Page 357 - Canine Lameness

19.5 Patellar uxation 329

19.5 Patellar Luxation

The patella is the largest sesamoid bone in the canine body. Its function is to redirect the forces of
the quadriceps muscle during stifle ROM (i.e. to allow the quadriceps to act as the major stifle
extensor). Patellar luxation is defined as dislocation of the patella outside (medial or lateral) of the
trochlear groove. Contraction of the quadriceps muscle results in pulling the patella onto a straight
line from the origin (proximal femur and ventral ilium for the rectus femoris muscle) to the inser-
tion (tibial tuberosity) of the quadriceps muscle. As such, the location of the origin and insertion
of this muscle determines whether the patella luxates.
Patellar luxation is one of the most commonly encountered orthopedic disease in dogs. It may be
of congenital origin (i.e. present at birth) but in most cases, it is a developmental disease (i.e. devel-
ops after birth) that has frequently been described as having a congenital etiology because it is
thought that abnormal skeletal development (with an underlying congenital etiology) results in
dislocation of the patella during development. Although several hypotheses have been suggested
(and been partially refuted), the etiology of patellar luxation remains somewhat unclear (Kowaleski
et al. 2018). A prominent explanation is that MPL originates from primary hip skeletal abnormali-
ties, including coxa vara (decreased femoral neck inclination) and relative retroversion of the fem-
oral neck (i.e. a decreased anteversion angle). The resultant genu varus (i.e. bow-legged deformity,
where the knees are too far apart while the hocks are too close together) may be accompanied by
femoral deformities including distal external femoral torsion and femoral varus, hypoplastic or
absent medial trochlear ridges, and a hypoplastic trochlear sulcus. Displacement of the patella STIFLE REGION
medially draws the tibial apophysis medially, resulting in medial rotation of the entire joint, medial
torsion of the proximal tibia, and medialization of the tibial apophysis. In the severest cases, the
articular surfaces of the femoral and tibial condyles may be deformed and hypoplastic medially.
The quadriceps muscle’s resting tension causes it to follow the shortest possible path along the
thigh, whereby acting like a bow string, it pulls the patella further out of the trochlear groove,
exacerbating skeletal abnormalities in the growing dog. The joint capsule becomes adhered and
contracted medially and stretched laterally, adding an overarching internal rotation of the entire
stifle joint. Dogs with MPL may have a patella that rides proximally in the trochlear groove, termed
“patella alta” (Mostafa et al. 2008).
Lateral patellar luxation (LPL) is associated with an opposing suite of skeletal abnormalities,
including coxa valga, genu valgus (i.e. knock-knee deformity, where the knees are too close together
while the hocks are too far apart), femoral valgus, an undersized lateral trochlear ridge, a laterally
rotated joint, lateral tibial tuberosity torsion, lateral bowing of the proximal tibia, and medial tor-
sion of the distal tibia. Dogs with LPL may have a patella that rides distally in the trochlear groove,
termed “patella baja” (Mostafa et al. 2008).
Patellar luxation may occur concomitantly with CCLD in up to 25% of dogs with MPL (Campbell
et al. 2010). It is of great importance to determine whether the CCL is also affected since dogs with
the combination of CCLD and patellar luxation generally respond less favorably to nonsurgical
treatment. The determination of patella alta or baja and the degree of skeletal deformities are par-
ticularly important to decide if surgical treatment should be considered. Various treatment options,
including surgical and nonsurgical treatment options have been described (Di Dona et al. 2018).
Most commonly employed surgical treatments include soft tissue reconstruction (e.g. release of
the retinaculum on the side of luxation and imbrication of the opposing side), tibial tuberosity
transposition (to realign the extensor mechanism), trochleoplasty (deepening of the trochlear
groove), and corrective osteotomy of the distal femur (to correct underlying femoral varus or valgus