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Journal of the Anatomical Society of India

Transmission of the weight through the neural arch of lumbarvertebrae in man

Author(s): Aruna. N, *Rajeshwari, T; Rajangam, S.

Vol. 52, No. 2 (2003-07 - 2003-12)

Department of Anatomy, St. John's Medical College, *Bangalore Medical College Bangalore. INDIA.

Abstract

In the present study an attempt has been made to find out the relative magnitude of the weight passing through the neural arch of lumbar vertebrae through its two weight bearing columns by comparing the mean inferior articular facet area and the mean cross sectional area of the lamina (lamina index) of the same vertebra which represent the two parameters of the posterior coloumn. These measurements were done on forty-four sets of adult male lumbar vertebrae (total 220). It was observed that at L1 level posterior column carried the load in the range of 14.76 to 18.24% of the total load at that level, while at L5 level the range was 21.3 to 23.29%. Since both anterior and posterior columns are involved in load bearing, this study recommends the preservation of the integrity of the articular facet joints in laminectomy. The neural arches at L4 and L5 levels are involved in transmission of considerable load, this indicates that these joints between the articular facets could be the sites for low back pain.

Key words: Lumbar vertebrae, neural arch, weight transmission, articular facets.

Introduction:

The vertebral column forms the central axis comparable to a pillar, which forms the main support for the bones and muscles. It is specially adapted to protect the spinal cord and to support the weight of the body and transmits the same to the ground through the pelvic girdle and inferior extremities. Neural arch component is also involved in weight bearing (Denis, 1983; Louis, 1985 and Pal and Routal, 1985, 1986). Thus, weight is transmitted through two columns: anterior column formed by the bodies and inter vertebral discs and posterior column formed by the successive articulations of laminae at their articular facets together with their posterior ligamentous complexes. The vertebral system of column is reinforced by horizontal struts namely the pedicles, which at the level of each vertebra firmly join the columns to each other (Louis, 1985) and transfers the weight between each other according to the line of gravity. The vertebral column performs the important function of weight bearing and transmission, should one or more of vertebrae be diseased, a factor comes into play which is not seen anywhere else in the body, that is the diseased segments are crushed by the superincumbent weight (Decker and du Plessis, 1975).

The resistance to pressure by a uniform structure depends on its cross sectional area. If one assumes that the size of a given portion of vertebra is related to the magnitude of forces acting upon it, then comparison of the size of the portion in different vertebra from the same individual should provide an assessment of the relative magnitude of those forces at different levels. Hence, this study has been attempted to find out the relative magnitude of the compressive forces passing through the vertebral column through its two columns so that this knowledge could be applied to explain some of the clinicopathological conditions of the spine.

Materials and Method:

The present study was done on dry vertebral bones procured from the collection of Bangalore Medical College. Forty-four sets of male vertebrae were studied constituting of 5 lumbar vertebrae in each set, making a total number of 220 vertebrae. The following parameters were taken in each vertebra:

Inferior Body surface Area: The area of the inferior surface of the body in each vertebra was measured using graph paper method. The outline of the inferior surface of the vertebral body was, traced on to a thin tracing sheet which was then transferred on to a graph sheet with the help of a tracing paper and the area was measured in square centimeters by counting the number of squares covered.

Inferior articular facet area: The surface area of the inferior articular facets were also measured using the graph paper method. The mean area of the two sides were then calculated to obtain the mean inferior articular facet area of each vertebra. This represents the first parameter of the posterior column.

Lamina index: The lamina index represents the cross sectional area of the lamina, it is obtained by the product of transverse distance and mean thickness of the laminae on both sides,just above the inferior articular facets measured with vernier calipers. This measurement represents the second parameter of the posterior column.

Table-1: The mean inferior body surface area,mean inferior articular facet area and mean lamina index at each vertebral level

Vertebral levels Mean inferior
body surface
area (cm2)
Mean inferior
articular facet
area (cm2)
Mean lamina
index
L1 10.22 ± 1.33 1.14 ± 0.20 1.77 ± 0.27
L2 10.89 ± 1.47 1.34 ± 0.27 1.99 ± 0.32
L3 11.64 ± 1.31 1.38 ± 0.28 2.22 ± 0.30
L4 12.57 ± 1.55 1.49 ± 0.25 2.53 ± 0.48
L5 11.13 ± 1.27 1.69 ± 0.31 3.02 ± 0.68

The mean of all these parameters were calculated at each vertebral level. The results obtained were tabulated and the correlation matrices were calculated.

