Experimental Studies On The Three-dimensional Stability And Safety Of Anterior Atlantoaxial Transarticular Locking Titanium Plate Screw Fixation System

Atlantoaxial instability is that the structures of atlantoaxial are destroyed by vertebral trauma, deformity, tumor, inflammation, which lose normal physiological function and stability, lead to atlantoaxial dislocation or vertebral subluxation, and finally lead to the stimulation of spinal cord, nerve root, vertebral artery and compression symptoms. The causes of atlantoaxial vertebral instability are as follows: ①Congenital abnormalities:such as atlantoaxial vertebral dysplasia, odontoid malformation, relaxation, missing or asymmetric of intervertebral joints and ligaments structure, etc.; ②Traumatic:car accidents, high falling injury leading to atlas-pivot vertebral body fracture, atlantoaxial subluxation, Jefferson fracture, Hangman fracture, odontoid fracture, chronic trauma leading to the odontoid pseudarthrosis formation, acute ligament injury leading to true dislocation, chronic ligament damage leading to C1 and occipital fusion, C2 and C3 of vertebral fusion, etc.; ③Pathological:rheumatoid arthritis, ankylosing spondylitis, upper cervical tuberculosis, etc.; ④Tumor lesions; ⑤Degenerative disease:osteoarthritis, etc. Traumatic atlantoaxial vertebral instability of which is the most common. Clinical performance of atlantoaxial vertebral instability: ①Pillow neck pain; ②Torticollis; ③Cervical spinal, medulla oblongata damage:paralysis, hard breathing and even life-threatening;④Vertebral artery injury:affecting the brain blood flowing;⑤ Nerve root injury:corresponding signs of the innervation area;⑥The long course of the disease can lead to a slow progressive pectoral girdle and upper extremity and hand intrinsic muscle atrophy. There is a serious risk in atlantoaxial instability to cause the signs and symptoms of compression of upper cervical spinal cord, nerve roots, vertebral artery. Therefore, it usually should be treated by surgery.Clinical surgery on atlantoaxial instability were divided into posterior fixation and anterior fixation currently. There were some main posterior fixation such as the C1-2 lamina wire technique, lamina clamp technique, magerl screw technique, posterior atlas lateral mass pedicle screws and rods technique. The mechanical properties of the C1-2 lamina wire technique, lamina clamp technique which were used early were poor, demanded for completing atlantoaxial rear structure, had unreliable fixation; The mechanical properties of "magerl" screw technique and posterior atlas lateral mass pedicle screws and rods technique improved, had reliable fixation. However, there were some special cases such as congenital incompleting atlantoaxial rear structure, the variation of the vertebral artery, gooseneck deformity, iatrogenic posterior bone structure resection, the anatomic variation on position and size of the atlantoaxial which should not be taken into posterior but anterior fixation and fusion. At the same time, posterior surgery is helpless in compression and adhesion which come from the front of the spinal cord, requiring anterior decompression and release, Ⅰ or Ⅱ period posterior fixation, turning over in surgery might cause fatal spinal cord injury, while increasing the cost of hospitalization of patients.Anterior surgery is mainly applied to the special cases such as congenital incompleting atlantoaxial rear structure, the variation of the vertebral artery which could not apply posterior surgery in the past. However, with the development of science and technology in recent years, it was applied to various atlantoaxial instability and had the advantage that posterior surgery do not have. Anterior transoral atlantoaxial titanium plate fixation could release, decompress and fix from the anterior side at the same time, which have been widely used for fresh and old traumatic atlantoaxial instability clinically, and have achieved a certain degree of efficacy. But there were fatal deficiencies that the surgical operation was in the case of pollution, susceptible, intraoperative injury of dural caused cerebrospinal fluid leakage, then infection would be difficult to control, so that threatening life. Therefore, anterior submandibular approach becomes a new choice, but there are not any satisfactory fixation devices currently. If atlantoaxial vertebral atlas lateral mass screw fixation is chosen, larger upper angle of the screw is required, which could damage the spinal cord and the vertebral artery, in addition, high atlantoaxial integrity is required. The channel of anterior atlantoaxial transarticular