1.1 Introduction

Sagittal balance of the spine is a recent concept developed by pioneers in the latter part of the 20th century and which has become well established currently in beginning of the 21st century. Before the modern age, many concepts on spinal curvatures were described by Hippocrates in ancient Greece, ¹ and are still used in the majority of anatomical publications. Lordosis and kyphosis have Greek etymologies, and it was probably Galen ² who first used the terminology “ithioscoliosis,” to describe the natural curves of the spine in the sagittal plane.

During the 16th century, human dissection brought to light precise descriptions of vertebrae anatomy, although it was centered on local description rather than Hippocrates curve segmentation. Pelvic positioning as a compensatory method secondary to certain spinal pathologies was first analyzed by some authors in the 19th century as a way of balance compensation. Global changes in the aging population became well known during this era. But it was not until the 20th century, with the advent of X-ray images of the spine, that it began to come into focus. During the golden era of the 1970s and 1980s, a plethora of information on spinal biomechanics was published, however only with a segmental focus. ³

Few authors expressed interest in global assessment of the spine either with relevance to anatomy or pathology. In 1953, Delmas, ⁴ a French anatomist, described the changes in spinal curvatures in asymptomatic individuals. His point of view had very few applications in the traditional treatment of spinal pathologies at that time. Spinal curvature concepts were exclusively relegated to manual therapy proposed by some investigators as a global rehabilitation technique, albeit as an empirical approach based on clinical assessment.

During et al ⁵ revolutionized spinal biomechanics by proposing a new pelvimetry that integrated both the spine and pelvis with respect to their shapes and positioning. Although this landmark paper was published in Spine in 1985, it failed to receive wide acceptance in the scientific community. Not until 2008, with Duval-Beaupère’s scientific paper that detailed the sagittal layout of the spine and pelvis, was sagittal parameter catapulted to the forefront of spinal research and accepted as one of the most fundamental criteria for understanding spinal pathologies and their treatment. ⁶

In this chapter, the authors pursue the historical background of spinal theory from the earliest Indus Valley civilization of ancient India to the Greeks and, finally, to the beginning of modern concepts to enlighten our readers on how ideas on spinal balance progressively evolved to become an undeniable element in understanding the spine.

1.2 Ancient India

The oldest reference available is written in the ancient Hindu mythological epics Srimad Bhagvat Mahapuranam ⁷ (3500 BC and 1800 BC), where it is mentioned how Lord Krishna corrected the hunchback of one of his devotees, Kubj, by pressing down on his feet and pulling up on his chin. ^{8 ,​ 9} A detailed description of the human body, including spinal anatomy, was known to the religious Aryans between 2500–2000 BC up to 750–500 BC. ¹⁰ In the Vedic period, which began in 1800 BC mainly after the age of the Brahmanas, the term “scoliosis” was never used, rather it was described in Sanskrit.

1.3 Ancient Greece

For the Greek philosopher Plato (427–347 BC), ¹¹ nature was a perfect divine creation and motion was a unique attribute of animals. He emphasized the flexibility of the spine, which was created to allow for body movements. Spinal flexibility was seen as being in harmony with a perfect body. Aristotle (490–430 BC) ¹² considered spinal flexibility an absolute necessity for bipedal motion. Among the animal kingdom, by comparing birds and human bipedalism, Aristotle demonstrated that human bipedal capability was the source of human superiority. However, these considerations were more philosophical than practical.

Hippocrates (460–370 BC), the father of medicine, ^{13 ,​ 14 ,​ 15} was born on the Greek island of Kos (Fig. 1‑1). He was responsible for changing medicine from what was previously religious and supernatural phenomena into a scientific and observational discipline. As human dissections were forbidden at that time, his observations were extracted from animal dissections, bodies in motion at gymnasiums, and cadavers on the battlefield. In his famous opus, On Articulations, he described the spinal curvatures following their sagittal orientation:

Regarding the rachis, it is inflected along its whole length: from the sacral extremity to the great vertebra (fifth lumbar), with which the lower limbs are connected, spine is convex backward. From there to the diaphragm insertion, the spine is convex forward for the whole length. This area is covered anteriorly by muscles: the psoas. From there to the great vertebra above the shoulders (seventh cervical), rachis is on the whole length, convex backward; but due to the length of the spinous processes, longer in the middle of the back, it appears more bent than in reality. Regarding the cervical area itself, it is convex forward…. (French translation by Émile Littré 1844)

Hippocrates identified spinal deformities with the term “scoliosis.” He showed that the forward bending of the spine changes with age and work culture. More than a century before Percivall Pott, Hippocrates related progressive severe kyphosis with the presence of tubercles in the lungs. He described many systems of treatment for deformities using a combination of traction and pressure. His description of the spinal curvatures remains in use to this day and will continue to be the basic mainstay of global spinal anatomy forever.

