The Knee as a Biologic Transmission With anEnvelope of Function

Dr.prashanth kumar 2nd yr ortho junior resident

• The knee is the largest and most complex of human joints

• Achieving full restoration of fully damaged joint can be a formidable challenge

• The knee of a pt with ruptured ACL is an archetyical ex of just such a damaged system

• Full function of the ACL deficient knee is not currently being surgically restored with predictability ,despite yrs of intense basic scinece research ,clinical research,and dedicated clinical practice

• In addition to outright postop structural and biomechanical failure,revealed by increased laxity and functional instability ,recent studies by daniel et al,and fritcshy et al,using follow up scintigraphic and radiographic imaging ,have manifested persistent loss of osseosus homeostatis and early degenerative changes even in the presence of normal instrumented laxity data and other biomechanical measurements

• These same studies reveal ,along with work of buss et al,that the knees of certain pts with documented ACL deficiency may remain asymptomatic and free from denerative changes without surgery if the functional demands across these joints are sufficiently low.

• In acessing the current state of ACL surgery ,gillquist observed that maybe the only effect of ACL reconstruction is”to give the pt enough security to reach the goal of going back to strenuous sports and then ruining the knee”

• From a purely biomechanical perspective,if one is able to achieve normal post op biomechanical parameters in an ACL deficient knee(normal instrumented laxity , negative pivot shift,normal ROM,normal muscle strength) the knee should manifest full restoration of function to preinjury levels.

• The fact that many such biomechanically well reconstructed knees cannot withstand preinjury loading levels,without persistent loss of tissue hemostasis and risk of early degenerative changes,reveals the limitations of the primarily biomechanical and structural view

• A broader conceptual framework taking into account other extent factors is needed to explain these findings and the observations of many orthopaedics,that full restoration of function is not predictably bieng achieved in ACL reconstructed knees

• Discovering the reasons for failing to achieve full function may lead toward deeper understanding of the knee and of other joints and musculoskeletal systems

KNEE AS BIOLOGIC TRANSMISSION

• After much consideration and discussion with the other members of the international orthopaedic community ,the author has come to believe that this analogy is an appropriate and valuable concept .

• A transmission ,such as in an automobile ,for ex is an assembly of parts designed to accept and differentially transfer loads b/w mechanical components.

• The knee can be viewed as a complex assemblage of living asymmetrical moving parts whose purpose is to accept, transfer ,and ultimately dissipate often high loads generated at the ends of the long mechanical lever arms of the femur and tibia.

• In this analogy various ligaments represent sensate ,nonrigid,adaptive linkages within the transmission

• The articular cartilage surfaces represent bearings,with the menisci as special mobile sensate bearings within the transmission

• The muscles in the analogy represent the both living engines providing motive forces ,and brakes and dampening system under complex cerebellar proprioceptive neurologic control mechanisms

• The purpose of neuromuscular subsytems is to provide dynamic control of the load applied across the knee

• Winter has shown that in normal ambulation ,the muscles about the knee actually absorb more energy than they generate in motive forces.

ENVELOPE OF FUNCTION

The range of loads that can be accepted ,transfered ,and dissipated by an individual joint in a given period without either macrostructural or supraphysiological failure is represented by an area that can be termed the envelope of function.

Discomfort,tenderness, swelling and

warmthEx:early phase of

stress #

Most activities of daily living and certain sports like bycycling but would still show +lachmans n

pivot shift test

Factors determining the envelope of function

• GENERAL :AGE AND NUTRITION• 4 SPECIFIC CATEGORIES: a)ANATOMIC

b)KINEMATICc)PHYSIOLOGICd)TREATMENT

ANATOMIC FACTOR

• Morphologic• Structural integrity• And biomechanical characteristics of all

anatomic components of the knee includes ligaments,retinacula,tendons,miniscal and articular cartilage,muscle ,bone,and limb alignment,ht and wt

KINEMATIC FACTORS

• PATTERN OF RECUITMENT OF ACL FIBERS• DYNAMIC FUNCTION OF ALL COMPLEX

NEUROMUSCULAR CONTROL SYSTEM• Includes a)propioceptive sensor output• b)cerebral and cerebellar sequencing

of motor units• c)spinal reflex mechanisms and

muscle strength

PHYSIOLOGIC FACTORS

• Effectiveness of the genetically determined mechanisms of molecular and cellualar homeostasis and the ability of such systems to repair damaged tissues.

• There are clearly phenotypical variations of tissue healing properties among individuals.

• All the people are born with their own unique set of molecular and cellular reparative mechanisms

• Early denerative changes following a biomechanically well performed ACL reconstruction may develop in some pts with a family h/o of degenerative joint disease

• whereas the knee tissues of a pt with robust molecular and cellular maintenance mechanisms may be able to withstand even higher loading and still maintain tissue homeostasis without ACL reconstruction surgery

• Some pts may be the victim of genetic sabotage with certain hereditary inflammatory arthopathies,hemophilia or arthogryposis that may effectively eliminate a safe envelope of function for a given joint.

