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Medial Patellar Luxation Grades Explained

Medial Patellar Luxation Grades Explained

Medial Patellar Luxation

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Clear guide to medial patellar luxation grades (I–IV) in dogs. Learn what each grade means, symptoms to watch for, and treatment options based on severity

By 

Sustainable Vet Group

Updated on

June 21, 2025

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What Is Medial Patellar Luxation (MPL)?

Medial Patellar Luxation (MPL) is a condition where a dog’s kneecap (patella) slips out of its normal position inside the femoral groove and moves toward the inside of the leg. This causes pain, limping, or skipping while walking. Over time, it can lead to joint damage or arthritis if not treated.

Commonly Affected Breeds

MPL is more common in small and toy breeds, though large breeds can also be affected.

  • Pomeranians
  • Yorkshire Terriers
  • Chihuahuas
  • Miniature and Toy Poodles
  • Boston Terriers
  • Pekingese
  • Shih Tzus
  • Some large breeds like Labrador Retrievers and Akitas

In many cases, the condition is inherited and may appear in both legs.

Why Grading Matters for Treatment

MPL is classified into four grades based on how easily the kneecap moves out of place. Grading helps the veterinarian decide the best treatment plan.

  • Lower grades may not need surgery and can be managed with lifestyle changes.
  • Higher grades often require surgery to correct the joint and improve function.

Understanding the grade is key to choosing the right care and predicting recovery success.

Understanding the MPL Grading System

Grade I Medial Patellar Luxation

In Grade I MPL, the patella can be moved out of place during a physical exam but returns to its normal position on its own. Most dogs do not show obvious signs of pain or discomfort.

Typical signs:

  • Intermittent “skipping” gait on one back leg
  • Kneecap slips briefly, then goes back into place
  • No swelling or pain when resting
  • Dogs remain active and playful

Is surgery needed?

  • Usually not required.
  • Vets often recommend monitoring, weight control, and joint supplements.
  • Surgery may be considered if the condition worsens over time.

Most dogs with Grade I live comfortably without surgery, especially if they stay active, lean, and avoid rough play. Early diagnosis helps prevent progression to higher grades.

Grade II Medial Patellar Luxation

In Grade II, the kneecap slips out more often and may stay out until it is manually repositioned. Dogs may limp or have periods of discomfort after exercise.

Common symptoms:

  • More frequent skipping or limping
  • Patella may remain out for several steps
  • Discomfort when running or turning quickly
  • Muscle loss may begin over time if not treated

When is surgery considered?

  • Surgery is recommended if symptoms worsen or become regular.
  • Dogs that limp often or avoid using the leg benefit from correction.
  • Joint damage may occur over time if left untreated.

Some dogs with Grade II improve with conservative care, but many eventually need surgery to avoid arthritis and improve leg use.

Grade III Medial Patellar Luxation

With Grade III MPL, the kneecap is out of place most of the time but can be pushed back into the groove during an exam. Dogs often have an abnormal gait and show clear signs of discomfort.

Functional impact:

  • Frequent limping or stiffness, especially after activity
  • “Bunny hopping” gait or dragging one leg
  • Muscle wasting on the affected leg
  • Joint swelling or changes in alignment

Surgical recommendation:

  • Surgery is strongly recommended to restore leg function.
  • If left untreated, arthritis and permanent joint damage may occur.
  • Recovery may take longer, but outcomes are usually very good.

Grade III dogs often need both soft tissue and bone procedures. Timely surgery greatly improves their quality of life and movement.

Grade IV Medial Patellar Luxation

Grade IV is the most severe form. The patella is always out of place and cannot be manually moved back into position. The leg often appears twisted or underdeveloped due to poor use.

Severe signs:

  • Constant lameness or inability to use the leg
  • Abnormal bone shape and knee structure
  • Significant pain, joint swelling, or deformity
  • Difficulty standing, sitting, or walking normally

Need for advanced correction:

  • Advanced surgery is required, often involving bone realignment.
  • In some cases, staged procedures are needed to fully correct the limb.
  • Recovery is longer and more closely monitored.

Even though Grade IV is complex, many dogs improve with surgery. While some may still have a limp, pain is usually reduced and mobility increases. Early treatment offers the best chance for comfort and stability.

How Vets Diagnose the Grade of MPL

Veterinarians use a mix of hands-on exams and imaging to diagnose the grade of Medial Patellar Luxation (MPL). The goal is to check how easily the kneecap moves out of place and how stable the joint is during movement.

Key Diagnostic Methods

  • Physical exam: The vet checks for signs of discomfort, joint looseness, and muscle loss in the hind legs.
  • Manual luxation test: While the dog is relaxed, the vet gently moves the patella to see if it slides out of place, how easily it moves, and whether it returns to the groove.
  • Radiographs (X-rays): Used to check bone shape, joint alignment, and other knee problems like arthritis or bone rotation.
  • Limb alignment studies: In advanced cases, special X-rays help measure bone angles and rotation, especially for surgical planning in Grade III or IV.

