The History of Spinal Biomechanics
Abhay Sanan, M.D., Setti S. Rengachary, M.D.
Department of Neurosurgery, University of Minnesota Hospital System, Minneapolis, Minnesota
The history of spinal biomechanics has its origins in antiquity. The Edwin Smith surgical papyrus, an Egyptian document written in the 17th century BC, described the difference between cervical sprain, fracture, and fracture-dislocation. By the time of Hippocrates (4th century BC), physical means such as traction or local pressure were being used to correct spinal deformities but the treatments were based on only a rudimentary knowledge of spinal biomechanics. The Renaissance produced the first serious attempts at understanding spinal biomechanics. Leonardo da Vinci (14521519) accurately described the anatomy of the spine and was perhaps the first to investigate spinal stability. The first comprehensive treatise on biomechanics, De Motu Animalium, was published by Giovanni Borelli in 1680, and it contained the first analysis of weight bearing by the spine. In this regard, Borelli can be considered the "Father of Spinal Biomechanics." By the end of the 19th century, the basic biomechanical concepts of spinal alignment and immobilization were well entrenched as therapies for spinal cord injury. Further anatomic delineation of spinal stability was sparked by the anatomic analyses of judicial hangings by Wood-Jones in 1913. By the 1960s, a two-column model of the spine was proposed by Holdsworth. The modern concept of Denis' three-column model of the spine is supported by more sophisticated testing of cadaver spines in modern biomechanical laboratories. The modern explosion of spinal instrumentation stems from a deeper understanding of the load-bearing structures of the spinal column.
(Neurosurgery, 39:657669, 1996)
Key words: Biomechanics, Borelli, Hanging, Hippocrates, History, Spinal devices, Spinal injury, Spine
Spinal biomechanics has a long and fascinating history. For the sake of convenience, its history can be divided into three eras. The ancient era was characterized by only a rudimentary knowledge of spinal anatomy and biomechanics. The premodern era was ushered in by the Renaissance, which brought with it an understanding of the anatomy of the spine and the first detailed inquiries into its mechanical nature. The modern era is characterized by the application of biomechanical principles to the treatment of spinal injuries and deformities. Landmarks in the history of spinal biomechanics are listed in Table 1. To properly trace the history of spinal biomechanics, the relevant history of spinal anatomy and the management of spinal trauma must be discussed because spinal
biomechanics is so inextricably entwined in the latter two subjects.
Edwin Smith surgical papyrus
The earliest clinical accounts of spinal injury are contained in the Edwin Smith surgical papyrus, originally written during the Egyptian Old Kingdom when the great pyramids were being built (26002200 BC) (5, 6, 11, 14, 15, 29). The papyrus is actually a copy of a copy; the originals are considered lost to antiquity. The exact author of the original papyrus is shrouded in mystery, but it is possible it was penned by Imhotep, the great physician-architect at the court of Pharaoh Zoser of the Third Dynasty. Whether this papyrus was used as a lecture book or as a guide for practicing surgeons is unknown, but it is probable that this papyrus was the Secret Book of the Physician referenced in other ancient Egyptian writings.
Approximately 500 years later, many words in the papyrus became archaic and a series of glosses that defined difficult words and phrases was added by yet another unknown physician. This revised document was then copied in the 17th century BC by a Theban scribe, a master of penmanship who was more concerned with the beauty of the document than its content. Unfortunately, the scribe did not finish the copy; he abruptly stopped in the middle of a case, in the middle of a sentence, in the middle of a word. The papyrus systematically covers injuries to the body and starts with the head and neck. Sadly, the ancient copyist stopped his work just as the papyrus was beginning to discuss injuries to the thoracic and lumbar spine. The unfinished scroll then lay untouched for 3500 years before it was sold to the American Egyptologist Edwin Smith in 1862. It was posthumously donated to the New York Historical Society in 1906.
Five of the 48 recorded "cases" describe cervical injuries. It is clear that the ancient Egyptians recognized that misalignment of the bony vertebral column could have disastrous consequences. The papyrus' descriptions of spinal injury have not been improved upon for 4500 years (Fig. 1) (6): "Instructions concerning a crushed vertebra in his neck. If thou examinest a man having a crushed vertebra in his neck thou findest that one vertebra has fallen into the next one, while he is voiceless and cannot speak; his head falling downwards, has caused that one vertebra crush into the next one; and shouldest thou find that he is unconscious of his two arms and his two legs because of it Thou shouldest say concerning him 'One having a crushed vertebra in his neck; he is unconscious of his two arms and his two legs and he is speechless. An ailment not to be
FIGURE 1. The Edwin Smith papyrus was written in hieratic, a rapid, cursive form of hieroglyphs. Column X of the papyrus discusses injuries to the cervical spine. Both the hieratic form (A) and its hieroglyphic transliteration (B) are shown
(reproduced from Breasted JH: The Edwin Smith Surgical Papyrus [published in facsimile and hieroglyphic transliteration, with translation and commentary, in two volumes]. Chicago, The University of Chicago Press, 1930 ).
The Egyptians' knowledge of anatomy was greatly advanced in part by the examination of the dead during the extensive mummification process and in part by treating the injured during the construction of the pyramids and temples. They clearly recognized that a fracture-dislocation was associated with a poorer prognosis than a simple fracture of the spine. Remarkably, the author makes a specific point of discussing the specific spinal fracture produced despite that the outcome was seen as hopeless for all spinal injuries. A wenekh (disjoining) of the vertebrae (subluxation) is distinguished from a sehem (crush) of the vertebrae (compression fracture). Furthermore, a sehem of the vertebrae was known to occur with axial loading such as falling on
one's head. These statements demonstrate that the original author had a keen interest in pathophysiological correlation.
