| Background:Craniocervical junction, also known as the occipitoatlantoaxial complex, connects the occipital bone and subaxial cervical vertebra. Its anatomical structure and movement statement are special and different from the other parts of the spine. Currently, research on injury mechanism, biomechanics and treatment methods of craniocervical junction has been a crucial issue in orthopedics.Classifying diseases at the craniocervical junction is rather complicated and involves multiple factors. The most commonly used treatment methods are posterior atlantoaxial arthrodesis and occipital-cervical fusion, in which the primary purpose of these internal fixation designs is to provide greater mechanical stability. However, stiffness of metal implants is far beyond the requirements for bone fusion, and it has been reported in clinical studies that an optimum effect may not be obtained by using rigid internal fixation.Stress concentration caused by rigid internal fixation could result in fatigue failure or fracture of the fixation system and early degeneration of the adjacent segment due to increased stress. Additionally, stress shielding effects can hinder bone regeneration and induce bone-screw interface loosening as a result of bone loss at the fixation area. Furthermore, the use of a rigid internal fixation in osteoporotic spine may cause the occurrence of bone cutting.It is considered that further increasing the mechanical strength of internal fixations does not accelerate bone fusion or improve bone fusion rate in conditions where it is sufficient for the internal fixation to achieve mechanical stability of the fixed segment. However, the exact strength required for bone fusion of the craniocervical junction remains unclear.Flexible or dynamic fixation is a newly developed method in spinal fixation, in which an ideal environment is obtained for the fixed and adjacent segments by controlling the movement of the surgical segment.It has been shown from finite element analyses and biomechanical studies that flexible or dynamic fixation could reduce stiffness and increase load transfer onto the anterior column of the spine. In addition, stress on the bone-screw interface and load on the screws could also be reduced,and influence on the pressure of the adjacent segment disc and facet joint is decreased; alleviating the occurrence of adjacent segment degeneration.The application of flexible or dynamic fixation techniques in treating lower cervical and lumbar degenerative diseases has been reported in a series of studies, and its clinical effect is basically equivalent to that of rigid internal fixations in short term follow ups. Although it was indicated from mid-term or long term follow up studies that dynamic fixation techniques could result in an increased rate of screw loosening, fracture or revision; degeneration of the adjacent segment is not significantly reduced and the advantage of using flexible or dynamic fixations could still be confirmed such as reduction of loading on the internal fixation, increase of loading distribution in the anterior and middle columns of the spine, and improvement in fusion rate. Based on the above-mentioned conclusions, it is considered that flexible or dynamic fixation does not act solely as a non fusion technology; which can also achieve segment fusion even with higher quality and a faster rate.Nowadays, there are no flexible or dynamic fixation devices available with regard to specific craniocervical junctions. Additionally, to the best of our knowledge, no studies have been reported that investigated whether flexible or dynamic fixation could be applied in treating craniocervical junction diseases with the achievement of effective stability and bone fusion.In this thesis, Chapter One and Two focuses on investigating the comparison of different flexible or dynamic fixation modes used for craniocervical junction via in vitro biomechanical experiments with the aim of elucidating functions of different flexible or dynamic fixations and obtaining optimal methods for posterior fixation of the craniocervical junction.Subaxial transfacet screw fixation is a simple, safe and economical method that has been used in clinical situations as an alternative or supplement to lateral mass screw and pedicle screw fixations. However, there is lack of research in studying the biomechanical properties of subaxial transfacet screw fixations. Therefore, Chapter Three of this thesis investigates the biomechanical characteristics of transfacet screws in comparison with that of lateral mass screw-rod construct fixations used in two