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 with tension band construct
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
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