A 19-year-old female patient was referred with anterior knee pain and a palpable hard mobile mass above the patella. Her medical history included a twisting injury 4 years prior to imaging. No ecchymosis or bruising was observed clinically. MR imaging was suggested.
MR imaging was performed with a 1.5 Tesla scanner using standard sequences. Axial T2-w (Fig. 1), sagittal PD (Fig. 2) and coronal T1-w images (Fig. 3) demonstrated an osseous fragment located posterosuperiorly and medially to the upper pole of the patella. There was also a small amount of intra-articular fluid (Fig. 1, 3). The coronal fat suppressed PD-w (Fig. 4) MR image, showed the presence of bone marrow oedema within the patella at the site of the avulsed bipartite fragment.
A. Patella is the largest sesamoid in the body [1]. It is a cartilaginous structure, which begins to ossify at 3 years of age from its centre to periphery until the age of 6 [2]. At the age of 12, secondary ossification centres develop and they are fused in adolescence. If this process is not completed, a bipartite patella results [1]. Bipartite patella is a normal developmental variant of the ossification centres, occurring in 1−2% of the general population and is more common in men [1, 3, 4]. Pathophysiology of this entity is not clear yet. Some authors suggest that bipartite patella is the result of repetitive microtrauma due to abnormal mobility at the fibrocartilaginous interface between ossification centres [1]. Other authors consider that there is non-union of disseminated ossification centres due to stress at the insertion site of the vastus lateralis [3]. Rarely, a tripartite or multipartite patella may be seen.
B. This entity is usually asymptomatic and is frequently discovered incidentally [2]. However, a painful bipartite patella in adolescents and young adults has been described [4]. The cause of pain is due to mobility of the synchondrosis between the patella and the bipartite fragment. MRI is important in this respect, because it reveals the presence of bone marrow oedema and explains the localised tenderness [1, 4]. A traumatic separation of a bipartite patella has been reported as rare complication [5]. Migration of the avulsed fragment, as shown herein, is extremely rare.
C. This entity could be recognised with X-rays, MR imaging or computed tomography. MR imaging is the gold standard for diagnosing symptomatic patients presenting with anterior knee pain, as it depicts the bone marrow oedema and the peripatellar soft tissue oedema. It has been reported that scintigraphy could be an alternative option of MRI [2]. A migrated avulsed fragment can easily be depicted with all imaging modalities. Imaging is important in differentiating a traumatic fracture from bipartite patella, the later showing irregular corticated borders.
D. Treatment of bipartite patella is usually conservative with nonsteroidal anti-inflammatories and physiotherapy and the prognosis is very good. In patients with persisting symptoms, surgical retinacular release, resection of the fragment with subsequent tendinous reattachment are the treatment options [2]. In our patient, the treatment included surgical reattachment and fusion of the fragment.
E. Bipartite patella may be complicated by avulsion of the fragment and may simulate the presence of a knee joint mass.
Avulsed bipartite patella presenting as mass causing anterior knee pain.
The patient is a 19-year-old female presenting with anterior knee pain and a palpable hard mass with good mobility located above the patella. Based on the provided MRI images, the following findings are noted:
Based on the above imaging findings and clinical history, the following diagnoses or differential diagnoses should be considered:
Considering the patient's age, anterior knee pain, a palpable mobile bony mass, and the MRI findings of a separated bone fragment with sclerotic borders and mild edema at the superior pole of the patella, the most likely diagnosis is: “Bipartite Patella with an Avulsed Bone Fragment Displacement.” This diagnosis is consistent with literature reports and represents a rare complication of bipartite patella caused by chronic wear or trauma.
For bipartite patella that is asymptomatic or mildly symptomatic, conservative treatment is generally adopted. However, in this case, due to a clearly detached or displaced fragment accompanied by pain, surgical intervention may be considered. The specific treatment plan may include the following steps:
Rehabilitation and Exercise Prescription Recommendations (FITT-VP Principle):
Individualization is key. If severe knee pain or fragment instability persists, reduce or pause weight-bearing training until inflammation subsides, then resume exercise.
Disclaimer: This report is a reference-based analysis based on current imaging and clinical data and does not replace an in-person consultation or professional medical advice. If you have any doubts or if your symptoms worsen, please seek medical attention promptly.
Avulsed bipartite patella presenting as mass causing anterior knee pain.
