Recurrent knee pain and effusion in a child: synovial chondromatosis

Summary

Chronic monoarticular joint pain in paediatric patients is prevalent, yet the aetiology often remains unidentified when initial imaging yields no abnormalities. This case highlights a rare but significant differential diagnosis: synovial chondromatosis. Early identification is essential to prevent joint damage, but diagnosis is frequently delayed due to subtle clinical manifestations and inconclusive initial imaging. Notably, mechanical symptoms and recurrent joint effusions are key indicators. In such cases, persistent pain with recurrent effusion should prompt early consideration of synovial pathology even when initial radiographs are unremarkable. Repeat imaging may be valuable in demonstrating interval development of ossified loose bodies; however, this case suggests that the underlying disease can be present well before calcification becomes radiographically visible. Therefore, clinical persistence and mechanical symptoms should drive timely escalation of evaluation. When concern remains, earlier use of targeted synovial assessment and advanced imaging should be considered.

Background

Synovial chondromatosis is an infrequent, benign proliferative disorder characterised by metaplasia of the synovial membrane into cartilaginous nodules, which may detach and form loose bodies. The reported incidence is approximately 1.8 cases per million individuals annually. This disorder primarily affects adults aged 30–50, with a male-to-female ratio of 2:1.¹ Synovial chondromatosis is distinctly uncommon in children and adolescents, which may contribute to low initial clinical suspicion and delayed diagnosis in paediatric practice. It typically occurs intra-articularly, particularly within the synovium, although it can also manifest extra-articularly, such as within the synovium of bursae or tendon sheaths.^{2 3} The pathogenesis of these nodules remains unclear; however, the core mechanism may involve metaplastic changes in the synovial tissue, manifesting as the transformation of synovium into cartilaginous tissue.^{4 5}

Synovial chondromatosis can be broadly classified into primary and secondary forms (table 1). Primary synovial chondromatosis arises in an otherwise normal joint and is driven by intrinsic synovial cartilaginous metaplasia, whereas secondary synovial chondromatosis develops in association with pre-existing joint pathology, such as trauma, osteoarthritis, osteochondritis dissecans or osteonecrosis, in which loose bodies arise secondary to degenerative or mechanical intra-articular change.⁶ Clinically, primary disease tends to occur in younger patients and is more likely to produce numerous, relatively uniform loose bodies, while secondary disease more often occurs in older patients and is accompanied by features of the underlying arthropathy. This distinction is clinically relevant because treatment of primary disease focuses on loose body removal with synovectomy, whereas management of secondary disease should also address the underlying joint disorder. Milgram categorised the disease process into three distinct stages in 1977.⁷ Stage I involves dysplasia, with lesions developing within the synovial lining, accompanied by active synovitis and nodule formation, but without calcification. In stage II, nodular synovitis and loose bodies are found in the affected joint, with the loose bodies remaining predominantly cartilaginous. In stage III, the synovitis resolves, but the loose bodies persist.

Table 1. Concise comparison between primary and secondary synovial chondromatosis.

Feature	Primary synovial chondromatosis	Secondary synovial chondromatosis
Underlying joint status	Usually arises in an otherwise normal joint	Occurs in a joint with pre-existing pathology
Aetiology	Idiopathic	Secondary to underlying joint disease or mechanical injury
Pathogenesis	Intrinsic synovial cartilaginous metaplasia with formation of nodules that may detach as loose bodies	Loose bodies develop in association with degenerative, traumatic or other pre-existing intra-articular pathology
Typical clinical setting	Tends to occur in younger patients; may rarely present in children	More often seen in older patients with evidence of underlying arthropathy
Loose bodies and imaging features	Often multiple, small and relatively uniform loose bodies	Loose bodies are more variable in size and are often accompanied by degenerative joint changes
Histopathological features	Disorganised cartilaginous nodules with features consistent with primary synovial cartilaginous metaplasia	Loose bodies often form around osteochondral fragments, with changes reflecting the underlying joint disorder
Treatment approach	Removal of loose bodies with synovectomy; arthroscopic or open approach depending on extent and location	Removal of symptomatic loose bodies plus treatment of the underlying joint pathology; synovectomy according to synovial activity and symptoms

