A 50-year-old female suffered from intermittent left knee pain for several weeks, especially when climbing the stairs and squatting. She denied previous trauma on the affected knee. Physical examination revealed local tenderness over the anterior left knee, with a positive patellar-grinding test. There was no evidence of patella subluxation or dislocation. Ultrasound examination over the left knee revealed no effusion in the suprapatellar pouch. Quadriceps and patellar tendons were also intact. Irregular contour and 2 separated segments were seen on both patellar bones. Compared with the right side (which only displayed cortical gap), marked irregularity without increased vascularity was noticed over the bony cortex on the left side, and it also corresponded to the tender point during sonopalpation (Figure 1). Bilateral bipartite patella was confirmed by plain radiography (Figure 2) as well. She was given acetaminophen and physical therapy. Her symptooms gradually subsided a few weeks after treatment.

Bipartite patella is a normal developmental variant of ossification. The patella ossifies from multiple foci which coalesce to a single ossification center between 4 and 6 years of age. The ossifi cation later expands the margins and may become irregular, resulting in several accessory ossifi cation centers during adolescence. Bipartite or multipartite patella is seen when the ossifi cation centers fail to fuse, i.e., having continuity with the cartilaginous but not osseous portion [1].

Bipartite patella is often thought to be an incidental finding. It accounts for approximately 0.2%– 6.0% in the general population and results in knee pain or instability in only less than 2% of the cases. Bipartite patella is mainly seen in males, and the incidence of bilateral involvement varies from 25% to 83% [2-6]. It may also contribute to patellofemoral pain syndrome whereby patients usually suffer from anterior knee pain during/after vigorous activities requiring repetitive knee bending (e.g., jumping, squatting, walking, or climbing stairs). Physical examination frequently yields localized tenderness over the fragments [2].

The relevant pain varies among different age groups. Subjects with painful partite patella are generally younger than the asymptomatic group— possibly because of increased physical activities [7]. Young adults and adolescents, especially who engage in sports, are vulnerable to pain owing to mobility between different segments of the patella. Middle-aged and old adults, on the other hand, may suffer from painful bipartite patella because of fragment displacement after trauma. Weakened quadriceps muscles along with separated bipartite fragments after severe injury can also cause pain in elderly patients [2].

Ultrasound is a reliable imaging modality for the evaluation of painful bipartite patella. It is also useful to assess common etiologies of anterior knee pain (e.g., patellar/quadriceps tendinitis and peri-patellar bursitis). Ultrasound examination has shown good inter- and intra-observer reliability for diagnosing bipartite patella, with the accuracy higher than 90% [8]. It is even reported to have 100% sensitivity and specificity in this regard [9]. In addition to simultaneous/easy comparison with the asymptomatic side, dynamic evaluation, Doppler imaging, and sono-palpation would all be contributory as regards prompt understanding of the details of the painful condition.

Most cases with bipartite patella can be treated with conservatively, and surgical management can be considered only in refractory cases or perhaps in physically active subjects. Conservative management includes restriction of activities, immobilization and bracing, medication, and physical therapy. Local (corticosteroid) injections have also been effective [10] whereby ultrasound imaging/guidance can defi nitely be a prompt router. Surgical interventions (e.g., excision of the fragment, lateral retinacular release, vastus lateralis release, open reduction and internal fi xation of the painful fragment) have been described. Surgical outcome was reported to be more successful in athletes—especially for better return to activity [11] with a median of 5–12 weeks postoperatively [12,13].

In conclusion, we demonstrated a case of anterior knee pain related to bipartite patella. For sure, ultrasound could be the preferred tool for examination—also to better facilitate its management.

Figure 1. Ultrasound Imaging and Schematic Drawing of the Transverse View for the (A) Left Painful and (B) Right Asymptomatic Patellar Bones

Marked irregularity with tiny fragments (arrows) was observed besides the gap (asterisk) between the two bony patellar segments in the left knee.
On the contrary, there was only a small bony concavity (arrowhead) on the right patella.

Figure 2. The Radiographs (A) and With Schematic Drawings (B) for Bilateral Bipartite Patella

A clear gap (arrowheads) was visualized between the two segments (asterisks) of the patellar bone at the left but not the right knee.

The authors declare no conflict of interest.

This work was funded by National Taiwan University Hospital, Bei-Hu Branch; Ministry of Science and Technology (MOST 106-2314-B-002-180-MY3 and 109-2314-B-002-114-MY3); and the Taiwan Society of Ultrasound in Medicine.