Man’s concepts of engineering and the subcategory, Bioengineering, are tooted in the Egyptian, Greek and Roman concepts of engineering and architecture, fighting the compressive forces of gravity with col umns and massive blocks of stone.

It is only in recent history when we hate developed newer materials that we have recognized that tension forces can play a significant role in the integrity of structures. However, engineers use tension mainly as a support system for compression loads. In humans, MacNab(7), Farfan(1), White(13) and others recognize that tensional components of muscles and ligaments prob ably play a role in spinal support, but only Kirkby(6), and Robbie(11) felt that at times tension may ht the major support force of the spine. Robbie(11), however, still believes that the spinal column is capable of function ing only as a ?stack of blocks? and Kirkby(6) feels that only when the body is properly “balanced” in the gravi tational field does tension function as the major sup port.

It is the author’s contention that only in failure does the spinal column function as a “stack of blocks.” The support system of the spine, and indeed the remainder of the body as well is a function of continuous tension. discontinuous compression, so that the skeleton. rather than being a frame of support to which the muscles and ligaments and tendons attach, has to be considered as compression components suspended within a continuous tension network.

Since the spine is a mechanical structure, inves tigators have used mechanical models to attempt to study spinal kinematics and kinetics. Until now. all models. mathematical or actual, have been based on the axial-loaded compression support system. The problem of such a construct is that they are unidirec tional, so that, like a ?stack of blocks?, or the Great Pyramid, they would lie pulled apart by the very forces that were conscripted to hold them together if tilted out of plumb. The mechanical laws of leverage that operate in the compressional system would create forces that far outstrip any strength of biologic materi als. We could not use such a systent to walk on our two legs, crawl on all fours, walk on our hands or stand on our heads without the addition of tensional forces to hold its together. Such a system is only as strong as its weakest link.

The structural system of continuous tension, discon tinuous compression. hereafter referred to as Tenseg rity, and described by Buckminster Fuller(2), can be used as a model to understand the physiological sup port systems of the body.

The understanding of tensegrity structures has many distinct advantages when applied to biological systems. These structures are multidirectional arid are stable in any direction and independent of gravity. When applied to animated beings the structural sys tem is maintained whether functioning as a biped or quadriped; prone. supine or standing upside down: out the ground, under water or in a spaceship. The laws of leverage act differently when applied within the tensegrity system so that forces generated are dissi pated and may actually strengthen the structure much as prestressed concrete or a wire under tension. External forces applied to the system are dissipated throughout it so that the “weak link” is protected. The forces generated at heelstrike as a 200 pound linebacker runs down the field, for example. could not be absorbed solely by the os calcis but have to be distributed-shock absorber-like-throughout the body.

Does the tensegrity system function in nature? The methane molecule, one of the most basic organic sub stances, has in itself the physical shape and properties of a tensegrity structure. Examination of radiolaria clearly demonstrates the basic structural model. In higher forms of life, we can examine the scapulo thoracic articulation. The entire support system of the upper extremity is a tension system being supported by the musculature interweaving the spine, thorax and upper extremity into a tension support system. I he scapula does not press on the thorax. The clavicle has been traditionally recognized as acting more as a com pression strut, as it would in a tensegrity model. In fact the cat family it is no more than a floating tensegrity strut. Although in humans the upper extremity is no weight-bearing, if we recognize that the same mechan ism is used in bearing weight in all quadripeds, then we can readily see that the tension support system is utilized in vertebrates.

The anatomist Grant(3). in his classic book Methods of Anatomy describes the sacroiliac joint, the major sup porting joint between the pelvis and spine and its superimposed structures. He states that the sacrum behaves not as a keystone but as the reverse of a keystone, and tends, therefore, to sink forward into the pelvis. The spine and its superimposed structures are. of course, supported by the massive ligaments so that the sacrum-and all that is above it-is “slung” in the pelvis and not dependent on axial-compressive sup port.

