## 1 Introduction  
A trend of innovative thinking has brought about enormous changes in knee replacement surgeries over the last decade. Since the introduction of kinematic alignment (KA) [1] and the concept of constitutional alignment [2], we have seen a remarkable increase in the scientific output calling for a personalised approach in total knee arthroplasty (TKA) surgeries. As a result, alternatives such as the KA and its restricted option, functional alignment, adjusted mechanical alignment, inverse kinematic alignment, and its restricted version are replacing the \"one-size-fits-all\" almost dogmatic approach of mechanical alignment [3; 4]. These different alternatives have generated enormous interest in the information related to the different phenotypes fundamentally studied in the coronal plane [5; 6; 7]. It is necessary to measure different angular values reliably and accurately, basically on weight-bearing, full-length anteroposterior X-rays of the lower limbs (LLRs), to establish classifications and obtain pre- and post-operative information on the specific alignments of each patient [8; 9; 10; 11; 12]. The mechanical hip-knee-ankle (mHKA) angle measures the angulation between the femur and tibia mechanical axes in the coronal plane [13]. The mHKA helps quantify the degree of valgus or vurus alignment in the knee [2; 14]. Quantifying misalignment is essential as it has been implicated in the pathogenesis of knee osteoarthritis [14; 15; 16; 17]. The mHKA is also used to assess post-operative knee alignments after total knee replacements and is necessary for angular correction surgeries and assessments of the corrections achieved [18]. Knowledge of the mHKA is also helpful in the analysis of the causes of TKA failure and the planning of revision surgery [19]. In addition, advances in digital technology in radiology in the last decade have enabled advantages such as reduced radiation exposure, more efficient image comparisons, variable contrast scales, and ease of storage. These advantages have encouraged the development of software tools for evaluating medical images [20]. Today, we have different software tools that allow us to measure medical images more easily, quickly, precisely, and accurately, with less intra- and inter-observer variability. There are even systems based on artificial intelligence which can automatically detect anatomical references on X-rays to establish determined angular measurements [21; 22; 23; 24; 25; 26; 27]. These advances in angle information management have not always been accompanied by the validation of accuracy and reliability, with few exceptions [28; 29; 30; 31; 32].  
An open-source, computer-aided measurement system (TraumaMeter v.873, [PERSON] and [PERSON], reg. no. 08/2021/374, Murcia, Spain) has been developed. Initially, the software was designed to improve the manual measurement of variables necessary to study vertebral misalignments, such as the Cobb angle and axial vertebral rotation (AVR) on digital radiographic images, without introducing the scale of the radiographic study performed [33; 34; 35].  
Our research aimed to evaluate the validity and absolute reliability (according to the [PERSON] criteria [36]) of an extension of the initial TraumaMeter v.873 software for measuring the angular values of LLRs. This validation, which was the first to be carried out, was necessary for the usefulness of the software in clinical practice to be confirmed.