Mao Yujiang, Song Jie, Wei Jie, Wang Manyi
Department of Orthopaedic trauma, Beijing Jishuitan Hospital, 4th Hospital of Beijing University, Beijing, China.
Hip Int. 2010 Oct-Dec;20(4):547-50. doi: 10.1177/112070001002000421.
Unrecognized joint penetration (UJP) by screw penetration through the articular surface undetectable on routine anteroposterior (AP) and lateral radiographs can cause serious complications. We have developed a geometric model to analyze UJP, and methods for the prevention of the problem. A Steinmetz Solid (SS) is the overlapping portion between two identical, vertically intersecting cylinders. The AP and lateral radiographs of a femoral head (simplified as a sphere) are projections of two cylinder-shaped images. A screw that appears to be within the femoral head in fact only lies within the cylinder. A screw apparently within the femoral head on both AP and lateral images is only confined to the SS generated by two cylinders, but not necessarily confined to the femoral head itself. We have therefore analyzed UJP using a geometric model based on SS. The geometric basis of UJP lies in the fact that the SS is larger than the sphere (femoral head) with a volume ratio of 4: π. The theoretical risk of UJP for any screw therefore can be as high as 21.5% ((4-π)/4). In reality, screws are always carefully placed to ensure a distance between the screw's tip and the edge of femoral head (tip-to-edge distance, or TED). This TED effectively lowers the risk of UJP by reducing the size of the screw-confining SS. When the SS entirely fits into (internally tangential to) the femoral head, the risk of UJP approaches zero. A TED fulfilling this requirement can be regarded as safe (approximately 0.29 x femoral head radius). With a femoral head diameter of 5 cm, the safe TED is approximately 7 mm.
螺钉穿透关节面但在常规前后位（AP）和侧位X线片上无法检测到的未识别关节穿透（UJP）可导致严重并发症。我们开发了一种几何模型来分析UJP以及预防该问题的方法。施泰因梅茨立体（SS）是两个相同的、垂直相交的圆柱体的重叠部分。股骨头（简化为球体）的AP和侧位X线片是两个圆柱形状图像的投影。一颗看似在股骨头内的螺钉实际上仅位于圆柱体内。在AP和侧位图像上看似都在股骨头内的螺钉仅局限于由两个圆柱体生成的SS内，但不一定局限于股骨头本身。因此，我们使用基于SS的几何模型分析了UJP。UJP的几何基础在于SS大于球体（股骨头），体积比为4:π。因此，任何螺钉发生UJP的理论风险可高达21.5%（(4 - π)/4）。实际上，螺钉总是被小心放置以确保螺钉尖端与股骨头边缘之间有一定距离（尖端到边缘距离，或TED）。这个TED通过减小限制螺钉的SS的尺寸有效地降低了UJP的风险。当SS完全贴合（内切于）股骨头时，UJP的风险接近零。满足此要求的TED可被视为安全的（约为0.29×股骨头半径）。对于直径为5 cm的股骨头，安全的TED约为7 mm。
