Summary
The knee functions as a type of biological transmission whose purpose is to accept and transfer a range of loads between and among the femur, patella, tibia, and fibula without causing structural or metabolic damage.Arthrit- ic knees are like living transmissions with worn bearings that have limited capacity to safely accept and transmit forces. A new method of representing the functional ca- pacity of the knee and other joints is the “envelope of function”, a load and frequency distribution that delin- eates the range of loads a given joint can sustain while still maintaining homeostasis of all tissues. The purpose of joint replacement surgery, therefore, is to maximize the envelope of function for a given joint as safely and pre- dictably as possible.
A fundamental principle of all orthopedic treatment is to restore, as much as possible, normal musculoskele- tal function.Following minor trauma to a previously nor- mal joint such as the knee (e.g., contusion, mild medial collateral ligament sprain), the process of healing – the result of over 400 million years of vertebrate evolution- arily designed molecular and cellular mechanisms [1] – is most often accomplished without the necessity of any therapeutic intervention. True restoration to the full pre- injury functional status is expected and most often occurs.With more substantial trauma to the knee,such as occurs with a complete rupture of the anterior cruciate ligament treated with a reconstruction, restoration to the full pre-injury physiological functional status is more problematic and often does not occur despite modern surgical techniques [2–4]. Even well-reconstructed knees have unfortunately demonstrated the development of early arthrosis if the joint is exposed to sufficiently high levels of loading, such as occurs with soccer and other similar pivoting sports. One can say that the pre-injury functional capacity of such an anterior cruciate ligament reconstructed knee has not been fully restored.
In the case of knees with advanced degenerative arthrosis which undergo joint replacement surgery, the principle of functional restoration may be more proper- ly stated as maximization of the functional capacity of the knee.As effective as current joint replacement techniques
are at achieving pain relief and often associated increas- es in muscle strength and control, knees that have had joint replacement surgery do not replicate the functional status of a healthy, uninjured, adult joint. No one with a total knee replacement, for example, should run marathons or play tackle football. Since the goal of total knee replacement surgery is to maximize joint function, what, then, is the function of the knee?
The Knee as Biological Transmission
Over the past decade or so, a new concept of joint func- tion has been developed that appears to provide a better theoretical description and therefore understanding of the function of the knee, and, by extension, of all diarthroidal joints. In a leap of insight, Menschik of Vienna communicated to me (A. Menschik (1988), per- sonal communication) that the knee could be best con- ceptualized as a type of “step-less transmission”, the purpose of which is to accept and redirect repeated bio- mechanical loads between the femur, patella, tibia, and fibula,and eventually through the ankle and foot,into the ground. Following much consideration and discussion with other individuals within the international knee community, it became clear that this view of the function of the knee as a kind of biological transmission was not only accurate, but represented a substantial advance in conceptual thinking with potential implications for the entire field of orthopedic surgery [5]. In this analogy of the knee as biological transmission, the ligaments can be visualized as sensate, nonrigid, adaptive linkages, articu- lar cartilage as bearings, and the menisci as mobile, sen- sate bearings [6]. The patellofemoral joint can be seen as a large slide bearing within the biological transmission that is exposed to the greatest forces,both in compression and in tension, of any component of human joints. The muscles in this analogy can be conceptualized as cellular engines that, in concentric contraction, provide motive forces across the knee and,in eccentric contraction,act as brakes and dampening systems, absorbing shock loads.
The importance of eccentric contraction to knee function has been demonstrated by Winter [7],who has shown that
2 Knee Arthroplasty to Maximize the Envelope of Function
S. F. Dye
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Chapter 2 · Knee Arthroplasty to Maximize the Envelope of Function – S. F. Dye 15
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the muscles about the knee actually absorb more than three times the energy that is generated in motive forces.
