Potential discrepancy between plain films and CT scans in Brooker classification of heterotopic ossification

Abstract

Objectives: The Brooker classification is a commonly used tool to characterize the extent of heterotopic ossification (HTO), which is the dystrophic formation of mature lamellar bone in non-osseous tissues such as muscles, connective tissue or nerves. We aim to provide illustrative cases to describe the limitations and challenges of this system.

Methods: A retrospective review was conducted in all patients who had retrieved hip and/or pelvic radiotherapy for HTO prophylaxis at Sunnybrook Health Sciences Centre between July 1998 and August 2016. An independent musculoskeletal radiologist who was blinded to the condition of the patient and the clinical outcome evaluated the imaging studies based on the Brooker classification.

Results: 22 patients were identified for analysis. Three patient cases were presented to illustrate the discrepancies between the Brooker classification and radiographic images, CT images and different radiographical projections. Two patient cases were presented to demonstrate the inability of the Brooker classification to account for volumetric differences in HTO and arthroplasty lengths.

Conclusion: Although the Brooker classification for HTO is a widely used quantitative and qualitative assessment tool given its simplicity and familiarity, it possesses several limitations. Utilization of other radiographic modalities, such as CT, and orthogonal projections may reduce ambiguities from using the HTO system. Development of a classification method that can appropriately correspond to the clinical outcomes such as functional capabilities to further HTO development is warranted. Advances in knowledge: The present study critically evaluated the Brooker classification system, and identified areas in which improvement is warranted on characterizing HTO. This is important in further research that aims to improve the accuracy of HTO classification guidelines.

Keywords: Brooker Classification; Heterotopic ossification.

Figure 1.

The Brooker classification of hip heterotropic ossification.^,

Figure 2.

AP radiograph of the pelvis demonstrating heterotopic ossification in both hips with Grade III Brooker classification in the right hip and Grade IV in the left hip.

Figure 3.

Coronal reformatted CT image (a) demonstrating incomplete fusion of ossification bilaterally with a corresponding absence of ankyloses at the left hip on the sagittal reformatted image (b).

Figure 4.

AP radiograph of the pelvis demonstrating acetabular reconstruction plate with multiple screw fixations and heterotopic ossification (Brooker Grade I) at left hip.

Figure 5.

Coronal reformatted CT image (a) and axial CT image (b) demonstrating extensive HTO (Brooker Grade III) spanning the left proximal femur to the acetabular surface without evidence of complete fusion.

Figure 6.

Anteroposterior projection (a) of the pelvis demonstrates an apparent fusion of the bony ossification at the right hip. A subsequent left posterior-oblique projection of the pelvis (b) demonstrates an incomplete heterotrophic ossification fusion at the right hip.

Figure 7.

Anteroposterior radiograph of the left hip demonstrating HTO (Brooker Grade II) immediately post-operatively (a). A subsequent AP radiograph 2 months following the surgery illustrates an overall reduction in HTO volume, despite lack of reduction in separation distance between opposing bony surfaces (b).

Figure 8.

Anteroposterior radiograph (a) of the right hip demonstrating a cement mold femoral hemiarthroplasty with associated heterotrophic ossification (Brooker Grade II).(b) demonstrates interval surgery with removal of cement from mold arthroplasty and placement of a non-cemented total arthroplasty and apparent widening of opposing ossification surfaces (white arrow).