J. Anat. (1979), 129, 4, pp. 753-758 With 2 figures Printed in Great Britain
753
The compressive strength of lumbar vertebrae W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
Division of Engineering, The Polytechnic of Central London, 115 New Cavendish Street, London W1M 8JS
(Accepted 7 December 1978) INTRODUCTION
During daily activities the spine is required to sustain compressive and shear forces. In the lumbar region the magnitude of the shear forces acting on the vertebrae is small in comparison with the compressive forces and it is in the flexed rather than erect posture that the vertebrae are subjected to the greatest compressive forces (Cyron, 1977). A simple calculation gives a good estimate of what these forces must be. Figure 1 shows a photograph of a 24 years old man weighing 75 kg, and holding a 750 N weight. We can calculate the forward bending moment acting at the L5-S1 intervertebral joint by taking moments about the centre of rotation of this joint which lies within the lumbosacral disc. The centre of gravity of the 750 N weight acts 45 cm anterior to the centre of rotation and the weight of the trunk and arms above L5 (404 N) acts 36 5 cm anterior to it; these latter two figures are derived from anthropometric data. Consequently, the bending moment generated is given by: 750 x 0-45 + 404 x 0-365 = 485 Nm. For equilibrium this has to be opposed by an equal moment generated principally by the extensor muscles of the lumbar spine. If these muscles act 5 5 cm posterior N (i.e. 8-8 kN). to the centre of rotation they must produce a force of o The total compressive force acting on the lumbosacral joint is given by the sum of the forces in the extensor muscles and the components of the upper trunk weight (404 N) and the load being lifted (750 N). Because in this posture the plane midway through the lumbosacral disc makes an angle of 800 to the horizontal, these latter two forces exert only a small component of compression at the lumbosacral level (404 + 750) cos 800 = 200 N. The total compressive force on L5 is thus calculated to be 9 0 kN and is almost entirely attributable to the extensor muscle activity. There have been a number of previous studies on the compressive breaking load of lumbar vertebral bodies and the average value we can find for the 20-30 age group ranges from 7-1 to 8-8 kN for L5 (Ruff, 1950; Eie, 1966; Yamada, 1970). It would appear, therefore, that our man is in some danger of developing a compression fracture of his L5 vertebra. It is generally agreed that intra-abdominal pressure offers some relief to the lumbar spine. However, the pressure can only be sustained at the expense of abdominal muscle activity and it is, therefore, difficult to arrive at an accurate value for the relieving moment on the lumbosacral joint. Using the radio pills described by Davis, Stubbs & Ridd (1977), the man shown in Figure 1 developed an intraabdominal pressure of 100 mmHg during the dynamic part of the lift, but this 0021-8782/79/2828-6820 $02.00 © 1979 Anat. Soc. G.B. & I. 48
ANA I29
754
W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
Fig. 1. Photograph of a man, in the fully flexed posture, holding a 750 N weight.
dropped to zero when the weight was off the ground and the man held the position shown in Figure 1. Similar results were obtained from a second subject (male, age 41). Thus, intra-abdominal pressure makes no contribution in the static posture being considered. A further suggestion has been made that in vivo the posterior longitudinal ligament together with intra-abdominal pressure act to restrict the outflow of blood from the vertebral body and so strengthen it hydraulically (Farfan & Lamy, 1977). This will be discussed later. The question remains as to whether in vivo the lumbar vertebrae are stronger than the results of experimental compression testing would suggest, or whether some physiological mechanism exists to mitigate the high forces that our calculation calls for. With this in mind, experiments were carried out to determine compressive breaking load of lumbar vertebrae under different conditions of loading. EXPERIMENTAL METHOD
Fifty-eight lumbar vertebrae were obtained at routine necropsies from 16 subjects aged between 17 and 65 years. The specimens were stored in sealed polythene bags at a temperature of -20 °C until required for testing. All specimens were free of bone disease and were extracted from subjects who, prior to death, were fully mobile. Before dissection the specimens were placed in a refrigerator for 24 hours to allow for slow de-freezing. Each vertebra was carefully separated from the rest of the vertebral column by halving the discs and cutting the ligaments. The neural arch was then sawn off across the partes interarticulares and the vertebral bodies were
Strength of lumbar vertebrae
755
10
8
Z 6
4-
2-
.
