“Fish head” sign on magnetic resonance imaging in knee synovitis – An observational pilot study

INTRODUCTION

Synovitis is a condition characterized by the inflammation of the synovial membrane within joints.^[1-4] The synovium is an intricate membrane enveloping diarthrodial joints, tendon sheaths, and intraarticular structures, such as the fibrous joint capsule, fat pads, and intraarticular ligaments, and plays a pivotal role in maintaining joint health.^[5] It synthesizes crucial synovial fluid responsible for joint lubrication, chemical balance, and nourishment to chondrocytes, thereby ensuring the well-being of articular cartilage.^[5] However, certain structures such as articular cartilage, menisci, labra, and the small bone bare areas remain exempt from its coverage.

The causes of synovitis encompass a diverse range of potential differential diagnoses, including inflammatory, infectious, degenerative, traumatic, hemorrhagic, and neoplastic etiologies.^[2-4] Synovitis has been shown in various studies to lead to intraarticular structural damage, particularly affecting cartilage, as part of pathological processes involving the synovium, ultimately contributing to irreversible joint deterioration.^[2,6]

Hence, early detection of the disease holds critical significance.^[3,7] Imaging plays an important role in identifying synovial diseases before they progress to irreversible joint impairment.^[1,8,9] Developing familiarity with distinct imaging findings associated with synovial disorders aids in accurate diagnosis and timely intervention.^[8,9] While clinical assessment forms the foundation for diagnosis, incorporating advanced imaging modalities significantly enhances diagnostic accuracy and facilitates continuous monitoring of synovial diseases. Among various imaging tools, gadolinium-enhanced magnetic resonance imaging (MRI) has emerged as a valuable tool for evaluating and monitoring synovitis.^[10] In addition, ultrasonography (US), radiography, and dual-energy computed tomography have proven to be useful.^[11,12] This study introduces a novel sign referred to as the “fish head” sign, a unique identifier of synovitis presence in the knee on MRI. This distinctive sign, observed within knee synovitis cases, offers a fresh perspective to comprehend the intricacies of synovial pathology. Hence, this study aims to describe this sign, thereby contributing to the evolving understanding of synovitis.

MATERIALS AND METHODS

Description of the “fish head” sign on MRI

The “fish head” sign manifests as a noticeable alteration in the morphology of the anterior horn of the lateral meniscus, evident on sagittal PDFS images. This alteration is characterized by a reduction in vertical height, rounding of the superior and inferior surfaces of the anterior horn of the meniscus, as well as an anterior bowing of the anterior horn of the lateral meniscus, resembling that of a fish head. This contrasts with the classical triangular-shaped appearance of the anterior horn seen in normal cases. A visual representation of this unique sign is provided in [Figures 1 and 2].

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Figure 1:

Sagittal proton density-weighted fat-suppressed image of the knee showing (a) normal morphology of anterior (arrow) and posterior horns of lateral meniscus and (b) “fish head” sign of the anterior horn of lateral meniscus (arrow).

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Figure 2:

Sagittal proton density-weighted fat-suppressed image of the knee highlighting (a) normal morphology of anterior (arrow) horn of lateral meniscus and (b) “fish head” sign of anterior horn of lateral meniscus with schematic overlay (arrow).

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RESULTS

Among the 50 patients comprising our cohort, a notable female predominance was observed, with a gender ratio of 2.6 females to 1 male (36 females and 14 males). The average age of our study population was 55.68 years, ranging from 17 to 83 years. Of the entire cohort of 50 patients, 20 individuals exhibited the “fish head” sign on MRI, whereas the remaining 30 patients did not manifest this sign.

Among the 20 patients with the fish head sign, 15 were female, and five were male patients. Furthermore, based on disease severity, 17 of these patients were identified as having mild synovitis, one patient exhibited moderate synovitis, and two patients had severe synovitis. Fourteen patients with synovitis did not demonstrate the sign. One possible explanation for this is that synovitis did not involve the anterior horn of the lateral meniscus.

There was no “fish head” sign in patients without synovitis. The fish head sign was seen in the anterior horn of the lateral meniscus only. Our study suggests that the presence of this sign is more likely to be observed in patients with mild synovitis. There was good agreement between the two readers.

DISCUSSION

Synovial inflammation has been implicated in contributing to knee joint dysfunction, pain, and disability in patients with osteoarthritis (OA) as well as inflammatory conditions, of which RA is most common.^[1,2,6,8,13] The hallmark of RA is characterized by synovial proliferation, leading to joint effusion, where an accumulation of exudate results from heightened vascular permeability within the inflamed synovial tissue.^[14,15] This sequence progresses to the development of synovial pannus, periarticular bone demineralization, cartilage degradation, and subchondral osseous erosions. Damage to other intra-articular structures, particularly cartilage, typically arises because of pathological processes originating in the synovium, ultimately leading to irreversible joint deterioration.^[2,6] In the context of OA, synovitis may manifest early in the disease’s progression, with an excess of synovial fluid stemming from mechanical irritation due to degenerated cartilage and bone.^[4] This triggers an amplified production of hyaluronic acid by intimal fibroblasts, further influenced by frictional forces inherent in OA.^[5]

Regardless of the underlying cause, a wealth of evidence from the literature substantiates the persuasive notion that targeting synovial inflammation offers a rational pathway for therapeutic intervention to effectively manage symptoms, particularly in patients with inflammatory conditions such as RA.^[15,16] Consequently, the significance of early diagnosis cannot be overstated, as it plays a pivotal role in ensuring that patients receive timely and appropriate management.