In general, spinal fractures were not treated because the prognosis was seen as dismal. There is good evidence that the Egyptians realized that the appropriate treatment of long-bone fractures was reduction and immobilization. Splints, dating from the Fifth Dynasty, have been discovered around compound extremity fractures, suggesting that bone
setting was an established art in ancient Egypt (38). However, there is no evidence to suggest that splinting was practiced with spinal fractures.
The ancient Hindus
The first account of treatment rendered for spinal deformity is recorded in the Srimad Bhagwat Mahapuranam, an ancient Indian epic written between 3500 and 1800 BC (27, 40). A passage describes Lord Krishna applying axial traction to correct the hunchback in one of his devotees, Kubja (Fig. 2) (27): "To shower the fruits of his blessings, happy Lord Krishna decided to straighten Kubja, who was deformed in three places. He pressed her feet by his foot, held her chin by two fingers and pulled her up. By the touch and pull of Lord, she became a beautiful straight woman."
It is probable that Kubja had kyphoscoliosis with the primary curve and the two secondary curves being regarded as the triple deformity. The veracity of the account may, of course, be placed into question. Nevertheless, the fact remains that this passage is the earliest known reference to the concept of axial traction, predating Hippocrates by over a millennium.
FIGURE 2. A, the hunchbacked woman, Kubja, requesting Lord Krishna to cure her. B, Lord Krishna applies axial traction by anchoring the woman with his well-placed foot and lifting her up with two fingers under her chin (reproduced from Kumar K: Did the modern concept of axial traction to correct scoliosis exist in prehistoric times? J Neurol Orthop Med Surg 8:310, 1987 [26b]).
Hippocrates II (460361 BC) of the island of Cos is the most celebrated physician of antiquity. Although the work Corpus Hippocraticum may not have been written in its entirety by Hippocrates, it certainly represents the sum of accepted ancient Greek medical knowledge. Judged by modern standards, Hippocrates' knowledge of anatomy was poor. Of course, humoral pathology rather than structural pathology was thought to form the basis of disease, so interest in anatomy was limited. He did, however, understand some basic concepts of the vertebral column. He realized that the bony column was held together by the discs, ligaments, and muscles. He noted that the spinous process could be broken without ill effect, but injury to the vertebral body was often deadly because of injury to the "spinal marrow" (spinal cord). He recognized spinal dislocation and realized that reduction was a desirable treatment.
Spinal manipulation as a treatment for spinal dislocation or deformity was widely practiced at the time of Hippocrates. Hippocrates recommended that kyphotic deformities be treated by subjecting the body to traction and applying pressure locally to the area of the kyphosis
(Fig. 3A) (24): "But the physicians, or some person who is strong, and not uninstructed, should apply the palm of the hand to the hump, and then, having laid the other hand upon the former, he should make pressure, attending whether this force should be applied directly downward, or toward the head, or toward the hips. And there is nothing to prevent a person from placing a foot on the hump, and supporting his weight on it, and making gentle pressure; one of the men who is practiced in the palestra would be a proper person for doing this in a suitable manner."
FIGURE 3. A, Hippocrates recommended that kyphotic deformities be treated by traction and the local application of pressure (in this case, a human foot) (reproduced from Schiötz EH, Cyriax J: Manipulation Past and Present. London, William Heinemann, 1975, p 9 [36a]). B, Hippocrates, however, thought that forcible compression over the apex of the kyphosis with a large board was the ideal treatment (reproduced from Guidi G: Chirurgia e Graeco in Latino Conversa. Bruxelles, Editions Culture et Civilization, 1970 [21a]).
If the application of the hand or foot was unsatisfactory, Hippocrates recommended another method (Fig. 3B) (24): "But the most powerful of the mechanical means is this: if the hole in the wall, or in the piece of wood fastened into the ground, be made as much below the man's back as may be judged proper, and if a board, made of lime-tree, or any wood, and not too narrow, be put into the hole, then a rag, folded several times or a small leather cushion, should be laid on the hump. When matters are thus adjusted, one person, or two if necessary, must press down at the end of the board, whilst others at the same time make extension and counterextension along the body, as formerly described." Hippocrates' methods were commonly used through the Middle Ages by medieval physicians, such as Henri de Mondeville (12601320) and Guy de Chauliac (13001368).
"Succussion" was a practice condemned by Hippocrates (Fig. 4). The patient was attached with ropes to an elevated ladder. By flinging the patient to the ground in an upside-down position, it was hoped that the jerk produced by sudden deceleration would realign any spinal deformity (24): "Wherefore succussion on a ladder has never straightened anybody, as far as I know, but it is principally practiced by those physicians who seek to astonish the mobfor to such persons these things appear wonderful, for example, if they see a man suspended or thrown down, or the like; and they always extol such practices, and never give themselves any concern whatever may result from the experiment, whether bad or good. But the physicians who follow such practices, as far as I have known them, are all stupid." Despite Hippocrates' warnings regarding the danger of this procedure, succussion was practiced through the 15th century AD.
