Location of Lesion
Among individuals with TBI the most common sites of HO are the soft tissues around the hip, elbow, shoulder and knee (Garland 1991; Garland et al. 1980; Van Kampen et al. 2011; Vanden Bossche & Vanderstraeten 2005). The hip is the most frequent site of ossification (Dizdar et al. 2013; Vanden Bossche & Vanderstraeten 2005); with total ankylosis of the joint occurring in 5-16% of affected hips (Stover et al. 1991). HO of the shoulder has been found to affect 5% of individuals with a brain injury (Cipriano et al. 2009), while the knee is a less common site for HO following a head injury (Sarafis et al. 1999). When HO is present in the knee, it is usually seen medially (Hosalkar et al. 2013). The distribution of HO around the elbow occurs most commonly either anteriorly in the flexor muscles or posteriorly in the extensors (Sarafis et al. 1999). Of the joints affected by HO after head injury, ankylosis is most likely to occur in the posterior elbow (Garland et al. 1980).
The onset of HO has been reported to vary between two and three weeks post injury (Watanabe & Sant 2001). More recently, clinical signs and symptoms have been said to develop 3-12 weeks post injury (Vanden Bossche & Vanderstraeten 2005; Zychowicz 2013). Pape and colleagues (2004) noted that clinical examination in the setting of HO may reveal a swollen, warm, painful joint which is often associated with a decreased range of motion. The earliest sign is typically a loss of range of motion in the involved joint (Watanabe & Sant 2001). Other findings then include erythema, palpation of a periarticular mass and fever (Varghese 1992). Because of the association with fever, it is sometimes difficult to differentiate HO from infection (Citta-Pietrolungo et al. 1992; Garland 1991; Garland et al. 1980). Moreover, the clinical picture may be confused with deep venous thrombosis (DVT), local trauma or fracture (Buschbacher 1992; Jensen et al. 1987). HO will then progress from these initial symptoms into a mass with stiffness and induration (Zychowicz 2013). Potential complications involved with HO include compression of blood vessels and nerves, breakdown of associated tissue, restricted motion and loss of function (Zychowicz 2013).
Investigations of Heterotopic ossification
During the initial presentation, plain radiographs may be negative and will usually remain normal until ossification begins at approximately 4-6 weeks post injury. Serum levels of alkaline phosphatase, a glycoprotein in the plasma membrane of osteoblasts, and the erythrocyte sedimentation rate may become elevated early on but are non-specific. The triple phase technicium-99 bone scan remains the diagnostic gold standard. The test is positive if there is an increased uptake during the first and second phases of the study. It typically becomes positive when clinical features appear (i.e., before an x-ray would be positive).
In the HO literature there are several classification systems, with the Brooker classification system being one of the most widely used. The Brooker Classification system is typically used to classify ectopic-bone formation after total hip replacement. The system is based on anteroposterior radiograph of the pelvis and the categorization of the progression of HO into classes (Brooker et al. 1973). Brooker et al. (1973) define Class I as islands of bone within soft tissues about the hip; Class II: bone spurs from the pelvis or proximal end of the femur with at least 1cm between opposing bone surfaces; Class III: bone spurs from pelvis or proximal end of the femur, reducing space between opposing bone surfaces to less than 1cm; and Class IV: apparent bone ankylosis of hip. The classification system has been criticized and consequently the system has been modified (Della Valle et al. 2002; Mavrogenis et al. 2012; Toom et al. 2005; Wright et al. 1994).
The hips, shoulders and elbows are most commonly affected by heterotopic ossification.
Clinical features include warm, swollen, painful joint with some restriction of range of motion.s.