applied sciences Article Detecting Anatomical Leg Length Discrepancy Using the Plug-in-Gait Model Sam Khamis 1,2, * ID , Barry Danino 3,4 , Shmuel Springer 5 ID , Dror Ovadia 3,4 and Eli Carmeli 2 1 Gait and Motion Analysis Laboratory, Department of Pediatric Orthopaedics, Dana Children’s Hospital, Tel Aviv Sourasky Medical Center, 6, Weizman St., Tel Aviv 64239, Israel 2 Department of Physical Therapy, University of Haifa, Haifa 31000, Israel; ecarmeli@univ.haifa.ac.il 3 Department of Pediatric Orthopaedics, Dana Children’s Hospital, Tel Aviv Sourasky Medical Center, 6, Weizman St., Tel Aviv 64239, Israel; barrydanino@hotmail.com (B.D.); droro@tlvmc.gov.il (D.O.) 4 Sackler Faculty of Medicine, Tel-Aviv University, Ramat Aviv 69978, Israel 5 Physical Therapy Department, Faculty of Health Sciences Ariel University, Ariel 40700, Israel; shmuels@ariel.ac.il * Correspondence: khamisam@gmail.com; Tel.: +972-50-5452699; Fax: +972-3-6947280 Received: 12 August 2017; Accepted: 4 September 2017; Published: 8 September 2017 Abstract: Leg length discrepancy (LLD) is a significant factor influencing several pathological conditions. Gait analysis is based on biomechanical gait models calculating joint kinematics; however, no previous study has validated its ability to detect anatomical LLD. The aim of the present study was to compare the validity of the Vicon ® Plug-in-Gait-model (PGM) in measuring femur and tibia segmental length discrepancy with measurements attained by X-ray. Fifteen participants with suspected leg length discrepancies underwent a lower limb X-ray and a standing calibration trial using a motion analysis system (Vicon ® , Oxford Metrics, UK). Femur and tibia segment lengths were deducted from both measurements. No differences were found when measuring the discrepancies between sides for the femur (p = 0.3) and tibia (p = 0.45) segmental length. A high correlation was found between methods (r = 0.808–0.962, p < 0.001), however, a significant difference was observed when measuring the femur and tibia length (p < 0.0001). PGM was found to be a valid model in detecting segmental length discrepancy when based on the location of the joint centers compared to X-ray. A variance was noted in the femur and tibial segmental length. The impact of this inconsistency in segmental length on kinematics and kinetics should be further evaluated. Keywords: gait model; Plug-in-Gait; joint center; segmental length 1. Introduction Leg length discrepancy (LLD) is a significant factor influencing several pathological and physiological conditions such as foot pathologies [1,2], low back pain [3,4], functional scoliosis in children [5], osteoarthritis of the hip and knee [6], impaired functional outcomes and patient satisfaction after total hip replacement [7,8]. Anatomic LLD is defined as the structural difference between the lengths of the two limbs measured from the femoral head to the distal tibia [9–11]. Various imaging techniques have been used to measure anatomic leg length [6,12]. Radiography is considered the gold standard for measurement, with accepted methods such as full limb radiographs, scanograms, computerized tomography and computerized digital radiographs. These methods are highly reliable and valid but are also expensive, not feasible for everyone and expose the subject to radiation, which limits their use in routine clinical settings. Three-dimensional motion analysis is extensively used today thus, improving a comprehensive understanding of gait in musculoskeletal disorders. It also assists in detecting gait deviations and impairments underlying reduced function, clinical decision making, quantifying rehabilitation Appl. Sci. 2017, 7, 926; doi:10.3390/app7090926 www.mdpi.com/journal/applsci