||Location effect of posterior tension band plate fixation for pelvic fracture
||Department of BioMedical Engineering
Pelvic ring fracture
Posterior tension band plate
骨盆環由兩塊髖骨及一塊薦骨所組成，其中薦骨作為脊椎的基石及骨盆環核心所在，為人體生物力學穩定的重要關鍵。在臨床上骨盆環骨折分為三種類型(Type A、B、C)，其中以第三類型骨折Type C最為嚴重，骨盆環前環及後環皆為不穩定骨折狀態，而本研究所探討的薦骨垂直性骨折(Denis fracture type)則會發生於Type C的狀況，此種狀況通常會發生於骨盆環受到劇烈撞擊的事故，如車禍、跌倒等…。
The aim of this study is to investigate the biomechanical influence of fixation location, S1 and S2 levels, in posterior tension band plate fixation for pelvic ring fracture. The general location for the plate to fixed is at S1 level which would irritation and make the patient uncomfortable. This study investigates the biomechanical effect of the plate fixed at S2 level in order to reduce the irritation feeling. The finite element analysis is employed to simulate the pelvic ring with intact model and three types of fracture pelvic models. The results showed that, S2 level plate fixation offers a similar biomechanical response, in terms of displacement and stress distributions, comparing with S1 level fixation, but the structure of S2 fixation is more stable than S1 fixation. Both S1 and S2 level fixations possess acceptable healing indices, fracture surface sliding and gap, while S2 fixation is better than S1 fixation. Both S1 and S2 level fixation possess high safety factor in terms of plate and screw stress while S2 fixation is again better than S1 fixation. Friction force assessments on the interface of bone screw indicated that S2 level fixation has less risk in screw loosing. The biomechanical results in osteoporosis models displayed similar outcomes in all the above indices, i.e., S2 level fixation is better than S1. To conclude: Although S1 level fixation is adopted clinically in most pelvic ring fracture treatment, S2 level fixation might provide better mechanical advantages in addition to its superior patient subjective feeling.
Key words: Pelvic ring fracture, Posterior tension band plate, Biomechanical evaluation
Clinically, using compression plate to fix posterior pelvic ring possesses a high success rate, except for osteoporosis patients. In general, the plate will be secured at the S1 level. This level however would induce significant uncomfortable for patient due to plate protrusion. Studies have been proposed to solve these problems. For example, grooves on the bilateral ilium and sacrum spinal are created with a chisel to decrease the plate protruding. However the induce of grooves would increase the unnecessary damage to the ilium and sacrum. Using locking plate with small plate is another way to provide a solution. However, in order to precisely position the screw, excessive soft tissue dissection is required which increase the infection risk comparing to the percutaneous screwing in traditional plate. This study proposed a simple and easy improving by secure the plate at S2 level, instead of S1 level, to reduce the patient discomfort. The objective of this study is to evaluate the biomechanical influence of this S2 level approach. The specific aims includes: (1) To evaluate the difference, biomechanically, between S1 level secure and S2 level secure; (2) To confirm the S2 level secure is not inferior than S1 level secure; (3) To evaluate whether S2 level secure can works on osteoporosis patient.
MATERIALS AND METHODS
Finite Element models
The pelvic ring computed tomography images from visible human project  were imported to Mimics, segmentation and modeling software to build the pelvic ring. The STL files, outputted from mimics, were input to Geomagic, modeling software, to smooth the pelvic model as well as converting from surface model to solid model. Cortex of bone was then generated by shrinkage the original solid pelvic model 1 mm inward. Finally, Solidworks, another modeling software, was employed to create the cartilage and L5 in the pelvic model. In addition to the intact model, three fracture models, different fracture sites as described below, as well as virtually implant the bone plate and bone screws to these fracture models were also carried in Solidworks. Finite element analyses were carried within ANSYS software. Spring elements were used in ANSYS to simulate the tension resistant ligaments.
All material properties in this study are assumed to be homogenous, isotropic and linear elastic. The simulated model includes L5, sacrum, pubic symphysis cartilage, and the entire pelvis with cortical bone and cancellous bone structures. Table 2.1 listed the mechanical properties of the normal as well as the osteoporosis cortex and cancellous bones[4, 5](TABLE PAGE 21). The plate, simulated as titanium, was assigned with a Young’s modulus of 114000 Mpa and a Poisson’s ratio 0.3.