situations:(1) posterior ligament complex (PLC) damage, and (2) in combination with anterior titanium plate fixation after anterior cervical corpectomy.Objectives:(1) To evaluate the influence of thin rod, polyetheretherketone (PEEK) rod, rotating pedicle screw and sliding pedicle screw on the stability and loading distribution of the craniocervical junction;(2) To analyze the influence of thin rod and different numbers of rotating pedicle screws on the stability and loading distribution of the occipital cervical region;(3) To compare the biomechanical characteristics of subaxial transfacet screw and lateral mass screw-rod construct fixations.Methods:1. Selection of several potential methods for flexible or dynamic fixation in the posterior occipital cervical region.(1) Thin rod:titanium rod with a diameter of 2.0 mm;(2) PEEK rod with a diameter of 3.5 mm;(3) Rotating pedicle screw:a modified ordinary polyaxial pedicle screw, in which rotation of the screw is achieved after locking of the nut (range:up to 15°);(4) Sliding pedicle screw:an ordinary monoaxial pedicle screw with a modified distal screw thread, in which longitudinal sliding between the screw and rod is achieved after locking of the nut with no limitation of distance.Rigid fixation employing a titanium rod with a diameter of 3.5 mm was used as a reference.2. Investigation of the influence of flexible or dynamic fixation on the stability and loading distribution of the atlantoaxial joint.In this part, a series of in vitro biomechanical tests were performed using six fresh adult cervical spines (occipital bone-C4 segment) to simulate fixation conditions in surgery including the (1) intact state; (2) injury state:type II odontoid process fracture; (3) rotating pedicle screw:two rotating pedicle screws were bilaterally inserted in the Atlas pedicle and the Axis was fixed by an ordinary polyaxial pedicle screw, which are connected by a titanium rod with a diameter of 3.5 mm; (4) sliding pedicle screw:two sliding pedicle screws were bilaterally reinserted into the Atlas pedicle with other internal fixation components unchanged in comparison with (3); (5) PEEK rod:atlantoaxial pedicles were fixed using ordinary polyaxial pedicle screws and connected by a PEEK rod with a diameter of 3.5 mm. The fixed state that used 2.0 mm and 3.5 mm diameter titanium rods was simultaneously measured while testing the occipitocervical fixed segments, and the range of movement based on data of atlantoaxial fixation segments were analyzed. Additionally, six pieces of strain gauges were positioned on the C1 lateral mass and in the C2-3 vertebrae bilaterally in each specimen, and were connected to a strain indicator and recorder.Biomechanical studies of samples were performed under intact, injury and various fixation statements using a spinal testing machine, while applying a constant moment of 1.5 Nm in flexion-extension, left-right lateral bending, and left-right axial rotation directions. A repeated measurement design was employed in all tests. Evaluation of experimental results mainly focused on the following two features. (1) Stability. Four infrared marking points were placed on the occipital bone and on the Cl, C2 and C3 vertebrae, which were connected by a K-wire. Movement of the marking points were measured consecutively by an Optotrak Certus 3D measurement system in order to analyze the range of motion (ROM) and neutral zone (NZ) of atlantoaxial segments. In addition, sliding distance of the sliding pedicle screw was measured when sliding screw fixation was used. (2) Loading distribution. Loading transfer in C1, C2 and C3 vertebrae were recorded by six pieces of strain gauges, which were used to reflect the change of loading in posterior internal fixation.3. Study on the influence of flexible or dynamic fixation on stability and loading distribution of the occipital cervical region.Biomechanical tests of posterior flexible or dynamic fixations in the occipital cervical region were performed using the same sample, with the fixation site extended from the occipital bone to the C3 vertebra. Testing conditions included the following: (1) injury state:a combination of type Ⅱ odontoid process fractures and transactions of the anterior atlanto-occipital membrane, apical odontoid ligament and alar ligament; (2) thin rod fixation:a 2.0 mm diameter titanium rod was used to fix occipital bone-C3 with locking connections between the screws and rods; (3) rigid fixation:a 3.5 mm diameter titanium rod was used to fix occipital bone-C3 with locking connections between the screws and rods; (4) fixation using different numbers