LUXATING PATELLA
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August 2014
Michel (Mike) B. Rubini
A luxating patella means a moving, or dislocated knee cap.
The patella, or knee cap, normally sits in a formed groove of the lower femur (thigh bone). A luxated patella occurs when the patella is dislocated from the groove of the femur. This is the most common knee joint injury for the canine breeds. It is more common in the miniature or ‘toy’ breeds (such as Pomeranians, Pekinese, Boston Terrier, Chihuahua etc.). While the condition still applies in a great majority to small breeds a troubling increase of luxated patellas is occurring in larger breeds.
The patella slides up and down the trochlear groove of the lower femur bone. The Trochlear groove is actually formed by two trochlear ridges (medial – toward the centre of the animal and lateral away from the animal) on either side of the groove. The patella is embedded in the quadriceps muscle and is attached to it with the quadriceps tendon. On its lower side it is directly attached to the tibia with the patellar ligament. As the quadriceps muscle contracts it pulls the tibia forward and so the patella guides the movement of the quadriceps muscles during leg motion. The patella, with its tendons and the quadriceps muscles in the rear legs form the ‘extensor mechanism’. When operating correctly the Patella increases the efficiency and stability of the canine stifle joint. Any failure of the extensor mechanism or malformation of the tibial and femur bone can eventually lead to a luxating patella.
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There can be several reasons for a luxating patella but nearly all are congenital / genetic in nature and
the disposition to the condition is likely inherited.
Genetic
A trochlear groove that is too shallow to contain the patella during leg motion.
There can be a weakness of the connective tissues or ligaments that causes instability of the
movement of the patella.
The tibial tuberosity that the patellar ligaments attach to on the tibia is misaligned.
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The rear skeletal anatomy is misaligned / malformed causing instability and stress in the stifle
joint movement. The structure of the leg and hips in some animals are inherited as a breed trait but the
resulting stress and bone angles can lead to luxating patella as well as other problems.
Disease – Joint diseases such as arthritis can damage the cartilage leading to the Patella luxation.
Trauma - A defect in bone angle or ligament damage can be caused by trauma to the knee and leg causing luxating patella. This is uncommon and rarely occurs.
In many, if not most cases, a luxating patella will be a result of a combination of the above problems.
Many animals will develop signs of a luxating patella early in life. The first signs of a luxated patella may occur during normal running or walking. The animal may ‘yelp’ at the pain as the patella luxates out of position and hold his leg straight out. Once the patella is luxated the animal will not usually feel pain.
After a short time the quadriceps muscles will relax and the patella may return to its normal position.
The condition is easily diagnosed by a veterinarian while the animal is still a puppy. Unfortunately, the earlier the condition develops the greater the severity of the condition and the resulting disability - including deformation of the rear skeleton. In some cases the condition may not appear until later in
life. Although this would not be strictly classed as ‘congenital’ the underlying causes were probably developed over the animal’s lifespan from certain defects that were present from a younger age.
Logically (as a general case) the smaller the animal then the smaller the bone structure will be. In many cases the underlying tendency to a luxated patella is often a normal consequence of having an insufficient bone structure to properly develop a strong enough extensor mechanism that can contain and centre the patella while the puppy is growing. There may be insufficient bone to properly develop the trochlear groove and ridges and tendon placement and still allow for stable mechanical containment of the patella against the twisting forces of leg motion.
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If the two trochlear ridges are insufficient in height then the groove that the patella slides in will be too shallow to guide the patella. The patella may then slide sideways out of its intended position in the trochlear groove. This is the type of patellar luxation that generally occurs (and is very common) in toy breeds. Female dogs have a 50% greater probability of developing Patellar luxation than do male dogs.
Because of normal motion, the mechanics and construction of the rear hind skeleton and extensor
mechanism the dislocation of the patella is generally MEDIAL, that is it will luxate toward the ‘centre’ of
the animal.
When (for whatever reason) luxation occurs while the animal is still a puppy and the bone structure is still growing the patella rubs over the medial trochlear ridge and quickly wears it down.
This causes the patella to luxate more frequently and further damages the trochlear ridge. Arthritis will
then set in and cause further joint damage. As the animal grows he will try to compensate for this
abnormality during normal walking and running and so the entire rear skeleton may develop abnormally.
It may also be the case that improper hind skeletal formation, either as a genetic defect or the results of
a breed trait will cause excessive stress on the trochlear ridges causing a luxating patella.
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It should be noted that normal leg motion is required for the patella to properly ‘set’ in its required
position as the puppy grows. During growth the normal movement of the patella increases the depth
and strength of the trochlear groove and ridges allowing the muscles and tendons to properly develop
around the patella. There is surgery available to deepen the trochlear groove (or replace it with and
artificial one),
For the extensor mechanism to operate properly it must be properly positioned and centered in the
stifle joint. Any weakness in the muscle or tendons associated with the patella can cause the patella to
luxate during movement. A cause of patellar luxation is that the patellar tendon which connects the
patella to the tibular tuberosity (or tibular crest) on the tibia is not placed properly. This causes a ‘pull’
towards the non-centered direction of the position of the tuberosity. This type of congenital anomaly
can be repaired by repositioning the tibular tuberosity.
