The Importance of Restoring Authentic, Evolved Stifle Biomechanics

Body symmetry applies first to the dimensions of the moving parts of a limb. Two limbs being equal in length helps an animal to walk normally. Discrepancies in limb length of a lesser nature make walking more difficult and uncomfortable for the animal.  Discrepancies in limb length of a greater nature would make walking nearly impossible. Body symmetry also unequivocally and strongly applies to the manner in which a limb moves or functions (biomechanics).

The hind limb of a dog is the sum of its parts, and how the entire limb moves overall greatly depends on the manner in which any one given joint moves. If a joint is injured or damaged, the joint cannot function or move as it would normally function and move. The overall “normal” movement of a given limb depends and relies on how harmonious all of the joints of the limb are moving together in a combined, coordinated, and synchronized manner.

When a joint is injured or damaged and cannot function or move as it normally would, this directly impacts and compromises the combined harmonious movement of all the joints within the limb.  Just as injury and dysfunction of a single joint impacts the function and movement of the limb as a whole, the entire limb’s ability to function in a combined and harmonious fashion with the opposite (uninjured) limb is in turn negatively impacted. The resulting discordant and unharmonious movement of the two (hind) limbs affects the animal’s overall ability to move its body in a normal straightforward and effortless manner—the coordination, synchronization, and fluidity of the animal’s gait is adversely affected and bilaterally symmetrical movement is lost.

Just as the hind limb of a dog is the sum of its parts, so is the body of a dog the sum of its parts. The hind limbs are connected to the pelvis, the pelvis is connected to the spine, and so forth. All the moving parts of the body are interconnected and comprise the locomotor system.  The interdependent components of the locomotor system work together to allow for confluent and harmonious movement of the animal, as well as support and stability in the body as a whole. In evolutionary terms, better movement translates to a competitive advantage for survival, and anything that impedes or impairs better movement is disadvantageous.

Bilaterally symmetrical movement is essential for not only allowing better movement but essential for optimizing better movement. Bilaterally symmetrical movement means that both limbs on either side of the body’s axis are anatomically and functionally the same, allowing for better coordination, synchronization, and fluidity of movement —as well as better speed and force of purposeful movement. The loss of bilaterally symmetrical and balanced movement of the hind limbs affects the animal’s ability to move its musculoskeletal system and body as a whole in a smooth, coordinated, and integrated manner. For any animal to have a body that is truly and unambiguously bilaterally symmetrical, all of the joints in both limbs on either side of the body axis must be bilaterally symmetrical in their anatomy and in their function or biomechanics.

The goal of any surgery for the repair of injury to a joint or other parts of the limb should be restoration of the prior anatomy, allowing for the parts to fit together well, and restoration of the prior function so that all the moving parts of the entire limb work in harmony as an interdependent and cohesive unit. After recovery, the surgically repaired limb should ultimately be functioning as a cohesive unit that works very much in the same way it worked before injury and works in the same way the opposite (uninjured) limb continues to work.

Both limbs must be working in the same way—for this to happen each joint must have the same biomechanics as its counterpart on the opposite limb—in order to restore the balanced and symmetrical limb function that has existed previously since birth and has evolved since the beginning of time. Balanced and symmetrical function of all the joints comprising both limbs serves to provide and maximize fluid and harmonious movement and to minimize discordant opposing frictional forces that are generated from abnormal and asymmetrical movement. Surgically restoring balanced and symmetrical joint and limb function allows for harmonious and fluid movement of not only the opposite limbs on the body’s axis, but for confluent movement of the entire musculoskeletal system and body overall and as a whole.

There have been research studies that link limb biomechanical asymmetry with greater risk of injury. A surgical repair of an injured joint that does not restore the same manner of function to the joint, but instead creates an entirely new and different way of functioning, cannot provide balanced, harmonious, and symmetrical movement between the surgically repaired limb and the normal (uninjured) opposite limb. As if oblivious to the injury and surgery sustained by the contralateral limb, the uninjured limb continues to function in the very same manner it has always functioned—and the same manner that the injured limb used to function prior to injury.

