統合CAD及有限元素方法模擬分析椎弓骨釘植入椎體

Integration of CAD and FE Method to Simulate the Pedicle Screw within the Vertebra

陳守義(Sou-I Chen) ； 張志涵(Chih-Han Chang) ； 林瑞模(Ruey-Mo Lin)

《Journal of Medical and Biological Engineering》 21卷3期 (2001/09) Pp. 175-182

摘要

對於胸腰椎不穩定的臨床病患，藉由手術置入椎弓骨釘內固定系統並輔以骨融合，在臨床上已是普遍採行的治療方式，固定系統除了能提供不穩定椎節術後立即的穩定外，同時也能協助骨骼的癒合達到長期固定的效果。然而目前在骨釘固定系統的使用上仍有失敗的案例發生，其主要失敗的情形為骨釘彎曲或斷裂、椎體骨骼的破壞，造成上述失敗的原因和機轉目前不是十分清楚，因此繼續椎弓骨釘的生物力學探討，將有助於往後固定系統的設計與分析，並減低手術後失敗的情形。本研究結合CAD系統及有限元素分析方法，模擬實際手術中椎弓骨釘植入腰椎的情形，藉由所建立之三維有限元素模型探討骨釘植入腰椎的內部力學機轉。研究結果顯示，骨釘主要透過四個區域將負荷傳遞至椎體骨骼，椎體最大壓應力值發生於骨釘尾端與脊椎交界的椎體部分（即第一支點發生位置）。骨釘主要承受的負荷形式為彎曲負荷，故於骨釘上緣部分主要承受張應力，最大張應力值發生於骨釘與螺紋交界處，此與臨床上觀察相合。另外，當骨釘與椎體之界面採用結合(bonded)以及接觸(contact)模擬時，兩者的應力的分佈結果有很大的差異，這可能是因為“結合”具有能夠傳遞壓力與張力之特性。由於本研究模擬手術植入腰椎之模式，需考慮骨釘螺紋之幾何外形，並利用接觸元素來表現骨釘鎖入椎體之情形，此有異於過去分析模型，將可較真實模擬骨釘植入椎體的力學行為，未來更可配合研究需要，分別以不同之植入位置或植入不同形式的骨釘，來探討各種臨床參數或手術方式，對整體固定器系統與脊椎之影響。

關鍵字

椎弓骨釘 ； 應力分佈 ； 有限元素模擬

並列摘要

The use of pedicle screw fixation systems for unstable spine has become common surgical procedure in orthopedics. These fixation systems could align the spine, maintain stability and speed up the fusion of bone graft. Recent clinical surveys have reported that the major failure modes of the pedicle screw fixation system were screw failure and vertebral body breakage. To avoid these failures, better understanding of the mechanical environment within the body and the implanted screws is essential. This study employed three dimensional finite element spine model to investigate the load transfer between the pedicle screw and the vertebrae body. CAD system was integrate into the meshing procedure to simulate the screw implantation procedure. Both contact and bonded interface condition were used to study the biomechanical behavior between the screw and body. The results shown that the major loading mode of the screw was bending and the peak stress region was occurred right before the first thread which was consistent with most clinical observations where the screw was broken. Bonded and contact interface conditions did play an important role in load transfer between the screw and body. For bonded interface condition, the screw stress distribution was more uniform and had lower peak stress value which was different from contact interface condition. In the simulation, the screw within the vertebra model included the geometry of the screw thread and employed the contact element to study the interface characteristic which had neglected by other FE simulation.

並列關鍵字

Lumbar spine ； Pedicle screw ； Finite element analysis