Medically diagnosed youth overuse injuries have increased in the United States (1), and comprise nearly 50% of all pediatric sports-related injuries (55). Increased sports involvement, high level of training intensity, and the complexity of sport-specific drills executed repeatedly at a young age carry a high risk for injury in youth (52). Although sport injuries are prevalent in preadolescents, about half of these injuries can be prevented with proper training programs and the use of protective equipment (4). Overuse injuries typically affect children more than adults and present a unique set of physical and physiological symptoms in children (1). This paper will focus on the traction apophysitis injury of the ankle also known as Sever's disease (6,19,40). The aim of this paper is to review the characteristics of Sever's disease and to provide strength and conditioning professionals and youth coaches guidelines to recognize, treat, and prevent potential cases of Sever's disease.
ANATOMY OF THE FOOT AND ANKLE
The ankle is the most commonly injured joint among young athletes accounting for about 30% of musculoskeletal injuries seen in children (44). The talus and calcaneus bones comprise the hind foot. The calcaneus, also commonly referred to as the heel bone, develops in a way similar to long bones forming a diaphysis, a metaphysis, a physis, and an apophysis (34). However, the calcaneus grows slower than most long bones and it has fewer proliferation cells. Sever's disease occurs at the apophysis of the calcaneus.
The apophysis is the attachment site for the Achilles tendon and is located in the posterior aspect of the bone. The apophysis and the nearby epiphyseal plate have different growth rates, as the greater tensile forces result in an increased number of collagen fibers in the apophysis (32). The muscles attached to the apophysis can be overly tight and become overstressed if repetitive traction forces are placed on these vulnerable growth centers. Typically until adolescence when the physis (growth plate) of the calcaneus finally fuses, the apophysis is vulnerable and can easily be a focus area for overuse injuries (14).
BIOMECHANICS ASSOCIATED WITH SEVER'S DISEASE
A child generally acquires the ability to walk without support at 15 months and run at 18 months of age (53). At 3 years of age children typically demonstrate a relatively mature gait and a stable walking pattern (12). Typically the development of walking and running is completed at 5 years of age (53). Sever's disease is most commonly associated with impact sports that involve excessive running (5,14,18,19). Children who are exposed to high volumes of running typically develop muscular imbalances (22) manifested through strong knee extensor and plantarflexor muscles with weak knee flexors and dorsiflexors. Such muscular imbalances can lead to calcaneal apophysitis in children (22). During running, the forces generated by the contractions of these stronger muscle groups can result in an increased strain on the bone (47). Because of the cumulative nature of microtrauma, running can lead to symptoms typically not developed from ordinary walking. If the repetitive strain is applied frequently and in high volume, the resulting forces lead to damage in the bone tissue, but typically are not enough to cause structural failure (47).
BIOMECHANICS OF THE HEEL STRIKE
Most children perform running movements with heel strike (53). Heel striking—also referred to as rear-foot striking (RFS)—is typically observed during running, and refers to a forceful ground impact, where the heel is the first point of contact with the ground (23). This occurs when the foot strikes the ground with the ankle dorsiflexed and the calcaneus is directly underneath the athlete's center of gravity (23) (Figure 1). With higher speed heel strike running, the ground reaction forces can generate vertical shock waves that may transition through the entire body (23). Although the soft tissue around the heel provides some energy absorption of the ground reaction forces, the resulting compression and the subsequent traction of the anterior and posterior calcaneal attachments pose substantial risks.
The high impact dynamic ground reaction forces generated with each heel strike are the primary reason for the development of foot injuries in youth (39). High magnitude of forces and rapid rates of ground reaction force development that the lower leg musculoskeletal system repeatedly experiences pose substantial risks for calcaneal apophysitis (26). A young runner may strike the ground over 600 times each kilometer of long-distance running, making youth runners particularly prone to repetitive injuries (27).
