Stress Fractures of the Hip and Pelvis

Stress fractures refer to overuse injuries of bone resulting from repetitive mechanical stress. Stress fractures of the hip and pelvic region, while relatively uncommon, have become increasingly recognized in certain populations, particularly long-distance runners and military recruits. The diagnosis of such injuries can be challenging, often hampered by a nonspecific physical examination and limited sensitivity of plain radiography. Early recognition is important to direct appropriate management, lessen time lost from sport, and avoid potential complications. The present article reviews the epidemiology, diagnosis, and management of bone stress injuries of the hip and pelvis, specifically the sacrum, pubic ramus, and femoral neck.

Key points

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Stress fractures of the hip and pelvis have become increasingly recognized in the literature, and are observed more commonly in long-distance runners and military recruits.
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The diagnosis of stress fractures in the hip and pelvic region necessitates a high index of clinical suspicion, often combined with advanced imaging modalities such as MRI.
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The sacrum and pubic ramus reflect lower risk sites of injury and are typically managed with activity modification to a pain-free level.
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Stress fractures of the femoral neck necessitate more aggressive management to prevent complication, including at a minimum strict non-weightbearing, and in some cases, surgical fixation.

Introduction

Stress fractures refer to overuse injuries of bone resulting from repeated mechanical stress, none of which alone would be sufficient to produce structural demise. Such injuries are relatively common in athletes, especially those engaging in repetitive physical exercise including endurance athletes, dancers, gymnasts, and military recruits. ^, Injury rates as high as 20% to 31% annually have been reported in runners and military recruits respectively. ^, Stress fractures are overwhelmingly more common in the lower extremities. The tibia, fibula, and metatarsals appear to be the most vulnerable. However, stress fractures of the hip and pelvis have become increasingly recognized in the literature.

Stress fracture development is multifactorial in nature, and various potentially contributing conditions have been identified. An excessively high volume or rapid escalation of training are some of the most consistently demonstrated contributors, especially in athletes. ^, Poor preparticipation fitness and a prior history of stress fracture are other common risk factors. Various anatomic influences have been identified including leg length discrepancy, genu valgum, pes planus or pes cavus deformities, and low bone mineral density and narrow bones in proportion to body size. Finally, female gender is frequently cited, although it remains unclear whether gender reflects an independent risk factor or the influence of sex-associated conditions such as those embodied by the female athlete triad, namely menstrual irregularities and disordered eating, as well as biomechanical differences.

Classically, a skeletal stress injury presents as localized mechanical pain that is not present at rest, but progresses with, or immediately following, activity. On physical examination, the most sensitive finding is localized tenderness over the involved bone. However, such assessment in the pelvic region is often challenged by the overlying soft tissues such that advanced imaging may be necessary to confirm diagnosis. MRI has generally become the imaging modality of choice in the diagnosis of bone stress injuries. Edema-sensitive sequences have utility to detect periosteal, muscle, or bone marrow edema, which may reflect the only finding in the initial stages of skeletal stress injury. As the injury progresses, edema may become apparent on T1-weighted sequences, and ultimately in cases of a frank fracture, a band-like, low-signal fracture line will be evident.

Early recognition of bone stress injuries is important to direct appropriate management, limit time lost from sport, and avoid potential complications. Management is largely dictated by the site of injury and associated risk of delayed union, nonunion, or progression to complete fracture. Injuries at high risk for these complications necessitate a more aggressive approach to management that includes at a minimum, a period of strict non-weight bearing, and in certain circumstances may require surgical fixation. ^, The femoral neck represents 1 such high-risk site. Lower-risk stress fractures, including the sacrum and the pubic ramus, may be managed less aggressively with activity modification to a pain-free level. ^, In all instances, it is critical to identify and address any modifiable risk factors to both promote recovery and prevent future injuries. This article will review in greater detail the epidemiology, diagnosis, and management of bone stress injuries involving the hip and pelvic region.

Stress fractures of the sacrum

Epidemiology

Located at the base of the spine, the sacrum is a triangular bone that articulates with the L5 vertebra superiorly, the coccyx inferiorly, and the ilium to either side by way of the sacroiliac joints. As the bridge between the spine and the iliac bones, the sacrum plays an important role in maintaining hip and pelvic stability. The sacrum may be divided into zones based upon regional anatomy that, when injured, can influence clinical presentation. Zone I refers to the sacral ala, or the area medial to the sacroiliac joint and lateral to the neural foramen. Zone II is the region of the neural foramen. Zone III is the area medial to the neural foramen and includes the spinal canal. Most sacral stress fractures are limited to Zone I. Moreover, the fractures typically develop vertically, parallel to the sacroiliac joint, as a result of repetitive axial load transmitted downward through the spine to the sacrum. ^, It has also been suggested that instability of the pelvic ring, as can be seen in the context of osteitis pubis for example, may result in abnormal shear forces that further predispose to sacral stress fractures.

Historically, stress fractures of the sacrum were considered relatively rare; however, it seems that these injuries may be more common than once thought. ^, Distance runners appear to reflect a particularly high-risk population. Sacral bone stress injuries have also been reported in tennis, basketball, gymnastics, cycling, track and field, badminton, and weightlifting, as well as in military recruits. ^, In addition to the general risk factors for bone stress injury outlined previously, it has been suggested that increased pelvic anteversion may represent a more specific risk factor for the development of sacral stress fractures.

