An Anatomical Comparison of the Human and Bovine Thoracolumbar Spine

Transcription

1 Journal of Orthopaedic Research 4: , Raven Press, New York Orthopaedic Research Society An Anatomical Comparison of the Human and Bovine Thoracolumbar Spine Paul C. Cotterill, J. P. Kostuik, G. D Angelo, G. R. Fernie, and B. E. Maki Department of Orthopaedic Surgery, University of Toronto, Orthopaedic Biomechanics Research Lab, Toronto General Hospital, West Park Research, Toronto, Ontario, Canada Summary: The bovine spine has been frequently selected as the model for in vitro mechanical studies. A comparative anatomical study has been presented describing important differences and similarities between selected areas in the adult human and calf thoracolumbar spinal segment. Differences in column length and curvature were observed. The total length of the adult bovine vertebral column was found to be on average 209 cm longer than the adult human spine. Equivalent throacolumbar lengths were obtained by selecting 6-8 week-old dairy calves, which because of their young age had the benefit of consistent bone density. The bovine spine was found to have only a single cervical lordotic curve and a single thoracolumbar kyphotic curve (2). A quantitative comparison of selected dimensions of vertebrae T6, T12, and L3 was performed, and significant findings are discussed. The bovine spinous processes were found to be on average 111% greater in length at level T6, which contributed to a 56% greater total anterior/posterior length at this level in comparison to the vertebrae of an adult male. A 23% greater bovine intertransverse length at level L3 was also shown. The orientation of the human and bovine superior facets, while being in the same general plane and direction, varied as much as 10.5 and 107% at vertebral level L3 on the two axes measured. These significantly different measurements were considered important factors that can influence experimental results when using the bovine spine as a model. Key Words: Anatomical comparison-human bovine-thoracollumbar- Spine. Much of the biomechanical testing of spinal implants has used an in vitro model. Calf spines have been used as they are easier to obtain than human spines and can be selected by age to be of comparable size. Little has been written however about the similarities between human and bovine spines (6,7). An appreciation of the differences in structure is essential for the correct extrapolation of the results of biomechanical testing to the human situation. Dyce and Wensing (3) reported that the bovine spine has two curves, i.e., a slight cervical lordotic curve in the sagittal plane followed by a gentle kyphotic curve in the thoracolumbar region. In contrast, the human spine has two additional curves, i.e., lordotic curves in the cervical and lumbar regions and kyphotic curves in the thoracic and sacral areas. The thoracic and sacral curves are primary, while the cervical and lumbar curves are secondary, appearing after birth in response to our upright posture (1,2). The total length of the vertebral regions in the calf spine is influenced by the relative shortness of the intervertebral discs, which contribute only 10% Address correspondence and reprint requests to Dr. J. P. Kostuik at Eaton North, Toronto General Hospital. 200 Elizabeth Of the length Of the in contrast to 25% in Street, Toronto, Ontario M5G 1L7, Canada. man (3). The total length of the adult bovine verte- 298

2 HUMANIBO FINE SPINE COMPARISON 299 bral column is approximately 280 cm on average (inclusive of intervertebral discs) as compared to an average of 71 cm for the adult man (4). However, it is possible to obtain equivalent thoracolumbar lengths by selecting 6-8-week-old dairy calves. There are vertebrae in cows, 35 if the tail is excluded, and 33 vertebrae in man. The breakdown of vertebral numbers is as follows: The bovine and human spines have seven cervical vertebrae. There are 13 bovine and 12 human thoracic vertebrae, six bovine and five human lumbar vertebrae, five sacral vertebrae in both species, and bovine caudal vertebrae with only four human vertebrae at this level. In cattle as in humans, movements in the thoracic region (flexion, extension, lateral rotation) are limited in extent by the rib cage. However, unlike humans, the greatest flexibility of the bovine trunk is found in the thoracic segments just cranial to T13 (5). Caudal to this level movement is more restricted, presumably because of the close proximity of the articular processes and the snugness of the surrounding capsules (3). The second major area of increased mobility is found at the lumbosacral joint. The lower overall flexibility of the adult bovine and calf spine relative to a human s is due in large part to the decreased height of the intervertebral discs (3). The bovine intervertebral disc is constructed, as in humans, with a central soft nucleus pulposus surrounded by a peripheral firm annulus fibrosis. Sue-A-Quan (6) reported that the shape of the disc in the adult bovine and calf spine is concavoconvex in the thoracic region, which differs to the biconvex shape in humans. However, the disc is similarly biconvex in the lumbar segments. A qualitative description of gross spinal characteristics is given, followed by a quantitative comparison of calf and adult human spines at three selected levels. of comparative photographs by studying the length, curvature, and overall shape of the two spines. I1- lustrations of human and calf vertebrae at levels T6, T12, and L3 were made to compare gross anatomical differences at each of the three levels selected. Vertebral levels T6, T12, and L3 were chosen for a qualitative and quantitative study as they were felt to be far enough apart on the spinal column to show unique characteristics pertaining to each vertebral level. A quantitative comparative study was performed by dissecting a total of ten human and ten 6-8- week-old calf spines. Measurements of 14 parameters were made with the use of a micrometer at vertebral levels T6, T12, and L3 (Fig. 6) with ten measurements taken at each level. A two-sided t test with a 0.01 confidence level was performed to test for significant differences between the means of the measurements. The orientation of the superior facets for each of the three levels studied was measured using the technique of White and Panjabi METHODOLOGY Human specimens were obtained from adult embalmed cadavers at the University of Toronto Anatomy Department, and dissected in our laboratories. The 6-8-week old calf spines were obtained fresh from a local abattoir, stored frozen, and then thawed for our studies. A qualitative comparison of the thoracic, lumbar, and sacral regions of the adult human and calf spines, aged 6-8 weeks, was made through the use FIG. 1. Lateral photograph of an adult human spine (left) compared to a calf spine (right) from vertebral level T1 caudally. J Orthop Res. Vol. 4, No. 3, 1986