Ligaments around the pelvic ring were simulated with spring element and only the major ligaments were considered which included: ASL, PSL, ISL, ST, and SS. According to the experimental study, which obtain the ligaments stiffness by in-vitro tension test[6-9], Table 2.2 listed the spring constants of each ligament structure(TABLE PAGE 22). It is noteworthy that the spring constant is equally distributed to individual spring forming each ligament.
In order to verify with existing experiment, vertical loading of 1000N was applied on top of L5 in intact pelvic ring model to meet the loading conditions of the experiment. Model was fixed at both sides of the acetabular fossa. The interface of sacroiliac joint was set to be frictionless while the pubic symphysis was set to be bonded. For the fracture simulation, three fracture types, representing Denis I, II, and III respectively, were generated with vertical fracture at different positions of left sacrum (FIGURE PAGE 24) and a friction coefficient of 0.3 was assigned to the fracture interface. Moreover, SS and ST ligaments on the left side, fracture half, of the pelvic ring were removed. The interface between plate and screws was set to be bonded to simulate the locking plate while interface between screw and bone was set to with friction coefficient 0.4 to investigate the loosing risk. Then 600N, instead of 1000N to simulate the upper body weight, was applied as the loading on fracture models.
RESULT AND DISCUSSION
For the intact validation, the load-displacement curves of the finite element result and experimental outcome were similar both in slope and values. For both S1 and S2 level fixation, the peak stress all occurred in screw regardless of the fracture type, and the highest stress arose in Denis III with S2 is safer than S1. (The safety factors are greater than 5 in most models.) For healing evaluations, the fracture interface sliding distance and gap opening in S2 level fixation are all less than those of corresponding S1 level fixation. The maximum interface gap opening occurred in Denis III fracture. This could due to fact that Denis III fracture possesses the longest fracture length and the maximum opening always occurred at the fracture tip. Unexpectedly, the maximum sliding distance occurred in Denis I fracture which possesses the shortest fracture length. This might due to the fact that fracture surface of Denis I suffered a larger moment with its longer level arm from the L5 loading site. Due to the fact that the threads were missing in the screw model, replaced by cylinder with large friction, the frictional force between the screw and bone is employed to evaluate the screw pullout risk. The results showed that S2 level fixation, in a relative sense, has less risk in screw loosing. However, it is noted that in this study the interface between plate and screw are boned to simulate a locking plate, this screw loosing assessment might not be that significant. Finally, in osteoporosis condition, all indices displayed identical outcomes as in normal bone condition, i.e., S2 level fixation is not inferior than the fixation at S1 level.
In this study, the finite element analysis is used to analyze the biomechanical effect of S1 and S2 level plate fixation. Five conclusions are given:
(1) S2 level plate fixation offers a similar biomechanical response, in terms of displacement and stress distributions, comparing with S1 level fixation, but the structure of S2 fixation is more stable than S1 fixation.
(2) Both S1 and S2 level fixations possess acceptable healing indices, fracture surface sliding and gap, while S2 fixation is better than S1 fixation.
(3) Both S1 and S2 level fixation possess high safety factor in terms of plate and screw stress while S2 fixation is again better than S1 fixation.
(4) Friction force assessment on the interface of bone and screw indicates that S2 level fixation has less risk in screw loosing.
(5) The biomechanical results in osteoporosis condition display similar outcomes in all the valuated indices, that is S2 level fixation is better than S1.
Although S1 level fixation is adopted clinically in most pelvic ring fracture treatment, S2 level fixation may provide better mechanical advantages in addition to its superior patient subjective feeling.
