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The importance of physiological relevance in biomechanical experiments
J
Bwmechonics.
1971. Vol. 10. pp. 61 l-612.
Pergamon
Press.
Printed
LETTERS
in Great
Bntam
TO THE EDITOR
THE IMPORTANCE OF PHYSIOLOGICAL RELEVANCE BIOMECHANICAL EXPERIMENTS One of the most promising approaches for investigating the skeleton’s adaptive response to mechanical stimuli is to replace a bone’s normal mechanical environment by an artificially imposed one, the components of which can be varied independently. A careful study of the changes in bone structure which result from known mechanical regimes should then give a valuable insight into the processes which influence bones’ mechanical adaptability. To some extent however a bone’s normal state is a reflection of its normal mechanical circumstances. Adaptive change will only occur if the artificially imposed circumstances differ from the normal ones. A knowledge of the normal is therefore a necessary prerequisite for any sensible study of adaptation to the abnormal. The recently reported experiments by White, Panjabi and Southwick (1977) admirably illustrate the importance of this point. In their experiments these authors compared the torsional strength of healing osteotomised tibiae in two groups of rabbits. In one group the healing bones were subjected to a constant axial compression of 20N. In the other group an additional cyclical load of a further 20N was also imposed. No significant difference in torsional strength was detected between the bones from the two groups. By attaching strain gauges to an isolated tibia from a 3-kg rabbit it can be easily shown that an axial load of 20N will only impose a strain at the bone’s midshaft of some 30 x 10d6 (Table 1). Strain gauges attached to the same site in uiuo demonstrate that this part of the rabbit tibia is normally subjected to some 300 x 10-s at each stride. It is unlikely therefore that White et al. were deforming the tibiae in their cyclically loaded group by any functionally significant amount. The bones from both their groups of animals were likely to have been atrophied from the disuse of being immobilised, and so it is not surprising that no difference in their torsional strength could be demonstrated. What is surprising, and disappointing, is that a number of authors continue to publish reports of mechanical investigations on bone where the conditions of the experiments were not related to those which normally occur. Before
Table 1. The force applied to an isolated fresh rabbit tibia through a transversely placed 3mm Kirschner wire, and the resulting compressive strain (mean f S.D. from 9 loading sequences) on the bone’s lateral and medial surfaces, as indicated by longitudinally arranged single element strain gauges attached in the region of the bone’s midshaft Load (N) 2 ;t!l 29 38 47 55 64
Lateral cortex (e x 106) 2* 14 + 28 + 42 + 57 + 71 k 87 + 100 *
0.8 1.9 1.2 1.9 2.4 2.7 1.7 2.4
Medial cortex (e x 106) 2+ 15 f 29 + 44 + 52 + 72 + 86 f 102 k
0.9 2.1 2.3 2.6 2.1 2.6 2.6 3.2
IN
the introduction of the strain gauge technique in oioo the natural range of bone strain, and strain rate, values could only be surmised. Since that introduction (Lanyon and Smith, 1969, 1970) there have been a number of reports (Cochran, 1972, 1974; Barnes and Pinder, 1974; Lanyon. 1973, 1974; Lanyon et al., 1975; Baggott, 1976; Turner et al., 1975; Daly et al., 1977) all of which suggest a range of functional strain values, in various limb bones, of between 300 and 2000 x 10e6 in tension or compression. Although the strain gauge technique has limitations, and it certainly provides no panacea it at present produces almost the only direct information of the mechanical circumstances of the skeleton in oiuo. It is in addition an extremely simple technique both in uiuo and in vitro and it requires no expensive apparatus that is not readily avaiiable in almost any laboratory. It is often feasible, therefore, in experiments on animal bone at least, to determine the normal in uiuo strain environment of any particular piece of bone under consideration. In all experiments