Epidemiology of Adult Ankle Fractures: 1756 cases identified in Norrbotten County during 2009–2013 and classified according to AO/OTA

Ankle fractures are one of the most common fractures among the adult population. It is the fourth most frequent fracture type after hip, wrist and hand fractures registered in the Swedish Fracture Register and it constitutes approximately every tenth fracture [1]. Similar numbers were found by Court-Brown et al. in the UK [2]. Ankle fractures are also the second most common fracture requiring hospitalization. However, the length of hospitalization decreases year-by-year [3, 4]. The incidence in different studies varies from 71 to 187 per 100,000 person-years [4–9]. It is generally accepted that the incidence of ankle fractures is rising, particularly among the elderly and female population [4–6, 8, 10].

Ankle fractures are commonly classified by the Danis-Weber, the Lauge-Hansen or the AO/OTA classification system [11]. The AO/OTA classification system of ankle fractures can be seen as a development of the Danis-Weber classification with both the height of the fibula fracture taken into account as well as the number of fractured malleoli and comminution of the fibula. The groups of the AO/OTA classification system can also be translated to the Lauge-Hansen system [12].

Ankle fractures are not regarded as the typical fracture associated with osteoporosis. While earlier studies on osteoporosis conclude that fractures of the wrist, humerus, vertebra and hip, for example have a significant relationship with low bone mass, ankle fractures do not [13, 14]. It is also stated that a general increase in age-related incidences is not found among ankle fractures [14, 15]. The majority of ankle fractures are found among patients under the age of 60. [2–4, 7]. In contrast to this, ankle fractures have been shown to increase the risk of a new fracture among postmenopausal women [16]. The epidemiology of ankle fractures is changing and while some earlier studies have reported a male predominance [6, 7] more recent studies now show a significantly higher incidence among females [4, 8, 17]. The age-adjusted incidence of ankle fractures in Finland amongst women over the age of 65 more than doubled between 1970 and 2000. [10]. The majority of studies show that there is a rise in incidence among women to the age of about 60 to 70 and thereafter it levels out or even declines. [3–7, 17–20]. Multimalleolar ankle fractures though, appear to increase with age amongst female patients [4, 5].

While ankle fractures are common, there are reasons to continue studying its epidemiology to better understand the fracture. The aim of this study is to examine the incidence of ankle fractures in Norrbotten County as well as the relationship between AO/OTA-classes of ankle fractures with age, gender and low-energy trauma.

This study shows an incidence of 179 ankle fractures per 100,000 person-years in the county. In comparison to both earlier and similar studies, this study shows a slightly higher incidence. The incidence of ankle fractures has been reported as between 71 and 187 per 100,000 person-years. [4–9]. We see a few reasons for this high incidence in Norrbotten County. Different presuppositions and methodology does, of course, make it difficult to make a completely identical comparison.

A major strength of this study is that we believe we have found almost every possible ankle fracture in the county during the period of study and validated them by examining and classifying every case. However, the strength of the study is also in a way its weakness. The population is relatively small, in a limited area in the far north of Sweden. One can question whether it is possible to generalize the results. Another weakness is that it is a quite low interobserver agreement of the subgroup level of the AO/OTA classification of ankle fracture that can affect the results in the smaller subgroups (e.g. A32 and A33). Even though we have gone to great lengths to find all fractures there may be scenarios where cases have escaped our search. One would be if a patient injured outside the county and did not return within the period of the cast immobilisation.

Elsoe et al. recently published a study on ankle fracture epidemiology in Denmark, reporting an incidence of 169 per 100,000 person-years [8]. In Norrbotten County the mean age of the population is about 2.5 years higher than Sweden’s general population [21]. Adjusted for this age difference, the incidence equates to 172 adult ankle fractures per 100,000 person-years. Our results are thereby very similar to those of Elsoe et al. and it seems that this is the true incidence for the period of the study and the region. We can expect similar countries to have a similar incidence.

There is general agreement that the incidence of ankle fractures increases with time, due to an ageing population [4–6, 8, 10]. We agree with this and believe it to be the main reason for the relatively high incidence in our population. The population in Norrbotten County is ageing relatively quickly, the mean age increasing from 40.5 years in 1998 to 43.7 years in 2015 [21]. Another factor that could explain a higher incidence is the long winter season in the county. The incidence increases markedly at the beginning and the end of the winter, when the average temperature is around zero degrees Celsius, resulting in icy and slippery roads and pavements. Our data shows an increase in incidence during the weekend, implying that a general increase in activity, as expected, is also a factor.