I
I
1
2
3 mm
I
I
I
4
5
6
Fig. 2. A typical force/deformation curve for a lumbar vertebra tested under compression. Specimen 4, L2.
cleaned of soft tissue and the remainder of the discs. The specimens were kept moist with physiological saline at all times during their preparation and testing. At this stage the vertebral bodies were not suitable for testing under compression because their end plates were concave and their anterior and posterior heights were unequal. The surfaces were, therefore, built up using surgical Simplex cement to give two parallel surfaces. Surgical Simplex was chosen because before hardening it can be shaped easily and after hardening it is of comparable stiffness to the cortical bone of the vertebral bodies. The compressive force was applied using a hydraulic servo-controlled testing machine at two different rates of displacement; 300 mm and 6 mm per minute respectively. The force applied and the corresponding displacement were recorded using an X-Y plotter. Thirty five vertebrae were tested at the fast rate of displacement and 23 at the slow rate. Seven of the 58 vertebrae were tested under an additional condition. In order to test whether trapped fluid can produce hydraulic strengthening of the vertebral bodies under compressive loads, they were coated with low melting point paraffin wax in an effort to restrict the outflow of fluid during the test. The breaking load was taken to be the load at which the force/deformation curve showed its first peak (Fig. 2). At this point the vertebral body was damaged beyond repair. RESULTS
The results are summarised in Table 1. The mean breaking load taken over all the 58 tests was 6475 N. The breaking load varied by a factor of nearly 20 between the weakest (810 N) and the strongest (15559 N). Paired t tests on the compressive breaking load of vertebrae from the same subject and tested at the same rate of 48-2
756
W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
Table 1. Compressive breaking loads of the vertebrae tested Rate of displacement (6 mm/min)
Rate of displacement (300 mm/min)
Specimen number Age
Sex
49 46 56 17 30 50 38 65 21 49 19 64 50 40 57 65 62
M M F M M F F F M F M M F M M F M
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
, Li
A
L2
L4
L3
4178 - 12931 2584 10298 10636 - 10632 10643 5215 1007
-
-
-
-
4604 -
5420 - 12349 2778
9858 9480 6635 6321 1254 6011 14919 15559* 6644 7324* -
7001
7832
-
-
-
-
-
-
2854
-
-
-
-
-
7010
2933 2030
L2
-
-
5965 6707
Li
L5
5193* 7402 8503 7142 3485 -
-
-
L3
L4
5603 2585 7914 8528 4357 810 3587 11073* 3741* -
-
-
10118
-
-
-
-
-
-
-
-
4556 6339
-
-
-
5799
-
-
-
5533
-
1767 5579
L5
9783
6107*
3846* 5925 6020
-
4631
Overall mean = 6475 N * Waxed.