FOREFOOT AND MIDFOOT STRIKING VERSUS REAR-FOOT STRIKING
Forefoot strike (FFS) refers to the toes and the ball of the foot touching the ground first, whereas in a midfoot strike (MFS) the toes, the ball of the foot, and the heel, all touch the ground at the same time (26). To achieve a proper FFS and MFS, athletes forcefully plantarflex their ankle before ground contact, whereas in RFS ground contact is achieved with a stiff and dorsiflexed ankle (26). Consequently, compared with RFS, ground contact in FFS and MFS involves considerable eccentric loads on the plantarflexors, and therefore, a mitigated effect on heel impact (26). FFS is most typically seen in sprinters, whereas most endurance runners' heels strike at low to moderate running speeds (26). During a heel strike at moderate running speeds, runners may experience impact forces up to 1.5–3 times their body weight (27). It is estimated that 62% of children heel strike when running, even if running barefooted (57). Therefore, barefoot running in children may lead to elevated loading rates and vertical ground reaction forces (30) and if coupled with poor form the likelihood of injury substantially increases (27).
CAUSES AND PREVALENCE OF YOUTH SPORT OVERUSE INJURIES
Overuse injuries are commonly caused by repetitive stress, which may lead to microtrauma on the tissue in a given body region (9,20,47). The microtrauma caused by repetitive sport activities most often affects the tendon, muscle, or bone tissue (35). In case of the Sever's disease, the repetitive stress comes from forces generated through the calcaneus (16). The microtrauma from the pull of the Achilles tendon attached to the calcaneus, along with the microcontusions from the repeated heel strike impacts may lead to calcaneal inflammation (6,16,17). Such repetitive high impact forces to the calcaneus can be commonly seen in soccer, track and field, and other activities that involve a high volume of fast running. In addition, this overuse condition may also be related to various biomechanical conditions, such as over pronation of the foot during running or reduced hip and ankle flexibility resulting from growth spurt limitations (22).
Another key aspect of the development of Sever's disease is the vulnerability of immature bones. A crucial difference between a child's skeletal system (immature skeleton) and an adult's (mature) skeleton is the presence of a physis, also referred to as growth plate, the part of the bone responsible for lengthening (21,43). Overuse injuries in youth athletes most commonly involve the physis of a bone and are the result of the inherent weaknesses of an immature skeleton (4).
Approximately 48% of youth athletes suffer an injury in a sport season, with about 80% of these injuries related to the musculoskeletal system (56). These injuries typically result in several days of activity loss and are the leading cause of restriction from sport activities (56). Physeal problems reportedly affect 18% of youth athletes (7). In the United States the estimated annual cost of overuse injury related treatment in youth is $1.8 billion (1).
Foot and ankle injuries account for about 30% of clinical injury treatment in youth (7), representing over a quarter of all sports-related injuries in youth (6). An estimated 87% of youth athletes who experience heel pain are involved in running and jumping sports (7). Running based high-volume endurance training regimens account for at least 60% of all overuse injuries in youth (34,42). Sever's disease is reported to comprise up to 16% of all youth musculoskeletal injuries (19,47,48), and young male athletes typically suffer physeal injuries 75% more frequently than females (14,43) because of the delayed skeletal maturity in men compared with women (10,16). Specific to Sever's disease, 2–3 times as many male athletes are affected as females (51), and in 61% of cases male athletes exhibit bilateral symptoms (10).
DESCRIPTION OF SEVER'S DISEASE
Sever's disease (calcaneal apophysitis or osteochondritis of the calcaneal apophysis) is the most common cause of heel pain in 5–11-year-old children (4,14,17), and in preadolescents (16,41,51), but does not affect young athletes after the closure of the physis, typically after puberty (49). Sever's disease most typically affects highly active youth with a tight gastrocnemius-soleus muscle complex (1). The overuse generated inflammation occurs on the calcaneus around the insertion of the Achilles tendon. Symptoms include the insidious onset of chronic and sharp heel pain typically aggravated by running (56).
Overuse injuries in youth are often perceived as minor discomfort. However, untreated physeal injuries may lead to permanent bone growth disturbance with possible long-term disability (9,10,45). In some rare cases, Sever's disease can even progress to a calcaneal avulsion fracture (14). Most typically, however, damage to the bone's epiphysis results in a long-term disruption of normal bone growth (52). Other long-term consequences of the disturbed physeal growth can include length discrepancy, angular deformity, or altered joint mechanics (3).
