The Epidemiology of Overuse Injuries in Ice Hockey: An Analysis from 29 Seasons in the Swedish Elite League

Background: An overuse injury is considered to be the result of iterated micro trauma or monotonous movements. Sports related to monotonous and repetitive movements have a high amount of overuse injuries. Ice hockey with its complexity of movements makes the player exposed to overuse injuries. The aim of this study was to analyze overuse injuries, and to study the long-term injury panorama. It is the first and essential step to obtain an injury-prevention method. Study Design: Descriptive epidemiological study. Method: Data from an elite ice hockey team was prospectively collected from 29 seasons. The definition of an overuse injury was a gradual onset injury caused by repeated micro trauma without a single identifiable event responsible for that injury and which resulted in absence from ice hockey. Result: 315 overuse injuries were registered and accounted for 16.2% of all injury registrations. An overuse injury led on average to 1.8 missed games, 6.5 missed training sessions and 8.4 days of absence. August and September were the most injury months for an injury, with a total of 38.7% of the events. The most affected body region was the hip and groin, as 67.3% of all overuse injuries arose there. The second and third most affected body regions were the back and the knee respectively. Conclusion: The present study reports that overuse injuries are a relevant problem. Primarily hip and groin are frequently affected. The beginning of the on-ice season has shown to be the most vulnerable period.


Introduction
An overuse injury is considered to be the result of iterated micro trauma or monotonous movements which cannot be related to any particular event. The absence of a specific trauma often delays the diagnosis [1][2][3][4]. These types of injuries are often ignored due to the limited amount of pain and functional loss. The athlete often continues the exercise without sufficient time to heal the affected area and with the result of increased dysfunction and severe pain [3].
Sports related to monotonous and repetitive movements are typically cycling, running, swimming and cross-country skiing [4,5]. In technical sports, as well as in team sports, overuse injuries are common due to the considerable amount of training hours and monotonous repetitive movements, such as skating [1,6]. Among sports, overuse injuries most frequently affect the hip and groin, shoulder, elbow, wrist, back and knee. Typically, these types of injuries include tendinopathies, myalgia, stress fractures and bursitis [7][8][9][10]. Regardless the sport, tendon and muscle injuries are the dominating overuse injuries [5]. Ice hockey is a team sport with great popularity, especially in North America, Europe and Russia. The sport has full body contact and the skating in highspeed results in a high risk for injuries [11].
In ice hockey, the majority of the injuries originate from traumatic events [7,12,13], although with its repetitive complexity of movements, make the players exposed to overuse injuries. This can be seen in goaltenders with the butterfly technique, defenders and forwards with flexion, adduction and external and internal rotation to the hip joint, all of which especially affect the hip and groin [6,[14][15][16][17][18].
Overuse injuries might be an underreported problem.
According to B Clarsen et al. [1] the reason could be due to the inclusion criteria, which involve both reduction of physical function and time loss. In the systematic review the "Definition and Usage of the Term Overuse Injury" [3], K.G. Roos

Methods
All injuries were diagnosed according to the ICD-10 standard.
Overuse injuries, which occurred during on-ice season, August to April, were included.
For the first 15 seasons data were primarily recorded on paper and were later exported into the injury registration system "International Sport Injury System" (ISIS) [19]. From the season 2001 and onward, injuries were directly recorded into ISIS. All games, including pre-season and play-off, as well as every training session were recorded. When a player was out of play or practice or if he had to stop playing due to injury, the cause was documented.
Consequently, only time-loss injuries were recorded into the system.

Definition
The definition of an overuse injury in this study was "a gradual onset injury caused by repeated micro trauma without a single identifiable event responsible for that injury" [4] and which resulted in absence from ice hockey. An overuse injury has a tendency to reappear. In our study, every injury, which gave absence, was counted. If the same type of injury at the same location reappeared the injury was counted as a relapsing injury.

Ethics
The study was ethically approved by the ethics committee in Umeå (registration number: 09/135M). Written consent was given from all participants.

Results
During the 29 seasons 267 players represented the club. There  The overuse injuries were classified into 13 different body regions, where 'hip and groin' was the most exposed in numbers and days of absence (Table 1)

Absence
The average player absence due to overuse injuries was 1.  The two months with the overall highest amount of both first time and relapsing overuse injuries were August and September, with 72 (22.9%) and 50 (15.9%) respectively (Table 2). April was the month with the fewest reported overuse injuries with 3 (1%). In 15 of the total 29 seasons, August was the month with the injury and 'game and training hours' maximum (Table 3), as well as the first month of ice-practice ( Figure 1). Among the 13 body regions, nine body regions were affected in August. 47 (65.3%) of overuse injuries in August were related to the 'hip and groin' region.

Injuries Per Month
The second and the third most affected body regions were the 'back' and the knee with nine (12.5%) and four (5.6%), respectively. The next three groups were 'abdomen', 'thigh', and 'head and neck'.

Discussion
The primary finding of this epidemiological study was that Our intention was to analyze the long-term injury panorama of overuse injuries among elite ice hockey players; to understand the rate and seriousness, to analyze the amount of time the player was out of play, and to analyze changes during seasons are the first and essential steps to obtain injury-prevention and to develop a method for strategy against overuse injuries.
Location to `Hip and groin´ as well the total numbers of injuries are overrepresented in the beginning of the on-ice season. Reasons for this might be a too intense start of the on-ice season and the sudden change to skating. This shift might involve other types of muscles, making particularly the hip and groin region more prone to develop an overuse injury. Emery and Meeuwisse [16] pointed out that less ice hockey specific training during the summer months increases the risk to develop a groin injury compared with players who had more focus on ice hockey specific training. The same study indicated that players older than 25 years and players with a history of groin injuries had a higher risk to develop a new injury. In accordance with their study, this study confirms that players older than 25 years were more susceptible to an overuse injury.
We were not able to analyze the off-ice training and we could not evaluate if the high number of overuse injuries was due to insufficient ice hockey specific or if the reason was the high number of training and game hours in the beginning of the on-ice season.
Possibly, a more efficient transition from the pre-season practice to the start of the on-ice practice could reduce the high amount of overuse injuries in August. But this remains to be shown.
During the 29 seasons, 'hip and groin', 'back', and 'knee' were the most exposed body regions to overuse injuries. Ice hockey especially on an elite level requires for example substantial lower extremity and core muscular to be able to function well and prevent overuse injury. We were not able to evaluate weather bad muscular function was a factor for the amount of overuse injuries.

Further Research
In our research, we have established that injuries in the hip and groin region are the major problem. We encourage future research to analyze the type and intensity of the training to further explain the high amount of overuse injuries in the beginning of the season.
More research is needed to investigate training methods, diet, and range of motion, as these are known to be important factors for avoiding overuse injuries.

Conclusion
Ice hockey is a sport with complex movements and with many directional changes, which requires great abdominal strength, and a balance between the thigh, hip and groin, and core musculature.
The present study reports that overuse injuries are a relevant problem; primarily the hip and groin regions are frequently affected. The beginning of the on-ice season has shown to be the most vulnerable period.

What are the New Findings?
a) The majority of overuse injuries appear in the beginning of the on-ice season.
b) Hip and groin injuries represent more than half of the total number of overuse injuries. c) There is almost a linear correspondence between game and training hours and overuse injuries.