43 - Patella Luxation

general

genetic → heritable

developmental disease → not congenital

65% are bilateral

80% are medial luxation

small breed

toy and mini breeds

medial luxation

large breed

lateral luxation → more complex to tx

increasing in prevalence

anatomy

patella is a sesamoid bone embedded within the patellar tendon

musculature → quadriceps insert on patella and continue as patellar ligament to insert on tibial tuberosity

normal patellar mechanism

provides smooth surface for tendon to glide over moving joint

acts like pulley mechanism → changes the direction of force + magnifies force with less effort

must be properly aligned for normal function → femoral shaft, trochlear groove, and tibial tuberosity

medial patellar luxation

luxation = malalignment of quadriceps mechanism

one of the most common rear limb lamenesses

often bilateral

MPL = higher incidence of concurrent CCLR

small breeds

usually initially detected in young dogs without CCLR

can be detected in older dogs with CCLR

no hx of trauma

“skipping” hindlimb lameness

NWB lamness can indicate MPL + CCLR

femoral malformations

coxa vara

decreased angle of femoral head

femur pulled up and tilted, may cause extensor muscle laxity

decreased angle of anteversion

femoral head/neck are tilted

limb rotates internally → knee and foot twist toward midline body

distal external torsion

tendon pulled out of alignment

MPL = external rotations

LPL = internal rotation

distal varus

rotated femur

hypoplasia of medial condyle

femur leans on malformed condyle

secondary to luxated patella during growth → no pressure on trochlear groove

tibial malformations

medial displacement of tibial tuberosity

most common

internal proximal tibial torsion

proximal tibial valgus

distal varus

less-common causes

secondary to trauma

patellar alta → patella sits too proximally in groove

tightness or atrophy of quadriceps muscles

exam findings

normal to decreased muscle mass in HL

normal to decreased weight bearing in HL

unilateral or bilateral skipping gait

intermittent lamness

dogs may limp then extend leg

discomfort with palpation and ROM

may feel grinding as patella moves

stifle effusion

crepitus with ROM

check for tibial thrust and/or cranial drawer

grading

increased grade associated with more severe femoral/tibial deformities

grade I

minimal malaignment

can be luxated by force but automatically return to placement

asyptomatic → often incidental finding

no treatment necessary

grade II

intermittent luxation

patella luxates easily on palpation but still wants to stay in groove

spends more time in place than out

surgery indicated in clinical signs present or are progressive

Grade III

patella permanently luxated but can be manually reduced → reluxates when released

significant lameness and gait abnormalities

surgery is indicated

Grade IV

may need to sedate to grade

permanent luxation and cannot be reduced manually

trochlear groove absent or severely eroded trochlear ridges

severe gait abnormalities

often has concurrent CCLRs

surgery indicated → guarded prognosis

surgical repair

assess trochlear groove

assess tibial tuberosity

case selection

small dogs with minimal to moderate limb deformities

large dogs with minimal to no major limb deformities

trochleoplasty

trochlear block recession

preserves more cartilage surface area

trochlear wedge resection

easy to perform with hand saw

preserves hyaline articular cartilage

deepens trochlear groove

shapes: wedge or block

abrasion sulcoplasty

removes cartilage and replaces with fibrocartilage → poorer overall quality

tibial crest transposition

re-aligns quadriceps by shifting insertion of patellar ligament

risks

may create LPL

implant failure

tibial tuberosity avulsion

fracture

soft-tissue aspects

releasing incison

anti-rotation suture if concurrent CCLR

retinacular imbrication

corrects retinacular laxity

excessive tension predisposes luxation in opposite direction → goal is equal tension medial and lateral

opposite side as direction of luxation

performed as final step of luxation correction