FIGURE 4. "Succussion" was a practice denounced by Hippocrates for being too rough. The victim was tied upside down and flung headlong to the ground, only to be stopped suddenly by a length of rope that suspended him. The sudden deceleration was believed to be therapeutic (A, reproduced from Commentaries of Apollonius of Chition on the Peri-arthron of Hippocrates. Florence, Laurentian Library, 11th century codex [7a]; B, reproduced from Guidi G: Chirurgia e Graeco in Latino Conversa. Bruxelles, Editions Culture et Civilization, 1970 [21a]).
The Middle Ages, aside from Galen, were practically devoid of any advancement in biomechanics. Galen (AD 131201) was a firm believer in the Hippocratic teachings and used spinal manipulation, much like before, to treat spinal problems. He did, however, make thorough studies of spine deformities and was the first to use the terms "kyphosis," "lordosis," and "scoliosis" (18). Unlike Hippocrates, he had a keen interest in anatomy but he extrapolated human anatomy, often incorrectly, from animal dissections. Despite this shortcoming in methodology, he correctly identified many anatomic features of the spinal column. He identified the number of vertebrae in each segment of the spinal column (7 cervical, 12 thoracic, and 5 lumbar) (37). He described the ligamentum flavum as a ligamentous structure distinct from the underlying dura and pia mater. He was able to correlate neurological findings with a specific spinal level, because he performed extensive vivisections (17): "If you cut through the spinal medulla behind the fifth vertebra of the head so that you dissever the origin of the nerve next following it, then both arms are paralysed and their movement will be completely abolished. And if you cut in behind the sixth vertebra then the nervous functions persist in all the subdivisions of the upper arm, being maintained in their normal state, with the exception of a small part. But the cut which falls behind the seventh vertebra, makes the forearm devoid of sensibility and of motion."
Italy in the 15th and 16th centuries rediscovered many ancient Greek and Roman texts that had been faithfully preserved by Arabic libraries. Classical works on medicine, philosophy, and the natural sciences were circulated throughout the universities of Europe. The study of mechanics was revived by studying On the Equilibrium of Planes by Archimedes (287212 BC) and De Architectura by Vitrovius (circa 25 BC) (32). The study of anatomy was developed, and, for the first time, separate academic chairs were founded for the subject. This rebirth of the sciences resulted in a reexamination of the natural world. Several creative thinkers began to contemplate the relationship between anatomy, mathematics, and mechanics, which were the first in-depth thoughts on biomechanics.
Leonardo da Vinci
Superlatives do not do justice to the genius of Leonardo da Vinci (14521519). The epitome of a Renaissance man, da Vinci was a master artist, engineer, and anatomist. His uncompleted work on human anatomy, De Figura Humana, reveals that he embraced a mechanistic approach to the study of the human body. As an introduction to the section on movement, he writes, " nature cannot give the power of movement to animals without mechanical instruments." da Vinci thought that mechanics was the paradise of mathematics, because in this garden mathematics bore its fruits (39).
da Vinci was the first to accurately describe the spine with the correct curvatures, articulations, and number of vertebrae (Fig. 5). Never before had the anatomy of the entire spine been examined with such attention to detail and proportion. He was also the first to suggest that stability to the spine was provided, in part, by the cervical musculature (Fig. 6) (26): "You will first make the spine of the neck with its tendons like the mast of a ship with its side-riggings, this being without the head. Then make the head with its tendons which gives it its movement on its fulcrum."
This statement is taken from da Vinci's later works. By that time, it seemed that he was no longer fascinated by simple dissection. He began to wonder how the body moved and how geometry and mechanics could further unlock the secrets of human physiology. For perhaps the first time, the biomechanics of the spine was contemplated.
FIGURE 5. Leonardo da Vinci studied perspective, geometry, and anatomy and paid great attention to detail. His representation of the spinal column was perhaps the first to show the correct curvatures, articulations, and number of vertebrae (reproduced from Todd EM: The Neuroanatomy of Leonardo da Vinci. Santa Barbara, Capra Press, 1983, p 114 [41a]. Courtesy of Edwin M. Todd).
FIGURE 6. A, this semischematic drawing of the cervical vertebrae shows that the cervical musculature stabilizes the cervical spinal column much like guywires (B) stabilize the mast of a ship. Although rudimentary, it is perhaps the first inquiry into the relationship of spinal anatomy and its mechanics (A, reproduced from Keele KD, Pedretti C: Leonardo da Vinci. Corpus of the Anatomical Studies in the Collection of Her Majesty the Queen at Windsor Castle. London, Harcourt Brace Jovanovich, 1979 ; B, reproduced from O'Malley CD, Saunders JB de CM: Leonardo da Vinci on the Human Body. New York, Henry Schuman, 1952 [32a]).
Vesalius (15141564) firmly occupies a place in the history of spinal biomechanics because of his superbly accurate descriptions of spinal anatomy. Although he followed da Vinci by a number of years, da Vinci's anatomic works remained unknown for centuries and so history has titled Vesalius the "Father of Anatomy." Before Vesalius, the often inaccurate anatomy of Galen was accepted as the standard. Vesalius' beautifully illustrated book, De Humani Corporis Fabrica (42), broke with Galenic teachings and presented the most integrated and accurate anatomy that had ever appeared, rendering all previous anatomy texts obsolete (Fig. 7).