We therefore call see in readily discernible anatomi cal studies that the tenscgrity system is utilized in two of the major support joints of the body, the scapulo -thoracic and the sacroiliac joints. Tension functions not as a support to a compressive system. Pmt rather as the only system in support of these joints.

Let us build a tensegrity model in its simplest form, the icosahedron. (Figure I.) The mechanical proper ties are described by Fuller.(2) First, it is omnisymmetri cal, one of only five onmisymmetric objects that can be constructed in spare. Only three of these are structurally stable, tire tetrahedron, the octahedron and the icosahedron. The icosahedron is structurally the most economical, containing the most volume for surface area. It is constructed of six Struts and twenty triangles. It is the basic building block of tensegrity structures, and they can lie combined in periodic ar rays to create towers or structures of infinite sizes and shapes, all units being integrated with one another. Second, the structure dots not behave in the war we expect most solid constructed structures at behave. If two opposite and parallel struts are pushed or pulled, all six members will move inwardly or outwardly, caus ing the icosahcdron to contract or expand in a sym metrical fashion. These compression members do not behave like conventional engineering beams. Ordi nary beams deflect locally, tending to contract the building in axial asymmetry. The tensegrity beam does not act independently but acts only in concert with the whole building. contracting symmetrically when the beam is loaded.

<img src=’https://novo.pedroprado.com.br/imgs/1982/1076-1.jpg’>
Figure 1

Stated simply, the whole structure compresses when under pressure from outside, and rotates slightly, re sembling the coupled motion in the spine as described by Panjabi(16). Because a tensegrity structure contracts symmetrically, the parts move symmetrically closer to one another. Therefore, gravity increases to the sec ond power and the whole system gets uniformly stronger. We thus have a system that is omnidirec tional, symmetrically compressible and expandable, and local-load distributing, like soccer balls and auto tires.

What about the biomechanical studies that have amassed over the years? They have all been based on the assumption. a point never proven, that the body is an axial-loaded, compression resisting structure. One of the principles of tensegrity is Synergy, defined by Fuller(2) as the behavior of integral, aggregate whole systems unpredicted by behaviors of any of their com ponents or subassemblies of their components taken separately from the whole. For example, one would not examine the properties of the metal sodium, and the gas chlorine, and predict the properties of the combination, salt. Tensegrity structures have the property of synergy. Therefore, any studies of subas semblies of vertebrae, discs, ligaments, motion segments and the like, have to be reassessed. Nachenson(8), in outstanding studies with transducers placed in discs of live subjects, does trot show compressive loads, but only pressure on the transducer which could be trans mitted by compression, shear, or tension.

There is a corollary to the concept of synergy known as the Principle of the Whole System, which states that the known behavior of some of the parts make possible the discovery of the presence of other parts and their behaviors, kinetics. structures, and relative dimensionalities.

We should, therefore, be able to predict behavior of certain structures. and make the following assertions:

1. Ligaments are under continuous tension.

Studies by Nachemson(9), Tkaczuk(12), Kazarian(4) and others have shown what is described as “pretension;” indicating that the ligamentum flavum, anterior lon gitudinal ligament, and posterior longitudinal liga ment are under tension when the spine is in a neural position. At no time are these ligaments completely lax, indicating a continuous tension state. Kazarian(4) descries that when the vertebral bodies are held to gether only by the intervertebral discs. after having cut the :interior and posterior longitudinal ligaments, the vertebral column expands, as would be expected in a tensegrity system if some of the tension members are cut.

We must recognize that all muscles attached to the spine bare a physiologic resting length, which means that they are tinder continuous tension. never becom ing completely lax. They, therefore, contribute to the continuous tension system as well.

