Bone can strengthen over time in response to loading, the same way that, conversely, astronauts lose bone mass when the stress of gravity and walking is removed. Stress fractures occur when a cycle of repetitive forces, none on their own sufficient to cause injury, is applied such that these forces exceed the bone’s ability to adapt and cumulatively damage the bone. In cases where the bone is entirely healthy, and the cause is simply too many cycles of load, the injury is denoted as a fatigue or stress fracture. Separately, fractures can also occur in bone that is not healthy, such as in the setting of osteoporosis, and does not stand up to even few cycles of repetitive forces. These are called insufficiency fractures. Repetitive activities such as walking, running and jumping can subject the bones of the foot to large forces that potentially lead to stress fractures, especially if these activities are started abruptly and without a ramp up period that allows the bone to effectively adapt. These injuries are commonly seen in the 2nd or 3rd metatarsal neck region, the base of 5th metatarsal (Jones Fracture), the sesamoid bones of the great toe, the navicular bone, or the calcaneus tuberosity.
Structure and Function
Bone is built to withstand load. Of course, a sufficiently high load applied at one time can cause bone to break. When a sudden injury generates a load higher than the bone’s strength, the bone will fracture. Bones can also be broken by cyclical application of subcritical loads, loads which do not cause damage if applied only a limited number of times. However, if too many cycles of such load are applied, or are applied quicker than the bone is able to adapt, stress fractures may ensue.
Repetitive application of a subcritical force causes microscopic damage to the bone. This damage is not a problem so long as the bone remodeling cycle (osteoclasts remove the damaged tissue and osteoblasts synthesize a healthy replacement) can maintain the structural integrity of the bone. There are, however, two instances where cyclical loads do lead to clinical problems. The first is when the bone metabolism is normal, yet the number of cycles is just too high for it to effectively adapt. Structural failure in the setting of normal bone remodeling that is unable to match the demands of high cycles of subcritical load is called a fatigue fracture. The second type of stress fracture is an insufficiency fracture. It occurs when the bone itself is abnormal, as in osteoporosis, and the cycle of loading would otherwise not have led to a fracture.
Initially in a stress fracture, the gross contour of the bone is normal and the damage is internal. This is sometimes described as a stress reaction. When internal damage accumulates the macroscopic architecture may fail and the bone may overtly fracture into two or more pieces. This is analogous to how paper clips are broken, wherein repetitive small loads are applied by bending the clip back and forth until enough damage accumulates, and the clip is broken in two.
Stress fractures of the feet occur in those bones, and often in the specific locations within those bones that are subjected to the highest repetitive loads. Each person’s foot absorbs force in a slightly different manner predicated by that person’s foot shape, alignment, foot stiffness, and gait pattern. Therefore, it is common for each foot to be disproportionally exposed to increased load in specific areas.
In certain foot shapes (typically flatfeet) the necks of the 2nd and 3rd metatarsals are subject to increased bending forces with walking. Thus, too much walking, especially with too little rest to allow repair, can cause stress fractures in these regions. This has been seen commonly in military recruits subject to long hikes with heavy backpacks, and therefore often called “march fractures.”
In patients with a high arched foot, which disproportionately tips the foot onto its lateral margin, running and sporting activities may subject the base of the 5th metatarsal to overload precipitating a stress fracture. These are frequently referred to as “Jones’ fractures” when they occur in the proximal metaphyseal/diaphyseal junction of the 5th metatarsal. This occurs because the high arch foot tends to be a stiffer foot and the heel tilts inward, shifting pressure onto the lateral border of the foot. These fractures may also be precipitated by an acute injury.
The navicular bone, too, is a site of stress fractures especially if the patient has a stiff, high arched foot, a relatively long second metatarsal, and participates in activities that involve dynamic repetitive loading through the forefoot such as sprinting. The mechanism of injury is felt to be repetitive loading through the second metatarsal and middle cuneiform into the navicular.
The sesamoid bones beneath the great toe can develop stress fractures, as found in patients who suddenly increase in running distance. The sesamoids see increased stress with push off activities involved in sprinting or jumping.
The calcaneal tuberosity can also develop stress fractures, usually in response to the repetitive heel strike inherent to some runners. This becomes especially true with sudden increase in mileage.
