- Email: [email protected]
Avoiding instability: The features that matter
SE
M I N A R S I N
AR
T H R O P L A S T Y
28 (2017) 76–81
Available online at www.sciencedirect.com
www.elsevier.com/locate/sart
Avoiding instability: The features that matter J. David Blaha, MD Department of Orthopaedic Surgery, University of Michigan Health Systems, Alfred Taubman Health Care Center, 1500 East Medical Center Dr, Ann Arbor, MI 48109-5328
article info
abstract
Keywords:
When replacing the human knee, we attempt to reproduce the stability of the normal knee
Total knee arthroplasty
so that the patient perceives the knee arthroplasty to feel and function similar to a normal
Stability
knee. Avoiding instability requires understanding the definition and implication of stability
Ligament
to the implant design features and the arthroplasty procedure. The purpose of this paper is
Compliance
to define the elements of stability in total knee arthroplasty and to explain the mechanics
Four-bar-link
of the knee that will translate to optimal knee stability after TKA.
Medial pivot
1.
Introduction
A foundational principle of successful total knee replacement is proper alignment and stability, but for the purpose of this paper, alignment is considered to be optimal and stability is the issue to examine. To answer the question, “which features matter?” in terms of instability after total knee arthroplasty (TKA), we first must define stability of the normal knee because it is that stability that we try to mimic in the replaced knee. Stability is a force vs. displacement measurement. When it requires little force to move one bone relative to another, the term is less stiff or more compliant. When motion requires more force, the term is more stiff or less compliant. When testing stability, a force is applied to the knee, and the resulting relative motion (e.g., tibia relative to the femur) can be measured. Engineers refer to the curves generated by this type of experiment as “stiffness,” but because stiffness is not a positive term in clinical followup, orthopaedists use the inverse term compliance instead.
& 2017 Elsevier Inc. All rights reserved.
2. The force vs. displacement (stress vs strain) of ligaments Because they are composed of longitudinal collagenous tissue, ligaments have a typical force vs. displacement behavior. In the resting state, the fibers of a ligament are wavy because they are not stretched tight. When loaded in this state, the ligament is less stiff (more compliant) and a small force longitudinally leads to greater linear displacement. This is called the toe-region of the ligaments force vs. displacement curve (Fig. 1). As the fibers become taut, the ligament becomes more stiff (less compliant) and requires more force to stretch the ligament. This is called the elastic portion of the force vs. displacement curve. As individual fibers begin to fail, the ligament elongates with less applied force, becoming less stiff in the plastic region of the curve [1].
3.
Compliance of the knee
A number of studies were published in the 1970s that explored stability of the normal knee. In a study by Markolf
Disclosure: The author received nothing of value for this presentation or the publication of this article. The author is a paid consultant and receives royalties from MicroPort Orthopaedics, and is a faculty member at the University of Michigan, Department of Orthopaedic Surgery, University of Michigan Health Systems E-mail address: [email protected] http://dx.doi.org/10.1053/j.sart.2017.07.003 1045-4527/& 2017 Elsevier Inc. All rights reserved.
S
E M I N A R S I N
AR
T H R O P L A S T Y
Figure 1 – The force–displacement test for ligaments is called a “stress–strain” curve and shows three regions of force– displacement response. Early in loading a small force causes considerable displacement. This is called the “toe region” of the curve. After a certain amount of displacement, the ligament enters the “elastic region” of the curve and becomes markedly more stiff. Finally, if enough force is applied, the ligament begins to fail at its “yield point”. Ligaments “live” in the toe region of the stress–strain curve. This can be seen clinically when, in response to varus– valgus and anteroposterior stress, the tibia moves relative to the femur until it is stopped by tension in the ligament. This is the ligament moving from the toe region into the elastic region. Reprinted with permission from Woo et al. [1].
