Incidence of Achilles tendon rupture: 25-year regional analysis with a focus on bilateral ruptures

Abstract

Objective

We aimed to fill the research gap regarding the incidence of Achilles tendon rupture (ATR), which is reportedly increasing, as there is a lack of data on the incidence of ATR and no data on bilateral ruptures in our region.

Methods

We determined the incidence of complete ATR among 273,485 people during 1991 to 2015.

Results

In the study period, 524 patients (486 [92.75%] men and 38 [7.25%] women, average age 39.03±10.86 [range 20–83] years, = sex ratio 12.8:1) were treated for ATR in our study area. The average incidence was 7.77 per 100,000 person-years, with an increasing trend until 2008 and peak incidence of 11.33 per 100,000 person-years. Most injuries (67.04%) occurred while performing sports activities. In total, 7 (1.34%) patients (six men [1.23%] and one woman [2.63%]) experienced ruptures on both sides, an average of 5.1 years apart; the average age at the second rupture was 57.71±16.69 (range 39–83) years, with a calculated incidence for bilateral ATR of 0.1 per 100,000 person-years.

Conclusions

The incidence of complete ATR in our study region increased gradually between 1991 and 2008, after which it declined. Bilateral ATR during that period was a very rare injury.

Introduction

Except for the quadriceps tendon, the Achilles tendon is one the strongest in the human body. ¹ Despite its ability to withstand four times the force produced by the calf muscles, the Achilles tendon is also vulnerable to rupture.^1,2 Several intrinsic and extrinsic risk factors including local, biomechanical, histological, and genetic factors, as well as medication, can influence Achilles tendon rupture (ATR). However, the true etiology of ATR is not completely known and seems to be multifactorial.^1–3

Despite increasing reports, the incidence of ATR in the general population is difficult to determine, with large variation among reports from different countries, from 1.8 to 51.3 per 100,000 person-years, and with an overall increasing trend.^4–22

Reports are lacking regarding the incidence of ATR, particularly in the Central European region, with no data on bilateral ruptures. Thus, the main focus of the present study was to determine and compare the incidence of ATR and the activities during which ATRs most often occur in the study region, with a special interest in ruptures on both sides or bilateral ATR.

Methods

We retrospectively analyzed data for all consecutive patients with ATR who were treated at the University Clinical Centre in Maribor, Slovenia between 1 January 1991 and 1 December 2015. Our center is the only institution for ATR treatment in the study region of Maribor. The inclusion criteria were: 1) patients aged 18 years or older; 2) closed ATR; 3) complete, first rupture (no re-ruptures); 4) rupture occurring in the tendinous portion, 2 to 8 cm proximal to insertion; and 5) written informed consent provided. This study was approved by the national ethics committee (KME 54/03/00). All procedures conformed to the tenets of the Declaration of Helsinki and all patient details were de-identified. The reporting of this study conforms to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. ²³

The diagnosis of ATR was based on the presented clinical criteria: a) palpable gap in the tendon; b) positive Thompson test result; ²⁴ and c) clinical signs of rupture (patient unable to raise on their toes or heel). In unclear cases such as regarding the extent and location of rupture, and in all cases of bilateral ATR, ultrasonography was performed to confirm the diagnosis. If doubt persisted in these cases, magnetic resonance imaging was routinely performed after the year 2000.

Patients who experienced bilateral ATR were additionally analyzed according to the type of treatment (conservative or operative) and functional assessment (American Orthopaedic Foot and Ankle Society [AOFAS] score), ²⁵ return to the previous activity level, complaints, and subjective assessment of treatment (good, fair or poor).^5,26

Objective factors specific to repair of an ATR were also examined, with a special focus on differences in the end results of both procedures. Complications were divided into two groups: major and minor.^5,26 The thickness and difference in the operated tendons was measured at the widest diameter with calipers. The neutral zero method was used for the assessment of ankle motion, with the maximum dorsiflexion considered to be 20° and plantarflexion to be 50°. ²⁷ Strength and endurance were tested clinically, using the standing heel-rise test, with 25 repetitions required for a grade of normal. ²⁸ Patients were instructed to stand straight and to rise up onto the balls of their feet and then to lower their heels, at a rhythm of 1 heel-rise every 2 seconds. Then, patients were asked to rise up 25 times onto their toes with both legs simultaneously (Test 1), and then to do so with the non-injured leg followed by the injured leg (Test 2). Patients were allowed to hold the edge of a table for balance but were not allowed to place weight on the hand.

