跟骨骨折之生物力學研究：有限元素分析

Abstract

跟骨骨折為最常見之跗骨骨折，手術復位的固定效果，對於跟骨骨折治療具有關鍵性的角色，開放性內復位手術(ORIF)是目前標準的治療方式，但ORIF採用骨板固定，會在跟骨外側產生L型的大範圍手術切口，可能導致軟組織的感染與潰瘍的發生，這樣的併發症促進了微創內固定手術(PSF)的發展。 ORIF使用固鎖骨釘(Locking screws, LS)與非固鎖骨釘(nonlocking screws, NS)，其生物力學效應上的差異仍相當不明確；此外，PSF的續發性關節炎發生率，明顯高於ORIF，意味著PSF的固定效果仍有待提升，以預防續發性關節炎的發生，避免再次手術治療。 跟骨骨折的骨折線與骨折形態相當複雜，其中Essex-Lopresti分類利用第二骨折線的形態，分成關節塌陷型與舌狀型兩種。 本研究之研究目的可分成三個部分：(1)分析ORIF治療關節塌陷型與舌狀型跟骨骨折，使用LS、NS於不同骨釘組合下的生物力學效應；(2)分析PSF治療關節塌陷型與舌狀型跟骨骨折，三種不同骨釘組合結構的固定效果差異；(3)利用最佳化方法，設計最佳化骨釘植入角度，以提升PSF的固定效果。 本研究首先建立三維立體的完整足部有限元素模型，此足部模型包含了骨頭、軟骨、足底筋膜與軟組織；在施加地面反作用力與阿基里斯腱拉力後，可合理模擬靜態站立狀態；此模型之驗證，乃藉由足壓量測系統的實驗值，與有限元素之足壓模擬值比對；驗證完成後，此模型被處理成骨折模型，用以分析ORIF與PSF的生物力學效應；最後本研究採用後足的局部模型，跟骨的鬆質骨分成兩種不同骨質，進行最佳化骨釘植入角度分析，目標函數為後關節面的最大垂直位移量之最小化。 經由分析，本研究發現關節塌陷型採用ORIF治療的結果，固鎖骨釘結構明顯優於非固鎖骨釘，而部分混合式骨釘結構與全固鎖骨釘的固定效果相似；對於舌狀型跟骨骨折，兩種骨釘都能取得良好的固定效果。 三種不同的PSF固定方式，在處理關節塌陷型與舌狀型跟骨骨折，本研究結果顯示其固定效果皆良好，而應力分析上，PP型的植入方式(兩根骨釘由跟骨粗隆下方平行植入至後關節面)能平均分散碎骨間的應力負載；此外針對舌狀型骨折，AS型的植入方式為兩根骨釘分別由跟骨粗隆後方上下分別植入至Sustentaculum tali與至跟骨前凸起)，其中由跟骨粗隆上方植入至至跟骨前凸起的骨釘，可降低舌狀碎骨上的應力。 本研究之後足局部模型，發現ORIF在不同骨質下，其固定效果相近，但PSF對於不同骨質，其固定效果落差很大；但本研究亦發現用最佳化方法規劃設計骨釘的植入角度，有機會降低骨質的影響力，提升骨質較差時，PSF的固定效果。 本研究之結果，可提供臨床醫師有用的資訊，以選擇最適合的植入物治療跟骨骨折。

Calcaneal fractures are the most common fractures of the tarsal bones. The stability of fixation is an important factor for the successful reconstruction of calcaneal fractures. Open reduction and internal fixation (ORIF) with a plate is the standard treatment for displaced intraarticular calcaneal fractures. However, ORIF with an extensive L-incision and fixed with a plate may be complicated by soft tissue issues and wound infection. These complications have led to the development of minimally invasive techniques using percutaneous screw fixation (PSF). For ORIF, the difference in the biomechanical effect between the locking screws and nonlocking screws used remains uncertain. Moreover, secondary arthrodesis is performed more frequently in PSF than ORIF. This means that PSF has to improve its fixation stability for the prevention of traumatic subtalar arthritis and secondary arthrodesis. Calcaneal fracture line patterns and the extent of comminutions can vary greatly. The Essex-Lopresti classification describes the location of the secondary fracture line, producing joint depression or tongue types. The purpose of this study was threefold: First, to analyze the biomechanical influence of plate fixation with different combinations of locking and nonlocking screws through ORIF for treating joint depression and tongue-type calcaneal fractures. Second, this study evaluated the stability differences offered by different PSFs in joint depression and tongue-type calcaneal fractures. Finally, an optimization tool was used to design the optimal screw orientation for improving the stability of PSF. A three-dimensional FE foot model was established, which comprised bones, cartilages, plantar fascia, and soft tissue. Static standing was simulated by applying ground reaction force and Achilles tendon force. To validate this intact foot model, the foot pressure, measured by a force-measuring system, was compared with those calculated from the intact foot FE model. After validation, this model was justified to evaluate the biomechanical influence of ORIF and PSF for calcaneal fractures. Finally, this study used a partial rear foot model for optimization analysis to determine the optimal screw orientations. This definition of the optimization objective function was employed to minimize maximal vertical posterior facet joint displacement. For treating joint depression-type calcaneal fracture with ORIF, this analysis showed that all the locking screws have greater stability than all the non-locking screws. Some hybrid screw configurations generated stability similar to that of all the locking screws. However, ORIF using each kind of screw provided good rigidity for tongue-type calcaneal fractures. This analysis also found that the three types of PSFs in this study all provided greater stability in joint depression or tongue-type calcaneal fractures. In the stress-based analysis, the PP type of PSF, the two screws of which were parallel-inserted from the posterior tuberosity inferior to beneath the posterior facet joint, might moderately alleviate stress between the fragments. In addition, one of the screws in the AS type of PSF for tongue-type calcaneal fracture, which was inserted from the posterior tuberosity superior to the anterior process, might reduce the stress in the tongue fragment. Finally, the analysis of the partial rear foot model showed that the stability of ORIF with different bone qualities was similar. However, the stability of PSF was significantly decreased with the poor bone quality. This study also revealed that the stability of PSF might be improved with the poor bone quality when using the optimization method to design the optimal screw orientation. The results of this study might provide surgeons with useful information for choosing the most appropriate implants for the fixation of calcaneal fractures.