et al [2], an unloaded cadaver knee was tested. The specimen was stabilized so that, in one instance, a varus–valgus force could be applied to the knee and, in another instance, an anterior–posterior force could be applied. The resulting graph (Fig. 2) shows that the knee is most stiff to varus–valgus force vs. displacement at 0 degrees of flexion, and the knee becomes markedly less stiff with flexion up to 135°. For anterior–posterior displacement (Fig. 3), the knee is most stiff in full extension and becomes less stiff at 20 degrees of flexion. In contrast to the stiffness to varus–valgus loads, the knee then becomes more stiff to anterior–posterior load at 45° and even more stiff at 90°. In another study by Markolf et al [3], published in 1978, the knees in living subjects were tested to evaluate in vivo knee stability. Of importance to this discussion, the protocol looked at the anterior–posterior stability of the knee with no muscle contraction and then with full voluntary muscle contraction. For both a male (Fig. 4) and a female (Fig. 5) subject, the knee became markedly more stiff when loaded by muscle contraction. In a study published by Hsieh and Walker [4], the anterior– posterior displacement of a cadaver knee was tested with increasing amounts of external load. When the same load was applied, the displacement for the knee at 30 degrees of flexion decreased markedly with increasing external load (Fig. 6). Their conclusions were, “Under load bearing conditions, although the soft tissue structures were still playing a part, there was an increasing contribution to stability from the joint surfaces themselves as the compressing load was
28 (2017) 76–81
77
Figure 2 – Graph showing that the unloaded cadaver knee is least compliant (i.e., most stiff) to varus–valgus in full extension with compliance increasing with increasing flexion. Reprinted with permission from Markolf et al. [2]. increased” [4]. And, “We believe that the geometrical conformity of the condyles to be the most important factor for the decreasing laxity under load-bearing” [4]. This conclusion is further supported by research from our lab in which we followed the motion of the knee in an open and closed kinematic chain model with intact and cut cruciate ligaments. The motion of the knee was nearly indistinguishable in trials completed with the cruciate ligaments intact and cut [5].
4.
“Take-home message” on stability
When external load is applied to the knee, either in the form of muscle contraction or weight bearing, the compliance of the knee decreases (i.e., it becomes more stiff and more stable). However, note that loading decreases the tension in the ligaments, yet the knee is less compliant. The only way this can happen is by the geometry of the surfaces (i.e., the congruence of the femur and tibia) imparting the stability with the ligaments doing little. Thus we can conclude that when moving in the plane of flexion–extension without external load, the stability of the knee is not dependent on the ligaments but rather on the geometry of the articular surfaces. Without external load
78
SEM
I N A R S I N
A
R T H R O P L A S T Y
5.1.
Figure 3 – Graph showing that the unloaded cadaver knee is least compliant to anterior–posterior motion at full extension, then becomes more compliant. Reprinted with permission from Markolf et al. [2].
the ligaments are in the toe-region of their force vs. displacement curve so they have little or no effect on stability. The ligaments are recruited to resist deforming forces in a varus– valgus or an anterior–posterior direction when that deforming force moves the ligaments into the elastic portion of the force vs. displacement curve.
5.
Two types of total knee prosthesis design
Most total knees are designed to have little or no congruence between the femur and tibia, likely because of the concern for “kinematic conflict” that dates to the four-bar-link model of knee motion first proposed by Zuppinger in 1904 [6] and expanded by others such as Goodfellow and O′Connor [7]. In these types of total knees, the ligaments are tensioned (i.e., balanced) to provide the stability that, in the normal knee, is provided by congruence. A few total knees are designed for congruence between the femur and tibia, either in the medial compartment only (i.e., medial pivot) or in both compartments (i.e., deep dish). Each has requirements to avoid instability.
