Your dog or cat starts limping on a back leg, so you schedule an appointment to have the leg checked. We perform a complete orthopedic exam and discover an unstable painful knee. Radiographs (X rays) are taken to evaluate the knee. We diagnose a torn cruciate ligament. Next we schedule surgery to fix the damage and return function to the knee.
We perform arthroscopic surgery on the knee to evaluate and treat the damage inside. We then perform surgery to stabilize the knee. We are aggressive with our pain management, and most patients are released from the hospital the evening of surgery day. At discharge a doctor or surgery technician meets with you to demonstrate the rehabilitation methods you will perform at home. Make sure they take it easy, leash walks only for elimination, no squirrel chasing! After 10 to 14 days, you return for suture removal and evaluation of healing progress. We discuss the next step in rehabilitation, gradually starting exercises to strengthen the leg. At about 6 weeks after surgery, you come back in for follow up examination and radiographs to evaluate healing. Most patients are then allowed to gradually return to normal activities. By six months after surgery most patients have returned to normal or near normal activity.
The Detailed Discussion
Injuries to the cranial cruciate ligament (CCL) are the most common orthopedic injury of the dog, and can happen in cats as well. The cruciate ligaments, along with the collateral ligaments, provide primary stabilization of the knee. Secondary stabilization is provided by the menisci (two cartilage pads) and the leg muscles. Dogs and cats are highly dependent upon the CCL for normal weight bearing and movement. The CCL prevents abnormal sliding motion in the knee ("drawer motion" or "tibial thrust"). The CCL additionally resists internal rotation and hyperextension of the knee. CCL injuries in dogs tend to result from chronic slow onset tearing or degeneration of the fibers of the ligament, followed by sudden rupture of the weakened ligament. This is in contrast to human cruciate (ACL in humans) injuries, which tend to be traumatic ruptures. Damage to the CCL causes instability of the knee which leads to lameness, pain, and arthritis. Without treatment, joint damage progresses causing further pain, arthritis, and in some cases disuse of the leg.
Ruptures of the CCL in dogs almost exclusively require surgical repair. Occasionally, in small dogs of less than 10 to 15 pounds, and many cats, the knee may heal without surgery. Larger dogs must have surgery to restore the stability of the knee and minimize future degeneration. Surgery involves evaluating the damage inside the knee, and restoring the stability which was lost with the damaged CCL. All dogs (and humans) with CCL injuries will eventually experience arthritis; however, prompt surgical repair minimizes the progression of arthritis, and returns normal function to the knee. Additionally, CCL injuries frequently (approximately 40% of the time) result in secondary injuries to the menisci, which need to be addressed at surgery when the knee is repaired. Occasionally, postliminary (after surgery) meniscus injuries can occur, which may lead to a second surgery to repair the menisci, although this is uncommon. Success rates range from 90 to 95%, which is defined as return to good to excellent function. It is important to note that between 40 and 60% of dogs who rupture the CCL in one leg will go on to rupture the CCL of the other knee often within 12 months.
Inside the Knee
There are two principal techniques used by veterinarians to evaluate and repair the damage inside the knee after CCL injuries. The joint can be operated in an open procedure (arthrotomy), or with an arthroscope (a tiny camera used to work inside the joint).
The most common method used in veterinary medicine for accessing the interior of the joint is the arthrotomy. This procedure involves surgically opening the entire knee joint with an incision of 4 to 6 inches through the entire joint. The open procedure is more painful and invasive for the patient due to the disruption of the entire length of the joint capsule. Due to the invasiveness of open approaches, long term degenerative changes result to the structures of the knee. Arthrotomy is technically easier to perform and uses standard surgical instruments, hence it is the most frequent method used by most veterinary surgeons. Some surgeons perform "mini" arthrotomies to minimize the invasiveness to the knee from surgery; however these techniques may not provide the full visualization of the internal structures of the knee.
Arthroscopy of the knee uses minimally invasive, tiny incisions (less than 1/8 inch) in the joint capsule to allow the use of tiny cameras and instruments inside the joint. It is far less invasive and less painful for the patient. However, it is technically demanding, and requires a great deal of high technology equipment. It allows far better visualization of the inside of the joint since the camera projects a bright magnified picture onto a large screen monitor. Use of the scope coupled with palpation of the menisci results in far better ability to identify and treat meniscal damage compared to arthrotomy. Currently few veterinarians perform arthroscopy. The primarily reason for performing an arthrotomy is due to the large investment in equipment and the long, steep learning curve required to become proficient with arthroscopy.
Humans who have had both an arthrotomy and arthroscopy report much less pain and quicker recovery with arthroscopy. Arthroscopic surgery is far preferred by human patients. Veterinary scientific literature supports this finding in dogs: quicker recovery, less pain, and less long term degenerative changes are seen with arthroscopic procedures.
With experience and advancements in arthroscopic equipment at Countryside Animal Hospital we perform arthroscopy instead of arthrotomy for all our CCL patients. Our selection of scopes allow us to enter even the smallest knee joints of small dogs and cats, as well as the biggest knees in larger dogs.
