The knee is the largest and most complex of human joints. Achieving full restoration of a damaged joint system can be a formidable challenge. The knee of a patient with a ruptured anterior cruciate ligament is an archetypical example of just such a damaged system. Full function of the anterior cruciate ligament deficient knee is not currently being surgically restored with predictability, despite years of intense basic science research, clinical research, and dedicated clinical practice. In addition to outright postoperative structural and biomechanical failure, revealed by increased laxity and functional instability, recent studies by Daniel et al,3 and Fritcshy et al7 using followup scintigraphic and radiographic imaging, have manifested persistent loss of osseous homeostasis and early degenerative changes even in the presence of normal instrumented laxity data and other biomechanical measurements. These same studies reveal, along with the work of Buss et al,2 that the knees of certain patients with documented anterior cruciate ligament deficiency may remain asymptomatic and free from degenerative changes without surgery if the functional demands across these joints are sufficiently low.
In assessing the current state of anterior cruciate ligament surgery, Gillquist8 observed that maybe the only effect of anterior cruciate ligament reconstruction is “to give the patient 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 postoperative biomechanical parameters in an anterior cruciate ligament deficient knee (normal instrumented laxity, negative pivot shift, normal range of motion, normal muscle strength) that 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 homeostasis and risk of early degenerative changes, reveals the limitations of this primarily biomechanical and structural view. A broader conceptual framework, taking into account other extant factors, is needed to explain these findings and the observations of many orthopaedists, that full restoration of function is not predictably being achieved in anterior cruciate ligament reconstructed knees. Discovering the reasons for failing to achieve full function may lead to a deeper understanding of the knee and of other joints and musculoskeletal systems.
KNEE AS BIOLOGIC TRANSMISSION
What is the function of the knee? The term function can be defined as the purpose for which an entity is specially fitted. (Alfred Menschik of Vienna, in a written communication in 1988, indicated that the knee could best be viewed as a type of stepless transmission, the purpose of which is to accept and transfer loads between and among the femur, tibia, patella, and fibula.) After much consideration and discussion with 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 example is an assembly of parts designed to accept and differentially transfer loads between 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, the various ligaments represent sensate, nonrigid, adaptive linkages within the transmission. The articular cartilage surfaces represent bearings, with the menisci as special mobile sensate bearrings within the transmission. The muscles in this analogy represent both living engines providing motive forces, and brakes and dampening systems under complex cerebellar-proprioceptive neurologic control mechanisms. The purpose of the neuromuscular subsystems is to provide dynamic control of the load applied across the knee. Winter10 has shown that in normal ambulation, the muscles about the knee actually absorb more energy than they generate in motive forces. (Cyril Frank, MD, of Calgary, Alberta, in a written communication in 1995, views the knee and other joints as systems to absorb and dissipate energy.)
ENVELOPE OF FUNCTION
After extensive consideration of the biologic transmission analogy, the author theorized that a method of representing the functional capacity of the knee and of other joints to accept, transfer, and dissipate loads could be depicted in a simple graph with increasing applied loads on the ordinate (vertical axis) and increasing frequency of loading on the abscissa (horizontal axis). The closely related parameter of time of loading also could be readily substituted for frequency of loading on the horizontal axis. The range of loads that can be accepted, transferred, and dissipated by an individual joint in a given period without either macrostructural or supraphysiologic failure is represented by an area that can be termed the envelope of function. This concept is similar to that of a flight envelope of an aircraft. Within the parameters of its' flight envelope (air speed, pitch) an aircraft can perform safely. Moving outside the envelope in any respect exposes the aircraft to unsafe conditions or even structural failure. Unlike the biomechanical concept of envelope of motion put forward by Blankevoort, Huiskes, and de Lang,1 which describes laxity parameters of the knee (internal and external rotation as a function of degree of flexion angle), the envelope of function describes a range of loading/energy absorption that is compatible with tissue homeostasis of an entire joint system. This envelope construct also encompasses loads that may be generated within muscle tissues.