PATHOPHYSIOLOGY OF THE DISEASE
THE GROWING BODY
Children's growing skeleton is more susceptible to mechanical load induced injury because of the bones being more porous and because of the weakened structure of the long bones near the epiphyseal plates (24). About 90% of apophyseal problems in youth athletes take place in those who have their epiphyses still open (7). Figure 2 provides an example of an open calcaneal physis typically seen in an adolescent. The physis represents the weakest link in children's skeletal structure. During growth spurts, the increased growth rate induces structural changes that ultimately lead to more fragile growth plates (3).
Epiphyseal plates in children's bones are composed of soft cartilage, which makes the growing skeleton uniquely weaker than the mature skeleton (20). Cartilage is present in children's bones not only at the epiphyseal plates, but also at the muscle-tendon insertion points, which are particularly vulnerable areas (20). In general, children's ligaments are stronger than their bones, but even their bones tend to be stronger than their soft cartilage tissue-rich epiphyseal plates (56). In comparison, the ligamentous structures in children may be as much as 2–5 times stronger than their physes, making the physes highly susceptible to physical activity induced mechanical stress (5,33).
Growth rates of bones and attached muscles are uniquely different in those children who are going through a growth spurt. During a growth spurt the long bones grow at a faster rate than the muscle-tendon units can stretch, which leads to tight muscles and in turn excessive stress on the tendon's attachment point to the bone (22). This is particularly evident in muscles that cross 2 joints, for example, the gastrocnemius and Achilles tendon complex (9). As such, children's flexibility during growth spurts drastically decreases because of the passive elongation and tightening of the muscle tissue (50,52), and imbalances that develop in agonist to antagonist muscle strength ratios. The muscle to bone growth rate difference imposes a crucial risk factor for physeal injuries, (32,41) as stress from repetitive sport activities can lead to the development of overuse injuries in the affected body regions (6,43).
In Sever's disease, in particular, the growth of the tibia and fibula exceeds the elongation capability of the gastrosoleus musculature and the Achilles tendon, which leads to the tightened Achilles tendon placing excessive stress on the calcaneal apophysis (16). However, as Sever's disease most commonly takes place during children's growth spurt, the risk of this condition substantially decreases after the fusion of the apophysis, typically around 15–16 years of age (6,10,16). The closure of the epiphyseal plate indicates ossification and is clinically recognized as bony maturation (11).
RELATED RISK FACTORS
The predisposing factors leading to overuse injuries are typically characterized as either intrinsic or extrinsic (20) and are summarized in Table 1. Repeated injuries may indicate insufficient rehabilitation or lack of knowledge of initial injury factors (6). As children develop, an insufficiency in muscle strength, endurance and motor skill leaves them susceptible to injury. Furthermore, this risk is seen in those who have alignment abnormalities and excessive ligamentous laxity. The state of equipment, for example, proper fit or overuse wear, as well as impact absorption (37), particularly when exercising on hard surfaces, are often associated with overuse injury in the young athlete. Improper technique, poor training approaches to skill development, as well as abrupt changes and unsound increases in training intensity and volume pose a risk for overuse injuries (6).
Sever's disease may be diagnosed by self-reported heel pain and by clinical assessment. Youth coaches and strength and conditioning professionals should be aware of the common symptoms and refer youth athletes to a medical professional when suspecting an overuse injury. It is important to note, however, that although tightness in the Achilles tendon and ankle weakness are eventually noticeable, the developing injury may not immediately interfere with children's sports performance. Therefore, youth coaches must regularly look for the common symptoms of Sever's disease, such as heel pain and impaired gate during fast running.
Patients with Sever's disease may report pain in the posterior heel during activity, indicating a need for ankle examination. Patients will often limp or walk on toes to prevent painful walking (14,48), as toe walking tends to relieve the pain (7,13,51). About 60% of Sever's disease patients show bilateral symptoms affecting both legs either sequentially or concurrently (4,7,16,17,28).
