Quantitative radiography of osteoarthritis.

Radiography is important in the diagnosis of osteoarthritis (OA) as the features described in the pathology of the disease can be visualised, with joint space narrowing generally thought to reflect cartilage loss.' Plain film radiography provides excellent detail of bony features but is generally poor at defining soft tissue structures, except with the help of an invasive procedure involving the introduction of a contrast medium. These characteristics together with the ready availability and ease of interpretation of radiographs has led to its use in confirming a clinically suspected diagnosis of OA, based on the appearance of characteristic radiographic features.2 Different methods of scoring the extent of these features have been developed and have been used as criteria both for diagnosis and to classify the base line status and progression of the disease in clinical studies. In spite of recent attempts at improving methods of scoring joint damage,3 4none of the different systems available3`8 have been widely adopted, largely due to the different emphasis that is placed on various radiographic features by different investigators, and the generally held notion that OA progresses slowly.9 Scoring systems, although an essential and widely used method for assessing disease progression, suffer from a number of limitations. They are based on two assumptions, first that the change in any one x ray feature is linear and constant during the course of disease, and second, that the relationship between the different x ray features is constant. Thus scoring radiographs does not take into account that different radiographic features may progress at different rates and at different times. To achieve an understanding of the relative significance of various radiographic features, a far better knowledge is required of the natural history and outcome of the disease, of which little or nothing is at present known."' In this instance all radiographic features need to be studied with equal importance. This can best be done by taking direct measurements of the distribution and size of each feature accurately and reproducibly and determining the relative rates of change in their dimension with time. Quantitative assessments of the structural changes in peripheral joints with OA are based on measurements of distance and area in the radiographic image. Such measurements are obtained either directly from the radiograph, or with the increased use of computer imaging, from digitised x ray films. Currently, direct measurement of x ray features are made from both standard and microfocal radiographs. The advantages and limitations of these two imaging modalities are described, and the results of their application in quantifying disease progression are outlined. Where it has been possible to obtain precise measurements of changes in x ray features, this has provided new insight into the natural history of OA and a number of these findings are described. Increasingly the concept of quantitative radiography is being accepted by rheumatologists and radiologists as the means of obtaining information about joint structure. To this end the last section in this paper outlines a range of factors which need to be optimised and standardised to improve quantitative standard radiography.