The cranial cruciate ligament is the primary stabilizer of the canine knee. This ligament prevents cranial translation of the tibia and internal rotation of the tibia. Dogs undergo degeneration of their ligament (mucinous degeneration) for reasons we do not understand. This degeneration leads to rupture or tearing of the ligament. Less important stabilizers of the knee are C-shaped cartilage structures called menisci. Once the cranial cruciate ligament is torn, the medial meniscus is damaged over time by the abnormal motion of cranial translation of the tibia or drawer.
The cranial cruciate ligament has two major bands, a caudolateral band, and a craniomedial band. Dogs can tear one or both bands. Once the ligament is torn it will not heal. Many dogs may also have a torn medial meniscus dependent on the length of the injury. Again if the meniscus is torn it will not heal.
Over time, in an attempt to stabilize the knee, dogs will develop what we refer to as medial buttress. This is an accumulation of fibrous tissue on the inside aspect of the knee. It spans the joint in an attempt to stop abnormal motion. Unfortunately, because this fibrous tissue is continuously stretched it cannot adequately stabilize a knee.
Once the cranial cruciate has torn, this abnormality will lead to the development of arthritis. While we cannot stop the arthritis from forming secondary to these joint abnormalities we can stop the painful aspect of this injury with an extracapsular stabilization procedure.
The surgery we perform to stabilize the knee involves joint exploration where the remaining cruciate ligament is excised and both the menisci are evaluated. If the medial meniscus is intact we perform a meniscal release to try and eliminate future impingement and tearing. If the medial meniscus is torn we remove it as it contains pain fibers and is an ongoing source of pain.
The TightRope is a vast improvement in the implant utilized in the MRIT or Lateral Suture procedure. The TightRope implant was developed several years ago by Arthrex. The procedure does not require an osteotomy(cutting the bone) as does the TPLO or TTA procedure. This implant only requires small bone tunnels in both the distal femur and proximal tibia. The implant is placed in the best isometric position through the tunnels and secured by placing buttons against the bone, knotting one side of the implant. Your goal is to achieve a joint without drawer or instability.
The implant is a FiberTape or Kevlar like material which is much stronger than monofilament nylon. This type of implant has been utilized in human surgery for quite some time.
This is an option for large breed dogs that have a large enough femoral and tibial bone to have a 3.5mm bone tunnel. The strength of the implant means they are unlikely to stretch or break the nylon. The implant also comes in a smaller size for younger smaller dogs. Small dogs must have a large enough femoral and tibial bone to have a 2.7mm bone tunnel.
Cranial cruciate ligament injury leads to a cascade of events including progressive osteoarthritis and medial meniscal tears. The instability results in synovitis (inflammation of the joint capsule), articular cartilage degeneration, periarticular osteophyte formation and capsular fibrosis (arthritis).
Progressive osteoarthritis continues even after stabilization of the knee regardless of the procedure used for stabilization. There are no studies supporting one method over another with respect to the progression of arthritis.
The most common complications are swelling at the incision site, seroma along the incision, premature staple removal by dog. These complications are fairly easily dealt with. If the dog removes his staples the incision should be flushed and closed and he should be placed on antibiotics. Swelling can be minimized by icing the incision 3 times a day. Seroma is a benign build up of fluid in the space between the skin and the fascia. Seromas tend to be self limiting but if they are very big they should be drained and the dog should be placed in a bandage. The concern with draining them is that you can introduce bacteria into the surgical site. Most of the time hot packing and time resolves the problem.
Infection is a potential problem with any surgery. It can occur for no reason just because of having surgery this happens in less than 1% of the cases. Infection most often occurs secondary to licking or chewing at the incision. This can be minimized by the use of an e-collar. If the infection is severe surgery may be necessary to flush the joint and obtain cultures.
The most concerning problem is implant failure and migration. Reported complications are suture failure either by elongation, failure at the fabella or breakage. The most common reason for these types of complications is an event that traumatizes the surgical repair, overactivity, falling, running, jumping ect. They have however been reported not in association with a traumatic event. These complications require revision surgeries to stabilize and repair damage.
Meniscal tears are fairly frequent in association with cranial cruciate tears. If the meniscus is not torn we release it from its caudal attachment. Subsequent meniscal tears occur in 6.3% of the cases.