Data used in the analysis were obtained from the Statistical Yearbook, 1991 and the Health Statistical Yearbook, 2016.^29,30

Statistical analysis

The data were analyzed using IBM SPSS 24.0 statistical software (IBM Corp., Armonk, NY, USA). To estimate risk, 2 × 2 tables were constructed. The odds ratio (OR) for rupture was calculated for by sex. Significance was determined using the Fisher exact test, with differences considered significant at p<0.05. The probability for rupture in the regional population of Maribor was calculated using the quotient of ATR in the entire regional population.

Results

Over a 25-year period in Maribor, Slovenia, 524 patients with 531 ATRs were recorded among a regional population of 273,485 people. The probability for ATR in the regional population was estimated to be 0.2%.

We identified 486 (92.7%) men out of 134,201 (49.1%) men in the regional population, and 38 (7.3%) women out of 139,284 (50.9%) women in the regional population, who experienced ATR. The average age of patients with ATR was 39.03±10.86 (range 20–83) years, with a sex ratio of 12.8:1. We found a statistically significant difference between men and women for ATR (p<0.001); the risk of ATR was 13.31-fold for men compared with women (OR: 13.31; 95% confidence interval [CI]: 9.57–18.53).

The average incidence of ATR was 7.77 per 100,000 person-years, with an increasing trend until 2008 and peak incidence (in 2004) of 11.33 per 100.000 person-years. The average 5-year incidence increased from 6.87 per 100,000 person-years to 8.84 per 100,000 person-years, after which it declined from 7.97 to 5.85 per 100,000 person-years. The number and incidence of ATR per year from 1991 to 2015 is presented in Figure 1.

Figure 1.

Number and incidence of Achilles tendon ruptures in Maribor region, Slovenia (per year).

In the study population, there were 286 (53.86%) left-side ruptures and 245 (46.13%) right-side ruptures. Sports activities were the cause of ATR in 356 (67.94%) patients, with soccer being the most frequent activity in 149 (41.85%) patients, followed by basketball in 108 (30.33%), tennis in 40 (11.23%), track and field activities in 23 (6.46%), volleyball in 13 (3.65%), and handball in 7 (1.96%) patients. Among the total patients, 15 (4.21%) were high-caliber athletes, and all were men.

During the 25-year study period, 7 (1.34%) of 524 patients experienced another ATR of the contralateral tendon (bilateral ATR). Therefore, the probability of contralateral rupture (after the first ATR) was estimated to be 1.3%.

In total, 6 (1.23%) out of 486 male patients and 1 (2.63%) out of 38 female patients experienced bilateral ATR an average 5.1 years apart (within 5 days to 12 years after the first rupture). The average age of patients with bilateral ATR was 57.71±16.69 (range 39–83) years, with a sex ratio of 6:1. The OR for ATR was 0.463 (95% CI: 0.05–3.94). The calculated incidence of bilateral ATR was 0.1 per 100,000 person-years.

Among the seven patients with bilateral ATR, four (57.14%) first sustained a left-side rupture and three (42.86%) first sustained a right-side rupture. Three of the seven (42.86%) patients sustained bilateral ATR during sports activities, with soccer the most common activity in two (66.66%) of these patients. There were no high-caliber athletes among patients with bilateral ATR. Patient data are summarized in Table 1.

Table 1.

Patient data.

	Single side	Bilateral
Number of patients	524	7
Men	486 (92.75%)	6 (1.23%)
Women	38 (7.25%)	1 (2.63%)
Average age (years), mean (standard deviation)	39.03±10.86	57.71±16.69
Sex ratio	12.8:1	6:1
Age range (in years)	20/83	39/83
Rupture side - left	286 (53.86%)	(first) 4 (57.14%)
Rupture side - right	245 (46.13%)	(first) 3 (42.86%)
Injured during sports activities	356 (67.94%)	3 (42.86%)
High-caliber athletes	15 (4.21%)	0
Type of sports activities:
Soccer	149 (41.85%)	2 (66.66%)
Basketball	108 (30.33%)	1 (33.33%)
Tennis	40 (11.23%)
Track & field	23 (6.46%)
Volleyball	13 (3.65%)
Handball	7 (1.96%)
Other sports	16 (4.49%)

