Cruciate ligaments are structures within your dog’s stifle (knee) that help support the joint during weight-bearing. If a cruciate ligament snaps or is injured, this can lead to a painful and unstable joint. This will cause your dog to limp or not be able to put weight on the affected limb at all, often causing them to hop on one back leg. This can be very worrying but rest assured your vet will be able to discuss all the treatment options available to your dog. Most dogs will return to full physical health with appropriate treatment. Keep reading to find out more about the cruciate ligaments. Read how they can become damaged and which surgery may be best to treat ruptured cruciate ligaments in your dog.

What are cruciate ligaments?

Cruciate ligaments are two tight fibrous bands that cross each other in opposite directions. They hold the bones above and below the joint in place and providing support to the stifle joint. These ligaments prevent the tibia (shin bone) from sliding forward relative to the femur (thigh bone). They are known as the cranial and caudal cruciate ligaments in dogs, according to where on the tibia bone they attach. If one of these fibrous bands tears or snaps then the stifle support is lost and your dog will limp. The cranial cruciate ligament is the most commonly affected ligament. 

How do they get damaged?

Rupturing of the cruciate ligaments in humans is a common injury seen in footballers, caused by trauma. However, this is not the case in dogs. Instead, there is a slow and progressive weakening of the ligament. This is a degenerative (ageing) process which can be likened to fraying of a rope. Tearing and rupture of the cruciate ligament is therefore seen with no trauma and during normal physical activity in dogs. Occasionally, a partial tear may cause your dog to limp for a couple of days and then get better. Unfortunately, due to progressive weakening of the ligament, these cases often progress to full and complete tearing, causing a long-term limp. This condition is also known as cruciate ligament disease.

We do not know the exact cause of this weakening, however certain breeds tend to be affected more than others, suggesting a genetic cause is to blame. Labradors, Boxers, Rottweilers and West Highland White Terriers are breeds commonly affected. Additional evidence to support a genetic link includes the fact that some patients can be affected bilaterally (both stifle joints), either at the same time or a couple of months to years apart. Other underlying factors may include obesity, individual conformation, hormonal imbalances, and early neutering in large-breed dogs. 

What is happening within the joint?

Fraying or complete rupture of the cruciate ligament within the stifle triggers an inflammatory response within the joint. This causes pain and limping. Unfortunately, by the time your pet shows symptoms, there will already be a degree of arthritis within the joint. Arthritis can often be detected on x-rays.

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When the cruciate ligaments are defective, the joint becomes unstable. This, combined with the shape of the tibia (that has a backwards slope in dogs), will lead to the femur bone sliding down the tibia when the dog bears weight on the limb. This can lead to crushing and tearing of the menisci (shock-absorbing cartilages within the joint) due to abnormal weight-bearing within the joint. 

How will the vet tell that my dog has a ruptured cruciate ligament?

The most obvious sign of cruciate ligament rupture is hindlimb limping. It is most commonly reported after no apparent injury. When both cruciate ligaments are ruptured concurrently, your dog may be unable to walk and this is sometimes confused with a neurological injury. In some cases, where there is damage to menisci, a clicking-sound may also be heard.

Your vet will examine the entire limb to assess where the problem lies. Often there will be signs of inflammation within the stifle joint, for example, swelling and heat. In cases of a complete rupture, your vet will be able to feel excessive movement within the stifle joint. In cases of a partial tear, the joint may still be stable. It will be necessary to x-ray the limb to confirm a diagnosis of ruptured cruciate ligament and rule out other causes of limping. X-rays are also necessary to plan future surgeries. Occasionally other tests may be required, for example, sampling the fluid from the joint. 

Does my dog need surgery?

The type of treatment recommended by your vet will depend on many factors, including the severity of your dog’s condition, their breed, conformation, weight and whether there are any other conditions present. Treatment of ruptured cruciate ligaments can involve surgery and/or non-surgical treatments.

