Measurement of the Medial Clear Space: An Intra-Observer Comparison of Various Measurement Methods
Authors: Guild Theodore T. , Mirochnik Karina , Xie Katherine K. , Lewis Lauren K. , Shah Amir-Ali , Ashkani-Esfahani Soheil , Kwon John 
Published: December 2023

Category: Ankle; Trauma Introduction/Purpose: The Medial Clear Space (MCS) is an anatomic space between the lateral cortex of the medial malleolus and the medial cortex of the talus. Measurement of the MCS is helpful in differentiating which ankle fractures are amenable to non- operative treatment, and which may require fixation. The MCS is often evaluated on radiographic views of the ankle, and several different methods of measuring the MCS have been described in the literature. There is little information on intra-observer reliability of MCS measurement, nor validation of the various medial clear space measurement techniques. As such, the aim of this study was to better understand intra-observer reliability of MCS via several of the most popularly described techniques. Methods: Patients with matched ankle X-rays and CT scans - either weight bearing or non-weight bearing imaging pairs - were identified via electronic medical record database review of a Level-1, academic medical system after IRB approval. Measurement of the MCS on CT scan was used as the gold standard against which radiographic measurements were compared given its superior osseous definition. Four independent reviewers measured the medial clear space on 20 weight bearing, and 20 non-weight bearing x-rays via the five most commonly described methods: 5mm inferior to the talar dome (MCS #1), half way down the medial articular surface of the talus (MCS #2), obliquely at the supero-medial corner of the talus (MCS #3), and at the level of the talar dome (MCS #4). Intraclass Correlation Coefficients (ICC) were then calculated for each of the four MCS measurements methods after compilation of all independent reviewer data. Results: The ICC varied between each of the four MCS measurement techniques. For measurements 5mm below the talar dome (MCS #1) the ICC was 0.041. Measured halfway down the medial talar articular surface, the ICC was 0.466. The superior oblique MCS measurement ICC was 0.725, and the MCS measured at the level of the talar dome had an ICC of 0.419. MCS measurement 5mm below the talar dome had poor intra-observer reliability, while measurement halfway down the medial articular surface and at the level of the talar dome had fair intra-observer correlation. Overall, measurement of the MCS at the superomedial talar dome had good intra-observer reliability, ranking as the most favorable measurement method. Conclusion: The commonality of ankle fractures demands accurate radiographic interpretation for the determination of need for surgical intervention. The medial clear space measurement is the most common metric used to determine the need for surgery. There is no consensus regarding the optimal method, nor reliability of the various MCS measurement techniques. The results of this study suggest that measurement of the medial clear space at the superomedial corner of the talar dome has the highest intra- observer reliability.