One third of the adult ankle fracture patients in the study were 65 years or older. We also observed an increase in incidence from the age of 30–39 to the age of 60–69 and this age group has the highest incidence. This rise in incidence is almost totally caused by the rise among females of nearly four times while men have a more even spread throughout their lives. Elsoe et al. show a similar pattern of incidence but report a decreasing trend among men [8]. We chose not to present the age span over 90 years of age separately, as this group is very small and hence the confidence interval is large.

There is controversy regarding ankle fractures. Should it be characterised as an osteoporotic fracture or not [2, 5, 8, 13, 14, 22–30]? Since a fracture is one of the strongest risk factors for another fracture this is still an important question [31]. A high frequency among women, low-energy trauma as a cause and the incidence increasing with age is seen in osteoporotic fractures. [23, 24]. The proportion of low-energy trauma, as a cause of fractures, was two-thirds in this study. Thur et al. report similar proportions, but in comparison to earlier studies it is slightly higher [4, 6, 9]. This is probably an additional result of an ageing population.

The increase of incidence with age is shown in a number of earlier studies where the highest incidence in females occurs after the age of 65 [4, 6, 8]. Furthermore, a study published recently show an association between lower bone mineral density and ankle fractures [30].

It is difficult to explain the pattern with the rising incidence among women both by age and time, without explaining it as being due to loss of strength in the bone. However, when using a definition of a fragile fracture due to low-energy trauma in an elderly person, they only constitute around one out of four fractures [8, 10]. With a prominent component of osteoporosis as cause of the fracture, this would contradict the plateau in incidence reached after 60 years of age.

We show some differences between the different classes of ankle fracture in the amount of low-energy trauma, gender and age. At one end of the spectrum are the multimalleolar trans-syndesmotic ankle fractures corresponding to B22–3 and B3 representing about every fifth ankle fracture. These occur in female patients in about three-quarters of cases and are caused by low-energy trauma in 70% of cases. It is difficult not to see bone fragility, at least partly, as a cause of this.

At the other end of the spectrum we found type A21 fractures and C fractures. A21 is equivalent to an isolated medial malleolus fracture and shows clear signs of being a non-osteoporotic fracture. It is the one subgroup in our population that stands out and has a low proportion of elderly people; caused in a majority of the cases by a non-low-energy trauma and the patients are mainly men. This is also in agreement with earlier studies [4, 6, 32].

The supra syndesmotic type C ankle fracture has been described as less of an osteoporotic fracture with a lower mean age and a lower frequency of low-energy trauma [6, 9]. Sakaki et al. demonstrate in their study in São Paulo of surgically-treated ankle fractures with a high amount of high-energy trauma that type C fractures compose about 37% of the fractures. More than half of them were high-energy traffic accidents [33]. Briet et al. show in a similar study that all Lauge-Hansen classes except supination external rotation fractures (SER) is, in a majority of the cases, caused by high energy trauma. [32]. In our population, type C fractures show a similar pattern with fewer elderly females and low-energy trauma. However, low-energy trauma is still the cause in over 60% of type C fractures and still one out of four patients are 65 years of age and older.

Court-Brown et al. present a theory where they give medial, lateral, bimalleolar, trimalleolar and supra-syndesmotic ankle fractures different incidence patterns [2]. Even though we do not see this somewhat simplified pattern, we noticed a tendency that supports the theory of an increase with age in lateral malleolus and bimalleolar and trimalleolär ankle fractures, but not in the same way in medial malleolar and supra-syndesmotic ankle fractures.

Compared with Thur et al., we see a lot of similarities in the incidence pattern, except for two differences. One is the decrease in incidence of bimalleolar and trimalleolar fractures after 70 years of age in their study. This can be due to the fact that type C is included in that category. The other difference is that the incidence of lateral malleolar fractures is very low. Our incidence of these is about 2–3 times higher, probably because of the limitation of only including inpatients [4].

This study shows an incidence of 179 adult ankle fractures annually per 100,000 persons. More than two thirds of the fractures were caused by a low-energy trauma and ankle fractures are more frequent among females. Females generally have an increased incidence during their life, mainly between the ages of 30 and 60. This is in contrast to men who have more of an even distribution throughout their life. Classification according to AO/OTA reveals some heterogeneity among the classes of ankle fractures in age and gender as well as the energy involved in the trauma.