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Seattle Veterinary Specialists Clinical Update
Patella luxation occurs due to a combination of
anatomic abnormalities in the rear limb that force
the knee cap out of it’s normal groove
The patella (knee cap) is part of the quadriceps muscle (large muscle group on the front of the thigh)
mechanism. The muscle originates at the top of the femur and inserts, via the patella ligament, onto
the tibial tuberosity. The quadriceps mechanism forms a straight line from origin to insertion. The
patella rides in a groove in the distal femur called the trochlea.
If the femur is slightly bowed and the tibial tuberosity (insertion point of the patella ligament) is medially
displaced, then the patella is pulled toward the medial side (inside) of the knee. The tissues on the
lateral side (outside) of the knee tend to stretch and the patella eventually dislocates medially. The
trochlea is often more shallow than normal.
Dogs with medial patella luxation (MPL) tend to intermittently
hold up the affected leg and hop or skip on the other back leg.
Sometimes the dog will straighten the leg behind them in order
to pop the patella back into place. This condition can occur in
one or both back legs. Most dogs are not lame when the
patella is within the trochlea (not luxated). MPL pre-disposes
dogs to tearing of the cranial cruciate ligment (CCL). When this
occurs, the degree of lameness typically worsens and the dog
may be non-weight bearing.
MPL is diagnosed by palpation (feeling the knee). Radiographs
(X-rays) are sometimes taken to assess the conformation of the
rear limb(s). A grading system is applied to MPL cases to assist with treatment options. The
grading system goes from Grade I to Grade IV. Grade I is the least severe and Grade IV is the most
severe. The condition can be progressive, meaning that a grade II may progress to a grade III (but
rarely to a grade IV). Most MPL patients seen at our hospital are either Grade II or Grade III. The
frequency of concomitant cranial cruciate ligament rupture (CCLR) is reported to be as high as 41%
of dogs suffering from MPL. The likelihood of having both MPL and CCLR increases in animals that
are older or have a higher grade of luxation (usually Grade III or IV).
Surgical treatment is recommended for dogs with Grade III or IV MPL, frequent lameness associated
with Grade II MPL, and animals with concurrent CCLR.
Treatment of MPL is directed toward overcoming the conformational changes that cause the patella
to dislocate in the first place. While surgery is tailored to the individual patient’s needs, it usually
involves some procedure to deepen the trochelar groove, re-align tibial tuberosity, and tighten the
stretched lateral joint tissues.involves some procedure to deepen the trochelar groove, re-align tibial
tuberosity, and tighten the stretched lateral joint tissues.
Surgery is recommended for dogs with
Grade III or IV MPL
1. Trocheloplasty:
There are several procedures that may be used to deepen the
trochlea groove and they are called “trocheloplasties”. The
surgeon will select the trocheloplasty that best meets your pet’s
needed correction.
2. Resection of Excess Lateral Tissue:
Before closing the joint, any excess (stretched) tissue on the
lateral side of the joint is removed or tightened. This helps to
stabilize the patella in its new groove.
3. Anti-rotational suture (also called MRIT):
A heavy gauge nylon suture is placed through a small bone
tunnel in the tibia and around the back of the femur on the
lateral (outside) aspect of the knee. When this suture is
tightened, the tibia is rotated towards midline to restore
alignment of the leg. This procedure is also used to stabilize the
knee (prevent cranial drawer) in cases where the cranial cruciate
ligament is also ruptured. Eventually scar tissue aligns itself
around the nylon sutures and ultimately it is this scar tissue that
holds the knee stable (since the suture will eventually stretch or
4. Tibial Tuberosity Transposition (TTT):
In addition to deepening the trochlea groove, the surgeon can
move the insertion point of the patella ligament by performing a
cut in the bone. The tibial tuberosity is re-positioned in a more
central location in order to re-align the quadriceps mechanism.
The tuberosity is repositioned and then held in place with small
This procedure is typically elected in larger dogs, dogs with
Grade IV MPL, or in dogs which have already had an antirotational suture.
Your pet will need to remain in the hospital for the night following surgery for monitoring and in order
to provide pain medication. Immediately following surgery, therapeutic laser therapy can be
performed over the incision and joint and has been shown to speed healing and decrease pain and
Seattle Veterinary Specialists 425.823.9111
inflammation. Ice/compression applied to the knee
joint every 6 hours during hospitalization also helps to
minimize swelling and pain.
Your dog’s activity must be restricted to leash walks,
with no running, jumping or playing with other dogs
for a period of at least 8 weeks after surgery.
However, physical rehabilitation is a crucial aspect of
the recovery process. We will recommend a
rehabilitation plan tailored to you and your pet.
Recheck appointments with the surgeon are required
at 2 weeks, 8 weeks and 3 months after surgery. Xrays are taken at least once if a TTT was performed to
assess bone healing.
Any time an animal (or human) undergoes anesthesia
there is the risk of adverse reactions to anesthesia,
including death. However, blood work is performed
prior to anesthesia in order to identify any underlying
medical conditions that may influence anesthetic
choices or preclude surgery. In addition, there are
board-certified anesthetists and an extremely
experienced staff of anesthesia nurses here at SVS
that will take exceptional care of your pet.
Complications associated with surgery are uncommon and include excessive bleeding, infection, pin
loosening (if TTT performed) and recurrence of luxation. Rare complications may require further
The prognosis for treatment of Grade II-III medial patella luxation is good. The surgery resolves the
clinical signs in most patients. Some patients may experience mild lameness depending on the
severity of osteoarthritis that may be present (particularly if they had concurrent CCLR).
Michael Mison, DVM, DACVS & Kristin Kirkby, DVM, MS, CCRT, DACVS
Hours of Operation
For after hours emergencies contact
Seattle Veterinary Specialists
11814 115th Avenue Northeast
Kirkland, WA 98034-6946