Malrotation Strongly Influences Posterior Tibial Slope Measurement on Lateral Radiographs of the Knee

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Background: Posterior tibial slope (PTS) is commonly measured on lateral radiographs, but there is an ongoing discussion on which criteria are essential as quality standards. Purpose/Hypothesis: The purpose of this study was to evaluate the effect of malrotation of the tibia on lateral radiographs for PTS measurement. It was hypothesized that malrotation alters the measured PTS. Study Design: Descriptive laboratory study. Methods: An infratuberositary osteotomy was performed on a tibial saw bone model, and 3 conditions—2 with increased PTS—were tested with the same experimental setup (series 1, closed osteotomy; series 2, 5-mm anterior opening; series 3, 10-mm anterior opening). Long lateral radiographs were taken from +12.5° of external tibial rotation (ETR) to −20° of internal tibial rotation (ITR), in steps of 2.5°. PTS was measured with the mechanical axis (MA) and the proximal anatomic axis (PAA). In addition, the distance between the medial and lateral posterior tibial condyles was measured (in mm). Results: In test series 1, between +12.5° to 0° of ETR and 0° to−20° of ITR, a change in PTS of 2.8°± 1.0 (ETR) and 3.7°±1.3 (ITR) for the MA and 3.7°± 1.4 (ETR) and 4.1°±1.5 (ITR) for the PAA was detected. In test series 2, changes of 1.5°± 0.5 (ETR) and 3.7±1.3 (ITR) for the MA and 1.6°± 0.6 (ETR) and 4.3°±1.4 (ITR) for the PAA were observed; likewise, in test series 3, changes of 1.8°± 0.7 (ETR) and 3.7°±1.3 (ITR) for the MA and 2.2°± 0.9 (ETR) and 4.2°±1.3 (ITR) for the PAA were observed. ETR of +12.5° resulted in a distance of 5 mm, and ITR of −20° resulted in a distance of 10 mm between the posterior tibial condyles in all testing conditions. There was no significant difference among the 3 testing conditions ( P > .05). A change of 1 mm in distance was equivalent to 2° (ITR) or 2.5° (ETR) of change in rotational alignment and 0.4° of change in PTS measurement, irrespective of whether MA or PAA was used. Conclusion: Tibial malrotation strongly influences PTS measurement on lateral radiographs. ETR leads to an underestimation, and ITR to an overestimation, of PTS. Every 1° of change in rotational alignment was equivalent to 0.2° of change in PTS measurement. For a reliable PTS measurement on lateral radiographs, the posterior tibial condyles should be superimposed exactly. Clinical Relevance: Dependent on PTS values, surgical correction of the PTS can be indicated. For a correct indication, it is of utmost importance to be able to determine the posterior slope exactly. This investigation demonstrates the effect of tibial rotation on PTS measurement and defines quality criteria, as well as a simple grading system for lateral radiographs for the measurement of the PTS. This helps surgeons to interpret radiographs correctly, allowing a better decision-making process, if and when a corrective osteotomy for PTS correction is indicated.

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