Patella Fracture - Trauma

Introduction
- A patella fracture is a traumatic knee injury caused by direct trauma or rapid contracture of the quadriceps with a flexed knee that can lead to loss of the extensor mechanism.
- Diagnosis can be made clinically with inability to perform a straight leg raise and confirmed with radiographs of the knee.
- Treatment is either immobilization or surgical fixation depending on fracture displacement and integrity of the extensor mechanism.

Epidemiology
- Incidence
  - common
    
    patella fractures account for 1% of all skeletal injuries
- Demographics
  - male to female 2:1
  - most fractures occur in 20-50 year olds

Etiology
- Mechanism of injury
  - direct impact injury
    
    occurs from fall or dashboard injury
    
    causes failure in compression
  - indirect eccentric contraction
    
    occurs from rapid knee flexion against contracted quads muscle
    
    causes failure in tension
    
    patella sleeve fracture
    
    seen in pediatric population (8-10 year olds)
    
    high index of suspicion required
- Associated injuries
  - high-energy dashboard injuries are associated with femoral neck fracture, posterior wall acetabular fracture, or knee dislocation

Anatomy
- Osteology
  - patella is largest sesamoid bone in body
  - superior 3/4 of posterior surface covered by articular cartilage
    
    articular cartilage thickest in body (up to 1cm)
  - posterior articular surface comprised of medial and lateral facets
    
    lateral facet is larger
    
    facets separated by vertical ridge
- Soft tissue attachments
  - quadriceps tendon and fascia lata attach to anterosuperior margin
    
    quadriceps tendon comprised of 3 layers
    
    superficial layer formed from rectus femoris tendon
    
    middle layer formed by vastus medialis and vastus lateralis tendons
    
    deep layer formed by vastus intermedius tendon
  - patellar tendon attaches to inferior margin
- Blood Supply
  - derives from anastomotic ring originating from geniculate arteries
  - most important blood supply to the patella is located at the inferior pole

Classification

Can be described based on fracture pattern

Fracture pattern classification
Nondisplaced
Displaced (step-off >2-3mm or fracture gap >1-4mm)
Transverse
Pole or sleeve (upper or lower)
Vertical
Marginal
Osteochondral
Comminuted (stellate)

Presentation
- History
  - direct blow to knee or extensor mechanism injury
- Physical exam
  - inspection
    
    palpable patellar defect
    
    significant hemarthrosis
  - motion
    
    unable to perform straight leg raise indicates failure of extensor mechanism
    
    retinaculum disrupted
    
    can aspirate hemarthrosis and inject local anesthetic if patient unable to perform due to pain
  - provocative tests
    
    perform saline load test to rule out open fracture

Imaging
- Radiographs
  - recommended views
    
    AP
    
    lateral
    
    best view to see transverse fx
    
    tangential
    
    best view to see vertical fx
  - findings
    
    fracture displacement
    
    degree of fracture displacement correlates with degree of retinacular disruption
    
    patella alta
    
    Insall-Salvati ratio >1
    
    indicates disruption of patellar tendon
    
    patella baja
    
    Insall-Salvati ratio <1
    
    indicates disruption of quads tendon
- CT
  - obtain if suspicion for patellar stress fracture, nonunion, or malunion
- MRI
  - obtain MRI if child has normal xrays but is unable to straight leg raise

Differential
- Bipartite patella
  - may be mistaken for patella fracture
  - affects 8% of population
  - characteristic superolateral position
  - bilateral in 50% of cases

Treatment
- Nonoperative
  - knee immobilized in extension (brace or cylinder cast) and full weight bearing
    
    indications
    
    intact extensor mechanism (patient able to perform straight leg raise)
    
    nondisplaced or minimally displaced fractures
    
    vertical fracture patterns
    
    early active ROM with hinged knee brace
    
    early WBAT in full extension
    
    progress in flexion after 2-3 weeks
- Operative
  - ORIF
    
    indications
    
    preserve patella whenever possible
    
    extensor mechanism failure (unable to perform straight leg raise)
    
