The Direct Anterior Approach to Hip Reconstruction

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CHAPTER 14 Role of Fluoroscopic Imaging and Image Guidance

to be moving much faster than the objects far away. This phenomenon can contribute to errors when using fluoroscopy and can cause differences in the interpreta tion of the image. In Figure 14.6 , for example, note how moving the C-arm beam cephalad or caudad will change surgeon perception of the true acetabular version. To maintain consistency in the intraoperative images, the surgeon must be aware of the position of the fluoros copy beam and attempt to maintain that position as the unit is brought in for imaging throughout the case. As users of fluoroscopy, we must be aware of the phe nomena of distortion and parallax and how they can each impact the interpretation of intraoperative measures. Because distortion alters the entire fluoroscopic image, any guidance tool should be in the field and subject to the same distortion in order to yield accurate results. The bi-ischial line used in the previously described tech nique has been shown to be the least reliable of pelvic landmarks to determine LLD compared with others, including the interteardrop line. 6 The reason this occurs is because of the presence of morphologic differences between the ischial tuberosity on each side that is fre quently encountered. This results in the intersection of the line across the lesser trochanters in different locations. Although radiographically this may indicate LLD, many times there is no actual measured LLD, and adjustments made solely from the bi-ischial line can lead to errors. Use of Bi-ischial Line to Determine Leg Length Discrepancy

In light of this, it must be emphasized that this line is used because it is easily obtained and reproduced flu oroscopically and its role is as a reference point only to calculate the change in limb length. Furthermore, it does not account for soft tissue contractures or spinal deformity or scoliosis that may alter the patient’s perception of limb length. 7 This becomes important because patients with perceived LLD after hip arthroplasty have been shown to have worse outcomes with lower Oxford and higher pain scores. 8 Given that perceived LLD can often correlate poorly with true orthoroentgenographic measurement, both the patient’s objective LLD on physical examination and the perceived LLD are used to determine the desired change in limb length. 9 This change in limb length, or “delta of correction,” has been found to be a useful pre dictor of outcome in THA, with smaller corrections being more favorable. 10 Therefore, we feel strongly that restor ing leg length is a multistep process that includes the per ception of the patient’s LLD; the assessment of any soft tissue, pelvic, or spinal abnormalities; the actual measure ment of leg length clinically; and the radiographic assess ment of LLD attributed to the hip joint. Combining this information gives the surgeon the best information on the amount of correction that is necessary. Underappreciating Anteversion With the Iliac Oblique View Throughout the procedure, the patient’s position and rotation on the operating table can shift or rotate. A common scenario is when the desired AP pelvis image

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A

B

FIGURE 14.6 The difference in acetabular cup positioning as the fluoroscopy unit moves in a caudad-cephalad direction. A, In the cephalic direction, the cup position appears more anteverted. B, In the caudad direction, the cup position appears less anteverted.