## 1 Introduction  
In the case of severe covarthrosis, pain and reduced joint function are the most common symptoms. Hence, pain relief and restoration of a physiological range of motion (ROM) are the objectives of a total hip arthroplasty (THA) [1]. Although this surgical procedure has a high success rate [1], particularly in the elderly population [2], a limited ROM at the replaced hip joint and lower-limb disability are still observed [3]. These drawbacks were associated with several critical factors [1; 3; 4; 5], but among these, the implant design and head-to-neck ratio were considered the most relevant. In particular, these factors seem to affect the hip joint ROM after replacement because of the possible undesired cup-to-neck prosthetic contact, also known as the stem-to-liner (StL) impingement [4; 6]. This introduces serious concerns when the indications for a THA are extended to younger patients, i.e., those subjects with a more active and demanding lifestyle, where a replaced hip ROM is expected to be large [7]. To cope with a limited ROM after a THA, prosthesis designs with larger diameter head bearings have been introduced to provide stability, a larger ROM, a smaller risk of dislocation, and to possibly prevent StL impingement. These claims have been supported in the literature [8; 9; 10; 11; 12], although concerns are still reported [13; 14; 15]. ROM and joint rotation have been extensively investigated in vivo using standard gait analysis (GA) [16; 17; 18; 19] and other simple but less accurate instruments, such as goniometers [20; 21] and inertial measurement units [18; 22; 23]. Due to the known technical limitations of these techniques, the ROM at the replaced hip joint is an approximated estimation and the exact cup-to-neck interaction cannot be determined with these instruments. In vitro [24; 25] and in silico [26; 27; 28; 29] evaluations have thus been performed as well, but these suffer from unrealistic soft tissue arrangements and overall motion and loading conditions. Today, very little is known about the ROM at the replaced hip in large and extreme joint movements. In particular, no accurate three-dimensional (3D) StL distances have been reported; these would be particularly interesting in the presence of large diameter heads, which are claimed to contribute to larger and smoother joint motion. To analyze this critical distance in daily living activities, as well as more demanding activities, a reliable 3D technique for component-to-component interaction is essential. The extent to which a large ROM is achieved and StL impingement is prevented should be assessed in real patients. Both the ROM and StL impingement depend on many conditions, including patient status, prosthesis design, component implantation, and the soft tissues. Careful 3D experimental studies in THA patients are therefore necessary, possibly involving combining full-body motion analyses and imaging at the replaced joint. Within the currently available techniques, stereophotogrammetry [16] and radiography [30] have potential regarding these in vivo analyses for both ROM and StL interaction analyses. The ROM at the replaced hip in extreme motion exercises has been analyzed to a limited extent [31; 32], as well as for StL impingement [6; 33], where there is a lack of studies on patients with large diameter heads. Whereas standard GA [34] can be easily adapted for more demanding motor activities, accurate 3D StL interactions can only be addressed using 3D videofluoroscopy analysis (VA). This allows for the reconstruction of the absolute and relative position and orientation of the prosthesis components in 3D by using 2D-to-3D spatial matching [35]. This has been widely adopted for assessing the surface-to-surface interaction of prosthesis components in total joint replacement [36; 37; 38; 35] and to a limited extent in THA [39; 40; 41; 42]. However, single-plane VA has been demonstrated to be a good compromise between patient safety and measurement accuracy [42].