The Hobbled Hound. How to Tell if Your Dog Has a Cruciate Ligament Injury

Introduction

Cruciate ligament injuries are a very common orthopedic problem in dogs, especially in larger breeds. The cranial cruciate ligament (CrCL) is one of the major ligaments that stabilizes the knee or stifle joint. Ruptures of this ligament are the leading cause of hind limb lameness and osteoarthritis in dogs.

Studies estimate that 1-3% of dogs will suffer from a CrCL rupture at some point in their lives, with breeds like Labrador Retrievers, Rottweilers, and Newfoundlands at higher risk. Over $1 billion is spent annually treating CrCL injuries in the United States alone [1].

There are several potential causes for CrCL rupture including obesity, trauma, anatomical factors, aging, and genetics. However, the exact cause is often unknown. CrCL injuries frequently occur in normal activity without any obvious inciting event.

Anatomy

The canine stifle joint is made up of the femur, tibia, patella, and fibula bones. It is a complex hinge joint that allows flexion and extension of the hind limb. The joint is stabilized by various ligaments including the cranial and caudal cruciate ligaments.

The cranial cruciate ligament (CrCL) connects the distal femur to the cranial intercondylar area of the tibia. It prevents excessive internal rotation and hyperextension of the joint (source: https://www.imaios.com/en/vet-anatomy/dog/dog-stifle). The CrCL is critical for stabilizing the stifle and preventing abnormal tibial thrusts. Damage to this ligament is a common cause of hind limb lameness in dogs.

Causes

There are several common causes of cruciate ligament injury in dogs:

Obesity – Carrying excess weight puts increased stress on the knee joint and cruciate ligaments, making injury more likely. According to the Association for Pet Obesity Prevention, over 50% of dogs are estimated to be overweight or obese (Source).

an overweight dog with a knee brace on

Age – Degenerative changes in the knee joint and ligaments occur as dogs get older, weakening the structures and predisposing them to injury. Most cruciate tears occur in middle-aged to older dogs between the ages of 4-8 years (Source).

Trauma – Direct trauma to the knee, such as falling or being hit by a car, can cause acute cruciate tears. Chronic minor trauma from repetitive activities like jumping or pivoting can also accumulate over time and lead to cruciate ligament degeneration and rupture (Source).

Symptoms

Dogs with a cruciate ligament injury often exhibit the following symptoms:

Lameness/limping: One of the most common signs of a cruciate ligament tear is sudden onset lameness or limping, usually in one of the dog’s hind legs. The lameness may worsen after exercise or become less noticeable after resting. According to the Veterinary Teaching Hospital at Colorado State University, the lameness may happen suddenly, often without major trauma. A minor traumatic event can cause a partially torn ligament to rupture completely, resulting in sudden lameness (1).

Difficulty standing up: Dogs with a ruptured cruciate ligament may have trouble standing up from a lying down position. They may be hesitant to put weight on the affected leg when trying to stand.

Swelling: Inflammation and swelling can occur around the knee joint. However, swelling may be minimal or not present in some cases.

Pain: The knee joint is likely to be painful, especially when manipulated. Dogs often resent the knee being touched or flexed.

Diagnostic Tests

There are several diagnostic tests veterinarians use to determine if a dog has a cruciate ligament injury:

Physical Exam

During a physical exam, the veterinarian will palpate the knee joint and surrounding area and perform orthopedic tests like the drawer test and tibial compression test to check for laxity and pain that may indicate an ACL tear (https://vetmedbiosci.colostate.edu/vth/services/orthopedic-medicine/canine-cruciate-ligament-injury/). The veterinarian will also assess the dog’s gait and look for instability, limping, or difficulty bearing weight on the affected leg.

Radiographs

X-rays allow the veterinarian to evaluate the knee joint for signs of osteoarthritis, joint effusion, and bone changes that often accompany cruciate tears (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC339306/). However, x-rays may appear normal in partial tears or early stages of injury.

MRI

An MRI can directly visualize the cruciate ligament and provide the most definitive diagnosis of a complete or partial tear. It can also help assess other soft tissue structures of the knee. However, MRI is more expensive than other diagnostic options.

Palpation

Palpating the knee joint is an important part of diagnosing cruciate ligament injury in dogs. The veterinarian will feel along the knee joint to check for instability, swelling, and pain that could indicate an ACL tear. According to research, “Patellar palpation and radiographic assessment showed excellent sensitivity, specificity, positive predictive value and negative predictive value” for diagnosing cruciate ligament ruptures (Carobbi et al., 2009).

