Risk Factors for Bone Stress Injuries: A Follow-up Study of 102,515 Person-Years

Abstract and Introduction

Purpose: The aim of the present study was to assess the risk factors for magnetic resonance imaging (MRI)-detected bone stress injuries in the pelvis, hip, thigh, and knee in a large cohort of Finnish conscripts during a follow-up of 102,515 person-years.
Methods: An epidemiologic prospective cohort study of 152,095 conscripts, including 2345 (1.5%) females, was conducted. Localized pain in the pelvis, hip, thigh, or knee resulted in an orthopedic surgeon's consultation and subsequent MRI examination at the Central Military Hospital, Helsinki, Finland. Risk factors were systematically collected from 1998 to 2004, including data on conscripts' physical fitness and body composition measured at the beginning of their military service.
Results: Altogether, 319 MRI-detected bone stress injuries of the pelvis, hip, thigh, or knee were identified in our cohort; thus, the incidence was 311 (95% CI: 277-345) per 100,000 person-years. The female:male ratio varied substantially, depending on the anatomic location of the injury; it was highest for sacral injuries (female:male ratio = 51.1) and lowest for injuries of the femoral condyle (female:male ratio = 0.8). In univariate Cox regression analysis, poor muscle strength and a poor result in a 12-min run were significantly associated with bone stress injuries. In multivariable analysis, the strongest risk factors for bone stress injuries were female gender (hazard ratio 8.2; 95% CI: 4.8-14.2) and higher age (hazard ratio 2.1; 95% CI: 1.4-3.1).
Conclusions: Female military trainees have a highly increased risk of bone stress injuries of the pelvis and hip compared with male conscripts. Sacral stress fractures are typical bone stress injuries in female military recruits. Physicians should remember the possibility of bone stress injury, especially when examining stress-related pain symptoms of the pelvic area in physically active young adult females.

Military trainees are often affected by bone stress injuries.^[9,13,31] Bone stress injury is usually defined as an overuse injury attributable to the repetitive loading of bone with vigorous weight-bearing activity, such as running, jogging, and marching. Symptoms of bone stress injuries can also be characterized as "a history of localized pain of insidious onset, which worsens with progressive activity and is relieved by rest".^[9] On the basis of whether the fractured bone is normal, two entities of stress fracture have been defined.^[1,11] A fatigue fracture occurs if normal bone is exposed to repeated abnormal stress, and an insufficiency fracture occurs if normal stress is applied to abnormal bone. Under a constant load, osteoclast resorption and osteoblast reconstruction of the bone tissue are balanced, resulting in normal bone remodeling. Under an increasing load, the process is augmented through increased bone resorption.^[10,27] Increased osteoclastic activity at sites of bone stress may cause local weakening of the bone, predisposing it to microdamage.^[24] If allowed to progress, these microfractures can lead to complete fractures.^[22]

The occurrence of bone stress injuries has been described mainly in military and athlete populations.^[9,13] The most common anatomic locations of bone stress injuries are the tibial shaft and the metatarsal bones;^[23,37] bone stress injuries to the pelvis, the hip, the thigh, and the knee are less common.^[39] Depending on their potential adverse consequences and long-term morbidity, bone stress injuries can be divided into high-risk and low-risk injuries.^[15] High-risk bone stress injuries are associated with a tendency for prolonged healing attributable to poor vascular supply.^[38] They commonly occur on the tensile side of the bone-that is, in the femoral neck and shaft.^[42]

Considering that high-risk bone stress injuries have a risk of progression to complete fracture, delayed union, and nonunion, they should be recognized and diagnosed promptly and treated aggressively. Low-risk bone stress injuries can be diagnosed by taking a thorough history, conducting a physical examination, and using plain radiography, which usually is an adequate imaging method. With early diagnosis and conservative treatment, their prognosis is favorable.^[7]

Several risk factors for bone stress injuries have been described and categorized into nonmodifiable and modifiable risk factors. The nonmodifiable risk factors shown to be associated with bone stress injury include female gender, Caucasian ethnicity, and high bone turnover.^[9,25] The association between age and bone stress injuries has remained inconclusive, because some of the studies indicate that the risk for bone stress injury increases with age, whereas others indicate the opposite.^[28] The significant modifiable risk factors previously shown to be associated with bone stress injuries include poor physical fitness,^[41] smoking,^[25] steroid use,^[12] low levels of sex hormones, low bone density, and footwear.^[9] Moreover, low body mass was found to be associated with bone stress injuries in a Finnish study.^[23]

Previous studies investigating the risk factors for bone stress injuries have analyzed bone stress injuries in different anatomic locations as one entity, leaving unanswered the question of whether the risk factors differ between different anatomic sites. Moreover, the target populations of these studies have been small. The most important limitation of the previous studies, however, is that the bone stress injury diagnoses were based on plain radiographs or scintigraphy instead of magnetic resonance imaging (MRI), despite the strong evidence that MRI is needed for accurate diagnosis of bone stress injuries.^[9,19,22] Furthermore, using MRI, the early stages of developing bone stress injuries can be detected,^[18,32] even though overestimation of the occurrence is possible if asymptomatic persons are studied.^[18] As a diagnostic imaging method for bone stress injuries of the lower limbs, MRI is as sensitive as scintigraphy and is highly specific in detecting soft-tissue damage.^[20]

In the Finnish Defence Forces, the medical care instructions require that all patients with high-risk bone stress injuries, even cases raising only the suspicion of the garrison physician, are transferred to the main military hospital. Thus, conscripts complaining about having or having recently had hip or groin pain suggestive of a bone stress injury are referred without delay for orthopedic consultation and MRI examination.

The aim of the present prospective cohort study was to assess the risk factors for MRI-identified bone stress injuries in the pelvis, hip, thigh, and knee in a cohort of 152,095 conscripts during a follow-up of 102,515 person-years.

Med Sci Sports Exerc. 2007;39(7):1061-1066. © 2007 American College of Sports Medicine

Cite this: Risk Factors for Bone Stress Injuries: A Follow-up Study of 102,515 Person-Years - Medscape - Jul 01, 2007.