Surgical treatments offer superior outcomes with a quicker return to normal limb use than non-surgical treatments. Surgery is still recommended for dogs with partial tears. They may get better with rest alone but as soon as they exercise again, the progressive weakening often leads to complete tearing. It is often only a matter of time. When surgery is performed, the joint is opened and the frayed cruciate ligament ends and meniscal cartilages are examined. The frayed ligament ends and damaged meniscal segments are removed, as these can act as a source of ongoing inflammation within the joint. 

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The sole use of non-surgical treatments is generally not recommended for management of this condition. Without surgery, recovery is rarely complete and can be delayed. Non-surgical approaches involve weight control, physiotherapy and anti-inflammatory pain medications. In some cases, there may be a good reason to avoid surgery, for example, if another pre-existing condition causes the general anaesthetic risk to be too great. A non-surgical approach is generally only suitable for dogs weighing less than 15kg since heavier dogs are likely to struggle to overcome the instability without surgery.

What are the surgery options available in dogs? 

Surgeries either aim to replace the deficient cruciate ligament; or else to render the ligament redundant by performing bone-cutting procedures that act to alter the forces within the joint. The techniques described below may be available at your vets’; however, some procedures may require referral to a specialist orthopaedic surgeon.

Ligament replacement techniques

The most commonly used and least invasive technique in dogs is that of a nylon lateral suture. The nylon suture is placed outside of the joint, around the fabella (a small bone located at the back of the femur) and through a small hole drilled in the front of the tibia. The suture is tightened either through the use of metal crimps or via the use of knots. 

The nylon lateral suture provides joint stability whilst allowing natural movement within the joint. This stability allows the body to produce fibrous tissue, which thickens around the joint like scar-tissue, to provide the joint with long-term stability. This is important, as the nylon suture will either snap or weaken with time. Therefore, this procedure is not generally recommended for large and overweight dogs, as premature suture failure is a possible complication. 

Essentially, this procedure replaces the ligament, but on the outside of the joint rather than inside.

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Bone-cutting procedures rendering the cruciate ligament redundant

These procedures aim to alter the angles between bones and therefore the forces which act within the joint, in turn removing the need for a functioning cruciate ligament to maintain stability. This is achieved by cutting the tibia (shin) bone and then fixing it in a new position. There are two main procedures currently employed:

  • Tibial Plateau Levelling Osteotomy (TPLO)
  • Tibial Tuberosity Advancement (TTA)

The TPLO procedure involves a curved cut being made in the tibia. This is rotated to create a flat top to the tibia which will help prevent the femur sliding down the tibia when weight is borne by the limb. The cut is fixed in place with a plate and screws.

The TTA procedure involves a cut being made in the front of the tibia, which alters the angle of forces running through the quadriceps muscles (large muscle group making up the thigh muscles). Like the TPLO procedure, the alteration of angles within the joint alters the forces acting through the joint, thus correcting the instability within the stifle joint. 

Which surgery is best?

The general consensus among orthopaedic surgeons is that the TPLO procedure offers the best outcome and recovery. This is especially the case for larger breed dogs. 

Other procedures may provide equally successful outcomes. The type of surgery that best suits a dog can sometimes vary according to their individual circumstances. It is important to have a thorough discussion with your vet to help you understand the options available and the potential associated complications. The bone-cutting procedures will typically cost more and often require referral to an orthopaedic surgeon. Potential complications will vary according to the procedure but may include infection, bleeding, fractures, late meniscal injuries, ongoing arthritis and lameness.

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Summary

Ruptured cruciate ligaments can commonly be seen in dogs, with some breeds being affected more commonly than others. It is caused by a poorly understood weakening of the ligament which when damaged causes knee joint instability and therefore limping, pain and inflammation. It is best treated by surgical interventions of which there are different options available. The option best suited to your dog will depend on many factors including their conformation, weight and whether other conditions are present. Typically, the bone-cutting procedures will offer superior outcomes and recovery times for larger breed dogs.