screw passes through the pivot vertebral body and lateral mass of atlas from anteromedial to posterolateral, which could avoid penetrating the spinal canal to injure spinal cord, far away from the vertebral artery, the risk of damaging vertebral artery getting lower, and reduce the risk of spinal cord vertebral artery injury. This internal fixation in our hospital had been popularized, and achieved satisfactory results, but the mechanical property of this technology should be further improved, for the reason that it demanded for high axial bone structure. Otherwise, the screw would loose and prolapse for osteoporosis, and difficult to operate. That comminuted fracture of the lateral mass of the atlas is not appropriate to be treated by this technology.Anterior atlantoaxial transarticular locking titanium plate screw fixation system (The Chinese patent No was ZL 2011 20134687.X) is a new anterior atlantoaxial internal fixation system we independently developed with independent intellectual property rights. There are some characteristic in the internal fixation system that have its own reposition function, a locking mechanism with screws, aiming function to form an integration. This technique used the anterior submandibular approach that could apply anterior decompression and I period fixation at the same time, then simplify the surgical operation, strengthen internal fixation reliability. This topic through establishing a operation model, to observe the internal fixation system, matching with atlantoaxial vertebral complex measuring fresh specimens of anterior atlantoaxial transarticular anatomic locking titanium plate screw fixation system atlantoaxial joint in flexion, in extension, in lateral bending, and in the axial rotation of range of motion, to explore the internal fixation system line atlantoaxial vertebral after internal fixation of three dimensional stability and safety, and provide theory basis for Clinical application of anterior atlantoaxial transarticular anatomic locking titanium plate screw fixation system.Part one Three dimensional stability study of anterior atlantoaxial transarticular locking titanium plate screw fixation systemObjective Through simulated surgery on fresh cadaver cervical spine specimens, we evaluate three dimensional stability of atlantoaxial vertebral after mechanics experiments on spinal three dimensional motion machine, and then provide the internal fixation operation biomechanical basis for further popularization and application in clinic.Methods 8 accidental death of adult male fresh specimens of cervical vertebra, eliminated the cervical vertebra disease, drawn out after the neck muscle tissue, keeped complete ligament and joint capsule, made Co～C4 complete experimental specimens. With poly-methyl methacrylate (denture acrylic) embedding, the embedding to Co above, below the embedding to the C4, putted into the refrigerator-20℃, and saved for later use.8 specimens were numbered, for each specimen for complete state, odontoid type Ⅱ fracture, anterior atlantoaxial transarticular screw locking titanium plate fixation and posterior atlantoaxial vertebral pedicle screw fixation of three dimensional motion range test of the four states. Each specimen first for three dimensional motion range test of the normal state, and then make specimens from atlantoaxial vertebral instability (odontoid type Ⅱ fracture), test three dimensional motion range of specimens under odontoid type Ⅱ fracture state. Again for each specimen respectively anterior atlantoaxial transarticular locking titanium plate screw fixation and posterior pedicle screw fixation, and test the three dimensional motion range under different states. To eliminate the influence due to the order of fixed way of on the experimental results of the same specimen, anterior fixation and posterior fixation order by randomly.Biomechanics test We test three dimensional motion in nondestructive way in spinal three dimensional motion tester Spine2000 (0.01 Nm), calibrating a specimen, then applying pure moment of couple of 2.0 Nm. Through a specimen loading and unloading load 2.0 Nm, measure the specimen under different states of flexion, extension, left lateral bending, right lateral bending, left axial rotation and right axial rotation. Before each direction load 3 times,2 times for the elimination of the influence of creep, the third time accurately measure the movement direction of angle displacement as a benchmark, to determine the range of motion of the movement of specimens. By a laser 3d scanner scanning atlas axis movement under zero load and maximum load state, record and input