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Functional Spinal Unit

Original Editor - Nehal Shah

Top Contributors - Nehal Shah, Kim Jackson and Vidya Acharya  

Introduction

Human spine consist of vertebrae of the cervical, thoracic, lumbar, sacral and coccygeal regions, the intervertebral discs, ligaments, rib cage, and spinal musculature. This will enable global view towards function of the spine, but will not enable specific features that are needed for understanding specific components like annulus fibrosus etc[1]

There are two methods to describe human spine[2]:

  • The finite element method - It represents a computer simulation methods. Its application enables understanding of bone biomechanics in normal and abnormal conditions like scoliosis, fractures, degenerative disc deseases and osteoporosis[3]It was initially proposed by Richard Courant in 1922[4]This method is mainly employed in surgical processes where it aids the process of operation planning and deeper vision into the common injuries of spine[2]. Its understanding helps designing the elements that can substitute disc or ligaments[5], study the effect of spinal stabilization[6]or in research that finds effects of external orthosis for spine[7]. In its making, it also considers spinal cord which is often ommitted in mechanical models[8]It is a versatile and complex model and hence though it can provide data on external and internal spinal devices, it is hard to design a new device only with the use of FEM[2]
  • The Functional Spinal Unit- The FSU is also known as a spinal motion segment or articular triad. It was conceptualized by Panjabi and White in 1990s. It consists of two vertebrae connected with passive elements like ligaments, a disc and each vertebra connected to multiple active elements like muscle system[2]It is the smallest physiological unit of spine and exhibit biocmechanical properties mimicking that of entire spine[9].

Human functional spinal unit (FSU) comprises of two adjacent vertebrae, the intervertebral disc (IVD), the cartilage endplates and the connecting ligamentous tissues[10]. Functional Spinal Unit is capable of Six Degrees of freedom[11] With aging, there will be changes in mechanical properties of trabeculae, cortical bone and IVD which can influence the movement of the FSU (displacements, rotations and disc bulges)[10]

Main Functions of FSU

Main functions of FSU are as under

  • Protect neural structures
  • Allows co-ordinated movements
  • Provides scaffolding for the rest of the muscles and ligaments of the body
Functional Spinal Unit[12]

Mechanics of Functional Spinal Unit

Spinal response to loading for eg Axial compression, flexion-extension bending, lateral bending and axial torsion are controlled by FSU[11]With aging, there will be changes in the mechanical properties of FSU which can affect its movements like displacements, rotations, disc bulges etc[13] The intervertebral disc stabilizes the FSU and reduces the biomechanical forces that are transmitted to the vertebral endplates

To describe spinal movements, International Organisation for Standardisation (ISO) and American Society for Testing and Materials (ASTM) for spinal testing have adopted a co-ordinate system and described movements about X - antero-posterior, Y - Lateral and Z - superior-inferior axes. Lateral Bending occurs about X-axis, flexion/extension occurs about Y axis and axial rotation occurs about Z axis.[11]There are regional variations in the loads and motions in FSU. Lumber vertebrae experience higher loads and reduced motion compared to cervical vertebrae. These differences are seen even within the same region. Additionally weight and activity levels differ from person to person and vertebral recruitment pattern also differ with different motions. To make this understanding more complex, motions are kinematically coupled. Eg - coupling of translational motion and flexion-extension and coupling of axial rotation and lateral bending[11]

Loading of Spine

Cervical spine is loaded by the head and lumbar spine by upper body weight. Additionally complex group of muscles and ligament responsible for stability and mobility add to this loading. Loading at cervical spine is much lesser than the lumbar spine. In the cervical spine this loading is usually upto three times the headweight which can increase manyfold in everyday activities which involve maximum muscle contraction like during contact sports or axial weight lifting. Lumbar spine on the other hand experiences loads of 1.5 to 2 times body weight even in inactivity state like standing and sitting. This increases with increasing activity levels like compressive forces rise to more than three times the body weight during level walking. With vigorous activity, extremes of motion, weight lifting etc, this can rise to 5-6 times the body weight[11]

Loading of FSU

Due to repeated loading of FSU in flexion and extension, there will be changes in the growth zones and endplates which are the early signs of fatigue and can lead to disc degeneration, disc herniations and ring apophyseal injuries[14]There will be displacement of FSU under every loading and this leads to increase in stiffness of FSU[15]With an increase in loading rates, IV disc's ability to dissipate energy reduces. This will ultimately make IV disc less effective in shock absorbing[11]Loading and boundary conditions in FSU can significantly effect stress-strain distributions within FSU.[16]Impact loading can affect the stiffness of a spinal segment. Studies have indicated that even during the shorter duration of impact, there will be high dynamic stiffness which will increase the stability of the spinal segment against the impact load. But there will be corresponding increase in the stresses within the vertebral body which may lead to endplate fractures. Hence this suggest that chances of fractures are more under shorter impact conditions. Depending upon the strength of that particular region, fracture will be in the endplate region or the posterior wall of the cortical shell. Impact duration does not affect nucleus pressure, rather it depends on the magnitude of the impact force[17]

Consequences of abnormal FSU

Understanding of FSU will enable easy understanding of various spinal conditions. An abnormal FSU can have a number of consequences.