Observations and Results:

As in table 1 the mean inferior body surface area showed gradual increase from L1 to L4 and the area of L5 was smaller than L4. The mean inferior articular facet area (table 1) showed an increase from L1 to L5. The mean lamina index (table-I) also showed an increase from L1 to L5.

The cross sectional area of a column at a particular level represents the megnitude of weight transmission. The surface area of the body and the right and left articular facets (or cross sectional area of the lamina) will represent the total weight transmitted at that level. The area of the body and the two articular facets were summed and considered as the total (100%) weight bearing area from which the percentage area of the body and the two facets were calculated separately. Similarly the area of the body was compared to the cross sectional area of the lamina; the second parameter of the posterior column. As in table 2 at L1 level, the posterior column carried the load in the range of 14.76 to 18.24% passing through the spine at that level, while at L5 level about 21.26% of the total weight was borne by the posterior column.

Table-2: Percentage area of inferior surface of the body in comparison with inferior articular facets and cross sectional area of the lamina at each vertebral level

Vertebral levels % inferior
body surface
area (cm2)
% inferior
articular facet
area (cm2)
% inferior
body surface
area (cm2)
% Cross
Sectional area
of lamina (cm2)
L1 81.76 18.24 85.24 14.76
L2 80.25 19.75 84.55 15.45
L3 80.83 19.27 83.98 16.02
L4 80.84 19.16 83.25 16.75
L5 76.71 23.29 78.66 21.30

Discussion:

Almost all textbooks of Anatomy indicate that the vertebral bodies and intervertebral discs sustain all the vertebral compressive forces. But Lewin (1964) has opined that the joints between the articular facets of lower lumbar vertebrae are most often arthritic since they carry higher loads than the upper lumbar vertebrae. It has also been reported that these joints may be the site of low back pain due to same reason (Nyde et al, 1980). Adams and Hotton (1980) have reported that on an average 16% of the force is carried by the apophyseal joints of lumbar vertebrae and therefore chronic overloading of articular cartilage of these joints may lead to osteoarthritis and pain. Yang and King (1984) tested the proportion of the load transmitted through the facet joints experimentally on lumbar vertebrae by means of load testing machine and found it to be between 3 and 25%. Sinohara (1997) has compared the transmission and distribution of load to adjacent vertebrae through the articular processes to regularly laid roof tiles among which pressure is transmitted and distributed through overlapping. All these reports indicate that neural arch is involved in the transmission of load.

Davis (1961) has reported that inferior surface area of L4 is greater than L5 and its inverse relationship with the size of the pedicle and transverse processes indicates that some part of the load from the neural arch is transmitted to the pelvis via ilio-lumbar ligaments. Similarly Pal and Routal (1986) have reported that the inferior surface area of L5 is smaller than L4 and the neural arch of L1 carried the load in the range of 11.02 to 16.10% and at L5 it was in the range of 17.62 to 21.52% from the total load passing at the vertebral level (table 3).

Table- 3:Comparative study of percentage of weight transmission by the neural arch in lumbar vertebrae

Sr No. Name of Worker (Year) % age of weight
transmitted by
posterior coloumn
in lumbar
vertebrae
1. Adams & Hutton (1980) 16%
2. Yang & King (1984) 3%-25%
3. Pal & Routal (1986) 17.62 % to 21.52 %
4. Present Study (2003) 21.3% to 23.29%

The transmission of load at each vertebral level is through the body and laminae. The cross sectional area at any particular level will represent their ability to resist the longitudinal compressive forces passing through the vertebral column at that level. The resistance to pressure by a uniform structure depends on its cross sectional area. Since the vertebral bodies are fairly uniform in internal structure it can be expected that their cross sectional area or more simply the areas of the upper and lower surfaces of the vertebral bodies would better represent their ability to resist the longitudinal compressive forces than do their linear dimensions. The inferior surface was preferred to superior surface because this surface was lowest. This parameter represents anterior column. The areas of the inferior articular facets were taken as the first parameter of the posterior column. The articular facets are incorporated in the laminae itself; hence the compressive forces acting at the superior articular facets are transmitted to the inferior articular facets through the laminae. Thus a cross sectional area of the laminae represents the magnitude of the forces transmitted through it. In other words, the compressive forces transmitted through the laminae are the same as that transmitted through the two articular facets and this was the second parameter of the posterior column. In this study these principles have been applied and they are similar to that applied by Pal and Routal (1985, 1986).

