In twelve feet, the stress injury was found to affect the calcaneus alone. In twenty-two cases, however, one or several other tarsal bones were also affected. The most commonly affected bones were the talus (Figs. 3-A and 3-B), the navicular, and the cuboid. Injuries of the upper part of the calcaneal body were associated with stress injuries of the talus, and injuries of the anterior part of the calcaneus were associated with stress injuries of the cuboid. In two of the thirteen feet with injuries in the upper region of the posterior part of the calcaneus, excess fluid was also noted in the retrocalcaneal bursa.
The examination by the orthopaedic surgeon revealed positive findings in twenty-one of the thirty-four feet. Ten feet had tenderness in the calcaneus, and eight had tenderness in other areas. Fifteen feet had soft-tissue edema around the ankle joint or heel. Pes planus was reported in association with three feet. Almost all of the ankles were stable; only two patients displayed minor amounts of joint laxity. For thirteen feet, the clinical findings were recorded in the medical record as normal. All patients were managed with reduced activity; none had cast treatment or surgery.
Twenty-five patients returned to duty after recovery and completed the service uneventfully. Of the five patients who were temporarily discharged from the training program, four resumed and completed the program uneventfully within two years after the injury. One patient had met the minimum length of the military service obligation at the time of the injury discharge and did not return to duty. The average time from the onset of pain to the date of diagnosis of a stress injury on magnetic resonance imaging was fifty-five days (range, twenty to 170 days).
With the numbers available, there were no significant differences between the patients with calcaneal stress injuries and the controls in terms of average height (178.7 cm for patients compared with 178.7 cm for controls, p = 0.9), weight (71.6 kg for patients compared with 73.8 kg for controls, p = 0.4), or body mass index (22.4 kg/m2 for patients compared with 23.6 kg/m2 for controls, p = 0.4). In addition, age (20.0 years for patients compared with 20.0 years for controls, p = 0.3), length of military service (nine months for patients compared with nine months for controls, p = 0.6), aerobic fitness (2503 m for patients compared with 2476 m for controls, p = 0.8), or muscle strength (16.2 points for patients compared with 15.0 points for controls, p = 0.2) did not reach significance as risk factors for stress injuries of the calcaneus.
All previous larger studies of calcaneal stress injuries were conducted more than thirty years ago when plain radiographs were the only imaging technology available. With use of magnetic resonance imaging and scintigraphy, stress injuries can be documented and characterized with higher accuracy than is the case with use of plain radiographs12. Injuries can be detected earlier throughout all parts of the bone. Notably, lower-grade (grade-I, II, and III) injuries associated with only edema can also be seen.
Stress injuries of the calcaneus are considered to be common. The authors of some studies of military recruits in the United States have claimed that such injuries represent the most common type of stress injury to the foot7,11. However, considerable differences in the incidence of such injuries have been described in different military recruit populations. The incidence in the present study was quite low but was consistent with that in a study of recruits in the Israeli army18. Both Israel and Finland maintain conscription forces, with the military service program being mandatory for all male citizens. The higher incidence reported in United States recruits may be explained by the inadequate shoewear that was used when those studies were conducted decades ago. This variation also can possibly be attributed to differences in military training programs, equipment, or the heterogeneity of the samples of recruits.
Previous studies have indicated that stress injuries of the calcaneus are almost always located in the posterior part of the bone3,8,10. Although most of the injuries in the present study were located in the posterior part of the bone, a considerable proportion (26%) of the injuries involved the anterior part of the bone and 18% involved the middle part of the bone. Only 56% of the injuries in the present study involved the posterior third of the bone, in contrast to 95% to 100% of the injuries in previous studies involving the use of conventional radiographs. The likely explanation for this difference is the greater sensitivity of magnetic resonance imaging in the detection of stress injuries in the middle and anterior parts of the calcaneus. The vast majority of the calcaneal stress injuries in the present study occurred in the upper part of the bone, an observation that was not reported in the previous studies conducted with plain radiographs.
It is noteworthy that magnetic resonance imaging detected lower-grade stress injuries, which accounted for 41% of the injuries in our study. These grade-I, II, and III injuries could not be detected with plain radiographs, yet they caused considerable pain for the patients. Of all stress injuries of the calcaneus that were detected with magnetic resonance imaging, only a small proportion (15%) were found with radiographs. Therefore, we believe that a magnetic resonance imaging scan should be acquired when physicians working with athletes or military recruits suspect a calcaneal stress injury, even if plain radiographs reveal normal findings.
Calcaneal stress injuries often were associated with stress fractures involving other bones of the foot and ankle. Notably, calcaneal stress injuries in the anterior and upper parts of the bone were associated with stress injuries of the cuboid and talus. Therefore, we may conclude that if a stress injury is seen in the calcaneus, one should be suspected in the other tarsal bones as well, particularly because stress fractures of the navicular and the talus can permanently damage the foot if left untreated19,20. No significant relationship was found between the incidence of calcaneal stress injuries and background variables such as weight and physical fitness. Three feet had a minor pes planus deformity that may have created a predisposition to the calcaneal stress injury. Other possible causes might include other abnormal foot structure patterns, the biomechanics of the foot, and/or limb-length inequality21,22. Apart from the three feet with pes planus, however, no other major abnormalities were found in the present study. This finding was expected, however, because men with major abnormalities of the feet are excused from military service.
Calcaneal stress injuries have been considered to be low-risk stress injuries as no displaced fractures have been documented in previous studies, to our knowledge3,8,15,23. On the basis of the present study, we agree that calcaneal stress injuries can still be regarded as benign, low-risk injuries. It is noteworthy, however, that they can cause considerable hardship for military recruits and athletes trying to focus on their training programs. Our patients were compelled to refrain from the training program for weeks or even months, and a substantial number were relieved from the military training program altogether.
In conclusion, calcaneal stress injury should be considered in the differential diagnosis of exercise-induced ankle or heel pain in soldiers and athletes. Although stress injuries of the calcaneus have been previously diagnosed with use of conventional radiographs, magnetic resonance imaging has a superior sensitivity for lower-grade injuries and injuries located in the anterior and middle parts of the bone. Roughly half of calcaneal stress injuries occur in the posterior third of the bone, and the other half occur in the middle and anterior thirds combined. Moreover, stress injuries to the different parts of the calcaneus are commonly associated with stress injuries to the surrounding bones. ▪
NOTE: The authors thank Harry Larni for the skillful artwork and the Radiological Society of Finland and the Pehr Oscar Klingendahl Foundation for personal grants supporting the study.
In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from the Scientific Committee of National Defense in Finland, the Radiological Society of Finland, and the Pehr Oscar Klingendahl Foundation. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at Central Military Hospital, Helsinki, Finland
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