Osteoarthritis (OA) is a degenerative disease characterized by loss and degradation of articular cartilage in addition to synovial inflammation, leading to joint stiffness, swelling, pain and loss of mobility.1 It is influenced by genetic and epigenetic factors,2,3 sex,4 ethnicity,5 age,1 obesity,4,6 sedentary lifestyle7 and joint injuries.1 For example, obesity increases joint loading1,4,6 and inflamatory mediators,1 which helps to explain its relationship with OA. In addition, sedentary lifestly increases the risk of having metabolic syndrome, type 2 diabetes and insulin resistance, which are also related to the development of the disease.8 According to epidemiological studies, the knee is the most frequently affected joint by OA, with prevalence ranging from 164 to 250 cases per 100,000 in the general population.9,10 This may be explained by the higher knee vulnerability to direct (knocked) and indirect (twisted) trauma,11 along with the high load supported by this joint.

Sports participation might be beneficial for knee joint cartilage and consequently protect against the occurrence of knee OA in former athletes. Jones et al.12 demonstrated that children who participated in more vigorous sports gained twice as much tibial articular cartilage as those who participated in less active sports, suggesting that articular cartilage positively responds to increased loads. Although studies with adults did not demonstrate effects of sports participation in cartilage thickness,13 most athletes start participating in sports in childhood or adolescence,14 which may act as a protective factor for knee OA later in life. It should be noted that sports participation also reduces body fat mass,15 a known risk factor for OA,9 prevents the loss of cartilage proteoglicans,16,17 which are responsible for cartilage's viscoelastic properties,18 and increases muscle mass and strength,19,20 which might reduce loading on joint passive components (e.g. cartilage).

On the other hand, athletes are under high physical demands during many years, which increases their susceptibility to knee joint injuries.21,22 Chen et al.21 demonstrated that the knee was the joint most frequently affected by sports injuries, with 58.8% of athletes having some type of knee injury. This may occur because the knee is particularly vulnerable to direct and indirect trauma.11 Knee injuries produce a local inflammation associated with degradation of the articular cartilage.23,24 Moreover, athletes are more frequently submitted to knee surgeries25–30 and often become overweight after retirement, which are also risk factors for knee OA.31 Therefore, considering that the knee injuries are common in athletes, former athletes might actually have a high prevalence of knee OA.

Previous reviews32–35 on this topic did not focus on knee OA35 prevalence or presented important limitations, and thus, prevalence of knee OA in former athletes is still unclear. For instance, they restricted date of publication32–34 and language32–34 of included studies. The primary aim of this systematic review with meta-analysis was to investigate the prevalence of knee OA in former athletes. The secondary aim was to investigate whether poor methodological quality of included studies, different diagnosis criteria, type of sport or sex impact on overall estimated prevalence of knee OA in former athletes. We hypothesized that former athletes would present high prevalence of knee OA, and that methodological quality of included studies, different diagnosis criteria, type of sport and sex would influence on this prevalence. Knee OA causes disability and consequently increases health care costs.36 Therefore, the results of this systematic review may help to improve understanding about the relationship between sports practice and knee OA prevalence and also provide sports organizations and health care systems important data on a highly prevalent health condition in the general population.

The protocol of this review was prospectively registered at PROSPERO (CRD42016050903) and methods followed recommendations from the Joanna Briggs Institute Reviewers’ Manual (The Systematic Review of Prevalence and Incidence Data),37 the Cochrane Collaboration38 and PRISMA reporting guidelines.39

Data extraction and study rating

Two independent reviewers extracted data (BAS and FOM), and disagreements were solved by consulting a third reviewer (RAR). Extracted data included study design, setting, study population, OA definition, diagnostic criteria and prevalence of knee OA. For prevalence, we extracted percentage and/or number of knee OA cases. As most of the studies did not clearly state in which of the knee joints the OA prevalence was investigated (i.e. tibiofemoral, patellofemoral or both joints), we decided to report prevalence of OA in both joints altogether. In addition, when studies reported prevalence of OA in different knee joints (i.e. tibiofemoral or patellofemoral joints), different lower limb sides, or using different diagnostic criteria, the higher prevalence data were computed and reported. Finally, when studies reported prevalence in different sport modalities, we computed and reported the general prevalence for that study.