  • Segmental Spinal Instability - Injury to the IV disc can lead to Segmental spinal instability. This is because IV disc stabilizes FSU[11].According to White and Panjabi, segmental spinal instability is the loss of ability of the spine to maintain normal relations among vertebras during physiological loading[18]
  • Chronic low back pain - Injury to the IV disc can also cause chronic low back pain.
  • Radiculopathy / Nerve irritation
  • Postural abnormalities - Kyphosis, Scoliosis etc

References

  1. ↑ Oxland TR. Fundamental biomechanics of the spine—what we have learned in the past 25 years and future directions. Journal of biomechanics. 2016 Apr 11;49(6):817-32.
  2. ↑ 2.0 2.1 2.2 2.3 Ciszkiewicz A, Milewski G. A novel kinematic model for a functional spinal unit and a lumbar spine. Acta of bioengineering and biomechanics. 2016;18(1):87-95.
  3. ↑ Naoum S, Vasiliadis AV, Koutserimpas C, Mylonakis N, Kotsapas M, Katakalos K. Finite element method for the evaluation of the human spine: a literature overview. Journal of Functional Biomaterials. 2021 Jul 31;12(3):43.
  4. ↑ Bowden A. Finite element modeling of the spine. Spine technology handbook. 2006 Aug 24.
  5. ↑ Borkowski P, Marek P, Krzesiński G, Ryszkowska J, Waśniewski B, Wymysłowski P, Zagrajek T. Finite element analysis of artificial disc with an elastomeric core in the lumbar spine. Acta Bioeng. Biomech. 2012 Jan 1;14(1):59-66.
  6. ↑ Filipiak J, Pezowicz C. Experimental investigation of durability of transpedicular screw-vertebra connection. Ortopedia, traumatologia, rehabilitacja. 2004 Apr 1;6(2):213-21.
  7. ↑ Clin J, Aubin CE, Parent S, Labelle H. Biomechanical modeling of brace treatment of scoliosis: effects of gravitational loads. Medical & biological engineering & computing. 2011 Jul;49:743-53.
  8. ↑ Czyż MA, Ścigała K, Będziński RO, Jarmundowicz WŁ. Finite element modelling of the cervical spinal cord injury–clinical assessment. Acta Bioeng Biomech. 2012 Dec 1;14(4):23-9.
  9. ↑ Ramachandran M, editor. Basic orthopaedic sciences. CRC Press; 2018 Sep 3.
  10. ↑ 10.0 10.1 Hammad A, Goel V, Ahmad AA. Biomechanics of the Thoracic Spine. InHandbook of Orthopaedic Trauma Implantology 2023 Nov 1 (pp. 1853-1870). Singapore: Springer Nature Singapore.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 Vicars R, Hall R, Hyde PJ. 7.14 Wear: Total intervertebral disc prostheses. InComprehensive Biomaterials II 2017 Jan 1 (pp. 246-264). Elsevier Korea.
  12. ↑ https://commons.wikimedia.org/wiki/File:Functional_spinal_unit_(FSU).webp
  13. ↑ Somovilla-Gómez F, Lostado-Lorza R, Corral-Bobadilla M, Escribano-García R. Improvement in determining the risk of damage to the human lumbar functional spinal unit considering age, height, weight and sex using a combination of FEM and RSM. Biomechanics and modeling in mechanobiology. 2020 Feb;19:351-87.
  14. ↑ Thoreson O, Ekström L, Hansson HA, Todd C, Witwit W, Swärd Aminoff A, Jonasson P, Baranto A. The effect of repetitive flexion and extension fatigue loading on the young porcine lumbar spine, a feasibility study of MRI and histological analyses. Journal of experimental orthopaedics. 2017 Dec;4:1-9.
  15. ↑ Asano S, Kaneda K, Umehara S, Tadano S. The mechanical properties of the human L4–5 functional spinal unit during cyclic loading: the structural effects of the posterior elements. Spine. 1992 Nov 1;17(11):1343-52.
  16. ↑ Talukdar RG, Mukhopadhyay KK, Dhara S, Gupta S. Numerical analysis of the mechanical behaviour of intact and implanted lumbar functional spinal units: Effects of loading and boundary conditions. Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine. 2021 Jul;235(7):792-804.
  17. ↑ Lee CK, Kim YE, Lee CS, Hong YM, Jung JM, Goel VK. Impact response of the intervertebral disc in a finite-element model. Spine. 2000 Oct 1;25(19):2431-9.
  18. ↑ Lazennec JY, Saillant G. Spinal Instability and Imbalance: Definition, Clinical Manifestations, and Biomechanics. Biomechanics and Biomaterials in Orthopedics. 2004:405-34.
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