EXPERIMENTAL, BIOMECHANICAL AND MATHEMATICAL MODELING OF METHODS OF CERVICAL INTERVERTEBRAL DISK PROSTHETICS WITH USE OF DYNAMIC ELASTIC AND MESH IMPLANTS MADE OF TITAN-CONTAINING ALLOYS
Polenov Russian Neurosurgery Institute,
Saint Petersburg, Russia
Moscow Aviation Institute,
Moscow, Russia
The orthopedic and neurosurgical techniques include multiple implants for surgical treatment of the cervical spine (CS) with traumatic or degenerative dystrophic lesions of one or several spinal motion segments (SMS) [1, 2, 3].
The tasks of our study were examination of biomechanical properties of SMS and comparative estimation of dynamic elastic nitinol implant (ENI) or endoprosthesis for the intervertebral disk in view of the single-turn spiral made of nitinol wire and the special titanium alloy cage – mesh titanium implant (MTI) in surgical treatment of patients with CS injuries with use of the methods of experimental biomechanical and mathematical modeling with subsequent estimation of clinical results.
From the anatomic and physiological point, the spine is the complex structure, which allows making extensions, flexions and twisting of the body in different planes. Owing to the modern achievements and multiple events, orthopedists and neurosurgeons are able not only perform prosthetics for lost spinal segments, but also to simulate their function [4, 5, 6].
Two main directions of neuro-orthopedic operations for the spine have developed. The first direction is creation of immobile spondylodesis with formation of interbody block between vertebrae. The second direction is dynamic stabilization with minimal changes in statics and dynamics in the operated region of the spine.
The principle of dynamic stabilization of the spine consists in the concept of biologically and mechanically compatible implants [6, 7].
The cervical spine is the most mobile segment with motions in three planes in compliance with inclination of articular areas of the plane. Directions of motions and rotations of vertebrae relative to each other are possible in view of flexion-extension, lateral flexion and rotation. Combined and spiral motions are also possible.
Motions around vertical axis are combination of rotation and cranial caudal displacement. About 45 % of rotation in the cervical spine arises in C1-C2 (by means of the atlantoaxial joint). Possible motions around sagittal axis are combination of lateral flexion with ventral or dorsal displacement. In case of lateral flexion between two vertebrae one can observe ventral or dorsal displacement between them. Motions around frontal axis are possible only with combination of flexion and extension with lateral displacement. In lateral flexion of C2, its rotation towards the same side appears, and after 15° lateral flexion the lateral displacement of the vertebral body of this vertebra towards the side contrary to flexion is possible.
The modern endoprosthetic devices are to be complied with biomechanics of motions in the intact spinal motion segment [8].
MATERIALS AND METHODS
In the present study we tried to set up some principles determining the requirements for implants made of titanium containing alloys, the features of development, manufacturing and usage.
Mathematical modelling uses the technique of the finite elements method as the part of Ansys software with reconstruction of bony, ligamentous and cartilaginous structures of CS and the analysis of biomechanical properties of the spinal model in flexion and extension [9].
The adequacy of the mathematical model was tested in comparison of the calculations and the experimental results of testing the anatomical samples of the cervical spine with preserved anatomic integrity of bony, cartilaginous and ligamentous structures. The data describing the mechanical behavior of the cervical spine was obtained with the anatomic samples which were exposed to transversal load before and after experimental installation of the nitinol intervertebral disk endoprosthesis and MTI.
The laboratory results were compared with the data of the clinical observations of the patients who received treatment of lesions of CS of various origin with use of ENI in view of the intervertebral disk endoprosthesis or MTI. The clinical follow-up was conducted at 6 months after surgery (the short term postsurgical period), at 12-18 months (the long term postsurgical period) and at 18 months (the late postsurgical period).
RESULTS AND DISCUSSION
At the first stage we estimated the mechanical mobility of the anatomic samples of the cervical spine in flexion and extension in 5 states:
1. The basic state with preserved bony, ligamentous and cartilaginous structures.
2. The basic state according to the item 1 with discectomy (further – disk resection) of C4-C5.
3. The state according to the item 2, additional stabilization with the cage (MTI) between C4-C5 (Fig. 1a).
4. The state according to the item 2, additional stabilization with the intervertebral disk endoprosthesis in view of the spiral turn made of nitinol with shape memory effect and superelasticity (Fig. 1b).
Figure 1
The extension and flexion tests of the anatomic sample of the cervical spine after resection and placement of titanium mesh implant (a) and the intervertebral disk endoprosthesis (b).
a 
b
The results of the tests are showed in the figure 2.
Figure 2
The results of the extension and flexion tests of the cervical spine sample in the basic state, after resection of the intervertebral disk and installation of the endoprosthesis or titanium mesh implant
The basic state of the anatomic sample of C2-C7 is characterized with rigidity about 0.8 N/mm in flexion and 1.9 N/mm in extension within the limits of the bend ±30° against the spinal axis in its mean physiological position. After discectomy at the level of C4-C5 we identified the kyphotic deformation (about 5°) in the state without additional load in dramatic decrease of resistance to flexion. The summary rigidity of the whole anatomic sample decreased to 0.25 N/mm. Resistance to extension changed insignificantly – to 1.5 N/mm. The analysis of resistance of the cervical part of the anatomic sample to flexion and extension in experimental implantation of the intervertebral disk endoprosthesis identified some minimal variations of the values in relation to the basic values: 1.2 and 1.6 N/mm correspondingly.
