Biomechanics and pathomechanics of scoliosis

biomechanics of scoliosis

1. Biomechanics and patho -mechanics of scoliosis Rashmita dash MPO NILD, kolkata 1

2. Introduction Scoliosis , ancient greek term means, “ a bending “ or “crooked” . Consistent lateral deviations of a series of vertebrae from the LOG in one or more regions of the spine may indicate the presence of a lateral spinal curvature in the frontal plane called a Scoliosis. OR Scoliosis is defined as an appreciable lateral deviation in the normally straight vertical line of the spine. 2

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4. Sagittal plane – lordosis & kyphosis Coronal or frontal plane – lateral curvature Axial or transverse plane – rotational deformity of vertebral column 4

5. Patho -mechanics of scoliosis:- Anatomical changes during scoliosis- Wedging of vertebrae caused by the Asymmetric pressure on the immature vertebrae causes the vertebral section on the concave side of the curve to decrease growth whereas the other convex vertebral section where less pressure is applied has normal or accelerated growth. Fadzan et al., Etiological Theories of Adolescent Idiopathic Scoliosis: Past and Present, 2017 5

6. Hueter -Volkmann’s law According to  Hueter -Volkmann Law, bone growth in the period of skeletal immaturity is retarded by mechanical compression on the growth plate and accelerated by growth plate tension. Hueter -Volkmann law is generally used to explain mechanism of scoliosis. Because of the physiologic curvature in the normal spine, compressive force is delivered on the ventrally located part of the vertebral column, whereas distractive force is delivered on the dorsally located part. 6

7. Shortening of the following soft tissues on the concave side:- the intervertebral joint capsule, which may lead to facet joint compression and ultimately osteoarthritis. the intervertebral muscles, the erector spinae , the quadratus lumborum , the psoas major and minor and the oblique abdominals shortening. The anterior and posterior longitudinal ligaments, the ligamenta flava and the interspinous ligaments also shorten to this side, and limit flexion towards the convex side. Fadzan et al., Etiological Theories of Adolescent Idiopathic Scoliosis: Past and Present, 2017 7

8. As the vertebrae rotate, the ribs, which are attached to the vertebrae by the musculoskeletal system, follow the rotational torque applied by the spine. They are pushed downwards as well as forwards on the concave side. This causes a crowding of ribs posteriorly on the concave side as well as a small hump on the anterior chest wall of the same side. Conversely, the ribs on the convex side become widely separated and are pushed backwards, creating a rib hump on the posterior chest wall. Associated with the posterior movement of the ribs is a narrowing of the rib cage on the convex side. The ribs on the convex side then push against the scapula and make it more prominent . 8

9. In patients with structural scoliosis, the anterior elements of the spine are indeed longer than the posterior elements. This condition is commonly called ‘relative anterior spinal overgrowth’ (RASO) 9

10. Vicious cycle theroy :- Stoke’s Vicious Cycle of Pathogenesis: A lateral spinal curvature produces asymmetrical loading of the skeletally immature spine, which in turn, causes asymmetrical growth and a progressive wedging deformity. Spinal loading asymmetry was dependent on neuromuscular activation strategy. 10

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12. Classification of scoliosis:- Structural scoliosis – scoliosis with a component of permanent deformity, vertebrae with sideways tilt and rotated along their long axis. Idiopathic scoliosis Congenital scoliosis Neuromuscular scoliosis others ref: Essential of orthopeadics , J Maheshwari 12

13. Classification of IS CHRONOLOGICAL CLASSIFICATION- JAMES proposed that scoliosis should be classified based on the age of the child at which the deformity was diagnosed. ANGULAR CLASSIFICATION- acc. to the cobb’s angle. 13

14. TROPOLOGICAL CLASSIFICATION- based on the anatomical site of the spinal deformity in the frontal plane. ref: SOSORT 2016 guidelines 14

15. Idiopathic scoliosis Idiopathic scoliosis - It is the commonest type of structural scoliosis. It may begin during infancy, childhood or adolescence. Infantile scoliosis begins in the first year of life, and is different from the other in that, it can be a progressive type. Scoliosis beginning later in life progresses at a variable rate, and leads to an ugly deformity. The deformity is most obvious in thoracic scoliosis because of the formation of a rib hump. Idiopathic curves progress until the cessation of skeletal growth. Ref: Essential of orthopeadics , J Maheshwari 15

16. Congenital scoliosis This type is always associated with some form of radiologically demonstrable anamoly of vertebral bodies. ( i ) hemi-vertebrae (only one-half of the vertebra grows) (ii) block vertebrae (two vertebral bodies fused) (iii) an un-segmented bar (a bar of bone joining two adjacent vertebrae on one side, thereby reventing growth on that side). These curves grow, often at a very fast rate. Sometimes, there are associated anomalies in the growth of the neural structures, leading to a neurological deficit in the lower limb. 16

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18. Neuromuscular scoliosis Neuromuscular scoliosis is an irregular spinal curvature due to abnormalities of the myo -neural (muscle-nerve) pathways of the body. It is generally most severe in non-ambulatory patients. Curve progression is much more frequent than idiopathic scoliosis, and may continue into adulthood . 18

19. Neuromuscular (1) Myopathic arthrogryposis muscular dystrophy (2) neuropathic upper motor neuron lower motor neuron 19

20. Others classification Marfan’s syndrome Myelomeningocele Syringomyelia 20

21. Non structural scoliosis - This is a mobile or transient scoliosis. Postural scoliosis: It is the commonest overall type, often seen in adolescent girls. The curve is mild and convex, usually to the left. The main diagnostic feature is that the curve straightens completely when the patient bends forwards. 21