displacement showed a significant but small increase in compressive strength at the lower level when tested at the fast displacement rate (P < 0-01) but no difference at the slow displacement rate. To determine the effect on compressive strength of different rates of displacement, pairs of adjacent vertebrae from the same subject were obtained in which one had been tested at the faster rate, the other at the slower rate. Of these 17 pairs, in 8 the upper vertebra of the pair had been tested at the fast rate and in the remaining 9 at the slower rate. A highly significant increase in breaking load was found at the faster strain rate (P < 0O001). The use of wax to delay the egress of fluid from within the vertebra, although not mimicking any physiological mechanism, might be expected to augment any tendency to hydraulic strengthening of the vertebra. Paired t tests between the seven waxed vertebrae and their nearest unwaxed neighbours tested at the same displacement rate failed to show any significant difference. The number of vertebrae tested in this way was too small to reveal any minor degrees of hydraulic strengthening that may be present. DISCUSSION
Compression testing of bony material poses some difficulties. The most important necessity is to distribute the compressive force uniformly throughout the specimen. In a structured body like a vertebra the distribution of stress in life may be far from uniform, the intervertebral disc transmitting some compressive force through the annulus to the compact bone of the vertebra and some through the nucleus pulposus to the vertebral end plate. One mode of failure in vivo is by vertebral end plate fracture, allowing the nucleus pulposus to herniate into the vertebral body. A second mode of failure seen in traumatic compressive damage to the spine is of collapse of the compact bone of the vertebral body, most frequently its anterior wall. It is
Strength of lumbar vertebrae
757
recognised that our method of compression testing involving the building up of the vertebral body with surgical Simplex to produce two parallel faces eliminates the former mode of failure, which is dependent on the presence of an intervertebral disc. However, the method does allow testing of those vertebrae, particularly L4 and L5, that are highly wedged, and where any attempt to compress the vertebra through those above and below it introduces extraneous forces. Bone as a visco-elastic material is strain rate dependent (McElhaney, 1966) and the vertebrae tested at the fast rate yielded higher values of compressive breaking load, the difference being 20-30 % in some cases. Thus, the visco-elastic properties of bone give increased strength during the fast application of forces, such as in lifting, when the acceleration will add a component to the weight being lifted. The results from the vertebrae which were waxed do not contribute to the support of the hypothesis that the vertebrae can be hydraulically strengthened. This theory seems less than likely considering the level of pressure inside the vertebral body necessary to give support. For example, if the vertebral end plate has an area of 13 cm2, a pressure of 1000 mmHg within the vertebral body only exerts a force on the end plate of 173 N. In any case, the subject illustrated in Figure 1 held the load for over 1 minute, and any hydrostatic pressure generated by the high compressive force could not be sustained for this length of time. Intra-abdominal pressure can reach levels of over 300 mmHg for very short periods during a heavy lift (Eie, 1973), and thus may be able to offer the vertebrae additional support by allowing an alternative loading route from the thorax to the pelvic floor. If, however, a weight is held for a long period and the subject no longer holds his breath, then the intra-abdominal pressure will fall to much lower levels. We consider it unlikely that the neural arch can resist any significant proportion of the compressive force, particularly in the flexed posture (Hutton & Cyron, 1978). The articular facets are at right angles to the plane bisecting the intervertebral disc thus allowing relative movement under the influence of a compressive force. The highest value for compressive breaking load that we could find reported in the literature was 12 kN. Five of the 58 vertebrae in our tests showed values greater than this. The highest value we obtained was 15-559 kN for an L5 tested at the slow rate (Specimen 2). It would have been interesting to have tested specimens from a group of practising athletes, but the vagaries of post-mortem collection prevent this. However, on the basis of the results obtained from two of our subjects (Specimens 2 and 9), lumbar vertebrae from some of the population evidently have compressive breaking loads in excess of the values reported previously. Although intra-abdominal pressure can support the lumbar vertebrae, the vertebrae themselves are much stronger than previously thought. The question remains, why are the previously reported values so low? It may be that the discrepancies are due to differing methods of testing, or that the postmortem material tested previously is less representative of that portion of the population who can and do lift heavy weights. Repeated strenuous activity is likely to produce bone hypertrophy resulting in a thicker vertebral body cortex and a more dense trabecular system. Inevitably, much cadaveric material is from subjects who were elderly, or whose terminal illness had resulted in reduced physical activity or long periods in bed. Even though our results suggest higher values for the compressive breaking load of lumbar vertebrae than those found previously, it is difficult to escape the conclusion that lumbar vertebrae in strenuous weight-lifting activities operate close to the limit