Two independent reviewers (BAS and FOM) also assessed risk of bias for each included study using “The Joanna Briggs Institute Prevalence Critical Appraisal Tool”.37 Each item was rated as ‘yes’, ‘no’ or ‘unclear’ according to information available in each study, with a maximum score of 10 points. To evaluate if sample size of each included study was appropriate, we used the following equation:

where p was the expected prevalence (19.0%), defined based on a previous study,40Z was the confidence level (1.96) and d was the precision (5.0%).41 Sample size estimation resulted in an appropriate sample size of 236 participants. A third reviewer (RAR) solved potential disagreements regarding risk of bias scoring.

This is the first systematic review with meta-analysis primarily aiming to investigate prevalence of knee OA in former athletes. Prevalence of knee OA was 30.0% (95% CI 20.0 to 40.0%). Current low-quality evidence suggests that estimated prevalence is likely to change with future high-quality studies.

Previous studies showed that the prevalence of knee OA in general population ranged from 19.0 to 28.0%.40,55 Thus, the prevalence of knee OA in former athletes may be higher than in general population, which may be related to higher frequency of knee injuries21,22,40,59–62 and increased knee loading56,58,59,61–64 during sports participation. Knee injuries, such as fractures, ligament ruptures and meniscal tears are strong risk factors for the development of knee OA.9,55,65,66 According to Kujala et al.,55 the risk of developing knee OA is almost five times higher in high-level male athletes with meniscal injuries, knee sprains or ligament ruptures than those without previous injuries. This finding may be explained by the articular cartilage degradation that follows joint injuries.1,62 During the degradation process, the chondrocytes are not able to repair the cartilage damage, making this process continuous and persistent.23,24 In addition, trauma to the knee might cause kinematic changes that modify the cartilage region of load bearing.59 For example, previous studies have demonstrated that patients following meniscectomy and anterior cruciate ligament repair have rotational and translational changes of the tibiofemoral posture.67–69 These kinematic changes shift loading to cartilage regions that typically sustain lower loads,59–61 initiating a cascade of degenerative changes that leads to cartilage thinning.59 Finally, prevalence of knee OA in former athletes might also be explained by repetitive and increased loading associated with sports participation,55,70 which may overtake muscles capacity to protect the knee, resulting in joint degeneration.34,71

The 30.0% mean prevalence of knee OA in former athletes seems to be similar to the prevalence demonstrated by other populations at higher risk of developing knee OA. For example, a previous study31 estimated 27.0% of knee OA prevalence in obese individuals. Moreover, one study showed association of squatting with increased prevalence of knee OA.72 More specifically, the prevalence of knee OA ranged from 30.2 to 37.5% in individuals who spend more than 60min squatting per day. Thus, the prevalence of knee OA in former athletes is comparable to that presented by populations that present risk factors for knee OA, such as increased body fat tissue and knee loading.

Sensitivity analyses suggested that methodological quality of the included studies and sex do not impact on the prevalence of knee OA in former athletes. However, analyses suggested that diagnosis using imaging may overestimate the prevalence of knee OA (40.0%; 95% CI: 20.0 to 60.0%) when compared to self-reported (30.0%; 95% CI: 20.0 to 30.0%) and clinical diagnosis (10.0%; 95% CI: 10.0 to 30.0%). It is possible that radiographic signs of knee OA are more prevalent than its clinical symptoms, such as pain and stiffness.73 In addition, radiographic signs may precede the clinical onset of knee OA,58 mainly in athletes. More specifically, it is suggested that the protective effects of increased muscle strength and better joint proprioception58,71 developed during sports participation increase athletes’ pain threshold,58,70 which contributes to the late clinical diagnosis after the onset of radiographic knee OA in this population.70 Sensitivity analyses also suggested that type of sports impact on prevalence estimates. Specific demands, such as the abrupt changes of direction and increased knee loading related to sprinting, squatting, cutting and pivoting may explain differences in knee OA prevalence between different sports.34,54