Extended Abstract II
第一章 緒論 1
1.1 前言 1
1.2 骨盆環解剖構造 2
1.3 典型骨盆環骨折及垂直性薦骨骨折 5
1.4 臨床治療方式 7
1.5 有限元素分析法 9
1.6 文獻回顧 11
1.6.1 骨盆環韌帶和身體重力對骨盆環整體穩定度之影響 11
1.6.2 骨盆環最大受力方式(薦骨垂直受力) 11
1.6.3 Type C不穩定骨盆環有限元素分析之相關模擬 12
1.7 研究動機與目的 13
第二章 材料與方法 15
2.1 研究流程 15
2.2 三維骨盆環有限元素模型之建立 17
2.2.1 三維骨盆環幾何模型之建立 17
2.2.2 三維骨盆環材料參數之建立 21
2.2.3 三維骨盆環網格化 23
2.2.4 骨盆環有限元素模型之負載及邊界條件 24
第三章 結果 25
3.1.1 薦骨受力後移動方式 25
3.1.2 薦骨受力後彈簧所承受之張力 27
3.1.3 薦骨受力後與兩側髖骨的作用力 29
3.2 有限元素模型與大體實驗之驗證 30
3.4 S1 level與S2 level之應力分布 32
3.5 薦骨垂直性骨折處之斷面滑移量與空隙 34
3.6 骨螺絲與骨頭間之摩擦力 36
第四章 討論 37
4.1 臨床治療方式探討 37
4.2 骨盆環骨折斷面接觸條件比較 38
4.3 骨板之等效應力分布 40
4.4 薦骨垂直性骨折癒合指標探討 41
4.4.1 薦骨垂直性骨折處之斷面滑移量探討 41
4.4.2 薦骨垂直性骨折處之斷面空隙探討 42
4.4.3 骨板固定位置之影響 43
4.5 骨板骨螺絲鬆脫探討 44
4.6 本研究之限制 45
第五章 結論 46
1. Krappinger, D., et al., Minimally invasive transiliac plate osteosynthesis for type C injuries of the pelvic ring: A clinical and radiological follow-up. Journal of Orthopaedic Trauma, 2007. 21(9): p. 595-602.
2. Acklin, Y., G. Marco, and C. Sommer, Double locking plate fixation of sacral fractures in unstable pelvic ring C-type injuries. Operative Orthopädie und Traumatologie, 2015. 27(1): p. 74-79.
3. NLM, U.S. The Visible Human Project.
4. Fu, S., et al., Comparison of the risk of breakage of two kinds of sacroiliac screws in the treatment of bilateral sacral fractures. Eur Spine J, 2014. 23(7): p. 1558-67.
5. Eichenseer, P.H., D.R. Sybert, and J.R. Cotton, A finite element analysis of sacroiliac joint ligaments in response to different loading conditions. Spine (Phila Pa 1976), 2011. 36(22): p. E1446-52.
6. Hao, Z., et al., The effect of boundary condition on the biomechanics of a human pelvic joint under an axial compressive load: a three-dimensional finite element model. J Biomech Eng, 2011. 133(10): p. 101006.
7. Zhang, L., et al., Biomechanical study of four kinds of percutaneous screw fixation in two types of unilateral sacroiliac joint dislocation: a finite element analysis. Injury, 2014. 45(12): p. 2055-9.
8. Phillips, A.T., et al., Finite element modelling of the pelvis: inclusion of muscular and ligamentous boundary conditions. Med Eng Phys, 2007. 29(7): p. 739-48.
9. N.Zheng:L.G.Watson:K.Yong-Hing, . Medical and Biological Engineering and Computing, 1997.
10. Comstock, C.P., M.C. van der Meulen, and S.B. Goodman, Biomechanical comparison of posterior internal fixation techniques for unstable pelvic fractures. Journal of orthopaedic trauma, 1996. 10(8): p. 517-522.
11. Tile, M., Acute Pelvic Fractures: I. Causation and Classification. J Am Acad Orthop Surg, 1996. 4(3): p. 143-151.
12. Marieb, Wilhelm, and Mallatt, Human Anatomy. Seventh ed.
13. Moore, K.L. and A.F. Dalley, Clinically Oriented Anatomy. Fifth ed.
14. Vaccaro, A.R., et al., Diagnosis and management of sacral spine fractures. Instr Course Lect, 2004. 53: p. 375-85.
15. Taguchi, T., et al., Operative management of displaced fractures of the sacrum. J Orthop Sci, 1999. 4(5): p. 347-52.
16. Tile, M., Plevic ring fractures: Should they be fixed? J Bone Joint Surg Br, 1988. 70: p. 1-12.
17. Tile, M., Classification, in Tile M (ed): Fractures of the Pelvis and Acetabulum, 2nd ed. Baltimore: Williams & Wilkins, 1995: p. 66-101.
