Rupture of the Anterior Cruciate Ligament in Dogs
The cruciate ligaments are located on the inside of your dog’s stifle (knee). These cruciate ligaments stabilize the connection between the femur and the tibia in a forward/backward direction. The caudal cruciate ligament is much thicker and stronger than the cranial cruciate and rarely tears.
In order for a joint to be stable, all of the forces that are applied within the joint need to be countered by an equal opposing force; if any of these forces become unequal the joint will become unstable. The forces that are applied in the normal canine or feline knee would cause the femur to slide off of the back of the tibia, but these forces are neutralized by the Anterior Cruciate Ligament (ACL). When the ACL is torn this neutralizing force is lost and the femur tries to slide off of the back of the tibia with every step that is taken. This movement is called Tibial Thrust or Drawer Motion and can be demonstrated with palpation while your dog is sedated. This instability causes pain because tissues that are now forced to hold the joint are not designed to do so, and in their attempt to hold the joint steady they are stretched and/or stressed with a load beyond their capacity.
Rupture of the ACL is one of the most common causes of rear limb lameness in dogs. They will typically become suddenly lame; sometimes they cannot put any weight on the limb at all. This cruciate ligament tear can be partial or complete. A partial tear may be able to provide some stability to your dog’s joint, but it is still very painful. This is because the ligament has pain receptors that are stimulated when a partially torn ligament is loaded. Most partial ACL tears will eventually become complete tears because the damaged ligament cannot heal adequately to carry a normal weight load. When the ACL is injured with either a partial or complete tear the damaged tissue releases chemicals into the joint, these chemicals will cause other changes in the joint that aggravate the disease process. These chemicals thin the joint fluid so it will not protect and lubricate the joint in a normal manner and the cartilage of the joint will be damaged and eroded, leading to other secondary arthritic changes. Left untreated the affected joint will be a source of continued pain for your pet. Over time, even though this pain does not disappear, it does seem that some patients will “get used to it.” However, the abnormal joint will continue developing arthritis, leading to cartilage loss and eventually bone on bone grinding. Therefore, stabilizing your dog’s knee with a torn ligament is important for both the long term and short term comfort of your pet.
Studies have shown that a dog will only bear 20-30% of their normal weight on a torn ACL. This puts the ACL of the other knee at risk for rupture because of the increased weight it must carry as a result of the pain in the damaged knee. Approximately 40% of the dogs that have torn one ACL will tear the ligament in their other ACL. For this reason it is important to seek care for a torn ACL as soon as possible and hopefully prevent the need to repair both.
The only way to stabilize a dog’s knee with a torn ligament is through some type of surgery. Over the years there have been several techniques developed to accomplish this stabilization; however, most have not met with satisfactory results on a long term basis. The most common procedures being performed at this time are the external capsular repair, the TPLO, and the TTA. Below is a more detailed description of these procedures.
The three most common corrective procedures for the repair of a torn ACL in a dog are: the external capsular repair, the TTA, and the TPLO.
The External Capsular Repair
The external capsular repair procedure involves stabilizing the joint by placing a very strong suture under the skin but on the outside of the joint to stop the Tibial Thrust that was explained above. This method relies on the production of scar tissue to eventually hold the joint stable. Even though there have been many patients that have had very successful outcomes with external capsular repair, the number of dogs that do not stabilize adequately is too high to consider it a suitable treatment for this injury. One of the factors that contribute to the frequent failure of this procedure is the length of time (4-6 months) that it takes for the stabilizing scar tissue to form, it is very difficult to control the activity of some dogs for such a long period of time. All it takes is one moment of high energy exertion or play to break down all of the healing that has taken place and return the dog to complete instability. In addition, this procedure does nothing to stop the forces that contribute to Tibial Thrust; the femur is still trying to slide off the back side of the tibia every time the dog puts weight on the leg.
The TTA and the TPLO both work off of the same premise of restructuring the stifle (knee) joint so that Tibial Thrust is neutralized. Both of these procedures have an advantage over external capsular repair because they neutralize Tibial Thrust, this means that during normal weight bearing the femur is not being forced off of the back of the tibia. The measurements used to develop the surgical plan for both the TTA (common tangent) and the TPLO (plateau angle) in a dog are very similar, but they are derived by using slightly different landmarks on your dog’s x-ray, which may cause some confusion as you compare these two specific surgeries.
The TTA or Tibial Tuberosity Advancement
The TTA surgery or the Tibial Tuberosity Advancement surgery was introduced in 2004 as an alternative method of correcting the pathology created when a dog’s ACL is torn. TTA Surgery has been used successfully in over 30,000 dogs. Some of the advantages of the TTA surgery over the TPLO surgery are that the bone cut (osteotomy) does not compromise the weight bearing surface of the tibia, it decreases the pressure applied to the Femur beneath the Patella which should stop or decrease further arthritic changes in this area of the joint. In addition, Patellar luxation can usually be corrected with the TTA surgery in a simple manner.
When viewed from the side, in a normal standing position, the rear limb of a dog is not straight. The angle between the femur and the tibia is less than 180 degrees (typically it is about 150 degrees). At this angle, there is a load on the ACL which contributes to tibial thrust. When the leg is flexed to approximately 110 degrees, the angle of the common tangent (between the femur and the tibia) intersecting the patellar ligament is 90 degrees (the ACL is no longer loaded) and tibial thrust is neutralized; this angle is called the “cross-over” point. Any time the dog’s knee is flexed at an angle between this cross over point and 180 degrees the ACL is loaded. Any time the dog’s knee is flexed below the “cross-over” point, the ACL is not loaded and Tibial Thrust has been neutralized. The purpose of TTA surgery is to change the cross-over point. When completed tibial thrust will be eliminated, and your dog’s knee will be comfortable and stable while standing in a normal position
Changing the cross-over point is accomplished by moving the Tibial Tuberosity (top, front of the shin bone) forward so that in a normal standing position the angle between the common tangent and the patellar ligament is 90 degrees. The tibial tuberosity is cut away from the tibia and a titanium cage is then inserted into the opening where the bone was cut to act as a spacer to hold the tuberosity forward from its original location. The tuberosity is secured in place with a titanium plate, and the opening between the tuberosity and the tibia (the osteotomy) is packed with a synthetic bone graft material that will provide a matrix to help the space fill with bone.
Your pet will need to be leashed for about 8 weeks from the time of the surgery to allow the osteotomy to fill with bone. During a portion of this time your dog’s leg will be immobilized in a custom made splint that is meant to decrease the likelihood of them stressing and damaging the surgery site before it gains adequate strength. A radiograph will be taken about 4 weeks after the surgery to check for adequate healing.
The TPLO or Tibial Plateau Leveling Osteotomy
The TPLO surgery or Tibial Plateau Leveling osteotomy procedure involves cutting away a portion of the top of the tibia, rotating it and reattaching it with a bone plate. This cut encompasses the weight bearing surface of the tibia. The TPLO surgery is technically more difficult to perform than the TTA surgery. It also increases the pressure that is exerted by your dog’s patella (knee cap) on the femur, which increases the risk of cartilage erosion and arthritis at this location. In a few cases, TPLO surgery has an advantage over the TTA surgery because some angular limb deformities might be corrected with the TPLO. Also, if the tibial plateau angle is too steep it may not be possible to advance the tuberosity far enough to negate tibial thrust. If either of these conditions is found on exam, you will be encouraged to discuss them with a TPLO surgeon.