It has been assumed that a ligament may support a load but it cannot move the load. This is not entirely true as the ligaments do act as rubber bands, and when they are deformed, tend to return to their resting length. When energy is imparted into the system by muscular action. or for that matter, by any other force. and the ligaments are stretched, they will absorb these Forces, contract, and then move the load. Ligaments under tension would therefore act as “movers” if their tension were not restrained by an equal and opposite force. The ligamentum flavum has the highest per centage of elastic fiber of any tissue in the body and, therefore, must do quite a hit of stretching and con tracting.

2. Certain structural configurations can he pre dicted.

Tensegrity structures use 60 degree coordination instead of 90 degree coordination. Kazarian(4) points out that the assumed 90 degree transmission of forces is wrong and a more acute angle of force should he used.

Anatomical studies of the fibers of the annulus of the disc show that they are 60-degree oriented.(13)

The relationship of the superior compressive member to the inferior compressive member in a ten segrity unit is such that an element of the superior member is inferior to the superior-most clement of the inferior compressive member. (Figure 1.) This rela tionship continues to exist even if the structure is ro tated in any position, as it is onnidirectional. This spatial relationship can be seen to exist in the vertebral column and, indeed, in essentially all the synovial joints in the body.

3. Although some of the rigid components of a ten segrity system may “kiss,” it does not mean that they are in compressive opposition to one another. Axial loads were applied to joints in live subjects under anes thesia during surgical intervention for a variety of conditions. Joint studies included the knee, ankle. elbow and metatarsal-phalangal joints. In our studies at no time could the articular-surfaces of these joints be forced into contact with one another as long as the ligaments remained intact. Although the study stay lack elements of sophistication, it is readily reproduci ble by any surgeon.

Conclusion: A rigid, axial-loading, gravitationally- oriented support system cannot be utilized as a model lit- animated structures, including the human spine. A model based on Buckminster Fuller’s(2) tensegrity icosahedron, which demonstrates the principle of continuous tension, discontinuous compression. may also be utilized to demonstrate the structural integration of the body. All our previous concepts of biomechanics of the body will leave to be reassessed in relation to this model and our therapeutic approaches to the musculo-skeletal system will have to be revised.

References

1. Farfan, H,F.: Muscular Mechanisms of the Lumbar Spine and the Position of Power and Efficiency. The Orthopaedic Clinic of North America, 6:135-144, 1975.

2. Fuller, R.B.; and Applewhite. E.J.: Synergetics, New York. The McMillen, 1975.

3. Grants Method of Anatomy, Eight Edition, Williams and Wilkins. Baltimore, MD, 1971.

4. Kazarian, L.E.: Creep Characteristics of the Human Spinal Column. Orthop. Clinics of North America, 6, January, 1975.

5. Kenner, H.; Buckey, A Guide Tour of Buckminster Fuller, New York, William Morrow and Company, 1973.

6. Kirkby, R.: The Probable Reality Behind Structural Integration. Bulletin of S.I., 5:5, 1975.

7. McNab, M.B.: Backache, Baltimore, Williams and Wilkins, 1977.

8. Nachemson, A.: Lumbar Intradiscal Pressure, Acta. Orthop. Scand., Suppl. 43, 1960.

9. Nachemson, A.; and Evans, J.; Some Mechanical Properties of the Third Lumbar Inter-laminar Ligaments. J. Biomechanics, 1:211, 1968.

10. Panjabi, M.M.; Krag. W.H.; White, R.A.; and Southwick, W.O.; Effects of Preloads on Load Displacement Curves of the Lumbar Spine. Orthop. Clin. North America, 8:181, 1977.

11. Robbie, D.L.: Tensional Forces in the Human Body. Orthop. Review, 1:45, November, 1977.

12. Tzaczuk, H.: Tensile Properties of the Human Lumbar Longitudinal Ligaments. Acta. Orthop. Scand., Suppl. 115, 1968.

13. White, R.A.; and Panjabi, M.M.: Clinical Biomechanics of the Spine. Philadelphia, J.B.Lippincott, 1978.Continuous Tension, Discontinuous Compression