Case Report: Idiopathic avascular necrosis of the femoral head following pregnancy

Introduction

Bone marrow cells and osteocytes die as a consequence of impaired blood supply to the bone affected. This pathological process is osteonecrosis, acknowledged as aseptic necrosis, ischemic necrosis, or avascular necrosis (AVN).¹ This impaired blood supply induces demineralization and trabecular thinning, leading to consequent joint surface collapse and subchondral bone fracture. It was first reported in 1957 by Pfeiffer.² AVN most commonly affects men between the ages of 30 and 50 years old.

Hip osteonecrosis has a number of known causes, including trauma, a genetic predisposition, sickle cell disorder, coagulation abnormalities, alcoholism, and high-dose corticosteroid therapy. Pregnancy is not a well-known risk factor for femoral head AVN, and it should be distinguished from one of the more prevalent hip pathologies in pregnancy, the so-called “Pelvic pain syndrome” and transient osteoporosis of the hip (TOH). Other than pregnancy, all other risk factors were ruled out for our patient. Not much is known regarding the development or management of AVN related to pregnancy since there have only been a few cases reported until now. Here, we have a case of neglected hip pain in a pregnant patient that resulted in AVN of the femoral head due to the uncommon association between pregnancy and the condition, which, if properly investigated early on, could have been treated more conservatively. According to the symptoms, examination, and imaging findings, a clinical diagnosis compatible with the disease is made.

Case report

Diagnostic assessment

Radiograph showed left femoral neck fracture with superolateral displacement of the greater trochanter and the rest of the shaft of the left femur (Figure 1). T1 and T2 weighted imaging sequences MRI of the femoral heads showed an irregular geographical area in the anterosuperior part of the left femoral head (Figure 2). T2, T1, and PD FatSat MRI images show the double line sign (Figure 3). Radiograph images showed a hip replacement implant prosthesis on the left side post-hip replacement surgery (Figure 4). The laboratory tests—complete blood count (CBC), erythrocyte sedimentation rate, liver function tests, c-reactive protein (CRP), creatinine, lipid panel, thyroid-stimulating hormone (TSH), rheumatoid factor and antinuclear antibody were all within normal limits. There weren’t any abnormities in the abdominal ultrasound. The patient was diagnosed with AVN of the femoral head on the left side and associated secondary osteoarthritis. One other pathology that could be confused with AVN is the TOH, which is self-resolving.

Figure 1. Radiogram antero-posterior view of the hip joint revealed fractured neck of the femur with the supero-lateral displacement of the femoral shaft on the left side.

Figure 2. Irregular geographical areas in antero-superior aspect of left femoral head appearing heterogeneously hypointense on T1WI and T2WI sequences.

T1WI, T1 weighted image; T2WI, T2 weighted image.

Figure 3. Irregular geographical areas in antero-superior aspect of left femoral head with the double line sign on T1WI, T2WI and PDFatSat sequences.

T1WI, T1 weighted image; T2WI, T2 weighted image.

Figure 4. Radiogram antero-posterior view of the hip joint revealed hip replacement implant prosthesis on the left side post-hip replacement surgery.

Discussion

The most common causes of atraumatic hip osteonecrosis include alcohol intake and corticosteroids.^3,4 Other risk factors include genetics, vasculitis, hyper coagulopathy, cocaine use, and micro emboli.⁵ While idiopathic AVN of the hip accounts for around 20% of cases,^3,4 femoral head osteonecrosis is rarely associated with pregnancy.

AVN of the hip associated with pregnancy has been linked to a number of pathophysiological mechanisms, but none of them have been proven to be the sole contributing factor. Pregnancy commonly results in increased coagulability and venous congestion.^6–8 Also, some endocrine changes that increase the levels of unbound cortisol during pregnancy have been linked to an increased risk of hip AVN.^6,7 Hyperplasia of parathyroid can also occur during pregnancy, which increases the parathyroid hormone levels thus increasing the risk for developing osteonecrosis.⁹ The endogenous lipoproteins can be destabilized by the placenta, which can eventually lead to fat embolism.¹⁰

The right common iliac artery lies right above left common iliac vein, making the vein more susceptible to compression with excessive weight gain and during pregnancy, especially on the left side. This mechanical factor has also been suggested as a contributing factor to the increased risk of hip osteonecrosis in pregnancy.^6,7,11,12 The literature indicates that the left hip is most frequently affected and this involvement mostly occurs in the third trimester or soon after delivery,^6,7,12,13 which supports the mechanical theory. In addition, hip AVN more commonly develops in primigravida,^7,14 and in older women^6,7,13,14 in contrast to our case, where the patient is young. From current studies, another cause may be ovulation induction, which activates both fibrinolytic and coagulation systems.^15,16 It has been reported that a difficult delivery or the compression of a growing uterus can lead to injury of the artery in the round ligament or the femoral joint.¹⁷ Biochemical, hematological and coagulation parameters were all within the normal limits in our patient.

The initial complaint is generally unilateral and can either be a gradually worsening or a sudden groin pain that radiates to the back, thigh or knee. Physical examination shows a restriction of active and passive joint movements associated with pain, more pronounced with rotation and less with flexion and abduction.¹⁸

TOH is another common hip pathology in pregnancy and is sometimes mistaken for hip AVN.^6,14,19–21 It is a rare cause of acute hip pain in pregnancy and most commonly occurs in the third trimester,¹⁹ characterized by sudden onset of pain in the hip that can be severe without movement restriction. TOH is a condition that is self-limiting and resolves gradually within seven to eight months. The management comprises of weight bearing restrictions.^6,21,22 By contrast, AVN onset is insidious, with pain increasing progressively, leading to movement limitation, and will eventually need early surgical intervention.^19,20,21 Hence it is very important to differentiate between these entities since the treatment and prognosis differ greatly.^6,12,19,21

Since the changes of osteopenia in TOH are evident only four to eight weeks from the onset of symptoms, the standard X-ray lacks sensitivity in distinguishing AVN and TOH.^20,21 With specificity and sensitivity of more than 99%, MRI is the gold standard imaging modality for diagnosing AVN in its early stages.²³ The MRI features of TOH are diffuse edema of the head and neck of the femur and, by contrast, MRI features of AVN include diffuse edema with focal defects and subchondral changes.^7,19–21 Double line sign is a pathognomonic MRI finding in AVN and refers to two adjacent serpentine lines demarcating the boundary between viable and devitalized bone marrow. The inner line corresponds to reparative granulation tissue, while the outer line corresponds to reactive sclerosis.²⁴

The best way to treat AVN associated with pregnancy is not very clear. Early diagnosis and advice are significant. Conservative methods such as physical therapy, restricted weight bearing, medication, electrical stimulation, extracorporeal shock-wave treatment, and hyperbaric oxygen treatment are usually started if there is no evidence of collapse.²⁵ The prognosis appears to be good following this conservative therapy, however surgical treatment would eventually be required for the secondary osteoarthritis or degenerative changes at a later age.¹⁸

Conclusions

Although AVN developing post-pregnancy is a rare occurrence, AVN of the femur should be one of the top differentials for post-partum hip pain. This could aid medical professionals in identifying the disease earlier before the collapse of the femur head occurs, which may aid in the preservation of the joint as opposed to replacement.

Consent

Written informed consent for publication of their clinical details and clinical images was obtained from the patient.