Inclusion Criteria:  Males and females aged over 18 years at time of surgery. Patients who are scheduled to receive Medacta GMK® Sphere system for primary TKA (class III, CE-marked). Patients with functional contralateral knee (i.e. without the need to use walking aids). Patients willing and able to provide written informed consent for participation. Patients willing to comply with the pre and post-operative evaluation schedule.  Exclusion Criteria:  Patients with one or more medical conditions identified as a contraindication defined by the labelling on any Medacta implants used in this study:  Patients presenting with progressive local or systemic infection Muscular loss, neuromuscular disease or vascular deficiency of the affected limb, making the operation unjustifiable. Severe instability secondary to advanced destruction of condralar structures or loss of integrity of the medial or lateral ligament Patients whose prospects for a recovery to independent mobility would be compromised by known coexistent, medical problems. Patients affected by concomitant spine, hip, ankle or contralateral knee pathologies that can affect walking capacity. Patients unable to understand and take action. Patients with known allergy to the materials used. Patients in which Medacta GMK® Sphere system is used in emergency interventions.