Patients with stress fractures will usually report focal aching in the affected area. They may give a history of an increase in their normal activity level. This pain typically increases with activity and decreases with rest. They may have a history of a condition that predisposes them to weaker bones such as osteoporosis (weak thin bone), amenorrhea (loss of normal menstrual cycle), or a history of smoking. Among young female athletes, the “female athletic triad” of an eating disorder, amenorrhea, and low bone density deserves special mention, wherein the presence of 2-3 components of the triad can potentiate stress fractures.
Stress fractures involving the lesser metatarsal bones (typically 2nd or 3rd) will often present with pain and swelling near the metatarsal neck, distally. Occasionally, high-level ballet and modern dancers will generate stress fractures at the base of the metatarsal near the midfoot. The foot type, in general, may be flat, often with a long second and possibly third ray. Early in the condition patients usually can walk without a limp.
Individuals who suffer a Jones’ fracture will initially report a dull aching pain on the outside of the midfoot. Patients often persist in their activities despite the pain until there is an overt failure of the bone. At that point they will have difficulty bearing weight and may walk with a pronounced limp.
Patients who develop navicular stress fractures will present with a chronic medial or central mid-foot ache. Although anyone can get a navicular stress fracture, the most common presentation is among athletes. The symptoms of a navicular stress fracture are often generalized to the mid-foot, and the relatively nonspecific location of the symptoms may make this condition difficult to diagnose.
Sesamoid stress fractures do occur, with the caution that in many instances “fracture” of the sesamoid is actually a normal bipartite sesamoid with superimposed sesamoiditis. The pain is usually isolated to under the great toe region.
Calcaneal stress fractures are often in runners and present with heel pain, often after an increase in their training regimen. A calcaneal squeeze test, wherein pain is generated by compressing the calcaneal tuberosity between both thenar eminences, can differentiate this condition from other conditions that often present in runners, such as plantar fasciitis or Achilles tendinitis.
A good history and physical examination may suggest a stress fracture; however, imaging studies are required to definitively diagnose a stress fracture in the foot. Plain x-rays may be diagnostic, although stress fractures often will not show up on x-rays unless there is enough structural failure of the bone to generate an overt crack, or a healing callus response is found – often weeks after the original stress fracture.
An MRI can demonstrate a stress fracture earlier due to its ability to detect reactive edema in the surrounding bones. Bone scans can also be positive, though their use has diminished with the ubiquitous availability of MRI. A normal MRI or bone scan effectively excludes the diagnosis of a stress fracture. These test, then, can help an athlete avoid unnecessary periods of enforced rest. Whereas, a positive MRI can provide strong, visual reinforcement to help the athlete understand the need for often unwanted periods of enforced rest. Lastly, an MRI can be used, if necessary, to monitor progress and allow timely return to sport (early, but not too early, that is). Navicular stress fracture especially benefits from the sensitivity of MRI for timely diagnosis.
Plain x-rays will identify a Jones’ fracture if it has progressed beyond a stress reaction (Figure 1). The fracture itself occurs at the metadiaphyseal area where the more flexible bone at the base of the 5th metatarsal meets the more rigid bone of the shaft of the metatarsal. The fracture is different from an avulsion fracture affecting the tip of the 5th metatarsal base.
Routine x-rays of the foot can be very helpful in distinguishing a stress fracture of the sesamoid from a bipartite sesamoid. The distinction is that the bone fragments of a bipartite sesamoid have a clearly identified smooth margin, whereas a traumatic fracture has ragged edges. An x-ray of the contralateral foot can also be helpful given that the majority of bipartite sesamoids are bilateral. In some patients, an MRI or CT scan may be required to differentiate a bipartite sesamoid from a sesamoid stress fracture.
Stress fractures are common and affect people of all ages. They are more likely to occur in females than in males, especially among females with the female athlete triad of amenorrhea, disordered eating, and osteoporosis. The estimated incidence in athletes and military recruits is 5-30%, depending on the sport and other risk factors. It is estimated that among people who run regularly for exercise, more than half will sustain an overuse injury that keeps them from running for 1 week. Stress fractures are among the more common injuries in recreational athletes. Stress fractures do not occur as often in the non-active population unless there is an underlying pathology that causes bone weakness or a sudden increase in activity level.
The main alternative diagnoses for stress fractures of the foot are trauma (including bone contusions) and tendinitis.
