The primary outcome of this study is to compare active range of motion values for the Triathlon PS Total Knee System.

Participants can have both knees replaced. Range of motion is measured for each knee, as such the number of knees evaluated can be greater than the number of participants.

The Knee Society Scores (KSS) at 1, 2, and 5-year visits will be compared. Additionally, comparison of scores at each post-surgery visit with baseline will be tested to see if any improvement is seen at each time point. The Knee Society Clinical Rating System is comprised of two distinct sub-scores: one for pain, ROM and joint stability, and one for functional parameters. Sub-scores range from a potential minimum score of 0 to a maximum score of 100 points. Although the specific scores are not distinguished as "excellent," "good," "fair," or "poor," a higher value represents a better outcome.

Maximum number of knees evaluated at any interval.

The SF-36 score at 1, 2, 3, 4 and 5-year visits will be compared at each post-surgery visit with baseline to see if any improvement is seen for each time point.The SF-36 includes a physical component and a mental component and is completed by the participant. Physical component and mental component scores were calculated on a scale ranging from 0 to 100. Low values represented a poor health state and high values represented a good health state.

Maximum number of knees evaluated at any interval.

The Western Ontario and McMaster Osteoarthritis Index (WOMAC) scores at 1, 2, 3, 4 and 5-year visits will be compared between groups, when data is available. Additionally, comparison of scores at each post-surgery visit with baseline will be tested to see if any improvement is seen for each time point. The WOMAC collects information specific to osteoarthritis outcomes. The questionnaire uses a visual analog scale for pain, measuring factors of general pain, stiffness, and physical findings. Pain is scored from 0 to 100 for each set of factors, with 0 indicating no pain and 100 indicating extreme pain. Total WOMAC scores range from 0 to 300. Lower values represent better outcomes.

Data for the WOMAC is only available at the 5 year interval due to typographical errors noted on earlier interval forms rendering them invalid for comparison.

The number of knees evaluated at 5 years.

The Lower-Extremity Activity Scale (LEAS) score at 1, 2, 3, 4 and 5-year intervals will be compared at each post-surgery visit with baseline to see if any improvement is seen for each time point.The LEAS is completed by the participant to assess activity level. Activity levels were ordered in terms of intensity from 1 to 18, with 18 indicating the highest activity level. A level of 1 indicated that the subject was confined to bed all day while a level of 18 indicated that the subject was up and about at will inside and outside of the house, and also participated in vigorous physical activity, such as competitive level sports, on a daily basis.

Maximum number of knees available at any interval.

Radiographic success/failure at 1, 2, and 5-year visits will be assessed. Radiographic failure is defined as a score of 10 or greater according to the Knee Society Roentgenographic Scoring System, regardless of symptoms. A migrating or shifting prosthesis with or without the disappearance of radiolucent lines is also a failure regardless of score.

Maximum number of knees evaluable at any interval.