3 300 P. C. COTTERILL ET AL. segment. Measurements of various parameters at T6, T12, and L3 have been determined and are illustrated in Fig. 6. Table l shows the mean and standard deviations of these measurements for the human and bovine vertebrae. Human-bovine differences in vertebral body parameters of width, length, height, and area were found at all three levels, however, there was no obvious pattern to these differences. Most similarities were found at T6. The antero-posterior dimension at vertebral body T6 was on average 56% larger in the bovine specimen (p < 0.01) accompanying a 11 1% greater bovine spinous process length. At T12 no significant difference was found, but at L3 the human dimension was significantly longer (p < 0.01). These differences can be attributed to the very long thoracic spinous processes in the bovine spine. The pro- T6 Bovine Human FIG. 2. Anterior photograph of an adult human spine (left) compared to a calf spine (right) from vertebral level T1 caudally. (8). The coordinate system used was as follows. The y axis passed vertically through the centre of the body and the x axis passed horizontally (in a coronal plane) through the centre of the body. Medial rotation about the y axis was defined as positive, and lateral rotation was negative. Only the superior facet on the left side was measured. RESULTS Figures 1 and 2 illustrate the adult human and calf spine inclusive of the thoracic, lumbar, and sacral regions. The calf spine is seen to have a thoracolumbar length similar to that of the adult human spinal segment. The kyphotic thoracic and lordotic lumbar curves in the adult human spine are very pronounced, with the calf showing only a gradual kyphotic curve over the length of the thoracolumbar region. Figures 3-5 show comparative illustrations at three vertebral levels of the thoracolumbar spinal FIG. 3. Level T6: Bovine-human comparison. Superior, lab eral, anterior views (top to bottom). J Orthop Res, Vol. 4, No. 3, 1986

4 HUMANIBOVINE SPINE COMPARISON 301 Bovine T12 Human FIG. 4. Level T12: Bovine-human comparison. Superior, lateral, anterior views (top to bottom). rected posteriorly. The bovine lumbar transverse process was significantly longer, directed anteriorly and curved cranially. For both groups, the superior articular facets at T6 were flat and almost vertical (-82.9' about the x axis for bovine and -83.5' * 4.2 for human) and were orientated slightly laterally and backwards (-27.3' & 8.4 about the y axis for bovine and c 2.6 for human). The negative y value denotes the lateral orientation for the left superior facet. At T12 there was no significant difference in angulation about the y axis between the human and calf facets. By level L3, the superior facet became curved and now faced slightly medially and backwards. At this level the human superior facets had a 107% greater amount of medial angulation and a 10.5% greater vertical orientation. Figure 7 shows a radiographic comparison of levels T6, T12, and L3 between a 6-8-week-old calf (right) and an adult human (left). The significantly longer spinous process of bovine vertebra T6 was obvious, as were the anterior facing transverse processes of L3 with respect to the human's shorter and posterior facing transverse processes. The calf vertebrae were more radiodense, with a thicker shell of cortical bone. cesses were longest in the upper thoracic region with a caudal slope gradually increasing to the tenth vertebra. By T12 the spinous process became much shorter, flatter, and horizontally directed. In comparison, the spinous process of humans was shorter and thinner as well as being more obliquely directed at the mid thoracic level. The obliquity of the process decreased below T7 such that by L3 they were short, thick, square cut, horizontally directed, and significantly larger than the bovine process. The lateral dimensions of the vertebrae at mid thoracic levels were similar, becoming significantly larger for the bovine specimens at T12 and L3. This may be attributed to the greater lengths of the transverse processes. The upper thoracic bovine transverse processes were thick, short, rounded, and slightly directed posteriorly. The human processes were thinner, larger, and directed more posteriorly. Below TI2 the processes were reduced to thick stunted projections, smaller than those of bovine origin. The largest variations occurred at L3, with the human processes being slender and di- Bovine L3 Human FIG. 5. Level L3: Bovine-human comparison. Superior, lateral, anterior views. (top to bottom). J Orrhop Res, Vol. 4, NO. 3, 1986

