Article
Changes in lower limb alignment due to flexion and rotation – a 3D simulation of radiographic measurements
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Authors
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Maximilian Jörgens
- Klinik für Orthopädie und Unfallchirurgie, Muskuloskelettales Universitätszentrum München (MUM), Klinikum der Universität München, LMU München, München, Germany
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Josef Brunner
- Klinik für Orthopädie und Unfallchirurgie, Muskuloskelettales Universitätszentrum München (MUM), Klinikum der Universität München, LMU München, München, Germany
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Hannah Kümpel
- StabLab, LMU München, München, Germany
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Maximilian Weigert
- StabLab, LMU München, München, Germany
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Wolfgang Böcker
- Klinik für Orthopädie und Unfallchirurgie, Muskuloskelettales Universitätszentrum München (MUM), Klinikum der Universität München, LMU München, München, Germany
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Julian Fürmetz
- Sporttraumatologie & Arthroskopische Chirurgie, Berufsgenossenschaftliche Unfallklinik Murnau, Murnau, Germany
| Published: | October 25, 2022 |
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Outline
Text
Objectives: Many radiographic lower limb alignment measures are dependent on patients' position, which makes a standardised image acquisition of long-standing radiographs (LSR) essential for valid measurements. The aim of this study is to investigate the influence of rotation and flexion of the lower limb on common radiological alignment parameters using 3D simulation.
Methods: Joint angles and alignment parameters of 3D lower limb bone models (n=60) generated from CT scans and projected into coronal plane to mimic radiographic imaging were assessed. All models were rotated in 5° steps around the longitudinal mechanical axis up to 15° inward and outward and additionally flexed in 10° steps along the femoral intercondylar axis up to 30°. This resulted in 28 models for each leg.
Results and conclusion: Relevant knee angles showed significant differences due to the position changes of the bone models. While the knee is extended, the MPTA decreases with internal rotation and increases with external rotation (-0.17° to 0.54°) and shows opposite effects when combined with flexion (2.07° to -5.54°). In the extended knee the impact of rotation and flexion on the MLDFA was smaller (-0.62° to 0.69°) with a mean deviation of 0.2° changes per 5° rotation but getting greater with additional flexion (-4.43° to 4.99°). The MAD is more laterally with external rotation and more medially with internal rotation and alters within a range of approximately 50 mm with combined effects (-25.45mm to 25.22mm). The HKA angle is barely influenced by flexion and rotation alone. With the knee extended, the mean deviation was less than 1° over all rotational steps, but increased towards more varus when external rotation was combined with flexion (8.41° to -8.48°).
Axial Rotation and flexion of the three-dimensional lower limb have a huge impact on the projected two-dimensional measurements in coronal plane. This must be considered when evaluating X-ray images. Extension deficits of the knee make LSR prone to error and this calls into question direct postoperative alignment controls.