758
W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
of their compressive strength. The calculated compressive force acting at the lumbosacral level of 9 0 kN when lifting a load of 750 N exceeds the measured compressive breaking load in 45 out of 58 vertebrae tested. Of the 17 spines tested, only five had vertebrae which exceeded a strength of 9 0 kN. CONCLUSION
Values for the compressive breaking load of lumbar vertebrae at physiological strain rates show enormous scatter across the population, ranging from 0-8 kN to nearly 16 kN. Increase in strength was found at faster strain rates, but differences in compressive strength between upper and lower lumbar vertebrae were not significant. Hydraulic strengthening does not appear to contribute to the compressive strength at the strain rates studied. The authors wish to thank Professor P. R. Davis, Department of Human Biology, University of Surrey, for allowing them to use his radio pills. The National Fund for Research into Crippling Diseases provided financial support. REFERENCES
CYRON, B. M. (1977). Mechanical factors in the etiology of spondylolysis. Ph.D. Thesis, The Polytechnic of Central London. DAVIS, P. R., STUBBS, D. A. & RIDD, J. E. (1977). Radio pills: their use in monitoring back stress. Journal of Medical Engineering and Technology 1, 209-212. EIE, N. (1966). Load capacity of the low back. Journal of Oslo City Hospitals 16, 73-98. EIE, N. (1973). Recent measurements of the intra-abdominal pressure. In Perspectives in Biomedical Engineering (ed. R. M. Kenedi), p. 121. London: The Macmillan Press Ltd. FARFAN, H. F. & LAMY, C. (1977). A mathematical model of the soft tissue mechanisms of the lumbar spine from Approaches to the Validation of Manipulation Therapy (ed. A. A. Buerger and J. S. Tobias). Springfield: Charles C. Thomas. HuT-rON, W. C. & CYRON, B. M. (1978). Spondylolysis - the role of the posterior elements in resisting intervertebral compressive force. Acta orthopaedica scandinavica 49, 604-609. MCELHANEY, J. H. (1966). Dynamic response of bone and muscle tissue. Journal of Applied Physiology 21, 1231-1236. RUFF, S. (1950). Brief acceleration: less than one second. In German Aviation Medicine: World War II, ch. VI-C, pp. 584-597. Washington, D.C: U.S. Government Printing Office. YAMADA, H. (1970). In Strength of Biological Materials (ed. F. G. Evans). Baltimore: The Williams & Williams Company.
753
The compressive strength of lumbar vertebrae W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
Division of Engineering, The Polytechnic of Central London, 115 New Cavendish Street, London W1M 8JS
(Accepted 7 December 1978) INTRODUCTION
During daily activities the spine is required to sustain compressive and shear forces. In the lumbar region the magnitude of the shear forces acting on the vertebrae is small in comparison with the compressive forces and it is in the flexed rather than erect posture that the vertebrae are subjected to the greatest compressive forces (Cyron, 1977). A simple calculation gives a good estimate of what these forces must be. Figure 1 shows a photograph of a 24 years old man weighing 75 kg, and holding a 750 N weight. We can calculate the forward bending moment acting at the L5-S1 intervertebral joint by taking moments about the centre of rotation of this joint which lies within the lumbosacral disc. The centre of gravity of the 750 N weight acts 45 cm anterior to the centre of rotation and the weight of the trunk and arms above L5 (404 N) acts 36 5 cm anterior to it; these latter two figures are derived from anthropometric data. Consequently, the bending moment generated is given by: 750 x 0-45 + 404 x 0-365 = 485 Nm. For equilibrium this has to be opposed by an equal moment generated principally by the extensor muscles of the lumbar spine. If these muscles act 5 5 cm posterior N (i.e. 8-8 kN). to the centre of rotation they must produce a force of o The total compressive force acting on the lumbosacral joint is given by the sum of the forces in the extensor muscles and the components of the upper trunk weight (404 N) and the load being lifted (750 N). Because in this posture the plane midway through the lumbosacral disc makes an angle of 800 to the horizontal, these latter two forces exert only a small component of compression at the lumbosacral level (404 + 750) cos 800 = 200 N. The total compressive force on L5 is thus calculated to be 9 0 kN and is almost entirely attributable to the extensor muscle activity. There have been a number of previous studies on the compressive breaking load of lumbar vertebral bodies and the average value we can find for the 20-30 age group ranges from 7-1 to 8-8 kN for L5 (Ruff, 1950; Eie, 1966; Yamada, 1970). It would appear, therefore, that our man is in some danger of developing a compression fracture of his L5 vertebra. It is generally agreed that intra-abdominal pressure offers some relief to the lumbar spine. However, the pressure can only be sustained at the expense of abdominal muscle activity and it is, therefore, difficult to arrive at an accurate value for the relieving moment on the lumbosacral joint. Using the radio pills described by Davis, Stubbs & Ridd (1977), the man shown in Figure 1 developed an intraabdominal pressure of 100 mmHg during the dynamic part of the lift, but this 0021-8782/79/2828-6820 $02.00 © 1979 Anat. Soc. G.B. & I. 48
ANA I29
754
W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
Fig. 1. Photograph of a man, in the fully flexed posture, holding a 750 N weight.