All seven patients with bilateral ATR were treated operatively, three using an open surgical approach under spinal anesthesia on both sides, and four percutaneously under local anesthesia. Postoperatively, patients wore a rigid immobilization or a functional orthosis for 6 weeks. Subsequently, patients started rehabilitation according to protocol, with full weight-bearing allowed 3 months after rupture. At the time of ATR, two patients (a 83-year man and 67-year-old man, with bilateral ATR 5 days and 2 months apart, respectively, and both treated percutaneous repair under local anesthesia) were receiving long-term peroral corticosteroid therapy owing to chronic obstructive pulmonary disease. No patients with bilateral ATR reported any previous tendinopathic problems. The seven patients were followed up for 2 to 10 years (the 83-year-old patient died 2 years after ATR), with no major complications including re-rupture, and one out of eight percutaneous procedures (12.50%) involving transient sural nerve injury. At the end of rehabilitation after the second rupture, all patients achieved the same functional result bilaterally, regardless of the type of treatment (open or percutaneous suturing). The mean AOFAS score was 94.42±7.56 (range 79–100) points. The oldest patients (a 76-year-old woman in the open surgery group and an 83-year-old man in the percutaneous group) had the worst AOFAS scores, but their problems were mainly associated with limitations owing to concomitant diseases. All seven patients returned to the same level of activity as that before the second ATR, except for the two oldest patients, who had limitations as previously mentioned; all subjectively assessed their treatment as good. All patients had a difference in the thickness of the operated tendons of <5 mm at the widest diameter. The two oldest patients above had 10° diminished dorsiflexion of the foot symmetrically on both sides at the final exam. Both of these patients passed Test 1 but were unable to pass Test 2. At the final examination 5 years after the second rupture, the 76-year-old woman could perform 15 heel raises within the allotted time. After 2 years, the 83-year-old man could only complete a couple of heel raises, with markedly diminished strength; this patient died soon after the last follow-up. All of the remaining patients passed Tests 1 and 2 at the final examination, symmetrically with both legs.

Discussion

ATRs are relatively common; however, the true incidence is difficult to determine. A report on the occurrence of ATR was published in 1954 by Christensen, who reported 57 ATRs among 70,000 patients treated in an orthopedic department between 1936 and 1954. ³¹ In 1960, Schönbauer reported approximately 151 cases of ATR among 97,000 patients admitted to the Unfallkrankenhaus in Vienna from 1925 to 1959. ³² Nillius first reported a large increase in the ATR incidence, from approximately 4 per 100,000 to approximately 17 per 100,000 population, with 229 ATRs diagnosed from 1950 to 1973 in the city of Malmö. ⁴ In the same city (population 230,000) from 1987 to 1991, Moller later found 153 cases of total ATR, with an incidence of 13 per 100,000 population. ⁵ From 1973 to 1977 in a population of 500,000 people in a Swedish region, Nistor reported an incidence of 5 ATRs per 100,000 people. ⁶

The incidence of ATR in Sweden between 2001 and 2012 was determined in a nationwide registry-based study including all adult inpatient and outpatient hospital visits. ⁷ A total of 27,702 patients with ATR were identified, with a reported incidence of 29.5 per 100,000 person-years in 2001 and 34.9 per 100,000 person-years in 2012. ⁷ Leppilahti investigated the incidence of ATR in Oulu (Finland) over a 16-year period, during which the population rose from 94,000 to 104,000. The author found 111 cases of ATR and an increased incidence from 2 per 100,000 inhabitants during 1979 to 1986 to 12 per 100,000 during 1987 to 1994, with a mean incidence of 7 per 100,000. The peak annual incidence was 18 per 100,000 in 1994. ⁸ Lantto expanded the observational period to 2011 and found that the overall incidence per 100,000 person-years increased from 2.1 (95% CI: 0.3–7.7) in 1979 to 21.5 (95% CI: 14.6–30.6) in 2011. ⁹

