Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066





The knee is a fairly complicated joint. It consists of the femur above, the tibia below, the kneecap (or “patella”) in front, and the bean-like fabellae behind. Chunks of cartilage called the medial and lateral menisci fit between the femur and tibia like cushions and there are an assortment of ligaments holding everything together allowing the knee to bend the way it should and keep it from bending the way it shouldn’t.

Forward View of the Canine Knee JointForward View of the Canine Knee Joint
(Photocredit: Public Domain Graphic via Wikimedia Commons)

There are two cruciate ligaments which cross inside the knee joint: the anterior (or, more correctly in animals, “cranial”) cruciate and the posterior (or, more correctly in animals, the “caudal”) cruciate. They connect from one side of the femur on top to the opposite side of the tibia on the bottom, the two ligaments forming an "X" (hence the name "cruciate") inside the knee joint.  They are named for their attachment site on the tibia (the cranial cruciate attaches to the front of the tibia and the caudal cruciate attaches to the back of the tibia).  This may be hard to visualize based on the description but the illustration above shows the orientation of the two crossing ligaments effectively. The anterior/cranial cruciate ligament prevents the tibia from slipping forward out from under the femur.


The ruptured cruciate ligament is the most common knee injury of the dog; in fact, chances are that any dog with a sudden rear leg lameness has a ruptured anterior cruciate ligament rather than something else. The history usually involves a rear leg suddenly so sore that the dog can hardly bear weight on it. If left alone, it will appear to improve over the course of a week or two but the knee will be notably swollen and arthritis will set in quickly. Dogs can be presented in either the acute stage (shortly after the injury) or in the chronic stage (weeks or months later).

The key to the diagnosis of the ruptured cruciate ligament is the demonstration of an abnormal knee motion called a “drawer sign.” It is not possible for a normal knee to show this sign.

Normal Bones and Cruciate
(original graphic by


The veterinarian stabilizes the position of the femur with one hand and manipulates the tibia with the other hand. If the tibia moves forward (like a drawer being opened), the cruciate ligament is ruptured.

Another test that can be used is the “Tibial Compression test” where the veterinarian stabilizes the femur with one hand and flexes the ankle with the other hand. If the ligament is ruptured, again the tibia moves abnormally forward.

If the rupture occurred some time ago, there will be swelling on side of the knee joint that faces the other leg. This is called a “medial buttress” and is a sign that arthritis is well along.

Drawer Sign
The drawer sign is exhibited when the tibia is able to
move forward from underneath the femur
(similar to a drawer opening).
If this action is demonstrated, the cruciate rupture is confirmed.
The motion is exaggerated here for the purposes of illustration.
(original graphic by

It is not unusual for animals to be tense or frightened at the vet’s office. Tense muscles can temporarily stabilize the knee preventing demonstration of the drawer sign during examination. Often sedation is needed to get a good evaluation of the knee. This is especially true with larger dogs. Eliciting a drawer sign can be difficult if the ligament is only partially ruptured so a second opinion with an orthopedic specialist is a good idea if the initial examination is inconclusive.

Since arthritis can set in relatively quickly after a cruciate ligament rupture, radiographs to assess arthritis are helpful. Another reason for radiographs is that occasionally when the cruciate ligament tears, a piece of bone where the ligament attaches to the tibia breaks off as well. This will require repair and the surgeon will need to know about it before beginning surgery. Arthritis present prior to surgery limits the extent of the recovery after surgery though surgery is still needed to slow or even curtail further arthritis development.

(Video Credit: Raintree Veterinary Center via


There are several clinical pictures seen with ruptured cruciate ligaments. One is a young athletic dog playing roughly who takes a bad step and injures the knee while playing. This is usually a very sudden lameness in a young large breed dog.

A recent study identified the following breeds as being particularly at risk for this phenomenon: Neapolitan mastiff, Newfoundland, Akita, St. Bernard, Rottweiler, Labrador retriever, Golden retriever, and Pit Bull terrier.

