Laboratory Studies
See the list below:
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Creatine kinase
This test is not specific for HO but is of value in determining the severity of muscle involvement and may be helpful in planning treatment of HO. Rossier and colleagues showed in 1973 that patients with an acute form of HO after SCI have elevated CK levels that correlate with histologic involvement of muscle. [14] Two subsequent studies found CK to be useful in the diagnosis and management of HO. Singh and coauthors reported significantly higher CK levels in patients with HO. [24] Data published by Sherman and colleagues indicated that a higher level of CK ultimately correlates with a more severe form of HO, suggesting more widespread involvement of surrounding muscle. [25]
These results are promising, because they indicate that CK may reliably predict a higher risk of HO development, can help to predict the severity of a patient's HO, and can be used to follow treatment success.
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C-reactive protein
The initial stage of HO is manifested by a prominent inflammatory response. This acute reaction is accompanied by changes in levels of cytokines that stimulate the production of acute-phase proteins, one of these being C-reactive protein (CRP).
A study by Estrores and colleagues indicated that the serum concentration of CRP correlates better than does the erythrocyte sedimentation rate with the inflammatory activity of HO after SCI. [26] In their study, the normalization of CRP in serum was accompanied by a resolution of the inflammation of soft tissue. It seems that administering nonsteroidal anti-inflammatory drugs (NSAIDs) in the early phase of HO, as well as monitoring the serum CRP level, may provide added benefit in reducing the inflammatory reaction that is proposed to be an important factor in HO's genesis.
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Alkaline phosphatase
The AlkP level, once a commonly used test, is not often employed today.
In many patients, serum AlkP levels are not elevated in acute HO.
The elevation can be nonspecific because of associated skeletal injuries or the surgical treatment of fractures.
The serum AlkP level is of little value in determining the maturity of HO prior to surgical removal.
Imaging Studies
Bone scintigraphy
Ideally, the use of diagnostic imaging should focus on the detection of nonmineralized HO, because the presently available medication, etidronate, can inhibit early mineralization. [27] In this respect, bone scintigraphy and ultrasonography are recommended imaging studies for the early diagnosis of HO.
Bone scintigraphy is highly sensitive in the early diagnosis of HO. This is the most commonly used diagnostic study for HO.
Freed and colleagues evaluated the 3-phase bone scan in the detection of HO and found that a marked vascular blush and increased blood pool about the hips preceded the development of clinical HO by 2-4 weeks. [28] The 3-phase bone scan using technetium-99m (99m Tc) diphosphonate is used in diagnosing and monitoring HO.
Ultrasonography
This is also used in the early diagnosis of HO about the hips. However, no data are available on the diagnostic value of ultrasonography in the diagnosis of HO in other joints (eg, knee, shoulder, elbow).
Radiography
While plain radiography is highly specific in the diagnosis of HO, this method lacks sensitivity in early diagnosis. Because soft-tissue calcification must occur for radiographic evidence of HO to be present, radiographs are not helpful in the early stages. Radiologic examinations do not show evidence of HO until a flocculent, patchy appearance develops, as calcium is deposited about 7-10 days after the onset of clinical symptoms.
This patchy appearance coalesces and enlarges on subsequent examinations, and by 2-3 months, the boundaries of the HO demarcate with the appearance of mature bone. Radiography, however, is not reliable at assessing the maturity of HO, because more mature areas may hide immature areas.
CT scanning and MRI
Computed tomography (CT) scanning and magnetic resonance imaging (MRI) may be useful in delineating local anatomy prior to resection.
The role of CT scanning and MRI in the evaluation of other aspects of HO has not been well established.
A retrospective study by Bachman et al indicated that CT scanning can be used prior to the excision of HO from the elbow to distinguish the paths of the radial and median nerves and to precisely determine the distance of these nerves from the ossification. In a study of 22 patients who had undergone removal of HO from the elbow, CT scan distinguished the radial nerve from the HO in 21 patients and the median nerve from the HO in 17 cases. The distance of HO from these nerves (3 mm and 9 mm from the radial and median nerves, respectively) was also determined. [29]
Three-dimensional (3D) stereolithography
This can be beneficial in the perioperative management of symptomatic HO. High-resolution CT scanning is used to create models that can assist during HO excision by allowing frequent intraoperative reference that contributes to the avoidance of iatrogenic neurovascular injuries. [30]
Other Tests
See the list below:
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Biopsy
Biopsy has no role in the diagnosis of HO, but it has been considered as a means of helping to determine maturity.
There is a possible risk of inadequate sampling, because mature and immature HO may be intermixed.
