The cranial cruciate ligament (CCL) is an important stabilizing ligament of the knee joint. Specifically, it prevents forward thrusting of the tibia whenever the animal steps. This tendency for forward thrusting of the tibia is a natural process due to the distribution of tendon forces exerting tension in the area. Rupture of this ligament leads to acute, severe instability of the knee joint. This injury occurs most commonly in medium to large breed dogs, but is seen occasionally in small dogs and cats. Predisposing factors that may lead to this injury are:
1.)High performance dogs, such as agility competitors.
3.)Sedentary lifestyle with only occasional periods of play.
4.)Genetically inherited steep tibial plateau angles sometimes seen in large breed dogs (more on this below).
Specifically, the affected dog or cat presents at first with toe-touching to non-weight bearing lameness. There is often significant swelling of the affected knee joint, with moderate to severe pain when the joint is manipulated. On physical examination, dogs and cats that have ruptured their cranial cruciate ligament have what is called a cranial drawer sign. This is when the femur (thigh bone) is held stationary, and the veterinarian can pull the tibia (chin bone) forward. This should not occur in a normal knee joint.
Left untreated, the instablility of the knee causes chronic pain, arthritis, and imminent degenerative joint disease. Treatment is surgical stabilization of the knee joint.
There are many different surgical procedures utilized for cranial cruciate ligament rupture. In the past generally accepted notion was that bone cutting techniques, such as tibial plateau leveling osteotomy (TPLO) or tibial tuberosity advancement (TTA), edge out other popular surgical techniques where stabilization is provided outside the joint capsule with a suture (extracapsular), in terms of overall effectiveness. It was generally accepted that the difference in effectiveness of TPLO or TTA over extracapsular suture was most evident with larger breed dogs 50 pounds or greater. However, the combination of the advent of a superior extracapsular suture technique called the tightrope, and surgical comparisons having been subject to extensive study, we now know differently.
The main difference in effectiveness of the bone cutting techniques versus extracapsular suture in the end has little to do with the patient’s size, but has more to do with the angle of the tibial plateau (top of the shin bone). Dogs with excessively steep tibial plateau angles of greater than 32 degrees tend to do somewhat better with bone cutting techniques. The reasons for this are the following:
1.)Steep tibial plateau angles predispose to CCL rupture in the first place due to the resultant pull the steep angles place on the CCL ligament. TPLO or TTA stabilize the knee by reducing excessively steep tibial plateau angles, changing the physics of the knee such that the joint no longer relies on the CCL ligament. This will be further clarified below as each procedure is described in detail.
2.)If a CCL rupture in a knee with excessive tibial plateau angle is repaired with an extracapsular suture, with the steepness of the tibial plateau always having the tendency pull on the suture, over time, this could cause the suture to stretch and lose some stabilization over time. This will also become more clear once each procedure is described below.
In patients with tibial angles of less than 32 degrees, however, I favor the extracapsular tightrope procedure for these reasons:
1.)Surgery is less invasive with no cutting of bone or plating necessary.
2.)Expense of surgery is less due to equipment considerations with TPLO or TTA.
3.)Tightrope carries a 5% less threat of complication than TPLO and TTA.
4.)In patients with tibial angle of less than 32 degrees, no studies have shown any better prognosis in CCL repair with TPLO or TTA in comparison with tightrope CCL repair, regardless of the size of the patient. In fact, statistics actually indicate that tightrope CCL carriers a higher prevalence of good to excellent prognosis in these patients, in comparison with TPLO or TTA.
In the end, the choice of procedure has nothing to do with the size of the patient, but the angle of the tibial plateau in the affected patient. Therefore, the most crucial part of a work up of a CCL rupture patient, is sedating the patient for proper positioning of x-rays of the affected knee joint so that tibial plateau angle can be measured. Tibial plateau angles of greater than 32 degrees, I generally recommend the owner consider TPLO or TTA. Less than 32 degrees, my recommendation is typically extracapsular tightrope.
Before explaining each of the aforementioned surgical CCL repair techniques, let me first discuss elements of CCL surgery that are common to all techniques. The first consideration before applying any fixation repair is to explore the knee. Any bone spurs present that may have resulted from degenerative changes that occurred from time of injury to repair, are cleaned out. Next, the cartilage of the inner section of the knee (called the medial meniscus) is inspected for concurrent injury. Statistical data indicates that 60% of CCL ruptures also have a concurrent tear of the medial meniscus. My own experience reflects this. If the medial meniscus is torn, that is repaired prior to fixation.
In the tightrope procedure, the knee is stabilized with a soft but incredibly strong (250 pounds of tensile strength) suture called fiber tape. This procedure is best illustrated in the picture below:
In TPLO, The philosophy behind the surgery is to change the angle of the Tibial Plateau, which is the area where the Femur and the Tibia interact and communicate. By doing so, this eliminates the need for the ACL ligament entirely and returns stability to the joint itself. In order to accomplish this, a portion of the head of the tibia is cut, rotated, and plated in place. The diagram below shows how this works:
In TTA, like TPLO, the angle of the tibial plateau is also lessened, but is a somewhat different manner. Instead of cutting and rotating the entire tibial head, a smaller portion of front part of the bone called the tibial tuberosity is cut and extended forward. A relevant analogy as to how this works would be to consider a steep ramp. By extending the length of the ramp, one can effectively decrease the steepness of the angle of the ramp. In the same manner, the TTA, decreases the angle of the tibial plateau. The image below provided by Dr. Daniel A. Degner, Board-certified Veterinary Surgeon DACVS (of http://www.vetsurgerycentral.com/ortho_TTA.htm) best illustrates how this works:
In the end, all of these procedures are well accepted means to repair CCL injuires in dogs and cats, regardless of size and physical characteristics. A wise surgical mentor of mine once said, “When it comes down to it, the best CCL repair procedure to choose for any given case, is the procedure the individual surgeon is most comfortable with.” These guidelines are presented to you here in this article to offer you a guide to choosing the procedure you are most comfortable with.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 9/18/2012