In the early phase of HO, triple phase bone scanning demonstrates increased uptake of osteotropic radionucleotides. Bone scanning has proven to be more sensitive than plain radiography in detecting early HO. Neurogenic HO becomes evident on plain radiography approximately two to six weeks after diagnosis using the triple phase bone scan (Orzel et al. 1985; Freed et al. 1982). However, bone scans have lower specificity than radiography (Freed et al. 1982). CT or MRI scanning may be a useful tool when considering surgery as it allows for better visualization of the heterotopic bone (Amendola et al. 1983). Some studies have examined diagnosis of HO through elevations in biochemical markers such as alkaline phosphatase (Singh et al. 2003; Tibone et al. 1978) and creatine phosphokinase (Singh et al. 2003; Welch et al. 1973; Rossier et al. 1973). The predictive value of alkaline phosphatase has not been validated (Singh et al. 2003; Welch et al. 1973; Rossier et al. 1973), although there is conflicting evidence of an association with HO and increased serum creatine phosphokinase levels (Singh et al. 2003; Welch et al. 1973). Schurch et al. (1997) studied individuals with acute SCI and found increases in the 24 hour prostaglandin E₂(PGE₂) urinary excretion a valid indicator of early HO formation.
The Brooker Classification Scheme is typically used to diagnose HO in the pelvic region (Zychowicz 2013). The system is based on an anteroposterior radiograph of the pelvis which classifies HO into one of five classes. The classes are based on the progression of ossification: Class 0 – no presence of ossification, Class 1 – islands of bone within soft tissue of any size, Class 2 – bone spurs from pelvis or femur with at least 1 cm between opposing bone surfaces, Class 3 – bone spurs from pelvis and femur reducing space between opposing bone surfaces to less than 1 cm, and Class 4 – complete ankylosis of hip (Zychowicz 2013).
The Brooker Classification Scheme has been criticized by some clinicians and adjustments to the traditional classification system have been proposed. Mavrogenis et al (2012) have suggest focusing on the location of the HO formation around the hip joint using the following scheme: Type 1 – anterior, Type 2 – posterior, Type 3 – anteromedial, Type 4 – circumferential HO. The adjustments are based on locating the anatomical position of HO which permits an estimation of the prognosis regarding blood loss, transfusion requirements, and recurrence. The new classification system improves ease of use and provides the opportunity for more rapid post-operative planning for surgical approach, evaluation, and prognosis (Mavrogenis et al. 2012). However, Citak et al. (2012) suggested the use of ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI) rather than radiograph in order to improve diagnosis and reduce the use of methods with less sensitivity for early diagnosis.