Idiopathic Transient Osteoporosis
A Hidden Cause of Hip Pain
Shawn Harrington, MD; Jay Smith, MD; Jeff Thompson, MD; Edward Laskowski, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 4 - APRIL 2000
In Brief: Idiopathic transient osteoporosis must be considered in active young adult and middle-aged patients who have spontaneous joint pain with acute onset. This often debilitating condition most commonly affects the hip; it is then marked by pain that intensifies with weight bearing or hip rotation. The knee, ankle, or foot can also be affected. Delayed diagnosis is common, in part because of a lack of physician familiarity with the condition. Physical exam tests can reproduce pain, and scintigraphy and MRI can help narrow the differential. Treatment is typically supportive and includes pain relief and physical therapy to maximize function. Two case reports illustrate management of the condition.
Idiopathic transient osteoporosis (ITO) is a relatively rare condition with distinct clinical and radiographic features. Differential diagnosis (table 1) includes avascular necrosis, stress fracture, septic arthritis, malignancy, soft-tissue injury, and radiculopathy. A detailed history and careful physical examination can narrow the list of diagnostic possibilities; however, laboratory and imaging studies are often required in cases in which a clear cause (eg, strain or sprain) cannot be readily identified.
TABLE 1. Differential Diagnosis for Idiopathic
Differentiation from bony avascular necrosis (table 2: not shown) is most problematic but has important prognostic and therapeutic significance. ITO, unlike avascular necrosis, is typically a self-limiting disorder with complete recovery as the rule. Treatment consists of observation, protected weight bearing, and pain-control measures. In severe cases, surgical core decompression has been successful.
Physicians who treat musculoskeletal conditions and injuries should be aware of ITO to avoid delayed diagnosis and to optimize treatment. ITO may be more common in middle-aged men than in the traditional "high-risk" group of women in the third trimester of pregnancy. The involvement of the former group in sports or other activities may confuse the diagnosis. This review will focus on idiopathic transient osteoporosis of the hip (ITOH), the most common type.
Terminology and Epidemiology
ITOH was first reported by Curtiss and Kincaid (1) in 1959. They described three patients who had hip pain accompanied by transient bone demineralization occurring in women during the third trimester of pregnancy. In 1968, Lequesne (2) coined the term "idiopathic transient osteoporosis of the hip" to describe the disorder. Since then, ITOH has been referred to as bone marrow edema syndrome (BMES), hip algodystrophy, transient bone demineralization, and, perhaps inaccurately, reflex sympathetic dystrophy of the hip.
The term "regional migratory (transient) osteoporosis" is used when multiple joints are affected in the same individual. The term BMES is currently reserved by some authors to describe a syndrome of a diffuse marrow edema pattern seen on magnetic resonance imaging (MRI) in the absence of radiographic evidence of osteopenia, and for which there exists no other clinical diagnosis to explain the disorder (3,4). ITO and BMES may indeed represent the same disorder identified at different stages.
Early reports suggested that ITOH primarily affected females during the second or third trimesters of pregnancy (5); however, men in their fourth through seventh decades of life currently account for more than two thirds of reported cases (6). ITO has also been reported, in decreasing frequency, in the knee, foot, ankle, and hand (7-10). ITO is extremely rare in children (11).
The cause of ITOH remains uncertain. Numerous theories exist based on observed clinical, pathologic, and radiographic features. The only known predisposing factors are late pregnancy and a history of transient osteoporosis.
The most commonly accepted theory is that microvascular injury causes tissue ischemia, resulting in marrow edema and limited cell injury, which also occurs in avascular necrosis. This theory is supported by the finding of edema of the bone marrow in biopsy specimens of ITOH and provides some rationale for using core decompression as treatment (3,12,13). Alternate theories include injury resulting from mechanical bone microtrauma, vasomotor instability as in reflex sympathetic dystrophy, microvascular thrombosis, and an unknown virally mediated insult (14).
