The Risk of Knee Osteoarthritis in Professional Soccer Players: A Systematic Review with Meta-Analyses
A systematic review with meta-analyses
Background: We address the question whether professional soccer players with and without macroinjury of the knee joint are at an elevated risk for knee osteoarthritis.
Methods: A systematic review with meta-analyses was conducted. The study protocol was prospectively registered (registration number CRD42019137139). The MEDLINE, EMBASE, and Web of Science databases were searched for relevant publications; in addition, forward searching was performed, and the listed references were considered. All steps of the process were undertaken independently by two reviewers, and any discordances were resolved by consensus. For all publications whose full text was included, the methods used were critically evaluated. The quality of the evidence was judged using the GRADE criteria.
Results: The pooled odds ratio for objectively ascertained osteoarthrosis of the knee was 2.25 (95% confidence interval [1.41–3.61], I2 = 71%). When only radiologically ascertained knee osteoarthrosis was considered, the odds ratio was 3.98 [1.34; 11.83], I2 = 58%). The pooled risk estimator in studies in which knee joint macroinjury was excluded was 2.81 ([1.25; 6.32], I2 = 71%).
Conclusions: A marked association was found between soccer playing and knee osteoarthritis in male professional soccer players. For female professional soccer players, the risk of knee osteoarthritis could not be assessed because of the lack of data. Knee injuries seem to play an important role in the development of knee osteoarthritis in professional soccer players.
Professional soccer players run a total distance of up to 10 km or even more during a game, with frequent abrupt stops and accelerations. Their knees are therefore subject to high levels of stress (e1, e2, e3). They are exposed to elevated risks of injury in the region of the knee joint (1, 2, e4), either macrotrauma (particularly ruptures of the cruciate ligaments or meniscus and fractures involving the knee joint) or microtrauma (as a consequence of sprains and contusions). For a professional soccer player, macrotrauma of the knee joint represents an occupational injury with benefit entitlement. If knee joint trauma in a professional soccer player leads to secondary osteoarthritis of the knee, this can be officially recognized as the consequence of an occupational injury (e5). However, primary osteoarthritis of the knee—in contrast to meniscopathy (coded OD no. 2102 in the German occupational diseases ordinance)—is not so far recognized as an occupational illness.
For the purposes of this publication, professional soccer players are those male and female players who earn their living from soccer. In Germany, soccer players who are paid, as opposed to playing as a hobby, have to be registered with statutory accident insurance funds by their clubs. One of the criteria is that the payments, including benefits in kind, have exceeded € 200 in each month of the contract period and amounted to at least € 8.50 per hour before deductions (e6).
A number of systematic reviews of the risk of knee osteoarthritis in professional soccer players have been conducted (3, 4, 5, 6, 7, 8, 9, 10), but they do not meet important quality criteria (e7, e8, e9) such as publication of the study protocol (3, 4, 5, 6, 7, 8, 9, 10), double study selection (3, 7, 9), double data extraction (3, 4, 5, 6, 7, 8, 9), assessment of study quality (3, 9), and disclosure of conflicts of interest (5, 8). Furthermore, some of the reviews explored the prevalence of knee osteoarthritis in professional soccer players alone, with no comparison to another group (4, 6, 8, 10). Moreover, the reviews failed to include central primary studies and did not consider the role of knee macrotrauma when evaluating the risk of osteoarthritis (4, 6, 7, 8, 9). The present systematic review is intended to help close this gap in our knowledge. The following research questions were posed:
- Research question 1: Are professional soccer players at higher risk of developing osteoarthritis of the knee?
- Research question 1a: Are professional soccer players with no macrotrauma of the knee joint at higher risk of developing osteoarthritis of the knee?
A systematic review was conducted following the PRISMA criteria (e10) (PROSPERO registration number: CRD42019137139 ). The population was defined as “male and female professional soccer players,” the exposure as “playing soccer,” the outcome as “knee osteoarthritis,” and the relevant study types as cohort studies, case–control studies, and cross-sectional studies (e11). A comprehensive literature search was conducted. Two reviewers independently carried out all steps of the process. Critical assessment of the methods comprised a risk of bias procedure (12, 13). Meta-analyses of the study results were performed (using random effects models and the heterogeneity measure I2 [e12]). The GRADE criteria (14) were utilized to assess the overall quality of the evidence, using an adapted version of the Navigation Guide for epidemiological observational studies (15, 16). Details of the methods can be found in eTable 7 (eMethods).