dropped to zero when the weight was off the ground and the man held the position shown in Figure 1. Similar results were obtained from a second subject (male, age 41). Thus, intra-abdominal pressure makes no contribution in the static posture being considered. A further suggestion has been made that in vivo the posterior longitudinal ligament together with intra-abdominal pressure act to restrict the outflow of blood from the vertebral body and so strengthen it hydraulically (Farfan & Lamy, 1977). This will be discussed later. The question remains as to whether in vivo the lumbar vertebrae are stronger than the results of experimental compression testing would suggest, or whether some physiological mechanism exists to mitigate the high forces that our calculation calls for. With this in mind, experiments were carried out to determine compressive breaking load of lumbar vertebrae under different conditions of loading. EXPERIMENTAL METHOD
Fifty-eight lumbar vertebrae were obtained at routine necropsies from 16 subjects aged between 17 and 65 years. The specimens were stored in sealed polythene bags at a temperature of -20 °C until required for testing. All specimens were free of bone disease and were extracted from subjects who, prior to death, were fully mobile. Before dissection the specimens were placed in a refrigerator for 24 hours to allow for slow de-freezing. Each vertebra was carefully separated from the rest of the vertebral column by halving the discs and cutting the ligaments. The neural arch was then sawn off across the partes interarticulares and the vertebral bodies were
Strength of lumbar vertebrae
755
10
8
Z 6
4-
2-
.
I
I
1
2
3 mm
I
I
I
4
5
6
Fig. 2. A typical force/deformation curve for a lumbar vertebra tested under compression. Specimen 4, L2.
cleaned of soft tissue and the remainder of the discs. The specimens were kept moist with physiological saline at all times during their preparation and testing. At this stage the vertebral bodies were not suitable for testing under compression because their end plates were concave and their anterior and posterior heights were unequal. The surfaces were, therefore, built up using surgical Simplex cement to give two parallel surfaces. Surgical Simplex was chosen because before hardening it can be shaped easily and after hardening it is of comparable stiffness to the cortical bone of the vertebral bodies. The compressive force was applied using a hydraulic servo-controlled testing machine at two different rates of displacement; 300 mm and 6 mm per minute respectively. The force applied and the corresponding displacement were recorded using an X-Y plotter. Thirty five vertebrae were tested at the fast rate of displacement and 23 at the slow rate. Seven of the 58 vertebrae were tested under an additional condition. In order to test whether trapped fluid can produce hydraulic strengthening of the vertebral bodies under compressive loads, they were coated with low melting point paraffin wax in an effort to restrict the outflow of fluid during the test. The breaking load was taken to be the load at which the force/deformation curve showed its first peak (Fig. 2). At this point the vertebral body was damaged beyond repair. RESULTS
The results are summarised in Table 1. The mean breaking load taken over all the 58 tests was 6475 N. The breaking load varied by a factor of nearly 20 between the weakest (810 N) and the strongest (15559 N). Paired t tests on the compressive breaking load of vertebrae from the same subject and tested at the same rate of 48-2
756
W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
Table 1. Compressive breaking loads of the vertebrae tested Rate of displacement (6 mm/min)
Rate of displacement (300 mm/min)
Specimen number Age
Sex
49 46 56 17 30 50 38 65 21 49 19 64 50 40 57 65 62
M M F M M F F F M F M M F M M F M
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
, Li
A
L2
L4
L3
4178 - 12931 2584 10298 10636 - 10632 10643 5215 1007
-
-
-
-
4604 -
5420 - 12349 2778
9858 9480 6635 6321 1254 6011 14919 15559* 6644 7324* -
7001
7832
-
-
-
-
-
-
2854
-
-
-
-
-
7010
2933 2030
L2
-
-
5965 6707
Li
L5
5193* 7402 8503 7142 3485 -
-
-
L3
L4
5603 2585 7914 8528 4357 810 3587 11073* 3741* -
-
-
10118
-
-
-
-
-
-
-
-
4556 6339
-
-
-
5799
-
-
-
5533
-
1767 5579
L5
9783
6107*
3846* 5925 6020
-
4631
Overall mean = 6475 N * Waxed.