FIGURE 7. Vesalius revolutionized the study of medicine with De Humani Corporis Fabrica. His diagrams of the cervical vertebrae were exquisitely accurate and revealed many of Galen's errors (reproduced from Vesalius A: Icones Anatomicae. Monachis, ex officina Bremenski, 1934 [42a]).
René Descartes laid the philosophical groundwork for a mechanistic view of the human body. Published in 1675, Tractus de Homine et de Formatione Foetus (13) stated that the body was a machine directed by a soul: "The body is a machine made by the hand of God." With this principle, Descartes argued that all of animal physiology could be explained by mechanics and paved the way for the experimental physiologists.
Giovanni Alfonso Borelli
Giovanni Alfonso Borelli (16081679) can be considered the Father of Biomechanics (Fig. 8). Borelli was the first-born son of a Spanish infantryman, Miguel Alonso, and a local Italian woman of Naples, Laura Porello. Although raised in turbulent times, Borelli quickly proved to be a bright boy. He obtained a public lectureship at Messina, Sicily, at the age of 23, and several years later he was recognized throughout Italy as being well versed in mathematics, physiology, chemistry, physics, and astronomy.
FIGURE 8. This posthumous portrait of Borelli was made by the painter Cavalier Ghezzi after an original sketch by Ciro Ferri. Ferri's sketch has yet to be discovered. The legend beneath the painting (not shown) reads "Giovanni Alfonso Borelli, who was an able and famous mathematician" (reproduced from Middleton WEK: A little-known portrait of Giovanni Alfonso Borelli. Med Hist 18:9495, 1974 [31a]).
In 1653, Borelli became a Professor of Mathematics at the University of Naples, and 5 years later he obtained the Chair of Mathematics at Pisa. Some of Borelli's more notable achievements were the discovery that some comets move in a parabolic path, a description of Jupiter's moons, and a concise review of Euclid's texts. Most notably, he was one of the founders of "iatromechanics," or the application of mechanics to physiology. This field is the forerunner of what we now call biomechanics.
Borelli's interest in human motion seems to have been sparked by Marcello Malphigi, professor of theoretical medicine at the University of Pisa. Apparently, the two men had a close relationship, as Malphigi recalls (30): "What progress I have made in philosophising stems from Borelli. On the other hand, dissecting living animals at his home and observing their parts, I worked hard to satisfy his keen curiosity." Borelli, who was not a physician, needed to work with Malphigi to ensure that his mechanical calculations made biological sense. The relationship between the two men was a forerunner of present collaborations between biomechanical engineers and spinal surgeons. Although Borelli's knowledge of mechanics was restricted to the principle of levers and the triangle of forces, he was able to generate an accurate and comprehensive account of muscle action (1).
His work, De Motu Animalium (3), underwritten by Queen Christina of Sweden, was published posthumously in 1680 and is the first comprehensive text devoted to biomechanics (Fig. 9). The book is split into two parts. The first part examines the external motions of the musculoskeletal system in animals from a mechanical viewpoint. The second part examines internal motions, such as the physiology of muscles and the circulation.
FIGURE 9. De Motu Animalium by Giovanni Alfonso Borelli is considered the first comprehensive text on biomechanics. A reproduction of the title page of the 11th edition is shown (reproduced from Borelli GA: De Motu Animalium, Rome, Angeli Bernabo, 1680 ).
One of the striking mechanical features of the body noted by Borelli was that the muscles act with short lever arms so that the intervening joint transmits a force that is a magnitude greater than the weight of the load. Borelli overturned older concepts of muscle action, which stated that long lever arms allowed weak muscles to move heavy objects (4). "Galen also states that a tendon is like a lever. He thinks that, consequently, a small force of the animal faculty can pull and move heavy weights. This general opinion seems to be so likely that, to my knowledge, nor surprisingly, it has been questioned by
nobody. Who indeed would be stupid enough to look for a machine to move a very light weight with a great force, i.e., use a machine or contrivance not to save forces but rather to spend forces? This seems strange and against common sense, I agree, but I can convincingly demonstrate that this is what happens and, given permission, that the upholders of the opposite opinion have been mistaken." This fundamental fact of human biomechanics was not realized again until 1935 when it was rediscovered by Friedrich Pauwels.
Borelli's evaluation of the spine illustrated a remarkable grasp of its biomechanics. He realized that the intervertebral discs acted like a viscoelastic substance by both cushioning the bones and acting like springs. He proposed that the discs must perform some load sharing because his calculations revealed an inability of the spinal musculature alone to support heavy weights. Remarkably, he was able to conduct detailed calculations describing the forces sustained by individual vertebrae when a load was carried on the neck (Fig. 10) (4): "If the spine of a stevedore is bent and supports a load of 120 pounds carried on the neck, the force exerted by Nature in the intervertebral disks and in the extensor muscles of the spine is equal to 25,585 pounds. The force exerted by the muscles alone is not less than 6,404 pounds. Therefore, the sum of the muscular forces GH which control the fifth lumbar vertebra and a third of the resistance of the intervertebral disc is equal to 826 pounds. The muscular forces are equal to 413 pounds and the forces exerted by the disc are equal to 1,239 pounds." These figures, produced by Borelli over 300 years ago, agree quite well with modern experimental calculations of spinal load sharing.
FIGURE 10. A, laborer is shown bent with the weight carried on his neck. B, Borelli analyzed the forces produced in the vertebrae with a schematic representation of the spinal column. For example, DF represents the lumbar vertebrae and GH represents the lumbar musculature (reproduced from Borelli GA: On the Movement of Animals [translated from the Latin by Maquet P]. Berlin, Springer-Verlag, 1989 ).
