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The elastic properties of the Achilles tendon are important for locomotion.1–3 The tendon has a capacity to withstand loads up to 12.5 times the body weight,4 but nevertheless many athletes sustain Achilles tendon disorders.5–7 The cause of Achilles tendon disorder is unclear,8 but it is suggested that the essential lesion of tendinopathy is a failed healing response.9–12 Achilles tendinopathy often manifests with a gradual onset of pain during tendon load (especially 2–6 cm proximal to the tendon insertion),13 morning stiffness and sometimes a localised swelling.6 ,14
Achilles tendon disorders are frequently seen in sports including running and jumping.7 ,8 ,15 ,19 ,20 Achilles tendinopathy or an Achilles tendon rupture can be career altering or career ending for professional athletes,6 ,17 ,21 ,22 but only a few studies have focused on Achilles tendon disorders in professional football players.17 ,23 ,24 Woods et al25 did not report Achilles tendon disorders as a separate diagnosis, but when evaluating ankle injuries during two seasons in English professional football, they found that 1% of all injuries were classified as tendonitis or paratendonitis. Nearly one-third of these occurred during the preseason period.24
The present study investigated the incidence of Achilles tendon disorders and the risk of reinjury in a large population of elite male football players.
Materials and methods
Participants and study cohort
The present study was performed within the framework of the prospective injury study UEFA (Union of European Football Associations) initiated among male elite level football teams in 2001, the UCL (UEFA Champions League) study.26 This portion of the study included 27 European professional football teams from 10 countries. The teams were followed over a varying number of seasons (1–11 seasons) between February 2001 and December 2011 and consisted of 1743 players (in an average 15 teams per season). All contracted players in the first team were invited to participate in the study. Players who left the team during the season (eg, because of transfer) were included during their time on the team. Teams were followed during the full season, including the preseason and competitive season.
Data collection and study definitions
The study design followed the consensus on definitions and data collection procedures in studies of football injuries outlined in the consensus document27 and by UEFA28 for studies of football. The validation of the injury and exposure reporting system and definitions has been described previously.28 To ensure high reliability of data registration, all teams were provided with a study manual describing the definitions used and procedures to record data, including explanatory examples. To avoid language problems, the manual and the study forms were translated from English into five other languages: French, German, Italian, Russian and Spanish. In addition, the study group checked all reports each month, and feedback was sent to the teams correcting any missing or unclear data. The definitions applied in the study are shown in table 1.
At the start of each season, player baseline data were collected. Individual player participation in training and matches (minutes of exposure) was recorded by the club contact person on a standard exposure form sent to the study group on a monthly basis. This included exposure with the first and the second teams, as well as any national team exposure, for all players. The training content was not recorded. The team medical staff recorded all time-loss injuries on a standard injury form that was sent to the study group every month. The injury form provided information about the diagnosis, nature and circumstances of injury occurrence. All injuries resulting in the player being unable to fully participate in training or match play (ie, time-loss injuries) were recorded, and the player was considered injured until the team medical staff allowed full participation in training and availability for match selection. Injuries were categorised under 4° of severity based on the number of days of absence from training and/or match. All injuries were followed until the final day of rehabilitation.
The present study included all recorded Achilles tendon injuries, including Achilles tendinopathies, partial Achilles tendon ruptures and total Achilles tendon ruptures. The diagnosis was based on the clinical examination by the team medical staff, and no specific diagnostic criteria were set out in advance.
For the 48 severe injuries (>28 days lost) in 43 players, specific data about surgery and location of injuries were collected retrospectively from the team doctors and the club medical records.
Anthropometric data (age, height, body weight and body mass index (BMI)) are presented with mean±SD and analysed with independent sample t test. Lay-off time is presented with mean±SD and corresponding median and quartiles (Q1 (25th percentile) and Q3 (75th percentile)). Owing to a skewed distribution in the lay-off time, group differences were analysed using the Mann-Whitney U test. Pearson's χ2 test was used to analyse association between categorical variables. Injury rates are reported as the number of injuries per 1000 player-hours and injury burden was calculated as the number of lay-off days per 1000 player-hours. The rate ratio (RR) with 95% CI was used for group comparisons of injury rates and injury burden, and significance was tested using z statistics. Seasonal trend, expressed as the average annual percentage of change, was analysed using linear regression with log-transformed injury rates as the dependent variables. A 2-year moving average (MA) approach, by summarising two consecutive seasons, was also used to smooth out the large seasonal variation. The reinjury risk after the short (0–10 days) and long (>10 days) recovery periods was compared using the risk ratio with 95% CI, and significance was tested using z statistics. Season injury prevalence was calculated as the number of injured players during a season/total number of players in that season ×100. All tests were two-sided and the significance level was set at p<0.05.