Neopolitan Mastiff

Neopolitan Mastiff
(Photocredit: Ewa Ziemska via Wikimedia Commons)


(Photocredit: DanDee Shots via Wikimedia Commons)


(Photocredit: Public Domain Image
via Wikimedia Commons

St Bernard mf

Saint Bernard



Labrador Retriever

Labrador Retriever

Golden Retriever

Golden Retriever

Pitt Bull Terrier

Pitt Bull Terrier


On the other hand, an older large dog, especially if overweight, can have weakened ligaments and slowly stretch or partially tear them. The partial rupture may be detected or the problem may not become apparent until the ligament breaks completely. In this type of patient, stepping down off the bed or a small jump can be all it takes to break the ligament. The lameness may be acute but have features of more chronic joint disease or the lameness may simply be a more gradual/chronic problem.

Larger overweight dogs that rupture one cruciate ligament frequently rupture the other one within a year’s time.

An owner should be prepared for another surgery in this time frame.

Poodle(Photocredit: The cranial cruciate rupture is not limited to large breed dogs. Small dogs can certainly rupture their ligaments as well and, while arthritis is slower to set in when the patient is not as heavy, there is an association with cruciate rupture and medial luxating patella that is very common in small breed dogs. With the patellar luxation, the kneecap flips in and out of the patellar groove. If the condition is relatively mild, it may not require surgical correction but it does stress the cranial cruciate ligament and can predispose to rupture and need to correct both conditions surgically.



Normal Xray

(almost) normal
(original graphic by


(original graphic by


Without an intact cruciate ligament, the knee is unstable. Wear between the bones and meniscal cartilage becomes abnormal and the joint begins to develop degenerative changes. Bone spurs called “osteophytes” develop and chronic pain and loss of joint motion result. This process can be arrested or slowed by surgery but cannot be reversed.

  • Osteophytes are evident as soon as 1-3 weeks after the rupture in some patients.

This kind of joint disease is substantially more difficult for a large breed dog to bear though all dogs will ultimately show degenerative changes. Typically, after several weeks from the time of the acute injury, the dog may appear to get better but is not likely to become permanently normal.

  • In one study a group of dogs was studied for 6 months after cruciate rupture. At the end of 6 months, 85% of dogs under 30 lbs of body weight had regained near normal or improved function while only 19% of dogs over 30lbs had regained near normal function. Both groups of dogs required at least 4 months to show maximum improvement.



There are 3 different surgical repair techniques commonly used today. Every surgeon will have their own preference for which technique is best for a given patient's situation.



This procedure represents the traditional surgical repair for the cruciate rupture. It can be performed without specialized equipment and is far less invasive than the newer procedures described below. First, the knee joint is opened and inspected. The torn or partly torn cruciate ligament is removed. Any bone spurs of significant size are bitten away with an instrument called a “rongeur.” If the meniscus is torn, the damaged portion is removed. A large, strong suture is passed around the fabella behind the knee and through a hole drilled in the front of the tibia. This tightens the joint to prevent the drawer motion, effectively taking over the job of the cruciate ligament.

  • Typically, the dog may carry the leg up for a good 2 weeks after surgery but will increase knee use over the next 2 months eventually returning to normal.
  • Typically, the dog will require 8-12 weeks of exercise restriction after surgery (no running, outside on a leash only including the backyard.
  • The suture placed will break 2-12 months after surgery and the dog’s own healed tissue will “hold” the knee.

In the Extra Capsular repair, an implant is used

In the Extra Capsular repair, an implant is used to tighten the joint. Although the implant is outside the joint, its angle mimics the natural cruciate ligament.
(original graphic by



This procedure uses a fresh approach to the biomechanics of the knee joint and was developed with larger breed dogs in mind. The idea is to change the angle at which the femur bears weight on the flat "plateau" of the tibia. The tibia is cut and rotated in such a way that the natural weight-bearing of the dog actually stabilizes the knee joint. As before the knee joint still must be opened and damaged meniscus removed. The cruciate ligament remnants may or may not be removed depending on the degree of damage.

This surgery is complex and involves special training in this specific technique. Many radiographs are necessary to calculate the angle of the osteotomy (the cut in the tibia). This procedure typically costs substantially more than the extracapsular repair as it is more invasive to the joint.

  • Typically, most dogs are touching their toes to the ground by 10 days after surgery although it can take up to 3 weeks.
  • As with other techniques, 8-12 weeks of exercise restriction are needed.
  • Full function is generally achieved 3 to 4 months after surgery and the dog may return to normal activity.

In the TPLO surgery, the plateau (flat top) of the tibia is carved away and repositioned

In the TPLO surgery, the plateau (flat top) of the tibia is
carved away and repositioned to change
the weight bearing angle of the knee to create stability.