of rotating pedicle screws:two, four and six rotating pedicle screws were applied to fix the C1, C2 and C3 vertebrae, respectively. Stability and loading transfer were evaluated under various flexible or dynamic fixations and rigid fixation.4. Biomechanical characteristics of subaxial transfacet screw fixation.Movement of fixation segments C5-C7 were measured using eight fresh cadaveric cervical specimens (C5-T1) with the application of a constant loading of 2 Nm in flexion-extension, lateral bending, and axial rotation directions under various fixation conditions; including intact, PLC damage in C5/6, transfacet screw fixation in C5-C7, lateral mass screw fixation in C5-C7, anterior titanium plate fixation after C6 cervical corpectomy, a combination of anterior fixation and transfacet screw fixation, and a combination of anterior fixation and lateral mass screw fixation. Additionally, the change of loading in the anterior column of the spine was compared under various fixation conditions by attaching strain gauges in C6 and C7.5. Statistical analysis.Experimental data were analyzed using the Statistica 7.1 software. Variance analyses based on repeated measures were performed to determine whether statistically significant differences existed, and SNK (Student-Newman-Keuls) test was used to compare the difference between groups.Results:1. Influence of flexible or dynamic fixation on the stability and loading distribution of the atlantoaxial joint.(1) In the atlantoaxial joint, ROM in Cl and C2 of the injury state caused by odontoid process fracture was significantly larger than the intact state in flexion, extension, lateral bending and rotation (P<0.05). ROM of fixation segments were significantly reduced in all directions when a 3.5 mm diameter titanium rod, a 2.0 mm diameter titanium rod, and a 3.5 mm diameter PEEK rod was used (P<0.05). In lateral bending, ROM of the PEEK rod was significantly larger compared with rigid fixation (P=0.005).(2) ROM of fixation segments was significantly reduced in flexion, extension, lateral bending and rotation directions when rotating and sliding pedicle screw fixations were used (P<0.05), compared with the intact state. A significant increase in ROM for rotating and sliding pedicle screw fixations in lateral bending was obtained, compared with rigid fixation (Protating screws=0.024, Psliding screws=0.001). Sliding distance of the sliding pedicle screw fixation had a range of 0.20-1.25 mm in each direction.(3) NZ in C1 and C2 of the injury state caused by odontoid process fracture was significantly larger compared with the intact state in flexion and extension (P=0.043); whereas in lateral bending and rotation, the difference was not significant, although an increase was obtained for the injury state (P>0.05). Compared with the intact state, NZs of fixation segments for rigid fixation, thin rod fixation, PEEK rod fixation, rotating pedicle screw fixation, and sliding pedicle screw fixation were significantly reduced (P<0.05). There were no significant differences among these fixations (P>0.05).(4) Strain test results revealed that standard deviation of data was relatively larger; and in most cases, there were no significant differences among the different groups.2. Influence of flexible or dynamic fixation on the stability and loading distribution of the occipital cervical region.(1) ROM in occipital bone-C3 of the injury state caused by the injury state (a combination of type II odontoid process fracture and the transaction of the anterior atlanto-occipital membrane, apical odontoid ligament and alar ligament) was significantly larger than that of the intact state in flexion, extension, lateral bending and rotation directions (P<0.05). ROM of fixation segments was significantly reduced in all directions when a 3.5 mm and 2.0 mm diameter titanium rod was used (P<0.05). ROM for thin rod fixation was significantly larger in all directions compared with rigid fixation (P<0.05).(2) When compared to the intact state, ROM in occipital bone-C3 with a fixation that used different numbers of rotating pedicle screws was significantly reduced in flexion, extension, lateral bending and rotation directions (P<0.05). The application of six rotating pedicle screws in C1-C3 significantly increased ROM for rotation in occipital bone-C3 than in rigid fixation (P=0.031).(3) Compared with the intact state, NZ in occipital bone-C3 of the injury state was significantly larger in flexion and extension (P=0.014); whereas in lateral bending and rotation, the difference was not significant, although an increase was obtained for the injury state (P>0.05). NZs of fixation segments for rigid fixation, thin rod fixation, and rotating pedicle fixation (different numbers) was significantly reduced in flexion, extension, lateral bending and rotation directions (P<0.05).