The various components of a living joint are constantly metabolically active, with the presence of complex mole- cular and cellular mechanisms that are designed to main- tain and restore tissue homeostasis under normal and in- jurious biomechanical conditions [8]. The concept of musculoskeletal function should therefore include the capacity not only to generate, transmit, absorb, and dissi- pate loads, but also to maintain tissue homeostasis while doing so.
The Envelope of Function
Mechanical transmissions are complex systems designed to differentially accept and redirect loads/torque between components. The functional capacity of a mechanical transmission can be represented by the range of torque that can be safely managed without structural failure or over-heating of the components. This range of loading can be represented by a torque envelope. Similarly, the functional capacity of the knee can be represented by a load and frequency distribution that I have termed the
“envelope of function”. The envelope of function was de- veloped as a simple method to incorporate and connect the concepts of load transference and tissue homeostasis in order to visually represent the functional capacity of the knee. It defines a range of loading that is compatible with and inductive of the overall tissue homeostasis of a given joint or musculoskeletal system. The envelope of function, in its simplest form, is a load and frequency distribution that defines a safe range of loading for a joint (⊡ Fig. 2-1).
The upper limit of the envelope represents a thresh- old between loads that are inductive of tissue homeosta- sis and loads that initiate the complex biological cascade of trauma-induced inflammation and repair (⊡ Fig. 2-2).
The area within the envelope can be termed the zone of homeostasis, or the zone of homeostatic loading. Loads that are beyond the threshold of the envelope but are low- er than those that induce macrostructural failure of a joint component are in the area that can be termed the zone of supraphysiological overload. Loading in this re- gion can induce the painful osseous remodeling associ- ated with the initial stages of a stress fracture, which is manifested as increased activity on technetium bone scans before any structural changes are noted on radi- ographs. These sites of increased osseous metabolic ac- tivity may return to documented homeostasis as shown by normal bone scans following nonoperative treatment, primarily involving a reduction of loading. If more ener- gy is placed across a joint, a second threshold is reached – the lower limit of the zone of structural failure. Such high loads result in overt structural failure of at least one
component of a joint or musculoskeletal system, such as a rupture of the anterior cruciate ligament or a fracture of the tibial plateau. An extended period of decreased loading, such as may occur with prolonged bed rest, can result in loss of tissue homeostasis, as evidenced by os- teopenia and muscle atrophy associated with disuse. This lower threshold demarcates the zone of subphysiological
⊡Fig. 2-1.The envelope of function for an athletically active young adult. The letters represent the loads associated with different activities.
All of the loading examples, except B, are within the envelope for this par- ticular knee. The shape of the envelope of function represented here is an idealized theoretical model. The actual loads transmitted across an indi- vidual knee under these different conditions are variable and due to mul- tiple complex factors, including the dynamic center of gravity, the rate of load application, and the angles of flexion and rotation. The limits of the envelope of function for the joint of an actual patient are probably more complex. (Reprinted with permission from [5])
⊡Fig. 2-2. The four different zones of loading across a joint. The area within the envelope of function is the zone of homeostasis. The region of loading greater than that within the envelope of function but insufficient to cause macrostructural damage is the zone of supraphysiological over- load. The region of loading great enough to cause macrostructural dam- age is the zone of structural failure. The region of decreased loading over time resulting in loss of tissue homeostasis is the zone of subphysiologi- cal underload. (Adapted from [3], reprinted with permission)
(B) Jump from 3-m height
(A) Jump from 2-m height
(C) 2 hours of basketball
(D) Walking 10 Kilometers (G) Bicycling for 20 minutes
(F) Swimming 10 minutes (E) Sitting in chair
FREQUENCY
ZONE OF
STRUCTURAL FAILURE
ZONE OF
SUPRAPHYSIOLOGICAL OVERLOAD
ZONE OF HOMEOSTASIS
ZONE OF
SUBPHYSIOLOGICAL UNDERLOAD
Envelope of Function
LOAD
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FREQUENCYⴑⴑⴑⴑⴑⴑⴑ䊳 ⴑⴑⴑⴑⴑⴑⴑ䊳LOADⴑⴑⴑⴑⴑⴑⴑ䊳
underload. It appears that most, if not all, musculoskele- tal systems respond to differential loading as depicted in these four regions.