Archimedes (287–212 BC) was a well-known mathematician, physicist, inventor, and astronomer of his age. He had the most profound influence on the future of spinal biomechanics through his work on the equilibrium of planes.

1.4 Roman Empire

Galen of Pergamon (130–210 AD) ¹⁶ was initially a physician of gladiators, which afforded him vast expertise in trauma. He was Greek but worked in Rome, serving Emperor Marc Aurèle and his son, Commode. He wrote several medical books where he reviewed Hippocrates. His anatomic doctrines became the basis for medical education for more than 1200 years.

It was assumed that Galen was the first to use the terms lordosis and scoliosis, dividing spinal deformities into lordosis, where the spine bends backward, kyphosis, where it bends forward, and scoliosis for lateral deformities. Because of variable translations and interpretations, it remains questionable whether Galen used the words lordosis and kyphosis in their modern significations. Lordosis (Greek λόρδωση), which can be translated into curvature, was used initially for any kind of normal curvature, bent forward or backward. Kyphosis (Greek κύφωση), translated as “hump,” had a pathological signification: abnormal forward curvature. This sagittal subdivision of the normal spine, initiated by Hippocrates, with its modern translation sacral kyphosis, lumbar lordosis (LL), thoracic kyphosis, and cervical lordosis, came to us without any change and still remains the gold standard in anatomical descriptions of the spine. As an official surgeon of gladiators in amphitheaters, he was accepted as “the father of sports medicine.” Galen also improved some devices described by Hippocrates for spinal trauma (Fig. 1‑2).

Oribasius (325–400 AD), ¹⁷ another Roman physician, added a bar to the Hippocratic reduction device and used it to treat both spinal trauma and spinal deformity.

Paul of Aegina (625–690 AD) ¹⁸ performed the first known laminectomy using a red-hot iron. He also collected the medical work of 1000 years in a seven-volume encyclopedia.

1.5 Middle Ages (330–1453 AD)

After a period of great activity during antiquity in medicine, contributions during the Middle Ages was paltry in comparison, especially in Europe. Translations of documents salvaged from ancient Greek were relegated to copyists in monasteries. Because of the proximity of Byzantium where most Greek archives were stored, Persian and Arabian physicians were able to translate Greek into Arabic and use and comment on the teachings of Hippocrates and Galen. The most famous, Avicenna (980–1037 AD), was a physician from what is presently Uzbekistan. Among his writings is the book, The Canon of Medicine, which became the reference book for students of medieval medicine. In the first volume, he analyzes the anatomy of the spine and precisely describes intervertebral motions in flexion, extension, and lateral flexion; spinal pathologies were treated in the third and fourth volume. ¹⁹ Following Hippocrates, he considered that kyphosis may have external (trauma) or internal causes and that good health was dependent upon a regular flux of humors (i.e., one of the four fluids entering into the constitution of the body). He considered degeneration and subsequent articular rigidity as a decrease of flux. He may have recognized spinal diseases such as Pott’s disease as an ankylosing spondylitis. ²⁰

Abulcasis (936–1013 AD), ²¹ a famous Arabian surgeon of the 11th century, wrote a treatise on surgery, At-Tasnif, in which he describes surgical disorders, including low back pain, sciatica, scoliosis, and spinal trauma, and advocates the use of chemical or thermal cauterization for several spinal disorders. He also developed a device to reduce the dislocated spine.

Şerefeddin Sabuncuoğlu (1385–1468 AD), ²² a Turkish physician, wrote an illustrated atlas of surgery, describing scoliosis, sciatica, low back pain, and spinal dislocations. He devised a technique for the reduction of spinal dislocations using a frame similar to that designed by Abulcasis.