Considering the high prevalence of knee OA in former athletes, sports organizations, teams and professionals should implement preventive strategies during and after athletes’ careers, which might help to reduce knee OA prevalence and its associated disabilities74 and socioeconomic impact.36 For example, prevention strategies such as neuromuscular training associated with warm-up,75 were able to reduce the occurrence of anterior cruciate ligament injuries.76,77 Olsen et al.76 showed that this program improves awareness and control of the knee during landing and spinning movements, reducing the occurrence of lower limbs injuries. Finally, previous studies have demonstrated that 78.0 to 80.0% of former soccer players are overweight,46,78 which increases the risk of developing knee OA.79 Therefore, educational programs related to healthy diet and maintenance of an active lifestyle may reduce weight gain after retirement and consequently reduce knee OA prevalence in former athletes.

Previous systematic reviews investigating the prevalence of knee OA in former athletes had methodological limitations, such as restricted date and language35 of publication, lack of protocol registration in PROSPERO and non-evaluation of the strength of the evidence.32–35 Moreover, no meta-analysis with the primary purpose of estimating the prevalence of knee OA in former athletes was conducted. The current systematic review with meta-analysis had its protocol prospectively registered at PROSPERO and the strength of the evidence evaluated using the GRADE system. The prevalence of knee OA found by the present study was higher than the prevalence demonstrated by one previous study34 and lower than two other studies.32,33 This may be explained by differences in the specific purposes and inclusion criteria between studies, which also help to explain the different number of studies included in each systematic review. Moreover, the procedures used to extract prevalence data were different. Moreover, some procedures used to evaluate quality of the included studies were different. For example, Driban et al.,34 unlike others reviews,32,33 assessed the quality of included studies using the Newcastle-Ottawa Scale (NOS). At this moment, considering the limitations demonstrated by previous systematic reviews, we believe that this review provides timely and relevant information on prevalence of knee OA in former athletes for researchers, clinicians, people with knee OA and policymakers.

Low-quality evidence suggests that future high quality studies with higher samples and clear and reliable inclusion criteria for knee OA are likely to impact on the estimated prevalence. The strength of the current evidence was downgraded due to imprecision and inconsistency.

This study had some potential limitations. It was not possible to define specific criteria to characterize former athletes, since most of the studies do not provide information regarding this construct. In order to solve this limitation, we only included studies that clearly stated that the sample was former athletes. Level of competition, OA onset and history of knee injury might influence on estimated prevalence of knee OA. These potential influences were not considered in the current study because most studies do not report them. Future studies should define specific criteria to operationalize former athletes, such as time of exposure to sport and time since retirement. In addition, they should also standardize the diagnostic criteria of knee OA. Level of competition, OA onset and history of knee injury may influence on estimated prevalence of knee OA. However, we did not consider these variables because most studies do not report them. Small number of studies was found, mainly investigating specific sports (e.g. former long distance runners, weight lifters and basketball, handball and table tennis players), which limited sensitivity analysis for some sports. Finally, most studies had small sample sizes30,32,34–37,39,41,44 and there is a lack of studies investigating former female athletes. Future studies including this population are warranted.

This systematic review with meta-analysis showed that the prevalence of knee OA in former athletes was 30.0% (95% CI 20.0 to 40.0%). Current low-quality evidence suggests that prevalence is likely to change with future high-quality studies. Increased knee loading and risk of injuries during sports participation might explain the high prevalence of knee OA in former athletes. Diagnostic criteria for knee OA and sport modality may impact on estimated prevalence. The high prevalence of knee OA in former athletes challenges researches, clinicians and policymakers to prevent and treat this health condition. It is possible that different strategies may reduce the knee OA prevalence and associated disabilities in former athletes.

The authors declare no conflicts of interest.