The study design underwent an ethical review at Linköping University, Linköping, Sweden and was approved by the UEFA Football Development Division and the UEFA Medical Committee.
In total, 1 057 201 h of exposure (888 249 h of training and 168 952 h of match play) were recorded. Overall, 8029 injuries were documented during the study period, 203 (2.5%) of which were Achilles tendon disorders. One hundred and ninety-four (96%) disorders were classified as gradual onset tendinopathies, and nine (4%) were acute onset partial or total ruptures. The injury rate was almost 22 times higher in Achilles tendinopathies compared with Achilles tendon ruptures (0.18 vs 0.01 injuries/1000 h, RR 21.6, 95% CI 11.0 to 42.1; p<0.001). As the mean total exposure time for a team of 28 players is about 7000 h per season, a team can roughly expect one Achilles tendinopathy every season, and one Achilles tendon rupture every 17th season.
Between-season and within-season variation of injury rates
The injury rate of Achilles tendon disorders has fluctuated, between 0.12 and 0.35 injuries/1000 h, over the 11 seasons. The highest injury rate was found in the February 2001 season and the lowest notations were found in the 2006 and 2007 seasons. The crude injury rates and the ‘smoother’ 2-year MA injury rates are illustrated in figure 1. The MA approach indicated an average annual decrease of 1.8% in the injury rate. The fitted regression model, however, indicated a non-significant seasonal trend in the log-transformed injury rates (R2=0.23, b=−0.056, 95% CI −0.131 to 0.020; p=0.132).
The within-season variation, measured as quarterly injury rates, are shown in figure 2 and table 2. A higher injury rate was found during the preseason (July–August) compared with the competitive season (September–May), 0.25 vs 0.18 injuries/1000 h (RR 1.4, 95% CI 1.1 to 2.0; p=0.027).
Injury severity and consequences
The severity of injuries and consequences in the form of lay-off time and injury burden are shown in table 3. The mean lay-off time for Achilles tendinopathies was 23±37 (median=10, Q1=4 and Q3=24) days, while a rupture of the Achilles tendon, on average, caused an almost seven times longer lay-off time, with 161±65 (median=169, Q1=110 and Q3=189) days of absence. Achilles tendon disorders account for 3.8% of the total lay-off time reported in the study. The injury burden was approximately three times higher in Achilles tendinopathies compared with Achilles tendon ruptures (4.24 vs 1.37 days lost/1000 h, RR 3.1, 95% CI 2.9 to 3.3; p<0.001).
Twenty-seven per cent of all Achilles tendinopathies were reinjuries (table 3). No early recurrences were seen among the nine Achilles tendon ruptures. The reported recurrence rate of all injuries is 12% in the cohort. A higher reinjury risk was found after short (0–10 days) recovery periods (31%) compared with longer (>10 days) recovery periods (13%), (risk ratio 2.4, 95% CI 2.1 to 2.8; p<0.001).
Players with Achilles tendon disorders were significantly older than the rest of the cohort, with a mean age of 27.2±4 years vs 25.6±4.6 years (p<0.001). No differences could be found in height (182.1±6.2 cm vs 182.3±6.4 cm, p=0.607), body weight (78.4±7.1 kg vs 78±7.1 kg, p=0.473) or BMI (23.6±1.3 vs 23.4±1.4, p=0.092).
Location and treatment of severe injuries
Information about surgery and location was provided for 34 of the 39 severe (>28 days lost) Achilles tendinopathies and information about surgery was received for eight of the nine severe Achilles tendon ruptures.
Twenty-three per cent of the proximal Achilles tendinopathies were operated on (3/13), two of them with resection of the plantaris tendon. Forty-five per cent (9/20) of the insertional tendinopathies were operated on. One player had surgery after problems both in the main body and at the insertion. The mean lay-off time for tendinopathy of the main body was 66±49 (median=43, Q1=35 and Q3=78) days, and for insertional tendinopathies 79±50 (median=61, Q1=41 and Q3=109) days (p=0.320). Surgery was performed on all seven total ruptures. The partial rupture, from which we received information about treatment, was not operated on.