For the analysis of activity of each SMS the mathematical modelling of the explored region of the spine was performed in the basic state, after discectomy, after installation of the intervertebral disk endoprosthesis and after installation of MTI (Fig. 3).
Figure 3
The results of mathematical modeling during flexion load in the cervical spine in the basic state (a), after resection of the intervertebral disk (b) and installation of the endoprosthesis
a 
b 
c
For estimation of stability of the segments we found the relationship of change in the angle between the endplate of the adjacent vertebrae in the states of injuries or stabilization by implants to the same change of the angles in the basic state in flexion or extension (the table 1). This ratio (stability ratio) is close to 1, if biomechanical behavior of the segment is adequate to the basic state (normal state) more than 1 in rigid fixation and less as result of instability.
| Table 1 | |||||||||||
| Сalculation of change in stability of the cervical spine strengthened by different types of implants | |||||||||||
The geometrical parameters of the model and physical and mechanical properties of tissues were corrected in such manner that basic mechanical behavior was adequate to the mechanical behavior of the uninjured anatomic sample.
The calculations showed the significant decrease in stability of the injured SMS to flexion load after resection of the intervertebral disk. The superior segment is characterized by two time lower mobility in comparison with the basic state.
After installation of the intervertebral disk endoprosthesis the stability is close to physiologic state in all SMS. It testifies adequacy of biomechanical behavior in the stabilized SMS in relation to the basic uninjured state.
After implantation of the cage the injured segment acquired rigidity to flexion loads (k = 11.2) that decreases stability of the superior segment. The stability to extension load changes within the narrow range (k = 1.4-0.85) and is close to the reference.
The analysis of the results of the anatomic examinations and the mathematical modelling was conducted by means of the analysis of the functional X-ray images from the patients who were previously operated in regard to cervical spinal osteochondrosis (at level of C5-C6 and C6-C7) and who received discectomy and the intervertebral disk endoprosthesis (Fig. 4).
Figure 4
The functional X-ray images in the patient G. during flexion (a) and extension (b) after disk resection and installation of the endoprosthesis
a 
b
The comparison of changes in the angles of the endplates of the vertebral bodies of the operated and the upper adjacent SMS showed that mobility of the examined SMS was statistically reliable in the early postsurgical period (the table 2).
| Table 2 | ||||||||||
| Analysis of functional X-ray images (discectomy with implantation of the endoprosthesis of the intervertebral disc) | ||||||||||
However after 12-18 months from the day of surgery the change in the angles of the stabilized SMS decreases by means of appearing fibrous block, which acts as the new intervertebral disk in combination with the intervertebral disk endoprosthesis, with preservation of necessary physiological mobility between the bodies, with imitation of function of the intervertebral disk. Changes in the height of the disks or position of the bone structures were not found.
At 18-24 months after implantation of the endoprosthesis the fibrous scar in the operated segment gradually alters to the bone block with formation of common interbody fusion, like in situation with transformation of own cartilaginous disks as result of degenerative processes, when blocking SMS appears as result of boundary osteophytes.
The analysis of the theoretical, experimental and clinical results show that restoration of normal biomechanics of the cervical spine is realized with dynamic stabilization of SMS with provision of its mobility within the limits of the functional norms, with the ratio closing to 1.
It is achieved by use of the implants supporting load in parallel with preserved spinal structures. It is possible in the case when rigidity of the implant is similar with rigidity of its structures. The most efficient way of creation of such implants is use of materials with low elasticity, for example, some polymers and alloys with shape memory effect and superelasticity (nitinol).
The necessary condition for stabilization of the injured SMS is use of ENI supporting load in parallel with preserved structures of SMS, which are compatible according to rigidity. The design of the implant should provide proper fixation in bone structures and durability in conditions of multiple load in functional motions. Such conditions are satisfied with ENI in view of the intervertebral disk endoprosthesis and MTI.
Despite of short term and long term development of fibrous scar and gradual decreasing mobility in the operated SMS, nitinol prosthetic devices of the intervertebral disk do not lead to overloading in adjacent segments; they prevent decrease in the height of intervertebral space after removal of the injured disk and realize moderate distraction of the vertebral bodies along the spinal axis with preservation of SMS mobility at 12-18 months after surgery.
CONCLUSION
The examinations showed that cervical discectomy with subsequent prosthetic surgery for the operated spinal motion segment with use of various implants lead to preservation of different degrees of mobility in adjacent SMS above and below the operated level.
Maximal rigidity (interbody block) appears in strengthening the injured spinal motion segment by means of the mesh titanium implant, but in this case the short term (up to 6 months) postsurgical period is associated with overloading and development of excessive mobility and instability in adjacent SMS.
The dynamic nitinol elastic implant (the intervertebral disk endoprosthesis) for the spinal motion segment prosthetic surgery preserves physiological biomechanics of the spine without overloading in the adjacent SMS in the short term and long term (up to 18 months) postsurgical periods.
Further progression of the degenerative process in the cervical spine appears in the late postsurgical period (more than 18 months). It results in formation the rough fibrous scar with boundary bone growth around the installed nitinol endoprosthesis, with further decrease in mobility in the operated SMS.