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Introduction

In a context of hip pain and suspicion of aseptic osteonecrosis of the femoral head, the role of the imaging device is, on the one hand, to confirm that there is indeed damage to the head and, on the other, to establish a diagnosis of osteonecrosis by eliminating at the same time the two great differential diagnoses which are algodystrophy and epiphyseal stress fracture due to trauma. These days, a virtually certain diagnosis will be based on nuclear magnetic resonance imaging, revealing the existence of a subchondral fracture, key to all the classifications, on the attentive examination of several views of standard radiographs and eventually of CAT scan image slices. Scintigraphy is no longer essential.

Diagnostic

Scintigraphy

A technique for earlier detection than the la radiograph or the scanner its sensitivity is high: 85%. However, it is not very specific and is difficult to interpret in the case of bilateral damage.

It will show a zone of hypointense fixation at an early stage representing the purely ischaemic phase, then a secondary hyperintense fixation translating the hyperaemic and reconstructive reaction of healthy bone around the necrotized bone (with the appearance of a rosette). Later on, the hyperfixation will prevail and mask the hypofixing avascular zone. It becomes more regular, localized or extensive and thus non-specific, a similar image being possible in the case of decalcifying algodystrophy.

 MRI

At present this is the technique offering the earliest detection and the most sensitive in detecting the disorder. In addition, it enables a pre-treatment assessment to be made with precise prognosis, as well as making the differential diagnosis with algodystrophies and subchondral stress fractures (Figures 1 to 12).

Figure 1 : MRI in weighted Spin echo sequence in T1 (T1) = normal hip

Figure 2 : MRI in weighted Spin echo sequence in T1 (T1) = normal hip

Figure 3 : MRI in weighted Spin echo sequence in T2 with suppression of fat (FAT) = normal hip

Figure 4 : MRI, appearance of necrosis in T1.

Figure 5 : MRI, appearance of necrosis in T2.

Figure 6 : MRI, appearance of necrosis in FAT.

Figure 7 : MRI, appearance of algodystrophy inT1.

Figure 8 : MRI, appearance of algodystrophy in T2.

Figure 9 : MRI, appearance of algodystrophy in FAT.

Figure 10 : MRI, appearance of a subchondral stress fracture in T1.

Figure 11 : MRI, appearance of a subchondral stress fracture in T2.

Figure 12 : MRI, appearance of a subchondral stress fracture in FAT.

Technique

• Weighted sequences in T1: for some their normality is sufficient to exclude them from the diagnosis.
  
• Weighted sequences in T2: they enable specificity of the diagnosis to be refined and to find intra-articular effusions, often the cause of pain.
  
• T2 sequences with suppression of fat (STIR and FATSAT) can usefully complete the examination by showing anomalies of bone marrow in the form of œdema in the head and neck of the femur.
  
• The injection of gadolinium is not systematic and even not useful in some cases. In particular it permits differentiation of osteonecrosis of stress fractures or of algodystrophy in uncertain cases.

The sequences are carried out in the frontal plane to enable a comparative study of both femur heads, and in the sagittal plane to show early anomalies in sphericity of the head. The axial plane will contribute to the spread assessment of necrotic lesions.

Symptomatology and MRI

In the majority of cases, MRI will reveal a characteristic appearance, very specific, common to all avascular osteoneocroses and never described in other disorders of the hip. However, one needs to know that the various abnormalities observed do not all have the same specificity.

The radiolucent demarcation line delimiting the zone of necrotic tissue translates into a hypointense signal in a band a few millimetres thick visible on the weighted sequences in T1, which runs imperatively from cortical to cortical whatever the image slice. This line is sinuous in most cases, but overall concave upwards. This radiolucent line corresponds to the interface of vascular reaction between the necrotized bone and the healthy bone and appears a few weeks after necrotic tissue is constituted. Its specificity is close to 100 % and its sensitivity to 80-90 %. The signal of this radiolucent line brightens initially after injection of gadolinium and its signal on the weighted sequences in T2 varies according to the degree of calcification: initially as a pure hyperintense signal, then, depending on the calcification, it evolves towards a hypointense signal. It is sometimes doubled in T2 by a parallel line in hyperintense signal (Mitchell’s classic double line) due to a phenomenon of chemical shift.

* The zone of necrotic bone delimited by the radiolucent line is variable in extent and seated in most cases at the superoventral part of the head. A focal compression of the necrotic zone is sometimes revealed through the various image slices. The signal of the necrotic zone varies according to the age of the lesion: at an early stage, the necrotic zone is a pure fat signal; it is not until some time after that the signal modifies as per Table I.

* The associated signs are in particular represented by the medullary œdema of the normal part of the head and neck of the femur. This œdema is frequent but completely aspecific, represented by a zone of hypointense signal T1 and hyperintense signal T2, or after intravenous injection of gadolinium, and better seen on the fat suppressed sequences (STIR or FAT SAT). The other sign classically associated is the presence of an intra-articular effusion which can be noted, quite visible on the weighted sequences in T2.

Figure 13 : Comparative MRI scan of both hips bringing to light a bilateral osteonecrosis (ARCO stage 3 to the right, Stage 1 to the left).