Results of the literature search
The results of the literature search are depicted in a flow chart (eFigure 1). The database search revealed 15 450 records, and supplementary searches found 3208 records. After removal of duplicates, 12 951 references were screened. Of the 39 full texts screened, 30 were excluded on various grounds (16, 17, 18, 19, 20, 21, 22, 23, 24, e13–e33; eTable 4).
Nine studies—a retrospective cohort study (26), three case–control studies (26−28), and five cross-sectional studies (1, 29, 30, 31, 32)—fulfilled the inclusion criteria of the systematic review. All of them had been carried out in Europe. Professional soccer players were included in six of these studies (1, 25, 29, 30, 31, 32). Three studies concerned themselves with professional and amateur soccer (26, 27, 28), and in one of these specific consideration of professional soccer was possible (27). Only in one study was exposure to soccer classified according to the total number of hours played (26). With the exception of one study in which the comparison group was made up of sports shooters (31), all studies drew their comparison groups from the population. Seven of the nine studies investigated only men, while two included women as well (27, 28). However, only effect estimators for men could be included in the meta-analyses. Operationalization of the outcome of knee osteoarthritis ensued via radiographs (1, 25, 28, 30, 31), knee joint replacement (27), hospital admission (25), or self-reporting of diagnosis of knee osteoarthritis by a physician or performance of joint replacement (29, 32). The characteristics of the individual studies are shown in eTable 5.
Results of the critical assessment of methods
Three studies (25, 26, 28) were found to have a low, four (29, 30, 31, 32) a high overall risk of bias. In two studies, different outcome-specific classifications of bias domains 4 and 5 led to two different assessments of the overall risk (1, 27). Information on the individual studies can be found in eTable 6.
The results of the individual studies can be found in eTable 7, those of the meta-analyses in the Table.
Research question 1
The prevalence of radiographically confirmed knee osteoarthritis in the tibiofemoral joint—graded ≥ 2 according to Kellgren and Lawrence (1957) (osteophytes, joint space narrowing, or joint deformity) (e34)—was 22.0% and 25.8% in professional soccer players, against 0% and 12.9% in the respective comparison groups (30, 31). In another study, joint space narrowing of the tibiofemoral joint as defined by Ahlbäck (e35) was found in 15.5% of the professional soccer players and in 2.8% of the members of the comparison group (1). The studies do not report prevalence separately for the medial and lateral portions of the tibiofemoral joint. One study described the rates of Kellgren and Lawrence grade ≥ 2 knee osteoarthritis of the patellofemoral joint as 16.1% in professional soccer players and 3.4% in the comparison group (31).
In the majority of the studies (n = 7 of 9), the relative risk estimators for the association between soccer and knee osteoarthritis were, at 1.5 to 12.3, statistically significantly elevated (1, 26, 27, 28, 29, 31, 32).
The pooled odds ratio (OR) for objectively confirmed osteoarthritis of the knee is 2.25 (95% confidence interval [1.41; 3.61], I2 = 71%, n = 6) (Figure 1) (1, 25, 27, 29, 30, 32). Looking only at radiographically confirmed osteoarthritis, the OR is 3.98 ([1.34; 11.83], I2 = 58%, n = 3) (Figure 2) (1, 25, 26, 27, 28, 29, 30, 32). Additional inclusion of studies that did not differentiate between professional and amateur soccer changed the OR only slightly (OR = 2.02 [1.37; 2.97], I2 = 77%, n = 8) (eFigure 2) (1, 25, 26, 27, 28, 29, 30, 32). Restricting analysis to studies with a high overall risk of bias, the OR is 2.08 [1.20; 3.62], I2 = 79%, n = 4) (eFigure 3) (27, 29, 30, 32). Excluding the two studies in which the outcomes were self-reported yields an OR of 2.12 ([1.35; 3.34], I2 = 24%, n = 4) (eFigure 4) (1, 25, 27, 30).
Research question 1a
The pooled risk estimator in studies that excluded macrotrauma of the knee joint is 2.81 ([1.25; 6.32], I2 = 71%, n = 3) (Figure 3) (1, 27, 30), against 2.48 ([1.22; 5.04], I2 = 75%, n = 4) (Figure 4) (1, 25, 29, 32) in studies that did not exclude such injuries. The pooled OR after adjustment for knee joint injuries is, at 2.71 ([1.55; 4.74], I2 = 41%, n = 4), much lower than that without adjustment (4.02 [1.63; 9.92], I2 = 82%, n = 4]) (1, 29, 31, 32) (eFigures 5, 6).