    open fractures
    
    fracture articular displacement >2mm
    
    displaced patella fracture >3mm
    
    patella sleeve fractures in children
    
    techniques
    
    mulitple techniques exist each with pros and cons including
    
    tension band construct with k-wires or screws
    
    cerglage wire
    
    plate and screws
  - partial patellectomy
    
    indications
    
    comminuted superior or inferior pole fracture measuring <50% patellar height ONLY if ORIF is not possible
  - total patellectomy
    
    indications
    
    reserved for severe and extensive comminution not amenable to salvage

Techniques
- ORIF with tension band construct
  - approach
    
    midline longitudinal incision centered over patella
    
    expose articular surface either through fracture site or retinacular rents
    
    can alternatively perform lateral parapellar arthrotomy and invertt patella if retinaculum is not damaged or if better visualization of articular surface is desired
  - soft tissue
    
    avoid extensive soft tissue dissection to preserve blood supply and viability of skin flaps
  - bone work
    
    remove devitalized fragments and loose bodies
    
    retain as much of patella as possible
  - instrumentation
    
    minifrag lag screw fixation for independent fragments
    
    tension band using 0.062 K wires with figure of 8 wire
    
    converts tensile forces generated by quads complex at anterior surface into compressive forces at articular surface
    
    figure-of-8 typically constructed using 18-gauge stainless steel wire
    
    difficult to manipulate and high reoperation rates due to painful hardware or wire migration
    
    can alternatively use braided polyester suture
    
    found to have 75% tensile strength of 18-gauge stainless steel wire
    
    longitudinal cannulated screws combined with tension band wiring
    
    plate and screw fixation noted to be biomechanically superior
    
    circumferential cerclage wiring
    
    good for comminuted fractures
    
    interfragmentary screw compression supplemented by cerclage wiring
    
    good for comminuted fractures
  - complications
    
    painful hardware/anterior knee pain
    
    important to tension wire at superior aspect of construct where more soft tissue coverage is available
    
    consider using braided polyester suture as opposed to 18-gauge stainless steel wire
    
    hardware failure
    
    important to tension wire in 2 places to apply equivalent tension in both sides of construct
    
    avoid overtensioning wire to prevent articular gapping or wire failure
    
    avoid prominent cannulated screw tips that can cause wire failure
  - outcomes
    
    modified anterior tension band wiring shown to produce best results
- Partial patellectomy
  - approach
    
    same as ORIF (see above)
  - soft tissue
    
    reattach quads or patellar tendon
    
    perform with transosseous tunnels or suture anchors with knee in hyperextension
    
    reattach as close to articular surface as possible
    
    prevents patellar tilt and minimizes contact stresses
    
    medial and lateral retinacular repair essential
  - bone work
    
    remove devitalized fragments and loose bodies
    
    retain as much of patella as possible
  - instrumentation
    
    transosseous tunnels or suture anchors to rettach quads or patellar tendon
    
    if necessary, reinforce with cerclage suture or wire from quads tendon to tibial tubercle
  - complications
    
    weakness
  - outcomes
    
    poor outcomes with removal of >40% patella
- Total patellectomy
  - approach
    
    same as ORIF (see above)
  - soft tissue
    
    restore integrity of extensor mechanism via imbrication
    
    medial and lateral retinacular repair essential
    
    consider advancing VMO
    
    found to have better strength and outcomes
  - bone work
    
    remove all bony fragments
  - complications
    
    weakness
    
    extensor lag
    
    avoid by performing sufficient imbrication
  - outcomes
    
    quadriceps torque reduced by 50%

Complications
- Anterior knee pain
  - more common with ORIF
- Symptomatic hardware (up to 50%)
  - most common
  - more common in open fractures, thought to be due to compromised soft-tissue envelope
- Hardware Migration
  - has been associated with tension band wiring with K-wires
- Weakness
  - more common with partial or total patellectomy
- Loss of reduction (22%)
  - increased in osteoporotic bone
- Nonunion (<5%)
  - can consider partial patellectomy
- Osteonecrosis (proximal fragment)
  - thought to be due to excessive initial fracture displacement
  - can observe these, as most spontaneously revascularize by 2 years
- Infection
  - more common in open fractures
- Stiffness
- Hardware failure
  - rare

Prognosis
- Osteonecrosis reported to occur in up to 25% but not found to affect clinical outcome