X-RAY DIAGNOSIS OF HETEROTOPIC OSSIFICATION. SOURCES OF THE ERRORS AND AMENDMENT OF THE ASSESSMENT SYSTEM.

Abstract

Heterotopic ossification (HO) is a frequent pathological phenomenon after total hip arthroplasty. Incidence of HO after the total hip arthroplasty is in average 43%. Most often classification from Brooker and co-authors has been used to assess the HO. Overall 47% of all studies published until the 1999 have used Brooker’s system, but there has been reported a “fair” reproducibility (Cohen’s kappa 0.5) of this system.

The aim of the present study was to estimate the influence of used classification system to the evidence of HO, to assess the reliability of the more often used classification systems, to determine the sources of errors in the assessment-process of HO and, to compose a new classification system with higher reliability.

Four investigators assessed HO in 111 patients applying most often used classification systems and the new system. Six investigators measured dimensionality of HO in 28 patients applying method of digital planimetry. Kappa statistics of all the compared classification systems were calculated. Main sources of errors were detected by dispersion model.

Average evidence of HO differed up to 1.4 times if the different systems were applied to assess HO. Therefore, using literature data, always the criteria of a system used should be taken into consideration.

Most important source of errors in the HO assessment was the error of diagnosing process. This source consisted of two components: inter-observer variation that formed 25.5% (+/−8.0%; p=0.0015) of total error and intra-observer variation that formed 60.9% (+/−7.3%; p< 0.0001). Technical performing error had less contribution in total error, namely 8,0% (+/−0.6%; p< 0.0001) and subspecialisation of the investigators did not cause any systematic bias having a proportion of 5.7% (+/−4.9%; p=0.2457).

Reliability of the Brooker’s system was lowest among the compared systems. Combining the Della Valle’s system which had high reliability with the Brooker’s system, we composed a new classification preserving high reliability. The results obtained using the new classification allow comparison with the results obtained using each of the parent classifications.