Two studies showed a higher risk of knee osteoarthritis for professional than for amateur soccer players. In Roos et al. the risk was 2.73 [1.07; 6.98] for amateurs and 11.47 [4.57; 28.79] for professionals (1), while in Sandmark and Vingard it was 1.10 [0.77; 1.56] for amateurs and 1.86 [1.05; 3.19] for professionals (27).
The “doubling dose”
On the basis of a study that concerned itself principally with amateur soccer players (26), the doubling dose was around 2600 h with categorized analysis, while with non-linear analysis of the continuous data it was 5900 h (eMethods).
The quality of the evidence on the link between professional soccer and objectively confirmed osteoarthritis of the knee was rated as moderate in the GRADE classification (eTable 9, Figure 1). When macrotrauma was excluded, the quality of the evidence on the association between professional soccer and objectively confirmed osteoarthritis of the knee (research question 1a) was formally rated low (eTable 9, Figure 3). Adjustment for knee joint injuries (eFigure 6) revealed—with relatively high consistency of the studies analyzed—a pooled risk estimator similar to that found when excluding macrotrauma (Figure 3); this further supports the evidence for affirmation of research question 1a.
This systematic review found that male professional soccer players have a 2.3-fold risk of knee osteoarthritis compared with the male general population (OR 2.25). For radiographically confirmed osteoarthritis, the risk is even higher, at fourfold (OR 3.98). However, no differentiation can be made between the tibiofemoral joint and the patellofemoral joint. Injuries to the knee seem to play a large part in the development of osteoarthritis of the knee joint in professional soccer players. Even after exclusion of or adjustment for macrotrauma of the knee joint, the risk of knee osteoarthritis is still increased 2.7-fold (OR 2.81 and 2.71, respectively). Given this elevated risk, it must be assumed, despite the formally limited quality of the evidence, that professional soccer players have a distinctly increased risk of knee osteoarthritis.
Based on the findings of a study that predominantly investigated amateur soccer players, the doubling dose, i.e., the cumulative time spent playing soccer that doubles the risk of knee osteoarthritis, is around 2600 h in categorized analysis and 5900 h with non-linear analysis of the continuous data. Owing to the focus on amateurs, however, this doubling dose cannot be assumed to apply to professional soccer players.
Three earlier systematic reviews also found elevated pooled risk estimators for osteoarthritis of the knee in professional soccer players (3, 7, 9). The results of relevant primary studies that we excluded because they were insufficiently representative were very heterogeneous in respect of research question 1—in contrast to the overall consistency of the findings in the studies we included (16, 17, 18, 19, 20, 21, 22, 23, 24). With regard to the prevalence of knee osteoarthritis in soccer players, two previously published reviews reported higher levels (40–80%  and 14–80% ) than found in our work (3–29%).
The results of numerous uncontrolled cross-sectional studies point to the influence of injuries to the knee joint—a frequent occurrence in professional soccer players (e36)—on the later development of knee osteoarthritis in these athletes (33, 34, 35, 36, 37, 38, 39, 40, e37, e38). The knee joint injuries commonly experienced in professional soccer include anterior cruciate ligament ruptures, meniscus injuries, and lateral ligament ruptures (35, e39, e40, e41, e42). A number of systematic reviews show that programs specifically designed to prevent such injuries in professional and amateur soccer, such as FIFA 11+, have proved effective in reducing the risk of injury in randomized controlled trials (e43, e44, e45, e46, e47). FIFA 11+ is a 20-min training unit that comprises running, stretching, and strengthening exercises.
In one study, osteoarthritis of the knee was the most frequently occurring manifestation of osteoarthritis in retired male professional soccer players—followed by osteoarthritis of the ankle joint, hip joint, and spinal column (e48). Other studies have reported osteoarthritis in joints other than the knee in professional soccer players: the ankle joint (e49), the hip joint (e50, e51), and the cervical spine (e52, e53). Ex-soccer players with osteoarthritis have a poorer quality of life than those without osteoarthritis (e48, e54, e55).
