Total knee arthroplasty (TKA) is a surgical procedure performed throughout the world in high numbers and at a high cost for health systems. Many countries have published national guidelines for the selection of a cost effective implant. The major criterion for this selection is the long term survival of the artificial joint. On the other hand, the evaluation of functional outcome and quality of life is also an important issue if the operation has to be proved cost effective. However, there is no general agreement related to the use of functional outcome tools in evaluating TKA [1].

Although the majority of patients report substantial gains in functional outcomes following primary TKA, the degree of improvement varies widely. In order to evaluate the potential role of preoperative pain due to other musculoskeletal conditions on postoperative functional outcomes, authors have attempted to quantify bilateral knee and low back pain before primary TKA and evaluate its effect on postoperative physical functional outcome. They concluded that the degree of functional improvement depends on the burden of musculoskeletal pain in other weight bearing joints [2–4].

There are several assessment tools which can be used in order to evaluate TKA functional outcomes. The majority of these functional scales are not patient oriented. Usually, there is a person (medical personnel), other than the patient, who performs the test by clinical examination alone or by a combination of clinical examination and the administration of a questionnaire of physical activities. However, patient satisfaction scores do not always correlate with clinical functional parameters recorded by the medical personnel [5, 6].

Mahomed et al. [7] developed a validated, self-administered satisfaction scale (very satisfied, somewhat satisfied, somewhat dissatisfied, very dissatisfied) which assesses overall satisfaction in terms of pain relief and the ability to perform daily and leisure activities. Wylde et al. [8] utilized this satisfaction scale in a comparison study of fixed versus mobile bearing TKAs (250 knees). While the authors found no differences in satisfaction between the two types of implants, they did note surprisingly low satisfaction rates for specific activities (66 % “very satisfied” for pain relief, 52 % for return to normal activities of daily living and 44 % for the ability to perform leisure activities) [8].

Thomsen et al. [9] investigated whether the achievement of a higher degree of knee flexion after TKA would result in a better patient perceived outcome. High flex compared to non high flex TKA showed increased knee flexion, but no significant differences were found in patient perceived outcomes. It was suggested that improved knee flexion (more than 110°) has little relevance for the patients due to the fact that pain free range of motion and high patient satisfaction were achieved with both types of TKA’s [9]. Boese et al. [10] have also investigated contemporary high flexion TKA knee designs which claim to provide more than 120° of flexion. Although a high degree of flexion is necessary for some activities of daily living there were no significant mid-term improvements in terms of function, patients’ overall satisfaction, flexion gained or lost, and the need for further surgery [10]. Chang et al. [11] assessed alterations in physical activity profiles of Korean patients after TKA and tried to determine whether postoperative physical activity level is influenced by patient socio-demographic factors and postoperative functional outcomes. They conclude that regular participation in physical activity should be encouraged to improve patient satisfaction [11].

Outcome measures used to evaluate an intervention, such as TKA, should be valid (measure the proper outcome), reproducible (the same value should be obtained on repeated assessments of a stable patient), and responsive to changes in a patient’s condition [12]. For assessing TKA, validated outcomes tools include those that are related to general health (SF-36, SF-12, Nottingham Health Profile, Sickness Impact Profile, & EuroQol), disease specific (WOMAC, Oxford Knee Score) and patient specific (MACTAR). A number of tests are available to assess functional outcomes after TKA. For functional capacity, in order to assess patients undergoing TKA, the 6 min walk and 30-s stair climb are commonly used. Other functional capacity tools include the KOOS, which is based on the WOMAC score but it has been expanded to include the outcomes of pain, activities of daily living, sport and recreation function, and knee related quality of life. Other functional outcomes of interest include the International Knee Documentation, the Lower Extremity Functional Scale, and the UCLA activity level rating [1]. The short form – 36 Health Survey (SF-36) is a 36 item questionnaire which has been used extensively and validated as a measurement of general health status. It generates scores in eight dimensions, namely physical functioning, role limitation due to physical problems, role limitation due to emotional problems, social functioning, mental health, energy/vitality, bodily pain, and general health perception. Other similar assessment tools have the disadvantage that they have been used up to 2 years post operatively and the sustainability of these outcomes in the longer term remains unknown. However, there are published efforts to evaluate changes in the SF-36 over a period of 5 years and at the same time to validate the effects of age and gender on the scores [13]. The SF-36 has been criticized when applied on an individual basis, but its extensive use in outcome analysis and its proven validity and reliability make it useful for comparison between different conditions. Sample size and duration of follow-up are always important. A designated person (nurse, student etc) should be responsible for the administration and collection of the questionnaires, ensuring the consistency and completeness of the database. Details about health status and outcomes may be lost with a simpler questionnaire (e.g. SF-12). The Oxford knee score (OKS) is a validated and widely accepted disease specific patient reported outcome measure, but there is limited evidence regarding long term trends in the score. Williams et al. [14] reviewed 5,600 individual OKS questionnaires (1,547 patients) from a prospectively collected TKA database in order to determine the trends in OKS over a 10 year period following TKA. The maximum post operative OKS was observed at 2 years, following which a gradual but significant decline was observed over the 10 year assessment. A similar trend was observed for most of the individual OKS components. Kneeling ability initially improved in the first year but was then followed by rapid deterioration. Pain severity exhibited the greatest improvement, although residual pain was reported in over two thirds of patients post-operatively. Peak improvement in night pain component did not occur until year 4. Post operative OKS was lower in women younger than 60 years and in those with a BMI >35 [14].