Stress Injuries of the Calcaneus Detected with Magnetic Resonance Imaging in Military Recruits

Sormaala, Markus J. MD; Niva, Maria H. MD, PhD; Kiuru, Martti J. MD, PhD, MSc; Mattila, Ville M. MD, PhD; Pihlajamäki, Harri K. MD, PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.E.01447
Scientific Articles
Abstract

Background: Calcaneal stress injuries are fairly common overuse injuries in military recruits and athletes. We assessed the anatomic distribution, nature, and healing of calcaneal stress injuries in a group of military recruits.

Methods: Military recruits who underwent magnetic resonance imaging for the evaluation of exercise-induced ankle and/or heel pain were identified from the medical archives. The magnetic resonance images, plain radiographs, and medical records of these patients were evaluated with regard to fracture type and the natural history of the injury.

Results: Over ninety-six months, magnetic resonance imaging revealed calcaneal stress injuries in thirty recruits in a population with a total exposure time of 117,149 person-years, yielding an incidence of 2.6 (95% confidence interval, 1.6 to 3.4) per 10,000 person-years. Four patients exhibited a bilateral injury. Of the thirty-four injuries, nineteen occurred in the posterior part of the calcaneus, six occurred in the middle part of the calcaneus, and nine occurred in the anterior part of the calcaneus, with 79% occurring in the upper region and 21% occurring in the lower region. The calcaneus alone was affected in twelve cases. In twenty-two cases, stress injury was also present in one or several other tarsal bones. A distinct association emerged between injuries of the different parts of the calcaneus and stress injuries in the surrounding bones. In only 15% of the patients was the stress injury visible on plain radiographs. With the numbers available, there were no significant differences between the patients with calcaneal stress injuries and unaffected recruits with regard to age, height, weight, body mass index, or physical fitness.

Conclusions: The majority of stress injuries of the calcaneus occur in the posterior part of the bone, but a considerable proportion can also be found in the middle and anterior parts. To obtain a diagnosis, magnetic resonance imaging is warranted if plain radiography does not show abnormalities in a physically active patient with exercise-induced pain in the ankle or heel.

Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.

Author Information

1 Centre of Military Medicine, P.O. Box 50, FIN-00301 Helsinki, Finland. E-mail for M.J. Sormaala: markus.sormaala@welho.com

Article Outline

Stress injuries of the calcaneus were first reported in the German literature in 19371,2. The first extensive study, published in 1944, described seventy-one cases of such injuries among military recruits in the United States3. Since then, only a few large studies have been documented4-8. Calcaneal stress injuries are considered to be fairly common, especially in military recruits9-11. In addition to military recruits, athletes are often affected12. In previous studies, nearly all calcaneal stress injuries were found to involve the posterior part of the bone3,7,10. Previous research has suggested several risk factors for calcaneal stress injuries, including poor physical condition and inadequate footwear3,13,14.

The previous large studies on calcaneal stress injuries are more than thirty years old and involved the use of plain radiographs as the diagnostic tool. Currently, scintigraphy and magnetic resonance imaging are considered to be the most reliable imaging methods for the diagnosis of bone stress injuries. Both methods have excellent sensitivity, but magnetic resonance imaging offers higher specificity and is considered to be the most sophisticated technology currently available with which to diagnose and describe bone stress injury12.

The purpose of the present study, therefore, was to assess the anatomical distribution, nature, clinical course, and healing of calcaneal stress injuries in a large military recruit population on the basis of magnetic resonance images and medical records. In addition, the risk factors for and the incidence of calcaneal stress injuries were assessed.

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Materials and Methods

The present study was conducted at a Finnish military hospital. Material was obtained from the magnetic resonance imaging archives of the hospital to cover the study period of eight years (from April 1, 1997 through March 31, 2005). We identified all recruits who had been referred for a magnetic resonance imaging examination because of exercise-induced ankle and/or heel pain. The criteria for inclusion were exercise-induced ankle and/or heel pain during military service, a physical examination by an orthopaedic surgeon, and evidence of a calcaneal stress injury on magnetic resonance imaging. We excluded patients with a known recent injury and those with an infection involving the ankle and/or foot. The original medical records and magnetic resonance images of all patients who had undergone magnetic resonance imaging were obtained for reevaluation to ensure that no stress injuries were missed. The study design was approved by the Medical Ethics Committee of the institution.

In Finland, military service is compulsory for all men but is voluntary for women. The duration of the service varies from six to twelve months. Men are expected to complete their service by their thirtieth birthday, but the majority enter and complete it before turning twenty-one years old. An average of 23,000 male and 370 female recruits enter into military training of the defense forces annually. To establish the total exposure time for the population at risk within the catchment area of the military hospital, dates of entry into and transfer or discharge from military service were recorded for every recruit. Using these dates, we reached the total exposure time of 117,149 person-years over the study period of ninety-six months. The equipment used and the military training program were constant for all recruits during the study period.