(4) Following fixation of occipital bone-C3 using thin rod and rotating pedicle screw, strain in the proximal vertebral body was significantly reduced in the Atlas than in the intact state in flexion, extension, and rotation directions (P<0.05); whereas in lateral bending, a decreasing trend was observed with no significant difference (P>0.05). Additionally, strain increased in different degrees in all directions for thin rod fixation and rotating pedicle screw fixation, compared with rigid fixation.(5) In the Axis, strain in the proximal vertebral body was significantly reduced for thin rod fixation and rotating pedicle screw fixation in flexion, extension, lateral bending and rotation directions, compared with the intact state (P<0.05). Strain was increased with different degrees for thin rod and rotating pedicle screw fixations in all directions, compared with rigid fixation; and a significant difference was obtained by both fixations in extension and thin rod fixation in rotation (P<0.05).(6) Following fixation of C3 using thin rod and rotating pedicle screws with different numbers, strain in the proximal C3 vertebral body was increased in flexion and extension than in the intact state, whereas strain was significantly reduced in lateral bending and rotation (P<0.05). The difference between various fixations was not significant (P>0.05).(7) ROM of each segment was comparable when fixed by different numbers of rotating pedicle screws in flexion, extension and lateral bending directions. In addition, ROM of each segment was comparable in rotation when thin rod fixation was used.3. Biomechanical characteristics of subaxial transfacet screw fixation.(1) After transfacet screw and lateral mass screw-rod fixations, ROM of fixation segments was significantly reduced, compared with the intact and injury states (P<0.05).(2) Transfacet screw fixation stability was significantly weaker compared with lateral mass screw-rod fixation in the flexion-extension direction (P=0.002), whereas the difference was not significantly different for these two fixations in lateral bending and rotation (P>0.05).(3) ROM following a combination of transfacet screw fixation with anterior titanium plate fixation was significantly reduced, compared with transfacet screw fixation in the flexion-extension direction (P=0.004); and the difference was not significant, compared with a combination of lateral mass screw-rod fixation with anterior titanium plate fixation (P=0.406).(4) The use of posterior instrument fixation reduced strains on the anterior vertebral column in the C6 level, compared with the intact state. Strain of the anterior vertebral column for lateral mass screw-rod fixation was smaller compared with transfacet screw fixation in the flexion directionConclusions:1. In the atlantoaxial joint, stability of using a 2.0 mm diameter thin rod fixation is similar to the stability of using a 3.5 mm diameter rigid fixation.2. In the atlantoaxial joint, stability of using a 3.5 mm diameter PEEK rod was comparable to rigid fixation in flexion, extension and rotation directions, but was weaker in the lateral bending direction.3. In the atlas, stability of using two rotating screws or sliding screws for fixation was weaker in the lateral bending direction, compared with rigid fixation.4. In the occipital cervical region, stability of using thin rod fixation was weaker than rigid fixation in flexion, extension, lateral bending and rotation directions.5. In the occipital cervical region, stability of using two or four rotating screw fixations in C1 and C2 is similar to rigid fixation in flexion, extension, lateral bending and rotation directions.6. Using six rotating screw fixations is as stable as a rigid fixation in flexion, extension and lateral bending, but weaker in rotation.7. In comparison with rigid fixation, flexible or dynamic fixation can increase ROM of fixation segments in different degrees.8. In comparison with rigid fixation, strain of the proximal C1 and C2 vertebral body increases in different degrees when thin rod fixation and different numbers of rotating pedicle screw fixations are used.9. Transfacet screw fixation is equivalent to lateral mass screw-rod fixation in axial rotation and lateral bending; however, it is weaker in terms of fixation in the flexion-extension direction. |
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