Fluoroscopic and RSA Evaluation of the Triathlon Total Knee Prosthetic Design

Inclusion Criteria: * Patient (minimum age of 18 years old, no maximum age limit) is diagnosed with osteoarthritis or rheumatoid arthritis and requires primary arthroplasty. * Patient is capable of giving informed consent and expressing a willingness to comply with this study * Patient has no major deformities (i.e. sagittal and coronal deformities are less then 15 degrees) * The ability to perform a lunge and step-up motion without the help of bars or a cane. * No or slight pain during activity according to the Knee Society Pain Score Exclusion Criteria: * The patient is unable or unwilling to sign the Informed Consent specific to this study * The individual has a functional impairment of any other lower extremity joint besides the operated knee * Patient has a flexion contracture of 15° and more * Patient has a varus/valgus contracture of 15° and more * Patients requiring revision arthroplasty * The patient does not understand the Dutch or English language good enough to participate. * The use of walking aids * The inability to walk more than 500 meters