The Establishment Of A Non-linear Thoracic Spine Modeland Its Application In The Research Of Proximal Junctional Kyphosis

【Backgrounds】The surgery to cure spinal disease has been widely performed among the world along with the progress of the surgical technique and the internal fusion instrument. Although it cured many patients, the rising complications of the surgery are getting more and more attention. Proximal junctional kyphosis(PJK) is one of the complications of spinal fusion surgery. It can lead to upback pain, low self-evaluation and a low score of life quality. PJK is getting more and more attentions from clinical doctors and has became one of the hotspot in the research field of spinal disease. Some studies show that clinical characteristics may predict the risk of PJK. But most of the studies were not validated by the basic experiments. It is still unclear of the cause of PJK and prevent methods.【Objectives】The study intend to collect the thoracic thin-section coronal CT images from a volunteer to built a three dimensional finite element model containing emulational ligaments by the reverse engineering software. Then we use engineering analysis software to analysis the model. We tested the spinal motion unit after destructed the proportions of the posterior ligament complex to study the change of the spinal unit. We studied the affect of interspinous ligament, superspinous liagment, and Smith-Peterson Osteotomy on the biomechanical situation of the proximal area after all pedicle screw system fixation. We also compared the biomechanical differences of all pedicle screw system, pedicle screw+proximal hooks, pedicle screw + proximal none fusion fixation and prolonged process hooks/none fusion fixation methods to test the biomechanical changes among the proximal junctional area. By these study, we hope to find out the best surgical plan to prevent PJK.【Methods】1. Establishment and validation of the three dimensional finite element model of the thoracic spine with emulational ligamentsCT supine thin slice scanning was given to a healthy female volunteer after bone, muscle, neural or any disease that might affect the result were excluded. The range was from T1-T8 and the interval was 0.5mm. The images were imported into mimics 15.0 and after we definited the bone signal range, fulfilled the signal defect and erased the abnormal signal, a finite element model comprise T1-T8 vertebral bodies was built. The intervertebral disc, end plate,ligaments were built according to anatomy and material knowledge. After assemble all of the structure and define polyline, mesh, quality control, a female three dimensional finite element thoracic model was built. We further validated the model with radiological images from the volunteer and other studies that with thoracic spine model2. The finite element study of the biomechanical effect of component from posterior ligament complex in the spinal unit and the proximal junctional area after fusion.The model was further divided into 7 spinal units. Stimulations were given to the unit after supraspinous ligaments, interspinous ligaments, ligamenta flava; posterior longitudinal ligament was destructed independently. The model was fixed by all pedicle screw system from T3-T8 and Smith-Peterson osteotomy was performed in different levels: A T6-T8, B T5-T8, C T4-T8 and D T3-T8. A forward(lateral) bending moment of 6Nm was given to the model. Stimulation of cutting T4-T5 supraspinous ligaments(E), T4-T5 intraspinous ligaments(F), T3-T4 supraspinous ligaments(G), T3-T4 intraspinous ligaments(H) were also given after T3-T8 fusion and was tested after forward(lateral) bending moment of 6Nm was given to the model. We compared the biomechanical change around the proximal area.3. The finite element study of the affect of different fusion instruments on the biomechanical change among the proximal junctional area.Pedicle hook model was built according to the instruction of pedicle hook and was optimized according to the finite element model. A simplified non-fusion system was also built by the instruction. Smith-Peterson osteotomy was performed from T6-T8 level on the model. Stimulations were performed on 5 different situation: 1: all pedicle screw fixation from T3-T8; 2:T4-T8 pedicle screw fixation, T3 process hook fixation(I); 3:T4-T8 pedicle screw fixation, T3 none fusion fixation(J); 4. T5-T8 all pedicle screw fixation, T3-T4 process hook fixation(K); 5. T5-T8 all pedicle screw fixation, T3-T4 none fusion method fixation(L). The biomechanical change among the proximal junctional area was collected after a forward(lateral) bending moment of 6Nm was given to the model.