displacement showed a significant but small increase in compressive strength at the lower level when tested at the fast displacement rate (P < 0-01) but no difference at the slow displacement rate. To determine the effect on compressive strength of different rates of displacement, pairs of adjacent vertebrae from the same subject were obtained in which one had been tested at the faster rate, the other at the slower rate. Of these 17 pairs, in 8 the upper vertebra of the pair had been tested at the fast rate and in the remaining 9 at the slower rate. A highly significant increase in breaking load was found at the faster strain rate (P < 0O001). The use of wax to delay the egress of fluid from within the vertebra, although not mimicking any physiological mechanism, might be expected to augment any tendency to hydraulic strengthening of the vertebra. Paired t tests between the seven waxed vertebrae and their nearest unwaxed neighbours tested at the same displacement rate failed to show any significant difference. The number of vertebrae tested in this way was too small to reveal any minor degrees of hydraulic strengthening that may be present. DISCUSSION
Compression testing of bony material poses some difficulties. The most important necessity is to distribute the compressive force uniformly throughout the specimen. In a structured body like a vertebra the distribution of stress in life may be far from uniform, the intervertebral disc transmitting some compressive force through the annulus to the compact bone of the vertebra and some through the nucleus pulposus to the vertebral end plate. One mode of failure in vivo is by vertebral end plate fracture, allowing the nucleus pulposus to herniate into the vertebral body. A second mode of failure seen in traumatic compressive damage to the spine is of collapse of the compact bone of the vertebral body, most frequently its anterior wall. It is
Strength of lumbar vertebrae
757
recognised that our method of compression testing involving the building up of the vertebral body with surgical Simplex to produce two parallel faces eliminates the former mode of failure, which is dependent on the presence of an intervertebral disc. However, the method does allow testing of those vertebrae, particularly L4 and L5, that are highly wedged, and where any attempt to compress the vertebra through those above and below it introduces extraneous forces. Bone as a visco-elastic material is strain rate dependent (McElhaney, 1966) and the vertebrae tested at the fast rate yielded higher values of compressive breaking load, the difference being 20-30 % in some cases. Thus, the visco-elastic properties of bone give increased strength during the fast application of forces, such as in lifting, when the acceleration will add a component to the weight being lifted. The results from the vertebrae which were waxed do not contribute to the support of the hypothesis that the vertebrae can be hydraulically strengthened. This theory seems less than likely considering the level of pressure inside the vertebral body necessary to give support. For example, if the vertebral end plate has an area of 13 cm2, a pressure of 1000 mmHg within the vertebral body only exerts a force on the end plate of 173 N. In any case, the subject illustrated in Figure 1 held the load for over 1 minute, and any hydrostatic pressure generated by the high compressive force could not be sustained for this length of time. Intra-abdominal pressure can reach levels of over 300 mmHg for very short periods during a heavy lift (Eie, 1973), and thus may be able to offer the vertebrae additional support by allowing an alternative loading route from the thorax to the pelvic floor. If, however, a weight is held for a long period and the subject no longer holds his breath, then the intra-abdominal pressure will fall to much lower levels. We consider it unlikely that the neural arch can resist any significant proportion of the compressive force, particularly in the flexed posture (Hutton & Cyron, 1978). The articular facets are at right angles to the plane bisecting the