on human, or inaccessible animal bone, it should be possible to relate the conditions of the experiment to the range of strain values already shown to occur in a number of bone sites in a variety of species. If biomechanics is to remain or become, a clinically relevant approach, those who work in this field must constantly examine the design of their experiments and relate them to the admittedly scant data at present available from the living situation. As far as the mechanical circumstances of bone are concerned most of these data are strain data. The literature would greatly benefit in consistency and relevance if bone strain data, not machine load or deflection data, could always be. quoted from experiments on cadaver specimens, and where possible from experiments concerning the living situation. L.E. LAPlYON J. A. O'CONNOR A. E. GOODSHIP REFERENCES Barnes, G. R. G. and Pinder, D. N. (1974) In uiuo tendon tension and bone strain measurement and correlation. J. Biomechanics I, 35-42. Cochran, G. V. B. (1972) Implantation of strain gauges on bone in uiuo. J. Biomechonics 5, 119-123. Cochran, G. V. B. (1974) A method for direct recording of electromechanical data from skeletal bone in living animals. J. Biomechanics 7, 563-565. Daly, W. R., Mills, E. J. and Hohn, R. B. (1977) In uiuo strain analysis of canine long bones and its application to internal fixation. Arch. 6, 11-15. Lanyon, L. E. (1973) Analysis of surface bone strain in the calcaneus of sheep during normal locomotion. J. Biomechanics 5, 277-281. Lanyon, L. E. (1974) Experimental support for the trajectorial theory of bone structure. J. Bone Jnt Surg. 568, 160-166. Lanyon, L. E., Hampson, W. G. J., Goodship, A. E. and Shah, J. S. (1975) Bone deformation record4 from strain gauges attached to the human tibia1 shaft. Acta orthop. scnnd. 46, 256-268. 611
612
Letters to the Editor
Lanyon, L. E. and Baggott, D. G. (1976) Mechanical function as an influence on the structure and form of bone. J. Bone Jnt kg. SBB, 43W3.
Turner, A. S., Mills, E. J. and Gabel, A. A. (1975) In oiuo measurement of bone strain in the horse. Am. .I. Vet. Res. 36, 15731579.
Bwmechonics.
1971. Vol. 10. pp. 61 l-612.
Pergamon
Press.
Printed
LETTERS
in Great
Bntam
TO THE EDITOR
THE IMPORTANCE OF PHYSIOLOGICAL RELEVANCE BIOMECHANICAL EXPERIMENTS One of the most promising approaches for investigating the skeleton’s adaptive response to mechanical stimuli is to replace a bone’s normal mechanical environment by an artificially imposed one, the components of which can be varied independently. A careful study of the changes in bone structure which result from known mechanical regimes should then give a valuable insight into the processes which influence bones’ mechanical adaptability. To some extent however a bone’s normal state is a reflection of its normal mechanical circumstances. Adaptive change will only occur if the artificially imposed circumstances differ from the normal ones. A knowledge of the normal is therefore a necessary prerequisite for any sensible study of adaptation to the abnormal. The recently reported experiments by White, Panjabi and Southwick (1977) admirably illustrate the importance of this point. In their experiments these authors compared the torsional strength of healing osteotomised tibiae in two groups of rabbits. In one group the healing bones were subjected to a constant axial compression of 20N. In the other group an additional cyclical load of a further 20N was also imposed. No significant difference in torsional strength was detected between the bones from the two groups. By attaching strain gauges to an isolated tibia from a 3-kg rabbit it can be easily shown that an axial load of 20N will only impose a strain at the bone’s midshaft of some 30 x 10d6 (Table 1). Strain gauges attached to the same site in uiuo demonstrate that this part of the rabbit tibia is normally subjected to some 300 x 10-s at each stride. It is unlikely therefore that White et al. were deforming the tibiae in their cyclically loaded group by any functionally significant amount. The bones from both their groups of animals were likely to have been atrophied from the disuse of being immobilised, and so it is not surprising that no difference in their torsional strength could be demonstrated. What is surprising, and disappointing, is that a number of authors continue to publish reports of mechanical investigations on bone where the conditions of the experiments were not related to those which normally occur. Before
