Genetic Welfare Problems of Companion Animals

An information resource for prospective pet owners

Yorkshire Terrier

Yorkshire Terrier

Patellar Luxation

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

Outline: Yorkshire terriers have the second highest prevalence of patellar luxation among dog breeds. In one survey, 26% of Yorkshire terriers were found to be affected. As a result of a developmental abnormality of the leg bones, the patella (kneecap) tends to become displaced from 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 range from 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 Yorkshire terrier they are looking to acquire.


Summary of Information

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1. Brief description

Patellar luxation involves sideways displacement of the patella (kneecap) from its normal position. It is usually the outcome of a developmental problem involving the whole of the back limb (Harasen 2006a). 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. 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 will lead to chronic pain due to arthritis (inflammation of the joint) 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 of dogs (Johnson et al 1994, Ness et al 1996). Medial patellar luxation has long been known to be most commonly seen in small and toy breeds such as the Yorkshire terrier (Roush 1993, Piermattei & Flo 1997, L’Eplattenier & Montavon 2002, Harasen 2006b).

The Orthopedic Foundation for Animals (OFA) scheme for registration of affected and unaffected animals lists the Yorkshire terrier as having the second highest prevalence of the condition (after the Pomeranian) with 26% affected (http://www.offa.org/stats_pl.html).

From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Yorkshire terriers (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 250,000. If, as suggested above, the prevalence of patella luxation is about 25%, then over 60,000 of these dogs in the UK are likely to be affected.

5. Diagnosis

The disease is diagnosed from clinical observation and a history of ambulatory problems. 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 Yorkshire terriers, 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 pattern of inheritance is unclear and the genes involved have 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).

Patellar Figure 1

Figure 1. A schematic diagram of the canine rear leg, illustrating the normal position of the patella.

 Patellar Figure 2

Figure 2. Side and front views of the knee/stifle. The patella is located within a tendon and connects the muscles of the femur (upper leg) to the front of the tibia (lower leg). (Image property of Phil Witte to whom we are grateful for permission to reproduce it here).

The patella protects the tendon as it slides over the front of the knee joint, as the knee joint is flexed and extended (see Figure 3). 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 this groove. 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 (See Figure 4).

Patellar Figure 3

Figure 3. The patella protects the knee as the leg is flexed and extended. It is prevented from moving from side-to-side as it rests within the trochlear groove. (Image property of Phil Witte to whom we are grateful for permission to reproduce it here).

Patellar Figure 4

Figure 4. A schematic diagram of patella luxation showing the displacement of the patella and the associated musculature from the centre to the side of the leg (compare its position in Fig 2).

Patellar luxation is usually a result of a developmental problem of the whole of the limb (Harasen 2006a). In Yorkshire terriers, displacements most often occur medially – sideways and towards the inside of the leg ( LaFond et al 2002, Shell 2007. (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 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 2006a). It is known that levels of oestrogen in the developing foetus 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 muscle, such that there is a tendency for it to pull to the medial side (L’Eplattenier & 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 & Montavon 2002)

The degree of patellar luxation often increases with time as the trochlear groove and the medial condyle - the bony round lump on the base of the femur where it forms the knee joint - become flattened. The patella may then luxate more easily or remain permanently luxated. In addition, the trochlear grove tends to become shallower 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 the bone 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 groove 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 grove.

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.

Patellar Figure 5 

Figure 5. Radiograph of a dog with grade III / IV patella luxation. Long-term displacement of the patella towards the medial side (inside) of the leg is resulting in a bowing deformity of the femur as well as the tibia. (Image property of Southern Counties Veterinary Specialists to whom we are grateful for permission to reproduce it here).

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, to then extend the leg several times, and afterwards to resume weight-bearing on all four legs, as the extension of the leg has returned the patella to its groove.

When the patella is permanently luxated, displaced out of position, there is usually obvious skeletal deformity. Affected 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. Such dogs often have great difficulty in locomotion especially if both legs are affected.

When the patella luxates and returns to the groove, damage is caused 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).

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.

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

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 (by causing stress and pain) 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. For example, non-steroid anti-inflammatory pain killers can cause gastrointestinal disturbances (Ramsey 2011).

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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 will lead to arthritis (inflammation of the tissues of the joint) 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 and owners may be unaware of the condition and consequently there is a delay in treatment.

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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 of dogs (Johnson et al 1994, Ness et al 1996). Medial patellar luxation has long been known to be commonly seen in small and toy breeds such as the Yorkshire terrier (Roush 1993, Piermattei & Flo 1997, L’Eplattenier & Montavon 2002, Harasen 2006b).

The Orthopedic Foundation for Animals (OFA) scheme for registration of affected and unaffected animals lists the Yorkshire terrier as having the second highest prevalence of the condition (after the Pomeranian) with 26% affected (http://www.offa.org/stats_pl.html).

From data on estimates of total dog population in the UK and on the percentage of all micro-chip registered dogs that are Yorkshire terriers (Lucy Asher, 2011, personal communication), we estimate that the UK population size of this breed may be around 250,000. If, as suggested above, the prevalence of patella luxation is about 25%, then over 60,000 of these dogs in the UK are likely to be affected.

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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 a return to normal walking 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 Yorkshire terriers, 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, neither the pattern of inheritance nor the genes involved have been determined.

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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 and which can lead to their offspring developing the condition even though they themselves are unaffected. There is currently no way of detecting these animals.

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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 and to Stephanie Kaufman for assistance in illustrating it.

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

Harasen G (2006b) Patellar luxation. Canadian Veterinary Journey 47: 817–818

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

Piermattei DL and Flo GL (1997) Handbook of Small Animal Orthopedics and Fracture Repair, 3rd ed. Philadelphia: WB Saunders, 516–534

Ramsey I (2011) Firocoxib. In: BSAVA Small Animal Formulary 7th edition, British Small Animal Veterinary Association, Cheltenham UK pp 143

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=449&FindingList=377290. Accessed 20th July 2011

http://www.offa.org/stats_pl.html accessed 14th July 2011

© UFAW 2011


Credit for main photo above:

http://depositphotos.com/13265541/stock-photo-yorkshire-terrier-in-city-park.html ©Depositphotos.com/tandemich