ReviewFundamental biomechanics of the spine—What we have learned in the past 25 years and future directions
Introduction
Clinical problems of the human spine continue to be prevalent in our society. Examples include low-back pain, sciatica, spinal deformity in both adults and children, spinal tumors, and spinal injury, including trauma to the spinal cord. Given that these clinical problems remain largely unsolved and that the spine plays an important mechanical role in human function, it is thus not a surprise that biomechanical research on the spine has expanded at a rapid pace. A PubMed search in June 2015 with the search terms ׳spine׳ and ׳biomechanics׳ showed that the number of articles in this field has increased exponentially over the past 25 years.
The classic textbook, Clinical Biomechanics of the Spine by White and Panjabi, was last published in 1990 and the next edition of this book is in the final stages of preparation. In the preparation of this third edition, we have had the opportunity to conduct a detailed literature review on the salient biomechanics literature related to the human spine over the past 25 years.
The purpose of this manuscript is to review what we have learned over the past 25 years in regards to the fundamentals of spine biomechanics. The material is organized in three main areas—the Whole Spine, the Functional Spinal Unit, and the Spinal Components (e.g. vertebra, intervertebral disc, spinal ligaments). My approach will be to briefly review what we knew in 1990, to outline what we have learned since that time, and to suggest areas for future research. Detailed reviews of papers are not provided, but classic references on a topic along with key new manuscripts are included for the reader to review. Due to space limitations, the spinal components of rib cage, muscle, spinal cord, and nerve root are not addressed in detail, nor is a review of in vivo spine kinematics or mechanobiology included. Further, topics such as the clinical biomechanical aspects of spinal trauma, spinal deformity, and surgical devices, techniques and instrumentation used in spine surgery are not included.
Section snippets
Whole Spine
The Whole Spine consists of the vertebrae of the cervical, thoracic, lumbar, sacral and coccygeal regions along with the intervertebral discs, ligaments, rib cage, and spinal musculature. In addressing the Whole Spine, a global view is taken towards spinal function rather than the local view when we address specific features of a Component such as the annulus fibrosus of the intervertebral disc. A summary of the topics addressed for the Whole Spine is shown in Table 1.
Functional spinal unit
The functional spinal unit (FSU) is the basic building block of the spine, consisting of two adjacent vertebrae, the intervertebral disc, the facet joints, and the spinal ligaments. In this section, I address the physical properties and functional biomechanics of the FSU through experimental studies on human samples and I include relevant data from studies on multi-FSUs. The topics presented with respect to the FSU are summarized in Table 2.
Component—vertebra
The vertebra consists of the vertebral body anteriorly, the neural arch posteriorly and a series of processes that serve as connection points for ligaments and muscles. The outer shell of the vertebra is mainly cortical bone and the inner region a network of cancellous bone. While we knew much in 1990 about the form and function of the spinal vertebrae, substantial research over the past 25 years has shed additional light on the biomechanics of the vertebra. A summary of the topics addressed
Component—intervertebral disc
The intervertebral disc consists of the central nucleus pulposus surrounded by the annulus fibrosus peripherally and the cartilaginous endplates rostrally and caudally. The structure and function of the disc is one of the most heavily researched biomechanical topics over many decades, beginning with the classic work of Hirsch, Brown and Virgin in the 1950s (Hirsch, 1955, Brown et al., 1957, Virgin, 1951). A summary of the topics addressed here for the Intervertebral Disc is shown in Table 4.
Component—spinal ligaments
The spinal ligaments are uniaxial structures that connect adjacent vertebrae along the spinal column. They enable the spine to move within certain limits to avoid damage to the surrounding neurologic structures. A summary of the topics addressed here for the spinal ligaments is shown in Table 5.
Overview
The last 25 years has seen an explosion in the volume of new research in the field of biomechanics as it relates to the human spine. It is hoped that this review places the novel findings regarding the fundamentals of spine biomechanics into the context of what was known in 1990. While conducting new research is always important, it is critical that we acknowledge and build on the efforts of past investigators. Only in this manner will true, meaningful progress be made.
These are exciting times
Conflict of interest
The author has no conflict of interest related to the content of this manuscript.
Acknowledgments
The author would like to acknowledge Professor Manohar Panjabi for his guidance, for his insights on the topics addressed herein, and for the many discussions over the years. The unique perspective of Professor Augustus White on these topics and our many warm discussions are gratefully acknowledged. The author is thankful to Dr. Kevin McGuire for his many insights on the subject matter.
The author thanks Mr. Stephen Mattucci, Mr. Masoud Malakoutian, Ms. Angela Melnyk, Mr. David Volkheimer, and
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