ACL- Function
Title: Factors Contributing to Function of the Knee Joint After Injury or Reconstruction of the Anterior Cruciate Ligament

Reference: AAOS Instructional Course Lectures, Vol 48, 1999

Main Message

Homeostasis is the key to joint function. The injured ACL knee is not normal, and efforts to reconstruct the ligament rarely ever make it “normal”. There are anatomic, kinematic, and physiologic factors that influence joint function, and these somehow balance out to a homeostasis in the normal knee. It is this “cellular homeostasis” that is rarely achieved. Few reconstructed knees are returned to normal, and normal ligaments are not being created surgically.

Points of Interest

The purpose is to discuss the concept of musculoskeletal function and to consider the various factors that contribute to the restoration of knee function after injury or reconstruction of the ACL.

Some studies have shown that the rate of degenerative arthritis is higher in those with ACL reconstructions! One study demonstrated that patients who were treated operatively had more advanced arthritis than those treated nonoperatively who also demonstrated more laxity in their knee than the surgical group at 10 years. (Probably a result of lifestyle differences.) Maybe the only effect of the reconstruction is to allow the athlete to “go back to strenuous sports and ruin the knee.”

Concept of Joint Function: joints are systems that are designed to transmit mechanical loads between components, and yet, by virtue of the fact that they are living structures, to maintain tissue homeostasis over a broad range of physical demands. The knee can thus be viewed as a kind of living, metabolically active, biologic transmission.

Concept of Envelope of Function: a range of loading that is compatible with, and probably inductive of, the overall tissue homeostasis of a given joint. Basically, a load and frequency distribution that defines a safe range of loading for a given joint. ACL injury to the knee redefines this envelope, such that the upper limit is probably not as high as before, but can be improved with rehab, etc.

Factors Contributing to Joint Function

Anatomic Factors: ligaments, tendons, menisci, articular cartilage, muscles, nerves, blood vessels, bone, limb alignment. Our current techniques do not recreate the normal macroanatomy (insertion sites, ligament morphology) or microanatomy (fibril size) of the native ACL. The neuroanatomy is not recreated either, making neuromuscular control different.

Kinematic Factors: tightening characteristics of the ACL fibers, neuromuscular control, muscle strength. Muscle tone influences the stability of the joint. Rehab may improve muscle strength and endurance, but may not restore normal levels of performance because of other factors. Proprioception is a major player.

Physiologic Factors: bone, cartilage, and soft tissue metabolism is altered in acute and chronic ACL knees. This can be documented in bone with bone scanning. Cartilage and soft tissues will respond to the mechanical environment that they are in – a certain amount of stress is good (helps to maintain tissue homeostasis) while too much stress disrupts this balance. The cellular response of chondrocytes and tenocytes is likely to be mediated through some way in which the mechanical environment alters the calcium concentrations within the cell, setting off signals for gene expression.


The real key is in homeostasis on a cellular level and “envelope of function” on a system/joint level. This is a concept brought forth by the authors. We neither understand the response on the cellular level, nor do we really understand where the boundaries of the envelope of function are and how they are moved by ACL injury or reconstruction. It is interesting that the authors twig to the fact that the mechanical environment will have cellular effects.

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