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Updated: Oct 12 2022

Bipartite Patella

Images 1.jpg 2.jpg type iii xr.jpg prone.jpg
  • summary
    • Bipartite Patella is a congenital knee condition caused by the failure of the patella to fuse and is often an incidental finding on radiographs.  
    • Diagnosis is confirmed radiographically with most commonly an unfused patella at the superolateral pole. 
    • Treatment is observation and most often does not require treatment as the condition is typically asymptomatic.
  • Epidemiology
    • Incidence
      • 2-3% of the population
    • Demographics
      • no notable sex predilection
    • Anatomic location
      • most often found in the superolateral region (Type III)
      • bilateral in 50%
  • Etiology
    • Etiology
      • Normal patella variant representing a failure of fusion
        • often confused with patella fractures
    • Pathophysiology
      • considered a developmental variation of ossification
      • painful bipartite patella following injury
        • direct or indirect injury results in disruption of the fibrocartilaginous zone between the main patella and accessory fragment
        • fibrocartilaginous zone cannot heal by bony union, resulting in persistent pain
          • lack of arterial penetration from patella to osteochondral fragment
          • vastus lateralis contributes to traction force in fragment separation and nonunion
    • Associated conditions
      • nail-patella syndrome
      • patella fracture
        • compared with patella fractures, bipartite patellas:
          • are located superolaterally
          • have smooth, rounded borders
          • may have similar findings on contralateral knee radiographs (50%)
  • Anatomy
    • Osteology
      • the patella is the largest sesamoid bone
      • arises from a single ossific nucleus
      • ossification
        • males at 4-5 years old
        • females at 3 years old
        • accessory ossification center appears between 8-12 years
          • separate fragment attached to patella by fibrocartilaginous tissue
    • Function
      • fulcrum for the quadriceps
      • protects the knee joint
        • articular cartilage of patella is thickest in body (up to 1cm)
      • enhances lubrication of the knee
      • see complete knee biomechanics
    • Blood supply
      • blood supply to patella is predominantly from distal to proximal
      • 6 arteries contribute
        • from popliteal artery
          • superior lateral geniculate artery
          • superior medial geniculate artery
          • inferior lateral geniculate artery
          • inferior medial geniculate artery
        • from superficial femoral artery
          • supreme geniculate artery
        • from anterior tibial artery
          • anterior tibial recurrent artery
  • Classification
      • Saupe Classification
      • Type
      • Incidence
      • Location
      • Type I
      • 5%
      • Inferior pole
      • Type II
      • 20%
      • Lateral margin
      • Type III
      • 75%
      • Superolateral pole
  • Presentation
    • Symptoms
      • most are asymptomatic and discovered incidentally
      • only 2% become symptomatic
        • anterior knee pain from
          • direct trauma (e.g. fall, kick to the knee)
          • indirect trauma or overuse injuries (e.g. cycling, hill climbing)
          • aggravated by squatting, jumping, climbing stairs
        • giving way
    • Physical exam
      • localized tenderness over accessory fragment (typically superolateral patella)
      • hematoma
      • quad inhibition
      • unusual patella prominence or palpable defect
      • larger than normal patella
  • Imaging
    • Radiographs
      • recommended views
        • AP knee radiograph
          • best view to visualize bipartite patella
        • skyline view
          • prone position (non-weight-bearing)
          • squatting position (weight-bearing)
            • may show displacement of the accessory fragment
        • consider radiographs of the contralateral knee for comparison
      • findings
        • smooth edges (helps differentiate from fracture)
        • weight-bearing skyline (squatting) view demonstrates increased separation of fragments compared with non-weight views (prone)
        • 50% have bilateral bipartite patella
    • MRI
      • indications
        • assessment of painful bipartite patella to determine if pain is attributable to the bipartite patella
      • findings
        • edema around the fragment may indicate that it is the cause of symptomatic knee pain
    • Bone scan
      • indications
        • equivocal radiographs with high suspicion for symptomatic bipartite patella
      • findings
        • increased uptake along superolateral aspect
      • utility somewhat controversial
    • CT scan
      • will clearly demonstrate fragment, but does not demonstrate edema
  • Studies
    • Histology
      • interposed tissue between accessory and main fragment
        • composed of fibrocartilage > fibrous > hyaline cartilage
        • avascular
      • adjacent bone
        • scalloped surface with numerous osteoclasts
        • well-vascularized
  • Treatment
    • Nonoperative
      • rest, immobilization, NSAIDS, and physical therapy
        • indications
          • generally, non-operative, symptomatic management is indicated for bipartite patella for at least 6 months
        • modalities
          • rest and restriction of sports activities
          • NSAIDs
          • isometric quadriceps strengthening exercises
          • immobilization with the knee braced in 30° of flexion
          • local corticosteroid injections
        • non-operative management may be less successful in younger, athletic patients, possibly due to non-compliance
    • Operative
      • open excision of the accessory fragment
        • indications
          • failed nonoperative treatment >6 months or in cases of a displaced fragment requiring reduction, direct trauma resulting in the onset of pain, or significant impairment in daily activities
        • most common treatment technique, typically good results
          • may lead to poor results if large, articular fragment due to patellofemoral incongruity
      • arthroscopic excision
        • thought to lead to expedited recovery and avoids disrupting the quad tendon
        • limited evidence to support but good results in case reports
      • lateral retinacular release
        • indications
          • superolateral fragment (to remove the traction force of the vastus lateralis on the fragment)
      • vastus lateralis release (subperiosteal)
        • indications
          • superolateral fragment
          • to avoid long lateral retinacular release
        • open vs arthroscopic
          • quicker recovery and less effusion noted in arthroscopic patients
      • ORIF
        • indications
          • for large fragments
          • limited support in the literature, controversial
  • Complications
    • Patellofemoral maltracking
      • due to excision of a large fragment or lateral retinacular release
      • may lead to patellofemoral degenerative changes
    • Effusion
      • may require arthrocentesis following fragment excision
    • Persistent knee pain
      • may see following lateral retinacular release
    • Quadriceps weakness
    • Osteonecrosis
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