28 (2017) 76–81
Non-conforming components
Because of his analysis of the knee generated from observations of radiographs of the knee made not long after the x-ray was invented [6], Zuppinger concluded that the knee was guided by a rigid, crossed four-bar-link. An absolute requirement of the four-bar-link model is that the femur must move bodily backward on the tibia with flexion and anteriorly with extension. (This motion has been called “rollback” but this term is a misnomer because very little rolling actually occurs.) Although many speakers say they do not believe in the four-bar-link model, it is an accepted principle in total knee joint design that anything that might prevent the anterior–posterior motion of the femur would lead to a “kinematic conflict” and that, in turn, would lead to loosening of the implant components. When the statement, “too much constraint causes loosening” is made, the underlying reason is the four-bar-link. Thus most tibiofemoral articulations are designed to have little or no conformity between the femoral and tibial components to allow rollback and avoid kinematic conflict. Because no conformity exists between the femoral and tibial components, these prostheses must rely on ligament tension to stabilize the knee to the considerable anteriorposterior forces (such as the posterior to anterior forces during gait) (Fig. 7). Ligament balancing is the art of tensioning the ligaments so that the resultant elasticity of the ligament leads to high enough contact force between the femoral and tibial components to provide frictional resistance to anterior–posterior travel. Tensors and “Smart trials” are two examples of devices introduced to measure and control the contact stresses between the femoral and tibial components [8,9]. Ligaments are normally in the toe region of their force vs. displacement curve. This can be seen in the curves from Markolf’s 1978 study [3]. To stabilize a non-conforming implant, the ligaments must be tensioned into the elastic portion of the force vs. displacement curve. The ligaments must then remain in that region indefinitely without stretch or component migration. Since ligaments do have viscoelastic properties [1] and it has been shown that components migrate [10,11], the tension obtained in the OR is unlikely to be maintained. If the ligaments are made too tight and they do not relax sufficiently with viscoelasticity and/or migration, the knee may not demonstrate easy range of motion and feel “stiff” to the patient. If the ligaments are left or become too loose, the knee may be unstable and functionally compromised. The target to hit for ideal tension is small and may be different for each patient.
5.2.
Conforming components
In recent years, components with a more conforming surface have been introduced, designed to provide for congruence throughout a normal range of motion [5]. The component that arguably has the asymmetric conformity and compliance most like the normal knee is a medial ballin-socket between the femur and tibia, commonly referred to as a “medial pivot” prosthesis. This type of prosthesis relies on the conformity of the articular surfaces for the
S
E M I N A R S I N
AR
T H R O P L A S T Y
28 (2017) 76–81
79
Figure 4 – Graph showing that the in vivo compliance of the knee of a male subject decreases markedly (i.e., becomes more stiff) with increasing active muscular force. Reprinted with permission from Markolf et al. [2].
anterior–posterior stability of the total knee, leaving the ligaments in the “toe-region” of the force vs. displacement curve more like the condition in the normal knee (Fig. 8). Surgical technique can leave the ligament in the “toe-region”
of the force vs. displacement curve. If external loads are applied, the ligaments can move into the elastic region to resist those loads much the same as occurs in the normal knee. Reports of the use of this type of component have
Figure 5 – Graph showing that the in vivo compliance of the knee of a female subject decreases markedly (i.e., becomes more stiff) with increasing active muscular force. Reprinted with permission from Markolf et al. [2].
80
SEM
I N A R S I N
A
R T H R O P L A S T Y
28 (2017) 76–81
Figure 8 – Ligament force–displacement graph with target for a conforming implant. Because stability is provided by the congruent geometry of the components, the ligament is left in the “toe-region” of the curve. “Out-of-(flexion-extension)plane” loads can tense the ligament into the elastic region of the curve similar to the condition in the normal knee.
Figure 6 – Graphs showing that in a test of AP compliance in a cadaver knee, increasing axial load makes the knee less compliant (more stiff). Reprinted with permission from Markolf et al. [2]. shown that the predicted “kinematic conflict” with loosening and implant failure has not occurred [5,12–15]. After 17 years of using a conforming medial-pivot type of prosthesis, I no longer try to balance ligaments actively. For
the prosthesis I use, the flexion tension must be considerably less than the extension tension, relying on the conformity of the surfaces to provide stability. Clinical results are under evaluation for patients who have had the conforming medial-pivot protheses implanted at our institution. The data are not available for presentation in this manuscript, but as the clinician who evaluated the history and performed the physical examinations, I have observed that patients seem more satisfied when their knee is asymmetrically balanced (medial side tighter than lateral side) and with medial and lateral opening that might be considered mid-flexion instability in arthroplasties does with non-conforming implants.