Patients with intraarticular joint synovitis often exhibit non-specific symptoms, mainly pain, arising from the synovitis itself or concurrent joint issues.^[17,18] These symptoms require a comprehensive understanding of their underlying causes. Recent studies have illuminated the intricate connection between knee pain and specific joint synovitis, as observed through MRI, especially in OA and inflammatory joint conditions. These investigations reveal that changes in pain scores correspond well with effusion and synovitis fluctuations over time.^[11,17-20] Moreover, they emphasize the independent link between effusion-synovitis and knee symptoms, highlighting its capacity to trigger symptoms apart from other structural issues. In RA-associated synovitis, clinical presentations vary. Patients may experience non-weight-bearing pain, night-time discomfort, and classic inflammation signs in the affected joint, often extending to other joints due to the disease’s systemic nature.^[16] Thus, understanding its diverse clinical manifestations is essential for precise diagnosis and tailored treatments.

Various methods are employed to detect and characterize synovitis, including imaging techniques and histologic assessments, especially in early synovial disease stages, before joint damage becomes irreversible.^{[10-12,17,19]} While plain radiography and US assist in diagnosis, MRI is the established standard for diagnosing and tracking synovial diseases.^[10,17] This recognition is backed by initiatives such as Outcome Measures in Rheumatology and the European League Against Rheumatism, emphasizing standardized magnetic resonance assessment of synovial conditions.^[21,22] MRI comprehensively assesses joint components, encompassing synovium, cartilage, bone marrow, and other structures. It non-invasively evaluates synovial volume and active inflammation, often through dynamic contrast-enhanced techniques highlighting heightened vascularity. Fat-saturated T2-weighted and post-contrast fat-saturated T1-weighted sequences provide insights into synovitis tenosynovitis and adjacent tissues. Moreover, post-contrast fat-suppressed T1-weighted sequences efficiently distinguish synovial proliferation from joint effusion.^[23]

Gadolinium-enhanced MRI has emerged as a valuable diagnostic tool for identifying synovial disorders, concurrently detecting bone marrow edema and erosions.^[10] Furthermore, the use of dynamic MRI plays a pivotal role in highlighting active disease, effectively presenting a synovial enhancement pattern.^[11] However, in situations where contrast-enhanced MRI accessibility is limited, recent evidence suggests assessing synovitis alongside effusion. This poses a challenge to radiologists in distinguishing inflamed synovium from the joint fluid within the synovial cavity, as both exhibit equivalent high signal intensities.^[24] Alternatively, Hoffa’s synovitis offers an indirect avenue for assessment, characterized by hyperintense signal changes within Hoffa’s fat pad on non-contrast fat-suppressed proton density (PD) or T2-weighted fast spin echo sequences. It is important to note that signal changes within Hoffa’s fat pad are not exclusive to synovitis; they may also encompass edema, cysts, or Hoffa’s ganglion.^[25]

In the present study, we introduce a novel and previously unreported imaging sign, termed the “fish head” sign, observed on sagittal PDFS MRI images during the early stages of synovitis in the knee among patients with RA. We hypothesize that in early synovitis, there is a thickening of the synovium, which causes a mass effect, particularly on the anterior horn of the lateral meniscus, resulting in the “fish head” sign. This was not noted in the posterior horn of the lateral meniscus or medial meniscus, and this could be due to the relatively greater space around these structures. This distinctive sign has the potential to serve as an adjunct diagnostic marker for early synovitis.

We anticipate that this unique imaging feature holds promise for future applications, aiding in the prompt diagnosis and effective management of patients presenting with knee synovitis. By enhancing diagnostic accuracy and offering a practical solution, the fish head sign may contribute significantly to improving patient outcomes and advancing the understanding of synovitis-related pathologies.

CONCLUSION

Our study describes the novel “fish head” sign seen on sagittal PDFS MRI images of early knee synovitis among RA patients. This sign could serve as a standalone diagnostic tool or adjunct for synovitis cases without the need for contrast-enhanced imaging. The sign presents a promising avenue for early diagnosis and effective management of knee synovitis in RA. Further research is needed to validate its broader applicability and potential impact on patient care. This new imaging feature could contribute to improved outcomes and a deeper understanding of synovitis-related conditions.