Besides the above-mentioned injuries as risk factors for knee osteoarthritis in professional soccer, other sport-specific phenomena are also thought to constitute stress factors for the tibiofemoral and patellofemoral joints. These include running fast (e56, e57, e58, e59, e60, e61), stepping sideways when dribbling the ball around an opponent (e61, e62), shooting the ball (e63), and continual accelerations and abrupt stops (e3, e64). During a 90-min game of soccer, male players cover a mean distance of around 11 km, including 0.7–0.9 km running fast (20–25 km/h) and 0.2–0.3 km sprinting (> 25 km/h) (e1, e3). The distance covered per game depends on the player’s position: it is lowest for goalkeepers, followed by central defenders, strikers, left/right fullbacks, and midfielders (e3, e65, e66). Female professional soccer players cover a mean distance of around 10 km per game, including 2.5 km at high speed (12.2–19.0 km/h) and 0.6 km sprinting (> 19 km/h) (e64).
Only a small number of studies on knee osteoarthritis in female professional soccer players have been published, none of them fulfilling the criteria for inclusion in this systematic review (22, 36, 39, e38), although female players make up around 1 116 000 of the 7 132 000 members of the German Football Association (DFB) (e67). The only one of these studies that featured a control group found a fivefold risk for osteoarthritis of the knee in female soccer players (22). The prevalence of knee osteoarthritis in the non-controlled cross-sectional studies was in the range 13.8–51 % (36, 39, e38).
The methods of this systematic review
One strength of this systematic review is exclusion of studies that were insufficiently representative owing to the response being unreported or very low (< 10%) and/or use of convenience sampling, in order to avoid including studies affected by selection bias in the data evaluations and meta-analyses.
In the course of critical evaluation of the methods, body mass index was not included as a relevant confounder in the assessment of possible distortion of the study results by confounders, because it can occur not only as a confounder but also as an intermediate factor in the association between soccer and knee osteoarthritis.
Self-reporting of knee joint replacement was included as an outcome in the meta-analysis of objectively confirmed knee osteoarthritis, because recall bias is improbable.
The meta-analysis of radiographically confirmed knee osteoarthritis included one study in which the members of the comparison group were not probands drawn from population, but professional sports shooters (31). The corresponding risk estimator (12.30) was much higher than the risk estimators of the other studies included in this meta-analysis (1, 30) (OR 1.90 and 6.32), which may explain why the pooled risk estimator was higher for radiographically confirmed knee osteoarthritis (OR 3.98) than for objectively confirmed knee osteoarthritis (OR 2.25).
The review includes studies that investigated both amateur and professional soccer (14, 26, 27, 28) without differentiating between the two in their presentation of the results with regard to the occurrence of knee osteoarthritis. These results were excluded from the meta-analyses, which were restricted to risk estimators for professional soccer.
The findings of this systematic review show a clear association between soccer and the development of knee osteoarthritis. The evidence quality is moderate, and some aspects of the results display high heterogeneity. Even after exclusion of or adjustment for macrotrauma of the knee joint, the risk of knee osteoarthritis incurred by male soccer players was still 2.7 times higher (OR 2.81 and 2.71, respectively) than in probands drawn from the population. The data are too sparse to permit any conclusions on the risk of knee osteoarthritis in female soccer players.
This study was conducted on behalf of the German Federal Ministry of Labor and Social Affairs (BMAS) with expert assistance from the Federal Institute for Occupational Safety and Health (BAuA).
Conflict of interest statement
The authors declare that no conflict of interest exists.
Manuscript received on 26 February 2020, revised version accepted on 24 August 2020
Translated from the original German by David Roseveare
Dr. rer. medic. Alice Freiberg, MPH
Institut und Poliklinik für Arbeits- und Sozialmedizin
Medizinische Fakultät Carl Gustav Carus
Technische Universität Dresden
Fetscherstr. 74, 01307 Dresden, Germany
Cite this as:
Freiberg A, Bolm-Audorff U, Seidler A: The risk of knee osteoarthritis in professional soccer players—a systematic review with meta-analyses. Dtsch Arztebl Int 2021; 118: 49–55. DOI: 10.3238/arztebl.m2021.0007
For eReferences please refer to: www.aerzteblatt-international.de/ref0421
eMethods, eTables, eFigures: www.aerzteblatt-international.de/21m0049
Division of Occupational Health, Department of Occupational Safety and Environment, Regional Government of South Hesse, Wiesbaden: Prof. Dr. med. Ulrich Bolm-Audorff
Extraordinary Chair of Occupational Medicine, University of Gießen: Prof. Dr. med. Ulrich Bolm-Audorff
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