6.
Conclusions
Every total knee prosthesis has its own geometry. Most are incongruent between the femoral and tibial components but have various cams and spines to promote stability. You must know the prosthesis you use—subtle differences occur among prostheses by different manufacturing companies that make it unwise to consider a cruciate-retaining or posterior-stabilized prosthesis from one company equivalent to that from another company. You must know the suggested technique for “balancing” a given prosthesis and follow that technique carefully. Failure to use the proper technique can lead to an unstable joint.
Figure 7 – Ligament force–displacement graph with the target and limits for ligament tensioning needed with a nonconforming total knee implant. The optimum would place the ligament in the elasic portion of the curve but viscoelastic relaxation and prosthesis migration could potentially decrease this tension.
re fe r en ces
[1] Woo LY, Younf EP, Kwan MK. Fundamental studies in knee ligament mechanics. In: Daniel DM, Akeson WH, O’Connor JJ, et al., eds. Knee ligaments structure, function, injury and repair. New York: Raven Press; 1990. 115–34.
S
E M I N A R S I N
AR
T H R O P L A S T Y
[2] Markolf KL, Mensch JS, Amstutz HC. Stiffness and laxity of the knee—the contributions of the supporting structures. A quantitative in vitro study. The Journal of Bone and Joint Surgery. American Volume 1976;58:583–94. [3] Markolf KL, Graff-Radford AD, Amstutz HC. In vivo knee stability. A quantitative assessment using an instrumented clinical testing apparatus. The Journal of Bone and Joint Surgery. American Volume 1978;60:664–74. [4] Hsieh HH, Walker PS. Stabilizing mechanisms of the loaded and unloaded knee joint. The Journal of Bone and Joint Surgery. American Volume 1976;58:87–93. [5] Blaha JD. The rationale for a total knee implant that confers anteroposterior stability throughout range of motion. The Journal of Arthroplasty 2004;19:22–6. [6] Zuppinger H. Die aktive Flexion im unbelasteten Kniegelenk. Züricher Habil Schr. Wiesbaden: Bergmann; 1904;703–63. [7] Goodfellow J, O’Connor J. The mechanics of the knee and prosthesis design. The Journal of Bone and Joint Surgery. British Volume 1978;60:358–69. [8] Gustke KA, Golladay GJ, Roche MW, Jerry GJ, Elson LC, Anderson CR. Increased satisfaction after total knee replacement using sensor-guided technology. Bone & Joint Journal 2014;96:1333–8.
28 (2017) 76–81
81
[9] Meere PA, Schneider SM, Walker PS. Accuracy of balancing at total knee surgery using an instrumented tibial trial. The Journal of Arthroplasty 2016;31:1938–42. [10] Pinskerova V, Maquet P, Freeman MAR. Writings on the knee between 1836 and 1917. The Journal of Bone and Joint Surgery. British volume 2000;82:1100–2. [11] Ryd L. Micromotion in knee arthroplasty. A roentgen stereophotogrammetric analysis of tibial component fixation. Acta Orthopaedica Scandinavica, Supplementum 1985;220:1–80. [12] Anderson MJ, Kruse RL, Leslie C, et al. Medium-term results of total knee arthroplasty using a medially pivoting implant: a multicenter study. Journal of Surgical Orthopaedic Advances 2009;19:191–5. [13] Karachalios T, Roidis N, Giotikas D, et al. A mid-term clinical outcome study of the Advance Medial Pivot knee arthroplasty. The Knee 2009;16:484–8. [14] Macheras GA, Galanakos SP, Lepetsos P, et al. A long-term clinical outcome of the Medial Pivot Knee Arthroplasty System. The Knee 2017;24:447–53. http://dx.doi.org/10.1016/ j.knee.2017.01.008. [15] Schmidt R, Ogden S, Blaha JD, et al. Midterm clinical and radiographic results of the medial pivot total knee system. International orthopaedics 2014;38:2495–8.