Affected individuals most often present with progressively worsening and disabling unilateral pain in the hip region, often most intense in the groin. The discomfort may localize to the medial or anterior thigh. Radiation below the knee is atypical. Symptom evolution is acute and spontaneous without significant preceding trauma or injury. The pain is most often described as a deep ache that intensifies with weight bearing or with passive or active hip rotation. An antalgic limp is common.
Pain with weight bearing progresses and typically peaks within 4 to 8 weeks. At the height of symptoms, patients often experience significant impairments and functional disabilities. The pain abates when the patient rests, contrasting with the pain at rest often seen with malignancy, avascular necrosis, or infection. Back pain, numbness and tingling, and bowel or bladder dysfunction suggest a neurologic lesion and should be investigated appropriately. Simultaneous multijoint presentations are atypical and should prompt investigation into alternative causes.
Given the broad differential diagnosis, a complete history should include screening for avascular necrosis and its risk factors, such as corticosteroid or alcohol use, collagen vascular disorders, chronic illness, sickle cell disease, and numerous metabolic disorders (table 1) (14).
The degree of discomfort experienced by patients with ITOH can be quite striking, and often the impairment caused by pain overshadows objective clinical findings. On inspection, gait is antalgic, and a compensated Trendelenberg gait pattern may be appreciated. Late in the course, disuse atrophy of the thigh and gluteal muscles may be observed, especially if weight-bearing activities have been limited. No external signs of infection or inflammation are present.
Palpation reveals tenderness over the greater trochanter or pubis and/or in the proximal adductor and hip flexor muscle groups. The most common finding is guarding during hip range of motion, especially with abduction or rotation. Manual muscle testing may reveal only a pain-associated giving way caused by weakness about the hip, and neurologic examination is usually normal. Recommended provocative testing includes the FABER test (figure 1) and resisted straight-leg raising, in which anterior or medial groin pain is elicited with manually resisted straight-leg raising at 30° of hip flexion. Pain is also elicited by compression or rotation loads on the joint. Examination of the spine and knee will be normal in ITOH.
Role of Diagnostic Testing
In the appropriate clinical setting, specific testing should be obtained to exclude alternative diagnostic possibilities. In ITOH, laboratory testing is generally normal. Plain radiographs to rule out stress fracture, osteoarthritis, and tumor are usually obtained and can be normal very early in the course of ITOH. By 4 to 8 weeks after symptom onset, various degrees of diffuse osteopenia and cortical thinning will be present in the femoral head and neck and the intertrochanteric region.
If osteopenia is severe, radiographs may give the appearance of a "phantom joint," but films most often exhibit reduced bony trabeculation and accentuation of force lines. In contrast, radiographs in avascular necrosis typically progress from sclerosis to articular surface flattening, and finally to subchondral collapse and end-stage degenerative joint disease (15).
Bone scanning is often abnormal earlier than plain radiographs. Focal increased radionuclide accumulation occurs in a homogeneous pattern and reveals a target-like image about the femoral head and neck (figure 2). Bone scanning can be used to follow the disorder; images typically normalize parallel to symptom resolution (16). In avascular necrosis, radionuclide scanning may reveal a photopenic lesion rather than increased tracer uptake.
MRI is most useful to exclude alternative diagnoses, but with ITOH it reveals a characteristic but nonspecific pattern of diffuse, homogeneous bone marrow edema that can appear even very early in the disease course. Marrow edema appears as decreased signal intensity on T1-weighted images and increased signal intensity with T2 weighting, with the distribution of changes best appreciated in the coronal plane (figure 3). The affected areas are similar to those seen with bone scanning.
In the hip, mild changes can be noted in the adjacent acetabular bone, but prominent acetabular abnormalities should prompt consideration of an alternate diagnosis. In ITOH, the bone cortex may appear thinned but is otherwise intact, and there is no evidence of a subchondral defect as in avascular necrosis (figure 4). In a malignant lesion, a focal cortical disruption or more widespread marrow infiltration can be appreciated with MRI or computed tomography.