1.6 15th to 17th Centuries

The Renaissance was a fertile period where humanists rediscovered Ancient Greek and Arabian works and spurred development of a new way of thinking in all arts and sciences. Until Andreas Vesalius (1514–1564), ²³ medicine and anatomy described by Greek authors were considered an absolute truth. He was the first to criticize Galen in his book, De Humani Corporis Fabrica, where he made accurate revisions of spine anatomy using direct and precise observations on human cadavers, a technique not allowed in Ancient Greece and limited to animal dissections, especially monkeys. Andreas Vesalius’ work, however, was limited to vertebrae description; nothing was written on the global assessment of the spine. Even the rich iconography of his work on human dissection was done in an artistic manner where spinal shape was not identifiable (Fig. 1‑3).

In only one drawing did Leonardo da Vinci (1452–1519) ²⁴ give a perfect lateral view of the whole spine, respecting Hippocrates’ orientation and segmentation of the spinal curvatures with the right number of vertebrae (Fig. 1‑4). He sensed the mechanical interaction of bone and muscles and, for him, the power of motion in animals was necessarily the result of mechanical means.

Although lesser known, Giovanni Alfonso Borelli (1608–1679) ^{25 ,​ 26} is considered the “father of biomechanics.” In his book, De Motu Animalium, he begins the introduction with these words: “Bodies and movements are the subject of mathematics. Such scientific approach is exactness of geometry….” His main observation was that muscles act on bony articulations in a manner akin to short lever arms. By extrapolation on the spine, he was able to calculate the forces acting on each intervertebral disk under a charge on the neck (Fig. 1‑5). He detailed the contact force on the fifth lumbar vertebra as the addition of weight-bearing and muscle forces. He was the first to experimentally define the position of the body’s center of gravity.

1.7 18th to 19th Centuries

The 18th century is marked by the advent of a scientific approach that was focused apart from the mirage that the human body as a godly creation. Even Vesalius, when describing perfectly the anatomy of vertebrae after human dissection, was describing in effect a perfect work of God.

This philosophical change of this period was formulated by Immanuel Kant (1724–1804) in his book Critique of Pure Reason. He showed that in a scientific approach, instead of placing the subject (actor of research) under the object rules (presupposition), it is the subject who applies his own rules onto the object under observation. In other words, when a scientific analysis presents a contradiction with previous observations, it is necessary to criticize objectively and, over time, change former ideas and understanding.

During the 18th century, few studies were undertaken with regard to the anatomy and biomechanics of the spine. Leonhard Euler (1707–1783) ^{27 ,​ 28} was a Swiss mathematician who, while studying the curvature of the spine, effectively demonstrated that a curved structure had a superiority in terms of resistance to a straight structure. Later, Jean Cruveilhier, in his Treatise of Descriptive Anatomy published in 1833, uses this relation to explain the spinal resistance: R = N ² + 1, where N is the number of curvatures of the structure, and gave a ratio of 1:16 for the normal spine resistance.

Two fundamental clinical descriptions of spinal disease were written. Bernard Connor in 1697 described the spontaneous fusion of the sacrum and vertebrae in Ankylosing Spondylitis. Percivall Pott (1714–1788) ²⁹ was the first to describe the arthritic tuberculosis of the spine and its severe kyphotic evolution.

Because of an explosion of engineering and biomechanical advancements in the 19th century, many of these methods were introduced into the science of the spine. Center of gravity positioning in the human body was the most pursued challenge of the time. It must be taken into account that radiology had not yet been invented. Ernst and Wilhelm Weber, ³⁰ in 1836, published a description on the movement of center of gravity during gait in their landmark publication Die Mechanik der Menschlichen Gehwerkzeuge (Mechanics of the Human Gait), ³¹ which laid the foundation of the modern concept of locomotion. In 1891, Wilhelm Braune and Otto Fischer, ³² in their book Der Gang des Menschen showed the positions of the center of gravity for each segment of the body in a tridimensional study. Using frozen cadavers and a special balance plate, they determined the body mass center for each body segment measured. A hundred years later, Duval-Beaupère ³³ performed the same measurements by way of a barycentremeter using X-ray measurements. Hermann von Meyer, a German anatomist, and Karl Culmann ³⁴ used a crane model to explain the mechanical effect of muscles on bony structures. In 1892, Julius Wolff ³⁵ published, The Law of Bone Remodelling, stating that, “…Structure is nothing else than the physical expression of function under pathological conditions. The structure and form of the parts change according to the abnormal conditions of force transmission.”