Frost’s extensive work regarding homeostatic proper- ties and principles of tissues, particularly bone, indepen- dently corroborates and complements the concept of the envelope of function [9, 10]. Frost’s view of excessive microdamage corresponds to the loading of tissues with- in the zone of supraphysiological overload [11]. Too little loading over time, resulting in disuse osteopenia, is re- flected in his concept of minimum effective strain or minimum effective signal as a lower threshold limit [12].
Virtually all symptomatic knees with radiographically iden- tifiable arthrosis sufficient to be considered for joint re- placement surgery will also manifest loss of osseous home- ostasis with technetium scintigraphy [13] (⊡Fig. 2-3a,b – left knee). Following well-performed total knee replacement surgery, the inflamed subchondral osseous tissue that is the source of abnormal scintigraphic activity (and, one also presumes, much of the nociceptive output from the arthritic knee) has been operatively removed. The com- ponents of a total knee are thus placed against (without cement) or near (with cement) living bone that was in most cases formerly homeostatic. A new level of meta-
bolic activity of the living bone under the components needs to be achieved following total knee replacement surgery [14]. Postoperative technetium scintigraphy is an excellent method of objectively tracking this process.The desired outcome is for the scintigraphic activity under the components to eventually become minimal and indefi- nitely remain so (⊡ Fig. 2-3a,b – right knee). Findings of increased uptake in one or more geographical regions in- dicates loss of osseous homeostasis and can be an in- dicator of current or eventual overt radiographically identifiable loosening [15, 16] (⊡ Fig. 2-4a,b – left knee).
Knees that have undergone joint replacement surgery do not necessarily have all of the possible nociceptive sources of pain removed or addressed at the time of surgery.Tissues such as inflamed synovium often remain following total knee replacement surgery,and can thus be a possible source of persistent pain, effusion, and dys- function, despite well-placed components. The goal of treatment is to maximize the load transference capacity of a knee that has had joint replacement surgery, in other words, to maximize the postoperative envelope of func- tion for that joint. The indicators that a joint is being loaded within its postoperative envelope of function are the absence of pain, swelling, and warmth, an excellent 16 I . Essentials
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⊡Fig. 2-3a, b. a A technetium 99m methylene diphosphonate 3-h delayed bone scan of a 78-year-old man, 6 years following total joint replacement of the right knee and advanced degenerative arthrosis on the left knee, manifesting minimal subcomponent activity indicative of relative homeostasis of the right knee. The marked increased activity noted in the left knee corresponds to the pathophysiological metabolic activity associated with the advanced de- generative arthrosis. b Radiographs of the same patient showing a total knee replacement on the right and advanced degenerative arthrosis on the left knee
a b
⊡Fig. 2-4a, b.a A technetium bone scan of a 68-year-old woman, 9 months following joint replacement surgery on the right knee and 3 years fol- lowing joint replacement surgery on the left knee, manifesting expected low-level metabolic activity associated with the right knee components and increased metabolic activity under the medial aspect of the tibial component of the left knee, consistent with possible loosening. b Radiograph of the same patient, manifesting acceptable total knee replacement on the right and evidence of possible loosening under the medial aspect of the tibial component on the left knee
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17 Chapter 2 · Knee Arthroplasty to Maximize the Envelope of Function – S. F. Dye
range of motion and muscle control, and a minimal level of subcomponent scintigraphic activity.
I have often found it valuable to draw out both the pre- operative and expected postoperative envelopes of func- tion for patients prior to surgery (⊡ Fig. 2-5a,b). Most patients can readily grasp the concept of the envelope, and therefore can have a better understanding of what function is to be expected postoperatively. By this method, they can more readily understand that joint re- placement surgery is not designed to restore a knee to full, normal physiological function. Patients have a re- sponsibility, as well, to do all that they can (by partici- pating in pre- and postoperative physical therapy, for ex- ample) to maximize their envelope and, once this is achieved, to not exceed the functional capacity of the joint following surgery by avoiding activities associated with supraphysiological loading. For most total knee patients,this information is much appreciated and is well within their expectations.