The mechanical philosophy of the 16th and 17th centuries altered the study of medicine altogether. The prevailing view held that man was the center of the universe and above the physical laws. But just as Copernicus subjected the earth to the laws of celestial mechanics, so Borelli subjected the human musculoskeletal system to the laws of physical mechanics. The human body was converted from an inscrutable, perfect creation of nature to an amazing collection of biological systems that followed the laws of the universe. Borelli's contribution to this change was not trivial. He showed that human motion could be explained in mechanical terms, a concept that remains fundamental to spinal biomechanics.
Fabricus Hildanus, a German surgeon, proposed a method of spinal reduction that was very advanced for his time. In 1646, he described a method for reducing cervical fracture-dislocations that is similar in concept to modern cervical traction (23). A silver needle was passed through the interspinous ligaments and made to hook under a spinous process. Forceps were attached to the needle and traction was applied to the neck, in the hope of reducing the fracture (Fig. 11). Apparently, his method did not become popular because his work was not referenced by later surgeons and his emphasis on controlled spinal reduction did not surface again for another 200 years.
FIGURE 11. A method to reduce cervical dislocations as described by Fabricus Hildanus. A silver pin was placed through (or between) the spinous processes and controlled traction was applied (reproduced from Hildanus WF: Wilhelmi Fabricii Hildani Opera, Francofurti ad Moenum, 1682 [23a]).
After the basic anatomy and fundamental biomechanics of the spinal column had been described during the Renaissance, the premodern era focused on applying biomechanical principles to therapy in a scientific manner.
Leonhard Euler (17071783), Swiss mathematician and one of the founders of pure mathematics, made several important contributions to geometry, physics, calculus, astronomy, and mechanics. The human spine, like a column, is designed to support a compressive load. At a point known as the "critical load," a column will buckle because of instability. The mathematical stability of a column was first described by Euler in 1744 as being a function of column height and stiffness (8, 16). The spine has undergone analysis as a Euler column, and its Euler stability has been experimentally derived. Although Euler did not address spinal biomechanics per se, his mathematical studies had a direct relevance to biomechanical models of the spine.
The contribution of Julius Wolff (18361902) to spinal biomechanics was indirect. Wolff, a German orthopedic surgeon, was engrossed with the relationship between the form and function of bone (Fig. 12) (44). Based on his own experiments and the work of others, he published Das Gesetz der Transformation der Knochen (The Law of Bone Transformation) in 1892 (43). This monograph contains what is now known as Wolff's law: "Every change in the function of a bone is followed by certain definite changes in internal architecture and external conformation in accordance with mathematical laws." This statement has important implications for spinal biomechanics and explains why an intervertebral bone graft will fuse when subjected to
loading and why loose bone fragments in the canal after a lumbar burst fracture will resorb.
FIGURE 12. A portrait of Julius Wolff, who made fundamental contributions to the relationship between the form and function of bone (reproduced from Keith A: Menders of the Maimed: the Anatomical and Physiological Principles Underlying the
Treatment of Injuries to Muscles, Nerves, Bones and Joints. London, Oxford University Press, 1919 [26a]).
Burrell, a turn of the century surgeon at Boston City Hospital, made several seminal observations on clinical biomechanics and spinal cord injury. The prevailing attitude of Burrell's contemporaries toward spinal cord injury was one of extreme pessimism, a view not entirely unjustified given a mortality rate >70% at that time. Burrell, however, noted that there were patients presenting with spinal fractures and neurological deficits who recovered if immediate intervention was taken. Therefore, he strongly argued for the
biomechanical principle of immobilization, admonishing the police, hospital attendants, medical students, and even house officers to keep their "hands off" patients with spinal cord injuries. Even the surgeon was told to examine the patient only by turning the body as a whole. He then broke further from conventional thought by emphasizing the urgent need for spinal reduction and external immobilization. Writing in 1887 on his experiences with 16 patients using this therapeutic philosophy, he remarked: "First: That, in the immediate correction of the deformity and fixation with plaster-of-Paris jacket or other means, we have a rational method of treating a large number of cases of fracture of the spine. My own belief regarding the status which the procedure should occupy in surgery is, that it will occasionally be a life-saving measure and that apart from the chance of recovery offered to the patient by this means, it will almost invariably make the patient more comfortable, in that he can be handled more easily" (italics are
His results reflected the soundness of his thinking. Ten of the sixteen patients treated in this manner were greatly improved. The need for immediate reduction became so important in Burrell's mind that by 1905, he was advocating open reduction if closed reduction failed.
"If the kyphos is very marked, or if upon extension, it does not readily reduce, an immediate operation, unless there is some contraindication, such as shock, should be done, that the reduction may take place while the cord is under the surgeon's eye" (7).
Burrell's conclusions are sound and for the most part are followed in the modern management of patients with spinal cord injuries. The major shortcoming of Burrell was his inability to divide spinal fractures into different categories based on the mechanism of injury and the pathological abnormality produced.
Execution by hanging prompted the first investigations into the clinical biomechanics of the spine. Eventually, specific forces acting on the cervical spine were seen to produce a specific type of cervical fracture and the study of clinical biomechanics was born.
