The knee, at first glance, seems like a pretty simple thing. It’s a joint that, unlike the hips or the neck, only moves along one axis. A couple of bones come together and swing back and forth. Simple, right?

Wrong. Given that even a single cell in the body is almost unfathomably complex, it should come as no surprise that a structure such as the knee (which is called the stifle in medical terms) is a complicated one, and that any slight alteration or issue with the structure can lead to problems. Take the knee cap (patella), for instance. A simple joint does not require a special bone like the patella whose function is to help smooth out its motion.

So, indeed, the knee is complex. It also is one of the main workhorse joints of the body. Knees undergo marked stress in the course of daily life and especially exercise. Knee injuries are quite common in people and dogs.

In both species, a series of structures called ligaments are especially prone to injury. The most common ligament injury in knees involves a special ligament called the cruciate ligament.

Knees have two cruciate ligaments. In dogs they’re called the cranial and caudal cruciate ligaments. In humans the terms anterior and posterior are substituted for cranial and caudal respectively. Cranial cruciate ligament injury is the most common significant knee injury in dogs.

Cruciate ligaments help the knee to remain stable. The cranial cruciate ligament prevents the tibia and fibula (the large bones of the lower leg) from moving cranially (forward) when the knee is in motion. It is a structure that is under nearly continuous strain, and it is highly prone to failure.

Humans most frequently suffer cruciate ligament injuries when they experience sudden severe trauma. A serious football tackle or a ski fall in which the binding does not release can strain the knee and tear the ligament.

Dogs sometimes suffer cruciate ligament injuries in the same manner. Some dogs jump up for a frisbee and land wrong; others may be tackled by larger dogs. In either case, the dog is likely to vocalize and stop using the affected leg immediately.

However, cruciate ligament injuries in dogs also occur frequently as chronic issues. The cranial cruciate ligament may degenerate over time, and may suffer from repeated partial tears. These small injuries to the ligament can sometimes progress suddenly to a complete tear of the ligament.

Limping on the affected limb is the most common symptom of cruciate ligament injury. The knee may become swollen, and over time may develop a knobby irregularity on its inside aspect called a medial buttress. The muscles of the limb may shrink due to reduced use.

There are several risk factors for cruciate ligament injury. Obesity, which places extra strain on the ligament, is a leading one. Genetics likely play a role. Large breeds, especially those with long legs, are predisposed. Early spaying and neutering have been implicated. Dogs who suffer cruciate ligament injury in one knee are at significant risk of injury to the other.

The rupture of a cruciate ligament unfortunately sets in motion a cascade of events that leads very often to arthritis in the joint. Force plate analysis of limbs that suffer cruciate ligament injuries show that dogs often chronically favor them.

Cruciate ligament injuries are usually diagnosed through a combination of radiographs (X-rays) and physical exam findings. Dogs may be sedated in order to assess for irregular movement (so-called laxity) in the knee when they are very relaxed.

The treatment of cruciate ligament injuries has been the subject of no small amount of debate in the veterinary world over the years. In many cases of partially torn ligaments, activity restriction, weight loss (if applicable), and pain killers are all that are recommended. Although it’s not recommended, I have seen some dogs with complete ruptures return to good use of the leg with this protocol as well (this has happened when the owners could not afford surgery; evidently scarring in the joint helps to stabilize it).

Surgery is generally recommended for dogs with complete tears of the cranial cruciate ligament. Over the years many different surgical tactics have been explored. One of them, the tibial plateau leveling osteotomy (TPLO), has gradually become the most favored.

Early surgical interventions sought to replace the ruptured ligament with sutures of varying materials. These techniques did not stand the test of time and were prone to poor outcomes. Another technique, called lateral fabellar suturing, works to stabilize the knee by tightening a structure called the joint capsule. This technique has generally been quite successful in helping dogs use the affected limb in a satisfactory fashion.

The TPLO technique works by way of a different principle. The bones below the knee are cut and then rotated to new angles using a special bone plate. The technique met substantial resistance at its outset, in part because of its unconventional nature, and in part because the inventor of the technique did something highly unusual in the world of veterinary medicine at the time: He sought to make a profit off of his idea.

Some studies (such as this one) of TPLOs implicated the technique with higher rates of complications than other surgical procedures. However, a more recent study indicated that TPLOs in general lead to better outcomes as measured by force plate analysis and owner satisfaction surveys.

Although the pendulum may in time swing in another direction, the TPLO procedure has been steadily gaining traction and also gaining a reputation as the best treatment for dogs with injuries to their cruciate ligaments. I hope that your dog never suffers such an injury. But if she does, be sure to talk about the various available treatment options with your vet.

Has your dog had a cruciate ligament injury? How did you deal with it?

Learn more about dogs with Dogster:

• The 10 Biggest Misconceptions About Guide Dogs for the Blind
• 6 Things to Remember When You Have a Fearful Dog
• Four Things You Should Know About Your Dog’s Growl

Got a question for Dr. Barchas? Ask our vet in the comments below and you might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)