Immediately after the onset of pain, usually no remarkable abnormal findings are detected unless collapse of the femoral head has already occurred. With time, osteopenia in the proximal femur and/or subchondral fracture line can be detected together with femoral head collapse when it occurs, but not always (Figs. 29.1, 29.3, 29.4 and 29.6c). In insufficiency-type fractures of the elderly, joint space narrowing is frequently observed, but the mechanism is not known (Fig. 29.2). The subchondral fracture line is usually more apparent on frog-leg view as in ONFH (Figs. 29.3, 29.5c and 29.6c).

Bone scintigraphy always shows focal or diffuse increased uptake in the femoral head (Fig. 29.3). It is nonspecific for the diagnosis but valuable as a screening test.

MR imaging is the most valuable and indispensible diagnostic tool (Figs. 29.1, 29.2, 29.3 and 29.4). Characteristic findings are subchondral abnormal intensity band and surrounding bone marrow edema pattern. The subchondral abnormal signal intensity band representing the fracture line is an essential finding for diagnosis. This irregular linear band is low signal intensity on T1-weighted images and variable abnormal signal intensity on T2-weighted images. The area of bone marrow edema pattern can be localized to the fracture line or diffuse in the femoral head and sometime extends to the intertrochanteric area. Bone marrow edema pattern decreases gradually and disappears completely together with the symptoms as the fracture heals. The collapse of the femoral head or thinning of the articular cartilage is identifiable when existing, and joint effusion is a frequent finding.

No or protected weight bearing is a basic initial treatment for SSFFH with no or mild collapse of the femoral head. The duration of limitation in weight bearing has not been established.

The management for the collapsed femoral head is also not clear. When detected early, it is possible to elevate the collapsed area and maintain it until fracture healing by impaction of strut bone graft (Fig. 29.1); a few reports have described this approach [8, 29]. Recently, Yamamoto et al. reported successful short-term results of transtrochanteric rotational osteotomy for collapsed cases [44].

When symptoms persist with or without improvement with conservative treatment, usually in insufficiency-type fractures, THA is necessary.

Symptoms decrease gradually and disappear completely within several months. Symptoms tend to persist longer in collapsed cases, up to 9 months. When the fracture heals without collapse of the femoral head, no subsequent problems arise. In fatigue-type fractures, cases healed with moderate-to-severe collapse have been reported to do well without any significant symptom for more than 10 years (Fig. 29.4) [50].

Unlike fatigue-type fractures, conservative treatment is not successful in the large proportion of insufficiency-type fractures. Almost half of the reported cases received total hip arthroplasty because of inadequate improvement with conservative treatment [32, 45, 52]. The reason for this poorer result than fatigue-type fracture is not clear, but old age and joint space narrowing have been suggested to be associated with the poor result. Development of rapidly destructive coxopathy subsequent to insufficiency-type fractures has been reported [53–55].

As was pointed out in the first report of insufficiency-type SSFFH, the differentiation of ONFH and TOH from SSFFH is both necessary and important. ONFH has similar clinical and image findings, but the prognosis is very different from that of SSFFH; after the onset of pain, collapse of the femoral head and subsequent joint destruction progresses, and most cases need total hip arthroplasty.

Subchondral fracture and bone marrow edema also occur in ONFH. However ONFH differs in two aspects (Fig. 29.5). In ONFH, an additional abnormal signal intensity band is detected outside the subchondral fracture line. The outer abnormal signal intensity band is the outer margin of the necrotic lesion, the reactive interface between dead and living bones [56, 57]. The reactive interface is apparent as a low signal intensity band on T1-weighted images and the band is the first detectable abnormal MR finding of ONFH [58–60]. In the early stage of ONFH, the reactive interface appears as an inner high and outer low signal intensity band, the so-called double line sign, on T2-weighted images. The other point of difference is the extent of the bone marrow edema [8, 29, 43, 61]. In ONFH, bone marrow edema occurs only outside the reactive interface and not in the necrotic area. On the contrary, bone marrow edema extends up to or over the subchondral fracture line in SSFFH. The convexity of the subchondral fracture line was proposed as a differential point because subchondral fracture lines are usually convex to the articular surface, and reactive interface lines are usually concave to the articular surface [62, 63]. However, both the concave subchondral facture lines and the convex reactive interfaces are detected occasionally (Figs. 29.5a and 29.6) [64]. Osteonecrotic lesions without subchondral fracture are always painless, and two bands of subchondral fracture and reactive interface are usually observed in symptomatic ONFH. When the reactive interface is detected clearly as a sclerotic line on plain radiographs, a differential diagnosis can be made without MR images (Fig. 29.5c).

The clinical course and image findings of TOH, a condition of unknown etiology, are identical to those of SSFFH without femoral head collapse [25–31]. The only difference is that the subchondral fracture line is not detected in TOH. Miyanish et al. [28] detected suggestive findings of subchondral fracture in a retrospective review of MR images of cases previously diagnosed as TOH; subchondral fracture line was recognized in the MR images of many past publications of transient osteoporosis of the hip [65–76]. The differential diagnosis depends solely on the detection of the subchondral fracture line, but sometimes the fracture line can be detected only on some slices of MR images made with various techniques including gadolinium enhancement. In addition, it is likely to be more difficult or even impossible to detect the fracture line when it is incomplete. Recently, Kim et al. reported a case of bilateral TOH in which serial MPR CT findings suggested callus formation in the subchondral trabeculae followed by remodeling [31]. These propose that subchondral trabecular injury is likely the etiology of TOH.

SSFFH and ONFH are similar but definitely different diseases. The prognosis of SSFFH is much better than that of ONFH. Therefore, it is extremely important to make a differential diagnosis and it is possible with MR images. TOH may well be the milder form of subchondral trabecular bone injury.