Spondylarthritides and related entities: Entheses and hypotheses

Ultrasound assessment of elbow enthesitis in patients with seronegative arthropathies

BACKGROUND

Enthesopathy is an evolving area for applied clinical research. MRI is the gold standard in the diagnosis of elbow joint pathology, but recent reports indicate that ultrasound imaging is more sensitive and accurate than MRI in detecting enthesopathy of the heels and knees. Too many patients are under-diagnosed and/or misdiagnosed because the early pathological changes of enthesitis in the different types of seronegative arthropathies are not detected.

OBJECTIVES

This study was undertaken to describe the ultrasound features of elbow enthesitis in patients with seronegative arthropathies.

METHODS

We studied 38 diseased elbows in 38 patients with spondyloarthropathies (26 men and 12 women, mean age 32 years). All had elbow enthesopathy without typical conventional radiographic findings. Patients with histories of degenerative changes and/or local steroid injections were excluded. An HDI 3000 ATL ultrasound machine was used with a 5-12 MHz linear transducer to examine the affected elbow joints. The elbows of 10 normal healthy individuals were examined as normal controls. The patients were examined in the supine position with the elbow flexed 30°-50°. Longitudinal and transverse scans were obtained of the radiohumeral joint, the ulnahumeral joint, and the olecranon fossa. Two independent observers unaware of the clinical diagnosis read the ultrasound images and assessed the collateral ligaments, intratendinous echogenicity, tendon calcification, tendon thickness, presence of fluid, synovial proliferation, and bony changes. The reliability of the sonographic images was assessed by review of video recordings of the ultrasound examinations.

RESULTS

Ultrasound revealed loss of the fibrillar echopattern (100 %), lack of a homogenous pattern with loss of the tightly packed echogenic dots (100 %), peritendinous edema with flaring of the tendon margins (84.2 %), irregular fusiform tendon thickening (100 %), and hyperechoic intratendinous lesions with ill-defined focal defects (18.4 %). Ultrasound also detected intratendinous calcifications of both the common extensor and common flexor tendons (52.6 %). Bony erosions were seen at the tendon insertions into the lateral epicondyles (13.15 %).

CONCLUSION

Ultrasonographic features of elbow enthesitis differed from those described in knee and heel enthesitis. Ultrasound clearly showed early signs of tendon calcification, tendon edema, peritendinitis, and bony entheseal erosions. However, in elbow enthesitis the early bone erosion was associated with bone marrow edema, and the common extensor tendon was diffusely thickened. Ultrasound is a reliable, reproducible bedside imaging procedure. It improves the documentation of disease activity, progression, and treatment responses in patients with spondyloarthropathies. We recommend its use for the diagnosis and post-treatment follow-up of patients with enthesitis and seronegative spondyloarthropathies.

The enthesis organ concept and its relevance to the spondyloarthropathies

A characteristic feature of the spondyloarthropathies is inflammation at tendon or ligament attachment sites. This has traditionally been viewed as a focal abnormality, even though the inflammatory reaction intrinsic to enthesitis may be quite extensive. We argue that the diffuse nature of the pathology is best understood in the context of an 'enthesis organ concept'. This highlights the fact that stress concentration at an insertion site involves not only the enthesis itself, but neighbouring tissues as well. The archetypal enthesis organ is that of the Achilles tendon where intermittent contact between tendon and bone immediately proximal to the enthesis leads to the formation of fibrocartilages on the deep surface of the tendon and on the opposing calcaneal tuberosity, but similar functional modifications are widespread throughout the skeleton. Many entheses have bursae and fat near the insertion site and both of these serve to promote frictionless movement. Collectively, the fibrocartilages, bursa, fat pad and the enthesis itself constitute the enthesis organ. However, it also includes both the immediately adjacent trabecular bone networks and in some cases deep fascia. The concept of a synovio-entheseal complex (SEC) and of a 'functional enthesis' are complimentary to that of an enthesis organ and also have important implications for understanding spondyloarthropathy. The SEC concept emphasizes the interdependence between synovial membrane and entheses within enthesis organs. It draws attention to the fact that one component (the enthesis) is prone to microdamage and the other (the synovium) to inflammation. If an enthesis is damaged, any ensuing inflammatory reaction is likely to occur in the synovium. The concept of a 'functional enthesis' serves to emphasise anatomical, biomechanical and pathological features that are shared between true fibrocartilaginous entheses and regions proximal to the attachment sites themselves where tendons or ligaments wrap around bony pulleys. Such'wrap-around regions' are well documented sites of pathology in SpA-for tenosynovitis is a recognized feature. Stress concentration at the enthesis itself is dissipated at many sites by fibrous connections between one tendon or ligament and another, close to the insertion site. At a microscopic level, enthesis fibrocartilage is of paramount importance in ensuring that fibre bending of the tendon or ligament is not focused at the hard tissue interface. Normal enthesis organs are avascular in their fibrocartilaginous regions, but tissue microdamage to entheses is common and appears to be associated with tissue repair responses and vessel ingrowth. This makes the enthesis organ a site where adjuvant molecules derived from bacteria may be preferentially deposited. This microdamage and propensity for bacterial molecule deposition in the context of genetic factors such as HLA-B27 appears to lead to the characteristic inflammatory changes of AS. Understanding the enthesis organ concept helps to explain synovitis and osteitis in spondyloarthropathy. An appreciation of the complex anatomy of 'articular enthesis organs' (e.g., that associated with the distal interphalangeal joints) is helpful in understanding disease patterns in psoriatic arthritis. In this chapter, we review the extent and types ofenthesis organs and show how a patho-anatomic appreciation of these structures leads to a new platform for understanding the pathogenesis of SpA.