XI Международная студенческая научная конференция Студенческий научный форум - 2019

Problem

Scoliotic spine disease is one of the common diseases in adolescence due to the technological progress and reduced motor activity. Scoliosis is curvature of the spine in the plane of the back.

According to James classification scoliosis is divided into: thoracic, lumbar and thoracolumbar (Pic.1) [1]. The most severe and common type of scoliosis is thoracic scoliosis.

One of the forms of surgical treatment of scoliosis is the use of metal devices that provide correction of spinal deformity during treatment [1]. All metal devices used in the treatment of scoliosis can be divided into two main types: internal (submersible) devices (Pic. 2)

nd external fixation devices (Pic.3).

Internal mechanical devices for the treatment of scoliosis include various types of clamps-ties, plate clamps. Clamps-ties are most often used in compression fractures of the vertebral column and provide reliable fixation of bone fragments.

In the case of pronounced instability, the most rigid fixation is provided by plate clamps.

The designs of internal distractors by scientists Harington, Kazmin, Rodnyansky were widely used for correction of scoliotic deformation of the spine [1]. The advantage of such distractors is the ability to set corrective action on the vertebrae and the preservation of this effect for a long time without surgery. At the same time internal fixators are inferior to external efforts to control the entire process of treatment.

External fixation devices of the spine provide a more flexible mode of correction in accordance with the state of the spine and allow to control the magnitude and direction of the impact on the individual vertebrae. The beginning of these research works in this direction refers to the 70th years of the last century. The inventors of these devices are Bytovyh, Chikunov, Usikov [ 1].

Application of devices for external fixation of the spine in case of mechanical damage or deformation of the vertebrae was first realized in 1982.

General problem

Alongside with all the variety of designs of devices of external fixation of the spine and effectiveness of their use for various tasks, large-scale introduction of such devices in medical practice is constrained by the need for high qualification of the surgeon using the devices.

Formulation of the article objectives

Taking into account the prospects of using the apparatus of external fixation of the spine at the present stage, it is necessary to develop algorithms and methods of its application, based on scientific studies of the stress-strain state of the parts of the apparatus and vertebrae carrying this apparatus. The purpose of this article is to set tasks for further improvement of the apparatus of external fixation of the spine, taking into account the mechanical stresses arising in the details of the apparatus and vertebrae.

Main material

The external fixation device is a spatial plate-rod structure with a high degree of static uncertainty [6]. The presence of a large number of additional connections between the parts of the apparatus leads to an increase in stresses in the parts of the apparatus and bone structures, which can reach dangerous values [4].

Mechanical connections between the parts of the device must provide rigidity and controllability of the structure. At the same time, in some elements, for example, the distraction rods can be introduced swivel joints in mutually perpendicular directions. This will ensure a reduction in the degree of static uncertainty and, consequently, a reduction in mechanical stresses. If necessary, it is possible to provide a rigid fixation of such hinges, which would be equivalent to the transfer of parts into a rigid connection. For the application of such a design it is necessary to conduct research on the effect of hinge joints on the overall correction process.

When installing the external fixation device on the spine due to misalignment between the parts of the device, as well as between the holes in the vertebrae with screw rods, significant mounting stresses may occur. The magnitude of such stresses can reach the limit values immediately after the assembly of the device, leaving no margin for strength during the correction. The solution to this particular problem is also possible through the use of hinges with the possibility of rigid fixation after Assembly [4, 7].

The advantage of the apparatus of external fixation of the spine is the variety of forms of influence on the spine, which is achieved by a different combination of corrective actions in the distraction rods together with different layout solutions of the entire apparatus.

At the same time, a high degree of static design uncertainty in some cases does not allow to find an obvious and unambiguous solution for the application of control actions. The creation of compression or distraction forces in some rods can lead to the emergence of force factors in the connection of the vertebra - rod-screw in the other completely unpredictable blocks of the apparatus.

Elastic-plastic deformities that occur in bone tissues and ligaments during the correction of spinal deformity, make their component in the overall picture of the stress-strain state of the apparatus. In this regard, the correction should take into account the dynamics of such deformations [8].

With the complex design of the apparatus of external fixation of the spine, the variety of tasks to be solved, the occurrence of additional introductory adjustments during surgery before the correction of spinal deformity and in the process, it is necessary to conduct mathematical modeling to optimize the control actions and predict the final result [8]. The development of such a mathematical model will allow not only to carry out the correction process more effectively, but also to optimize the design of the device at the stage of its design and assembly for specific tasks.

Conclusions from this study and prospects

Along with the improvement of the medical aspects of application of external fixation devices requires the solution of a number of tasks on optimization of stress-strain state of parts of the apparatus, reduce the redundancies of the design and optimization of controlling impacts on the spine from the side of the machine. The interaction of the above components should be carried out taking into account the medical and biological parameters of the patient, the actual curvature of the spine.

Bibliography

1.Piven V. V., Lyulin S. V., Kovalenko P. I., Mushtaeva Yu. A. Mechanics of external fixation device of spine: reducing the mounting stress. IOP Conference Series: Materials Science and Engineering MEACS 2017 327 (2018) 042078 doi: 10.1088/1757-899X/327/4/042078.

2. Sergeev K. S., Piven V. V. The substantiation of the elastic–viscoplastic model of the human spine for modeling the correction process of kyphoscoliotic deformation. Journal of Craniovertebral Junction and Spine. 2018, 9 (1). Pp. 32 - 36. Doi: 10.4103/jcvjs.JCVJS_156_17.

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