Luxating patella in dogs

Overview

The kneecap's role and direction of displacement

The patella is a small bone within the tendon of the thigh muscle that straightens the knee. It normally moves within the trochlear groove at the end of the thigh bone. The patellar tendon attaches to a prominence on the front of the shin bone just below the knee. The extending thigh muscle, patella and tendon normally share the same line of pull. Medial luxation moves the kneecap out towards the knee's inner side. Lateral luxation moves it towards the outer side. One or both knees may be affected.

English primary care records examined during 2009–2014 recorded patellar luxation in 1.30% of dogs. Among 722 dogs with the site of a new finding recorded, 273 had both knees affected. These were veterinary diagnoses, excluding directly traumatic luxations.

Causes

Hind limb growth and knee structure

Luxation usually relates to abnormal hind limb development during growth. A shallow groove provides less sideways support. Tibial rotation can move the tendon attachment away from the normal line of pull. Medial luxation can accompany outward bowing of the lower thigh bone. Lateral luxation can accompany external tibial rotation.

Several genes influence knee structure and inherited susceptibility. An abnormality may be noticed only after an accident despite pre-existing susceptibility. Rarely, direct trauma can displace a previously stable kneecap.

In a bow-legged hind limb, thigh muscle pull can direct the kneecap inwards during extension. Supporting tissues on either side can gradually stretch under constant sideways pull.

Risk factors

Medial luxation is most common, particularly in small breeds. Lateral luxation is less common but occurs relatively more often in large dogs. Signs often start before one year old.

English records identified patellar luxation in 6.5% of Pomeranians. The Yorkshire Terrier proportion was 5.4%. Chihuahuas had 4.9%. Crossbreeds had 1.2%.

One set of breeding screening data from northern Europe found luxation in 44% of examined Pomeranians born during 2014–2018. Corresponding screening data found it in 52% of examined Yorkshire Terriers. It occurred in 31% of screened Chihuahuas. Screening also detects asymptomatic knees, so proportions can exceed clinical records.

Typical symptoms

Dogs may skip for a few steps when the kneecap slips out. Mild luxation can be an incidental health-check finding without home lameness.

• Your dog may briefly hold one hind leg flexed off the ground while trotting.
• The leg may extend or flick before movement resumes.
• Lameness may become more frequent and then persistent.
• Severe cases may walk with knees continually bent.
• Bilateral disease can produce a low, crouched gait.

Sudden worsening can also reflect concurrent cranial cruciate ligament injury.

A growing puppy's hind legs may look bowed with severe medial displacement. Large dogs with lateral luxation may look knock-kneed.

Complications and consequences

Cartilage damage and surgical outcomes

Repeated slipping can wear away joint cartilage. Damage can cause knee osteoarthritis and persistent pain. Instability can increase cruciate ligament loading. Prolonged severe luxation during growth can alter the whole hind limb's shape.

A surgical series from northern Europe found no remaining luxation in 64 of 65 knees after tibial tuberosity transposition secured with a plate and pin. Knees were examined 33–648 days after surgery. Post-operative complications affected 14 of 65 knees. Major complications affected six of 65.

Another follow-up of dogs with bilateral medial luxation found 11 of 119 operated knees needed revision for complications. Reluxation occurred in seven of 119 knees. After correction of abnormal tibial rotation, normal tracking returned in 51 of 53 cases. Radiographic bone healing after rotational correction occurred at an average of 2.7 months.

Major complications affected 11% of tibial rotational correction cases. A fixation pin can migrate and irritate surrounding tissue. Bone healing can sometimes be delayed after realignment involving bone movement.

Diagnosis at the vet

Examination and imaging

The vet watches walking and trotting to assess lameness. Palpation checks whether the kneecap leaves its groove during flexion or extension. Both knees are examined because luxation can be bilateral. Cruciate ligament integrity can be assessed separately in a painful knee.

Increased fluid can accompany concurrent cruciate injury. The vet also checks for a grating sensation during movement. Sedation sometimes helps if pain prevents handling. X-rays can identify existing osteoarthritis. Suspected major alignment abnormalities may require imaging the entire femur and tibia. Three-dimensional CT images can help plan bone realignment.

Kneecap stability from zero to four

Some official screening schemes use the Putnam scale, where zero means the kneecap does not leave its groove. At grade one it can be displaced during examination but returns spontaneously. At two it can slip out during movement and remain displaced briefly. At three it is usually displaced but can temporarily be replaced manually. At four it is permanently displaced and cannot be manually reduced.

Putnam grades describe stability, so symptoms also inform surgery decisions. Grade three can involve 30–60° rotation of the upper tibia. The most severe grade can involve up to 90°.

Treatment

Monitoring and symptom relief

Mild, asymptomatic luxation can usually be monitored by a vet. Weight management in overweight dogs reduces knee loading. A prescribed NSAID can relieve pain when symptoms occur. Rehabilitation can strengthen the knee-extending thigh muscles. Non-surgical care relieves symptoms but does not change bony structure.

When is surgery considered?

Surgery is usually considered for grades two to four with clear lameness or progressively impaired movement. Correction during growth can aim to limit worsening bone alignment. Moving the tibial tuberosity can improve tendon pull relative to knee movement. Deepening the trochlear groove can improve kneecap stability. Soft tissues pulling in the wrong direction can be released. Significant rotational or angular long-bone deformity can also be corrected.

Recovery after bone realignment

Movement is usually restricted for six to eight weeks after bone movement or realignment. Early outings are brief, quiet lead walks. Follow-up X-rays often assess healing at six to eight weeks. Normal activity resumes gradually once bone healing is confirmed.

Rehabilitation may include exercises maintaining knee range of movement.

Prevention

In some breeding screening schemes, dogs must be at least 12 months old for an official patellar assessment. Identification is checked during examination. In one such scheme, results issued before age three remain valid for two years. Results issued at three or older do not expire. A knee operated on for luxation is recorded as operated.

One Norrbottenspets breeding programme requires both parents to have valid official knee assessments at mating. Its registration limit is grade one. A grade-one dog must be paired with a grade-zero dog.

Official screening in these schemes is performed without sedation for standardised assessment. A valid report may be required before mating under the breed's health programme.

Important for dog owners

Film briefly if leg lifting occurs only at home or during walks. Keep outings short and on-lead until the knee is examined. Avoid jumping and stairs meanwhile. After surgery, follow your dog's individual exercise and rehabilitation plan.

Contact the vet promptly for wound discharge or marked surrounding redness. A protective suit or collar can prevent licking.

Film movement from the side and, if possible, front and rear so the affected limb can be identified.

Frequently asked questions

Does briefly lifting a hind leg indicate luxation?

Luxation can cause brief lifting followed by normal walking. Other hind limb problems can also cause short episodes. Diagnosis requires veterinary assessment of knee stability.

Does grade two always need surgery?

Symptom-relieving non-surgical care is sometimes suitable with little or no lameness. Persistent or worsening lameness may prompt surgery to correct tracking.

Must both knees be operated on if both are affected?

Bilateral findings do not always require bilateral surgery. A minimally affected or asymptomatic knee can be monitored. Both knees can sometimes be operated on together or in separate procedures.

Can a symptomatic puppy be examined before 12 months?

Yes, symptomatic knees can be examined before 12 months. The minimum age concerns official breeding reports in the relevant scheme. Early assessment is advisable if hind limb shape changes during growth.

When can normal walks resume after surgery?

Activity is usually restricted for six to eight weeks after bone realignment. The vet may check healing radiographically at six to eight weeks after bone movement or correction. Loading increases gradually once bone heals.