A significant increase in the incidence of ATR among 4201 patients during the period from 1980 (4.7 ruptures per 100,000 people) to 1995 (6 ruptures per 100,000 people) was reported in Scotland. ¹⁰ Levi reported approximately 213 ATRs among 209 patients in Copenhagen from 1978 to 1995, with an overall incidence of 13.4 per 100,000 inhabitants per year. ¹¹ In a Danish county, among five hospitals serving 220,000 people, 718 ATRs were reported, and the annual incidence of ATR increased from 18.2 per 100,000 inhabitants in 1984 to 37.3 per 100,000 in 1996. ¹² In another study among a Danish cohort of 470,000 people, the reported yearly incidence of ATR increased from 22.1 per 100,000 person-years in 1991 to 32.6 per 100,000 person-years in 2002, with a mean incidence rate of 27.3 per 100,000 person-years. ¹³

The National Patient Registry was retrospectively searched to find the number of acute ATR in Denmark during 1994 to 2013. Regional population data were retrieved from the services of Statistics Denmark. During this 20-year period, 33,160 ATRs occurred in Denmark, revealing a statistically significant increase in the ATR incidence (p<0.001; range 26.95–31.17 per 100,000 per year). ¹⁴ In a study from Oslo, Norway, 168 acute ATRs were reported between 1990 and 1997 in a population of 320,000 inhabitants, with an estimated average incidence of approximately 8 per 100,000 inhabitants per year. ¹⁵

An overall mean incidence of 8.3 (range 5.5–9.9) ATRs per 100,000 people in the period from 1998 to 2002 was determined in a study from Edmonton, Canada. ¹⁶ In a study performed in the province of Ontario, Canada from 2003 to 2013, 27,607 patients who sustained ATR were identified. ¹⁷ The annual incidence density rate increased from 18.0 to 29.3 per 100 000 person-years. ¹⁷ In New Zealand, 412 ATRs were reported in the national insurance scheme among a population of 4 million in 2003; the incidence of ATRs rose from 4.7 per 100,000 in 1998 to 10.3 per 100,000 in 2003. ¹⁸ In a cohort of 177,000 people in Dunedin, New Zealand, the average incidence from 1999 to 2008 was estimated to be 24 per 100,000 people. ¹⁹

There were 1441 spontaneous ATRs reported among active duty servicemembers of US Armed Forces between 1998 and 2001, with a reported average incidence of 30.9 per 100,000 person-years (range 22.7–51.3 per 100,000 person-years (representing the highest incidence per year found in the literature. ²⁰

In the US, 14,127 ATRs were recorded between 2005 and 2011 within the population enrolled in Medicare. ²¹ The reported incidence of ATR increased from 6.7 per 100,000 patients in 2005 to 10.8 per 100,000 patients in 2011 (p<0.01) ²¹ Another study in the US using the National Electronic Surveillance System database revealed 854 ATRs, representing a national estimate of 32,906 ruptures. ²² There was a significant increase in the incidence of ATR from 1.8 (95% CI: 1.7–1.8) per 100,000 person-years in 2012 (the lowest incidence found in the literature) to 2.5 (95% CI: 2.4–2.5) per 100,000 person-years in 2016 (p < 0.01), for an overall incidence of 2.1 (95% CI: 2.0–2.1) per 100,000 person-years. ²²

There are several possible reasons for the widely varying ATR incidence reported in different countries and continents. One reason is methodological; many studies were from orthopedic (surgical) departments in hospitals, where patients were treated operatively; thus, conservatively treated patients would be omitted. In some studies, the authors did not report the exact period or size of the population served by the departments concerned. In many studies, the true criteria were not completely defined; therefore, re-ruptures, partial ruptures, or ruptures in the musculotendinous part of the triceps could be included in these studies. In other studies, only data from specific subgroups were analyzed (such as on-duty servicemembers of the US Armed Forces), or patients younger than age 18 years were included. Some studies only included people with certain types of medical insurance (e.g., Medicaid), those who were employed, or mostly older people. On the contrary, in some countries with numerous private hospitals and practices, many people are treated in these institutions; therefore, no actual epidemiological (population) data are available.

