Through recent technological advances, ultrasound allows high-resolution visualisation of inflammatory and destructive changes in tendon and joint structures. Over the last few years, the added value of the use of ultrasound for evaluating entheseal involvement in spondyloarthritis (SpA) patients has been demonstrated. Several studies have described the ultrasound features of enthesitis in SpA, revealing the high frequency of clinically asymptomatic abnormal findings. It is, therefore, highly relevant to consider the validity of ultrasonographic measures of entheseal inflammation and damage. This article focusses on ultrasound appearance of peripheral enthesitis, and underlines the advantages and current limitations of the technique for the management of SpA.
Over the last few years, ultrasound has proved to be a highly sensitive and non-invasive tool, especially for assessing tendon and joint involvement . Although most of the recent data are based on rheumatoid arthritis (RA), there is an increasing interest and evidence for the use of ultrasound for the evaluation of SpA. Ultrasound can visualise most of the relevant musculoskeletal SpA-associated pathologies such as enthesitis, bone erosions, synovitis, bursitis and tenosynovitis. The exception is osteitis, since the ultrasound beam is not able to penetrate the bone cortex. While conventional radiography allows a clear documentation of the later stages of inflammatory changes of joint involvement, ultrasound, both in grey scale and in power Doppler, is sensitive enough to also detect early inflammatory lesions.
Ultrasound appearance of musculoskelal lesions of SpA
Ultrasound manifestations of synovitis, erosions and tenosynovitis in SpA patients are not different from those observed in other inflammatory arthritis including RA. The main difference seems to relate to the ultrasound appearance of enthesitis, that is, inflammation at the insertion of tendons, ligaments and capsules into the bone, which is seen as a primary lesion that may underlie all SpA skeletal manifestations .
The extensive description of entheseal involvement by ultrasound in SpA patients was made, for the first time, by Lehtinen and colleagues in 1994 , followed by Balint and colleagues in 2002 and D’Agostino and colleagues in 2003 . The first two authors described in grey scale the ultrasound abnormalities of lower limb enthesitis of SpA, revealing the high frequency of asymptomatic findings.
Grey-scale ultrasound permits the depiction of both signs of acute and chronic inflammation of enthesis as well as structural damage. Grey-scale enthesitis is characterised by the loss of normal fibrillar echogenicity of tendon insertion with or without an increase of the thickness, or by intralesional focal changes of tendon insertion, such as calcific deposits, fibrous scars and periosteal changes (erosions or new bone formation). Additionally, a clear involvement of the body of tendon, far from the enthesis, and of the adjacent bursae can be observed, even if these two processes can be seen in the absence of enthesitis in other inflammatory and non- inflammatory diseases. Recently, the use of power Doppler for visualising abnormal vascularisation and hyperaemia of soft tissues in inflammatory joint diseases was extensively demonstrated . The first description of the utility of power Doppler ultrasound for studying enthesitis was published by D’Agostino and colleagues . The landmark finding on power Doppler ultrasound of enthesitis in SpA patients was the presence of abnormal vascularisation at enthesis insertion ( Figs. 1 and 2 ).
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