The ankle examination begins with inspection for swelling, bruising, or any obvious deformity. Passive and active range of motion and strength should be assessed (44). On examination, tenderness may be reported after compression of the medial/lateral posterior calcaneus. In addition, tenderness has been reported in the plantar fascia and proximal to the insertion of the Achilles tendon (7,46,47). Careful palpation of the painful area is critical in identifying affected tissue (6).
The squeeze test is the most commonly used clinical evaluation method whereby pain is induced by medial and lateral compression of the heel (54) as shown in Figure 3. Swelling and hardening of this area is also usually present with Sever's disease (7,16). Radiographs can be used primarily to exclude other potential sources of heel pain, such as fractures. Radiographically the apophysis may appear thickened and fragmented (13) as shown in Figure 4.
Radiographic images may show fragmentation of the secondary nucleus of the calcaneus, which can also be diagnosed through ultrasound (5) or by ultrasonography to avoid excess radiation for children (15). Magnetic resonance imaging (MRI) can also show the apophysis and the pull of the Achilles tendon. An MRI has the capability to demonstrate edema in the apophysis and additional areas such as the calcaneus and heel pad (5,13). Although rarely used in the diagnosis of Sever's disease, MRI imaging techniques have the capacity to exclude other conditions such as stress fractures or small bone fusion in the heel (5,13).
Treatment aims to not only advance tissue healing, but also to strengthen the tissue to a level sufficient for active sport participation. Patients with Sever's disease can make full recovery and return to sports after about 2 months with the inclusion of stretching and strengthening exercises (14). Rehabilitation exercises focused on calf and Achilles stretching and eccentric strengthening of the calf can help decrease traction forces on the apophysis (14). Failure to receive adequate treatment can delay recovery or possibly increase the risk of new injury (10).
With regard to physeal overuse damage, rest, as the first therapeutic measure is the key which ultimately will decrease pain (22). With adequate rest, patients have reported activity with minimal to no pain in as little as 3–6 weeks (4,7,16,18), although, for some patients several months of rest may be needed (6). Rest provides protection to the damaged tissue by decreasing mechanical stress from the load, ultimately terminating the scenario of overuse (6). Immobilization in a short leg cast may force rest in patients who are inclined to return to sports immaturely and may provide an improvement of symptoms in about 4 weeks (12,16). Immobilization may also be needed for patients not responding to conservative therapy (5).
In addition to rest, treatment of Sever's disease may consist of icing and oral medication (18,19,46). If rest and ice are deemed insufficient, Acetaminophen has been shown acceptable for discomfort relief (19). Besides basic analgesic effects, nonsteroidal anti-inflammatory drugs have shown no benefit in overuse injuries (6).
Prolonged stretch of the calf musculature is vital for treatment. A practical way of preventing relapse of injury and pain management is through education of proper stretching technique (40). If a growth spurt has occurred before symptoms, the tendon may not yet have adapted to the increase in skeletal structure (6). This scenario gives rise to tightness in the Achilles tendon and the muscles of the calf, ultimately decreasing the ankle dorsiflexion range of motion. Therefore, strengthening the dorsiflexors and improving the ankle range of motion through prolonged plantarflexor stretch is critical (7,40,46). Stretching of the plantarflexors should be practiced daily (1). Strengthening the ankle dorsi and plantarflexors should be practiced both concentrically and eccentrically (7).
Heel lift shoe inserts are another treatment option for Sever's disease. The purpose of an insert is to position the ankle in a slightly plantarflexed alignment, which alleviates some of the tension the Achilles tendon places on the calcaneus (7). Using ¼ to ½ inch heel lifts to reduce the amount of pull by the calcaneus have been recommended (58). The use of orthotics has been shown to provide benefits by altering lower body joint alignment and gait movement patterns, and lead to reduced stress to the active and passive soft tissues of the foot (38).
To effectively reduce the prevalence of youth overuse injuries preventive measures are critical from both medical and economic standpoints (42). This is particularly true for youth ankle overuse injuries, given that an estimated 50% of these injuries could be prevented (20,55). Establishing and regulating associated risk factors are critical for the successful prevention of overuse injuries (6). Early medical screenings, proper warm-up, enhancing proprioceptive awareness, increasing flexibility, age appropriate muscle strengthening, pre-exercise taping, sport relevant protective equipment, and education intervention are effective prevention strategies (42).