* One of the interests of an MRI scan is to check the often bilateral nature of the osteonecrosis of the femur head (Figure 13) and to note that the pain is linked to the intra-articular effusion and the oedema of the femur neck, these two anomalies not being found in asymptomatic hips.

* Differential Diagnosis: You need to be well acquainted with two differential diagnoses: algodystrophy, where the oedema is present but in most cases involves extensive plaques, or even the whole of the femur head with frequent extension towards the neck, but these plaques are not delimited by a radiolucent line (Figures 7, 8 and 9); fractures through impaction and insufficient bone or due to stress, where a radiolucent line with a hypointense signal not far from the subchondral bone and parallel to it can be found, but the radiolucent line is not joined to the cortical bone (Figures 10, 11 et 12);

Figure 10 : MRI, appearance of a subchondral stress fracture in T1.

Figure 11 : MRI, appearance of a subchondral stress fracture in T2.

Figure 12 : MRI, appearance of a subchondral stress fracture in FAT.

* Measurement of surface and volume: Analysis of MRI image slices enables one to measure more precisely than Kerboul’s angle the surface of the necrotic zone and even its volume using 3D analysis of the digitized image slices (Cf. Mazières article on prognosis).

Search for subchondral fracture

Standard Radiographs

The period before the appearance of radiographic signs in relation to clinical symptoms can be quite long because the radiological aspect of dead bone cannot initially be distinguished from healthy bone. It is the reaction of the tissue at the junction of dead and live bone which will permit the appearance of initially discrete signs and a perfected x-ray technique is required to reveal them.

- Lateral pelvis.

- Comparative AP and lateral views (lateral urethral or Ducroquet’s medical profile) centred on the femoral heads.

In the case of strong clinical suspicion, if the previous views are normal, additional x-rays should be taken:

- Lateral view at an ascending 30° ray with discrete external rotation of the head. It has the advantage of being tangent to the superoventral quadrant of the head which is most frequently the object of subchondral compression fractions. In addition, it unmasks the edge of the acetabulum.
- Views under traction which can increase subchondral bone dissection and sometimes show up a void.
- Views under compression so as to improve the quality of the images.

Depending on the extent of the radiograph signs, aseptic osteonecrosis of the femoral head can be classified in four stages according to the radiological criteria defined by Ficat:

Figure 14. X-ray Ficat 1- ARCO 0 and 1

Figure 15 : RX Ficat 2- ARCO 2

[Click image to enlarge] Figure 16. X-ray Ficat 3-ARCO 3

Figure 17. X-ray Ficat 3- ARCO 4

Figure 18. X-ray Ficat 4

CAT Scan

It is more sensitive than the standard radiograph in detecting stage II condensing anomalies in the form of bands or plaques of sclerosis which sit in the superoventral part of the head around the necrotic zone. There is therefore an irregularity of the femoral head and early alterations of normal trabeculation of the head (asterisk sign) (Figure 24). The CAT scan can also show a subchondral fracture or a flattening of the head which is invisible on standard x-rays (Figure 29). In this case, sagittal 3D reconstructions can be very useful.

Figure 24. Stage 2 confirmed by CAT.

Figure 29. CAT Stage 3 Ficat- 3 ARCO

However, although the symptomatology of the CAT scan needs to be known because something could be found incidentally during an exploration of the hip, the scan, as with the old tomographies, is not at present the examination to be asked for systematically when faced with osteonecrosis of the hip. It should be reserved for cases where there is doubt concerning the existence of a subchondral fracture upon examination of the standard radiographs.

Diagnostic pitfalls

When the femoral head appears to be spherical without a radio-visible subchondral fracture on the AP and lateral radiographs, the contours of the femur head on the AP view in flexion at 30° needs to be examined attentively, along with the various CAT image slices (Figures 19 and 20, Figures 21, 22, 23 and 24, Figures 25 and 26, Figures 27, 28 and 29).

Figure 19. AP x-ray: slight displacement = stage 3 Ficat, 4 Arco

Figure 20. Three months later.

Figure 21. AP x-ray Stage 2 ?

Figure 22. Lateral x-ray Stage 2 ?

Figure 23. X-ray flexion 30° Stage 23.

Figure 24. Stage 2 confirmed by CAT scan

Figure 25. AP X-ray Stage 2 ?

Figure 26. X-ray flexion 30° Stage 3 Ficat-4 ARCO

Figure 27. AP x-ray Stage 2 ?

Figure 28. Lateral x-ray Stage 2 ?

Figure 29. CAT Stage 3 Ficat- 3 ARCO.

Classification de l'arco

Many classifications have been suggested (cf. article B. Mazières: Prognosis concerning osteonecrosis). The ARCO classification (Table II) takes into account clinical date and all paraclinical tests, the existence, or not, of a remaining subchondral fracture, which, as in all the other classifications, is a determining element.

Conclusion

Magnetic resonance imaging has become crucial in the diagnosis and management of osteonecrosis of the femoral head. It also permits a better grasp of the pathological modifications in the head and neck of the femur and the joint cavity, enabling at the same time an explanation of clinical symptoms. The improvement of surface antenna, the possibility of studying in a sequential manner the contrast uptake in the epiphysis and spectroscopy should enable optimization of the technique in the future.
Radiographs are still indispensable to classify the stage of necrosis and thereby chose the line of treatment.