Rupture of the Canine Cranial Cruciate Ligament

Rupture of the cranial cruciate ligament is the most common orthopedic injury and cause of hind leg lameness in the dog. It affects dogs of all sizes and activity levels and can be slow or sudden in onset. Unlike the situation in humans where the majority of cruciate ligament tears are traumatic sports-related injuries, breakdown of the canine cruciate ligament tends to be gradual and progressive. Mechanical forces within the knee play a major role in cruciate ligament breakdown but other diseases of the joint and connective tissues can contribute. Rupture of the ligament leads to instability in the knee and the main goal of surgical treatment is to restore the stability of the knee so that your dog can return to full function.

Surgical repair of a canine cranial cruciate ligament can be performed in a variety of ways depending on the age, weight and intended use of the dog. Considerable attention must be given to the dog’s conformation (limb alignment and shape of the proximal tibia) and mechanical forces anticipated at the knee. This requires a detailed orthopedic and radiographic examination of the hind leg to best assess which technique for repair would be most appropriate. Surgical techniques are divided into mechanical stabilization and suture techniques.

TPLO/TTA – mechanical stabilization techniques.

Extra-capsular Suture/Tight Rope – suture techniques.

Understanding the TPLO:

1. Anatomy and Mechanics of the Canine Knee (Stifle) Joint:

To understand what the TPLO does you must first understand something of the anatomy and mechanics of the canine knee. The anatomy of the dog’s knee is almost identical to that of the human (see illustration). The knee is a hinge joint and therefore has ligaments to hold the two bones together during motion. There are collateral ligaments down the outside of the joint and two cruciate ligaments (“cruciate” means “cross” and these two ligaments cross over one another in the middle of the joint). It is the cranial cruciate ligament that is most often torn in the dog.

Although the anatomy of the knee is very similar to that of people the mechanics of a dog’s knee is quite different. The dog stands on tip-toe with the ankle off the ground and both knee and ankle bent. This puts the muscles of the hind leg in constant tension in order to stop the leg collapsing down with the dog’s weight (try standing like this yourself and you will feel how tight the muscles of your thigh become).

The thigh muscles attach to the tibia just below the knee and pull on the tibial bone. These muscle pulls create a force in the knee known as tibial thrust that would like to push the tibia forward. The cranial cruciate ligament resists this forward thrust and stops forward translation of the tibia in the normal knee. However, we have come to understand that tibial thrust is one of the main mechanical factors in breaking down the cranial cruciate ligament.

Once the cranial cruciate ligament becomes damaged or is completely torn (A), there is an imbalance within the knee. Due to the cranial tibial thrust, this results in forward motion of the tibia. In addition, if the ligament becomes partially torn, the repeated strain results in an eventual complete tear in most dogs. With either partial or complete tears, the stifle becomes progressively unstable resulting in pain, damage to the medial meniscus (B) and arthritis.

In people, cruciate ligament injury is most often traumatic – playing soccer, hockey or football. In dogs, the ligament most often breaks down slowly, largely because of the mechanical strain of tibial thrust wearing away at the ligament. There are other factors such as obesity, hypothyroidism, rheumatoid disease and other causes of arthritis in the stifle joint, limb alignment issues, etc., but the final common pathway is usually mechanical breakdown of the ligament. Because of these factors the usual history with cruciate disease is that of slowly progressive and often intermittent signs of hind limb lameness which may have progressed to an acute lameness during normal exercise.

2. What does a TPLO do?

Traditional methods of cruciate repair in dogs focused on replacing the ligament as was done in people. While traditional repair can be effective, the repaired ligament is under the same strain from tibial thrust as the original ligament. This can result in stretching the repair, return of instability, progression of arthritis and in some cases further damage to the meniscus and acute lameness. Essentially, a TPLO neutralizes cranial tibial thrust and so stablizes the knee during motion. This results in minimal progression of arthritis and a full return to vigorous exercise. The ultimate test is returning dogs to athletic endeavours such as hunting, agility and field trialing which can be achieved with TPLO.

The tibial plateau is the flat part of the tibia in contact with the femur. Normally it slopes backwards about 25 degrees. TPLO is an acronym for Tibial Plateau Leveling Ostoeotomy – the tibial plateau is leveled – made more perpendicular to the tibial axis - using an osteotomy or bone cut. The proximal tibia is cut in a curve and then the top part of the tibia is rotated to achieve the leveling of the plateau. It is held in place with a steel plate and screws.

An analogy which is helpful in understanding how the TPLO works is demonstrated by the illustrations below. Figure A can be used to represent a normal dog's knee. The wagon wheels represent the round condyles of the femur (thigh bone); the hill represents the tibial plateau slope. The rope (F) represents the cranial cruciate ligament which overcomes the tibial thrust (arrow D). As the dog bears weight (arrow C), additional force is added to arrow D because of the hill. One can see that if the rope breaks (F), the wagon will roll downhill, just as the femoral condyles slide down the back of the sloped tibial plateau with a torn cranial cruciate ligament. The medial meniscus (E) is not strong enough to support the weight of the wagon alone and can become crushed. In the past, conventional procedures have focused on replacing the function of the rope, but this is a large force to overcome. Over time, these repairs typically loosen resulting in motion in the knee and increased arthritis. The TPLO (illustration B) gets rid of the slope which allows the wagon to stay on the surface of the tibial plateau without sliding backwards.

