**6. Avascular osteonecrosis**

Osteonecrosis, also known as aseptic necrosis, avascular necrosis, ischemic necrosis, and osteochondritis dissecans, is a pathological process that has been associated with numerous conditions and therapeutic interventions. The exact prevalence of osteonecrosis is unknown. In the United States, there are an estimated 10,000 to 20,000 new patients diagnosed per year, and osteonecrosis is the underlying diagnosis in approximately 10 percent of all total hip replacements. The male-to-female ratio of this disorder is 8:1, but varies with different comorbidities (Jones, 2011). The mechanisms by which this disorder develops are not fully understood. Compromise of the bone vasculature leading to the death of bone and marrow cells (bone marrow infarction) appear to be common to most proposed etiologies. The process is most often progressive, resulting in joint destruction within three to five years if left untreated. A variety of traumatic and nontraumatic factors contribute to the etiology of

from 20% to 90% and the reported accuracy is around 50-70% (El-Maghraby et al., 2006; as cited in Rosenthall et al., 1982, Turpin & Lambert, 2001, Wilson, 2004). The combination of 67Ga-citrate and Tc-99m-MDP bone scans has better results, with accuracy around 70-80% (El-Maghraby et al., 2006). But lower results for this combination has also been reported because the uptake of both tracers can be found not only in infection, but also in the postoperative patient, in heterotopic bone formation, and loosening or inflammatory reaction to cement

Labelled leukocyte imaging is, at least theoretically, the ideal technique for diagnosing the infected prosthesis because in general, white cells do not accumulate at sites of increased bone mineral turnover in the absence of infection (Palestro, 1998). However, labelled-leukocytes yield false positive results due to reactive or displaced bone marrow as a result of surgery are present up to more than 24 months after implantation (El-Maghraby et al., 2006; as cited in Oswald et al., 1990). Labelled leukocytes accumulate not only in infection but in the bone marrow as well. This problem has been overcome by the addition of complementary bone marrow imaging, which is usually performed with Tc-99m sulfur colloid. Both, labelled leukocytes and sulfur colloid accumulate in the bone marrow, but only labelled leukocytes accumulate in infection. In contrast to the results reported for labelled leukocyte imaging alone, the results of combined leukocyte-marrow imaging of prosthetic joints have been uniformly excellent, with an accuracy of 90% or greater (Love et al., 2001; as cited in Palestro 1990, 1991) and has become the method of choice to evaluate surgical prostheses (El-Maghraby et al., 2006; as cited in Love et al., 2001; Turpin & Lambert, 2001). Although extremely accurate, leukocyte-marrow scintigraphy is hampered by significant limitations. The in vitro labelling process is labor intensive, is not always available, and requires direct contact with blood products. The need for marrow imaging adds to the complexity and cost of the study and is an additional inconvenience to patients, many of whom are elderly and debilitated (Love et al., 2001). In an effort to maintain the accuracy of the study while reducing or eliminating the disadvantages, several methods of labelling leukocytes in vivo have been investigated, but

FDG-PET has been extensively investigated, the high-resolution tomographic images, availability of the agent, and rapid completion of the procedure are all desirable traits. Published results to date, however, are inconclusive in this setting (Love et al., 2009; as cited in Chacko et al., 2002; Joseph et al., 2001; Love et al., 2004; Manthey et al., 2002; Pill et al.,

Osteonecrosis, also known as aseptic necrosis, avascular necrosis, ischemic necrosis, and osteochondritis dissecans, is a pathological process that has been associated with numerous conditions and therapeutic interventions. The exact prevalence of osteonecrosis is unknown. In the United States, there are an estimated 10,000 to 20,000 new patients diagnosed per year, and osteonecrosis is the underlying diagnosis in approximately 10 percent of all total hip replacements. The male-to-female ratio of this disorder is 8:1, but varies with different comorbidities (Jones, 2011). The mechanisms by which this disorder develops are not fully understood. Compromise of the bone vasculature leading to the death of bone and marrow cells (bone marrow infarction) appear to be common to most proposed etiologies. The process is most often progressive, resulting in joint destruction within three to five years if left untreated. A variety of traumatic and nontraumatic factors contribute to the etiology of

fixators (El-Maghraby et al., 2006; as cited in Turpin & Lambert, 2001).

their role in prosthetic joint infection has not been established.

2006; Reinartz et al., 2005; Stumpe et al., 2004; Zhuang et al, 2001).

**6. Avascular osteonecrosis** 

osteonecrosis. Glucocorticoid use and excessive alcohol intake are reported to be associated with more than 90% of the cases. Osteonecrosis usually occurs in the anterolateral femoral head, although it may also affect the femoral condyles, humeral heads, proximal tibia, vertebrae, and small bones of the hand and foot. Many patients have bilateral involvement at the time of diagnosis, including disease of the hips, knees, and shoulders. The most common presenting symptom of osteonecrosis is pain and patients may have eventually limitation on range of motion. A limp may be present late in the course of lower extremity disease. A small proportion of patients are asymptomatic. In these cases the diagnosis is usually incidental. Asymptomatic involvement contralateral to a symptomatic site is frequently noted.