computer, respectively to calculate complete state, odontoid type Ⅱ fracture, anterior atlantoaxial transarticular screw locking titanium plate fixation and posterior atlantoaxial vertebral pedicle screw fixation in the ROM.Results The ROM of anterior atlantoaxial transarticular screw locking titanium plate fixation and posterior atlantoaxial vertebral pedicle screw fixation are much smaller than the one of complete state and odontoid type Ⅱ fracture(P<0000); There are significant differences between the atlantoaxial vertebral ROM of flexion, extension, lateral bending and axial rotation of complete state and odontoid type Ⅱ fracture(P<0000). There is no significant difference between anterior fixation and posterior fixation. According to the biomechanical testing, anterior atlantoaxial transarticular locking titanium plate-screw fixation provides satisfied stability, which provides the same effect as that by posterior pedicle screws fixation. It will be another choice alternative for the treatment of atlantoaxial instability.Conclution The ROM of anterior atlantoaxial transarticular screw locking titanium plate fixation are much smaller than the one of complete state and odontoid type Ⅱ fracture(P<0000), and has no significant difference with posterior atlantoaxial vertebral pedicle screw fixation. The three-dimensional ROM of anterior atlantoaxial transarticular locking titanium plate-screw fixation group was (1.39±0.26)°in flexion, (1.40±0.22)°in extension, (1.43±0.23)°in lateral bending and (1.77±0.34)°in the axial rotation,they can immediately stabilize atlantoaxial vertebral complex, obvious decrease atlantoaxial joint range of motion of each direction and has a good biomechanical performance.Part two Anatomic safety study of anterior atlantoaxial transarticular locking titanium plate screw fixation systemObjective Through anterior atlantoaxial transarticular locking titanium plate screw fixation simulation surgery on fresh cervical instability specimens, we analyze anatomical measurement data from CT three-dimensional reconstruction, and evaluate the anatomic safety of this internal fixation system in the aspect of clinical application.Methods 8 accidental death of adult male fresh specimens of cervical vertebra, disposed like part one, simulated surgery of anterior atlantoaxial transarticular screw locking titanium plate fixation, then the surgical models were scanned by Multislice Spiral Computer Technology (MSCT) on 2.0mm thickness postoperatively, the raw data acquired was delivered to Vitrea 2 workstation to conduct Multi-planar Reconstruction (MPR) and Volume Rendering (VR) of Three Dimension CT Reconstruction, the distance between atlantoaxial transarticular screw and vertebral artery, spinal cord in different planes, the upper articular surface of the lateral mass of atlas of Multi-planar Reconstruction (MPR) which related to anterion atlantoaxial transarticular anatomic locking titanium plate screw fixation system were measured. At the same time DR was applied postoperatively to analyze the match between fixation system and atlantoaxial, then comprehensive analyzing with the data from biomechanical experiments, to evaluate the safety in the aspect of clinical application. Results After anterior atlantoaxial transarticular screw locking titanium plate fixation surgery, X-ray examination reads locking titanium plate match well with the axis body, screws are within the bone, not into the spinal canal and transverse process holes, The shortest distance between atlantoaxial transarticular screw and vertebral artery, spinal cord was in horizontal plane A(the upper edge of the transverse foramen of atlas),with interval of (5.35±1.02)mm and (8.55±0.93)mm respectively; with an interval of (3.45±0.64)mm between atlantoaxial transarticular screw and the upper articular surface of the lateral mass of atlas. The distance between atlantoaxial transarticular screw and vertebral artery, spinal cord was (5.55±0.99)mm and (8.15±1.03)mm respectively in horizontal plane B(the lower edge of the transverse foramen of atlas),was (9.75±1.21)mm and (10.95±1.02)mm respectively in horizontal plane C(the lower edge of atlantoaxial upper articular surface).Conclusion Anterior atlantoaxial transarticular screw locking titanium plate fixation system can stabilize atlanto-axial vertebral complex immediately, there was enough screw placement space for Chinese atlantoaxial specimens, the anterior atlantoaxial transarticular anatomic locking titanium plate screw fixation system in atlantoaxial fixation was anatomicly safe and feasible for Chinese. Therefore, it has a good safety in clinical application.