One Thing Leads To Another: Why The Second ACL Often Goes Too

by Susan E. Davis, PT

Have you ever wondered about the impact of the improper use of an injured limb on the rest of the body and how other areas are affected?

The canine knee or “stifle” immediately comes to mind as it is the most problematic joint in the dog.

Source: The Dog Health Handbook
It is a complex joint starting with the rounded shape at the end (like knobs) of the femur, the plateau of the tibia, the patella, meniscus, cruciate and collateral ligaments, tendons, joint capsule.

Because of this, some veterinarians and therapists embrace the concept of this joint being an “organ”.

Here’s how the structures work:  the capsule protects the joint surfaces by being a mechanical barrier, providing blood supply and nutrition to the joint and has a lubricating fluid which gives viscosity for the joint to work. Ligaments provide support to the capsule and “bind” the joint together. The patella or “kneecap” rides in a smooth canal formed a valley in the femur and provides a fulcrum for mechanical advantage to the quadriceps muscles when they contract. The menisci or shock absorbers are triangular wedge-shaped cartilage structures that transfer stress off the joint surfaces.

Now let’s examine what happens and why things can go wrong with the stifle.

The cranial cruciate ligament (CCL), also often referred to as ACL, confronts unique biomechanical forces in canine “down on all fours” gait compared to upright 2-legged human gait. The human ACL tears more acutely, mainly from trauma.

The canine CCL tears from repeated movements, non-trauma-related and acute tears are less common. 

This type of tear is usually a slow gradual degradation followed by complete rupture. It is not only due to different forces on stifle during canine gait but also from the shape and angle of the canine tibial plateau.

In humans, the tibial plateau is essentially flat and sits in a level transverse plane almost parallel to the ground.

In dogs, the plateau is more sloping in shape and sits in a plane at an acute angle to the ground. 

Therefore gravity affects the canine CCL differently with more shearing forces on it during “walking on all fours” than on the human ACL in upright gait.  Now you can see why “TPLO” surgery stands for Tibial Plateau LEVELING Osteotomy!

Once the stability of the CCL becomes compromised, the stifle joint is lax (or loose) and the menisci can tear or degenerate because of repeated stress from the laxity.

Patellar luxation in a medial or inward direction can also develop.  With these problems, the first outward sign is non-weight bearing (NWB) or partial (PWB) lameness in the affected limb.  In turn, this causes increased weight on the other “sound” limbs, both front, and back.

“Overuse” syndrome can then occur, leading to inflammation, irritation, and crepitus (sounding “crunchy”) during passive range of motion.

Inflammation left untreated causes tissue damage by dilation of the blood vessels with the release of histamine, activation of enzymes with the release of free radicals that digest matrix and contribute to loss of cartilage and formation of unwanted scar tissue.  This can progress into arthritis, or degenerative joint disease (DJD).

That is why dogs with a ruptured CCL have a 20-40 % chance of tearing the other side!

The incidence is also variable by obesity and by certain breeds that have some underlying genetic predisposition, but it is not just the large breeds!  There is a correlation between body structure—“straighter leg” dogs of both large and small breeds have more chance of CCL tears than “bowed”.

Breeds with straighter bones have more direct force on the ligament as opposed to breeds with more bowed bones like a bulldog, dachshund or corgi.  

If the injured CCL goes untreated or has surgery and takes longer to recover, the more likely the other side may “wear and tear” beyond the 40%. If the injured side becomes arthritic there is a 60-70% incidence the other side will tear within 12-16 months (ref:  Doverspike et al, JAAHA.)

The key to prevention of “other side” tears is in the TIMING:  early diagnosis and surgical correction for complete tears, early post-op rehab, healthy diet and ideal body weight.

Otherwise, excessive weight shifted onto the sound leg will increase the possibility of that CCL tearing. If the tear is just partial and you see only intermittent lameness, surgery may not be needed but PT with modalities such as laser, functional electrical stimulation and massage can be of great benefit, along with other treatments advised by your veterinarian. Further information on these modalities will be provided in a future post.


Susan E. Davis (Sue) is a licensed Physical Therapist with over 30 years of practice in the human field, who transitioned into the animal world after taking courses at the UT Canine Rehabilitation program.  She is located in Red Bank, New Jersey.

She has been providing PT services to dogs and other animals through her entity Joycare Onsite, LLC in pet’s homes and in vet clinics since 2008.

She also provides pro bono services at the Monmouth County SPCA in Eatontown, NJ.  Sue is the proud “dog mommy” to Penelope, a miniature Dachshund with “attitude”.  For more information see her website, or follow on Twitter @animalPTsue.

Sue is also the author of a fantastic book on physical therapy, Physical Therapy And Rehabilitation For Animals: A Guide For The Consumer.  

Physical therapy can do so many great things for your dog. Understanding all the possibilities physical therapy can offer will change your dog's life. This book definitely belongs on the shelf of every dog lover.

Articles by Susan E. Davis:
Functional Strengthening Exercises: the What, Why and How
One Thing Leads To Another: Why The Second ACL Often Goes Too
Compensation: An Attempt To Restore Harmony
Paring Down to the Canine Core
Canine Massage: Every Dog ‘Kneads’ It”
Photon Power: Can Laser Therapy Help Your Dog?  
Physical Therapy in the Veterinary World  
Reiki: Is it real? 
Dog Lessons: Cooper  
The Essentials Of Canine Injury Prevention: 7 Tips For Keeping Your Dog Safer 
It's Not Just Walking, It's Therapy! 
Treatment And Prevention Of Canine Intervertebral Disc Disease (Part I)
Treatment And Prevention Of Canine Intervertebral Disc Disease (Part II Physical Therapy)
Range Of Motion: It’s A Matter Of Degree…
The Weight Of Water And How It Helps Dogs 
By Land or By Sea? A Comparison of Canine Treadmills 
Unraveling The Mystery Of Fascia And Myofascial Trigger Points (Part I)
Unraveling The Mystery Of Fascia And Myofascial Trigger Points (Part II) 
Scar Tissue: Is it Too Much of a Good Thing? 
Physical Therapy Tip Of The Month: Ramps! 
Physical Therapy Tip Of The Month: Indoor Duo Dog Exercises!
Physical Therapy Tip Of The Month: Best Practices After Your Dog’s Surgery 
Physical Therapy Tip Of The Month: Ideas to Chew on - Can Physical Therapy Help with my Dog’s Digestive Problems? 

Related articles:
Talk To Me About ACL Injuries
ACL Injuries in Dogs: Non-Surgical Alternatives?
ACL Injuries in Dogs and Stem Cell Regenerative Therapy
Newest Surgery For Ruptured ACL In Dogs
Preventing ACL Injuries In Dogs
ACL Injuries In Dogs: Xena's Story 
ACL Injury Conservative Management: Sandy's Story
Surviving The Post-Op: After Your Dog's ACL Surgery
Talk to Me About Arthritis
Don't Forget the Physical Therapy 
My Love Is Sleeping At My Feet: ACL Surgery Complications 
Coco's TPLO Post-Op Diary 
Small Breeds Can Hurt Their ACL Too: Star's Naughty Knee


  1. Great article. Tucker's blew his at age 5. 5 years later, the other. You never know. He was a mixed breed dog with probably Chow, Lab and ? Never overweight, he was quite athletic.

    1. Yes, there are a number of reasons why the cruciate goes. How did you treat it? Want to share Tucker's story here?


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