Conclusion
Joint replacement surgery is designed to expand the en- velope of function of symptomatic arthritic knees as safe- ly and predictably as possible.Properly utilized,total knee replacement surgery is capable of substantial increases in the functional capacity of a given arthritic joint, but it is not designed to restore the full physiological function of a normal, uninjured adult knee. Future developments in the therapeutic management of arthritic knees may even- tually involve biological approaches that could result in further improvements in maximizing the post-treatment envelope of function over what can be achieved with the current technique of using artificial components. By tracking the loss of osseous homeostasis in knees start- ing at a time prior to the development of overt radio- graphically identifiable degenerative changes, an im- proved understanding of the natural history of arthrosis
could be achieved. Such an improved understanding of the natural history of knee arthrosis could have broad implications for the early detection, control, and ulti- mately prevention of arthrosis in all joints.
References
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2. Daniel DM, Stone ML, Dobson BE, Fithian DC, Rossman DJ, Kaufman KR (1994) Fate of the ACL-injured patient. A prospective outcome study. Am J Sports Med 22:632–644
3. Dye SF, Wojtys EM, Fu FH, Fithian DC, Gillquist J (1998) Factors contribut- ing to function of the knee joint after injury or reconstruction of the an- terior cruciate ligament. J Bone Joint Surg 80A:1380–1393
4. Garrick JG, Requa RK (2003) Sports fitness activities: the negative conse- quences. J Am Acad Orthop Surg 11:439–443
5. Dye SF(1996) The knee as a biologic transmission with an envelope of function. Clin Orthop Rel Res 325:10–18
6. Dye SF, Vaupel GL, Dye CC (1998) Conscious neurosensory mapping of the internal structures of the human knee without intra-articular anes- thesia. Am J Sports Med 26:773–777
7. Winter DA (1983) Energy generation and absorption at the ankle and knee during fast, natural, and slow cadences. Clin Orthop 175:147–154 8. Guyton AC, Hall JE (1996): Textbook of medical physiology. W.B. Saunders,
Philadelphia
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10. Frost HM (1989) Some ABCs of skeletal pathophysiology. II: General mediator mechanism properties [editorial]. Calcif Tissue Int 45:68–70 11. Frost HM (1989) Some ABCs of skeletal pathophysiology. IV: The tran-
sient/steady state distinction [editorial]. Calcif Tissue Int 45:134–136 12. Frost HM (1983: A determinant of bone architecture. The minimum
effective strain. Clin Orthop 175:286–292
13. Dye SF(1994) Comparison of magnetic resonance imaging and tech- netium scintigraphy in the detection of increased osseous metabolic ac- tivity about the knee of symptomatic adults. Orthop Trans 17:1060–1061 14. Brand RA, Stanford CM, Swan CC (2003) How do tissues respond and adapt to stresses around a prosthesis? A primer on finite element stress analysis for orthopedic surgeons. Iowa Orthop J 23:13–22
15. Henderson JJ, Bamford DJ, Noble J, Brown JD (1996) The value of skeletal scintigraphy in predicting the need for revision surgery in total knee replacement. Orthopedics 19:295–299
16. Smith SL, Wastie ML, Forster I (2001) Radionuclide bone scintigraphy in the detection of significant complications after total knee joint replace- ment. Clin Radiol 56:221–224
⊡Fig. 2-5a,b. aExample of a preoperative envelope of function of a patient with symptomatic knee arthrosis, showing severe restrictions of func- tional capacity. ADLs, Activities of daily living. bExample of a postoperative envelope of function, showing substantial increases in the functional ca- pacity following successful total knee replacement, but not restoration to full physiological function of an asymptomatic normal knee
a b
