Patella Fracture - Trauma

Introduction

Overview
- a patella (kneecap) fracture is a traumatic injury caused by direct trauma or rapid contracture of the quadriceps with a flexed knee.
  - treatment is either immobilization or surgery depending on diplacement and knee extension function.
  - it is a serious injury with the potential for long term sequelae.
Epidemiology
- incidence
  - patella fractures account for 1% of all skeletal injuries
- demographics
  - male to female 2:1
  - most fractures occur in 20-50 year olds
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
Prognosis
- osteonecrosis reported to occur in up to 25% but not found to affect clinical outcome

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
- 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 wires shown 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