How to diagnose ACL tears on X-rays

With experience, you will pick up on subtle changes that can help you diagnose early ACL tears

The left thigh is significantly wider than the right on this VD view. Photos courtesy Phil Zeltzman
The left thigh is significantly wider than the right on this VD view.
Photos courtesy Phil Zeltzman

“You do not need radiographs to diagnose an ACL tear.” While this statement is generally true, since a positive drawer sign is pathognomonic for a torn ACL, radiographs are very useful for several reasons.

  1. They help confirm the diagnosis.
  2. They help rule out other conditions, such as tumors.
  3. They confirm the suspicion when there is no drawer sign or the ACL is partially torn.
  4. To take measurements before a TPLO or a TTA.
  5. To document the degree of DJD.
  6. To image the opposite knee to document changes related to a contralateral ACL tear.

The following are 10 signs you can recognize on radiographs of the stifle.

1) Effusion

One of the early signs of a partially torn ACL is effusion, i.e. fluid accumulation in the stifle joint.

2) Fat pad sign

A direct consequence of stifle effusion is compression of the infrapatellar fat pad.

The fat pad is displaced cranially. Instead of appearing triangular on a lateral radiograph, it appears flattened craniocaudally. This is called the fat pad sign.

This is one of the most classic signs of a tear; therefore, this is the reason X-rays are so important as part of your work-up of a hind limb lameness.

3) Caudal capsule

A direct consequence of stifle effusion is caudal displacement of the joint capsule. Other ways to describe include:

  • Caudal displacement of the gastrocnemius facial plane
  • Caudal fascial strip due to displacement of caudal joint capsule
  • Synovial distention

4) Osteophytes

Osteophytes are outgrowths of bone at the margin of the articular surface as a response to joint stress.

Depending on the degree of chronicity, osteoarthritic changes can appear in multiple locations of all bones involved in the stifle joint: tibia, patella, femur, and fabellae.

On a lateral view of the stifle, locations can include:

  • Femoral trochlear ridges (peritrochlear osteophytes)
  • Apex and base of the patella (peripatellar osteophytes)
  • Fabellae
  • Cranial and caudal tibial plateau

On a ventro-dorsal view of the stifle, locations can include:

  • Femoral epicondyles
  • medial and lateral tibial plateau regions
  • Fabellae

5) Medial buttress

The medial buttress, a palpable and visible bulge on the medial aspect of the stifle, is a sign of chronicity. It is due to thickening of the medial collateral ligament (and not the joint capsule).

It can be visible on a ventrodorsal view of the stifle.

6) Cranial tibial thrust

In a standard lateral “TPLO view,” i.e. the classic 90-90 position (90 degrees of flexion in both joints), you can sometimes see a black and white demonstration of the cranial tibial thrust.

This happens when there is cranial displacement of the proximal tibia relative to the distal femur.

Also called the Cazieux sign, this subluxated position is similar to what you would do to perform the tibial compression stress.

7) Devil’s horns

This is not an officially or universally recognized sign. It stems from the observation that degenerative joint disease secondary to a torn ACL may lead to osteophytes at the lateral and medial margins of the tibial plateau on a VD view.

The osteophytes may look sharp and spiky, leading to the appearance of one or two “devil’s horns.”

8) Popliteal sesamoid

This is a subtle, but very useful sign, to pay attention to. In a 1999 article, (JSAP Vol 40, N 7), de Rooster and van Bree write:

“Distal displacement of the popliteal sesamoid is a useful parameter in the interpretation of tibial compression radiographs in cases of cranial cruciate ligament rupture in the dog. An accuracy of 99 percent and a specificity of 100 percent were achieved by assessing the localization of the sesamoid bone in the diagnosis of cruciate disease.”

9) Muscle atrophy

Ideally on a symmetrical ventrodorsal radiograph of the pelvis and thighs (OFA style), you can measure the width of the thigh muscle. In cases of subtle or obscure lameness, you can visualize muscle atrophy in one thigh, which confirms disuse of a leg.

It is not pathognomonic for an ACL tear, but it does confirm abnormal leg use. Of course, this can also be measure during the orthopedic exam.

10) Sclerosis

Careful observation of the subchondral bone, mostly on the tibial plateau, can reveal sclerosis. It is even more obvious by comparing one affected stifle to a contralateral normal stifle.

The list above is not exhaustive. There can be other changes such as enthesophytes, mineralization of the menisci, joint mice, subchondral bone cysts, etc.

The 10 changes are rarely all noticeable on a single stifle. With experience, though, you will pick up on subtle changes that can help you diagnose early ACL tears.

If in doubt, you can take two views of the opposite stifle, and compare one knee to the other.

Once you have diagnosed a torn ACL, the next step is to recommend the best procedure for your particular patient.

Phil Zeltzman, DVM, DACVS, CVJ, Fear Free Certified, is a board-certified veterinary surgeon and serial entrepreneur whose traveling surgery practice takes him all over Eastern Pennsylvania and Western New Jersey. He also is cofounder of Veterinary Financial Summit, an online community and conference dedicated to personal and practice finance ( Zee Mahmood, a practitioner in Austin, Texas, contributed to this article.

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