The current data regarding gender are contradictory. The 2003 NIH consensus statement concludes gender was not strongly associated with short-term functional outcomes as assessed by the WOMAC, SF-36, or KSS [14]. In a large prospective study of 7,326 primary TKAs there were equal pain relief and walking improvements for both sexes during a 5-year follow-up period [50]. In four prospective studies of less than 300 patients each, gender was not associated with any difference in prevalence of substantial pain up to six [51–53] and 12 months post TKA [35]. On the contrary, a multinational randomized study of 860 TKA patients reported worse pain in women on the WOMAC pain scale at 1 and 2 years post TKA, but no age differences [54]. In a large retrospective observational study, women were 45 % more likely to report moderate to severe pain 2 years after primary TKA [55].

Data related to the effect of age on PROMs are also contradictory. While some studies report better outcomes in older patients [36, 55, 56], others have reported similar outcomes in all age groups [35, 51–53, 57] or poorer in older age [54, 58, 59]. It is unclear whether the potentially better outcome in older patients is due to less demand on the replaced joint (e.g. higher pain tolerance or less scar formation with less stiffness) or to different levels of expectations compared to younger patients. On the other hand, older patients are likely to have more co-morbidities and less room for improvement than younger people. Well selected elderly patients can derive as much benefit from TKA as younger recipients [60].

The correlation between knee osteoarthritis and obesity has been recognized for several years [61]. The negative effects of obesity following total joint arthroplasty, such as increased morbidity and mortality, have also been well documented in the literature. However, the association between obesity and outcome after TKA is disputed and little is known about whether specific body mass indices can be used as cut offs to determine which patients are most at risk for having a poor postoperative outcome [62]. Some studies indicate that obese individuals experience lower quality of life and performance after TKA [63–66]. In a large data set, including 1,011 primary TKAs, the effect of obesity was investigated in five groups of patients based on BMI 1 year postoperatively. It was concluded that the performance of the obese individuals quantified by the WOMAC and SF-36 outcomes tools was significantly lower. Additionally, 1-year follow up indicated that higher BMI negatively affected the ascending and descending capabilities of these individuals [63]. In a prospective study including 535 primary TKAs with a mean follow up of 9.2 years, the Hospital for Special Surgery (HSS) scores were significantly lower in obese individuals compared to peer-matched non-obese patients [64]. In another prospective study, 445 consecutive primary TKAs were followed up to 9 years. Clinical outcomes of non-obese (BMI <30) were compared to obese (BMI >30) patients. Significant improvements in outcome were seen and sustained in all groups 9 years after TKA. However, lower function scores were seen at all follow-up periods prior to 9 years in the highly obese subset with BMI >35 [65]. In a systematic review of the literature identifying 24 studies with a mean 5-year follow-up, the postoperative mean objective and function KSS were significantly lower in morbidly obese patients compared to non-obese. However, obese patients did not have significantly lower KSS (objective and function scores) compared to non-obese. Morbidly obese patients also had significantly lower implant survivorship than obese and non-obese patients [62]. There are, however, other investigations reporting contradictory findings with no significant differences observed between obese and non-obese individuals in regard to the outcome of TKA [67–70]. They report that body weight did not adversely influence the outcome of TKA in the short term [68, 69] or in the long term although there was a trend for obesity to influence the rate of aseptic loosening [67, 70]. The degree of functional improvement following TKA in the obese population remains controversial. It appears that obese patients have similar satisfaction rates as the non-obese population following total joint arthroplasty. As BMI increases (>40), however, the functional improvement becomes less and/or occurs more gradually and is tempered by the associated increased complication profile [71].

Medical factors that are highly predictive of a poor outcome after TKA include a greater number of co-morbidities and a worse pre-operative status i.e. high pain and disability [72]. Depressive symptoms are also significant predictors of both pain and functional limitations after TKA [35, 73]. Low self-efficacy is related to higher intensity pain in arthritis [74], and expectations for complete pain relief after TKA exert a strong influence on achieving better function and less pain postoperatively [75]. Physical and psychological issues may influence the success of TKA, and understanding patient differences could facilitate the decision making process before, during, and after surgery, thereby achieving the greatest benefit from TKA [14].

Despite the increased success of TKA, questions have arisen regarding the materials and implant designs that are most effective for specific patient populations. Implant design has evolved over time, with improvement in success rates [14]. A number of knee prosthesis designs are on the market today, however their relative merits are generally unclear. The mobile bearing compared to the fixed bearing design has the theoretical advantages of decreased wear and improved kinematics, which should result in an improvement in functional outcome and a decrease in long-term failure rate. The main theoretical disadvantage is instability and dislocation of the bearing [76]. According to many RCTs, meta-analysis and systemic reviews, no statistically significant differences could be identified in any of the PROMs between patients who received the fixed-bearing or the mobile-bearing knee post-operatively [77–80]. The polyethylene components of modern prosthetic designs appear to be quite durable [14]. Another common variation is the design of the tibial component. Use of a metal-backed base plate has theoretical advantages in that it distributes load more evenly across the bone implant interface than an all polyethylene tibia, and thus should decrease the risk of loosening. Limited comparisons between non-metal backed and metal backed components have been performed, and no definitive difference has been reported [81, 82]. There is considerable variability in patellar resurfacing; many surgeons routinely use it while others choose patellar retention. Although there is no clear evidence as to which approach is best, the role of soft tissue balancing and patella-friendly prosthetic designs is recognized.

