Pelvic heterotopic ossification: when CT comes to the aid of MR imaging
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This article compares various imaging aspects of magnetic resonance (MR) and computed tomography (CT) of heterotopic ossification (HO) in the pelvic soft tissues in paraplegic patients. Our aim is to highlight the benefits of integrating MR and CT imaging in the diagnosis of immature HO, which may be challenging with MR images alone.
Paraplegic patients examined on the same day by contrast-enhanced 0.4-T pelvic MR and unenhanced CT for pressure-sore-related infections were selected. MR imaging was performed on a Hitachi-Aperto 0.4 T; the Open Magnet served as a more favourable configuration for the required limb positioning of these patients. CT images were attained on a six-slice Siemens-Somaton-Emotion.
MR images of HO differ according to the degree of bone maturity. The more immature the HO process, the more heterogeneous is the signal, characterised mostly by focal iso-hypointensity on T1-weighted images and hyperintensity on T2-weighted/short TI inversion recovery (STIR). These characteristics correlate to different CT patterns.
MR and CT features of pelvic HO in paralysed patients were reviewed with a focus on the different aspects associated with the degree of ossification. Based solely on the MR findings, immature heterotopic ossification may be difficult to differentiate from other soft tissue pelvic lesions.
• The pelvis and hip are common locations of heterotopic ossifications (HO), often occurring in paraplegic patients.
• With respect to HO, MR imaging allows for a confident diagnosis in mature ossified lesions only. The MR aspect of immature ossification may be confused with other pathologies.
• Plain radiographs and CT may show various phases of ossification: amorphous calcification, immature and mature ossification.
• Integrating MR with CT can help recognise HO foci and differentiate them from infections and other soft tissue lesions.
KeywordsMusculoskeletal Neuro CT MR
Heterotopic ossification is the development of bone in abnormal sites. It may originate at any extraskeletal location where undifferentiated mesenchymal cells are found.
While its etiopathogenesis is unknown, it is likely that more than one factor (vascular stasis, low tissue oxygenation, microtraumas) contributes to activation of osteoblast and chondroblast progenitor cells with a consequent precipitation of calcium salts within the connective tissue [1, 2, 3]. Clinically, heterotopic ossification (HO) occurs in rare congenital disorders and, more commonly, in acquired conditions such as after surgery or related to spinal cord or head lesions. This latter condition, called neurogenic heterotopic ossification, is the topic of this article.
Ossifying lesions run along an ossification continuum: amorphous calcification, to immature ossification, to mature ossification.
MR imaging alone allows easy recognition of mature ossifications; however, amorphous calcification or immature ossification shows non-specific signal and contrast enhancement features. Thus, since enhanced MR imaging is routinely performed on paraplegic patients with infection of the pelvis, the differential diagnosis between HO and other soft tissue diseases can be problematic. Plain radiographs and CT can help in identifying the ossifying process showing peculiar morphological characteristics related to the degree of maturity.
In this review article, we describe our experience with pelvic imaging in paraplegic patients. We focus on different HO features through a simultaneous comparison of MR and CT findings.
All examined patients underwent both a contrast-enhanced MR scan and an unenhanced CT scan on the same day. All patients were scanned on a 0.4-T open magnet to accommodate required patient positioning. Our goal in this review is to underscore the importance of integrating MR imaging with CT to differentiate HO from other processes in the clinical management of paraplegic patients. More generally, integrated imaging can be helpful whenever an ossifying process may be expected in the pelvis and/or around hip joints.
HO in paraplegia
HO is frequently observed in paraplegic patients with spinal cord lesions of traumatic or atraumatic origin. Its incidence varies from 15 % to 35 % [1, 2, 3]. This phenomenon is mostly observed about larger joints, hips being the more affected site.