In our study region, there have been practically no changes in recent decades with respect to demographic data. The population diminished from 1991 to 2015 by 1.66% (4561 people). Our institution is the only center in the study region where treatment for ATR is provided. Our strict inclusion criteria, with no inclusion of partial or repeat ruptures and no ruptures in the insertional area or musculotendinous part, might have influenced the lower incidence of ATR found in our study, particularly in comparison with Scandinavian countries. However, our findings can be compared with those from Scotland ¹⁰ and some studies from Canada, the US, and New Zealand.^16,18,21 Another potential reason for the lower incidence in our study might be that despite the typical clinical picture of complete ATR, this injury can be overlooked in up to 25% of cases owing to extensive swelling or hematoma filling in the tear, which later heals with no surgical intervention but only patient-induced immobilization (such as for an ankle sprain), and surprisingly, only minor pain reported. ³³ It could therefore be of interest to further examine the incidence of ATR in our region as well as other regions and countries with respect to the potential impact of the COVID-19 pandemic on the incidence of ATR.

Despite all possible biases and methodological concerns among the published studies, all findings showed an increased incidence of ATR in all study countries and on all continents in recent decades. There could be several reasons for this increase. The main risk factors involve changes in people's general lifestyle that may lead them to become more sedentary and less physically active, or increased leisure time leading to increased recreational and competitive sports activities.^2,9,18,22 Although most ATRs can be attributed to participation in sporting activities, other factors such as sex, genetic factors, blood group, hyperthermia, drugs, diseases, intrinsic structural variations, extrinsic factors, and biomechanical changes related to aging may all contribute as well.^{2,3,7,10,11,14} According to the literature, 44% to 83% of ATRs occur during sports activities whereas only 8% to 20% occur among competitive athletes.^22,34–36 The prognosis after ATR is generally very good, with a full return of function in approximately 80% (95% CI: 75–85) of patients. However, an ATR can have a devastating outcome for high-caliber athletes in whom ATR has prevented a return to play in up to 30.6% of professional athletes in the US.^22,34–36

A long-term epidemiological study reported that the incidence of ATR increased in Oulu, Finland by more than 900% (from 2.1 to 21.5 per 100,000 person-years) during 1979 to 2011 and by approximately 80% (from 12 to 21.5 per 100,000 person-years) during 1994 to 2011. ⁹ In the abovementioned studies from Sweden, the ATR incidence increased during 1950 to 1973 by more than 300% (from approximately 4 to approximately 17 per 100,000 population) ⁴ and then by 18%, from 29.5 in 2001 to 34.9 per 100,000 person-years by 2012. ⁷ A similar increase of 16% (from 26.95 to 31.17 per 100,000 person-years) during a similar period (1994–2013) was reported in Denmark. ¹⁴ In our study, the 5-year incidence first increased from 6.87 to 8.84 per 100,000 person-years (12.86% in 15 years), after which it decreased to 5.85 per 100.000 person-years in the following 10 years. The real reason for this surprising result could reflect changes in lifestyle during this period in our study area, with more private companies hiring more migrant day laborers from neighboring countries after changes in local working regulations, as well as less leisure time and thus, less time for intensive recreational sporting activities. Additional follow-up is warranted to further investigate the reasons for this change.

The specific sporting activities that can result in ATR vary among countries but mainly include ball and racquet sports. The most common causes for ATR in Denmark are badminton and soccer,^12,14 badminton and volleyball in Finland,^8,9 basketball and tennis overall in the US, ³⁶ basketball and soccer among men and volleyball and basketball among women in the US, ²² women’s gymnastics and men’s basketball among collegiate-level athletes in the US, ³⁷ soccer and volleyball in Canada, ¹⁶ basketball and netball in Australia, ³⁸ and netball and squash in New Zealand. ¹⁹ In our study, sporting activities were the main reason for ATR in 356 (67.04%) patients, with soccer being the most frequent activity, followed by basketball. These findings are likely to simply reflect the most popular sports among the population of each country. In our study, 15 (2.86%) high-caliber athletes sustained an ATR (none had bilateral ATR), which is fewer in comparison with other studies.^7,9,14,22,36 The reason for this might be owing to the fact that there are fewer professional athletes in Slovenia.

