A 13-year-old female patient presented at our hospital with a history of painless tumefaction on the medial side of her left knee, starting a few weeks before.
Radiographs (Fig. 1) showed soft tissue densities with chondroid-like calcifications adjacent to the medial tibia. CT examination (Fig. 2) additionally evidenced a small thorn-like exostosis at the medial tibia, representing either a pes anserinus bony spur or an osteochondroma. This lesion was surrounded by a distended anserine bursa, which contained four loose bodies with “ring-and-arc” type calcifications. On MR imaging (Figs. 3, 4), these loose bodies were isointense on T1WI and hyperintense on T2WI, with hypointense areas in all pulse sequences related to calcifications. After contrast administration, peripheral enhancement of the loose bodies was evident.
Surgery and histologic analysis were performed. The surgical specimen included four chondroma-like well-defined cartilaginous nodules, some with calcified matrix. No mention of synovial chondrometaplasia was made. The exostosis was not resected.
This combination of radiologic and pathologic findings is consistent with the diagnosis of bursal synovial osteochondromatosis secondary to the exostosis.
Exostoses occurring in the pes anserinus region may or may not be osteochondromata [1, 2, 3]. In our case, there was a pedunculated lesion angled away from the joint, with cortical and medullary continuity with the parent bone, thus suggesting osteochondroma. Nevertheless, it arose in the pes anserinus location and was very small and thorn-like, a classic appearance for a pes anserinus spur [1, 3, 4].
Synovial osteochondromatosis (SOC) is an uncommon disorder characterized by the formation of multiple cartilaginous nodules or osseous loose bodies within the synovium. It can be classified as primary (benign neoplastic process) or secondary (associated with joint abnormalities, such as mechanical or arthritic conditions that cause intraarticular chondral bodies). Rarely, it may involve extraarticular sites, arising in synovium about the tendons or bursa [5, 6].
Imaging features of SOC are frequently pathognomonic [5, 6]. Radiographic features include multiple intraarticular (or intrabursal, as in this case) chondral bodies with “ring-and-arc” chondroid mineralization and extrinsic erosion of bone. CT is the optimal radiologic modality to identify and characterize these calcified intraarticular fragments and extrinsic erosions. MR imaging appearance is variable owing to the degree of mineralization and ossification of the chondral bodies, although the extent of involvement is exquisitely depicted. The noncalcified regions of hyaline cartilage typically demonstrate hyperintensity on T2WI because of the high water content of this tissue.
In the unusual case presented here, we believe that the presence of a tibial exostosis played a significant role in the development of bursal SOC [7, 8, 9, 10]. Despite its small size, it is possible that the exostosis in this case, given its specific location, caused chronic mechanical impingement of the anserine bursa. During this process, trauma to the bursal wall could have produced synovitis and subsequent fibrin coagula within the bursal cavity, which, in turn, could have undergone chondrification. An alternative potential source for intrabursal osteochondral bodies is synovial chondrometaplasia [8]. Also, if the exostosis represents an osteochondroma, cartilaginous debris from its tip could have been shed into the bursa, where, nourished by bursal fluid, they continued to grow and proliferate [7].
The pathologic appearance of SOC may be misleading because of significant histologic atypia and radiologic correlation is vital for correct diagnosis, as it allows the distinction of this condition from more aggressive chondroid neoplasms [5, 7, 8]. A multimodality imaging approach is desired to fully characterize this condition and to allow for optimal patient management.
Synovial osteochondromatosis of the anserine bursa secondary to a tibial exostosis
Based on the provided X-ray, CT, and MRI imaging data, the following findings are noted:
1. A small protruding lesion is observed in the proximal medial tibia of the left knee, with a stalk-like appearance, continuous with the tibial diaphyseal medullary cavity and cortex, consistent with the typical features of an osteochondroma (bony exostosis).
2. Multiple round or oval calcifications or ossifications are clearly visible along the medial tibia, distributed near the pes anserinus (tendinous insertion of gracilis, sartorius, and semitendinosus) or within its bursal area. CT shows “ring-and-arc” cartilage calcifications, suggesting cartilaginous proliferation.