M I N A R S I N
AR
T H R O P L A S T Y
28 (2017) 76–81
Available online at www.sciencedirect.com
www.elsevier.com/locate/sart
Avoiding instability: The features that matter J. David Blaha, MD Department of Orthopaedic Surgery, University of Michigan Health Systems, Alfred Taubman Health Care Center, 1500 East Medical Center Dr, Ann Arbor, MI 48109-5328
article info
abstract
Keywords:
When replacing the human knee, we attempt to reproduce the stability of the normal knee
Total knee arthroplasty
so that the patient perceives the knee arthroplasty to feel and function similar to a normal
Stability
knee. Avoiding instability requires understanding the definition and implication of stability
Ligament
to the implant design features and the arthroplasty procedure. The purpose of this paper is
Compliance
to define the elements of stability in total knee arthroplasty and to explain the mechanics
Four-bar-link
of the knee that will translate to optimal knee stability after TKA.
Medial pivot
1.
Introduction
A foundational principle of successful total knee replacement is proper alignment and stability, but for the purpose of this paper, alignment is considered to be optimal and stability is the issue to examine. To answer the question, “which features matter?” in terms of instability after total knee arthroplasty (TKA), we first must define stability of the normal knee because it is that stability that we try to mimic in the replaced knee. Stability is a force vs. displacement measurement. When it requires little force to move one bone relative to another, the term is less stiff or more compliant. When motion requires more force, the term is more stiff or less compliant. When testing stability, a force is applied to the knee, and the resulting relative motion (e.g., tibia relative to the femur) can be measured. Engineers refer to the curves generated by this type of experiment as “stiffness,” but because stiffness is not a positive term in clinical followup, orthopaedists use the inverse term compliance instead.
& 2017 Elsevier Inc. All rights reserved.
2. The force vs. displacement (stress vs strain) of ligaments Because they are composed of longitudinal collagenous tissue, ligaments have a typical force vs. displacement behavior. In the resting state, the fibers of a ligament are wavy because they are not stretched tight. When loaded in this state, the ligament is less stiff (more compliant) and a small force longitudinally leads to greater linear displacement. This is called the toe-region of the ligaments force vs. displacement curve (Fig. 1). As the fibers become taut, the ligament becomes more stiff (less compliant) and requires more force to stretch the ligament. This is called the elastic portion of the force vs. displacement curve. As individual fibers begin to fail, the ligament elongates with less applied force, becoming less stiff in the plastic region of the curve [1].
3.
Compliance of the knee
A number of studies were published in the 1970s that explored stability of the normal knee. In a study by Markolf
Disclosure: The author received nothing of value for this presentation or the publication of this article. The author is a paid consultant and receives royalties from MicroPort Orthopaedics, and is a faculty member at the University of Michigan, Department of Orthopaedic Surgery, University of Michigan Health Systems E-mail address: [email protected] http://dx.doi.org/10.1053/j.sart.2017.07.003 1045-4527/& 2017 Elsevier Inc. All rights reserved.
S
E M I N A R S I N
AR
T H R O P L A S T Y
Figure 1 – The force–displacement test for ligaments is called a “stress–strain” curve and shows three regions of force– displacement response. Early in loading a small force causes considerable displacement. This is called the “toe region” of the curve. After a certain amount of displacement, the ligament enters the “elastic region” of the curve and becomes markedly more stiff. Finally, if enough force is applied, the ligament begins to fail at its “yield point”. Ligaments “live” in the toe region of the stress–strain curve. This can be seen clinically when, in response to varus– valgus and anteroposterior stress, the tibia moves relative to the femur until it is stopped by tension in the ligament. This is the ligament moving from the toe region into the elastic region. Reprinted with permission from Woo et al. [1].
