materials Review Materials for the Spine: Anatomy, Problems, and Solutions Brody A. Frost 1, Sandra Camarero-Espinosa 2,* and E. Johan Foster 1,* 1 Department of Materials Science and Engineering, Macromolecules Innovation Institute, Virginia Tech, Blacksburg, VA 24061, USA; [email protected] 2 Complex Tissue Regeneration Department, MERLN Institute for Technology-inspired Regenerative Medicine, Maastricht University, P.O. Box 616, 6200MD Maastricht, The Netherlands * Correspondence: [email protected] (S.C.-E.); [email protected] (E.J.F.) Received: 29 November 2018; Accepted: 5 January 2019; Published: 14 January 2019 Abstract: Disc degeneration affects 12% to 35% of a given population, based on genetics, age, gender, and other environmental factors, and usually occurs in the lumbar spine due to heavier loads and more strenuous motions. Degeneration of the extracellular matrix (ECM) within reduces mechanical integrity, shock absorption, and swelling capabilities of the intervertebral disc. When severe enough, the disc can bulge and eventually herniate, leading to pressure build up on the spinal cord. This can cause immense lower back pain in individuals, leading to total medical costs exceeding $100 billion. Current treatment options include both invasive and noninvasive methods, with spinal fusion surgery and total disc replacement (TDR) being the most common invasive procedures. Although these treatments cause pain relief for the majority of patients, multiple challenges arise for each. Therefore, newer tissue engineering methods are being researched to solve the ever-growing problem. This review spans the anatomy of the spine, with an emphasis on the functions and biological aspects of the intervertebral discs, as well as the problems, associated solutions, and future research in the field. Keywords: spinal anatomy; intervertebral disc; degenerative disc disease; herniated disc; spinal fusion; total disc replacement; tissue engineering 1. Human Spinal Anatomy The spine, or vertebral column, is a bony structure that houses the spinal cord and extends the length of the back, connecting the head to the pelvis [1]. The most important function of the spine is to protect the spinal cord, which is the nerve supply for the entire body originating in the brain [1]. Along with this major function, others include supporting the mass of the body, withstanding external forces, and allowing for mobility and flexibility while dissipating energy and protecting against impact. The spine is connected to the muscles and ligaments of the trunk for postural control and spinal stability [2]. It can be separated into five distinct sections, the cervical spine, the thoracic spine, the lumbar spine, the sacrum, and the coccyx, all of which are comprised of independent bony vertebrae and intervertebral discs [3], Figure1. To describe the differences between the spinal column sections, each one has been further discussed. Materials 2019, 12, 253; doi:10.3390/ma12020253 www.mdpi.com/journal/materials Materials 2019, 12, 253 2 of 41 Materials 2019, 12, x FOR PEER REVIEW 2 of 40 (a) (b) FigureFigure 1.1. OverviewOverview of the of the vertebral vertebral column column with with each each specific specific section section labeled labeled for clarification for clarification (a). The (a ). greenThe greenhighlighted highlighted section section refers refers to the to part the of part the of spine the spine that contain that contain individual individual vertebrae, vertebrae, as well as wellas intervertebralas intervertebral discs discs (IVD). (IVD). The The structure structure of the of the vertebrae vertebrae and and IVD IVD (green (green highlighted) highlighted) have have bee beenn addedadded for for better better visualization visualization (b (b) )[[4].4]. 1.1. Cervical Spine 1.1. Cervical Spine TheThe cervical cervical section section of of the the spine spine consists consists of of seven seven vertebrae vertebrae (C1 (C1–C7)–C7) and and six six intervertebral intervertebral discs, discs, andand extends extends from from the the base of thethe skullskull toto the the top top of of the the trunk, trunk, where where the the thoracic thoracic vertebrae vertebrae and a ribnd cagerib cagestart start [3] Figure[3] Figure1. 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This can lead to feelings of weakness, numbness, tingling, and potentially loss of feeling. 1.2. Thoracic Spine The thoracic section of the spine consists of twelve vertebrae (T1–T12) and twelve intervertebral discs, and extends from the bottom of the cervical spine to the beginning of the lumbar spine [3], Figure1. The thoracic spine’s major functions include heavy load bearing and protection of the spinal cord, supporting posture and stability throughout the trunk, and connection of the rib cage that houses and protects vital organs, such as the heart and lungs [10]. This connection poses a significant decrease in mobility, as compared to the cervical spine section, and a greater stability and support of the entire trunk, usually leading to fewer cases of disc degeneration [10,11]. The vertebrae that make up the thoracic spine have body sizes (thickness, width, and depth) that drastically increases descending from T1 to T12, corresponding to an increased load bearing that is transferred from the vertebra above [12]. All other features stay relatively the same throughout, except for the T11 and T12 vertebrae, in which no ribs are connected. Along with this change towards the end of the thoracic spine, the T12 plays an interfacial role and has distinct thoracic characteristics superiorly and lumbar characteristics inferiorly for articulation with the L1 vertebra, allowing rotational movements with T11 while disallowing movements with L1 [12]. The thoracic spine contains nerves that are much less specialized