The patients in the study were first managed nonoperatively in the primary health-care unit. As the result of an unclear diagnosis and prolonged pain, they were then referred to an orthopaedic surgeon. The surgeon examined the patients to determine the onset and duration of pain and its relation to physical activities. The maximum areas of tenderness in the ankle and heel were identified, and plain radiographs of the affected area were made. The standard views were anteroposterior, mortise, and lateral views of the ankle; lateral oblique and anteroposterior views of the foot; and lateral and superoinferior views of the heel. The patients were then evaluated with use of a 1.0-T magnetic resonance scanner with an extremity coil (Signa Horizon; GE Medical Systems, Milwaukee, Wisconsin). Magnetic resonance images of the ankle were acquired in at least two different planes; of these, the sagittal and axial T1-weighted spin-echo sequence images (repetition time, 500 to 680 msec, with two signals averaged; echo time, 10 to 15 msec, with two signals averaged; 256 × 192 to 224 matrix) and T2-weighted fast-spin echo sequence images with fat suppression (repetition time, 4400 to 6000 msec, with two signals averaged; effective-echo time, 80 to 90 msec, with two signals averaged; echo train length, 8 to 12; 256 × 224 matrix) were the most common. The field of view was 18 to 20 × 18 to 20 cm, and the slice thickness was 3.0 to 4.0 mm, with a 0.5 to 1.0-mm interslice gap. Additional sequences, such as a STIR (short-tau-inversion-recovery) sequence (repetition time, 5400 msec, with two signals averaged; echo time, 17 msec, with two signals averaged; tau inversion, 140 msec, with two signals averaged; 256 × 224 matrix; field of view, 32 to 48 × 24 to 48 cm; slice thickness, 4.0 to 5.0 mm; 0.5 to 1.0-mm interslice gap), were acquired as well. Two patients also underwent scintigraphy.

All magnetic resonance images were reevaluated by a musculoskeletal radiologist (M.K.). For evaluation purposes, the calcaneus was divided into three anatomic regions: the anterior part, the middle part, and the posterior part (Fig. 1-A). The calcaneus was also divided into upper and lower regions to determine the location of injury as accurately as possible (Fig. 1-B). Calcaneal stress injuries were classified into lower-grade (grade-I, II, or III) injuries, which were associated with periosteal, endosteal, and muscle edema, and more advanced (grade-IV) injuries, which were associated with a visible fracture line on magnetic resonance imaging15-17. The lower-grade injuries were evaluated together because of the difficulty of differentiating between such injuries in the calcaneus, which consists essentially entirely of trabecular bone. All other positive magnetic resonance imaging findings, including stress injuries to other tarsal bones, were also reported.

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Background information, such as military service data and physical fitness, was retrieved for all recruits in the catchment area of the hospital (117,149 person-years). The available computer-based statistics included data on the age, gender, height, weight, and length of military service of every recruit. The body mass index values for all of the patients and unaffected controls were determined from these statistics by dividing the body weight (in kilograms) by the square of the body height (in meters). Aerobic and physical fitness levels were ascertained during the first weeks of service. The aerobic fitness level for all subjects was measured with use of a twelve-minute running test, and the muscle strength level was assessed on the basis of five measures (the distance of horizontal jump and the number of sit-ups, push-ups, pull-ups, and back-lifts), which were then used to determine the individual physical fitness scores for all subjects in the study.

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To test differences between groups, the Kruskal-Wallis test was used for the continuous skewed data and the independent-samples t test was used for the continuous data. Differences in the cross-tables were determined with the Pearson chi-square test. The level of significance was set at p ≤ 0.05. The statistical analysis was performed with use of SPSS software (version 12.0.1 for Windows; SPSS, Chicago, Illinois).

The incidence of stress injuries of the calcaneus was calculated by dividing the number of recruits with a stress injury of the calcaneus (as identified on magnetic resonance imaging) by the total exposure time. The confidence interval was set at 95%, and the incidence is presented per 10,000 recruits per year.

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Results

On the basis of magnetic resonance imaging, thirty male recruits with a mean age of twenty-one years (range, eighteen to twenty-six years) exhibited a stress injury of the calcaneus over the period of eight years. This finding yields a person-based incidence of 2.6 (95% confidence interval, 1.6 to 3.5) per 10,000 person-years. The occurrence of stress injuries in the calcaneus was 1.4 per 10,000 recruits. Four patients had bilateral involvement; thus, a total of thirty-four different calcaneal stress injuries were evaluated.

Because of the calcaneal stress injury, five of the thirty patients were relieved from the military training program and two were transferred to military duties that did not require physical performance. The patients were suspended from physical training for an average of twenty-four days (range, three to seventy-three days). Patients became symptomless and returned to normal training activity within an average of seventy-seven days (range, twenty to 223 days) after first seeking medical attention. This length of time includes leaves and failed attempts to resume the training program. In only five cases (15%) was the stress injury found or suspected on the basis of plain radiographs and then confirmed with magnetic resonance imaging (Figs. 2-A and 2-B). In the cases of the other patients, there was no indication of a stress injury on the plain radiographs. The median time from the beginning of military training to the onset of pain was sixteen days (range, three to 250 days).

With regard to the anatomical region of involvement, nineteen stress injuries (56%) occurred in the posterior part of the calcaneus, six (18%) occurred in the middle part, and nine (26%) occurred in the anterior part (Fig. 1-A). In five cases the injury involved both the posterior and middle parts of the bone, and in three cases the injury extended from the upper region to the lower region of the posterior part of the bone. In these eight latter cases in which the injury involved two anatomic areas, it was considered as one injury and the location was assigned on the basis of the predominant area of involvement. When the entire bone was divided into upper and lower regions, twenty-seven (79%) of the thirty-four injuries involved the upper region and seven (21%) involved the lower region (Fig. 1-B).

In nineteen cases the stress injury was located in the right foot, and in fifteen cases it was located in the left foot. There was no relationship between the anatomic distribution of bone stress injuries and the side of involvement. Twenty (59%) of the thirty-four injuries were higher-grade (grade-IV) stress injuries with a fracture line that was visible on magnetic resonance imaging (Figs. 2-B, 3-A, and 3-B), but fourteen injuries (41%) were lower-grade (grade-I, II, or III) injuries that were manifested only as bone marrow edema. The higher-grade (grade-IV) injuries predominated in the posterior part of the calcaneus (representing fourteen of nineteen injuries) and the anterior part of the calcaneus (representing five of nine injuries). In contrast, the lower-grade (grade-I, II, and III) injuries predominated in the middle part of the bone (representing five of six injuries).

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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.

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Discussion

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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