When the two hind limbs are functioning in two completely different ways, harmonious and fluid movement is not possible. This newly established dichotomy results in asymmetrical and discordant movement generating frictional forces and physical stresses on all the moving parts—the bones, joints, ligaments, muscles, and tendons—of both hind limbs. In turn, these frictional forces and resulting physical stresses on the moving parts of both hind limbs can manifest as problems elsewhere in the body’s musculoskeletal system. The greater the dichotomy in anatomy and/or function, the greater the abnormal forces and physical stresses that will be generated with movement.

NAHAH’s MFLS surgical services use the cranial cruciate ligament itself as the most ideal model for providing both stability and natural movement for the stifle joint. Surgical procedures that do not restore the original and natural stifle joint function, but create a new and different way for the surgically repaired stifle joint to work, leave the patient with an obvious dichotomy in stifle joint function and so inherently a dichotomy in hind limb function. The patient has a surgically repaired limb that works in an entirely new and different way, and an opposite (uninjured) limb that has not been changed at all and is still functioning the way it has always functioned since birth—with authentic evolved stifle biomechanics. The resulting constant asymmetrical and discordant movement generates significant frictional forces and in turn physical stresses on all the moving parts and this has consequences over time.

There are pre-existing and progressive degenerative changes in the CrCLs of most dogs prior to tearing a CrCL, and these degenerative changes are a contributing factor in most CrCL tears, including opposite limb CrCL tears. Progressive degenerative changes are often a contributing factor, however, all ligaments and tendons in all mammals undergo age-associated degenerative changes as the animal grows older. While degenerative changes in the CrCL are one contributing factor in CrCL tears, degenerative changes in the CrCL appear to be a much less significant contributing factor when it applies to opposite limb CrCL tears in the same patient. What appears to be the contributing factor of much greater significance is the failure to restore authentic evolved stifle biomechanics with the surgical repair of the first or original CrCL injury. If authentic and evolved stifle biomechanics are not restored surgically and the natural original balanced and symmetrical hind limb function is not re-established, the resulting unharmonious movement that prevails creates discordant frictional forces in the hind limbs that may contribute more substantially to the tearing of the opposite limb’s CrCL than pre-existing degenerative changes that may be present.

When the CrCL tears, the injured stifle joint becomes unstable and the joint no longer functions properly or the way it used to function. After the injury, the injured and dysfunctional stifle joint is then functioning in a new and unnatural manner. The injured stifle does not and cannot function the same way it used to function, and so can no longer function symmetrically and in balance with the way the opposite limb stifle joint is still functioning.  With injury itself comes dichotomy in function—once injury occurs, there now exists a clear dichotomy in how the stifle joints are functioning in relation to each other. After injury the other (uninjured) stifle joint is working fine and still working the way it has always worked and has evolved to work – while the injured stifle joint is unstable and dysfunctional, and now working—or not working—in an entirely new and abnormal way. Normal stifle joint biomechanics have been altered by injury and cease to exist, and the injured stifle joint’s function has morphed into a very unstable and irregular movement that without the CrCL is mechanically modified and further altered and improvised step-by-step in response to pain.  This new manner of functioning through injury is completely different from normal. The patient’s natural, balanced, and bilaterally symmetrical stifle function has been compromised by injury and so no longer exists.

Anything that compromises or alters natural, balanced, and bilaterally symmetrical stifle function and results in a significant dichotomy in how one stifle joint works in relation to the other stifle joint—be that injury itself, or surgery to repair injury that does not restore the natural balanced and symmetrical stifle function present prior to injury— may significantly contribute to unharmonious movement and discordant frictional forces and may ultimately contribute significantly to tearing of the opposite limb’s CrCL. Alteration of bilaterally symmetrical and balanced stifle joint function in and of itself, whether the bilaterally symmetrical and balanced stifle joint function is altered by injury or by subsequent surgery (i.e., not restored by surgery), and the resulting dichotomy in stifle joint function is likely a significant contributing factor to CrCL tears in the patient’s opposite hind limb.

The importance of restoring natural authentic evolved stifle biomechanics to re-establish and preserve natural balanced and bilaterally symmetrical hind limb function cannot be over-emphasized. If nothing is done surgically to correct the asymmetrical stifle function that exists after injury or the surgical repair procedure fails to correct this asymmetrical function and creates greater dichotomy by establishing an entirely new manner of stifle joint function, the percentage of patients that will suffer an opposite limb CrCL tear may be negatively and substantially affected and altered to be a much greater number of patients.