Distinguishing ITOH from avascular necrosis can be problematic. Avascular necrosis, however, produces some classic features on diagnostic imaging that are not apparent in ITOH. These include (1) a focal subchondral femoral head defect on radiographs and MRI (usually in the anterosuperior region) (figure 4) with surrounding marrow edema on MRI, and (2) the classic "double-line" or "crescent sign" on T2-weighted coronal MRI (a high-intensity rim inside a low-intensity margin surrounding the necrotic lesion). These findings are pathognomonic for avascular necrosis.
Some authors believe BMES to represent the earliest MRI findings of avascular necrosis, and that ITOH may be part of a spectrum of disorders ranging from BMES to avascular necrosis (17). Ultimately, the diagnosis of ITOH is proven by its transient and relatively benign nature; therefore, close clinical and radiographic follow-up is necessary, especially in equivocal cases.
When the clinical history and exam suggest alternative diagnoses such as infection, inflammation, or malignancy, then bone biopsy, joint fluid analysis, and/or open exploration may become necessary. In cases of uncomplicated ITOH, joint fluid is sterile with only a mild degree of lymphocytosis. Biopsies taken during therapeutic core decompressions or to exclude malignancy reveal mild synovial thickening with normal cartilage and varying degrees of porotic trabecular bone, reactive bone formation, osteoclastic bone resorption, rare lipid cysts, and pronounced marrow edema (18). However, the finding of limited fat necrosis in some cases of presumed ITO likely represents the earliest stages of avascular necrosis. This has led some investigators to hypothesize that a more limited ischemic insult may lead to ITOH, while a more extensive insult results in osteocyte cell death and avascular necrosis (12,14).
Pain Relief and Physical Therapy
The natural history of ITOH is one of an eventual symptom plateau followed by a gradual, spontaneous resolution occurring over 6 to 10 months (19). No specific treatments have been shown to alter the natural course of the disorder; therefore, management is generally supportive once a clear diagnosis has been established.
Pain management strategies include protected weight bearing and nonsteroidal anti-inflammatory medications, although narcotics are sometimes necessary. Alendronate, a bisphosphonate, has been successful in treating hip pain in a case of postpartum bilateral ITOH (20), and deflazacort, a bone-sparing corticosteroid, has also been used in similar situations (21).
No reports regarding calcitonin in the treatment of transient osteoporosis are available, although its use in the treatment of pain associated with primary osteoporotic compression fracture is generally well accepted (22). Surgical core decompression for the relief of increased intramedullary pressure, as advocated for avascular necrosis, is sometimes used in severe or refractory cases. Several authors have reported pain relief and early return to function after core decompression (13,19).
Physical therapy should be initiated in all cases and incorporate gait retraining with a cane, crutches, or other aide. Protected weight bearing is particularly important during the peak of symptoms. There is no conclusive evidence to suggest that weight bearing as tolerated will adversely affect outcome, but protected weight bearing usually facilitates pain control and increased function.
Early implementation of a gentle isometric strengthening, flexibility, and range-of-motion program will minimize deconditioning. As symptoms allow, the program can be progressed to isotonic exercise and nonimpact or low-impact cross-training. Most patients can tolerate aquatic therapy such as water walking or running. Because peak symptoms can be so disabling, clinicians should address issues of employment and disability access early in the disease course.
When to Refer
The decision to order other diagnostic testing or imaging beyond plain radiographs may be best managed by a qualified specialist in cases of atypical hip pain. Uncomplicated cases of confirmed ITOH can be followed by a nonsurgical specialist or primary care physician comfortable with using the above outlined principles.
Although joint contractures can occur, insufficiency fracture of the femoral neck is considered the most serious complication of ITOH. Subcapital fractures of the femoral head require orthopedic referral and may require surgery (23,24). An apparent osteochondral defect consistent with avascular necrosis deserves prompt referral to a surgical specialist because early operative intervention is often recommended. A protracted clinical course or intractable pain may warrant core decompression, although the exact role of this technique in the management of ITOH remains to be defined. These decisions are best approached by a specialist familiar with the condition.