ADJUSTING RISK FACTORS
Recent findings have shown that addressing certain risk factors can reduce overuse injuries incidence by 15–50% (8). Injury prevention strategies may include education, environmental, and enforcement intervention (1). Such interventions should include appropriate supervision by sport-educated parents and coaches, proper training and officiating, safe equipment, and suitable play locations (1). Successful prevention measures often require collaboration between physicians, physical therapists, parents, athletic trainers, coaches, and the young athletes, thus decreasing the (re)injury rate and enhancing athletic performance (9).
Education of coaches and parents about sports injuries are needed (59). Coaches should inform parents and children, especially those new to a sport, on the type of equipment for the given sport and the proper fit (6). High quality shoes that offer firm support and soles designed for shock absorption can help, but excessive running or repetitive high impact landing on hard surfaces should still be generally avoided (13). Preparticipation exams could potentially reveal injury history and provide an opportunity to evaluate cognitive and physical maturation (1,6). Overuse injuries can generally be identified from the answers for the exercise history questions (55). In addition, the preparticipation examinations provide “teachable moments” to address injury prevention strategies (1).
Although proper running gait is one of the most important elements of injury prevention (26), the role of proper shoes in youth injury prevention is also critical. Children are able to walk and run faster when wearing shoes (57). Children's perception of safety when wearing shoes likely provides more confidence when running. Shoes also function as sensory filters by impairing feedback from proprioceptive systems, ultimately leading to modifications in gait pattern for improved stability (26). The increased sole width of the shoe also allows the child to increase the base of support (26).
In some competitive running shoes, cleats, spikes, and racing flats the cushioning is drastically decreased to facilitate improved performance through less weight (30). Although necessary in specific scenarios, this shoe design should be used with caution in children (30). Heel strike running in cleats, spikes, and flats lead to substantially increased external loads compared with well-cushioned running shoes (30). Modern running shoes are designed to make RFS running less hazardous for injury by using elastic materials in a large heel to absorb some of the transient force and spread the impulse over more time (27). The elevated elastic heels in these shoes dampen the magnitude of the peak impact by approximately 10%, leading to smaller loading rates and reduced risk for overuse injuries in children (26).
Overall, rapid fluctuations in intensity, frequency, and volume of training activity have shown a strong correlation with overuse damage (22). Gradual progression is a key principle in the prevention of youth overuse injuries (20). Running based training for young children should be minimized and sport training should be geared toward general fitness and motor skills development. The 10% rule should be used, limiting the increase in training frequency, intensity, and volume to no more than 10% per week (6,20). The purpose of the 10% rule is to allow the body gradual acclimation to the stress of the activity (55).
For most sports, general off-season and preseason training programs for children focus on muscular strength and endurance, balance, coordination, and flexibility training. To reduce the risk of Sever's disease development, lower body flexibility training should be emphasized not only in the precompetition phase, but in the competition season as well. There are some key exercises to improve flexibility in athletes to prevent or treat Sever's disease (Table 2). A flexibility program that emphasizes the lower leg and is a focused hamstring-to-heel cord regimen can help prevent the occurrence and recurrence of Sever's disease (13) (Figure 5). Stretching exercises should only be performed after a thorough warm-up to decrease the risk of muscle pulls (37).
Although often overlooked in youth sports, youth training programs should be built on the principles of periodization similarly to adult sports. Periodization is an effective strategy not only to improve performance, but also to reduce the development of overuse injuries through planned rest periods (20). Young athletes should begin a general fitness or conditioning program at least 2 months before the start of the competition. Successful youth injury prevention programs should typically include frequent flexibility training, the gradual application of lower body plyometrics, and functional strength exercises, as well as balance and body control activities (9). During training sessions, coaches should look out for proper neuromuscular control, core body stabilization, hip musculature control, lower extremity joint alignments, and appropriate bilateral landing techniques demonstrated by young athletes (9). Adolescent athletes should incorporate at least 2–3 nonconsecutive months of non–sport-specific training or active rest per year, ensuring adequate recovery from sports-related injuries. At least 2 days of rest each week is necessary for children to provide proper recovery and adaptation to the taxing demands of the sport (6).