The cause of HO in paraplegia is unknown. Tissue denervation, oedema, and vascular stasis are all considered related to the formation of HO that usually occurs without precipitating trauma. Nevertheless, some authors have hypothesised that microtrauma may play a role in the etiopathogenesis of neurogenic HO stemming from passive physiotherapy joint manipulation .
Initial clinical findings of HO relate to the inflammatory process, and are characterised by local swelling, redness, muscle spasms and pain. Early in the clinical setting HO may be confused with infection, fracture or neoplasia . The inflammatory process then evolves to the formation of a connective mass with progressive deposition of bone tissue. In this phase, HO can cause motion restriction, joint ankylosis and decubiti. In fact, as many as 5–10 % of patients develop complete hip ankylosis [1, 2, 4, 5]. Typically, HO develops between 1 and 5 months after spinal injury, and can continue to be active even 5 years after the onset of paraplegia ; in some cases, it never reaches complete maturity . On radiographic imaging, HO foci vary in size and shape, and can range from a thin, 1-mm-long strip to a mass of many centimetres in diameter.
HO distribution and sites of origin
The most common sites of neurogenic pelvis ossification are the iliac, ileopsoas or vastus muscle areas .
Our patient population showed immature HO either in the muscle and fascia or in the bursae; however, we never found mature HO limited to the bursae. We assume that with increased maturity, HO formations tend to coalesce and adhere to the contiguous bone, which fades anatomic landmarks. CT imaging can easily identify calcium deposits inside a muscle or calcification/ossification at the anatomical site of a bursa, as is usually observed in initial soft tissue ossification.
Imaging can assist in the identification of three degrees of HO: amorphous calcifications, immature calcifications and mature calcifications .
Both plain radiographs and CT imaging are the standard references to assess HO maturity, since CT more accurately defines the different stages of ossification than do plain radiographs.
When MR imaging reveals a lesion with an indeterminate aspect, CT can aid the diagnosis of HO by detecting amorphous calcification or immature ossification inside the lesion.
Differential diagnosis and pitfalls
Furthermore, as shown in Figs. 9 and 10, soft tissue infection and ossification formation may show peculiar behavioural patterns with respect to the superficial tissue. In paralysed patients, deep infection of the soft tissues is usually contiguous to skin ulcers and is always associated with inflammation of subcutaneous tissue since infection spreads through the skin . Conversely, HO originates from deep in soft tissues; involvement of superficial subcutaneous layers is secondary to fibrotic retractive changes.
Contrary to immature HO, mature HO is easily recognisable by MR imaging since its signal intensity is similar to that of normal cancellous bone outlined by cortex. When these features of HO appear on MR images, CT is not required, as it offers no additional information.
HO may lead to a series of complications, including nerve entrapment, pressure ulcers and joint ankylosis. Consequently, early disease detection is extremely important.
Diphosphonates and non-steroidal anti-inflammatory drugs (NSAIDs) are used for prohylaxis or HO treatment .
After diagnosis, passive range-of-motion exercises to maintain joint mobility are prescribed .
When the ossification formation restricts mobility, surgical resection may be performed to increase joint function. During surgery planning, assessment of the degree of ossification is crucial, as resection must be performed only when HO is mature. Resection of an ossification process that has reached maturity implies less intra-operative complications, such as haemorrhage or a minor risk of post-operative recurrence . On the other hand, excessive surgical delay may expose the patient to development of joint ankylosis. Imaging plays an important role in optimising the timing of surgery, and plain radiographs and CT imaging are the most reliable ways to evaluate advanced ossification foci.
Pelvic MRI of paraplegic patients is a diagnostic challenge. Based on MR findings, immature HO may be difficult to differentiate from other pathologies, such as soft tissue pelvic infection, abscess and septic bursitis. The radiologist should be aware of the different aspects of HO at MR imaging. Correlation between MR and CT features is critical to accurate diagnosis. When dubious findings are present, CT scan follow-up is advisable. In advanced ossification, imaging is key to correctly time surgical resection.
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