There is no pathognomonic feature of osteonecrosis. A clinical diagnosis is appropriately made in a symptomatic patient when imaging findings are compatible with this disease and other causes of pain and bony abnormalities are either unlikely or have been excluded by appropriate testing. The evaluation for suspected osteonecrosis should begin with plain film radiography, althought it can remain normal for months after symptoms of osteonecrosis begin. Features of osteonecrosis on plain film radiographs, radionuclide scans (**Fig. 3**), and MRI are helpful diagnostically and provide the basis for classification and staging systems. Early diagnosis of osteonecrosis is crucial: the earlier the stage of the lesion at the time of diagnosis, the better the prognosis. Clinically, early diagnosis and treatment of osteonecrosis might prevent unnecessary surgery (Pape et al., 2004). Therefore, early diagnosis and location of osteonecrosis have prognostic value and determine the therapeutic alternatives.

Fig. 3. Radionuclide bone scan of the pelvis in a 68-year-old man with hip pain. Bilateral central area of diminished uptake surrounded by a zone of increased uptake in the femoral head consistent with avascular necrosis.

Currently, MRI is the technique of choice for the diagnosis of avascular osteonecrosis in the early stages. This technique has been proven to be a highly accurate method both for early diagnosis (changes can be seen early in the course of disease when other studies are negative) and for staging of the disease (Malizos et al., 2007). MRI is far more sensitive than plain radiographs or bone scanning, with an overall reported sensitivity of 91% (Jones, 2011; as cited in Chang et al., 1993). Nevertheless, 99mTc bone scintigraphy also plays an important role in the early diagnosis of avascular necrosis and whole body bone scan is useful in patients with suspected polytopic osteonecrosis. The characteristic distribution of the radiopharmaceutical

Nuclear Medicine in Musculoskeletal Disorders: Clinical Approach 109

provided by the combination of three signs: Increase activity ratio in the blood pool phase performed at 5-15 min, diffuse uptake in the carpus o tarsus and periarticular uptake in all the small joints (Murray, 1998). Decreased radiotracer accumulation has also been described, especially in children and adolescents (Driessens et al., 1999; Love et al., 2003). Bone scintigraphy is of major importance for the diagnosis in order to clearly differentiate from other conditions which are incorrectly diagnosed and treated as RSD. If the bone scan is not suggestive of RSD, the clinical picture, radiological examination and vascular scan may lead to the correct diagnosis. This may be a pseudodystrophy, in which a hypovascularization is found right from the start, while in true RSD there is initially a hypervascularization. Other conditions which may be confused with RSD are causalgia, neurotic compulsive postures, hysterical conversion, malingering and even self-mutilation (Driessens et al., 1999). Bone scintigraphy has a high sensitivity in the initial stage of Sudeck's syndrome, but after 26

Osteoporosis is defined as a systemic skeletal disease characterized by low bone density and microarchitectural deterioration of bone tissue, with an increase in bone fragility and susceptibility to fractures. Despite an increase in bone turnover that is usually present in osteoporosis, the bone scan has no role in the diagnosis of uncomplicated osteoporosis, but the Tc-99m-MDP bone scan is most often used in established osteoporosis to diagnose fractures, particularly at sites that are difficult to image with plain film radiography (eg, sacrum, ribs), and may be particularly useful in the diagnosis and timing of vertebral fractures. It also has an important role in assessing suspected fractures where radiography is unhelpful, either because of poor sensitivity related to the anatomical site of the fracture (eg, sacrum **Fig. 5**) or because adequate views are not obtainable because of the patient's discomfort (Fogelman & Cook, 2003). The characteristic appearance of these fractures is discussed elsewhere in this chapter. If a patient complains of back pain with multiple previous vertebral fractures noted on

Fig. 5. Posterior and anterior Tc-99m-MDP bone scan showing a typical "H-shaped" pattern

in the sacrum, indicating a sacral insufficiency fracture.

weeks, it loses accuracy (Benning & Steinert, 1988; Lee & Weeks, 1995).

**8. Metabolic bone disorders** 

**8.1 Osteoporosis** 

in the affected area (cold area surrounded by a hyperfixation rim) enables early diagnosis, before the appearance of anatomical changes, which only show up later with radiography (Jones, 2011; as cited in Feggi et al., 1987 and Maillefert et al., 1997). The presence of this tipical pattern may increase the diagnostic accuracy to distinguish between osteonecrosis and transient osteoporosis, which usually has a diffuse pattern of tracer uptake, with no cold area. The accuracy of scintigraphy can be improved by using SPECT in patients with suspected avascular necrosis of the femoral head but have concomitant changes that may show up as false positives, such as severe acetabular osteoarthritis (Jones, 2011; as cited in Collier et al., 1985). It also may help us to avoid overlooking a subchondral fracture.
