Download Medial Patella Luxation

Medial Patella Luxation
Anatomy
The Patella is the large sesamoid bone (Kneecap) in the stifle joint. It forms part of the quadriceps
muscle mechanism which is the main muscle group responsible for extension of the stifle joint. The
patella ligament extends from the patella and inserts on the tibial tuberosity which is the anchor point
against which the quadriceps muscle pulls.
Pathogenesis
Medial patella luxation arises from medial displacement of the quadriceps muscle mechanism. As a
result the quadriceps muscle pulls the patella out of the groove onto the inside of the leg. This leads to
poor development of the patella groove which remains quite shallow. It often occurs as a result of
curvature of the femur and tibia (bowed legs) which leads to the insertion point of the patella ligament
remaining in a medial (inside) position.
Predisposing factors
Breed – dogs with naturally bowed legs (Staffordshire Bull Terriers, Bulldogs, Shih Tzu’s, Pugs, King
Charles Cavalier Spaniels etc) are predisposed to this condition.
Weight – extra weight in dogs leads to additional loading on the stifle joint which increases the chance
of traumatic luxation.
Clinical symptoms
Dogs with Medial Patella Luxation display varying degrees of lameness. Most commonly dogs will “skip”
on the affected limb. This will be seen as a dog that intermittently lifts the leg for a few steps then puts
it back down. It is often not painful at the time they lift it but it has essentially popped out of the groove
and then spontaneously pops back in.
In cases of traumatic luxation they will be very lame and quite swollen and painful around the stifle
joint.
Diagnosis
Diagnosis is achieved with a combination of examination and X-rays. Often the patella can be felt
spontaneously popping in and out as the dog moves during examination. In some well-muscled dogs or
very tense or nervous patients sedation is used to facilitate examination and diagnosis.
Grade 4 MPL
Grades of MPL
Medial Patella Luxation is graded from 1-4 and helps determine the severity, treatment options and
prognosis.
Grade 1 – Patella occasionally pops out and immediately returns to normal position. Manual
manipulation can displace the patella but it returns to normal position very quickly.
Grade 2 – Patella pops out more frequently and on manipulation is easy to luxate and hold out but will
return to normal position when released.
Grade 3 – Patella is luxated more often than in its normal position. It can be returned to normal position
but will luxate again soon afterwards.
Grade 4 – Patella is permanently luxated and difficult to return to normal position. If achieved it reluxates almost immediately.
Treatment
Treatment is based on the grade of MPL and the clinical lameness that is displayed by the patient.
Grade 1 – In small patients or those that are seldom lame we treat conservatively.
Grade 2 – In larger patients or patients that are intermittently lame these require surgical repair.
Grade 3 and 4 – Dogs with Grade 3 and 4 MPL generally require surgical repair.
I have often had clients comment that dogs are not in pain when they have been diagnosed with MPL.
This is true (apart from traumatic patella luxation) but the consequence of not repairing the MPL leads
to progression of osteoarthritis which in later life can be crippling to pets.
Surgical techniques
The elements of surgery are aimed at correcting the poor alignment of the quadriceps muscle
mechanism. They include the following
Trochleoplasty – deepening of the trochlea groove in which the patella sits to prevent it from
spontaneously popping out of the groove.
Medial desmotomy – opening of the joint capsule on the medial (inside) aspect of the stifle joint to
release the soft tissues that are pulling on the patella.
Lateral imbrication – suturing closed the lateral (outside) joint capsule to help pull the soft tissues back
into alignment.
Tibial transposition – moving the point of insertion of the patella ligament on the tibia to a more normal
position so that the pull of the quadriceps muslces on the patella is in a straight line.
Correctional osteotomies – bone cuts in the femur and tibia to help re-align the bony components of the
quadriceps mechanism. This is a very extreme correct of MPL and is often only indicated in very severe
cases of grade 4 MPL.
Surgical repair often entails a combination of techniques to achieve the desired outcome.
Recovery from surgery
Most dogs recover well from surgery but it is very important that they are kept confined while the repair
heals. In dogs that have had tibial transposition or correctional osteotomies time is required for the
bone to heal. Patients are confined for 8 weeks after surgery with slow return to full activity over th
months that follow.
Potential complications
Wound infection – as with all surgeries this is a possibility. Increased activity and patient interference
increase the chances of wound infection.
Joint infection – this is rarely seen but as a result of opening the joint it remains a potential
complication. Peri-operative antibiotics are given to reduce the chance of joint infection.
Device failure – In instances where plates (corrective osteotomies) and pins (tibial transposition) are
used to support bone while it heals these devices are at risk of failure. The more active a patient is after
surgery the more likely this may occur. Device failure occurs infrequently.
Return of luxating patella – this can occur with intermediate frequency but often only occurs with more
advanced disease. In most cases we improve the grade of MPL (for example from grade 3 to grade 1)
which generally does not require revision surgery. In some instances however further surgery is required
to better align and stabilize the patella.