Cruciate ligament injury in dogs – a torn cranial cruciate ligament

Overview

The cranial cruciate ligament lies inside the knee joint and connects the femur to the tibia. It prevents excessive forward sliding of the tibia.

It also limits overextension of the knee and inward rotation of the tibia. Cruciate ligament injury is one of the most common causes of back-leg pain and lameness.

It is the most common joint disease in dogs requiring surgery. A partial tear can gradually progress to a complete tear.

Symptom severity partly depends on whether the tear is partial, acute or longstanding. Osteoarthritis may develop in the knee before the ligament tears completely.

Causes

The ligament most often weakens gradually through degeneration, rather than a completely healthy ligament tearing in a single accident. A partial injury may progress over weeks or months.

A weakened ligament may eventually tear during ordinary running or jumping. Injury to a healthy ligament generally requires major trauma.

Genetics, limb conformation and immune-mediated processes within the joint may contribute to the disease. When the ligament no longer supports the joint, the tibia can move abnormally relative to the femur.

The medial meniscus is a cartilage-like cushion between the bones on the inner side of the knee. Persistent instability can trap and tear it.

The lateral meniscus is injured less often. The body tries to stabilise a knee that has been loose for a long time by forming scar tissue around it.

The menisci also help distribute load through the knee joint. They also contribute to the joint's sense of position.

Extensive scar tissue formation can reduce the knee's range of movement.

Risk factors

Cruciate injuries occur in dogs of all ages and sizes. The average age at diagnosis is about seven years.

In large breeds, the injury is usually diagnosed younger. Predisposed breeds include Rottweilers, Labrador Retrievers and Newfoundlands.

Excess weight and poor physical fitness increase the risk. Insufficient angulation of the hindlimbs may predispose a dog to injury.

Kneecap dislocation and osteochondrosis can also increase the risk of cruciate injury. Injury associated with osteochondrosis can cause symptoms while a dog is still growing.

Underlying conditions in an older dog may include hypothyroidism. Degenerative injury is also more common in neutered dogs and in females.

Inherited susceptibility has been demonstrated in Newfoundlands and Labrador Retrievers, for example. Other predisposed breeds include Akitas, Mastiffs, St Bernards and Chesapeake Bay Retrievers.

Greyhounds, Dachshunds, Basset Hounds and Old English Sheepdogs have been described as less commonly affected. Heritability varies from moderate to high between populations.

Typical symptoms

• After an acute tear, your dog is lame in a back leg and does not bear weight on it properly.

• Getting up from sitting becomes difficult.

• Sitting down becomes awkward or unusual.

• Jumping into the car or another previously easy effort becomes difficult.

• Willingness to play and activity levels decrease.

• A firm thickening is visible or can be felt on the inner side of the knee.

• A click may be heard from the knee, which can be associated with meniscal injury.

With a partial tear, lameness may become more noticeable during exercise and improve with rest.

With a complete tear, lameness may persist despite rest.

A longstanding injury can reduce thigh muscle mass.

Your dog may be painful even without whimpering or constantly holding the leg off the ground.

Lameness may be particularly obvious after a long rest.

The knee may become stiff and lose range of movement.

The event that started a longstanding injury is not always noticed.

Complications and consequences

Osteoarthritis and the meniscus

Persistent knee instability leads to osteoarthritis. Concurrent meniscal injuries have been reported in 20–77% of dogs with cruciate tears, and sometimes more.

Later meniscal injury after surgery has occurred in about 2–30%, depending on the method. Lameness that had improved may therefore suddenly worsen again if the meniscus is injured.

Surgery achieves substantial improvement in about 85–90% of cases. Good or excellent limb function returns after surgery in more than 80%.

Osteoarthritis may nevertheless progress even after successful surgery. Surgery to stabilise the knee generally slows its progression compared with medication alone.

The other knee and surgical risks

Injury to the opposite knee is also common: the estimated later risk is 40–60%. In up to 50% of dogs, the other knee may be affected within the first 12–18 months.

Another estimate reports tears in both knees in 30–50% of dogs, occurring 1–2 years apart. The overall complication rate for surgical stabilisation has been reported as 15–28%.

Possible complications include infection, implant breakage and bone fracture. Inadequate aftercare can increase the risk of implant failure.

Surgery can also involve bleeding. An extracapsular suture can be associated with compression of the fibular nerve.

A sideways shift of the knee after surgery may be associated with residual instability.

Diagnosis at the vet

Assessing knee instability

The vet looks for abnormal movement of the tibia relative to the femur. Finding a cranial drawer movement supports the diagnosis of cruciate injury.

Sedation may help relax the muscles if the dog is painful or tense. Instability may be harder to detect with a partial injury.

Extensive scar tissue and osteoarthritis may reduce the drawer movement that can be felt. The assessment also considers increased joint fluid, thickening on the inner side and range of movement.

Another finding indicating instability is forward movement of the tibia during a test that simulates loading.

X-rays and tests before surgery

X-rays show osteoarthritic changes and joint swelling. Imaging also rules out conditions such as a fracture or bone tumour.

X-rays are needed to plan surgery. Particular views may be needed for TPLO and TTA planning.

Blood tests assess health before anaesthesia or the use of certain painkillers. The meniscus is also checked during surgery.