Hanging was probably introduced to the British in the 5th century AD by invading Germanic tribes (Angles, Jutes, and Saxons) and was subsequently employed as the preferred method of execution for the common criminal, decapitation being the privilege of the nobility. Early English hangings can only be described as cruel. The prisoner was hoisted off the ground by a rope tied around the neck and left to struggle until strangulation produced death. Sometimes the hangman would pull on the feet of the condemned or climb on his back to hasten death (36). Guidelines for execution were absent; the technical details of hanging were left to the hangman whose " vanity is equaled only by his ignorance" (31). It would take centuries before English hangings became more humane.
The "drop" method of hanging was probably introduced in 1784 at
Newgate Gaol in London. Unfortunately, the length of the drop was
variable and drops that were too short produced strangulation whereas
drops that were too long produced decapitation. It must be remembered
that hangings were public spectacles and neither result was
satisfactory to the audience; strangulation was too tedious and
decapitation too gruesome. It was not until 1875 that the drop length
was standardized by Samuel Haughton. A table was calculated that gave
the appropriate size of rope and drop length for a given weight of a
Standardization of drop length prevented decapitation, but some prisoners still died of suffocation. This prompted an investigation into knot position. John J. deZouche Marshall viewed several hangings and concluded in 1886 that a knot placed under the chin (submental knot) was the most lethal and even devised a contraption that forced the knot to remain below the chin (Fig. 13) (31). Marshall presented his finding to the Committee of Capital Sentences; he was, much to his chagrin, ignored. Although Marshall's observations were correct, he did not study the underlying mechanism or
pathological findings in the cervical spine produced by hanging.
FIGURE 13. Marshall's device consisted of a chin trough that kept the knot in a submental position. He was convinced that this knot position produced rapid death by cervical injury (reproduced from Marshall JJDZ: Judicial executions. Br Med J 2:779782, 1888 ).
A.M. Paterson in 1890 was the first to describe the pathological outcome produced by hanging with a submental knot (34). The findings of a "hangman's fracture" were described. The pedicles of C2 were fractured, and the anterior and posterior longitudinal ligaments were ruptured. The body and odontoid of C2 migrated cephalad with the cranium while the neural arch migrated caudad.
Frederic Wood-Jones, director of anatomy at the London School of Medicine for Women, can be credited with the first systematic study of clinical biomechanics. His interest began in 1908 when he was allowed to examine the exhumed remains of 102 Nubian men executed by hanging in the Upper Nile Valley during the Roman occupation (2000 yr earlier) (45). Many of the victims still had the ropes tied to their necks, but Wood-Jones noted a curious observation; not a single case of cervical fracture was found. The majority of the victims had disruptions of the cranial base as the only finding.
Wood-Jones hypothesized that the knot position may have been the determining factor in the pathological findings produced by hanging. It was his impression that a large knot was placed below the angle of the jaw and mastoid process in the Roman hangings. This knot position, referred to as a subaural knot, transmitted a force through the cranial base as the body jerked upon hanging.
Wood-Jones then had the exceptional fortune, in 1913, to examine other skeletons whose original owners had been executed by hanging. First, he found a description of the skull of Dr. Pritchard, a Scottish prisoner known to be executed in 1865 by a subaural knot. The skull had the same peculiar cranial base fracture as the Nubians, confirming that a subaural knot was used in the ancient Roman hangings (Fig. 14). Again, as in the Nubians, the cervical spine was intact. He next examined five skeletons donated by Captain C.F. Fraser, superintendent of the Rangoon Central Jail. These prisoners were known to have died instantaneously, and Captain Fraser assured Wood-Jones that a submental knot was used. Examination of the cranium revealed no fractures, but all five sets of cervical spines had identical fractures (Fig. 15) (46): "It is to be noted that the odontoid plays no part in producing death, but that the posterior arch of the axis is snapped clean off and remains fixed to the third vertebra, while the atlas, the odontoid process, and the anterior arch of the axis remain fixed to the skull. This lesion is produced by the violent jerk which throws the man's head suddenly backwards and snaps his axis vertebra."
FIGURE 14. The cranial base fracture produced in the ancient Nubians and in the criminal Dr. Pritchard using a subaural knot. No cervical fracture was seen (reproduced from Wood-Jones F: The ideal lesion produced by judicial hanging. Lancet 1:53, 1913 ).
FIGURE 15. The pathological outcome in the cervical spine produced by the drop method of hanging when a submental knot was used. This fracture was called a "hangman's fracture" by later authors (reproduced from Wood-Jones F: The ideal lesion produced by judicial hanging. Lancet 1:53, 1913 ).
Wood-Jones, like Marshall, argued on the grounds of mercy that the submental knot should be used in all English judicial hangings because it produced death efficiently, cleanly, and rapidly. His report is also a landmark in the history of spinal biomechanics because specific forces, namely hyperextension and distraction, were seen to produce a specific pathological outcome in the spine, in this case, traumatic spondylolisthesis of C2 or "hangman's fracture."
Clinically useful biomechanical models of the spine are a relatively recent development. Sir Frank Holdsworth proposed a two-column model of the spine in 1962 (25). Holdsworth served an area with a large coal miner population so he had the opportunity to study a large number of spinal fractures, particularly at the thoracolumbar junction. He published his report after treating >1000 cases of spinal fracture with neurological deficit and an even greater number without neurological deficit. Holdsworth's anterior column consisted of the anterior longitudinal ligament, the vertebral body, and the posterior longitudinal ligament and the posterior column consisted of the pedicles, lamina, spinous processes, facets, and the surrounding ligaments. Holdsworth thought that disruption of the posterior column was necessary for spinal instability. Hence, his model predicted that compression and burst fractures were stable whereas fracture-dislocations were unstable.
