The purpose of this study is to clinically demonstrate a phenomenon of measurement error that can occur during placement of a specific type of total knee replacement prosthesis (single radius femoral component) using a certain type of surgical technique (flexion/extension gap balancing).

Inclusion Criteria:

• Scheduled to undergo primary total knee arthroplasty performed by the principal investigator
• Use of a single radius posterior-stabilized TKA prosthesis is planned
• Spinal anesthetic is planned
• Diagnosis of osteoarthritis
• Has normal knee ligaments with no history of previous injury or surgery to ligaments, or other pathologic condition
• Has at least 100 degrees knee flexion preoperatively (measured by long goniometer)
• Gives valid informed consent
• Age 18 years or older

Exclusion Criteria:

• Patient receives general anesthetic
• Diagnosis other than osteoarthritis (e.g., rheumatoid arthritis, avascular necrosis, post-traumatic arthritis)
• History of any prior knee ligament injury, or any surgery to ligaments
• Any connective tissue disease affecting integrity of the ligaments (must have normal ligaments)
• History of long bone fracture in the affected leg
• Flexion contracture that would interfere with ability to establish extension gap reliably (judgment of investigator)
• Maximum pre-operative knee flexion is less than 100 degrees.
• Patient is non-ambulatory
• Any preclusion or inability to use navigation
• Treatment with single radius posterior stabilized TKA prosthesis is not indicated
• Participation is not in best interest of patient for any reason
• Cannot or does not give valid informed consent
• Female patient who is currently pregnant
• Age less than 18 years
• Body mass index greater than 40 kg/(m*m)
• Incarcerated

Clinic based population (patients undergoing total knee replacement at an orthopedic surgery clinic).