diagnostics Article Computer-Assisted System in Stress Radiography for Anterior Cruciate Ligament Injury with Correspondent Evaluation of Relevant Diagnostic Factors Chien-Kuo Wang 1, Liang-Ching Lin 2, Yung-Nien Sun 3, Cheng-Shih Lai 1, Chia-Hui Chen 4,* and Cheng-Yi Kao 1,* 1 Department of Medical Imaging, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, No. 1 University Rd, Tainan 704, Taiwan; [email protected] (C.-K.W.); [email protected] (C.-S.L.) 2 Department of Statistics, National Cheng Kung University, No. 1 University Rd, Tainan 704, Taiwan; [email protected] 3 Department of Computer Science and Information Engineering, National Cheng Kung University, No. 1 University Rd, Tainan 704, Taiwan; [email protected] 4 Department of Medical Imaging and Radiological Sciences, I-Shou University, No.8, Yida Rd., Jiaosu Village, Yanchao District, Kaohsiung City 82445, Taiwan * Correspondence: [email protected] (C.-H.C.); [email protected] (C.-Y.K.); Tel.: +886-7-6151100#7802 (C.-H.C.); +886-6-2766108 (C.-Y.K.); Fax: +886-7-6155150 (C.-H.C.); +886-6-2766608 (C.-Y.K.) Abstract: We sought to design a computer-assisted system measuring the anterior tibial translation in stress radiography, evaluate its diagnostic performance for an anterior cruciate ligament (ACL) tear, and assess factors affecting the diagnostic accuracy. Retrospective research for patients with both Citation: Wang, C.-K.; Lin, L.-C.; knee stress radiography and magnetic resonance imaging (MRI) at our institution was performed. A Sun, Y.-N.; Lai, C.-S.; Chen, C.-H.; complete ACL rupture was confirmed on an MRI. The anterior tibial translations with four different Kao, C.-Y. Computer-Assisted System methods were measured in 249 patients by the designed algorithm. The diagnostic accuracy of each in Stress Radiography for Anterior method in patients with all successful measurements was evaluated. Univariate logistic regression Cruciate Ligament Injury with analysis for factors affecting diagnostic accuracy of method four was performed. In the inclusive 249 Correspondent Evaluation of patients, 177 patients (129 with completely torn ACLs) were available for analysis. Mean anterior Relevant Diagnostic Factors. tibial translations were significantly increased in the patients with a completely torn ACL by all four Diagnostics 2021, 11, 419. https:// doi.org/10.3390/diagnostics11030419 methods, with diagnostic accuracies ranging from 66.7% to 75.1%. The diagnostic accuracy of method four was negatively associated with the time interval between stress radiography and MRI as well as Academic Editor: Ingrid Moller force-joint distance on stress view, and not significantly associated with age, gender, flexion angle, intercondylar distance, and force-joint angle. A computer-assisted system measuring the anterior Received: 24 December 2020 tibial translation in stress radiography showed acceptable diagnostic performance of complete ACL Accepted: 26 February 2021 injury. A shorter time interval between stress radiography and MRI as well as shorter force-joint Published: 2 March 2021 distance were associated with higher diagnostic accuracy. Publisher’s Note: MDPI stays neutral Keywords: knee; anterior cruciate ligament; ACL; stress radiography; computer-aided diagnosis with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction The anterior cruciate ligament (ACL) is the main restraint of anterior tibial translation. It may be torn when taking unexpected heavy loading, e.g., strenuous exercise or traffic Copyright: © 2021 by the authors. accident, thereby causing knee instability [1]. Therefore, early diagnosis of ACL tear Licensee MDPI, Basel, Switzerland. is crucial. Magnetic resonance imaging (MRI) with high resolution and good imaging This article is an open access article quality is helpful for confirming the diagnosis of the ACL tear, but it is expensive and distributed under the terms and time-consuming [2]. Clinical diagnosis of ACL tear is still important. conditions of the Creative Commons The clinical diagnosis of ACL tear is based on anterior tibial translation by applying Attribution (CC BY) license (https:// a different force to the knee. The anterior drawer test is subjective, unmeasurable, and creativecommons.org/licenses/by/ 4.0/). not well reproducible [3]. Stress radiography of the knee offers a relatively objective and Diagnostics 2021, 11, 419. https://doi.org/10.3390/diagnostics11030419 https://www.mdpi.com/journal/diagnostics Diagnostics 2021, 11, 419 2 of 11 measurable approach to quantify the anterior tibial translation when applying force to the knee [4–7]. A prior study has focused on the effect of knee position on the reproducibility of measurements taken from stress radiography [8]. A different knee flexion angle, injury extent, muscle guarding, and femoral rotation affect the laxity measurement using stress radiography [4,9,10]. Another study compared different measuring methods and assessed the intra-class correlation coefficient of each method to evaluate the knee instability [11]. However, the reliability of manual measurements done by different readers and different methods is questionable. Several automatic methods for objective measurement of joint space or angle using computer-assisted systems have recently been developed [12–14]. Automatic computer image assessment and analysis system can segment the femur, tibia, and patella using an edge-based method. Then, some landmarks to calculate tibial translation distance can be defined using a segmentation algorithm. In this study, we designed a computer-assisted system that automatically segmented bone contours of the knee and, subsequently, measured the distance of anterior tibial trans- lation in stress radiography. The diagnostic performances of different methods for ACL injury were evaluated. It was hypothesized that we could use our computer-assisted sys- tem to (1) evaluate the diagnostic performance of stress radiography for ACL insufficiency by different measurement methods, and (2) assess the effects of age, gender, time interval between stress radiography and MRI, and technical factors on the diagnostic accuracy. 2. Materials and Methods 2.1. Patients Institutional Review Board approval was obtained, and the requirement to obtain informed consent was waived. A retrospective search of the Picture Archiving and Com- munication System (PACS) for patients with both knee stress radiography by Telos device (Metax, Hungen, Germany) and MRI at our institution was performed. The MRI was reviewed by one musculoskeletal radiologist, and the status of ACL was classified into complete tear, partial tear, or no tear. If the review did not match the previous report, a musculoskeletal radiologist with more than 20 years of experience made the final decision. The patients were divided into one group with a complete ACL tear, and another group without a complete ACL tear. The time interval between stress radiography and MRI was recorded. A total of 399 patients with stress radiography and MRI for evaluation of ACL injury were included in this study, from which 150 patients were excluded (Figure1). In addition, 28 patients were diagnosed as multiple ligamentous injuries on MRI, while 15 patients with the torn meniscus had displaced fragments. Nine patients were excluded due to long interval (>9 weeks) between stress radiography and MRI. Furthermore, 27 patients were excluded due to anatomic shape changes by severe osteoarthritis, fractures, or intraar- ticular loose bodies, 36 patients were excluded due to a previous knee surgery, 18 patients were excluded because of unsatisfactory device position to obscure the segmentation, and another 17 patients were excluded due to poor image quality. Imaging data processing and a tibial translation measurement were performed in the remaining 249 patients. Diagnostics 2021, 11, 419 3 of 11 Diagnostics 2021, 11, x FOR PEER REVIEW 3 of 11 FigureFigure 1. TheThe flow flow chart chart of of patient inclusion and exclusion. 2.2. Knee Segmentation 2.2. Knee Segmentation Lateral projection views of the knee before and after exerting a force of 15 daN (1 Lateral projection views of the knee before and after exerting a force of 15 daN (1 daN = 10 N), in accordance with a previous study [15], posteriorly on the tibia by a daN = 10 N), in accordance with a previous study [15], posteriorly on the tibia by a stress stress device were taken. In the automatic measurement of knee translation in stress device were taken. In the automatic measurement of knee translation in stress radiog- radiography, the most important and challenging part was knee segmentation (Figure2). raphy,We first the performed most important down sampling and challenging of the images part was to improve knee segmentation the efficiency (Figure of following 2). We firstpre-processing. performed down Then, thesampling Gaussian of the smoothing images to is improve applied tothe the efficiency images forof following noise removal, pre- processing.making the Then, edge ofthe the Gaussian bone relatively smoothing clear is by applied removing to the artifactimages infor the noise image. removal, Since makingthe knee the joint edge is located of the bone between relatively two major clear bony by removing structures the (femur artifact and in tibia),the image. a less Since bony thestructure knee joint meant is located less brightness. between two For everymajor rowbony in structures the image, (femur