A three-column model was proposed by Francis Denis in 1983 after he studied >400 radiographs of spinal fractures (12). He discovered that burst fractures, considered by Holdsworth to be stable, were actually unstable. This led him to describe an additional column, a middle column consisting of the posterior vertebral body, the posterior annulus fibrosis, and the posterior longitudinal ligament (Fig. 16). Disruption of two columns was required for instability. Therefore, he considered compression fractures as stable and burst fractures, Chance fractures, and fracture-dislocations as unstable, a paradigm consistent with today's treatment strategies. Denis' model has undergone modification by many authors, but the concept of three columns in the spine has withstood more than a decade of scrutiny (33).
FIGURE 16. The anterior, middle, and posterior columns of the spine. AF, annulus fibrosis; SSL, supraspinous ligament; PLL, posterior longitudinal ligament; ALL, anterior longitudinal ligament; ISL, interspinous ligament; LF, ligamentum flavum; C, capsule (reproduced from Denis F: The three
column injury and its significance in the classification of acute thoracolumbar spinal injuries. Spine 8:817831, 1983 ).
Finite element method
The finite element method is a mathematical modeling system, originally designed for structural engineering, that has been applied to the spine for >25 years. In this powerful method, the spine is divided into a large number of geometric forms (e.g., bars, cubes, plates) referred to as "elements." These elements interact with each other at junctions called "nodes" (20). By breaking the continuum of the spine into a finite number of discrete units, the stresses, strains, and forces at any given location can be calculated using a computer (Fig. 17).
FIGURE 17. A finite element model of the lumbar spine is shown. The large number of elements and nodes required for clinically meaningful results currently requires an impractical amount of computer time (reproduced from Goel VK, Gilbertson LG: Applications of the finite method to thoracolumbar spinal research: Past, present, and future. Spine 20:17191727, 1995 ).
The future of this approach lies in its ability to assess the degree of stability imparted to the spine by a construct before implantation. Furthermore, a scenario can be envisioned in which magnetic resonance images can be directly converted into a three-dimensional computer model and subjected to analysis. Currently, the major limitation of the method is the enormous computer time and storage required to faithfully represent the complexities of the spine.
EVOLUTION OF TREATMENT MODALITIES BASED ON BIOMECHANICAL PRINCIPLES
As the understanding of spinal anatomy and its biomechanics became more refined, treatment of spinal injury became more sophisticated. Devices were invented that could perform the intended therapeutic goals. Spinal manipulation was approved by Hippocrates and devices to accomplish it reached their zenith in the 16th and 17th centuries (21). The Hippocratic scamnum used a series of windlasses that were connected to the injured part of the body using ropes. By cranking the opposing windlasses, a great deal of force could be applied to any part of the body (Fig. 18).
FIGURE 18. The scamnum was recommended by Hippocrates as a device to reduce fractures. It reached its height in popularity and complexity in the 17th century. Ropes attached to windlasses generated the required traction. In this figure, traction as well as compression is being applied to a gibbus deformity (reproduced from Scultetus J: Armamentarium Chirurgicum. Leyden, 1665 [36b]).
Several centuries passed before any significant improvement was made to the scamnum like devices. Taylor (41) in 1929 was among the earliest to use controlled cervical traction for the reduction and immobilization of cervical injuries. He used a halter device that used the occipital protuberance and mandible as purchase points (Fig. 19). Crutchfield (9), in 1933, was forced to use an alternate means of traction in a patient that presented with a C2C3 dislocation and a compound fracture of the mandible. Crutchfield inserted Edmonton extension tongs into the outer table of the cranium and attached the tongs to a pulley and weight system (Fig. 20). Over the next several years, Crutchfield made several modifications to the system and his instrument became the standard for cervical traction (2, 10, 28). One major disadvantage of Crutchfield tongs was that the pins needed to be placed near the cranial vertex, which limited the amount of traction that could be safely applied. Another inconvenience was the requirement of a limited burr hole through the outer table to obtain sufficient bony purchase.
FIGURE 19. The halter device of Taylor obtains purchase on the mandible and external occipital protuberance (reproduced from Taylor AS: Fracture dislocation of the cervical spine. Ann Surg 90:321340, 1929 ).
FIGURE 20. A diagram of the Edmonton tongs used by Crutchfield in his first article. Although the tongs underwent several modifications, a small distance remained between the points of the instrument. This required placement near the vertex and limited the amount of traction that could be applied (reproduced from Crutchfield WG: Skeletal traction for dislocation of the cervical spine. South Surg 2:156159, 1933 ).
Gardner (19), in 1973, devised a set of tongs that improved upon Crutchfield's system. The tongs were larger, which allowed placement of the pins below the cranial equator. The pins were directed toward the vertex, and heavy traction would only drive the pins further into the cranium. The pin design was particularly noteworthy. Sharp pins were used to allow for direct placement through the skin. Penetration of the inner table was prevented by the taper; as the pin was driven further into the cranium, an exponential increase in the
metal-bone surface area limited further entry. The system became known as Gardner-Wells tongs (Fig. 21), a name that credited the surgeon and the manufacturer (Trent Wells).