3. What is involved in a TPLO:

a. Meeting the surgeon:

You and your dog will meet with the surgeon who will examine your dog and confirm the diagnosis and check for any other abnormalities in your dog. The procedure and potential complications will be discussed and an accurate quotation provided. If you are coming from out of town surgery should be scheduled ahead of time to ensure that you can have the surgery performed expediently at the same visit. Usually (unless by special arrangement) surgery will be performed next day and your pet will be discharged the following day.

b. Chosing between arthrotomy and arthroscopy:

When the cruciate ligament breaks it can quickly cause some considerable pathology within the joint. Hence it is important to get your pet diagnosed and treated as quickly as possible. Most importantly, there can be damage to the medial meniscus, which is a cartilage pad between the femur and tibia on the inside of the leg. This needs to be dealt with and can be treated either arthroscopically or via a mini-arthrotomy (surgical incision into the joint). Also, the remnants of the cruciate ligament are debrided, which arguably may help to slow progression of arthritis.

Arthrotomy- a small incision is made in the joint which allows the cruciate remnants to be removed and the menisci to be inspected. If the medial meniscus is damaged it will have to be treated, which may include partially removing it. The goal is to preserve as much of the meniscus as possible as it has an important function within the joint and removal can lead to more arthritis on that side of the joint over the long term.

Arthroscopy – this means looking into the joint with a small (2.4 or 2.7mm) scope and is the standard of care in human orthopedics, because it gives great visualization of the joint (with magnification) and is very atraumatic and thus less painful postoperatively. It is now a rapidly growing subspecialty in veterinary medicine and allows us to inspect and treat the knee joint at the time of TPLO without causing the trauma of an arthrotomy. It does add a small amount of extra cost to the procedure.

c. Performing the surgery:

X-rays of the leg are taken prior to surgery and under the same anesthetic. These are important to allow accurate measurement of your pet’s own tibial plateau angle and to assess the leg for bow-leggedness. The procedure is then performed through an incision on the inside of the leg. The bone is cut and rotated a specific amount, based on your pet’s own tibial plateau angle, and then fixed with a steel plate and screws. Normally, we use a 6 hole plate and 6 screws but in some larger dogs we will use an 8 hole plate. Postoperative x-rays are taken to confirm the accuracy of derotation of the tibial plateau and correct placement of the implants.

d. Postoperative pain relief:

Cutting a bone can be a painful thing and so we take great care in performing good postoperative pain relief. Our hospital is staffed 24 hours a day, 7 days a week and our staff are trained in both pain recognition and treatment. With appropriate pain relief the patient tends to be less stressed and can recover quickly.

e. Postoperative Instructions:

We have a detailed list of postoperative instructions for the home care of your pet. The day after surgery we will make an appointment with you to go over these instructions and make sure you are comfortable with how to look after your pet.

Expected Outcome and Potential Complications of TPLO

The TPLO has been a highly successful surgery for the treatment of cranial cruciate disease in dogs. There is a steep learning curve with this surgery but our surgical team has a wealth of experience with this procedure and most of the technical issues have been worked out. However, dogs can be both good and bad patients and no surgery is perfect. We expect the majority of our patients to return to good or excellent function. But, there is the potential for complications.

Infection: strict sterile technique is used during the surgery but there is a small chance of infection in any clean orthopaedic surgery (there is a similar risk of postoperative infection in human surgical patients). Superficial infections can be readily treated with antibiotics but deeper infections may require implant removal to clear up. This is not done until bone healing is complete. Infection is not necessarily introduced at the time of surgery but can be introduced after surgery by the dog licking the wound. Therefore, it is vital to keep your pet away from the wound by using the Elizabethan collar provided.

Problems with bone healing: this can occur if your dog is overactive in the early postoperative phase (first 8 weeks). It is critical to keep your dog confined to gentle leash walks only. Strenuous bursts of activity could result to damage to the tibial tubercle or even implant loosening. This should be avoided with careful restriction.

Patellar ligament strain: following TPLO surgery the patellar ligament will have a significantly increased pressure exerted on it and may become strained if your pet is overactive during the healing phase. Patients with severe quadriceps muscle atrophy may be predisposed. Rest and anti-inflammatory medication are used to resolve this problem.

Arthritis: this is present to some degree in most dogs that have a cranial cruciate ligament rupture. The stability achieved with a TPLO makes most dogs very comfortable and functional and minimizes the progression of arthritis. However, some dogs have advanced arthritis at the time of presentation and this level of arthritis may cause the dog not to use the leg well even after surgery. Also, arthritis is more likely to progress with time. Medications can be used to help relieve these clinical signs.

Meniscal tear: this is a complication that occurs in 3-6% of dogs following TPLO surgery and may necessitate another operation. Sometimes it can be managed medically.

TPLO in Small Dogs:

TPLO can be performed in most dogs down to 5-7 kg depending on bone size by using smaller blades and smaller implant sizes. It was previously thought that small dogs only needed to have the traditional extracapsular repair of a ruptured cruciate to do very well. However, the same mechanical principles of cruciate disease apply equally to small and large breed dogs. In addition, some small breed dogs can have quite excessive tibial plateau slopes (some as high as 45 degrees) and quite severe bow-leggedness. Both of these conformational issues can lead to less than ideal results with the traditional repair techniques. So TPLO can be applied equally to small and large breed dogs and can lead to early and full return to limb function.

TPLO and Bow-leggedness:

Dog breeds come in all shapes and sizes. Some breeds, such as bulldogs, bull terriers and some rottweillers and Labrador retrievers are particularly curved in their hind legs. If you look at your dog from behind you can sometimes appreciate their bow-leggedness– their hind legs bow outwards. During motion this limb shape can lead to a “twisting” type of gait where the leg twists during motion. This twisting motion can put a lot of stress on the knee and especially the cranial cruciate ligament. This may be one of the predisposing factors in early rupture of the cruciate ligament and in some cases needs to be corrected at the time the TPLO is performed. This can often be achieved at the level of the tibial cut prior to fixing it with a plate. In extreme cases there may be other corrections needed.