Table 1. The force applied to an isolated fresh rabbit tibia through a transversely placed 3mm Kirschner wire, and the resulting compressive strain (mean f S.D. from 9 loading sequences) on the bone’s lateral and medial surfaces, as indicated by longitudinally arranged single element strain gauges attached in the region of the bone’s midshaft Load (N) 2 ;t!l 29 38 47 55 64
Lateral cortex (e x 106) 2* 14 + 28 + 42 + 57 + 71 k 87 + 100 *
0.8 1.9 1.2 1.9 2.4 2.7 1.7 2.4
Medial cortex (e x 106) 2+ 15 f 29 + 44 + 52 + 72 + 86 f 102 k
0.9 2.1 2.3 2.6 2.1 2.6 2.6 3.2
IN
the introduction of the strain gauge technique in oioo the natural range of bone strain, and strain rate, values could only be surmised. Since that introduction (Lanyon and Smith, 1969, 1970) there have been a number of reports (Cochran, 1972, 1974; Barnes and Pinder, 1974; Lanyon. 1973, 1974; Lanyon et al., 1975; Baggott, 1976; Turner et al., 1975; Daly et al., 1977) all of which suggest a range of functional strain values, in various limb bones, of between 300 and 2000 x 10e6 in tension or compression. Although the strain gauge technique has limitations, and it certainly provides no panacea it at present produces almost the only direct information of the mechanical circumstances of the skeleton in oiuo. It is in addition an extremely simple technique both in uiuo and in vitro and it requires no expensive apparatus that is not readily avaiiable in almost any laboratory. It is often feasible, therefore, in experiments on animal bone at least, to determine the normal in uiuo strain environment of any particular piece of bone under consideration. In all experiments on human, or inaccessible animal bone, it should be possible to relate the conditions of the experiment to the range of strain values already shown to occur in a number of bone sites in a variety of species. If biomechanics is to remain or become, a clinically relevant approach, those who work in this field must constantly examine the design of their experiments and relate them to the admittedly scant data at present available from the living situation. As far as the mechanical circumstances of bone are concerned most of these data are strain data. The literature would greatly benefit in consistency and relevance if bone strain data, not machine load or deflection data, could always be. quoted from experiments on cadaver specimens, and where possible from experiments concerning the living situation. L.E. LAPlYON J. A. O'CONNOR A. E. GOODSHIP REFERENCES Barnes, G. R. G. and Pinder, D. N. (1974) In uiuo tendon tension and bone strain measurement and correlation. J. Biomechanics I, 35-42. Cochran, G. V. B. (1972) Implantation of strain gauges on bone in uiuo. J. Biomechonics 5, 119-123. Cochran, G. V. B. (1974) A method for direct recording of electromechanical data from skeletal bone in living animals. J. Biomechanics 7, 563-565. Daly, W. R., Mills, E. J. and Hohn, R. B. (1977) In uiuo strain analysis of canine long bones and its application to internal fixation. Arch. 6, 11-15. Lanyon, L. E. (1973) Analysis of surface bone strain in the calcaneus of sheep during normal locomotion. J. Biomechanics 5, 277-281. Lanyon, L. E. (1974) Experimental support for the trajectorial theory of bone structure. J. Bone Jnt Surg. 568, 160-166. Lanyon, L. E., Hampson, W. G. J., Goodship, A. E. and Shah, J. S. (1975) Bone deformation record4 from strain gauges attached to the human tibia1 shaft. Acta orthop. scnnd. 46, 256-268. 611
612
Letters to the Editor
Lanyon, L. E. and Baggott, D. G. (1976) Mechanical function as an influence on the structure and form of bone. J. Bone Jnt kg. SBB, 43W3.
Turner, A. S., Mills, E. J. and Gabel, A. A. (1975) In oiuo measurement of bone strain in the horse. Am. .I. Vet. Res. 36, 15731579.