ITOH is a relatively rare condition that is likely underappreciated. The demographics of ITOH are changing, and the condition is increasingly diagnosed in middle-aged, athletically active individuals (see "Illustrative Cases of Idiopathic Transient Osteoporosis," below). The clinical picture is characteristic, but not entirely specific. Timely diagnosis requires a high index of suspicion, accurate history taking, and consideration of a broad differential. Physicians treating active middle-aged individuals who have unexplained hip pain should be aware of ITOH and its association with significant, yet transient, functional limitations.
Illustrative Cases of Idiopathic Transient Osteoporosis
Case 1. An otherwise healthy 37-year-old male recreational runner reported 3 weeks of discomfort in his left upper thigh and hip region. The pain began during a long hiking trip and had escalated. He described a deep aching sensation in the groin with sharp pain on weight bearing. At the time of the initial evaluation, he was unable to ambulate without limping and had been off work for about a week.
On examination, pain was exacerbated by weight bearing and with passive hip internal and external rotation. Detailed neurologic examination and nerve tension signs were unremarkable. Provocative testing with FABER and resisted straight-leg raising (see the "Physical Examination" section above) reproduced his familiar groin pain.
Plain radiographs of the hips were unremarkable. Bone scanning to exclude a stress fracture revealed a focal target-like increased uptake at the femoral head and neck (figure A). Magnetic resonance imaging (MRI) taken 8 weeks from symptom onset (figure B) revealed diffuse T1 and T2 signal changes without focal cortical abnormality, consistent with idiopathic transient osteoporosis of the hip (ITOH). The patient was treated nonoperatively with protective weight bearing, work restrictions, physical therapy for maintaining range of motion, isometric and isotonic strengthening, and a nonsteroidal anti-inflammatory drug (NSAID).
Symptoms reached a plateau and began to slowly improve. Follow-up radiographs at 4 months revealed subtle decreased trabeculation within the femoral head and neck, supporting the diagnosis of transient osteoporosis of the hip. By 6 months, the patient had returned to normal activities, including recreational sport, with only mild residual discomfort. Repeat MRI revealed complete resolution of the bony edema (figure C).
Case 2. A 46-year-old man was referred to a sports medicine clinic regarding a 3-week history of spontaneously occurring and gradually worsening ache in his right anterolateral thigh and groin. His pain was exacerbated by active hip flexion or rotation—especially when getting in and out of his car—and could be alleviated only with rest and avoidance of hip flexion. He reported no trauma or associated numbness, tingling, weakness, or low-back pain. The patient was recreationally active and jogged regularly, and he denied any recent changes in his training regimen.
Evaluation prior to referral included plain radiographs of the lumbar spine, hips, and pelvis; no abnormalities were noted. Treatment had consisted of NSAIDs. Examination revealed no acute distress at rest. Gait was antalgic, and he tended to keep his affected limb "stiff." Palpation revealed no focal tenderness. Passive range of motion elicited considerable discomfort during abduction and adduction and rotation. He was unable to obtain the FABER position (figure 1 on page 88) due to pain. Straight-leg raising on the affected side caused groin pain at 60° of elevation. Neurologic and spine examinations were normal.
An MRI was obtained to rule out avascular necrosis. It revealed abnormal T1 and high T2 signal changes in the femoral head and contiguous femoral neck (figure D). A tiny amount of effusion was present in the hip joint. The findings were consistent with ITOH.
The patient was placed on crutch-assisted ambulation and off-work status. A physical therapy program was initiated to prevent soft-tissue contractures and maintain strength. A water activities program was begun, and by 6 months the patient reported 90% improvement in symptoms and had returned to low-intensity jogging. A follow-up radiographic examination was not deemed necessary, and he returned to full activities 10 months after symptom onset.
Dr Harrington is senior resident in the Department of Physical Medicine and Rehabilitation at the Mayo Clinic in Rochester, Minnesota. Dr Smith is an assistant professor and Dr Thompson and Dr Laskowski are associate professors of physical medicine and rehabilitation at the Mayo Medical School in Rochester, Minnesota. Address correspondence to Jay Smith, MD, Mayo Clinic, Dept of Physical Medicine and Rehabilitation, E-10, 200 1st St SW, Rochester, MN 55905; e-mail to [email protected].
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