Stress fractures in a female athlete might be part of the Female Athletic Triad: namely, disordered eating, amenorrhea, and osteoporosis. Disordered eating (which may be manifest as compulsive exercising, as an indirect means of purging) leads to decreased body fat. Fat is a precursor of estrogen, therefore the level of this hormone is often much lower in emaciated women. Low levels of estrogen may lead to irregular menses and excessive bone resorption, as may be seen with osteoporosis. A poor diet also leads to inadequate bone maintenance owing to inadequate calcium and vitamin D intake as well.
Treatment Options and Outcomes
Most stress fractures respond to rest. After all, stress fractures are overuse injuries, and rest (which may be thought of as “under-use”) removes the harmful cause. For a stress fracture of the foot, immobilization and decreased weight-bearing which can be accomplished with crutches and a CAM walker boot may be helpful. For early injuries, cessation of the offending activity may be sufficient, often coupled with more supportive shoewear. For more involved stress fractures casting and non-weight-bearing may be necessary. Once healing has occurred, a gradual return to activity in a stiff-soled shoe is advised.
Certain stress fractures may benefit from surgery to aid in healing or prevent non-healing (i.e., non-union) or re-fracture. These “high risk” stress fractures include the Jones' fracture of the 5th metatarsal, navicular stress fractures, and those in other locations that have recurred despite adequate rest.
Surgery for a Jones’ fracture stabilizes the fracture site by placing a screw through the canal of the bone (Figure 2). The screw itself resists deforming forces and the drilling needed to place it potentially stimulates blood flow to help healing. A patient that has a non-union of the Jones’ fracture or a recurrent fracture after it had appeared to have healed, such as those with a significantly high arch, may need more involved reconstructive foot surgery to realign the foot and offload the fifth metatarsal more permanently.
Navicular stress fractures can also be difficult to treat due to the relative lack of blood supply to the navicular. Nonetheless, treating non-displaced navicular stress fractures with casting and prolonged non-weight-bearing has been reported to be successful in 90% of cases if patients are compliant. Despite the lack of supportive evidence of efficacy, some doctors may also recommend use of a bone stimulating device which may employ electrical and electromagnetic stimulation, ultrasound, to extracorporeal shock waves to possibly encourage bone formation and growth. Early surgery may be recommended for some patients even without an initial period of non-operative treatment if there is any sign of fracture displacement, the period of needed immobilization is unacceptably long, or because the consequences of displacement may be too high. Surgery may be associated with a faster return to sports, and may include drilling across the fracture, placement of one or more screws, and possibly the addition of bone graft to improve healing.
Sesamoid stress fractures may be treated with an orthotic with a recess under the base of the first metatarsal head to transfer forces away from the sesamoids. This is combined with a cushioned insole and a stiff soled shoe with a rocker bottom contour to allow for a smoother dispersion of the force away from the base of the great toe. If this does not lead to symptom resolution, more aggressive immobilization and non-weight-bearing may be needed. The results of surgical treatment are unpredictable and thus surgery, in the form of repairing the fracture or removing part or all of the sesamoid bone, is often a treatment of last resort.
Risk Factors and Prevention
Risk factors for stress fractures include intrinsic mechanical factors (e.g. decreased bone density, foot structure), nutritional factors, hormonal factors, physical training, and extrinsic mechanical factors (e.g. footwear, running surface). Patient's with metabolic bone diseases such as osteoporosis, osteomalacia, osteogenesis imperfecta, Paget's disease, and fibrous dysplasia are at a higher risk for stress fractures (insufficiency fractures).
Preventative measures to prevent stress fractures include gradually increasing training intensity, eating a diet with adequate amounts of calcium and vitamin D, and wearing the proper footwear for the desired activity as well as replacing footwear at appropriate intervals. For example, female military recruits given calcium and vitamin D supplements have been shown to be less likely to sustain a stress fracture than recruits that did not take supplements (PMID: 18433305). Runners may be able to prolong the longevity of their running shoes by considering them to be sports equipment using them specifically for running -and not for routine daily activities.
Stress fracture; Bone remodeling; osteoporosis; female athlete triad; Jones’ fracture; navicular stress fracture; sesamoid stress fracture, calcium; vitamin D; amenorrhea; insufficiency fracture, fatigue fractures,
Bedside skills for the diagnosis stress fractures include the ability to take a detailed but focused history and perform a thorough musculoskeletal examination.