Objective: 

To investigate the degree of error due to parallax during intraoperative rotational imaging involving the distal femur.

Methods: 

Twelve, fresh-frozen, lower-extremity cadaveric specimens were studied. The limbs were positioned supine and rotated until the posterior femoral condyles were superimposed using a C-arm. The C-arm was then repositioned to place the femoral condyles at the anterior and posterior margins of the image intensifier. The rotation necessary to resuperimpose the femoral condyles due to parallax was recorded. A second C-arm was then used from the contralateral side to simulate the rotational imaging technique to determine the magnitude and directionality of combined parallax.

Results: 

All 12 specimens demonstrated parallax, resulting in rotational discrepancies. Compared with central field of view alignment, the mean rotational discrepancy was 5.6 and 5.5 degrees for anterior and posterior positions, respectively. Contralateral imaging resulted in an additive effect with rotational differences of 9.0 and 12.6 degrees for bilateral anterior and posterior positions, respectively.

Conclusions: 

Parallax causes rotational discrepancies with fluoroscopic imaging of the distal femur when the posterior femoral condyles are off-center in the imaging field of view. If femoral condyles are not centered when obtaining intraoperative rotational imaging of the uninjured and injured femurs, parallax can result in potentially clinically significant rotational deformity even if the femurs seem perfectly matched at the conclusion of surgery. We recommend perfectly centering of the posterior femoral condyles in the imaging field of view when obtaining lateral images of the distal femur to prevent this potentially overlooked contribution to malrotation.

中文翻译：

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术中比较股骨旋转成像：不要忽视视差

客观的： 

探讨股骨远端术中旋转成像过程中视差导致的误差程度。

方法： 

研究了十二个新鲜冷冻的下肢尸体标本。四肢仰卧并旋转直到股骨后髁使用 C 形臂叠加。然后重新定位 C 臂以将股骨髁置于图像增强器的前缘和后缘。记录由于视差而重新叠加股骨髁所需的旋转。然后从对侧使用第二个 C 臂来模拟旋转成像技术，以确定组合视差的大小和方向性。

结果： 

所有 12 个样本都表现出视差，导致旋转差异。与中心视野对齐相比，前部和后部位置的平均旋转差异分别为 5.6 度和 5.5 度。对侧成像产生附加效应，双侧前部和后部位置的旋转差异分别为 9.0 度和 12.6 度。

结论： 

当股骨后髁在成像视野中偏离中心时，视差会导致股骨远端透视成像出现旋转差异。如果在获得未受伤和受伤股骨的术中旋转成像时股骨髁未居中，即使股骨在手术结束时看起来完全匹配，视差也会导致潜在的临床上显着的旋转畸形。我们建议在获取股骨远端的横向图像时，将股骨后髁在成像视野中完全居中，以防止这种可能被忽视的对旋转不良的影响。