Acute ATR is particularly common in recreational (sporadic) athletes or “weekend warriors” aged 30 to 49 years.^2,7,12,19,21 There are some reports on the bimodal distribution of ATRs with respect to age, with a second peak after age 60 years.^7,14 ATRs are much more frequent in aging athletes, with a possible role of concomitant exposure to drugs like fluoroquinolones or corticosteroids, particularly in elderly individuals. However, the importance of these risk factors in the overall distribution of ATRs remains uncertain as no bimodal distribution in this regard has been reported.^3,8,21,22,39

According to demographic characteristics reported in 142 studies from 1953 to 2017, the mean age at which ATR occurs has been increasing by at least 0.721 years every 5 years and has progressed from age 36.98 years before 1970 to age 42.09 years up to 2017. ³⁹ Among active athletes, the reported average age at which ATR occurs is generally lower than that in the overall population, perhaps owing to the physical and competitive abilities of athletes in different age groups, particularly in sports that involve jumping, running, and sudden accelerations with rapid footwork such as soccer, tennis, or basketball.^22,33–38 The mean age of 39.09 years in our patients with ATR could be considered comparable to that of patients with ATR in the above cited studies.

The sex difference in the rates of ATR is a well-known, but this phenomenon has not been entirely explained.^37,40 Men generally experience ATRs at a higher rate than women, with reported ratios varying from 1.18:1 to 30:1.^{10–12,14,17,19,22,37,41} This could reflect the greater prevalence of men than women who are involved in sports, although there may be other as yet unrecognized factors. The sex ratio in studies from North America and New Zealand is lower in comparison with that in studies from Scandinavian countries, which may be owing to the greater involvement of women in sports in Scandinavia, particularly among professional athletes.^8,19–22,40 In our study, an overall sex ratio of 12.8:1 was found, which was in the middle range of ratios reported in other studies and might be correlated with the ratio of women participating in sports in Slovenia.

The left Achilles tendon is ruptured more frequently than the right one, possibly because of the higher prevalence of individuals who are right-side dominant and who therefore push off with the left lower limb. ⁴² There was a minor predominance of ATRs on the left side in our study as well.

Bilateral ATR is a rare injury. Up to 2004, Hayes et al. found only 26 cases of bilateral ATR in the literature. ⁴³ Most patients with bilateral ATR in the published literature had associated risk factors and some even had rupture of other tendons, such as the quadriceps tendon. However, there are no data on the incidence of bilateral ATR across a longer period.^43–51 Risk factors associated with bilateral ATR include use of pharmacological agents such as quinolone antibiotics and corticosteroids;^3,13,44,45 anabolic steroids; ⁴⁶ renal failure (with dialysis) and organ transplantation;^43–45,47 diabetes mellitus;^44,45 systemic disorders such as Cushing disease, rheumatoid arthritis, and systemic lupus erythematosus;^48,49 hyperthyroidism and psoriatic arthritis; ⁵⁰ limb ischemia; ⁵¹ and previous ATR. ¹⁵

In our region between 1991 and 2015, 7 out of 524 (1.34%) patients had bilateral ATR. The calculated incidence of bilateral ATR was 0.1 per 100,000 person-years, which is comparable to the estimated average incidence of ATR in the entire Maribor region of 7.77 per 100,000 person-years. Therefore, bilateral ATR in our study region is considered to be a very rare condition. The average age of patients at the time of a second rupture on the contralateral side was older (57.71 years) than the average age of all patients with ATR (39.03 years). Patients with bilateral ATR had the second rupture an average of 5.1 years apart and while performing the same activity as that in the first event, in all our cases. Apart from two patients who were receiving peroral corticosteroid therapy, no other clinically important comorbidities, risk factors (except previous ATR), or previous tendinopathic problems were identified in any patients who experienced bilateral ATR in our series. All of these patients eventually returned to the same level of activity as that before the second rupture, what suggests a promising outcome in patients with bilateral ATR.

Some study limitations should be taken into consideration, such as the small number of patients who sustained bilateral ATR and data collected from a single institution in a single region of Slovenia. The results should therefore be interpreted with caution.

Conclusions

The reported incidence of acute ATR varies widely among different countries but has been reportedly increasing in recent decades. In our 25-year study period in the Maribor region of Slovenia, the incidence of ATRs increased gradually between 1991 and 2008, after which it declined. Bilateral ATR in our region during the period from 1991 to 2015 was very rare. According to the results of our study, the functional outcomes in patients who experience bilateral ATR might be considered promising, despite characteristics of a “repetitive” injury with required repeated therapy and rehabilitation, which might be discouraging for such patients.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.