Case presentation

A boy in middle childhood presented to our institution with a 12-month history of intermittent right knee pain, which was aggravated by physical activity and accompanied by mild swelling. There was no history of trauma, fever, systemic symptoms or involvement of other joints. At the time of symptom onset 1 year earlier, the patient had been evaluated at a local hospital. Physical examination then revealed mild joint-line tenderness without redness and full range of motion. Plain radiographs obtained at that time were interpreted as normal, and empirical treatment with non-steroidal anti-inflammatory drugs was initiated. However, the symptoms persisted without improvement. Functionally, the pain had begun to affect his participation in higher-demand activities: he continued to attend school, but was no longer able to take part in physical education lessons or running and ball games because of knee discomfort. He was referred to our tertiary centre after a prolonged symptomatic course and prior unremarkable radiographs obtained elsewhere. On referral to our hospital, a retrospective review of the initial radiographs revealed subtle, poorly defined loose bodies in the posterior compartment of the knee (figure 1A). Due to worsening discomfort, repeat imaging was performed. The radiograph now clearly demonstrated intra-articular loose bodies. MRI revealed synovitis, synovial thickening, joint effusion and confirmed multiple intra-articular loose bodies.

Figure 1. Imaging findings of the right knee. (A) The initial lateral radiograph of the right knee appeared normal; however, a retrospective review identified a faint loose body in the posterior compartment (indicated by a blue arrow). (B) Repeat radiograph showed multiple high-density spherical loose bodies (indicated by a blue arrow). (C) Three-dimensional CT reconstruction demonstrating multiple intra-articular loose bodies (indicated by a blue arrow). (D–F) Axial, coronal and sagittal CT images displayed clustered cartilaginous loose bodies within the infrapatellar fat pad and popliteal fossa (indicated by blue arrows). Blue arrows indicate the pathological findings across the images.

On physical examination, the patient exhibited mild joint line tenderness, particularly pronounced on the lateral aspect, accompanied by slight joint effusion. The knee demonstrated a full range of motion (130° flexion to 0° extension) without any signs of erythema, increased warmth or instability. Observation of his gait revealed a normal, non-antalgic walking pattern with no limp or asymmetry. Ligamentous and meniscal special tests, including the McMurray, Lachman, anterior drawer and posterior drawer tests, yielded negative results. Considering the chronicity of symptoms, imaging findings and intra-articular mechanical signs, a diagnosis of synovial chondromatosis of the right knee was established. The patient subsequently underwent arthroscopic surgery.

Investigations

Routine laboratory investigations, including complete blood count, erythrocyte sedimentation rate (ESR), C reactive protein (CRP), antinuclear antibody (ANA) and rheumatoid factor (RF), were all within normal limits.

Repeat radiographs revealed multiple intra-articular loose bodies predominantly located in the posterior compartment of the knee (figure 1B). CT imaging further identified multiple nodular loose bodies within the infrapatellar fat pad and popliteal fossa, consistent with cartilaginous density (figure 1C–F).

Postoperative histopathological examination of the loose body showed a nodular fragment of hyaline cartilage with central cavitation and irregular peripheral endochondral ossification, without cytological atypia or malignant features, consistent with primary synovial chondromatosis (figure 2C).

Figure 2. Intraoperative view, gross appearance loose bodies and histopathological findings. (A) An arthroscopic view showed multiple white loose bodies in the posterior compartment. (B) Gross specimen with loose bodies placed beside a metric ruler (largest ~10 mm). (C) Histopathological section shows hyaline cartilage nodules within synovial tissue, consistent with primary synovial chondromatosis. The section was stained with H&E stain, scale bar: 500 µm.

Differential diagnosis

In paediatric patients presenting with chronic monoarticular knee pain, several differential diagnoses should be considered, including juvenile idiopathic arthritis (JIA), pigmented villonodular synovitis (PVNS) and chronic nonspecific synovitis.

JIA typically manifests with prolonged joint pain, morning stiffness, swelling and limited range of motion, often accompanied by elevated inflammatory markers or positive autoimmune antibodies. However, in this case, the patient had no morning stiffness, systemic symptoms or laboratory abnormalities (such as ESR, CRP, ANA or RF elevation). Furthermore, imaging revealed discrete intra-articular loose bodies rather than diffuse synovial thickening or erosive changes, effectively ruling out JIA.

