Potential sources of inaccuracy in leg length discrepancy (LLD) measurements commonly arise due to postural malalignment during radiograph acquisition. Preoperative planning techniques for total hip arthroplasty (THA) are particularly susceptible to this inaccuracy, as they often rely solely on radiographic assessments. Owing to the extensive variety of pathologies that are associated with LLD, an understanding of the influence of malpositioning on LLD measurement is crucial. In the present study, we sought to characterize the effects of varying degrees of lateral pelvic obliquity (PO) and mediolateral limb movement in the coronal plane on LLD measurement error (ME). A 3-D sawbones model of the pelvis with bilateral femurs of equal-length was assembled. Anteroposterior pelvic radiographs were captured at various levels of PO: 0°, 5°, 10°, and 15°. At each level of PO, femurs were individually rotated medio-laterally to produce 0°, 5°, 10°, and 15° of abduction/adduction. LLD was measured radiographically at each position combination. For all cases of PO, the right-side of the pelvis was designated as the higher-side, and the left as the lower-side. At 0° PO, 71% of tested variations in femoral abduction/adduction resulted in LLD ME < 0.5-cm, while 29% were ≥ 0.5-cm, but < 1-cm. ME increased progressively as one limb was further abducted while the contralateral limb was simultaneously further adducted. The highest ME occurred with one femur abducted 15° and the other adducted 15°. Similar magnitudes of ME were seen in 98% of tested femoral positions at 5° of PO. The greatest ME (~ 1 cm) occurred at the extremes of right-femur abduction and left-femur adduction. At 10° of PO, a higher prevalence of cases exhibited LLD ME > 0.5-cm (39%) and ≥ 1-cm (8%). The greatest errors occurred at femoral positions similar to those seen at 5° of PO. At 15° of PO, half of tested variations in femoral position resulted in LLD ME > 1-cm, while 22% of cases produced errors > 1.5-cm. These clinically significant errors occurred at all tested variations of right-femur abduction, with the left-femur in either neutral position, abduction, or adduction. This study aids surgeons in understanding the magnitude of radiographic LLD ME produced by varying degrees of PO and femoral abduction/adduction. At a PO of ≤5°, variations in femoral abduction/adduction of up to 15° produce errors of marginal clinical significance. At PO of 10° or 15°, even small changes in mediolateral limb position led to clinically significant ME (> 1-cm). This study also highlights the importance of proper patient positioning during radiograph acquisition, demonstrating the need for surgeons to assess the quality of their radiographs before performing preoperative templating for THA, and accounting for PO (> 5°) when considering the validity of LLD measurements.

中文翻译：

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骨盆倾斜度和肢体位置对影像学腿长差异测量的影响：Sawbones 模型

腿长差异 (LLD) 测量不准确的潜在来源通常是由于在 X 射线照片采集期间的姿势不对中引起的。全髋关节置换术 (THA) 的术前计划技术特别容易受到这种不准确性的影响，因为它们通常仅依赖于影像学评估。由于与 LLD 相关的病理种类繁多，因此了解错位对 LLD 测量的影响至关重要。在本研究中，我们试图描述不同程度的骨盆外侧倾角 (PO) 和冠状面中侧肢体运动对 LLD 测量误差 (ME) 的影响。组装了具有等长双侧股骨的骨盆 3-D 锯骨模型。在 PO 的不同水平上拍摄前后位骨盆 X 光片：0°、5°、10° 和 15°。在每个 PO 水平，股骨分别向中外侧旋转以产生 0°、5°、10° 和 15° 的外展/内收。LLD 在每个位置组合进行射线照相测量。对于所有 PO 病例，骨盆右侧为上侧，左侧为下侧。在 0° PO 时，71% 的测试股骨外展/内收变异导致 LLD ME < 0.5-cm，而 29% ≥ 0.5-cm，但 < 1-cm。随着一侧肢体进一步外展，同时对侧肢体进一步内收，ME 逐渐增加。最高的 ME 发生在一个股骨外展 15° 和另一个内收 15° 的情况下。98% 的测试股骨位置在 5° PO 处观察到类似程度的 ME。最大的 ME (~ 1 cm) 发生在右股骨外展和左股骨内收的极端。在 PO 的 10°，LLD ME > 0.5-cm (39%) 和 ≥ 1-cm (8%) 的病例发生率较高。最大的错误发生在股骨位置，类似于在 PO 5° 看到的那些。在 PO 的 15° 处，一半的测试股骨位置变化导致 LLD ME > 1-cm，而 22% 的病例产生的误差 > 1.5-cm。这些临床上显着的错误发生在所有测试的右股骨外展变化中，左股骨处于中立位、外展或内收。这项研究有助于外科医生了解不同程度的 PO 和股骨外展/内收产生的 X 线 LLD ME 的大小。在 PO ≤ 5° 时，股骨外展/内收的变化高达 15° 会产生边缘临床意义的误差。在 PO 为 10° 或 15° 时，即使是内侧肢体位置的微小变化也会导致临床上显着的 ME（> 1 厘米）。