Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

Labrador Retriever Labrador Retriever

Patellar Luxation

Related terms: (kneecap) dislocation, medial luxation of the patella (kneecap)

Outline: Labrador retrievers have been found to be three times as likely as other breeds to have patellar luxation. As a result of a developmental abnormality of the leg bones, the patella (kneecap) tends to become displaced out of the groove in which it is normally positioned on the front of the knee. This severely affects the function of the limb and can cause pain due to stretching of ligaments. Damage to the patella, and the groove in which it runs, leads to osteoarthritis of the knee causing chronic pain, which may be mild to severe. The disease can be hard to correct surgically. To help avoid perpetuating the problem, prospective dog owners may wish to ascertain that the disease does not occur or is very rare in the lineage of the Labrador they are looking to acquire.


Summary of Information

(for more information click on the links below)

1. Brief description

Patellar luxation involves sideways displacement of the patella (kneecap) from its normal position and it is usually the outcome of a developmental problem involving the whole of the back limb (Harasen 2006). The cause may involve malformation of the upper leg or shallowness of the groove on the end of the femur (thigh bone) in which the patella is normally positioned, although other factors may be involved. In affected dogs, the degree of patellar luxation and leg deformity often increases with time.

Patellar luxation causes lameness and, because of the displacement of the patella, pain. It leads to degenerative joint disease (osteoarthritis), particularly in the knee (stifle) joint, a condition which itself leads to chronic pain and decreased mobility (Shell 2007).

A variety of surgical procedures have been used to correct the deformities that cause the luxation. These range from surgically loosening or tightening the soft tissue around the joint, to breaking and resetting leg bones in more normal positions. Long-term medication for degenerative joint disease (osteoarthritis) may be the only possible treatment in some cases. This is not always successful and also may cause significant side effects and welfare issues.

2. Intensity of welfare impact   

Luxation of the patella can cause pain at the time of displacement (due to stretching of the ligaments and the abnormal position of the bone), and in time leads to chronic pain due to arthritis which, depending on the circumstances, can be severe. The level of pain experienced depends on the nature, extent and duration of the luxation and on the nature of the underlying deformities.

3. Duration of welfare impact

The disease does not correct itself and therefore is of life long duration unless successfully corrected or treated. Most affected dogs will have at least minor pain and disability for long periods of time.

4. Number of animals affected

Patellar luxation is one of the most common orthopaedic conditions encountered in dogs (Johnson and others 1994, Ness et al 1996) and Labrador retrievers have been found to be 3.3 times more likely to be affected than other breeds (Gibbons et al 2006).They were also commonly represented in case series reported by Hayes et al (1994) and Remedios et al (1992). However, as far as we are aware, there are no data on the proportion of Labrador retrievers affected by this condition. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Labrador retrievers (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 1 million.

5. Diagnosis

A diagnosis is made from clinical observation and a history of previous orthopaedic problems in the limb. Radiography is used to assess the degree of deformity of the leg bones and to assess the severity of degenerative joint disease (osteoarthritis).

6. Genetics

The relatively high incidence of the disease in Labrador retrievers, and the high proportion of cases in which both knees are affected, strongly suggests that it has a genetic basis (LaFond et al 2002, Shell 2007). It is likely that multiple genes are involved and that there are also environmental influences. However, the exact genetic basis of the disease has not been determined.

7. How do you know if an animal is a carrier or likely to become affected?

It is likely that many apparently normal animals are carriers of genes that may contribute to the condition. There is currently no way of detecting these animals.

8. Methods and prospects for elimination of the problem

As far as we are aware, there are no breeding schemes aimed at reducing the incidence of patella luxation. Generally it seems sensible not to breed from individuals affected by the disease or from their relatives.

 

For further details about this condition, please click on the following:
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1. Clinical and pathological effects

The patella (kneecap) is normally situated at the front of the knee joint (stifle). It is the largest sesamoid bone in the body. Sesamoid bones are found in some tendons, where the tendon passes over a joint. They act to protect the tendon and to increase the mechanical effect of the attached muscles. The patella is in the tendon that attaches the main muscles at the front of the upper leg (the quadriceps muscles) to the top of the main lower leg bone (the tibia).

The patella protects the tendon as it slides over the front of the knee joint as the knee joint is flexed and extended. It rests inside the trochlear groove, the notch in the front of the lower part of the thigh bone (the femur). Side-to-side movement of the tendon and the patella is normally prevented by the sides of the groove in which it runs. However, this depends on the correct, straight, alignment of the quadriceps muscle and its attachment to the limb bones, above and below the knee and, thus, on the shape of these limb bones. If the tendon is not aligned correctly and the pull of the quadriceps (front of thigh) muscles is not in a straight line over the front of the knee, there is a risk that the patella will jump out of the groove causing sideways displacement of the kneecap, or luxation.

Patellar luxation is usually a result of a developmental problem of the whole of the limb (Harasen 2006). In Labrador retrievers, displacements most often occur medially - towards the inside of the leg (Gibbons et al 2006).(Patellar luxations can also be caused in normal dogs by major trauma).