A team of 28 players can roughly expect one Achilles tendon disorder every season, and older players are more frequently affected. Most absence periods for Achilles tendinopathies were short (median of 10 days), but the recurrence rate was high (27%). There was a significantly higher risk of sustaining a reinjury if the player rested for less than 10 days. Finally, Achilles tendinopathy was more common during the preseason period.
Achilles tendinopathies are common overuse injuries—Achilles tendon ruptures are rare
The season prevalence of Achilles tendinopathies in this study was 2.1–5.1%. Woods et al25 did not report Achilles tendon disorders as a separate diagnosis, but when evaluating ankle injuries during two seasons in English professional football, they found that 1% of all injuries were classified as tendonitis or paratendonitis. Fredberg et al17 reported that 15% of the players reported time loss attributable to Achilles tendinopathy in Danish professional football players. The difference in injury prevalence may reflect a true difference in injury prevalence or differences in study methodology, reporting thresholds or varying treatment regiments (ie, to let players with symptoms carry on playing or take them out of training). Achilles tendon ruptures were rare at 0.01 injuries/1000 h of exposure.
Higher risk of Achilles tendinopathy during preseason
The incidence of Achilles tendon disorders was significantly higher during the preseason. All the teams in this cohort followed an autumn–spring season with preseason training in July–August, and a competitive season between September and May. One team had a longer break between December and February and contributed half the injuries in January, but the injury incidence was still significantly higher in the preseason.
More overuse, especially tendon injuries, occurs during the preseason period.24 ,29–31 Woods et al24 reported that 32% of all Achilles tendon disorders occurred during the preseason, but this study did not account for exposure, so the high number of injuries could just reflect more training sessions. The intense preseason follows a period of rest. These sudden changes in intensity, training load and training type are suggested to be risk factors for overuse injuries such as Achilles tendinopathies,6 ,7 ,14 ,20 ,24 ,32 because the Achilles tendon may not be able to adapt fast enough to changes in loading pattern.14
Varying absence periods for Achilles tendinopathy
The absence period for Achilles tendinopathy in this study varied, with a median of 10 days and an average of 23 days. Both Sankey et al18 in rugby (mean=30 days) and Oztekin et al23 in Turkish professional football (median=30 days) reported longer absences. This may reflect differences in methodology and in the demands of the sport.
The variations in lay-off time can be explained by the nature of the injury. Achilles tendinopathy often manifests with a gradual onset of pain,6 ,13 ,14 and it can be difficult to decide if and when a player should rest. Many players continue to participate in training even with symptoms of Achilles tendinopathy,33 ,34 especially if they want to play an important match.35 This affects the rehabilitation time needed. The player's importance to the team can also explain why some players rest less than others.
Achilles tendon ruptures cause long absences from football
The median absence period for the Achilles tendon rupture in this study was 169 days (mean=161). Both professional rugby players (mean=185.5 days)18 and professional Turkish football players (median and mean=180 days)23 reported longer absences. This may result from differences in methodology and study size, or in rugby, differences between sports. Also, with only nine ruptures in 11 years, the average is vulnerable to outliers.
