How To Confirm Partial ACL Tear

Here are multiple ways to determine if a partial ACL tear has taken place or not.

Few things are as frustrating as trying to diagnose a partial anterior cruciate ligament tear. A dog presenting with hind limb lameness but no obvious drawer sign is a common situation.

Bernard Paré, DVM, Dipl. ACVS, practices at Veterinary Specialty Center in Buffalo Grove, Ill.

We asked surgeon Bernard Paré, DVM, Dipl. ACVS, of Veterinary Specialty Center in Buffalo Grove, Ill., to share tips on how he diagnoses partial ACL tears.

History and Signalment

It starts with the history. Clients may describe mild to moderate on-and-off lameness that has lasted for weeks, months or years. They may interpret the lameness as exercise intolerance. The patient may be stiff in the morning or after rest or exercise.

Patients with bilateral ACL tears may appear to have a neurological problem such as lumbo-sacral disease.

Stifle degenerative joint disease in a 2-year-old male Greyhound.

The signalment may be helpful as well: Most patients are 5 to 7 years old on average. Breeds commonly afflicted with ACL tears include Labradors, Rottweilers, Akitas, mastiffs and St. Bernards. Some breeds, like Akitas, often have hyperextended hocks and stifles, which increases stress on the ACL. Other breeds, like bulldogs, tend to be toed-in. This internal tibial rotation increases stress on the ACL. Being overweight could be a contributing factor.

Orthopedic Exam

The next step is a physical exam and an orthopedic exam with the patient awake. The patient may shift weight onto the “good leg” while standing in the exam room. It is important to differentiate knee pain from hip dysplasia and lumbo-sacral disease. Granted, a patient may have several issues, which can confuse the situation.

One study showed that 32 percent of dogs referred to a surgeon for hip dysplasia treatment had, in fact, a torn ACL (Journal of the American Veterinary Medical Assn., 2005, Vol. 227). Interestingly, 94 percent of those dogs with an ACL tear had concurrent radiographic signs of hip dysplasia.

The exam may reveal mild to moderate lameness and variable degrees of thigh muscle atrophy.

A medial buttress may be palpable at the medial aspect of the knee. The medial buttress represents thickening of the medial joint capsule from chronic instability.

Joint effusion may be palpated along the patellar tendon, medially and laterally. This is typically done with the knee in extension, but our colleague believes that palpating with the stifle in flexion is very helpful as well. The patellar tendon becomes difficult to identify when there is effusion and loses the sharp contour it normally has.

The drawer sign should be tested with the stifle in extension and in flexion. It may be hard to perform this test with the patient awake, especially if you have small hands or the patient is very large. The cranial tibial thrust can, however, be performed in the awake patient in almost all cases.

A favorite secret of Dr. Paré is the stress test and its two variations. Forced internal rotation of the tibia or hyperextension of the stifle may cause enough discomfort to elicit lameness. With an ACL tear, the test will increase or reveal the patient’s lameness.

Another classic observation is the sit test. “Hip dysplasia patients will usually sit square—i.e. symmetrically with both knees flexed—whereas ACL dogs will often have one leg extended because their stifle hurts in flexion,” Paré explains.

Under Sedation

If the diagnosis is still not obvious, the exam should be repeated under sedation.

With light sedation, testing the drawer sign may cause pain and increase the respiratory rate. Under deep sedation or anesthesia, muscle relaxation and analgesia may allow a positive drawer sign. Again, the test is performed both with stifle flexion and extension.

When in doubt, compare the suspicious side to the presumed normal side. You can do it several times until you appreciate the difference between the two knees.

Joint Tap

While the patient is sedated, a joint tap can be performed.

“Arthrocentesis is a very simple, affordable and informative test,” Paré says. In a patient with an ACL tear, the joint tap reveals more joint fluid than normal. It is typically less viscous and straw-colored. With an acute partial ACL rupture, joint effusion is typically present before degenerative joint disease can be observed on radiographs.

The fluid can be submitted for cytology to differentiate an ACL tear from immune-mediated disease or septic arthritis. With an ACL tear, the white blood cell count is less than 5,000 cells/mm3. Most of these cells are mononuclear. This is in contrast with immune-mediated joint disease, where many cells are polymorphonuclear.

Try an X-Ray

Radiographs can be taken with the patient awake or sedated. The lateral radiograph is most useful. With an ACL lesion, an X-ray will reveal stifle effusion, caudal capsular distension, various levels of degenerative joint disease and sometimes calcification of the insertion of the ACL on the tibial plateau.

“Saying that ACL tears are not diagnosed on radiographs is not totally accurate,” Paré says. “Radiological changes can be suggestive of ACL rupture.”

Radiographs can help rule out tumors such as osteosarcoma and synovial cell sarcoma.

On the lateral view, the popliteal sesamoid bone is often displaced distally if tibial compression is performed while the X-ray is being taken.

The antero-posterior view is less useful but can show degenerative joint disease or an osteochondritis dissecans lesion.

Other Options

An MRI may be useful in diagnosing a partial ACL tear. Arthroscopy is another way to study the joint and evaluate lesions of the ACL and the meniscus in a minimally invasive way.


When all else fails, surgery may be the only way to visualize a partial tear. Paré prefers to perform “a lateral arthrotomy if I plan to use fabello-tibial nylon sutures or a medial mini-arthrotomy if I intend to perform a tibial plateau leveling osteotomy.”


Diagnosing a partial tear of the ACL can be frustrating, even for a board-certified surgeon. When in doubt, these tips above may help pinpoint the diagnosis.

It is important not to ignore this condition, which invariably leads to advanced degenerative joint disease and in most cases a complete tear of the ACL.

Phil Zeltzman, DVM, Dipl. ACVS, is a small-animal board-certified surgeon at Valley Central Veterinary Referral Center in Whitehall, Pa. His website is

This column first apeared in the September 2009 issue of Veterinary Practice News


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