. 4.3). Nevertheless, less apparent alterations in the tissue structure and composition not discernible by means of conventional biomedical imaging were also found to take place.³⁶

Fig. 4.3 Healthy and degenerated discs with increasing degrees of degeneration (after Wilke et al²⁴). Macroscopic changes and inward and outward bulging of the nucleus.

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