TREATMENT FACTORS

• NONOPERATIVE : a) rehabilitation b) bracingc) medication d) and icing

• OPERATIVE : a)ACL reconstruction b) meniscus repair c) chondroplasty

d)osteotomy e)joint replacementf)and other procedure

THREE DIMENSIONAL ENVELOPE OF FUNCTION

DISCUSSION

• What are the implication for the practicing orthopaedist of this theoretical construct ???

• If just seen as a purely academic concept ,it would primarly offer only intellectual and aesthetic interest.

• However the use of this paradigm of joint function can result in a more rational clinical approach to pts, currently and in future

• The author has found that pts readily grasp the concept ,and thereby have a more realistic understanding and expectation of the possible ultimate capabilities of their injured joint .

• Healing of tissue damage is a complex and rate limited biologic phenomenon

• Respecting the limitations inherent in tissue reparative mechanisms through incremental increases of loading with the time is an ex of a rational clinical approach

• In dealing with an ACL deficient knee ,the injury can be described to a pt as a functional loss of a complex ,sensate linkage within the transmission that may result in limitation of their envelope

• The knees of pts who have had ACL reconstruction are described as rebuilt transmissions with probably less capability.

• Even with current techniques ,a new linkage may not be as strong as a normal ACL ,and is probably asensate

• Pts who have had a partial meniscectomy can be told that a portion of an important transmission bearing is missing following such surgery ,and that may result in limitation of their envelope

• Pts receiving such explanations of their condition are likely to be more willing participants in a rational therapeutic program designed to elicit maximal healing potential of their injured knee without recurrent subversion of the underlying complex reparative systems by premature excessive loading .

• Pts with patellofemoral pain often are symptomatic due to supraphysiological loading of anatomically normal anterior knee components.

• Limiting loading ,atleast temporariliy,to within such a knee’s restricted envelope allows normal homeostatic reparative mechanisms to proceed most rapidly

• Repetitive loading out of the envelope through inappropriate activities( including muscle strengthening programs resulting in increased patellar pain),only subverts normal molecular and cellular healing mechanisms

• Appropriate strengthening of muscles should be accomplished in such a way (painless straight leg raising,patellar taping,swimming)so as not to supraphsiologically overload the knee components.

• The concepts of joints as biological transmissions also can apply to the treatment of pts with estabilised degenerative arthosis.

• Knees with estabilised denerative changes can be described to pts as transmision with worn bearings that now posses smaller envelopes of function .

• Often simple modification of activities of daily living will lead to loading more within their own reduced envelope.

• An ex of such modification would be restriction of excessive stair climbing

• Degenerative arthosis is often a process of punctuated equilibrium with periods of osteophytes formation and dormancy ,controllable in part by the degree of loading appilied to the system

• By recommending that the pts stay more within their own envelope of function through avoidance of certain loading activities ,the author is not advocating a sededntary therapeutic appraoch.

• On the contrary ,it is desirable that the pt be as active as possible within the upper threshold limits of their own specific envelope

• Even pts with quite restricted envelopes often can participate safely in an aerobic swimmimg and bicyccling program that effectiviely maintains muscle strength ,tone,joint flexibility,cardiovascular condtioning and endorphin production ,without supraphysiological overload of the joint as a whole

• Loading more in their own envelope along with icing, medications,and appropriate minimally invasive surgery can extend the functional capacity of such joints for years.

• Viewing a joint as a living biologic transmission also can provide a conceptual framework to help analyze and interpret disparate clinical and basic research data for an improved understanding of the knee’s complex systems.

• Any process ,such as the development of posttraumatic arthrosis,that can be defined and ultimately controlled in the knee at the molecular and cellular level,may be similar in other joint systems.

• This theory also appilies to conditions of repetitive injury syndromes,as exemplified by chronic lateral epicondylities.

• For ex ,an initial injury with loads outside the envelope for an elbow system ,can result in dimmuniiton of its envelope.

• Normal activities of daily living then can be become recurrent supraphysiogical loads subverting the pts homeostatic mechanisms,resulting in chronic persisting symptoms

• In the author’s view,orthopaedic surgeons should try to broaden the envelope of function of a given musculoskeletal system to the maximum as predictably and safely as possible

• Orthopaedic surgeons have also responsibility to advise pts that their joint may have lowered threshhold of function following injury and subsequent treatement,which could put them at risk of early degenerative changes with certain loading activities.

• Normal biomechanical parameters alone are insufficient to prove restoration of function.

• Improvements in the treatment of ACL injuries and other orthopaedic conditions should result from a more through understanding of the complex anatomy,kinematic,and physiologic factors extant in living human knees

THANK YOU…