PVNS is another consideration, especially in children presenting with recurrent non-traumatic knee swelling and effusion. PVNS is characterised by haemosiderin-laden villonodular proliferation, often producing low-signal intensity on both T1-weighted and T2-weighted MRI sequences. In the present case, MRI showed multiple cartilaginous loose bodies with isointense to low-signal features and no evidence of haemosiderin deposition, making PVNS unlikely.

Chronic non-specific synovitis or post-traumatic synovial inflammation is a frequent benign cause of knee pain in children. However, the presence of distinct intra-articular nodules and progressive calcified loose bodies, confirmed by CT and arthroscopy, clearly indicated synovial chondromatosis rather than simple synovitis. Overall, the chronic mechanical symptoms, characteristic imaging findings and histopathological confirmation supported a final diagnosis of primary synovial chondromatosis.

Treatment

In view of the persistent monoarticular knee pain and imaging evidence of multiple intra-articular loose bodies consistent with synovial chondromatosis, the patient was scheduled for arthroscopic loose body removal with partial synovectomy.

Under general anaesthesia, arthroscopic surgery was performed with the patient in the supine position using standard anterolateral and anteromedial portals. Diagnostic arthroscopy revealed diffuse synovial hypertrophy and multiple smooth, white cartilaginous loose bodies, mainly in the posterior compartment and around the infrapatellar fat pad. All visible loose bodies were removed using graspers and an arthroscopic shaver, and a partial synovectomy of macroscopically diseased synovium was carried out while preserving normal tissue. Representative loose bodies were collected and sent for histopathological examination. Intraoperatively, significant synovial hyperplasia was observed. Multiple oval-shaped loose bodies (largest ~10×5 mm) were found in the posterior compartment (popliteal fossa). These arthroscopic findings were suggestive of synovial chondromatosis. Synovial shaving and complete removal of loose bodies were performed (figure 2A,B). Subsequent histopathological analysis confirmed the diagnosis of primary synovial chondromatosis (figure 2C). Together, the intraoperative appearance and histological findings confirmed the diagnosis of primary synovial chondromatosis.

Outcome and follow-up

Subsequent follow-up demonstrated excellent clinical outcomes, with complete symptom resolution and no clinical evidence of recurrence. At the 6-month postoperative follow-up visit, he reported no knee pain in daily life or during sport and was actively engaged in a home-based rehabilitation programme. He had returned to full participation in school, including physical education lessons and age-appropriate running and ball games, without perceived limitation, and clinical examination confirmed a full, pain-free range of motion and normal gait. Both the patient and his parents expressed satisfaction with the functional outcome.

Discussion

Synovial chondromatosis is rare in paediatric patients, and this rarity often makes diagnosis difficult. In paediatric populations, diagnosis is frequently delayed due to nonspecific symptoms and early radiographs that are normal or radiographically occult.⁸ In this case, radiographs obtained 12 months earlier at another institution were reported as normal despite recurrent knee pain and effusion, likely reflecting limited radiographic sensitivity in early disease (prior to mineralisation) and the posterior compartment location of the pathology. By the time the child was referred to our centre, ossified loose bodies were already visible on plain radiographs, representing disease progression rather than new onset. This timeline highlights that clinicians should not rely on interval ossification on repeat radiographs as the diagnostic turning point; instead, persistent symptoms should trigger a synovium-focused evaluation and timely escalation to ultrasound or MRI.