An illustration of an idealized envelope of function for the knee of an athletically active young adult over a period of 12 hours is depicted in Figure 1. An example of a single high load event within the capacity of this idealized knee, with all subsystems intact and operative, would be the load applied across the knee following a jump from a 2-m height (A). The actual loads transmitted across an individual knee under such a circumstance are no doubt variable and are due to multiple complex factors, including weight of the individual, dynamic center of gravity, rate of load application, angle of flexion and rotation, sequencing of motor unit contractions, structural and biomechanic integrity of components, and other factors. Such loads would be nearly impossible to measure. In an athletically conditioned individual, however, such a single loading event could occur without resulting in either overt or covert damage. In another event, a jump from a 3-m height (B) could easily result in loading sufficiently high so as to result in overt structural failure of knee components, for example, fracture of bone, ligament rupture, torn meniscus, torn muscle, and other lesions. Such a load would be out of the envelope of function for this knee.
Decreasing the applied load, but increasing the frequency of loading, could be represented by an activity such as participation in basketball for 2 hours (C). Such an activity involves sudden high accelerations, decelerations, and rotational forces applied across the knee. The knee of a conditioned athlete, however, could accept such loading without damage to the system.
Decreasing the load still further, but increasing the load cycles would be represented by walking 10 km (D). Still lesser loading activities that would be well within such an idealized envelope of function are sitting still in a chair (E), swimming for 10 minutes (F), and bicycling on an exercise bicycle for 20 minutes (G). Loads applied across a knee that are within its envelope of function would be compatible with and probably even inductive of tissue homeostasis.
The upper limit of a given knee's envelope of function represents a threshold between homeostatic loading and loading sufficiently great so as to initiate the complex biologic cascade of trauma induced inflammation and repair that can be manifested clinically by discomfort, tenderness, swelling, and warmth. The range of loads sufficient to activate such a physiologic response, but insufficient to cause macrostructural failure, can be termed the zone of supraphysiologic overload (Fig 2). The early phase of a stress fracture in a long distance runner, resulting in increased osseous metabolic activity detectable by technetium scintigraphy before radiographic changes, is a clinical example of repetitive loading within this zone. With the further increase of applied load, a second threshold may be reached, resulting in actual macrostructural failure of 1 or more knee components such as a tibial plateau fracture, or an acute rupture of an anterior cruciate ligament in a skiing accident. Such forces would be into the zone of structural failure (Fig 2). A lower limit of loading probably exists that, if persistent, could result in loss of joint homeostasis, exemplified by disuse atrophy of soft tissue and osteopenia with prolonged bed rest (not shown).
The envelope of function for a given injured joint can vary significantly. An idealized representation of the dynamic character of the envelope of function of an injured knee is represented in Figure 3. Figure 3A represents the preinjury envelope of function for the knee of an athletically conditioned adult soccer player. Two hours of playing soccer (X), represents a load that is within this knee's envelope of function. A load sufficiently great to rupture the anterior cruciate ligament is represented by (Y). Figure 3B represents the envelope of function immediately after acute anterior cruciate ligament rupture, and is dramatically reduced in area. Such acutely injured knees can withstand only a narrow range of loading without further damage or increased symptoms. Figure 3C represents the envelope of function 9 months following an intensive rehabilitation program alone, without surgery. In this particular instance, there has been an increase in the area within the envelope of function, but only sufficient to encompass most activities of daily living and certain sports (such as bicycling) within its zone of homeostasis. Such a knee would still show abnormal static laxity on clinical examination, (positive Lachman's and pivot shift tests,) but would be functionally stable secondary to, among other reasons, proper motor unit sequencing with loads within its current envelope. These protective mechanisms can be sufficient for straight ahead sports, but may not be sufficient for turning, jumping, and pivoting activities. Figure 3D represents an envelope of function of such a knee 1 year following anterior cruciate ligament reconstruction and postoperative rehabilitation. (The immediate postoperative envelope for this knee would be reduced temporarily, but is not represented in this graph.) In this instance, the envelope of function is increased over that achieved by rehabilitation alone, but the envelope has not been restored to the preinjury status. The shaded area between the preinjury envelope of function (A) and the postsurgical envelope of function (D) now represents a zone of supraphysiologic overload or possibly even structural failure for this knee. If a patient with this knee returns to high impact sports (X) (2 hours of soccer) that previously were within the preinjury envelope of function, the knee would be supraphysiologically overloaded and, with repetitive activity, could develop early degenerative changes. Such a knee may even manifest gross strucural failure of the anterior cruciate ligament graft, which for a variety of biologic and mechanical reasons, may possess insufficient strength to withstand such loads.