3. MRI indicates multiple abnormal signal nodules in that region. The non-calcified portions appear as high signal intensity on T2-weighted images, with mild enhancement post contrast. Given the frequent location in or around a bursa, these lesions are indicative of bone-cartilage-like bodies within the synovium (or bursa).
4. There is no evidence of large-scale swelling or tendon injury in the surrounding soft tissue. However, localized thickening or reactive changes are noted in the bursa.
Taking the patient’s age, medical history, and imaging findings into consideration, the main diagnoses and their rationales include:
1. Osteochondroma: A lesion continuous with the diaphyseal medullary cavity, featuring a cartilaginous cap on its surface; a common benign exostosis in adolescents.
2. Pes anserinus spur: May similarly manifest as a spur-like protrusion on the proximal medial tibia, often related to chronic irritation at the fascia or tendon insertion site.
3. Synovial Osteochondromatosis (SOC): Characterized by multiple cartilaginous or ossified nodules within a joint or bursa, with “ring-and-arc” calcifications on imaging and typically presenting as numerous scattered small nodules.
4. Secondary Factors in Synovial Chondromatosis: Chronic bursitis may arise when an osteochondroma grows directionally, or mechanical irritation persists, leading to cartilaginous and ossified bodies within the bursa.
5. Chondrosarcoma: Should be considered if the cartilaginous cap is excessively thick or the lesion appears invasive. However, in this case, the findings are more indicative of a benign process.
Based on the patient’s adolescent age, mild symptoms, the presence of a stalk-like bony exostosis continuous with the tibial medullary cavity, and multiple small bone-cartilage nodules around the lesion, the most likely diagnosis is:
“Medial tibial osteochondroma (or small exostosis) with local synovial osteochondromatosis (SOC).”
In this case, the small tibial exostosis/spur likely exerts chronic mechanical irritation on the pes anserinus bursa, resulting in multiple cartilaginous or bony bodies forming within the synovium or bursa. If definitive confirmation is required, a histopathological examination may be performed to exclude the rare possibility of malignancy.
1. Conservative Treatment: In the absence of significant pain, functional impairment, or neurovascular compromise, conservative management is recommended. This includes oral NSAIDs to control chronic bursitis and local physiotherapy to alleviate pain and inflammation.
2. Surgical Treatment: In cases of recurrent symptoms (e.g., repeated joint locking, escalating pain, restricted range of motion), or if malignancy is suspected, surgical excision of the lesion (including removal of the exostosis/osteochondroma and intra-bursal bone-cartilage fragments) may be considered to eliminate mechanical irritation and prevent progression.
3. Rehabilitation/Exercise Prescription:
- Early Phase (acute phase or immediate postoperative phase):
• Focus on maintaining joint range of motion and muscle strength. Incorporate passive and limited active exercises while protecting the knee from secondary injury.
• Limit each exercise session to about 10–15 minutes, performed 2–3 times per day, maintaining low intensity.
- Middle Phase (chronic stable phase or mid-postoperative period):
• Emphasize strengthening of knee-surrounding muscles such as the quadriceps and hamstrings with isometric and isotonic exercises.
• Gradually increase both exercise intensity and frequency, for example, 15–20 minutes per session, 3–4 sessions per week, progressively adding workload.
• Introduce light squats or seated knee extensions to improve joint stability and muscle strength.
- Late Phase (functional recovery phase):
• If there is no pronounced pain or inflammation, incorporate single-leg balance drills and controlled knee compound movements (e.g., low-impact jumping) to enhance functional recovery.
• Increase exercise frequency to 4–5 times per week, about 20–30 minutes each session, supplemented with low-impact aerobic exercises such as swimming or cycling.
- Throughout the entire rehabilitation process, adjust exercise frequency, intensity, duration, and mode according to the patient’s tolerance (the FITT-VP principle), with careful attention to knee joint stability.
- For patients with osteoporosis or a weaker constitution, pay extra attention to injury risks, and, if necessary, conduct rehabilitation under professional guidance.
This report is based on the currently provided medical history and imaging information for reference purposes only and does not replace an in-person consultation or professional medical opinion. Specific treatment and rehabilitation plans should be determined by clinical physicians and rehabilitation therapists according to the patient’s actual condition.
Synovial osteochondromatosis of the anserine bursa secondary to a tibial exostosis