In the present study the body surface area gradually increased from L1 to L4 indicating, that from above downwards more and more weight is borne by the anterior column. The smaller area of L5 is diverted before it reaches its inferior surface. This being so, a part of the compressive forces is transmitted to the pelvis by some other mechanism. Transverse process of L5 is thick and pyramidal unlike the transverse processes of other lumbar vertebrae, having an enlarged base. The massive size of the transverse processes of L5 and its inverse relationship to inferior body surface area indicates that the part of the compressive force from the body of L5 is carried through the transverse process, via the ilio-lumbar ligaments to the pelvis (Davis 1961).

On the other hand the inferior articular facet area and lamina index showed a gradual increase from L1 to L3 followed by a sudden increase at L4 and L5. This indicates that lower portion of the posterior column transmits more compressive forces than the upper portion and these zygoapophyseal joints are highly loaded elements, a cause for low back pain.

On the basis of the above measurements, the compressive forces in the lumbar region were transmitted through two columns, one anterior (formed by the bodies and inter-vertebral discs) and one posterior (formed by the successive articulations of laminae with each other at their articular facets). At L1, the posterior column carried the load in the range of 14.76 to 18.24%, while at L5 level the range was 21.3 to 23.29%. Table III gives a comparison of percentage of weight transmission by the neural arch in the lumbar vertebrae as calculated by different workers. The range was higher in the present study, may be because the methodology was different from that of Adams & Hutton (1980) and Yang & King (1984), and also due to racial and ethnic differences in the people involved in the study.

Summary and Conclusion:

To conclude from the observations of this study, it appears that about 20% of the load passes through the laminae in the lumbar vertebral region. Interference with this column when a lamina is destroyed or weakened by disease, injury or surgery leads to excessive strain on the anterior column, which eventually fails, explaining the etiology of instability in the lumbar region. This study strongly recommends tthe preservation of the integrity of the articular facet joints in laminectomy. The loaded articular facet joints is one of the reason for low back pain in addition to other reasons like root pain, muscular spasm etc.

References:

  1. Adams , M.A. and Hutton, W.C. (1980): The effect of posture on the role of apophyseal joints in resisting intervertebral compressive force. Journal of Bone and Joint Surgery. 62 B : pp 358-362.
  2. Davis P.R. (1961) : Human lower lumbar vertebrae; some mechanical and osteological considerations. Journal of Anatomy. 95 : pp 337 - 344
  3. Denis, F. (1983): The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine.8: pp 817 - 824.
  4. Decker, G.A.G. and du Plessis, D.J.. Lee McGregor's Synopsis of Surgical Anatomy. 11th edition, John Wright and sons Ltd. Bristol: pp 124 - 125 (1975).
  5. Lewin T. (1964): Osteoarthritis in lumbar synovial joints. Acta Orthopaedica Scandinevia (supplementary): pp 73 - 75.
  6. Louis, R. (1985): Spinal stability as defined by three column spine concept. Anatomica Clinica. 7: pp 33 - 42.
  7. Nyde, S. Bell E; Wike B.D. (1980): The innervation of the lumbar spinal joint and its significance. Journal of Bone and Joint Surgery (Br). 62-B : pp 255- 258.
  8. Pal, G.P. and Routal, R.V. (1985): A study of weight transmission through the cervical and upper thoracic regions of the vertebral column in man. Journal of Anatomy. 148 : pp 245-261
  9. Pal, G.P. and Routal, R.V. (1986): Transmission of weight through lower thoracic and lumbar regions of the vertebral column in man. Journal of Anatomy 152: pp 93-105.
  10. Sinohara, H. (1985): Changes in the surface of the superior articular joint from the lower thoracic to the upper lumbar vertebrae. Journal of Anatomy. 190: pp 461-465.
  11. Yang, K.H. and King, A.I. (1984): Mechanism of facet load transmission as a hypothesis for low back pain. Spine. 9: pp 557 - 565.
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