78 C. A. L. Bassett 



Electro-mechanical Factors Regulating Bone Architecture '• 



C. A. L. Bassett '••■' 



The Orthopaedic Research Laboratories, Columbia University, College of Physicians and 



Surgeons and the New York Orthopaedic Hospital, Columbia Presbyterian Medical Center, 



New York, N. Y., U.S.A. 



Bone has been defined as a hard substance, a mineralized connective tissue, the 

 structural material upon which muscles and ligaments are hung. From early child- 

 hood, when man becomes conscious of the meaning of a skeleton, he is aware of the 

 permanence of bone. The surgeon saws it, drills it, screws it, nails it and otherwise 

 treats it very much as he would a piece of oak or pine in his workshop. In fact, its 

 desirable physical properties, such as hardness, elasticity and durability, have 

 prompted men to employ bone in the construction of many diverse items. It has been 

 used in the hunt, as arrowheads and corset stays, and in the pleasures of the parlor, 

 as dice and toothpicks. Truly, bone is a most remarkable substance! Its enduring 

 behavior as an inanimate material, however, is not matched by its conduct in the 

 animate state, for living bone is changing bone. 



The capacity of living bone to adapt its structure to meet mechanical demands 

 has been a source of wonder through much of recorded history and forms the basis of 

 many tribal customs. Centuries ago, the concept that skeletal growth could be shaped 

 by properly applied external forces was incorporated in the practice of orthopaedics 

 by men such as Andry (1741). The famous etching, included in his text of 1741, 

 epitomizes this concept; as the twig is bent, so grows the tree — as the bone is bent, 

 so grows the bone. In more recent times, the close relationship between bone 

 architecture and mechanical principles has stimulated much analysis and speculation. 

 Of the many writings on the subject, those of Wolff (1892) are most universally 

 known and are referred to as Wolff's law. Most simply stated by Jansen (1920), 

 this law holds that, "The form of the bone being given, the bone elements place or 

 displace themselves in the direction of the functional pressure". As pointed out by 

 Thompson (1963), the placement and displacement of bone elements is such that they 

 are best oriented to resist compressive and tensile stresses, while being out of the line 

 of shearing stresses. This adaptive mechanism permits the organism to achieve harmony 

 with its surroundings so that it is not limited to a skeletal shape or size pre-deter- 

 m.ined by inflexible genetic and hormonal factors. Furthermore, in order to meet the 

 challenge of a changing environment, a vertebrate must have the capacity not only 

 to reorient bony elements but also to alter their mass (Wunder et al., 1960). In view 

 of this latter statement, Wolff's law might be improved if amended to state "The 

 form of the bone being given, the bone elements place or displace themselves in the 

 direction of the functional pressure and increase or decrease their mass to reflect the 

 amount of functional pressure". The purpose of this paper is to consider new data 

 that may help clarify the mechanisms by which bone meets the functional demands 

 of mechanical stress. It is hoped that the speculations based upon these data will 

 stimulate a new approach to certain aspects of bone physiology and pathology. 



«■ Supported in part by grants from the U.S. Public Health Service, TI AM 540S, and AM 07822, and the 

 Easter Seal Research Foundation. 



'■'■■ Career Investigator, Health Research Council of the City of New York. 



