背景：对于股骨远端扭转所致髌股关节不稳的患者，如何确定合适的股骨远端截骨平面以纠正股骨扭转畸形仍是临床一大难题。本研究拟借助简化的三角几何函数方法于股骨标本上构建模型，以明确不同截骨平面的情况下冠状面力线的变化情况，从而确定出合适的截骨面并指导临床。本研究假说为无论股骨中段弯曲如何，截骨面应垂直股骨干解剖轴线，方可避免术后膝外翻畸形。
方法：本研究构建一种新的数学模型，即立柱式悬臂模型（Pillar-Crane-Model，图1），该模型可预测不同截骨平面时，冠状位下肢力线变化情况。研究选取2例人体尸体共4例股骨，并分别模拟8个不同的股骨扭转角度。股骨远端截骨面垂直解剖轴（图2），经0度、10度、20度以及30度4个不同的去旋转角度后，分别计算冠状位力线情况，并与去旋转前相比较。力线情况的评估包括：解剖-机械轴夹角（anatomical mechanical axis，AMA），股骨远端机械外侧角（mechanical lateral distalfemur angle，mLDFA）以及股骨远端解剖外侧角（anatomicallateral distal femur angle，aLDFA）。
图1 Pillar-Crane-Model：立柱式悬臂模型示意图。Bowing of midshaft：股骨干弓度；virtual anatomical axis：股骨解剖轴线；Cutting plane：截骨面。截骨面垂直解剖轴线，股骨去旋转后，AMA增加、mLDFA增加。
图2 股骨远端截骨面垂直解剖轴。Cutting plane：截骨面；anatomical axis：解剖轴；middle of the greater trochanter：大转子中点；middleof proximal femur: 股骨近端中点。A为侧面观，B为冠状面观。
结果：该模型显示，当截骨线垂直解剖轴时，可观察到 AMA（图3）以及mLDFA增加，即出现轻度的力线内翻。经去旋转30度后，可见AMA由4.8度增加至6.3度、mLDFA由85.2度增加至86.7度（图4），而aLDFA基本保持在80.4±0.4度。
图3 截骨面处AMA角度计算方法。
图4 冠状位X线，同一标本不同扭转角度：A 扭转=37°, AMA = 4.9° , mLDFA = 84.6°; B扭转 = 27°, AMA =5.4°,mLDFA = 85.7°;C扭转= 17°, AMA = 5.8°,mLDFA = 86.1°; D扭转= 7°, AMA = 6.4, mLDFA = 86.4。
结论：经该模型进行股骨远端去旋转截骨术前设计，可有效避免术后下肢力线外翻。当股骨截骨线垂直股骨解剖轴时，可增加mLDFA（膝内翻），避免膝外翻畸形。(译者概括，垂直股骨轴线的股骨去旋转截骨会导致轻度的膝内翻，可以改善下肢力线；一般情况下避免过度内翻。)
How to avoid unintended valgus alignment in distal femoral derotational osteotomy for treatment of femoral torsional malalignment - a concept study
BACKGROUND: Defining the optimal cutting plane for derotational osteotomy at the distal femur for correction of torsion in cases of patellofemoral instability is still challenging. This preliminary study investigates changes of frontal alignment by a simplified trigonometrical model and demonstrates a surgical guidance technique with the use of femur cadavers. The hypothesis was that regardless of midshaft bowing, a cutting plane perpendicular to the virtual anatomic shaft axis avoids unintended valgus malalignment due to derotation.
METHODS: A novel mathematical model, called the Pillar-Crane-Model, was developed to forecast changes on frontal alignment of the femur when a perpendicular cutting plane to the virtual anatomical shaft was chosen. As proof of concept, eight different torsion angles were assessed on two human cadaver femora (left and right). A single cut distal femoral osteotomy perpendicular to the virtual anatomical shaft was performed. Frontal plane alignment (mLDFA, aLDFA, AMA) was radiographically analyzed before and after rotation by 0°, 10°, 20°, and 30°. Measurements were compared to the model.
RESULTS: The trigonometrical equation from the Pillar-Crane-Model provides mathematical proof that slight changes into varus occur, seen by an increase in AMA and mLDFA, when the cutting plane is perpendicular to the virtual anatomical shaft axis. A table with standardized values is provided. Exemplarily, the specimens showed a mean increase of AMA from 4.8° to 6.3° and mLDFA from 85.2° to 86.7 after derotation by 30°. Throughout the derotation procedure, aLDFA remained at 80.4° ± 0.4°SD.
CONCLUSIONS: With the use of this model for surgical guidance and anatomic reference, unintended valgus changes on frontal malalignment can be avoided. When the cutting plane is considered to be perpendicular to the virtual anatomical shaft from a frontal and lateral view, a slight increase of mLDFA results when a derotational osteotomy of the distal femur is performed.
文献出处：Imhoff FB, Scheiderer B, Zakko P, Obopilwe E, Liska F, Imhoff AB, Mazzocca AD, Arciero RA, Beitzel K. How to avoid unintended valgus alignment in distal femoral derotational osteotomy for treatment of femoral torsional malalignment - a concept study. BMC Musculoskelet Disord. 2017 Dec 29;18(1):553. doi: 10.1186/s12891-017-1904-7.
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