FIGURE 21. Gardner-Wells tongs could be placed below the cranial equator. The position and design of the pins allows for heavy traction without slippage or penetration of the inner table (reproduced from Gardner WJ: The principle of spring-loaded points for cervical traction. J Neurosurg 39:543544, 1973 ).
Halo immobilization was introduced by Perry and Nickel (35) in 1959 as an alternative to Minerva jackets in patients undergoing total cervical fusion to treat head instability secondary to poliomyelitis (Fig. 22). The halo produced better postoperative
immobilization, was more comfortable, and allowed positional control of the head in all three directions.
FIGURE 22. Lateral view of the original halo device used by Perry and Nickel (reproduced from Perry J, Nickel VL: Total cervical-spine fusion for neck paralysis. J Bone Joint Surg Am 41:3760, 1959 ).
Harrington (22), in 1962, developed a rod and hook system that represented a major technical advance in spinal surgery (Fig. 23). Harrington instrumentation, like the halo device, was invented in an effort to better treat patients with polio. Scoliosis of the thoracic spine was treated by carefully studying the preoperative roentgenograms and then applying the biomechanical principle of three-point bending to achieve the desired result. The current explosion in spinal segmental fixation devices is a direct result of enhanced understanding of spinal biomechanics and increased sophistication in applying these principles to treat spinal deformities.
FIGURE 23. Harrington rods ushered in the revolution in spinal internal implants. The principle of three-point bending is illustrated (arrows) (reproduced from Harrington PR: Treatment of scoliosis. J Bone Joint Surg Am 44:591610, 1962 ).
From antiquity, civilizations such as the Egyptians have realized the importance of preserving the integrity of the spinal column. Greek and Roman physicians forged some basic ideas of spinal anatomy and physiology. The Renaissance produced a number of key figures, most notably Giovanni Borelli who was able to merge mechanics with anatomy and physiology to produce "iatromechanics," the forerunner of biomechanics. From his work, the study of spinal biomechanics owes its direct existence. The turn of the 20th century was a proving ground for many biomechanical ideas of how best to treat spinal trauma. The clinical investigation into the biomechanics of the spine can be traced
to the study of judicial hangings in the early 1900s. The modern era has produced some comprehensive clinical and mathematical models of the spine. The history of spinal biomechanics is worthy of study because it notes our progress and reveals our shortcomings in the clinical treatment of patients with spinal injuries.
We thank Maria Falconer from the Wangensteen Historical Library for assistance in obtaining historical texts and Priya Sanan for assistance in manuscript preparation. This work was partly supported by National Institutes of Health Grant 5T32NSO7361.
Received, March 25, 1996.
Accepted, April 12, 1996.
Reprint requests: Abhay Sanan, M.D., UMHC Box 96, D429 Mayo Memorial Building, 420 Delaware Street S.E., Minneapolis, MN 55455.
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Occasionally, it is well worth our time to sit back and contemplate how it is that we arrived at "where we are." Sanan and Rengachary eloquently provide a means for us to do this with regard to biomechanics, the root of spinal surgery. From the Egyptian surgical
papyrus to the three-column concept and from the calvarium to the sacrum, Sanan and Rengachary have documented and illustrated the roots of biomechanics. Predominantly cranial and predominantly spinal surgeons alike will be served well by gaining an understanding of "where we were," "where we are," and how we
arrived at "where we are." Perhaps then we can apply biomechanical principles more effectively in our clinical practices.
Edward C. Benzel
Albuquerque, New Mexico
Sanan and Rengachary present a special and interesting article on the historical development of concepts in spinal mechanics. Starting with antiquity, they write of the remarkable understanding our early brethren had of the significance of injuries to the spinal
column, some so severe that treatment need not be performed. Reviewing the work of our Renaissance predecessors, we find remarkably accurate drawings of the spine and its structures. By the 17th century, people like Borelli were working out significant concepts on the biomechanics of the spine. Within this article are a number of wonderful anecdotes;
one that immediately comes to mind is the discussion of hanging and the evolution of how high to place the criminal and where to place the knot. A "hangman's fracture" will never have the same meaning for me after reading this article. The authors go on to provide an excellent historical review of recent 20th century contributions to the treatment and stabilization of the spine. All of this has been illustrated to provide visual clues of the development of this subject for the reader. This is an article all neurosurgeons should read, enjoy, and learn from, and it should be kept very close at hand for our colleagues in training.
James T. Goodrich
Bronx, New York
When this article first came across my desk, I groaned, expecting an hour or so of boring reading. How wrong I was. After several paragraphs, I realized that I could not put it down. Until I read this historical review, I thought that the study of biomechanics began in the mid-20th century. This engrossing article shows how much we owe to early investigators, such as da Vinci, Borelli, Hildanus, Burrell, and others who paved the way for the modern practice of spinal surgery. The title of this scholarly article is a bit of a misnomer, because the authors have chosen to define biomechanics in the broadest possible sense of the term. In fact, this is more a mini-history of the management of spinal disease with an emphasis on the contributions of anatomists, early clinicians who made accurate observations of the nature of spinal disease (especially trauma), and those who treated spinal disease without operation. A disproportionately small amount of space is given to the enormous contributions made by a variety of investigators in the last 30 or 40 years. This is appropriate, because the more recent advances are familiar to most neurosurgeons and, although more relevant in our contemporary management of patients, they are perhaps less interesting than what transpired in the several millenia that preceded the 20th century.
Paul R. Cooper
New York, New York
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