Accurate grading is key to choosing the right treatment. Early diagnosis allows for better planning and can prevent the condition from getting worse over time.

Signs and Symptoms That Help Determine the Grade

MPL affects dogs differently depending on the grade. Symptoms often start mild but can worsen if the kneecap slips more often or stays out.

Common Symptoms by Grade

  • Grade I: Mild, with occasional skipping or brief lameness. The dog usually walks normally between episodes.
  • Grade II: More frequent skipping or limping. Some dogs start avoiding exercise or show leg stiffness after activity.
  • Grade III: Constant limping or altered gait. Dogs may hop with both legs or show signs of pain during touch or movement.
  • Grade IV: Severe dysfunction. Dogs often drag one leg or walk with bent knees. The limb may look twisted or underdeveloped.

Other Key Signs

  • Skipping or bunny-hopping gait: Common in Grades II–III
  • Bilateral signs: Both knees are often affected in small breeds
  • Unilateral signs: Seen more often when injury causes MPL on one side

Observing the dog’s movement helps the vet match symptoms with the correct MPL grade and decide on treatment.

Treatment Options by Grade

Treatment for MPL depends on the grade, the dog’s age, weight, and symptoms. Lower grades may improve with non-surgical care, but higher grades usually need surgery.

Conservative Management (Grade I–II)

  • Weight control
  • Joint supplements
  • Physical therapy
  • Limiting jumping or rough play
  • Pain medication, if needed

Dogs with mild signs often live comfortably without surgery if their condition stays stable.

Surgical Treatment (Grade II–IV)

When symptoms worsen or the patella stays out often, surgery is usually recommended.

  • Trochlear sulcoplasty: Deepens the groove where the kneecap sits
  • Tibial tuberosity transposition (TTT): Realigns the patellar tendon
  • Soft tissue adjustments: Tighten or release tissues to improve tracking

Advanced Surgeries

  • Needed for Grade III–IV
  • May include bone cutting, plate fixation, or staged corrections
  • Tailored to the dog's unique joint shape and deformity

Surgical plans vary, but the goal is always to keep the patella in place and restore comfortable movement.

Prognosis Based on Grade

The long-term outlook after MPL surgery is usually very good, especially when the condition is treated early and post-op care is followed closely.

Grade-Based Outcomes

  • Grade I–II: Excellent prognosis; many dogs return to full activity and never need surgery
  • Grade III: High success rates, but there is a slightly higher risk of reluxation or needing a second procedure
  • Grade IV: Outcomes vary depending on joint damage. Dogs often improve, but some may keep a limp or limited motion

Reluxation Risk

  • Occurs in about 10–21% of cases
  • More common in Grade III and IV
  • Risks decrease with proper rehab and weight control

Benefits of Early Treatment

  • Helps prevent arthritis and joint damage
  • Improves surgical success
  • Shortens recovery time

With early diagnosis, a tailored surgical plan, and careful recovery, most dogs live active, pain-free lives after MPL correction.

FAQs

How do vets grade a dog’s patellar luxation?

Vets grade MPL by gently moving the kneecap during a physical exam to see how easily it slips out and whether it goes back into place. They also check how the dog walks and may use X-rays to study bone shape and joint alignment. Grades range from I (mild) to IV (severe and permanent).

What is the difference between Grade I and Grade IV MPL?

Grade I MPL is the mildest form, where the kneecap only slips out during an exam and quickly returns on its own. Dogs usually show no pain or limping. Grade IV is the most severe. The kneecap is always out, cannot be pushed back in, and the leg may look twisted or underused, often needing complex surgery.

Can Grade I MPL become worse over time?

Yes, Grade I MPL can worsen if the dog gains weight, gets injured, or has weak joint support. Over time, the kneecap may start slipping more often, leading to joint wear and pain. Regular checkups, weight control, and joint care can help prevent it from progressing to higher grades.

Is surgery always needed for Grade II MPL?

Not always. Some dogs with Grade II MPL respond well to weight control, joint supplements, and restricted activity. But if the dog shows regular limping, pain, or reduced movement, surgery may be the better option. Your vet will decide based on symptoms, age, and how the joint is behaving.

What happens if MPL is left untreated?

If MPL is not treated, it can lead to chronic pain, joint damage, and arthritis. The dog may limp more often, avoid using the leg, or develop muscle loss. In higher-grade cases, untreated MPL can result in permanent joint deformity or the need for more complex surgery later.

Are some breeds more likely to have higher-grade MPL?

Yes, small breeds like Pomeranians, Chihuahuas, Yorkies, and Toy Poodles are more likely to have MPL, and some may develop higher-grade luxation due to their bone shape and genetics. However, large breeds can also be affected. Early screening in at-risk breeds helps catch problems before they get worse.

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