et al [2], an unloaded cadaver knee was tested. The specimen was stabilized so that, in one instance, a varus–valgus force could be applied to the knee and, in another instance, an anterior–posterior force could be applied. The resulting graph (Fig. 2) shows that the knee is most stiff to varus–valgus force vs. displacement at 0 degrees of flexion, and the knee becomes markedly less stiff with flexion up to 135°. For anterior–posterior displacement (Fig. 3), the knee is most stiff in full extension and becomes less stiff at 20 degrees of flexion. In contrast to the stiffness to varus–valgus loads, the knee then becomes more stiff to anterior–posterior load at 45° and even more stiff at 90°. In another study by Markolf et al [3], published in 1978, the knees in living subjects were tested to evaluate in vivo knee stability. Of importance to this discussion, the protocol looked at the anterior–posterior stability of the knee with no muscle contraction and then with full voluntary muscle contraction. For both a male (Fig. 4) and a female (Fig. 5) subject, the knee became markedly more stiff when loaded by muscle contraction. In a study published by Hsieh and Walker [4], the anterior– posterior displacement of a cadaver knee was tested with increasing amounts of external load. When the same load was applied, the displacement for the knee at 30 degrees of flexion decreased markedly with increasing external load (Fig. 6). Their conclusions were, “Under load bearing conditions, although the soft tissue structures were still playing a part, there was an increasing contribution to stability from the joint surfaces themselves as the compressing load was
28 (2017) 76–81
77
Figure 2 – Graph showing that the unloaded cadaver knee is least compliant (i.e., most stiff) to varus–valgus in full extension with compliance increasing with increasing flexion. Reprinted with permission from Markolf et al. [2]. increased” [4]. And, “We believe that the geometrical conformity of the condyles to be the most important factor for the decreasing laxity under load-bearing” [4]. This conclusion is further supported by research from our lab in which we followed the motion of the knee in an open and closed kinematic chain model with intact and cut cruciate ligaments. The motion of the knee was nearly indistinguishable in trials completed with the cruciate ligaments intact and cut [5].
4.
“Take-home message” on stability
When external load is applied to the knee, either in the form of muscle contraction or weight bearing, the compliance of the knee decreases (i.e., it becomes more stiff and more stable). However, note that loading decreases the tension in the ligaments, yet the knee is less compliant. The only way this can happen is by the geometry of the surfaces (i.e., the congruence of the femur and tibia) imparting the stability with the ligaments doing little. Thus we can conclude that when moving in the plane of flexion–extension without external load, the stability of the knee is not dependent on the ligaments but rather on the geometry of the articular surfaces. Without external load
78
SEM
I N A R S I N
A
R T H R O P L A S T Y
5.1.
Figure 3 – Graph showing that the unloaded cadaver knee is least compliant to anterior–posterior motion at full extension, then becomes more compliant. Reprinted with permission from Markolf et al. [2].
the ligaments are in the toe-region of their force vs. displacement curve so they have little or no effect on stability. The ligaments are recruited to resist deforming forces in a varus– valgus or an anterior–posterior direction when that deforming force moves the ligaments into the elastic portion of the force vs. displacement curve.
5.
Two types of total knee prosthesis design
Most total knees are designed to have little or no congruence between the femur and tibia, likely because of the concern for “kinematic conflict” that dates to the four-bar-link model of knee motion first proposed by Zuppinger in 1904 [6] and expanded by others such as Goodfellow and O′Connor [7]. In these types of total knees, the ligaments are tensioned (i.e., balanced) to provide the stability that, in the normal knee, is provided by congruence. A few total knees are designed for congruence between the femur and tibia, either in the medial compartment only (i.e., medial pivot) or in both compartments (i.e., deep dish). Each has requirements to avoid instability.