Just such a circumstance illustrates the experience of Daniel et al,3 Buss et al,2 and he observations of Gillquist.8 Despite current surgical treatment, in many cases with interior cruciate ligament insufficiency, the envelope of function may not be restored to preinjury capacity, even though postoperative instrumented laxity measurements may be within normal limits. If one encourages such a patient to retun to a high level of activity, that individual may be loading outside their envelope of function, which has not been fully restored. Certain types of overly aggressive rehabilitation may, for example, result in loading out of the envelope for some knees. Evidence of the presence of increased microosseous remodeling can be detected scintigraphically. Eventually degenerative changes may occur. Conversely, patients who show abnormal laxity may not have arthrosis develop, barring a new injury, if they remain within their newly smaller envelope.
INDICATORS OF THE ENVELOPE OF FUNCTION
Indicators that a given joint is being loaded out of its envelope of function would include pain, discomfort, functional instability, and the presence of effusion, warmth, and tenderness as well as a positive bone scan. Indicators that loading is within the envelope would be the absence of these signs and symptoms, along with a normal technetium bone scan and normal long term radiographs.
At present, technetium scintigraphy is the most readily available technique capable of sensitively manifesting homeostasis of the osseous components. The capability of a positive bone scan to identify regions of eventual overt degenerative changes in anterior cruciate ligament injured knees, at a time when results of the radiographs are still normal, has been validated in a lapine model by McBride et al.9 No such imaging technique is currently available to sensitively assess the metabolic/homeostatic characteristics of musculoskeletal soft tissues, although positron emission tomorgraphy or a related imaging technology has the potential to do so in the future. Mailine Chew, MD, and the author have shown that restoration of osseous homeostasis documented by technetium scintigraphy is possible following anterior cruciate ligament reconstructive surgery using bone-patellar tendon-bone autograft and incremental post-operative rehabilitation techniques.4,5 (Peter Fowler, MD, of London, Ontario, in a verbal communication in 1995, indicated that he currently uses the sensitive indicator of a normal or near normal technetium scintigraphic study to determine when an athlete with an initial osseous injury revealed by magnetic resonance imaging, often associated with ligament damage, may begin training to higher levels of sport.)
FACTORS DETERMINING THE ENVELOPE OF FUNCTION
In addition to general factors such as age and nutrition, there are at least 4 specific categories that in combination determine the envelope of function, or zone of homeostasis, for a given joint. These categories would include anatomic factors, kinematic factors, physiologic factors, and treatment factors. There may be other factors yet to be discovered. The following are factors for the knee.
Anatomic factors encompass such characteristics as the morphology, structural integrity, and biomechanical characteristics of all anatomic components of the knee to include ligaments, retinacula, tendons, meniscal and articular cartilage, muscle, bone, and limb alignment, height, and weight.
Kinematic factors determine the actual derived motion of a given knee's specific anatomic components under load and would include the pattern of recruitment of anterior cruciate ligament fibers, and the dynamic function of all the complex neuromuscular control systems. These systems include proprioceptive sensor output, cerebral and cerebellar sequencing of motor units, and spinal reflex mechanisms and muscle strength.
Physiologic factors include the effectiveness of the genetically determined mechanisms of molecular and cellular homeostasis and the ability of such systems to repair damaged tissues. There are clearly phenotypical variations of tissue healing properties among individuals. All people are born with their own unique set of molecular and cellular reparative mechanisms.
Early degenerative changes following a biomechanically well performed anterior cruciate ligament reconstruction may develop in some patients with a family history of degenerative joint disease, whereas the knee tissues of a patient with robust molecular and cellular maintenance mechanisms may be able to withstand even higher loading and still maintain tissue homeostasis without anterior cruciate ligament reconstructive surgery. Some patients may be the victim of genetic sabotage with certain hereditary inflammatory arthropathies, hemophilia, or arthrogryposis that may effectively eliminate a safe envelope of function for a given joint.
Treatment factors include both nonoperative factors such as rehabilitation, bracing, medication, and icing, and operative factors, such as an anterior cruciate ligament reconstruction, meniscus repair, chondroplasty, osteotomy, joint replacement, and other procedures.