It is thought that congenital medial luxation is, in some cases, a result of there being an abnormal angle between the femoral neck (the top of the femur bone which connects the ball part of the hip joint to the shaft of the femur) and the femoral axis (the main shaft of the bone). This distorts the position of the whole of the lower leg causing the muscles of the quadriceps and its tendons and connections to the bones, both above and below the stifle joints, to move medially. The result is that, the patella is partly dragged medially when there is tension on the tendon.

Luxation can also occur if the groove on the front of the knee in which the tendon runs is abnormally shallow and poorly developed (Hulse 1993, Harasen 2006). It is known that levels of oestrogen can affect the depth of the tracheal groove but the significance of this to the development of patella luxation is unknown (Roush 1993).

Additional factors that may be involved include primary abnormalities of the muscles. It has been noted that some animals with congenital medial luxation of the patella have structural abnormalities in part of the quadriceps muscle group – the rectus femoris, such that there is a tendency for it to pull to the medial side (L’Eplattenier and Montavon 2002). Affected animals also tend to have abnormal positioning of the attachment of another upper limb muscle, the sartorius, to the pelvis (L’Eplattenier and Montavon 2002). 

The degree of patellar luxation often increases with time as the trochlear groove and the medial condyle become flattened. The patella may then luxate more easily or remain permanently luxated. In addition, the trochlear grove tends to flatten due to disuse if the patella is not in its normal position.

Once the patella starts luxating, the whole upper leg can progressively deform with the femur becoming curved. The pulling of the quadriceps muscle over to the inside leg of the dog, rather than being directed straight down over the front of the knee, causes the medial side of the femur to grow less than the lateral, outer, side leading to bowing. It is very difficult to know if such limb distortion is a primary cause of the luxation or whether it is an effect secondary to luxation caused by defects in the position or shape of the trochlear groove.

Various schemes have been used to grade patella luxation. Gibbons et al (2006) listed the following grades:

Grade 1: The patella was easily luxated with manual pressure but returned to the femoral trochlea when released.

Grade 2: The luxation occurred with rotation of the paw and flexion of the stifle, but the patella returned to the femoral trochlea.

Grade 3: The patella was permanently luxated but could be put back in place with manual pressure.

Grade 4: The patella was permanently luxated and could not be manually put back in place.

Luxation of the patella seriously affects the function of the knee joint and causes disability due to mechanical lameness.  In time it also leads to degenerative joint disease (osteoarthritis), particularly in the tissue of the knee, which may have further impacts on mobility as a result of pain and decreased joint function (Shell 2007).

The extent of these problems partly depends on the degree of luxation. Temporary luxation may cause a dog to hold its leg off the ground for a short time whilst continuing to run, extend the leg several times, and then, after extending the leg a few times, to resume weight-bearing on all four legs when the patella has retuned to its groove.

When the patella is permanently luxated, displaced out of position, there is usually obvious skeletal deformity. The dogs tend to walk with a flexed stifle and a bow-legged gait. It may not be possible for the knee to be fully extended. There is often great difficulty in locomotion especially if both legs are affected.

When the patella luxates and returns to the groove, there is damage to the cartilage of the patella and the medial side of the groove in which it runs (the medial condyle of the femur). This leads to degenerative joint disease (osteoarthritis). Degenerative joint disease is also common when the patella is constantly out of position (Shell 2007).

When the patella can be slid from the trochlear groove by manual pressure (Grade 1 luxation), although this is highly abnormal, the dog is less likely to have severe clinical signs than if it is permanently luxated and cannot be returned to the correct position even when manually pushed (Grade 4).

Degenerative joint disease (osteoarthritis) is associated with pain and disability that varies from mild to severe. It is irreversible and treatments largely rely on relieving pain and lifestyle modifications.

A variety of surgical procedures have been used to try to correct the deformities associated with luxating patella. They range from surgically loosening or tightening the soft tissue around the stifle joint, to breaking and resetting the leg bones in more normal positions. These surgical interventions can, themselves, have significant welfare consequences and are not always successful. Long-term medical treatment for degenerative joint disease (osteoarthritis) may be the only possible treatment in some cases. This is not always successful and also may cause significant side effects.

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2. Intensity of welfare impact

Luxation of the patella can cause pain at the time of displacement (due to stretching of the ligaments and the abnormal position of the bone), and in time leads to arthritis which causes chronic joint pain. The level of the pain depends on the nature, extent and duration of the luxation and on the nature of the underlying deformities and can be severe. When both legs are affected the welfare implications are greater both because of pain and loss of function in both legs. When both legs are affected bi-lateral lameness may mask obvious signs of limping which can lead to owners being unaware of the condition and consequently a delay in treatment.

