The primary objective of this study was to determine patient reported outcomes and implant survivorship for the Persona knee implant system used in primary cemented total knee arthroplasty.
Within two years after surgery, one patient had a periprosthetic joint infection for which debridement was performed and the insert was exchanged, resulting in a survival estimate of 0.99. Although a survival estimate of 0.99 is good, we will need a longer follow up to draw definite conclusions on the performance of the Persona Knee System regarding the implant survival. Survival rates of total knee arthroplasties have improved in the last decades; the Swedish Knee arthroplasty Register showed an improvement in 10-year survival from 89% for TKAs performed during 1985 to 1994 to 96% during 2005 to 2014 . Furthermore, from the Norwegian Arthroplasty Register it was concluded that the 10-year survival had improved to 94% in the period 2005–2015.  The explanation for these increasing survival rates is multifactorial, with adjusted patient selection, improved implant designs and altered education.
The number of complications in this study is low, which is in accordance with literature [14, 15]. Pain after total knee replacement is a known problem; between 7 and 20% of all operated patients have persistent pain after TKA .
Patient reported outcome measures were promising in this study; all measures improved significantly up to one year after surgery. The OKS, one year after surgery in this study (41.1 points) is considerably higher when compared to other TKA cohorts [17,18,19]. Moreover, the results of the OKS in the current study are comparable to the results of the OKS of a partial knee arthroplasty [20, 21].
An increase by more than the minimal clinically important difference (MCID) is more important than a significant difference since it reflects the clinical relevance of the increase. According to the study of Monticone et al., the MCID of KOOS for patients who underwent a TKA is 16.7 for Pain, 10.7 for Symptoms, 18.4 for ADL, 12.5 for Sports and 15.6 for Quality of Life.  However, Collins et al. conclude in a review that a change of at least 20 for all subscales represents a true change in older patients.  The results of the present study show that after six months, all subscales had an increase of at least 20 points. The MCID of the OKS is 5 points according to Clement et al. (2014) and 9 points according to Beard et al. (2015), therefore the increase of the OKS in the current study from before surgery to one year after surgery can be considered clinically relevant. [24, 25] No further increase between one and two year follow up could be observed. This is in accordance with the study of Matharu et al. (2014), who stated that one year after a TKA no relevant changes in OKS can be seen.  Nilsdottir et al. (2009) concluded that PROMs concerning pain and physical functioning are best after 12 months and that there is a decline in outcome from 1 to 5 years after TKA, although results are significantly better compared to before surgery. In the current study no decline in PROMs from one to two years after surgery was observed.
KSS also improved by more than the MCID, 6 and 12 months after surgery. The MCID of the KSS is 34.5 points  and the mean improvement of KSS was 34.5 and 40.1 points, respectively.
Judge et al. (2012) considered a 11-point change or more on OKS, six months after surgery, to be associated with high patient satisfaction. Furthermore, a score of 30 points or more on the OKS was related to the highest level of patient satisfaction six months after surgery .
In the current study, 126 patients (86%) had a 11-point change or more on the OKS or a score of 30 points or more, six months after surgery, indicating that the majority of patients was highly satisfied with the results of the surgery.
Patient related outcome measures become increasingly important in defining success of TKA . According to Baker et al. , implant brand and hospital type were the only surgical factors influencing the improvement of PROMs after TKA, although the effect of these factors was small and not as pronounced as several patient factors.
Several limitations of this study need to be addressed. First, since this is a observational cohort study, comparison with other prosthesis is difficult. Second, although results of the current study are promising, the number of patients with two years follow up is relatively low as well as the years of follow up. Longer follow up and further studies are needed to determine long term survival and long term clinical performance of the Persona Knee implant.