【Results】1. The female thoracic spine three dimensional finite element model with emulational ligaments was built successfully. The model contain 8 vetebral, 7 intervetebral nucleus pulpous, 7 annulus fibrosus,zygapophysial joints,anterior longitudinal ligament, posterior longitudinal ligament, ligamenta flava, intertransverse ligaments, interspinous ligament and superspinous ligament. The model contain 346197 node and 1661988 element. The finite elemnt mode can stimulate the normal female thoracic spine accurately after give the model different biomechanics characteristics according to the material feature of the tissue.2. The mean T2 shift under a forwad bending moment of 2Nm, in the intact spinal unit, after cutting superspinous ligaments,cutting interspinous ligaments, ligamenta flava,posterior longitudinal ligament was 0.349±0.013mm(0.33-0.37mm), 0.443±0.020mm(0.42-0.47 P=0.001); 0.429±0.020mm(0.39-0.45 mm,P<0.001); 0.363±0.018mm(0.34-0.39, P=0.025); 0.351±0.017mm(0.34-0.38, P=0.457) respectively The mean intervertebral pressure was 0.393±0.022Mpa(0.36-0.42Mpa); 0.431±0.029Mpa(0.38-0.47 Mpa,P=0.001);(0.434±0.016Mpa(0.41-0.46 MPa,P<0.001); 0.397±0.015Mpa(0.38-0.42 MPa, P=0.407); 0.383±0.019Mpa(0.36-0.41 MPa,P=0.062) respectively. Under the situation of A, B, C,D, after forward moments were given,the T2 shift was 1.04 mm, 1.02 mm, 1.33 mm, 1.42 mm respectively;The T2 pressure was 2.61 Mpa,2.65 Mpa, 2.95 Mpa, 3.18 Mpa respectively; The T2-3 intervetebral pressure was 1.95 Mpa, 1.97 Mpa, 2.08 Mpa, 2.27Mpa; Under lateral moment, the T2 shift was 2.11 mm, 2.10 mm, 2.33 mm,2.97 mm respectively. Under the situation of E, F, G,H, under the forward moment, the T2 shift was 1.04 mm,1.06 mm,1.20 mm,1.26mm; The T2 pressure was 2.63 Mpa, 2.68 Mpa, 2.81 Mpa, 2.86 Mpa respectively; the T2-3 intervetebral pressure was 1.96 MPa, 1.98 Mpa, 2.05 Mpa, 2.11 Mpa respectively; under the lateral moment, the T2 shift was 2.10 mm, 2.14 mm, 2.23 mm, 2.26 mm respectively.3. We tested the model of I and J under a forward moment of 6Nm, The T2 shif, T2 pressure, T-3 intervetebral pressure was 1.52 mm, 2.13mm; 2.23 MPa,1.92MPa; 1.78 MPa, 1.93 MPa respectively. Under a lateral moment, the T2 shift was 2.73 mm and 3.15 mm respectively. We also tested the model of K and Lunder a forward moment of 6Nm, The T2 shift, T2 pressure, T-3 intervetebral pressure was 1.63 mm,2.37mm; 2.18 MPa, 1.88MPa; 1.69 MPa, 1.90MPa; Under a lateral moment, the T2 shift was 2.80 mm and 3.54 mm respectively.【Conclusions】1. The study built a none- liner three dimensional finite element model with emulational ligaments of the thoracic spine successfully. The bone strength was built by the signal in the CT image and the intervetebral disc and ligament were built with super elastic model and can stimulate the tissue during the physiological situation successfully. The biomechanical characteristics of the model were close to the cadevur test, it can be used to the study the biomechanical problems of the thoracic spine under different situations.2. The posterior ligaments complex has a significant function in maintaining the stability of the spinal unit. During the posterior fixation and dusion surgery of the spine,surgeons should preserve the superspiners and interspiners ligament under the proximal fusion vertebral. It has little meaning to preserve the ligament below the level according to our study.3. Smith-Peterson Osteotomy should not be performed at the proximal 2 fusional vertebrals in the proximal area during posterior fixation and fusion surgery of the spine.4. The pressure on the first un-fused vertebral in the proximal area can be relived after the use of process hook or none fusion method on the up proximal vertebral. It can be used to prevent PJK.5. The pressure on the first un-fused vetebral can be futher relived after prolong the use of process hook/none fusion method in the proximal area. But it can also reduce the stability on the proximal area of the spine after surgery.