Owing to the relatively young age of the ultrasound modality, great efforts are still made to achieve standardisation in musculoskeletal sonography as well from a reference point of view as to the actual observer reliability of targeted regions.12,13
Despite its importance for mobility, the hip joint still seems to be under‐rated in this respect and to our knowledge, no standardised way of reporting ultrasound examinations of this joint has yet been developed for the evaluation of osteoarthritis.
From our experience with these examinations, we chose to test four ultrasound parameters covering the important aspects of hip osteoarthritis—that is, the bony and the synovial changes. In addition, the investigators were asked to give a global ultrasound evaluation, corresponding to an overall assessment. Different aspects of both the bony changes and the synovial evaluations showed a fair amount of consistency between the two evaluators. ICC was chosen as the primary outcome, owing to the ordinal character of our data and κ values, and absolute agreements were calculated as complementary information.
In clinical practice, the interpretation of which image is representative of the observed hip might be of great importance for the assessment. The distribution of the chosen frames in our study showed good concordance between the two observers when interpreting osteoarthritis. Secondary analysis of the outliers diminished the concerns of possible misinterpretations by showing widespread arthrotic changes of similar magnitude.
The intra‐observer repeatability in our study was good to excellent, with no ICC <0.69 and the κ results followed the same pattern. κ Values were comparable to those observed for the Kellgren score in radiological evaluation of the hip joint.14
Even though there were slightly weaker values for the assessment of soft tissue as opposed to that of bony structures, the overall ICC results of the partitioned scoring system showed stability comparable with global scores (ICC 0.70–0.72). The accuracy of CCD has been questioned when magnetic resonance imaging is used as the gold standard,5
and has therefore not been taken into account in this study.
According to our ultrasound examiner test, the global osteoarthritis score was the most closely related to the symptoms of the patients estimated by the retrospective test of the VAS activity, whereas the synovitis score had an independently significant, albeit small importance of its own (about 1% improved prediction of VAS activity), indicating a possible influence of inflammation on osteoarthritis of the hip. This result is in accordance with the notion of inflammation in osteoarthritis as indicated by ultrasound on examination of the knee.15
The interobserver ICCs were lower than their intra‐observer equivalents, yet still within the range of acceptability (ICC 0.45–0.65; κ 0.35–0.49). A trend in favour of bony contours was found, similar to the intraobserver variation, but here too, the partitioned scoring systems were found to be in the same range as the global score (ICC 0.58–0.56).
The presence of effusion is of the greatest interest when trying to expose synovitis. However, the present study showed a low ICC of 0.45 (effusion). Some of the disagreement could be due to a difference in cut‐off levels. The actual presence of joint exudates summed up (when dichotomising the fluid score into positive and non‐positive) into positive predictive values of 0.44/0.5 and negative predictive values of 0.881/0.83 for examiners A and B, respectively. Several explanations may be offered in this connection: large viscosity of the fluid, occlusion of the rather thin needle (21 G) and uneven distribution of the fluid away from the site of puncture are just some of the factors against using the aspirated volume as gold standard. On the other hand, the capsule may be so tight that a rather limited effusion may result in high intra‐articular pressure and patient discomfort. In this regard, it is interesting to note that in only a few of our patients did we observe fluid accumulation after intra‐articular injection of 3‐ml volume.
Ultrasound may allow visualisation of even very small fluid accumulations, although proof of this assumption can only be obtained by aspiration, or if the amount of fluid leads to a displacement of fluid under applied pressure. As a parameter in a general score of the hip in osteoarthritis, fluid is of limited value, although in the final diagnosis of the ultrasound examination it cannot be disregarded.
Osteophytes on the acetabulum were left out in this study, as we assessed their presence to be of minor importance and only for the impairment of joint motion.
The clinical association of the various imaging parameters was tested by univariate linear regression, analysing what percentage (R2) of our clinical outcomes (self‐reported pain on VAS) could be explained by each chosen parameter.
The high significance of the ultrasonographic findings confirmed the overall relevance of our choice of parameters.
The smallest degree of predictability was observed for pain at rest, with only 58.1% explained statistically by the ultrasound osteophyte score. The radiographic score was in this matter equipotent (R2
57%). By contrast, the synovial parameters (shape and effusion) were both below R2
Pain on walking, however, was explained with as much as R2
73.8% by the osteophyte score, which was similar to the result of the Kellgren score, R2
73.9%. Only the effusion score had no explanatory value.
We also wanted to show whether x ray and ultrasound were somehow associated and therefore possibly confounded. The stepwise multivariate regression analysis is designed to eliminate mutual predictive variables, thus pointing to independent variables representing a better prediction. With the natural exception of pain on walking, the second best predictive parameter of pain at rest was the femoral head score. Similarly, pain on walking was predicted by the Kellgren score, as well as the structural descriptive parameters of ultrasound—that is, the femoral head score and global osteoarthritis score (which was dependent on the osteophyte score more than on any other factor).
Radiographic and ultrasound scores presented independent factors in the stepwise, multivariate analysis, and the independence between radiographic and US scores in the assessment of osteoarthritis hips suggest that the two modalities register different characteristics of the disease, reflecting the difference in the perspective of the ultrasounnd findings.
In conclusion, this study suggests that ultrasound could be a reproducible method for the assessment of changes in the osseous surface and synovium‐related inflammation. The semiquantitative scoring system presented seemed to match the global assessment of a trained ultrasound investigator and might be used by moderately trained investigators—for example, clinicians—but only after a proper introduction to the procedures involved in a systematic evaluation of osteoarthritis.
Ultrasound is less often used in the adult hip compared with other major joints such as the shoulder and ankle, largely because of the relative inaccessibility.16
Clinical examination is impaired for the same reasons, and it is our concern that the osteoarthritis hip may have a tendency to be underdiagnosed. Ultrasound should not be regarded as a substitute for radiographic assessment, but rather as a supplementary source of information.
Future studies are needed to clarify the validity of the score in clinical situations.