THREE-DIMENSIONAL ENVELOPE OF FUNCTION
The envelope of function also can be represented in 3 or more dimensions by, for example, the addition of another parameter such as degrees of flexion on the Z axis. An example of a theoretical 3-dimensional envelope for an idealized knee is shown in Figure 4. Kevin Speer, MD, of Duke University, in a written communication in 1995, has applied this same envelope concept to the shoulder, and developed a similar 3-dimensional envelope graph to represent the functional capacity of various shoulder conditions.
What are the implications for the practicing orthopaedist of this theoretical construct? If seen just as a purely academic concept, it would primarily offer only intellectual and aesthetic interest. However, the use of this paradigm of joint function (that of living biologic transmission with an associated envelope of function) can result in a more rational clinical approach to patients, currently and in the future. The author has found that patients 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 time is an example of a rational clinical approach.
In dealing with an anterior cruciate ligament deficient knee, the injury can be described to the patient as a functional loss of a complex, sensate linkage within the transmission that may result in limitation of their envelope. The knees of patients who have had anterior cruciate ligament reconstruction are described as rebuilt transmissions with probably less capability than factory new transmissions. Even with current techniques, the new linkage may not be as strong as a normal anterior cruciate ligament, and is probably asensate. Patients who have had a partial menisectomy can be told that a portion of an important transmission bearing is missing following such surgery, and that this may result in limitations of their envelope. Patients 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 knees without recurrent subversion of the underlying complex cellular reparative systems by premature excessive loading. Patients often tell the author these principles make sense-that they are a common sense approach to management of their injury.
Patients with patellofemoral pain often are symptomatic due to supraphysiologic loading of anatomically normal anterior knee components.6 Limiting loading, at least temporarily, 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 the muscles (engines) should be accomplished in such a way (painless straight leg raising, patellar taping, swimming) so as not to supraphysiologically overload the knee components (transmission).
The concept of joints as biologic transmissions also can apply to the treatment of patients with established degenerative arthrosis. Knees with established degenerative changes can be described to patients as transmissions with worn bearings that now possess smaller envelopes of function. Often simple modification of activities of daily living will lead to loading more within their own reduced envelope. An example of such a modification would be restriction of excessive stair climbing. Degenerative arthrosis is often a process of punctuated equilibrium with periods of osteophyte formation and dormancy, controllable in part by the degree of loading applied to the system.
By recommending that patients stay more within their own envelope of function through avoidance of certain loading activities, the author is not advocating a sedentary therapeutic approach. On the contrary, it is desirable that the patient be as active as possible within the upper threshold limits of their own specific envelope. Even patients with quite restricted envelopes often can participate safely in an aerobic swimming or bicycling program that effectively maintains muscle strength, tone, joint flexibility, cardiovascular conditioning, and endorphin production, without supraphysiologic 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 science research data for an improved understanding of the knee's complex systems. The human knee is viewed as an excellent microcosm of all orthopaedic joint systems, and as such, represents a kind of orthopaedic Rosetta Stone. 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 applies to conditions of repetitive injury syndromes, as exemplified by chronic lateral epicondylitis. For example, an initial injury with loads outside the envelope for an elbow system, can result in diminution of its envelope. Normal activities of daily living then can become recurrent supraphysiologic loads subverting the patient's homeostatic mechanisms, resulting in chronic persistent symptoms.
In the author's view, the raison d'être of orthopaedic surgeons is to broaden the envelope of function of a given musculoskeletal system to the maximum as predictably and safely as possible. Orthopaedic surgeons also have a responsibility to advise patients that their joint may have a lowered threshold of function following injury and subsequent treatment, 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 anterior cruciate ligament injuries and other orthopaedic conditions should result from a more thorough understanding of the complex anatomic, kinematic, and physiologic factors extant in living human knees.
The author thanks the following individuals for their help and guidance in the preparation of this work: Geoffrey Vaupel, MD, Mailine Chew, MD, Hans Uli Stäubli, MD, Cyril Frank, MD, Ejnar Eriksson, MD, Edward Wojtys, MD, Kevin Speer, MD, Savio Woo, PhD, Peter Fowler, MD, John Feagin, Jr, MD, Werner Müller, MD, W. Dilworth Cannon, Jr, MD, Gregory Carlin, Ira Dye, Capt USN, (Retired), Ann Dye, RN, Christopher Dye, Gil Gardner, Lottie Applewhite, Anne Shew, and Rebecca Larsen.