Assessment of Lower Limb Length and Alignment by Biplanar Linear Radiography: Comparison With Supine CT and Upright Full-Length Radiography

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Abstract
The purpose of this article is to compare lower limb length and alignment measurements on supine CT, upright full-length radiography, and 3D models based on upright biplanar linear radiography. This study involved 51 consecutive patients (22 men and 29 women; mean age, 68.8 years; range, 43-92 years) who were scheduled for total knee replacement. Lower limb length and alignment angle were measured on CT, upright full-length radiography, and 3D models based on biplanar linear radiography with standard and composed leg methods by two independent readers. Descriptive statistics of each modality were calculated. Measurements of different modalities were compared by paired Student t tests. Agreement between readers and modalities was assessed by Bland-Altman analyses. Mean (± SD) limb lengths were 783 ± 56.1 mm (range, 639-927 mm), 785 ± 53.0 mm (range, 655-924 mm), 780 ± 55.4 mm (range, 633-921 mm), and 783 ± 55.9 mm (range, 636-924 mm) for CT, upright full-length radiography, and 3D models based on biplanar linear radiography standard and composed leg measurements, respectively. Mean alignment angles were 2.3° ± 5.5° (range, -12° to 20°) for CT, 2.5° ± 6.7° (range, -17° to 18°) for upright full-length radiography, and 3.4° ± 6.6° (range, -14° to 18°) for 3D models based on biplanar linear radiography. No significant differences among modalities for mean limb length were found when using composed leg measurements in biplanar linear radiography. Very small but significant mean differences in angle measurements were seen for CT (-1.1° ± 2.5) and upright full-length radiography (-0.9° ± 3.1) compared with biplanar linear radiography. Bland-Altman analyses showed no significant differences between readers, with the highest agreement for biplanar linear radiography length measurements. Measurements on 3D models based on upright biplanar linear radiographs allow lower limb length and alignment angle measurements that are interchangeable with supine CT scans and upright full-length radiographs but with superior interreader agreement.

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