intervertebral disc thus allowing relative movement under the influence of a compressive force. The highest value for compressive breaking load that we could find reported in the literature was 12 kN. Five of the 58 vertebrae in our tests showed values greater than this. The highest value we obtained was 15-559 kN for an L5 tested at the slow rate (Specimen 2). It would have been interesting to have tested specimens from a group of practising athletes, but the vagaries of post-mortem collection prevent this. However, on the basis of the results obtained from two of our subjects (Specimens 2 and 9), lumbar vertebrae from some of the population evidently have compressive breaking loads in excess of the values reported previously. Although intra-abdominal pressure can support the lumbar vertebrae, the vertebrae themselves are much stronger than previously thought. The question remains, why are the previously reported values so low? It may be that the discrepancies are due to differing methods of testing, or that the postmortem material tested previously is less representative of that portion of the population who can and do lift heavy weights. Repeated strenuous activity is likely to produce bone hypertrophy resulting in a thicker vertebral body cortex and a more dense trabecular system. Inevitably, much cadaveric material is from subjects who were elderly, or whose terminal illness had resulted in reduced physical activity or long periods in bed. Even though our results suggest higher values for the compressive breaking load of lumbar vertebrae than those found previously, it is difficult to escape the conclusion that lumbar vertebrae in strenuous weight-lifting activities operate close to the limit
758
W. C. HUTTON, B. M. CYRON AND J. R. R. STOTT
of their compressive strength. The calculated compressive force acting at the lumbosacral level of 9 0 kN when lifting a load of 750 N exceeds the measured compressive breaking load in 45 out of 58 vertebrae tested. Of the 17 spines tested, only five had vertebrae which exceeded a strength of 9 0 kN. CONCLUSION
Values for the compressive breaking load of lumbar vertebrae at physiological strain rates show enormous scatter across the population, ranging from 0-8 kN to nearly 16 kN. Increase in strength was found at faster strain rates, but differences in compressive strength between upper and lower lumbar vertebrae were not significant. Hydraulic strengthening does not appear to contribute to the compressive strength at the strain rates studied. The authors wish to thank Professor P. R. Davis, Department of Human Biology, University of Surrey, for allowing them to use his radio pills. The National Fund for Research into Crippling Diseases provided financial support. REFERENCES
CYRON, B. M. (1977). Mechanical factors in the etiology of spondylolysis. Ph.D. Thesis, The Polytechnic of Central London. DAVIS, P. R., STUBBS, D. A. & RIDD, J. E. (1977). Radio pills: their use in monitoring back stress. Journal of Medical Engineering and Technology 1, 209-212. EIE, N. (1966). Load capacity of the low back. Journal of Oslo City Hospitals 16, 73-98. EIE, N. (1973). Recent measurements of the intra-abdominal pressure. In Perspectives in Biomedical Engineering (ed. R. M. Kenedi), p. 121. London: The Macmillan Press Ltd. FARFAN, H. F. & LAMY, C. (1977). A mathematical model of the soft tissue mechanisms of the lumbar spine from Approaches to the Validation of Manipulation Therapy (ed. A. A. Buerger and J. S. Tobias). Springfield: Charles C. Thomas. HuT-rON, W. C. & CYRON, B. M. (1978). Spondylolysis - the role of the posterior elements in resisting intervertebral compressive force. Acta orthopaedica scandinavica 49, 604-609. MCELHANEY, J. H. (1966). Dynamic response of bone and muscle tissue. Journal of Applied Physiology 21, 1231-1236. RUFF, S. (1950). Brief acceleration: less than one second. In German Aviation Medicine: World War II, ch. VI-C, pp. 584-597. Washington, D.C: U.S. Government Printing Office. YAMADA, H. (1970). In Strength of Biological Materials (ed. F. G. Evans). Baltimore: The Williams & Williams Company.