A missed meniscal tear may explain persistent pain and impaired joint function. With a complete tear, forward displacement of the tibia may also be visible on an X-ray.

Another condition identified by blood tests may affect the choice of anaesthetic medicines.

Treatment

When is surgery needed?

Treatment choice depends on the dog's size, age, activity, conformation and degree of knee instability. A complete cruciate tear is usually treated with stabilising surgery.

Medical management may suit an elderly dog or one with a high anaesthetic risk, for example. A chronic condition that affects recovery may also influence the choice.

The vet prescribes medication for pain and inflammation as part of the treatment plan. Pain relief may improve the dog's condition while knee instability persists.

Restricted lead exercise reduces strain on the unstable knee. Information on the benefits of braces remains limited.

A brace may cause skin sores or ongoing discomfort. Conservative treatment or an extracapsular suture may suit a small dog better than a large dog.

With a partial tear, rehabilitation can strengthen muscular support around the knee and delay the need for surgery. Scar tissue may support the knee during medical management too, but an active dog can be injured again.

Operations to stabilise the knee

In TPLO, a curved cut is made in the upper tibia and the orientation of the joint surface is changed. This makes forces acting through the knee more stable without a functioning cranial cruciate ligament.

A plate and screws hold the cut bone in position while it heals. The plate is not usually removed later unless it causes problems.

In TTA, the front part of the tibia is moved forwards to balance forces within the joint. The patellar tendon attaches to the bone section moved during TTA.

TTA also requires fixation and time for bone healing. In extracapsular stabilisation, a strong suture supports the knee while scar tissue forms.

In large, active dogs, procedures that alter bone alignment often provide a better long-term result than suture stabilisation. A damaged part of the meniscus may be removed during the same operation.

CBLO is another procedure that changes tibial alignment, with a bone cut in a different location from TPLO. It can also be used in young animals whose growth plates remain open.

Recovery and rehabilitation

Tissue healing usually takes about 6–12 weeks, depending on the patient. Overall exercise restrictions may continue for at least three months.

Normal activity often resumes only after 3–4 months, once the bone and muscles have recovered. Keep the recovering dog on a lead, with no running, jumping or boisterous play.

A protective collar prevents wound licking and removal of stitches, usually for about two weeks. Bone healing may be checked with X-rays at 4–6 weeks and three months.

Supervised rehabilitation can speed recovery and improve the outcome. Exercises may include passive movements, balance exercises and controlled walking.

Comparisons of TPLO with suture stabilisation have not always found a clear difference in lameness at 2–6 months. In dogs studied after TPLO, walking and trotting matched healthy controls at six and 12 months.

After TTA, comparable normal walking was only achieved at 12 months. In these comparisons, neither TTA nor suture stabilisation restored trotting to the level of healthy controls.

Prevention

A lean weight and regular exercise to maintain fitness help reduce modifiable risk factors. Monitoring food amounts helps prevent excessive weight gain.

Inherited susceptibility and limb conformation nevertheless influence the disease. Some breed clubs recommend that a dog treated for cruciate injury should not be used for breeding.

Some breeding guidance recommends that, where a serious inherited disease occurs in a family, the mate should be healthy and have no close relatives with the same disease. A dog may already have been used for breeding before the cruciate injury is diagnosed.

Important for dog owners

Persistent lameness indicates pain even if your dog otherwise seems lively. A sudden setback during recovery may involve the meniscus.

Free exercise too early can cause a complication requiring another operation. Wound licking may cause infection at the surgical site.

Ask for a written exercise and rehabilitation programme before discharge. Record weight-bearing on the leg, wound condition and the effect of medication. Agree follow-up appointments and instructions for sudden worsening of lameness. Monitor use of the other back leg even when the operated leg is recovering well. Skin stitches or staples are usually removed about two weeks after surgery.

Increase the duration and difficulty of rehabilitation walks gradually according to the programme. Exercises strengthen the muscles supporting the trunk as well as the affected leg.

Frequently asked questions

Can a dog's cruciate ligament tear during an ordinary walk?

Yes. The ligament has often weakened gradually before the final tear. A weakened ligament can tear during running or jumping. Traumatic rupture of a completely healthy ligament is less common. The final step alone therefore does not explain the full background to the disease.

Can a cruciate injury improve without surgery?

Medication may relieve pain, but knee instability can remain. Medical management may be considered for a dog with a high anaesthetic risk, for example. Surgery is a common choice for a complete tear. Medical management or suture stabilisation may suit small dogs better than large dogs.

Can the cruciate ligament in the other knee be injured too?

Yes. The estimated later risk to the other knee is 40–60%. Injuries to both knees have also been reported in about 30–50%, occurring 1–2 years apart. A predisposition to degeneration affecting both knees often underlies the problem.

How do TPLO and suture stabilisation differ?

TPLO changes the orientation of the tibial joint surface and therefore the forces acting through the knee. With suture stabilisation, a strong suture supports the joint from outside while scar tissue develops. The method is chosen according to factors including the dog's size and activity. After TPLO, the bone must be allowed to heal before free exercise resumes.

When can a dog exercise normally after surgery?

Normal activity often resumes after about 3–4 months. Bone healing and muscle recovery are prerequisites. Until then, exercise is controlled and rehabilitation exercises are adapted to the dog's recovery. Check the treating vet's plan for permission to increase exercise.