A new classification of peri-articular heterotopic ossification of the hip associated with neurological injury: 3D CT scan assessment and intra-operative findings.

In this paper we propose a new classification of neurogenic peri-articular heterotopic ossification (HO) of the hip based on three-dimensional (3D) CT, with the aim of improving pre-operative planning for its excision. A total of 55 patients (73 hips) with clinically significant HO after either traumatic brain or spinal cord injury were assessed by 3D-CT scanning, and the results compared with the intra-operative findings. At operation, the gross pathological anatomy of the HO as identified by 3D-CT imaging was confirmed as affecting the peri-articular hip muscles to a greater or lesser extent. We identified seven patterns of involvement: four basic (anterior, medial, posterior and lateral) and three mixed (anteromedial, posterolateral and circumferential). Excellent intra- and inter-observer agreement, with kappa values > 0.8, confirmed the reproducibility of the classification system. We describe the different surgical approaches used to excise the HO which were guided by the 3D-CT findings. Resection was always successful. 3D-CT imaging, complemented in some cases by angiography, allows the surgeon to define the 3D anatomy of the HO accurately and to plan its surgical excision with precision.

Inter-observer reliability in the assessment of heterotopic ossification: proposal of a combined classification.

We applied four published classifications for assessment of heterotopic ossification after total hip arthroplasty (Arcq, Brooker, DeLee and Della Valle [1, 2, 5, 6]. The average incidence of heterotopic ossification varied from 19.8% to 27.7%. The inter-observer reliability of the various classifications was determined by Cohen's Kappa statistic. Kappa values ranged from 0.897 for Arcq's to 0.814 for Brooker's classification. In order to increase the reliability and consistency, we propose a new classification system combining Brooker's and Della Valle's classifications. This new classification preserves the high reliability of Della Valle's system and is comparable to previous publications since it includes Brooker's criteria.