# Detecting Anatomical Leg Length Discrepancy Using the Plug-in-Gait Model
## 4 Discussion
The ASIS trochanter distance, mean leg length, pelvis width, marker radius and fixed radians calculated the offset vectors of the two HJCs. However, using a more accurate method to define the hip joint center could lead to a more accurate estimation of the femoral length [26]. The KJC and AJC's location were also determined by the knee and ankle width measurements [18]. HJC localization based on a regression equation has been found to be inaccurate resulting in a 30 \(\pm\) 6 mm margin of error compared to a three-dimensional ultrasound, based on a population of healthy adults [17]. [PERSON] reported similar results found in a sample of 11 male adult able-bodied volunteers, where the HJC estimation was at a distance of 29 \(\pm\) 8 mm of a measurement obtained by a roentgen stereophotogrammetric analysis [27]. [PERSON] et al. found, a 25 mm \(\pm\) 10 between HJC calculated by a PGM and a reference calculation by EOS imaging in 11 typically developed children [28]. In a CP group, the difference was 21 \(\pm\) 10 mm. These findings can contribute to the differences in femoral length found in our study. The propagation of the errors in the localization of the hip joint center on kinematics and kinetics has been previously evaluated in the literature [29; 30; 31] and demonstrated that HJC misplacement could result in significant errors on both hip and knee kinematics and kinetics. Our results imply significant variance when calculating the KJC and AJC's location as determined by the PGM calculation. As for the KJC, the importance of an accurate definition of the location of anatomical landmarks has been shown to have a significant effect on sagittal and knee joint moments, with a 25% disparity [32]. [PERSON] el al found KJC localization to significantly affect coronal and transverse plane angulation using different KJC estimation techniques during walking among 12 male participants and weaker test retest reliability for the PGM [33]. As for the AJC, the PGM identifies the AJC based on the location of the knee and hip joint centers [18] and has previously produced errors when identifying the AJC. An AJC calculated by the PGM was found to significantly differ from the AJC calculated by a medial and lateral malleoli model [34]. The distances between two AJCs in the posterior-anterior, medial-lateral and inferior-superior directions were found to be approximately 6.3, 7.7 and 8.2 (mm). These results also concur with our findings, taking into consideration that the tibia segment is affected by the KJCs and AJCs location. However, the effect of our findings as to the tibia length and the byproduct of the KJC and AJC's location on the angulation calculations of the model are unknown. The impact of this variance in the localization of the KJCs and AJCs on kinematics and kinetics should be further assessed. In conclusion, PGM was found to be a valid model in detecting segmental length discrepancy based on the joint centers' location compared to X-ray measurement. A variance is present in the femoral and tibial segmental length as measured by the PGM compared to the X-ray. Impact of this inconsistency in segmental length on kinematics and kinetics should be further evaluated. [PERSON] is responsible for the design of the study, acquisition, analysis, and interpretation of data for the work, and writing the manuscript. [PERSON], is responsible for critically revising the manuscript. [PERSON], is responsible for study interpretation and final approval of submitted version. [PERSON], is responsible for critically revising the manuscript. [PERSON], is responsible for final approval of submitted version. The authors declare no conflict of interest.