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ACL Tears and TPLO Surgery

Torn “ACL’s” in our canine friends (known as cranial cruciate ligament ‘CCL’) is UNBELIEVABLY common, and it occurs for a different reason compared to humans.


  • Based on the dog’s INTERNAL knee biomechanics, they engage their ACL each and every step that they take.

  • Tears are most commonly caused by chronic “wear and tear”

  • Risk of their other knee experiencing an injury to the ACL is ~50%

Human ACL

  • The human’s ACL is engaged when an EXTERNAL force acts upon the joint

  • Tears occur from traumatic EVENTS

Understanding a bit more about the normal canine ACL

This is an X-ray (side view) of a dog’s NORMAL knee

  • The bones are labeled in red

  • The ACL is labeled as the red line

    • Notice the trajectory/path that this ACL takes

  • The joint surfaces are purple.

    • Notice how the bottom of the femur is round to permit flexion and extension of the joint on the flat part of the top of the tibia; however,
      the “flat” part is sloped (like a hill).

    • This also means that there is a shearing/slipping force that occurs during every step, as the “ball” is on a “sloped” hill

  • The force that results from the calf muscles is shown as the blue arrow

  • The primary force that the ACL counteracts during weight-bearing is shown as the yellow arrow

    • This force must be counteracted EVERY step that your dog takes because of their biomechanics – The round joint surface of the femur slips off the sloped joint surface of the tibia as the muscles pull through the joint.

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Understanding a bit more about the ABNORMAL canine ACL

This is an X-ray (side view) of a dog’s ABNORMAL knee. Although we cannot “see” the ACL on X-ray, there are some very important findings that raise our suspicion.

  • Joint effusion (“swelling”) is seen circled in red

    • This is actually excess fluid within the joint, which is the result of inflammation

  • Early evidence of arthritis is seen circled in green

    • This very subtle bright line on our X-ray are “bone spurs” that the joint begins to deposit once it recognizes that it has a serious injury

  • The joint is no longer centered properly, as shown by the two blue lines.

    • These two lines should be aligned; however, since this patient has a torn ACL, the tibia has slipped forward relative to the femur. See the previousexplanation of the “normal” knee to understand where this displacement originated from

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Understanding how the TPLO procedure works

This is a side view (left) and front face (right) X-ray of a dog’s knee AFTER their TPLO procedure.

  • The cut in the bone (“osteotomy”) is seen in red

    • This is a semi- circular saw blade that allows the joint surface to be rotated (left image).

    • This cut is made parallel to the joint in the front face (“frontal”) plane (right image)

  • The joint surface (“plateau”) is seen in purple

    • The plateau is rotated so when your pet bears weight (yellow arrows) and their muscles engage (blue arrows) all the forces are transmitted through the joint and there is no abnormal shearing/sliding motion within their knee joint.


  • The metal bone plate and screws (bright white on the X-rays) are implanted to stabilize the bone in its new position until it has healed.

    • The bone takes approximately 8 weeks to heal, and during that time it is very important to not overload this implant.

    • Once the bone has healed, this implant it no longer active and is just there for the “free ride”.

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