## 1 Introduction
### Current Methods for Assessing Limb Length Discrepancy  
In order to receive appropriate compensation, it is essential to accurately quantify LLD. The quantification of LLD is carried out through functional measurements [8] and several static methods described in the specialized literature. These solutions have been visually classified and hierarchized in this paper in order to provide an overview of the methods and devices used for LLD quantification. A limited selection of these solutions has been previously documented in [10; 11]. To critically track the current state of the techniques for LLD assessment, the idea diagram method [12] was used, which allowed for systematic classification and visualization of the methods and representative devices for determining leg length discrepancy found in the specialized literature on the subject. As illustrated in Figure 1, the methods and devices are categorized into two primary subdomains. Figure 1: Three-level classification of methods and devices used in LLD evaluation [11; 13; 14; 15; 16; 17; 18; 19; 20]. These subdomains are not confined to the following examples (as indicated by the use of \"?\"): (1) classical clinical methods (direct, indirect, and imaging-based), which are well-known and not emphasized in this paper, and (2) modern, automated solutions that are applied through indirect procedures with biomechatronic, electronic devices, or image-based solutions using specialized digital programs, sometimes combined with clinical imaging methods. The authors' primary focus is on the second category, a subject of particular relevance in the context of this paper. Contemporary indirect methods involve the implementation of electronic systems positioned beneath the subject's feet during analysis. These devices include both classical instruments, such as electronic scales, which find primary application in experimental settings, and advanced mechatronic systems capable of detecting changes in the center of mass (CoM) triggered by a shorter limb [11]. These systems are designed to compensate for the difference in weight, thereby restoring the subject's body to a state of symmetry. The value by which these systems re-balance the body is equivalent to the LLD value. A similar principle underlies plantar analysis, a functional assessment of LLD [13; 14; 15]. In the pursuit of non-invasive methods, alternative approaches have been developed, employing ultrasound or a combination of ultrasound and lasers. One such example is the utilization of an ultrasound probe affixed to a rod, accompanied by a laser system to measure the distance from the ground to the rod. This rod is then automatically maneuvered by a linear actuator, based on ultrasound images, to identify the highest point of the femoral head [16]. Other systems involve the loading of radiograms into machine learning or deep learning systems, which are then compared and classified to determine LLD [17; 18]. Intraoperative methods are also employed in specific surgical procedures [19]. Another LLD assessment method is based on computer vision, utilizing the video camera of a smartphone to detect a series of strategically placed markers on the subject. This method enables the rapid, non-invasive, and cost-effective quantification of LLD [20]. Regular evaluation of LLD is imperative for prosthetic patients. These individuals require periodic visits to medical specialists to ensure congruence between the prosthetic limb and the biological limb, thereby averting the onset of LLD and the ensuing complications. This is particularly salient in the case of pediatric patients undergoing growth, as their biological processes are subject to variability [8; 21; 22]. Special attention must be given to post-amputation patients when they begin using a definitive prosthesis, which no longer resembles the biological limb that was amputated. In such cases, patients initially use a temporary prosthesis until the residual limb's final shape is defined.