The most common techniques used to restore stability to the knee are placement of a tissue graft replacement ligament, placement of a synthetic replacement ligament (MRIT), Tibial Plateau Leveling Osteotomy (TPLO), or Tibial Tuberosity Advancement (TTA). All have their pros and cons, as outlined below.
Tissue Graft Methods
The use of live tissues to create grafts to repair the ACL is common in human orthopedic surgery. One frequently used technique takes a portion of the patellar tendon to replace the torn ACL. These techniques have been used in dogs, but have a poor success rate. The graft in a canine knee is fully loaded shortly after surgery, versus humans where the knee can be held in a brace to unload the graft during healing. In dogs and cats this early loading causes premature failure of the graft. As noted earlier, CCL injuries in dogs tend to start with slow, degenerative processes, where in humans, ACL injuries tend to be acute traumatic injuries. Because of the degenerative nature of CCL injuries in dogs the joint space tends to be too inflammatory, or hostile, for graft survival. Tissue graft methods have been scienticifally shown to produce less optimal repairs. Therefore, tissue graft procedures are not commonly used in veterinary orthopedics.
Synthetic Ligament Methods
Since live tissue grafts tend to fail in canine CCL surgery, many veterinarians use synthetic ligament replacement techniques. Variations of this technique have been used with success for many years, most notably the MRIT method. This involves replicating the function of the CCL with synthetic material, such as hard nylon or Kevlar type fiber. The most common methods involve securing the synthetic material between the femur and tibia using either the fabella (a small bone behind the knee) or a bone tunnel or screw anchor in the femur, and a bone tunnel drilled in the tibia. This requires a small incision on the side of the knee. The MRIT is a very effective method, however it requires diligent activity restriction for up to 6 months after surgery.
The TPLO method involves cutting a curved segment of bone from the weight bearing portion of the Tibia (shin bone), rotating it forward, and securing it back together with a bone plate. This is done through an incision into the side of the knee. The goal is to create a level platform within the knee which stabilizes drawer motion. Scientifically the TPLO causes a change in the contact mechanics of the knee, placing a higher load on the cartilage and on the patellar tendon than with other methods.
The TTA method involves cutting a segment of bone from a non weight bearing area in the front of the tibia, moving it forward, and securing it with a special plate and block. It, too, is accomplished through an incision on the side of the knee. This procedure causes a change in the force exerted by the muscles of the thigh, allowing the patient to dynamically stabilize the knee with muscle control. Current studies are showing comparable results and better contact mechanics than the TPLO.
When the TPLO was developed, surgeons were taught to perform a meniscal release to minimize the chance of postliminary (after surgery) meniscal tears. However, recent scientific evidence shows that meniscal release can actually produce more problems, and many surgeons are moving away from performing releases. The current evidence shows that we should carefully evaluate meniscal injuries and debride or repair them as they are discovered, thus preserving as much normal meniscal tissue as possible. The arthroscope provides much better ability to detect and treat meniscal injuries than conventional arthrotomies.
Prognosis and Complications
The prognosis for postoperative outcome is good to excellent in 90 to 95% of patients operated with either the MRIT, TPLO, or TTA. With the tissue graft methods, outcomes are worse, thus these techniques are not commonly used. Regardless of the surgical technique, all dogs will have some degree of long term arthritic changes in the knee. Slender, athletic patients who avoid explosive movements have the best outcomes in general. Overweight patients tend to have more long term arthritis. However, early surgical repair minimizes these degenerative changes, and restores good long term function.
Studies have shown that complication rates for MRIT, TPLO, and TTA are similar. The main concern with MRIT method is early disruption of the synthetic material. This has been shown to be more of a concern with young active patients, and overweight patients. Because of this, patient selection and activity restriction (up to 6 months) are critical when performing an MRIT repair. TPLO and TTA procedures tend to require a shorter period of activity restriction (6 to 10 weeks), as the bone heals the repair becomes very solid. The TPLO can be associated with thickening of the patellar tendon, and the TTA may produce a slightly quicker return to function. Both the TTA and TPLO produce similar outcomes when analyzed with force plate studies. Patient selection for TTA or TPLO are similar, with a few exceptions beyond the scope of this discussion (concurrent luxating patellas, excessive tibial plateau angles, etc).
Surgeons enjoy being on the forefront of medicine, performing the newest techniques. However, we must temper our enthusiasm by using scientific methods to base our decisions. Very clear scientific evidence supports arthroscopy as the method of choice for knee surgery compared to arthrotomy. The evidence for stabilization method is not as clear. The MRIT has a long track record; however it can have problems in highly active dogs. Many surgeons recommend the TPLO or the TTA, however, this choice is often based on surgeon preference. I prefer the TTA over the TPLO due to the less invasive nature of the procedure. The surgical methods and techniques are being continually refined, and as more studies become available we may find clear evidence favoring one procedure. At this time, based on current scientific evidence we perform arthroscopic treatment within the knee. We perform either the MRIT stabilization or the TTA stabilization based on the patient. The TTA is recommended for larger more active dogs, and dogs that are more excitable or overweight. The MRIT is reserved for smaller, less active dogs and cats.