## 1 Introduction
This prosthetic concept was developed by designer [PERSON] and is specifically designed for children with unilateral transtibial amputations in developing countries. To modify the length of the prosthesis during the child's development, additional modular rings are added to increase length as LLD emerges due to growth [19]. Similarly, the FIT concept proposed by designer [PERSON] promotes the idea of length adjustment through a central modular rod, allowing the user to extend or shorten the prosthesis as needed. This cost-effective concept is intended for manual adjustment to ensure a customized fit (over several years) for the residual limbs of children with amputations in developing countries [20; 21]. Additionally, a CAD version of a module consisting of two interlocking tubes with different diameters is available, which can be secured at the desired length using an elastic bushing [22]. As an attempt to address the issue of adjusting the length of lower-limb prostheses for proper fitting over several years as the patient grows, prosthetic assembly was developed. This assembly consists of a prosthetic foot attached to an adjustable rod secured with a bushing and screw [23]. The system also allows for the length adjustment of the foot by shifting a component that simulates the forefoot and attaches to the midfoot. The prosthetic rod, which is made entirely of mechanical components without any electronic systems, can be lengthened or shortened manually based on the growing child's needs. This adjustment is intended to be performed manually (without any measurement system), without the need for visits to a specialist or the involvement of expert advice [23; 24; 25]. The needs of patients in developing countries and the high cost of prosthetic devices make it impossible to frequently change the prosthesis in accordance with the development of a growing amputee child. Therefore, for a 9-year-old female patient with a transtibial amputation, an adjustable aluminum rod was developed (with an extension of up to 50 mm, with a 1 mm pitch), secured with a counter nut. The rod, suitable for use in modular prostheses, was placed between the classic prosthetic foot and the prosthetic socket [25]. A similar design for an adjustable prosthetic rod for the lower limb, with counter nut fixation, was designed to facilitate the adjustment of prostheses for disadvantaged children [26]. It was reported that the prosthesis was changed for 15 children (out of 28 monitored) on two occasions within a 6-month period, once after 3 months for 5 patients, while no adjustments were reported for the remaining patients or for those not monitored. Additionally, another prosthetic limb with adjustable length contains a screw-and-nut mechanism that allows for elongation when the screw is manually rotated (by the prosthetist or user), with rotational motion being converted into linear motion [27]. Additionally, the design of a passive lower-limb prosthesis has been reported, featuring incremental length adjustment through the manual operation of a system consisting of three interlocking rods. This system is functionally similar to other prostheses that allow for such adjustments. The rod would enable the user to adjust the length while using the same prosthesis over a period of several years (suitable for users up to 10 years old) [28]. All current prosthetic length adjustment devices are manually operated, lacking automated or autonomous components, which may introduce usage errors and require specific training for precise adjustments. Currently, the specialized literature and medical practice do not include an adaptive prosthesis model capable of detecting limb length discrepancy (LLD), eliminating these limitations, and simplifying the alignment process for the prosthetic patient.