28 (2017) 76–81
Non-conforming components
Because of his analysis of the knee generated from observations of radiographs of the knee made not long after the x-ray was invented [6], Zuppinger concluded that the knee was guided by a rigid, crossed four-bar-link. An absolute requirement of the four-bar-link model is that the femur must move bodily backward on the tibia with flexion and anteriorly with extension. (This motion has been called “rollback” but this term is a misnomer because very little rolling actually occurs.) Although many speakers say they do not believe in the four-bar-link model, it is an accepted principle in total knee joint design that anything that might prevent the anterior–posterior motion of the femur would lead to a “kinematic conflict” and that, in turn, would lead to loosening of the implant components. When the statement, “too much constraint causes loosening” is made, the underlying reason is the four-bar-link. Thus most tibiofemoral articulations are designed to have little or no conformity between the femoral and tibial components to allow rollback and avoid kinematic conflict. Because no conformity exists between the femoral and tibial components, these prostheses must rely on ligament tension to stabilize the knee to the considerable anteriorposterior forces (such as the posterior to anterior forces during gait) (Fig. 7). Ligament balancing is the art of tensioning the ligaments so that the resultant elasticity of the ligament leads to high enough contact force between the femoral and tibial components to provide frictional resistance to anterior–posterior travel. Tensors and “Smart trials” are two examples of devices introduced to measure and control the contact stresses between the femoral and tibial components [8,9]. Ligaments are normally in the toe region of their force vs. displacement curve. This can be seen in the curves from Markolf’s 1978 study [3]. To stabilize a non-conforming implant, the ligaments must be tensioned into the elastic portion of the force vs. displacement curve. The ligaments must then remain in that region indefinitely without stretch or component migration. Since ligaments do have viscoelastic properties [1] and it has been shown that components migrate [10,11], the tension obtained in the OR is unlikely to be maintained. If the ligaments are made too tight and they do not relax sufficiently with viscoelasticity and/or migration, the knee may not demonstrate easy range of motion and feel “stiff” to the patient. If the ligaments are left or become too loose, the knee may be unstable and functionally compromised. The target to hit for ideal tension is small and may be different for each patient.
5.2.
Conforming components
In recent years, components with a more conforming surface have been introduced, designed to provide for congruence throughout a normal range of motion [5]. The component that arguably has the asymmetric conformity and compliance most like the normal knee is a medial ballin-socket between the femur and tibia, commonly referred to as a “medial pivot” prosthesis. This type of prosthesis relies on the conformity of the articular surfaces for the
S
E M I N A R S I N
AR
T H R O P L A S T Y
28 (2017) 76–81
79
Figure 4 – Graph showing that the in vivo compliance of the knee of a male subject decreases markedly (i.e., becomes more stiff) with increasing active muscular force. Reprinted with permission from Markolf et al. [2].
anterior–posterior stability of the total knee, leaving the ligaments in the “toe-region” of the force vs. displacement curve more like the condition in the normal knee (Fig. 8). Surgical technique can leave the ligament in the “toe-region”
of the force vs. displacement curve. If external loads are applied, the ligaments can move into the elastic region to resist those loads much the same as occurs in the normal knee. Reports of the use of this type of component have
Figure 5 – Graph showing that the in vivo compliance of the knee of a female subject decreases markedly (i.e., becomes more stiff) with increasing active muscular force. Reprinted with permission from Markolf et al. [2].
80
SEM
I N A R S I N
A
R T H R O P L A S T Y
28 (2017) 76–81
Figure 8 – Ligament force–displacement graph with target for a conforming implant. Because stability is provided by the congruent geometry of the components, the ligament is left in the “toe-region” of the curve. “Out-of-(flexion-extension)plane” loads can tense the ligament into the elastic region of the curve similar to the condition in the normal knee.
Figure 6 – Graphs showing that in a test of AP compliance in a cadaver knee, increasing axial load makes the knee less compliant (more stiff). Reprinted with permission from Markolf et al. [2]. shown that the predicted “kinematic conflict” with loosening and implant failure has not occurred [5,12–15]. After 17 years of using a conforming medial-pivot type of prosthesis, I no longer try to balance ligaments actively. For
the prosthesis I use, the flexion tension must be considerably less than the extension tension, relying on the conformity of the surfaces to provide stability. Clinical results are under evaluation for patients who have had the conforming medial-pivot protheses implanted at our institution. The data are not available for presentation in this manuscript, but as the clinician who evaluated the history and performed the physical examinations, I have observed that patients seem more satisfied when their knee is asymmetrically balanced (medial side tighter than lateral side) and with medial and lateral opening that might be considered mid-flexion instability in arthroplasties does with non-conforming implants.