Technical factors in performing TKA surgery may influence both short and long term clinical outcomes. Navigation systems have been developed to improve the accuracy of alignment of the components in TKA. Proper alignment of the prosthesis appears to be critical in minimizing long term wear, risk of osteolysis and loosening of the prosthesis. Computer navigation may eventually reduce the risk of substantial malalignment and improve soft tissue balance and patellar tracking [83, 84]. However, the technology is expensive, increases operating room time, and the benefits on PROMs remain unclear [14]. Minimally invasive surgery (MIS) is widely promoted as a possible improvement over conventional TKA. It allows for faster recovery time, less pain, less need for assistive devices, better knee flexion during the early post-operative period, and improvements in function [85, 86]. However, concerns have risen over potentially increased complications associated with delayed wound healing and infections, and the learning curve required for successful accomplishment of MIS techniques [87]. Poor visualization during surgery could also affect long-term outcomes (e.g. component malalignment) [85, 86, 88]. A systematic review of RCTs comparing MIS and conventional TKA found that MIS resulted in longer operating times, early improvements in KSS (6 and 12 weeks, but not after 6 months), early improvements in knee range of motion (6 days after TKA), and a greater incidence of delayed wound healing and infection, although no greater incidence in overall complications and component malalignment [85]. Again, the benefits on PROMs remain unclear. A meta-analysis of RCTs comparing clinical and radiological outcomes following MIS and conventional TKA found that knee flexion was significantly greater following MIS and that there were trends for statistically significant improvements in quadriceps muscle strength during early follow-up, but not at later follow-up [86]. In a systematic review of the published literature on MIS TKA, patients tended to have decreased postoperative pain, rapid recovery of quadriceps function, reduced blood loss, improved range of motion (mostly reported as a short-term gain) and shorter hospital stay compared with patients undergoing standard TKA. However, an increased tourniquet time and increased incidence of component malalignment in the MIS TKA groups is also reported [88].

Patient satisfaction with the outcome of TKA is becoming increasingly used as a measure of the patient’s perception of TKA success and it has been recognized as an important measure of outcome because there is a well- documented discrepancy between clinician and patient ratings of health status [93–98]. On the other hand, the higher rates of success for this procedure have led younger and more active patients to undergo TKA, expecting to be more active and pain free after surgery. Therefore, it is important to evaluate patient satisfaction after TKA in more detail, although we do not have a gold standard method to measure it by. While the majority of patients are satisfied with the outcome following TKA, and show good improvement in function afterwards, some are dissatisfied with the outcome. Despite on-going advances in primary TKA patient selection, in implant design and surgical techniques, the rate of dissatisfaction following TKA ranges from 5.5 % to 19 % with patients citing either a lack of pain relief or lack of functional improvement [93, 99–106]. The Swedish Arthroplasty Registry has reported that 2–17 years after TKA, 81 % of the 25,000 patients were satisfied with the outcome of their surgery, 8 % were not satisfied and 11 % undecided [99]. The National Joint Registry for England and Wales found that 82 % of patients were satisfied at just over 1 year after primary TKA [106]. Other studies have found that 14–19 % of patients were dissatisfied with the outcome 1 year after primary TKA [14, 93, 100, 107]. Satisfaction with the outcome of TKA can vary depending on the domain being assessed. For example, in a cohort of 407 patients (523 knees), 73 % of patients were very satisfied with pain relief, but only 50 % of patients were very satisfied with their ability to perform leisure activities 10 years after TKA [41]. In another cohort of 1,703 primary TKA patients, satisfaction with pain relief varied from 72 % to 86 % and with function from 70 % to 84 % for specific activities of daily living 1 year after surgery [93]. In a small prospective study (n = 112) with 7 years of follow-up, 86 % of patients were satisfied with TKA, 80 % would undergo the operation again, and 56 % did regular physical activity and had better WOMAC pain and functional scores [108]. Determining which factors affect patient satisfaction after TKA is a very important clinical issue and depends on many factors, including pain relief and functional ability achieved postoperatively, the fulfilment of pre- surgical expectations and mental wellbeing [100, 109]. Achievement of pain relief and functional status are the most important predictors of satisfaction [100–111]. While pain relief is very relevant to patient satisfaction, it is not the sole driver. It is quite possible for patients to report good levels of pain relief and overall dissatisfaction or vice versa. Marrying of expectations and resultant perception of outcome has been suggested as a model for understanding satisfaction response. The failure to meet optimistic expectations is associated with dissatisfaction following joint arthroplasty, with the expectations of kneeling, squatting and ease of climbing stairs amongst the least frequently met expectations [100, 111].