Absence of hypointense areas in the subchondral bone marrow on MR images of the hip ensures reversibility of imaging findings and complete resolution of clinical symptoms. Occasionally though, thin hypointense subchondral lesions may be observed in patients with TOH due to subarticular insufficiency microfractures related to the demineralization [20]. When the thickness of these areas is equal to or exceeds 4 mm, there is a strong possibility for lesion irreversibility [3]. It must be stressed that the diagnosis of TOH is established with certainty only after demonstration of complete resolution of imaging findings and/or clinical symptoms (Fig. 7.2).

Osteonecrosis (ON) or avascular necrosis of the femoral head more commonly affects young adults. It is more frequent in men than women with a 7:3 ratio [21]. Osteonecrosis, either idiopathic or secondary to a variety of conditions, is characterized by local ischemia which leads to necrosis of trabecular bone and bone marrow and eventually to collapse of the articular surface. The femoral head is more prone to osteonecrosis compared to other osseous sites, because of its relatively poor vascular supply. At presentation, symptoms of ON of the femoral head may be insidious with vague pain at the hip, thigh, and gluteal region.

Osteonecrosis of the femoral head does not present with a pure bone marrow edema pattern. Bone marrow edema is a late feature of this disorder, associated with worsening of pain and a more dismal prognosis [22]. According to one study of 200 patients with early findings of ON of the femoral head, bone marrow edema never occurred before the appearance of the characteristic band-like sign which consists of a band of abnormal signal intensity at the interface of necrotic and normal marrow [23]. On non-fat-suppressed T2-weighted spin-echo images, the abnormal interface is seen as two rims, a low-signal-intensity outer rim and a high-intensity inner rim (double line sign) [24]. On STIR or fat-suppressed FSE T2-weighted images and on contrast-enhanced fat-suppressed T1-weighted images, it is seen as a single high-intensity rim (bright band sign). A subchondral fracture with high T2 signal (corresponding to the radiolucent crescent sign seen on plain radiographs) is also, when present, strongly indicative of ON.

Transient osteoporosis of the knee (transient BME of the knee) is a distinct clinical entity. It presents with acute onset of pain without prior trauma and manifests with a bone marrow edema pattern on MR images, which by definition resolves completely at follow-up. According to a large study comparing MRI findings at presentation with final outcome, features which favor the diagnosis of transient osteoporosis of the knee over osteonecrosis include extension of edema to the non-weight-bearing surface of the bone and sparing of the subchondral marrow (found, respectively, in 44.9 % and 37.7 % of patients with a final diagnosis of transient osteoporosis of the knee) (Fig. 7.3) [25]. In the same study, almost half of patients with BME of the knee associated with a T2 hyperintense subchondral fracture were finally diagnosed with transient osteoporosis; therefore, according to this study, in the knee (unlike the hip), the presence of a subchondral fracture does not exclude the diagnosis of transient osteoporosis.

Spontaneous osteonecrosis of the knee (SONK), first described by Ahlback in 1968, may present with acute bone marrow edema [26]. It mostly affects elderly women, has no known cause, and is not associated with any of the known risk factors for osteonecrosis. Development of microfractures in osteoporotic bone which then lead to osteonecrosis is the most popular theory for the development of SONK [25, 27]. Patients with SONK present with symptoms similar to those of osteoarthritis or meniscal injury. Indeed, osteonecrosis of the knee is present in 10 % of patients with osteoarthritis [28]. On MR images, SONK presents with diffuse bone marrow edema. It affects mostly the weight-bearing surface of the medial femoral condyle (in contrast to transient osteoporosis which often involves the non-weight-bearing parts of the knee). Articular collapse at presentation or during the evolution of the disorder is characteristic of SONK and excludes the diagnosis of transient BME [24]. Furthermore, generalized osteopenia is present in patients with SONK, but it is unusual in a patient with transient BME.

Treatment of transient osteoporosis of the hip includes analgesic drugs and restricted weight bearing. Bisphosphonates are routinely used as they may reduce pain and duration of symptoms [29]. The use of core decompression to expedite recovery and for patients with debilitating pain is controversial considering the self-limiting nature of this disorder [30]. In pregnant women with transient osteoporosis, restriction of weight bearing until reconstitution of normal bone density is strictly recommended to avoid fractures of the hip, which have, occasionally, been reported [31].