6.
Conclusions
Every total knee prosthesis has its own geometry. Most are incongruent between the femoral and tibial components but have various cams and spines to promote stability. You must know the prosthesis you use—subtle differences occur among prostheses by different manufacturing companies that make it unwise to consider a cruciate-retaining or posterior-stabilized prosthesis from one company equivalent to that from another company. You must know the suggested technique for “balancing” a given prosthesis and follow that technique carefully. Failure to use the proper technique can lead to an unstable joint.
Figure 7 – Ligament force–displacement graph with the target and limits for ligament tensioning needed with a nonconforming total knee implant. The optimum would place the ligament in the elasic portion of the curve but viscoelastic relaxation and prosthesis migration could potentially decrease this tension.
re fe r en ces
[1] Woo LY, Younf EP, Kwan MK. Fundamental studies in knee ligament mechanics. In: Daniel DM, Akeson WH, O’Connor JJ, et al., eds. Knee ligaments structure, function, injury and repair. New York: Raven Press; 1990. 115–34.
S
E M I N A R S I N
AR
T H R O P L A S T Y
[2] Markolf KL, Mensch JS, Amstutz HC. Stiffness and laxity of the knee—the contributions of the supporting structures. A quantitative in vitro study. The Journal of Bone and Joint Surgery. American Volume 1976;58:583–94. [3] Markolf KL, Graff-Radford AD, Amstutz HC. In vivo knee stability. A quantitative assessment using an instrumented clinical testing apparatus. The Journal of Bone and Joint Surgery. American Volume 1978;60:664–74. [4] Hsieh HH, Walker PS. Stabilizing mechanisms of the loaded and unloaded knee joint. The Journal of Bone and Joint Surgery. American Volume 1976;58:87–93. [5] Blaha JD. The rationale for a total knee implant that confers anteroposterior stability throughout range of motion. The Journal of Arthroplasty 2004;19:22–6. [6] Zuppinger H. Die aktive Flexion im unbelasteten Kniegelenk. Züricher Habil Schr. Wiesbaden: Bergmann; 1904;703–63. [7] Goodfellow J, O’Connor J. The mechanics of the knee and prosthesis design. The Journal of Bone and Joint Surgery. British Volume 1978;60:358–69. [8] Gustke KA, Golladay GJ, Roche MW, Jerry GJ, Elson LC, Anderson CR. Increased satisfaction after total knee replacement using sensor-guided technology. Bone & Joint Journal 2014;96:1333–8.
28 (2017) 76–81
81
[9] Meere PA, Schneider SM, Walker PS. Accuracy of balancing at total knee surgery using an instrumented tibial trial. The Journal of Arthroplasty 2016;31:1938–42. [10] Pinskerova V, Maquet P, Freeman MAR. Writings on the knee between 1836 and 1917. The Journal of Bone and Joint Surgery. British volume 2000;82:1100–2. [11] Ryd L. Micromotion in knee arthroplasty. A roentgen stereophotogrammetric analysis of tibial component fixation. Acta Orthopaedica Scandinavica, Supplementum 1985;220:1–80. [12] Anderson MJ, Kruse RL, Leslie C, et al. Medium-term results of total knee arthroplasty using a medially pivoting implant: a multicenter study. Journal of Surgical Orthopaedic Advances 2009;19:191–5. [13] Karachalios T, Roidis N, Giotikas D, et al. A mid-term clinical outcome study of the Advance Medial Pivot knee arthroplasty. The Knee 2009;16:484–8. [14] Macheras GA, Galanakos SP, Lepetsos P, et al. A long-term clinical outcome of the Medial Pivot Knee Arthroplasty System. The Knee 2017;24:447–53. http://dx.doi.org/10.1016/ j.knee.2017.01.008. [15] Schmidt R, Ogden S, Blaha JD, et al. Midterm clinical and radiographic results of the medial pivot total knee system. International orthopaedics 2014;38:2495–8.