Pain and reduced function are the most common symptoms of coxarthritis. Pain relief and normal range of motion (ROM) restoration are the objectives of a Total Hip Arthroplasty (THA) procedure. It is recognized that increased head-neck ratio results in increased ROM due to the fact that prosthetic impingement between neck and acetabular liner occurs with wider arcs of motion. This head-neck ratio has been investigated extensively only in vitro (on cadavers or sawbones) or using mathematical modeling in computer simulations. These studies are limited by the lack of soft tissues and muscle activations in the models. This study will clinically assess the effect of prosthetic head diameter on patient capability of performing movements which require extreme arcs of motion of the hip joint. Therefore, the research questions are:

• What role do soft tissues play in limiting ROM?
• Can prosthetic impingement really occur in a well positioned THA?
• Can a bigger head diameter alone reduce the risk of impingement?

This will be a prospective comparative randomized double-blind Study (both patient and the clinical engineer performing the gait analysis will be blind with respect to the prosthetic head diameter implanted). Patient recruitment will last 12 months and the Study will have a follow-up period of 12 months. A total number of 45 patients will be recruited. Patients will be randomly allocated into three groups: the first group will undergo THA with a 28mm diameter head (Pinnacle Acetabular System, with a ceramic on ceramic - CoC - bearing), in the second group THA will be performed using a 36 mm diameter head (DeltaMotion), finally patients in the third group will undergo THA with a 40 mm diameter head (DeltaMotion). Patients will be operated by three surgeons of the same hospital according to the same surgical procedure and using the same direct lateral approach. All patients will receive a Corail stem and will follow the same rehabilitation program.

• Primary endpoint: Gait analysis and fluoroscopy will provide quantitative information of prosthesis in vivo performance, e.g. what is the arc (degrees) of active or passive motion when prosthetic impingement occurs?
• Secondary endpoint(s): these analyses will also allow to: 1) assess ROM of the operated hip versus contralateral non operated hip and the effect of prosthetic head diameter and any correlation to hip functional scores validated and commonly used in the Literature; 2) set reproducible criteria for in vivo fluoroscopic analysis of ROM in THA patients.

Inclusion Criteria:

• Patients with unilateral primary or secondary hip arthrosis indicated for THA;
• Men and female, age between 35 and 55 years old;
• BMI smaller than 30;
• Patients able to understand the protocol and to sign the informed consent.

Exclusion Criteria:

• Child-bearing female;
• Patients with also controlateral hip arthritis;
• Neurological patients, because of possible alteration of neuro-muscular control of the replaced hip;
• Patients treated with joint replacement at any other lower limb articulation