Beyond normal muscular development, a proper resistance training program can increase the muscular strength of children and preadolescents (8,29). In general, resistance training for youth should begin with 8–12 common exercises that strengthen the upper, core, and lower body. Initially, youth should perform 1–2 sets of 8–15 reps of body-weight exercises, or using external resistance with a light to moderate work load (below 60% 1RM) while focusing on proper form and technique (2,8). Learning correct exercise techniques and safe training procedures should be the primary focus for young athletes as opposed to focusing on the training load. The use of child-sized equipment is important for children to properly and safely execute a movement with correct technique (29). Resistance training programs should start with simple exercises and gradually progress to more complex exercises as coordination and confidence in correct technique improve (2). Specific exercises that require neuromuscular proprioception, such as balance and coordination exercises should also be included.
There are specific resistance training exercises that will help prevent the development of Sever's disease (Table 3). Strengthening the weakened muscles is crucial for both recovery and prevention of repeated injuries (36). Ankle muscles should be targeted in general by performing both plantarflexion and dorsiflexion movements, and completing all exercises bilaterally, which in turn would promote strength gains in both extremities (36). For example, a resistance training session could consist of 3 sets of 15 repetitions of calf raises on a straight leg mainly loading the gastrocnemii (Figure 6), and 3 sets of 15 repetitions with knees bent loading the soleus (25). Dorsiflexor resistance training exercises should also be incorporated, whether with using dorsiflexion strength equipment, lifting the toes while squeezing a dumbbell between the feet in a seated position, applying manual resistance training by the coach, or using rubber bands or cable pulley (Figure 7). Symmetric hamstring sets, mini squats, and lunges should also be included to strengthen the lower body (31).
ROLE OF COACHES
The outcomes of injury prevention efforts substantially depend on the coaches' professionalism and the compliance of the youth athletes. Youth sports coaches should develop an understanding of general sports safety, sports techniques and skills as well as a thorough knowledge of children and adolescence, growth and development, and general medical concerns in youth (55). Qualified professionals should demonstrate an understanding of the mechanics of correct exercise techniques, basic principles of pediatric exercise science, and the pedagogical aspects of coaching youths (29).
Although in training, indications of soreness and low vigor must be constantly monitored by coaches. Additional practice and increased training load, although consistently prescribed in this scenario, may actually cause further damage (20). Fatigue from increased volume of training can increase muscle strain, which reduces the muscle's ability to absorb forces (20). A practice session that involves a variety of exercises and drills well distributed in time allows more time for recovery and reduces the likelihood of fatigue. Hence, muscles more effectively absorb force and the joints are better protected (20).
As kids advance from free play to the structured movement, requirements of sports participation, an epidemic of sports injuries is predictable (33). With an estimated 40 million children playing sports in the United States, the incidence of overuse injuries has paralleled the growth in youth sports participation (46). Overuse growth plate injuries occur because of a list of factors including physical and physiological differences, excessive movement repetition, improper training as well as inadequate equipment and footwear.
Children need careful assessment related to the common problems affecting a growing skeleton. Physical examinations should be accomplished before sports programs begin. Education needs to be available to parents, coaches, and athletes to bring awareness to overuse injuries. Once children are involved in sport participation, prevention of overuse injuries should be a key focus. Appropriate supervision, safety, and proper equipment should always be in place to prevent injury. Also, adequate rest between training sessions and sports seasons is needed to ensure the body's ability to repair tissues and avoid repetitive microtrauma (22). Coaches should become familiar with information outlined here regarding the injury incidence, causes of injury, and the modifiable and nonmodifiable risk factors associated with Sever's disease to develop relevant prevention or treatment programs. Strength, flexibility, coordination, and balance exercises should all be incorporated in the sport-specific training programs for youth. Ankle range of motion, particularly the proper flexibility and regular stretching of the plantarflexors is key in preventing Sever's disease, along with strengthening both the plantar and dorsiflexors. Also, youth coaches should build a broad repertoire of drills and training activities to avoid repetitive movements in practice.
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