Enthesitis M08.-

Enthesitis is an inflammation of the enthesis, i.e. the attachment point of ligaments or tendons to the bone. Achilles tendons, plantar aponeurosis, trochanter, ischium and iliac crest are particularly affected.

Fibrous enthesis occurs in the metaphysis and diaphysis, while fibrocartilaginous enthesis, which is of interest in rheumatology, occurs preferentially in the apophysis and epiphysis.

Fibrocartilaginous enthesis shows a characteristic four-zone structure consisting of collagenous connective tissue, non-calcified fibrocartilage, calcified fibrocartilage and bone (McGonagle D et al. (2007). "Fibrocartilaginous enthesitis is a cardinal symptom of spondyloarthritis. The characteristic predilection site is the Achilles tendon". However, the specificity is only 78% (Sieper J et al.2009).

Mechanical stress is considered a possible trigger of enthesitis. Other triggering factors may be genetic changes, microorganisms, immunological cross-reactions with proteoglycan molecule components (e.g. aggrecan), BMP-2 and BMP-6 as well as IL-23 expression in the tissue (Pittam B et al. 2020). At least in animal models, the pathophysiology of spondyloarthritic enthesitis involves the activation of local T lymphocytes through the release of IL-23, which induces the secretion of cytokines such as IL-17, TNF and IL-22, which subsequently leads to the typical symptoms of erosion, inflammation and new bone formation (7 Kehl AS et al. 2016).

Enthesitis is characterized by inflammatory pain with resting and persistent pain, which can be exacerbated by muscle provocation tests.

Sonographic and MR tomographic signs of enthesitis are not specific for SpA. In ultrasound diagnostics, only the occurrence of erosion appears to be associated with SpA. On MRI, bone marrow edema can provide an indication of enthesitis, but it is not a specific sign of spondyloarthritic enthesitis.10,11 Over 270 enthesitis sites have been identified, and the distribution of these can be documented using scores (Mease PJ et al. 2017).

The diagnosis of enthesitis is not difficult. The major difficulty lies in determining whether the diagnosed enthesitis is spondyloarthritis-associated.

The differential diagnosis of spondylarthritis-associated enthesitis must be differentiated from fibromyalgia or far more common degenerative changes. The most important differential diagnoses in dactylitis are crystal arthropathies and osteomyelitis.

In addition to NSAIDs and possibly local glucocorticoid injections, drug therapy for enthesitis in the context of SpA consists of various bDMARDs - in addition to TNF blockers, these are primarily IL-17 and IL-23 inhibitors - and tsDMARDs (apremilast and tofacitinib), which have been shown to be effective (Koppikar S et al. 2020). Due to the pain-related, reflexive muscle shortening caused by enthesitis of a tendon insertion, physical therapy measures that are offered to relieve pain and prevent loss of function are always recommended in parallel.

The term enthesitis was mentioned in ankylosing spondylitis as early as 1971, but it was not until the 2000s that it gained pathophysiological significance for the entire group of spondyloarthritides (SpA).

1. Kehl AS et al. (2016) Review: Enthesitis: New Insights Into Pathogenesis, Diagnostic Modalities, and Treatment. Arthritis Rheumatol 68:312-22:
2. McGonagle D et al. (2007): The concept of a "synovio-entheseal complex" and its implications for understanding joint inflammation and damage in psoriatic arthritis and beyond. Arthritis Rheum. 56:2482-2491.
3. McGonagle D et al. (2019) Pathophysiology, assessment and treatment of psoriatic dactylitis. Nat Rev Rheumatol 15: 113-22
4. Mease PJ et al: (2017) Performance of 3 Enthesitis Indices in Patients with Peripheral Spondyloarthritis During Treatment with Adalimumab. J Rheumatol 44:599-608.
5. Pittam B et al: (2020) Prevalence of extra-articular manifestations in psoriatic arthritis: a systematic review and meta-analysis. Rheumatology (Oxford) 59:2199-2206.
6. Sieper J et al.: (2009) New criteria for inflammatory back pain in patients with chronic back pain: a real patient exercise by experts from the Assessment of SpondyloArthritis international Society (ASAS). Ann Rheum Dis 68:784-788.