5 302 P. C. COTTERILL ET AL. DISCUSSION YY - I I" 9 FIG. 6. Foramina length (A) and width (B). Vertebral body height (C), width (D), length (E), pedical width (F), spinous length (G), intertransverse length (H), total anterioriposterior length (I), intertransverse superior/superior facet distance (J), and intertransverse superiorlinferior facet distance (K). \\ The ability to select the bovine calf spine as a comparative model to that of the human spine on the basis of accessibility, age, and homogeneity of bone make it a desirable research specimen option. Six- to eight-week-old calves have been used by other investigators (6,7) because of their consistent bone density and similar size. Our qualitative comparison has shown the 6-8-week-old calf spines as having a thoracolumbar length similar to the adult male while at the same time providing a homogenous radiodensity not always present in the adult human spine, but essential for standardized spinal experimentation. Calves as old as 3-4 months were excluded from study, as at this age, the individual vertebral height was over times the height of an average adult male giving a greatly increased total column length. The results of a quantitative comparison, made of various vertebral measurements between human and bovine vertebrae, found the bovine spinous processes to be 111% greater in length at level T6. This one feature was the most appreciably different vertebral measurement found in comparison to the human vertebrae. A second important finding was the bovine's 23% greater intertransverse length at level L3, which was attributed to the very long transverse process at this level. With any biomechanical studies performed on the bovine spine, the TABLE 1. Comparative measurements at three vertebral levels T6 T12 L3 Bovine Human Bovine Human Bovine Human Foramina" Width Length Bodya Height Width Length Areab Pedicle Width" Spinous processa Intertransversea Total ant/posta Inter sup/sup face" Inter sup/inf facet" Sup facet' x axis y axis 16.3 f f f f f 1.6d 23.1 f f f f f f I.ld 87.9 f f 4.4d 49.8 t t f t 8.4d 12.8 i i 2.3d 22.7 f f t f f 2.6d Mean f standard deviation (n = 10). Values are in millimeters. b Values are in square centimeters. Values are in degrees (x and y axes as described in text). Indicates significant difference at p < f t f t f ? f t 5.8d 22.6 f IT 3.4d 6.5 f t 1.9d f d 50.5 t d 64.0 f ? f 2.3d Sd i t 5.9d f f f f 1.7d 12.1 i lsd 26.4 f f f f 7.4d 24.8 f t 6.1d 7.0 f f 2.9d 8.5 f t d 91.2 f t 8.7d 61.8 f f 8.Id 27.3 f i f i f f 4.2d f d J Orthop Res. Vol. 4, No. 3, 1986

6 HUMANIBOVINE SPINE COMPARISON 303 a good research specimen (6). Our study has shown that although the superior facets are orientated in the same general axes as humans, the degrees of orientation vary, particularly at L3 where there is a 10.5% difference on the x axis and a 107% difference on the y axis. These differences in orientation are, additionally, important anatomical factors to be considered when interpreting the results of bovine spinal testing. It is hoped that the researcher will find this data helpful when extrapolating the results of in vitro studies on bovine calf specimens to humans. Currently, with the aid of a better understanding of the anatomical characteristics of the calf thoracolumbar spine, a fracture model is being developed for further biomechanical testing. Acknowledgment: This spinal research is supported by a grant from the Medical Research Council, grant #MA8904. Special thanks to Canada Packers Inc. for the donation of calf spines. REFERENCES FIG. 7. Radiographic comparison of human (left) and bovine (right) vertebrae at levels T6, T12, and L3 (top to bottom). greater length of the bovine spinous processes at level T6, and the greater length of transverse processes at L3, would be factors to consider important in influencing motion at each vertebral segment. The presence or absence of ligaments or muscles, additionally, will influence testing results. The contour of the facets of the bovine vertebrae had previously been felt to be a feature that made it 1. Anderson J: The back. In: Grant s Atlas of Anatomy ed by JE Anderson, Baltimore, Williams & Wilkins Co., 1978, pp Breathnach AS: Frazer s Anatomy of the Human Skelton, ed by AS Breathnach, London, J.A. Churchill Ltd., 1965, pp Dyce KM, Wensing CJG: Essentials of Bovine Anatomy. Utrecht, A Oosthoek, 1971, pp Gray H: Osteology: In: Anatomy ofrhe Human Body. Philadelphia, Lea & Febiger, 1973, pp Sisson S, Grossman H: Ruminant osteology. In: The Anatomy of the Domestic Animal, ed by R Getty, Toronto, W.B. Saunders Co., 1975, pp Sue-A-Quan EA: Anterior instrumentation of the spine. Thesis for Master of Surgery, University of Toronto, Wenger DR, Carollo JJ, Wilkerson JA, Wauters K, Herring JA: Laboratory testing of segmental spinal instrumentation versus traditional Harrington instrumentation for scoliosis treatment. Spine 7: , White AA, Panjabi MM: Physical properties and functional biomechanics of the spine. In: Clinical Biomechanics of The Spine. Toronto, J.B. Lippincott Co., 1978, pp 1-56 J Orthop Res, Vot. 4, No. 3, 1986