(original graphic by



The tibial crest where the patellar ligament attaches is angled
The tibial crest where the patellar ligament attaches is angled in the TTA surgery to create a stable joint.
(original graphic by

The TTA similarly uses the biomechanics of the knee to create stability though this procedure changes the angle of the patellar ligament. This is done by cutting and repositioning the tibial crest where the patellar ligament attaches and implanting a titanium or steel "cage," "fork," and plate as well as bone grafts to stabilize the new angle. Like the TPLO, bone is cut, special equipment is needed including metal implant plates. Similar recoveries are seen relative to the TPLO.


The TTA and TPLO are much more invasive, much more expensive, and require special equipment and specially trained personnel. They have greater potential for complication. Are they worth it? For dogs under 45 lbs, it generally accepted that there is no clear advantage of the newer procedures over extra capsular repair. For larger dogs, there is great controversy. For all the theory behind TPLO and TTA, results one year post-operative seem to be the same regardless of which of the three procedures the dog had performed. There is some evidence that recovery to normal function may be faster with the newer procedures. Controversy continues and there are very strong opinions favoring each of the three procedures. We recommend discussing options with your veterinarian in order to decide.


A dog with arthritis pain from an old cruciate rupture may still benefit from a TPLO surgery and possibly from the TTA. It may be worth having a surgery specialist take a look at the knee. Most cases must make do with medical management. Visit our section on arthritis treatment.



We mentioned the menisci as part of the knee joint. The bones of all joints are capped with cartilage so as provide a slippery surface where the bones contact each other (if the bones contact each other without cartilage, they grind each other down). In addition to these cartilage caps, the stifle joint has two “blocks” of cartilage in-between the bones. These blocks are called the menisci and serve to distribute approximately 65% of the compressive load delivered to the knee. The only other joint with a meniscus is the jaw (tempero-mandibular joint).

When the crucial ligament ruptures, the medial (on the inner side of the knee) meniscus frequently tears and must either be removed, partly removed, or ideally repaired. This is generally done at the time of cruciate ligament surgery and we would be remiss not to mention it.

meniscal cartilage

The meniscal cartilage covers the plateau of the tibia as shown. When the cruciate ligament tears, this cartilage can tear with it.
(Photocredit: Public Domain Graphic via Wikimedia Commons)

Pets with meniscal damage may have an audible clicking sound when they walk or when the knee is examined, but for a definitive diagnosis the menisci must actually be inspected during surgery. It is difficult to access the menisci and thus repairing a tear in the meniscus is problematic; furthermore, poor blood supply to the menisci also makes good healing less likely. For these reasons, removal of the damaged portion of the meniscus is the most common surgical choice. This leaves some meniscus behind to distribute the compression load on the knee but removes the painful, ineffective portion.

Areas of current research include techniques to improve blood supply to the healing meniscus so that repair can be more feasible. If meniscal damage has occurred in a cruciate rupture, arthritis is inevitable and surgery should be considered a palliative procedure.




Enhancing recovery post-operatively is largely about strict confinement early. This cannot be over-emphasized. Be prepared to crate your dog our employ a pen such as a child's playpen depending on the dog's size. A corral of sorts can be constructed with boxes and a baby gate.  Be sure you understand the instructions with regard to gradual return to exercise over several months.

Adequan® Injections

A series of adequan injections can help with joint inflammation as well as lubrication. Typically injections are given twice a week for a total of eight injections. For more information on this medication, click here.


Oral joint supplements such as glucosamine contain cartilage building blocks to help the body repair cartilage damage. This is an excellent time to begin supplementation and there are numerous brands. For more details, click here.

Weight Management

Overweight dogs have an increased risk for arthritis and for cruciate rupture. A weight management program can reduce the potential for arthritis and can reduce the risk of rupture of the opposite cruciate ligament. If your dog is overweight, ask your vet about a weight management plan that might be started during the recovery period.

Professional Rehab/Physical Therapy

Nothing compares to professional rehab for return to function. If you are lucky enough to have such a facility in your area, consider utilizing their services. A list of home exercises may be obtained and/or the dog can visit weekly or a few times weekly for exercise and treatment. Some facilities allow the dog to board and have daily treatment. Ask your vet about this option.

Page last updated: 10/13/2018
Page last reviewed: 3/9/2024