18. Denis, F., S. Davis, and T. Comfort, Sacral fractures: an important problem. Retrospective analysis of 236 cases. Clin Orthop Relat Res, 1988. 227: p. 67-81.
19. Routt, M.L., Jr., et al., Early results of percutaneous iliosacral screws placed with the patient in the supine position. J Orthop Trauma, 1995. 9(3): p. 207-14.
20. Griffin, D.R., et al., Vertically Unstable Pelvic Fractures Fixed with Percutaneous Iliosacral Screws: Does Posterior Injury Pattern Predict Fixation Failure? Journal of Orthopaedic Trauma, 2003. 17(6): p. 399-405.
21. Schütz, M. and N.P. Südkamp, Revolution in plate osteosynthesis: new internal fixator systems. Journal of Orthopaedic Science, 2003. 8(2): p. 252-258.
22. Chen, H., et al., Parallel analysis of finite element model controlled trial and retrospective case control study on percutaneous internal fixation for vertical sacral fractures. BMC Musculoskelet Disord, 2013. 14: p. 217.
23. Conza, N.E., D.J. Rixen, and S. Plomp, Vibration testing of a fresh-frozen human pelvis: The role of the pelvic ligaments. Journal of Biomechanics, 2007. 40(7): p. 1599-1605.
24. Fessler, J., Festigkeit der menschlichen Gelenke mit besonderer Berücksichtigung des Bandapparates...: als Habilitationsschrift zur Erlangung der venia legendi an der... Ludwig-Maximilians Universität zu München. 1894: M. Rieger'sche Universitäts-Buchhandlung.
25. Rothkotter, H.J. and W. Berner, Failure load and displacement of the human sacroiliac joint under in vitro loading. Arch Orthop Trauma Surg, 1988. 107(5): p. 283-7.
26. Varga, E., B. Dudas, and M. Tile, Putative proprioceptive function of the pelvic ligaments: biomechanical and histological studies. Injury, 2008. 39(8): p. 858-64.
27. Vrahas, M., et al., Ligamentous contributions to pelvic stability. Orthopedics, 1995. 18(3): p. 271-4.
28. Vukicevic, S., et al., Holographic analysis of the human pelvis. Spine (Phila Pa 1976), 1991. 16(2): p. 209-14.
29. Hammer, N., et al., Ligamentous influence in pelvic load distribution. Spine J, 2013. 13(10): p. 1321-30.
30. Vigdorchik, J.M., et al., Biomechanical stability of a supra-acetabular pedicle screw internal fixation device (INFIX) vs external fixation and plates for vertically unstable pelvic fractures. J Orthop Surg Res, 2012. 7: p. 31.
31. Simonian, P.T., et al., Biomechanical Simulation of the Anteroposterior Compression Injury of the Pelvis: An Understanding of Instability and Fixation. Clinical orthopaedics and related research, 1994. 309: p. 245-256.
32. Whyne, C.M., et al., Effects of bone density alterations on strain patterns in the pelvis: application of a finite element model. Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, 2009. 223(8): p. 965-979.
33. Yinger, K., et al., Biomechanical comparison of posterior pelvic ring fixation. J Orthop Trauma, 2003. 17(7): p. 481-7.
34. Humphrey, C.A., et al., Locked plates reduce displacement of vertically unstable pelvic fractures in a Mechanical Testing Model. J Trauma, 2010. 69(5): p. 1230-4.
35. Chen, H.W., et al., Treatment of unstable posterior pelvic ring fracture with percutaneous reconstruction plate and percutaneous sacroiliac screws: a comparative study. J Orthop Sci, 2012. 17(5): p. 580-7.
36. Drake, R.L., A.W. Vogl, and A.W.M. Mitchell, Gray's Anatomy for Students. second ed.
37. Reilly, M.C., et al., The effect of sacral fracture malreduction on the safe placement of iliosacral screws. Journal of Orthopaedic Trauma, 2003. 17(2): p. 88-94.
38. Kabak, S., et al., Functional outcome of open reduction and internal fixation for completely unstable pelvic ring fractures (Type C) - A report of 40 cases. Journal of Orthopaedic Trauma, 2003. 17(8): p. 555-562.
39. Hsu, C.C., et al., Increase of pullout strength of spinal pedicle screws with conical core: biomechanical tests and finite element analyses. Journal of Orthopaedic Research, 2005. 23(4): p. 788-794.