Bishop N E, Ferguson S, Tepic S
AO/ASIF Research Institute, Davos, Switzerland.
J Bone Joint Surg Br. 1996 May;78(3):349-56.
The fatigue failure of bone cement, leading to loosening of the stem, is likely to be one mode of failure of cemented total hip replacements. There is strong evidence that cracks in the cement are initiated at voids which act as stress risers, particularly at the cement-stem interface. The preferential formation of voids at this site results from shrinkage during polymerisation and the initiation of this process at the warmer cement-bone interface, which causes bone cement to shrink away from the stem. A reversal of the direction of polymerisation would shrink the cement on to the stem and reduce or eliminate the formation of voids at this interface. We have investigated this by implanting hip prostheses, at room temperature or preheated to 44 degrees C, into human cadaver femora kept at 37 degrees C. Two types of bone cement were either hand-mixed or vacuum-mixed before implantation. We found that the area of porosity at the cement-stem interface was dramatically reduced by preheating the stem and that the preheating temperature of 44 degrees C determined by computer analysis of transient heat transfer was the minimum required to induce initial polymerisation at the cement-stem interface. Temperature measurements taken during these experiments in vitro showed that preheating of the stem caused a negligible increase in the temperature of the bone. Reduction of porosity at the cement-stem interface could significantly increase the life of hip arthroplasties.
骨水泥的疲劳失效会导致柄部松动，这可能是骨水泥型全髋关节置换术的一种失效模式。有充分证据表明，骨水泥中的裂纹始于作为应力集中源的孔隙，尤其是在骨水泥-柄部界面处。该部位孔隙的优先形成是由聚合过程中的收缩以及该过程在较热的骨水泥-骨界面处引发所致，这会使骨水泥从柄部收缩离开。聚合方向的反转会使骨水泥收缩至柄部，并减少或消除该界面处孔隙的形成。我们通过将室温或预热至44摄氏度的髋关节假体植入保持在37摄氏度的人体尸体股骨中来对此进行了研究。两种骨水泥在植入前分别进行了手工混合或真空混合。我们发现，预热柄部可显著减少骨水泥-柄部界面处的孔隙面积，并且通过对瞬态热传递进行计算机分析确定的44摄氏度预热温度是在骨水泥-柄部界面引发初始聚合所需的最低温度。这些体外实验期间进行的温度测量表明，柄部预热导致骨的温度升高可忽略不计。减少骨水泥-柄部界面处的孔隙率可显著延长髋关节置换术的使用寿命。