Six of the nine players sustaining ruptures of the tendon had been followed in our study before the season when the rupture occurred, and none of these players had any Achilles disorders resulting in time loss before their rupture. Symptoms before ruptures are reported to vary between 10% and 25%.19 ,36–38 Since degenerative changes are found in almost all ruptured tendons,38 it seems plausible that tendons undergo changes during an asymptomatic period of several months, where the tendon might be injured but the player does not experience symptoms, before the pathology results in pain and the changes become symptomatic.17 ,38 Therefore, some tendons might rupture before signs of Achilles tendinopathy occur.38
Short absence time increases the risk of reinjury
The high recurrence rate for Achilles tendinopathy found in this study (27%) reflects the chronic and recurrent character of this condition. The frequent relapse of symptoms when players return to football after a short rehabilitation period could be explained if pain is only the tip of the iceberg. Even if pain recedes to below the threshold, the pathological changes in the tendon could still exist and could therefore become symptomatic when the player resumes his activity level.14 Players who abstained from training and matches for less than 10 days at their index injury experienced a significantly higher risk of reinjury than players who rested for more than 10 days (p<0.001). Inadequate rehabilitation, with a premature return to activity, is a risk factor for reinjury.30 ,39 ,40 Therefore, it could be suggested that a longer rehabilitation period at the first signs of Achilles tendinopathy could be beneficial to avoid recurrences. A functional test before returning to play might also reduce recurrences.41
There is little evidence on how to prevent these recurrences. Fredberg et al17 showed that prophylactic eccentric and stretching exercises did not reduce the frequency of ultrasonographic abnormalities in Achilles tendons in professional football players. On the other hand, the lack of effect might be because the training was not intense enough (2.25 times/week).17
Older players sustain Achilles tendinopathy
The players who sustained an Achilles tendon disorder were significantly older than the rest of the cohort (27.2 vs 25.6 years), as reported in other studies.6 ,22 The incidences of muscle calf injuries increase with age42 and, if the calf muscle is more exposed to injury, this could affect the risk of injury in the Achilles tendon as well. Further, it is well established that prolonged, strenuous endurance exercise induces muscle damage and impairs muscle function.43 Normally, the muscle tendon unit is able to repair and adapt, but there might be a limit to the regenerative capacity and adaptability, resulting in pathology.44
If the calf muscles of older players theoretically have impaired function, there is a risk that the muscles become more easily fatigued. Fatigued muscles lose their ability to absorb shock,6 which theoretically can put a greater stress on the Achilles tendon.
Treatment of the severe injuries
In accordance with other studies, all the total Achilles tendon ruptures in this study were managed surgically.18 ,23 Many Achilles tendinopathies can be treated conservatively, but around 25% require surgery.20 ,45 Of the severe (>28 days lost) tendinopathies in the present study, 38% were operated on.
Most severe tendinopathies in this study were located at the insertion (20 of the 34 we had information about). The insertional lesions also required surgery more often (45% vs 23%) and caused longer absences from football than the proximal lesions (mean=79 vs 66 days).
The prospective design and the 11-season study period are the strengths of the present study, together with the large and homogeneous sample of professional footballers.
However, this study also has some limitations. First, no treatment details were requested on the general injury card. Additionally, no substudy specific injury card was sent to the clubs as was done for some of our previous substudies. Distinction between insertional and proximal Achilles tendon disorders was therefore not possible, which was a limitation in the study. We did, however, retrospectively collect data about the location and surgery for injuries causing an absence of greater than 28 days and since most clubs preserve files with objective data such as images and surgery protocols of previous injuries, we did receive data for most of these injuries.
Second, our study manual did not provide the participating clubs with specific diagnostic or return-to-play criteria: it is therefore possible that the reported subclassifications, lay-off times and reinjury rates varied between clubs due to different diagnostics and treatment algorithms. Third, no reliable history of injury was collected at player inclusion; it was therefore not possible to compare the rates of Achilles disorders between uninjured players and players with prior Achilles disorders.
Fourth, with the current time-loss definition, the number of overuse injuries, such as Achilles tendinopathy, captured in this surveillance study might only be the tip of the iceberg.45 Overuse injuries might be underestimated, because many players with Achilles tendinopathy keep participating in training and matches despite having problems, and are therefore not captured in our time-loss definition.35 ,45 However, this definition was chosen for the general injury surveillance study to capture the injuries, which had a substantial effect on the players and the teams' ability to perform.35
What are the new findings?
Achilles tendon disorders account for 2.5% of all injuries and 3.8% of lay-off times in male professional football.
Achilles problems are more common in older players.
A majority (96%) of the disorders are gradual onset tendinopathies, and 4% are acute onset partial or total ruptures.
The incidence of Achilles disorders is higher during the preseason compared with the competitive season.
The median lay-off time for Achilles tendinopathies is 10 days and for ruptures it is 169 days.
Twenty-seven per cent of Achilles tendinopathies are reinjuries and these are more common after short (>10 days) recovery periods.
How might it impact on clinical practice in the near future?
Players with Achilles tendinopathies might need longer recovery periods since the reinjury risk is significantly higher after recovery periods of less than 10 days.
The team medical and coaching staffs should be aware of the increased risk of Achilles problems during the preseason training period and observant of overuse to Achilles tendons.
The team medical and coaching staffs should be observant of the higher risk of older players who suffer from muscle-tendon problems after repetitive hard training sessions.
The authors would like to thank the participating clubs, medical staffs and players.
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