To palpate, the veterinarian will gently manipulate the knee joint and surrounding area while the dog is standing and lying down. Signs of cruciate injury that may be detected during palpation include:

a vet examining a dog's hind leg

  • Joint instability or “drawer sign” – this indicates ligament rupture
  • Joint thickening or swelling
  • Pain when palpating the joint

Comparing both hind knees is important, as swelling or instability in one knee points to an injury. The veterinarian will also check for tenderness of the patella, joint laxity, and fluid in the joint capsule that could signal a cruciate ligament tear (Harasen, 2002). Careful palpation by an experienced veterinarian is crucial for accurately diagnosing cruciate ligament damage in dogs.

Drawer Test

The drawer test is a manual examination used to check for cranial cruciate ligament rupture in dogs. It is considered the most reliable and sensitive test for detecting cruciate ligament tears (Harasen, 2002).

To perform the drawer test:

  • The dog is positioned in lateral recumbency with the hind leg to be tested uppermost.
  • The examiner stands behind the dog and places one hand above the stifle (knee) to stabilize the femur.
  • With the other hand, the examiner cups the toes and slowly pulls the foot and tibia forward while feeling for abnormal cranial tibial movement or “drawer.”

A positive drawer sign indicates cranial tibial translation and cruciate rupture. The stifle should be assessed in extension, slight flexion, and 20-30 degree flexion during the drawer test as ligament tears may be felt at different joint angles (Palmer, n.d.).

performing a drawer test on a dog's hind leg

Comparing drawer movement between limbs will help determine if laxity is abnormal. Greater than 3-5 mm of forward tibial movement suggests complete cranial cruciate tear (Harasen, 2002). The examiner must pull firmly enough to overcome mild muscle tension, but not so hard as to cause pain or elicit a reflex muscle contraction.

While the cranial drawer test is very sensitive for cruciate tears, false positives are possible. Proper patient restraint, joint positioning, and experience performing the technique while assessing results are important.

Tibial Compression Test

The tibial compression test is performed to diagnose cranial cruciate ligament injury in dogs. It can be done with the dog standing or in lateral recumbency with the affected leg up. The stifle joint is flexed to 90 degrees while the tibia is stabilized. Downward pressure is applied to the tibial tuberosity causing a cranial tibial thrust. A positive test is when this causes abnormal cranial translation of the tibia, indicating rupture or damage to the cranial cruciate ligament 1. The test has high sensitivity and specificity for detecting cranial cruciate ligament tears in dogs.

If cranial translation of the tibia is felt during the test, it indicates damage or rupture of the cranial cruciate ligament. A normal response is for the joint to remain stable without abnormal forward movement of the tibia. The amount of cranial thrust can give an indication on partial versus complete tears. Orthopedic specialists recommend this test as the most reliable for suspected cranial cruciate injury.

Radiographs

Radiographs (x-rays) are an important diagnostic tool for confirming cruciate ligament tears in dogs. The vet will take x-rays of the knee joint from multiple angles or views. Common views include the mediolateral, craniocaudal, and skyline patellar views (Veterinary Practice News, 2022).

On the x-rays, the vet is looking for signs of arthritis, effusion or swelling in the joint, and any bone changes. Effusion is one of the earliest signs of a partial cruciate tear, seen as fluid accumulation in the joint (Veterinary Practice News, 2022). The “fat pad sign” may also be present, where the infrapatellar fat pad is displaced from its normal position due to effusion (Veterinary Practice News, 2022).

In chronic tears, the vet will look for osteoarthritis and meniscal calcification. There may also be evidence of a torn meniscus if bone chips are seen in the joint. An experienced vet can make an accurate diagnosis of a complete cruciate tear based on the x-ray findings and physical exam (CSU Veterinary Teaching Hospital, 2022). While x-rays don’t directly visualize the ligament, they show the consequences of a cruciate tear very well.

Treatment

The most common treatment for a ruptured or partially torn cruciate ligament in dogs is surgery. Surgery options include tibial plateau leveling osteotomy (TPLO), tibial tuberosity advancement (TTA), lateral fabellar suture (LFS), and extracapsular repair. These procedures aim to stabilize the stifle joint by altering joint mechanics or replacing the ruptured ligament function.

Most dogs will require 8-12 weeks of exercise restriction and rehabilitation after surgery. This allows the surgical site to heal properly and prevents re-injury. Rehabilitation includes range of motion exercises, strengthening, weight management, and controlled leash walking. The overall prognosis after surgery is good if post-op recommendations are followed.

a dog doing physical therapy after knee surgery

In some cases, small breed dogs may be treated without surgery if they are willing to strictly limit activity for 6-8 weeks. However, surgery is still recommended for large breed dogs or highly active dogs to prevent osteoarthritis. Surgery can minimize long-term lameness and joint changes compared to conservative management. Discuss options with your veterinarian to determine the best treatment plan.

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