22. 2. Compensatory scoliosis: In this type, the scoliosis is a compensatory phenomenon, occurring in order to compensate for the tilt of the pelvis (e.g., in a hip disease or for a short leg). The scoliosis disappears when the patient is examined in a sitting position (in case the leg is short) or when the causative factor is removed. 3. Sciatic scoliosis: This is as a result of unilateral painful spasm of the para -spinal muscles, as may occur in a case of prolapsed inter-vertebral disc. 22

23. Normal spinal alignment Spinous processes all line up in a straight line over the sacrum. Scoliosis is a combination of Angular displacement Lateral displacement 23

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25. Descriptive terms The side towards which the convexity of the curve is directed is designated as right or left. The involved location of the curve is as follows Cervical Cervico thoracic Thoracic Thoracolumbar lumbar 25

26. Simple curve- single spinal deviation Compound curve- displacement in right and left direction Primary curve- curve that develops first Secondary or compensatory curve- develops as a balancing response to the primary curve. 26

27. Non structural curve – curve is flexible and corrects by bending towards convex side Structural curve – curve is not corrected on bending on convex side. 27

28. Lenke classification 28

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30. Biomechanical considerations involved in prognosis Measurement of rib vertebral angles at the apex of the curve would be useful in determining the prognosis of idiopathic scoliosis. For infantile idiopathic scoliosis, the rib vertebral angle difference (RVAD ) was >20° in 80% of those curves which progressed and <20° in the remaining curves. The rib-vertebrae angles on the convexity and concavity of the spinal curve. 30

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32. Clinical biomechanics Cobb’s angle and its significance Furgeuson angle Facet apposition, interface distance, percentage canal occulusion , vertebral body height loss and maximum fragment retropulsion . 32

33. BIOMECHANICS OF SCOLIOSIS Creep and Relaxation: Creep is the deformation that follows the initial loading of a material and that occurs as a function of time without further increase in load. When a force is applied to correct a spinal deformity, and the force continues to work after the initial correction, the subsequent correction that occurs over a period of time as a result of the same load is due to creep . 33

34. Creep in scoliosis. F is a constant force applied with axial traction. The original length of the scoliotic segment L corrects and increases to L+ D as a function of time. D is the deformation or change in the length of the curved segment of spine. When a load is applied to a visco -elastic material and the deformation remains constant, the observed subsequent decrease in load with time is relaxation. 34

35. Comparison of Axial, Transverse, and Combined Loads for Scoliosis Correction The comparative efficiency of different types and combinations of loads applied to a scoliotic spine for correction. The scoliotic spine is modelled by three components: Two rigid links AC and BC, connected by way of a torsional spring C . 35

36. The scoliotic spine under axial load A simplified model of the spine being subjected to axial distraction force F. Free body diagram of the model link BC and the joint C. 36

37. An axial force is applied at the two ends of the spine segment, represented by points A and B in the model, to elongate and straighten the spine. The mechanism of angular correction by elongation is not due to tensile stresses in the spine but rather to the bending moments (stresses) created at the various disc spaces. It is these bending moments that correct the angular deformity. 37

38. The scoliotic spine under transverse loads. A simplified model of spine being subjected to three-point transverse forces. Free body diagram of the model link in BC and the joint C. 38

39. The lateral force is applied at C, and reactive forces half its size are taken up at points A and B. The angular correction is again obtained by creating corrective bending moments at the disc spaces. 39

40. The corrective bending moment at the apex of the curve is the axial force F multiplied by its perpendicular distance D to the apex of the curve. It is easily seen that the greater the deformity, the greater is the distance D . In other words, the correctional ability of the force increases with the severity of the deformity. 40

41. The components lie and move in the frontal plane. The links are oriented to simulate spine deformity in theta(°) degrees as measured by Cobb's method. The static behavior of this model is studied under three separate loading conditions- axial force, transverse force, and a combination of axial and transverse forces. 41

42. The corrective bending moment at the apex of the curve equals half of the force at the apex (the other half works on the other half of the spine) multiplied by D , the perpendicular distance to the apex of the curve. In contrast to the axial force, the corrective bending moment for the lateral force decreases as the deformity of the spine increases. 42

43. The scoliotic spine under combined axial and lateral loads. A simplified spine model being subjected to combined loading. Free body diagram of the model link BC and the joint C. 43

44. COMBINED LOAD- Using equal loads at the three loading points, the two end forces will have to be tilted 30 degree toward the centre force for the equilibrium of the spine Comparison of the efficiency of the three loading types can be made on the basis of the corrective bending moment produced at the disc space. The greater the bending moment, the greater is the angular correction obtained. 44

45. In conclusion of above loadings : Axial components provide most of the corrective bending moment when deformity is severe Transverse component takes over the corrective function when deformity is mild The combined load is most benificial for all situations 45

46. A graphic representation of "relative corrective moment ML" as a function of spine deformity in degrees (Cobb's method) for the three loading types. According to the theoretical model studied here, we note the following: The combined load is the most efficient for any degree of deformity; the axial load efficiency increases with the angular deformity; and the transverse load efficiency decreases with angular deformity. The deformity angle of 53° is a break-even point for the axial and transverse loads. Examples of two theoretical patients with mild (30°) and severe (70°) curves are shown. 46

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48. Considering two patients whose curve measure 30 and 70 degrees as per cobbs method. For Ө = 30 degrees , the values of M/FL for: Axial load = 0.26 Transverse load = 0.48 Combined load = 0.71 For Ө = 70 degrees , the values of M/FL for: Axial load = 0.57 Transverse load = 0.41 Combined load = 0.91 48

49. Based on this theoretical consideration, it can be concluded that patients with severe deformity should be treated with axial loading in the beginning and as the deformity decreases the loading should be changed to transverse type assuming that axial and transverse loadings can not be combined and applied simultaneously and vice versa . 49

50. Thank you 50