Presentation and Evaluation

The clinical presentation of sacral bone stress injuries is notoriously vague. Patients may describe acute or insidious onset low back, buttock, pelvic or hip pain. The pain tends to be exacerbated by weight-bearing activities and relieved by rest. Although the majority of sacral stress fractures are limited to the sacral ala, or Zone I, some patients may experience radicular symptoms because of involvement of the traversing lumbosacral nerve roots or fracture extension into Zone II. ^, On physical examination, reproducible tenderness to palpation in the sacral region is a common finding. ^, ^, A single leg hop test may also reproduce pain. Provocation maneuvers targeting the sacroiliac joint such as Gaenslen test and Patrick sign may be positive, but reflect a nonspecific finding. ^, It has been suggested that a prone fulcrum test, whereby the examiner applies an anteriorly directed force over the posterior sacrum, may have utility in diagnosis, although this has not been well validated. Finally, neural tension signs (eg, slump test or straight leg raise) may be positive in the setting of accompanying nerve root involvement.

Given the nonspecific nature of the history and physical examination, imaging studies are often necessary to confirm diagnosis. Plain radiographs are recommended initially to exclude potentially mimicking diagnoses. However, radiographs are poorly sensitive for the diagnosis of a sacral stress fracture. ^, MRI is currently considered the gold standard for diagnosis of sacral bone stress injuries. MRI is notably more sensitive than computed tomography (CT), particularly in detecting earlier stages of bone stress injury. Moreover, MRI can be useful to exclude alternative bone or soft tissue injuries. ^, Sacral bone stress injuries present with bone marrow edema limited to the sacrum, reflected by relatively high signal intensity on T2-weighted images, and often corresponding low signal intensity on T1-weighted images. When present, a fracture line appears as a vertical, hypointense signal involving the superolateral anterior cortex and extending inferiorly through the first few sacral segments ( Fig. 1 ). As previously mentioned, sacral stress fractures almost exclusively involve the sacral ala, or Zone I of the sacrum. In select cases, perpetuated strain may lead to transverse extension into Zone II with consequent neural foraminal involvement. ^, CT or radionuclide bone scans may be considered in situations wherein MRI is contraindicated. Radionuclide bone scans, while less specific, are more sensitive than CT in the diagnosis of sacral bone stress injuries.

Management

Relative rest, including cessation of offending and high-impact activities, is imperative in the management of sacral stress fractures. A brief, 1- to 2-week period of non-weightbearing or crutch-assisted ambulation is reasonable until the patient is able to walk without pain. Thereafter, nonimpact cross-training such as stationary cycling, swimming, or deep water running may be introduced, using caution to maintain a pain-free level both during and after sessions. Activity may be gradually advanced to include light-weighted exercise and sport-specific strengthening, followed by a progressive reintroduction of sport-specific activity. Antigravity treadmills, which incorporate an air-filled, pressure-controlled compartment that permits body unweighting below the level of the waist, have become increasingly popular in the rehabilitation of running athletes. These devices allow an athlete to run at a high intensity but with reduced loading to the lower extremities. One recent case report describes the successful return of a collegiate athlete with a pelvic stress fracture using this method.

During the initial recovery period, any underlying risk factors for bone stress injury should be identified and addressed, including education regarding training errors, reversal of energy imbalance, and correction of biomechanical deficits. Calcium and vitamin D intake should be optimized to promote bone health. Use of nonsteroidal anti-inflammatory drugs (NSAIDs) for pain control is somewhat controversial, although avoidance is generally recommended to limit any potential impairment of bone healing. Moreover, analgesic medications in general should be used with caution to avoid masking pain during rehabilitation and return to sport.

Recovery and return to sport following sacral bone stress injuries are variable, but may require longer periods than some other low-risk stress fracture sites. Fredericson and colleagues reported that return to preinjury training may take 3 to 6 months. Factors that can contribute to prolonged recovery include delay in diagnosis and the composition of the involved bone. ^, Bone stress injuries involving predominantly trabecular bone, such as the sacrum, appear to be associated with protracted recovery compared with those involving predominantly cortical bone.

Stress fractures of the pubic ramus

Epidemiology

The literature regarding stress fractures of the pubis is relatively sparse, although like sacral stress fractures, it has been submitted that pubic stress fractures may be under-recognized. One study involving military recruits over an 8-year period found that the pubic arch represented 8% of all diagnosed stress fractures. Another study comprised of military recruits with stress-related hip, buttock, or groin pain identified a total of 174 bone stress injuries involving the pelvis or proximal femur, of which 37 (21%) involved the pubic arch. Stress fractures of the pubic region most frequently occur within the inferior pubic ramus, near to the junction of the ischial ramus. This has been attributed to repetitive tensile forces imparted by the adductor magnus resulting in an avulsion-type fatigue fracture. ^,

Stress fractures of the pubis have been reported most commonly in runners and military recruits. ^, Select cases have also been described in bowling, gymnastics, and swimming. ^, Excessive stride length, or overstriding, has been posited as a possible risk factor for such injuries, especially in female military recruits. ^,

Presentation and Evaluation

Stress fractures of the pubic ramus can present with pain involving the inguinal, perineal, sacral, and/or gluteal regions. The pain is generally associated with physical exercise and relieved with rest. On physical examination, there may be tenderness to palpation over the anterior groin, inferior pubic ramus, or adductor muscle insertion. ^, The pain may be exacerbated by hip abduction and resisted hip adduction. ^, Thus, pubic stress fractures may be easily mistaken for groin or adductor strains.

Plain radiographs represent the initial imaging modality of choice and may demonstrate a hypointense transverse fissure and/or surrounding cloud-like callus, most often within the inferior pubic ramus ( Fig. 2 A). However, plain radiographs often fail to identify bone stress injuries of the pelvis including the pubic ramus, with a reported sensitivity of 47%. ^, Moreover, the symptoms of pelvic stress fractures often overlap with those of higher-risk sites, namely the femoral neck, such that advanced imaging is essential to attain an accurate diagnosis. MRI is generally favored and should include the entire pelvis and both proximal femurs ( Fig. 2 B).