Patellar luxation is one of the most common orthopaedic conditions in dogs.  The condition affects primarily small dogs.  The patella, commonly referred to as the knee cap, is usually located in the centre of the knee joint.  A luxating patella is a knee cap that moves out of its normal location, as indicated by the term 'luxating' which means out of place or dislocated.

 

This condition is usually noticed in dogs less than two years of age.  Signs range from mild to severe and include temporary or occassional lameness possibly accompanied by a hopping motion.  Eventually the dog may hold the leg permanently off the ground.  50% of affected dogs have both knees involved.

 

Patella luxation in toy and small-breed dogs is considered a heritable trait that can be passed on from parent to offspring, although the exact mode of inheritance is not fully known (it is poygenic, ie multiple genes are involved).  It is recommended that dogs with medial patella luxation should not be used for breeding.

Patellar Luxation 

Cruciate Ligament Rupture

There are two bands of fibrous tissue called the cruciate ligaments in each knee joint.  They join the femur and tibia (the bones above and below the knee joint) together so that the knee works as a hinged joint.

They are called cruciate ligaments because they 'cross over' inside the knee joint.  The cranial cruciate ligament serves to limit the forward movement of the lower leg bone (tibia) relative to the upper leg bone (femur).  It also serves to limit over straightening (hypertension) of the knee and the turning in of the lower leg bone. Traumatic cruciate damage is caused by a twisting injury to the knee joint.  This injury usually affects the anterior or cranial (front) ligament.  The joint is then unstable and causes extreme pain often resulting in lameness. 

 

Cruciate ligament rupture is a common knee injury and although sudden ruptures can occur in dogs they are much less common that a process called Cruciate Disease. 

Cruciate Ligament Disease

Most commonly in dogs a chronic form of cruciate damage occurs due to the weakening of the ligaments as a result of disease.  The reasons for this are only partly understood.  The ligament may become stretched or partially torn and lameness may be only slight and intermittent but a process of inflammation, or athritis, is occurring in the joint at the same time.  With continued use of the joint the condition gradually gets worse until rupture occurs in the course of normal activity.