Treatment of atopic dermatitis usually involves drugs. The most effective treatments are based on corticosteroids or ciclosporin, both of which can have significant side effects with their own welfare issues. Some atopic dogs become seriously ill due to the side effects of treatment. Frequent shampooing is often required which may be unpleasant for some dogs. Owners report that their dogs’ quality of life suffered not only from the disease but also the treatments, even though they considered these treatments helpful (Linek & Favrot 2010). Dogs are euthanized because of atopic dermatitis. This is often because diagnosis and treatment are time-consuming and expensive for owners and reasonable long-term control is often the best that can be achieved rather than there being any hope of cure.

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3. Duration of welfare impact

The disease does not correct itself and therefore is of life-long duration unless successfully corrected or treated. Most affected dogs will have at least minor pain and disability for long periods of time. 

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4. Number of animals affected

Patellar luxation is one of the most common orthopaedic conditions encountered in dogs (Johnson and others 1994, Ness et al 1996) and Labrador retrievers have been found to be were 3.3 times more likely to be affected than other breeds Gibbons et al (2006). They were also commonly represented in case series reported by Hayes et al (1994) and Remedios et al (1992). However, as far as we are aware, there are no data on the proportion of Labrador retrievers affected by this condition. From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Labrador retrievers (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 1 million.

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5. Diagnosis

A history of a “hopping” gait with a back leg being held up for a few steps and then walking normally is characteristic of a dog with a lower grade luxation. The other grades are diagnosed on the basis of physical examination and radiography. Radiography is used to assess the degree of deformity of the bones and to assess the severity of degenerative joint disease (osteoarthritis). There is not a close correlation between the extent of radiographic signs of degenerative joint disease and the degree of clinical signs shown by the affected dog.

Because luxation of the patella has become so common in some breeds of dogs, care must be taken that the luxating patella is not blamed when another, perhaps more acute and severe disease, is the cause. In this way, luxating patellas can ‘cause’ welfare problems through delayed diagnosis and treatment of the real, underlying disease.

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6. Genetics

The relatively high incidence of the disease in Labrador retrievers, and the high proportion of cases in which both knees are affected, strongly suggests that it has a genetic basis (LaFond et al 2002, Shell 2007). It is likely that multiple genes are involved and that there are also environmental influences. However, the exact genetic basis of the disease has not been determined.

Return to top

7. How do you know if an animal is a carrier or likely to become affected?

It is likely that many apparently normal animals are carriers of genes that may contribute to the condition. There is currently no way of detecting these animals.

Return to top

8. Methods and prospects for elimination of the problem

As far as we are aware, there are no breeding schemes aimed at reducing the incidence of patella luxation. Generally it seems sensible not to breed from individuals affected by the disease or from their relatives. Prospective dog owners may wish to avoid acquiring dogs of breeds in which there is a high prevalence of the disease (in order to help avoid perpetuating the problem) unless they are able to ascertain that the disease does not occur or is very rare in the particular lineage of the dog they are interested in acquiring

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9. Acknowledgements

UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section.

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10. References

Gibbons SE, Macias C, Tonzing MA, Pinchbeck GL and McKee WM (2006) Patellar luxation in 70 large breed dogs. Journal of Small Animal Practice 47: 3–9

Harasen G (2006) Patellar luxation: pathogenesis and surgical correction. Canadian Veterinary Journal 47: 1037–1039

Hayes AG, Boudrieau RJ and F Hungerford (1994) Frequency and distribution of medial and lateral patellar luxation in dogs: 124 cases (1982–1992). Journal of the American Veterinary Medical Association 205: 716-720

Hulse DA (1993) Medial patellar luxation in the dog. In: Disease Mechanisms in Small Animal Surgery. 2nd edn. Ed MJ. Bojrab. Lea and Febiger, Philadelphia. pp 808-817

Johnson JA, Austin C and Breur GJ (1994) Incidence of canine appendicular muscluloskeletal disorders in 16 veterinary teaching hospitals from 1980 through 1989. Veterinary and comparative. Orthopaedics and Traumatology 7: 56-69

Lafond E, Breur GJ and Austin CA (2002) Breed susceptibility for developmental orthopaedic diseases in dogs. Journal of the American Animal Hospital Association 38: 467-477

L’Eplattenier H and Montavon P (2002) Patellar luxation in dogs and cats: Pathogenesis and diagnosis. Compendium of Continuing Education for the Practising Vet. 24: 234–239

Ness MG, Abercromby RH, May C, Turner, BM and Carmichael S (1996) A survey of orthopaedic conditions in small animal veterinary practice in Britain. Veterinary and Comparative Orthopaedics and Traumatology 9: 43-52

Remedios AM, Basher AWP, Runyon CL and Fries CL (1992) Medial patellar luxation in 16 large dogs. A retrospective study. Veterinary Surgery 21: 5-9

Roush JK (1993) Canine patellar luxation. Veterinary Clinics of North America Small Animal Practice 23: 855–868

Shell LG (2007) Patellar luxation. Veterinary Information Network Associate. http://www.vin.com/Members/Associate/Associate.plx?DiseaseId=449and FindingList=408491,377290. Accessed 6th October 2010

© UFAW 2011


Credit for main photo above:

By Erikeltic at English Wikipedia [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons