The purpose of this study is to investigate the role of two-dimensional FEM models obtained from DXA images in the longitudinal assessment of periprosthetic bone, in subjects undergoing hip arthroplasty. Specifically, the study will focus on the joint analysis of changes in the parameters of bone mineral density (BMD) and Bone Strain Index (BSI) at the level of periprosthetic bone of the proximal femur in patients undergoing hip replacement surgery using DXA, evaluating the densitometric variation of these two parameters over time, starting from an immediately postoperative data and checking the change at 6 months to follow the patient through the postoperative course

Hip osteoarthritis is nowadays one of the most prevalent diseases in developed countries. Prosthetic replacement of the degenerated joint is the definitive treatment for more advanced stages of osteoarthritis.

Despite continuous improvements in prosthesis material, design, and implantation procedures that have led to a percentage reduction in failures, there is still an albeit low risk of hip prosthetic implantation failure today.

Preoperative planning is therefore a prerequisite in order to ensure the success of the implant and its longterm survival. This is generally based on radiographic investigations, with implant selection being based on "templating" the radiographs to replicate the patient's joint anatomy.

However, with preoperative radiographs (or possibly second-level investigations such as CT scans), only the geometry of the bone subjected to the implant through is evaluated, without obtaining information regarding the bone density or the load to which the implant will be subjected. However, the long-term stability of the implant depends on the load transfer to the adjacent bone. Non-physiological load transfer can cause stress-shielding, bone resorption, or fibrous tissue growth due to high micromovements at the implant-bone interface.

The study of images by finite element analysis (FEA) has been used in orthopedic biomechanics for fifty years, and the techniques have progressively evolved in terms of sophistication and ability to assess the postoperative mechanical environment The Bone Strain Index (BSI) software, obtained from computerized bone mineral imaging (DXA) surveys using FEA, automatically calculates the deformations and stresses to which a given bone segment is subjected from patient-specific loading conditions. Previous studies have evaluated changes in periprosthetic bone mineral density (BMD) around hip replacement, noting densitometric changes that could vary between subjects and as a function of treatment, suggesting periprosthetic DXA as a useful technique in early assessment of possible bone loss from periprosthetic resorption. However, there are no DXA-based studies in the literature that correlate BMD changes to periprosthetic mechanical stress.

In this study, the investigators aim to study periprosthetic hip bone by DXA technique, applying the FEA method to obtain two-dimensional models of bone resistance to loading, using BSI software.

This evaluation will be included in the context of the normal pre- and postoperative evaluation process, with a view to better understanding the therapeutic outcome and possibly providing a means of predicting early mobilization and thus possible surgical failure.

Patients will be recruited as part of the normal surgical procedure for patients referred for hip replacement surgery to be performed at the Galeazzi - Sant'Ambrogio Hospital. Subjects, after being informed about the study and giving their written informed consent, will provide the examining physician with the information needed to fill out appropriate biographical and medical history forms. Recruitment will be entrusted to the physicians who will perform routine clinical examinations.

Once enrolled, patients will have a lumbar and femoral DXA scan as a preoperative preliminary investigation; anthropometric evaluations, postoperative DXA scans (immediate postoperative and at 3,6,12 months), blood draws, and PROMs will be administered for each patient. The study will be conducted according to the guidelines of Good Clinical Practices

Inclusion Criteria:

Patients who are candidates for elective hip arthroplasty surgery (indication determined only by the Orthopedic Surgeon)

• Patients with noninflammatory degenerative hip joint disease, including osteoarthritis and the outcomes of congenital hip dysplasia.
• Hip joint deformity correction surgery
• Patients aged 50 to 90 years (completed)
• 18 ≤ Body Mass Index (BMI) ≤ 35 kg/m2
• Postmenopausal women (absence of menstrual cycle for at least 2 years)
• Willingness and ability to provide informed consent

Exclusion Criteria:

Taking drugs that may interfere with muscle or bone metabolism,

• Evidence of diseases known to interfere with bone or muscle metabolism
• Rheumatoid arthritis
• Patients undergoing revision hip replacement surgery
• Presence of infection or suspicion of infection at the hip
• Inability to provide informed consent
• Previous synthetic surgery for proximal femur fractures
• Severe obesity (BMI >35kg/m2)