Recurrent monoarticular symptoms in paediatric patients necessitate consideration of a mechanical differential diagnosis, even in the absence of trauma. In any child with persistent knee pain and effusion, the synovium should be considered a primary source of pathology until proven otherwise. Persistent symptoms should prompt a re-evaluation of common assumptions regarding overuse or growing pains. In cases of chronic recurrent knee pain, synovial chondromatosis should be considered within the scope of clinical practice, rather than solely focusing on simple synovitis.^{9 10} Early diagnosis presents certain difficulties, particularly in paediatric patients.^{10 11} As early lesions may lack calcification, conventional radiography struggles to detect abnormalities,¹² frequently leading to misdiagnosis as growing pains, sports injuries or other benign conditions. Accordingly, waiting for ossification to appear on radiographs may delay diagnosis; persistent or recurrent effusion and mechanical symptoms should prompt timely escalation of evaluation. In this context, escalation of evaluation primarily means proceeding to further imaging, particularly ultrasonography with Doppler and when indicated, MRI. Ultrasound with Doppler is widely available, non-ionising and inexpensive, and can assess effusion characteristics, synovial hypertrophy and vascularity, and may detect non-calcified loose bodies. MRI demonstrates high diagnostic sensitivity, excelling in identifying non-calcified chondroplastic nodules and synovial thickening, thereby serving as a vital tool for early detection.⁹

Similar diagnostic difficulties and the importance of early synovial evaluation and timely escalation to ultrasound or MRI have been highlighted in paediatric cases of synovial chondromatosis. In a 14-year-old girl with shoulder involvement and a 10-year-old girl with knee locking, initial radiographs were normal and the diagnosis was only made after ultrasound and/or MRI, followed by arthroscopy, demonstrated multiple loose bodies.^{8 12} In a 2-year-old girl with bilateral knee disease and a 6-year-old boy with hip pain and intermittent limping, early radiographs and laboratory tests were also non-diagnostic, and synovial chondromatosis was confirmed only when persistent symptoms prompted further imaging or exploratory surgery.^{10 11} Across these reports, the diagnostic turning point was not interval findings on repeat radiographs but escalation to synovium-focused assessment with ultrasound or early MRI (and arthroscopy when indicated), enabling detection and removal of loose bodies with good functional recovery, consistent with our patient’s outcome.

Clinically, heightened vigilance is warranted for the following red flag symptoms: persistent pain in a single joint persisting beyond several weeks (or recurring despite initial management) with negative or inconclusive radiographs; presence of mechanical symptoms such as joint catching, clicking or restricted movement; and recurrent non-traumatic joint effusion.¹³ In such cases, synovial pathology should be considered, and further evaluation for synovium should be considered without waiting for radiographic ossification. Ultrasound with Doppler may be a practical early adjunct, while MRI is particularly helpful when symptoms persist or mechanical features are present.

The primary approach to treatment involves arthroscopic synovectomy and the removal of loose bodies, which generally results in excellent functional outcomes in most cases.¹² However, recurrence may occur if synovial metaplasia persists, necessitating careful follow-up. Arthroscopy serves both diagnostic and therapeutic purposes, especially when non-invasive imaging is inconclusive.¹⁴ It offers visualisation of synovial and articular structures and enables the definitive removal of loose bodies.^{15 16}

To aid clinical decision-making, we have summarised a simplified diagnostic algorithm for persistent monoarticular knee pain with effusion in children:

1. Persistent knee pain with effusion warrants primary consideration of synovial pathology; normal plain radiographs do not exclude early synovial disease.

2. Initial evaluation comprises targeted history-taking and physical examination, plain radiographs and baseline laboratory investigations (complete blood count, ESR, CRP±ANA/RF, as clinically indicated).

3. In the presence of persistent or recurrent effusion or mechanical symptoms (eg, catching, clicking, locking or restriction of motion), early ultrasonography with Doppler may be considered as an early adjunct to assess synovial hypertrophy, vascularity, effusion characteristics and potential non-mineralised intra-articular loose bodies.

4. When ultrasonography demonstrates features suggestive of synovial pathology, or when symptoms persist despite non-diagnostic radiographs, timely MRI is indicated, rather than deferring assessment until radiographic calcification becomes apparent.

5. On confirmation of intra-articular pathology, arthroscopy provides definitive diagnosis and treatment (loose body removal±synovectomy), followed by clinical surveillance for recurrence.

Learning points.

• Persistent monoarticular effusion should raise suspicion for synovial pathology even when radiographs are normal.

• Waiting for ossification on radiographs may delay diagnosis by months.

• Ultrasound with Doppler is a practical early tool for assessing synovial abnormalities.

• MRI should be considered early when symptoms persist or mechanical signs are present.

• Arthroscopy remains both diagnostic and therapeutic.