Weak or misplaced attachments of the patellar lateral tendons in the stifle joint area may also result in a
luxated patella. Surgery in these cases to repair the tendons/muscle can be successful.
Larger dogs are less prone to luxated patella than are smaller dogs. Generally they will have larger,
stronger leg bones with plenty of room for a deep trochlear groove to contain the patella. However,
larger dogs can develop luxating patella’s as a secondary condition caused by hip problems (i.e.
Dysplasia). The Luxation in these cases can be lateral as well as medial.
A veterinarian can assess the dogs luxating patella and grade the severity of the condition during a
routine examination.
Grade 1 – the patella will luxate out of its normal position in the trochlear groove but returns to
its normal position right away during examination. This would be a recurrent problem.
Grade 2 – the patella will luxate out of its normal position in the trochlear groove but will not
always return to its normal position right away. This would be a recurrent problem.
Grade 3 – a luxated patella is normally luxated out of the groove, but can be manually placed
back to its normal position. This would probably be a permanent situation.
Grad e 4 – the patella will not stay in the groove. When forcing the patella into its normal
position in the groove the patella luxates out of the groove after pressure is released. This is a
permanent condition.
Not all Volpinos (or small dogs) will develop a luxated patella but a seemingly growing percentage will about 1% if that.
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The reasons are varied as outlined above. In most cases it should be considered ‘genetic’ in nature and
inherited even though the cause can be considered simply to be the size of the dog (which is, of course,
hereditary). In many cases where normally angled and developed rear hind skeleton is present then a
larger dog may dramatically reduce the instance of luxated patellas. A heavier and more substantial
bone structure in a puppy may allow for a stronger, more normal development of the canine stifle joint
and deeper groove with less instances of the luxated patella.
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At any rate it is important NOT to breed a Volpino that have developed or has a family tendency towards
a luxated patella. It should also be recorded how many of their offspring actually developed the anomaly. In many cases this will reveal a line that is troubled with the disease.
In many cases a luxated patella can be found during a veterinarian exam at 8 weeks old, while the puppy is still growing.
Please feel free to send any correction, comments or suggestion to me
Aug 2014
Michel (Mike) B. Rubini mike@rubini.ca
References
FCI Volpino Standard (English)
http://www.volpinoitaliano.ca/standardenglish.html
UKC Standard
http://www.ukcdogs.com/Web.nsf/Breeds/NorthernBreed/VolpinoItaliano
Primary Lens Luxation
1 –University of Missouri - http://www.caninegeneticdiseases.net/GLX/mainGLX.htm
2 – Animal Health Trust (AHT) , Cambridge, England - http://www.aht.org.uk/cmsdisplay/genetics_pll.html
3 – College of Veterinary Opthalmologists, ADAMTS17 Mutation associated with PLL
http://oz.dogs.net.au/nbtca/uploads/documents/ADAMTS17_mutation_associated_with_primary_lens_luxation.pdf
4 – Genomia Genetic Laboratory - http://www.genomia.cz/en/test/pll/
Progressive Retinal Atrophy (PRA)
University of Prince Edward Island
http://ic.upei.ca/cidd/disorder/progressive-retinal-atrophy
http://www.vetstreet.com/progressive-retinal-atrophy-in-dogs
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0021452
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OMIA – THE ONLINE MENDELIAN INHERITANCE IN ANIMALS - http://omia.angis.org.au/home/
BREED SPECIFIC HEALTH DNA TESTING – (ENGLISH)
http://www.thekennelclub.org.uk/media/14688/dnatestsworldwide.pdf
Digital Journal of Ophthalmology, Harvard,
http://www.djo.harvard.edu/site.php?url=/physicians/oa/381
Living with Blind Dogs by Carol Lewis
Canine Cataracts
http://www.merckmanuals.com/pethealth/special_subjects/poisoning/herbicide_poisoning.html)
http://www.1hope.org/glyphos8.htm
http://www.peteducation.com/article.cfm?c=2+2092&aid=407Luxating Patellas
(Papillion club of America – dated but nice summaries on genetic conditions)
http://www.pcagenetics.com/Articles.html#076
http://www.veterinarypartner.com/Content.plx?A=2448
http://www.vetsurgerycentral.com/patella.htm
http://www.vetinfo.com/dpatella.html
http://www.offa.org/pl_overview.html
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Note – Note all Types of the known inherited PRA is a autosomal recessive gene except these two exceptions. The Bull Mastiff PRA inheritance is an autosomal dominant inheritance. The Siberian Husky is a sex (X chromosome) linked recessive inheritance trait.