• Bernd Lutz, Angela Trubrich, Thomas Kappe, Heiko Reichel, and Ralf Bieger.
• Department of Orthopaedic Surgery, University of Ulm, Ober Eselsberg 45, 89081, Ulm, Germany. bernd.lutz@rku.de.
• Arch Orthop Trauma Surg. 2018 Sep 1; 138 (9): 1287-1292.

PurposeThe epicondylar ratio (ER) is used to restore the individual joint line (JL), especially in revision total knee arthroplasty. It was first described in magnetic resonance imaging (MRI) but is usually applied to a.p. radiographs of the knee for preoperative planning. The objective of the current study was to define reliable landmarks in MRI and X-ray images of the knee, which allow comparison of the image modalities. Furthermore, the correlation of the measured ER in MRI and X-rays of the knee was calculated.MethodsA consecutive series of 87 patients who underwent an arthroscopical intervention of the knee were included into the present study. The lateral epicondyle was defined as the most lateral and distal prominence. On the medial side, the measurement was aligned to the epicondylar sulcus. The medial and lateral ER were calculated by dividing the perpendicular distance from the JL to the epicondyle by the transepicondylar distance. One observer determined the ER twice to calculate the intramethod intraobserver agreement, and a second observer obtained the intramethod interobserer agreement. The ER obtained from X-ray and MRI was compared to calculate the intermethod correlation.ResultsThe average lateral ER was 0.29 on X-ray versus 0.28 on MRI. The average medial ER was 0.33 and 0.33, respectively. Intramethod agreement ranged from 0.66 to 0.88 and intermethod correlation from 0.49 to 0.57.ConclusionsThe ER can be determined reliably on MRI and X-ray images of the knee. The correlation of the ER in MRI and X-ray is fair.

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