Summary
The evolution of knee implant design reflects recognition of the principle that implant geometry, acting in concert with surrounding soft-tissue structures,determines joint stability, range of motion, interface forces, and material stresses. Interchangeable plateau geometries associated with modular knee designs permit the orthopedic sur- geon to optimize the articulation for a patient’s present- ing pathology within a single system. This chapter pre- sents performance characteristics for contemporary knee designs including intrinsic stability and surface stress dis- tributions.
Introduction
The introduction of modular total knee designs in the late 1980s addressed several contemporary concerns, both clinical and economic. With regard to the former, modularity has proved successful, as the orthopedic sur- geon can address a variety of presenting soft-tissue in- adequacies and bony pathologies with the use of a single knee system. Further, in revision situations where the tibial insert has failed but the femoral component and tibial tray are well fixed and their surfaces not damaged, an easier revision procedure involving only polymer component exchange is an option. Optimization of sys- tem instrumentation has facilitated an overall improve- ment in technical proficiency and clinical outcome. Fi- nally, hospitals were able to virtually eliminate their stock of multiple knee designs and abandon this cost- prohibitive practice.
However, there were unforeseen circumstances asso- ciated with the implementation of the process, including shelf storage of ultra-high-molecular-weight polyethyl- ene (UHMWPE) tibial insert components gamma irradi- ated in an air environment [1], material selection [2, 3], component finishing, component sizing, fixation methodology [4, 5], and device design. This chapter fo- cuses on the tibial-femoral articulating interface partic- ular to intrinsic stability and surface stress distributions for contemporary modular total knee systems.
Intrinsic Stability
Restoration of normal knee joint function through surgi- cal reconstruction is dependent upon load sharing be- tween the implant,surrounding ligaments,and other sup- porting soft-tissue structures. Excision, surgical release and progressive pathological weakening of ligamentous structures will result in an increased dependency upon the implant system for stability.
Stability is achieved in non-hinged total knee re- placements through geometric variation of the tibial- femoral condylar surfaces. The capacity of an implant to resist rotational, anterior-posterior, and medial-lateral displacement during physiological loading defines its in- trinsic stability [6].
Rotational Stability
Figures 58-1 and 58-2 present the rotational stability for con- temporary modular knee systems at 15° of flexion under 1900 N [4.45 N = 1 pound force (lbf)] axial compressive load [7-9].
Low-torque designs are characterized by flat tibial plateau surfaces whose resistance to rotation is due primarily to fric- tional forces between the metal and the UHMWPE bearing surfaces. Higher torques are noted in designs with either marked geometric congruity or a prominent intercondylar eminence, which abuts in rotation (
⊡ Fig. 58-1, 58-2).
Torques generated by rotation of implant systems un- der axial load are, of necessity, dissipated by transfer to both fixation interfaces and soft tissues. Excessive torque experienced at fixation interfaces may accelerate cement fixation failure or compromise bone ingrowth on porous surfaces. Despite a clinical demand for increasing anteri- or-posterior and medial-lateral constraint in modular knee designs, rotational constraint should be kept to a minimum to reduce the potential for loosening.
Posterior Stability
Figures 58-3 and 58-4 present the posterior stability for contemporary modular knee systems at 0° and 90° of flex-
58 Modular UHMWPE Insert Design Characteristics
A. S. Greenwald, C. S. Heim
ion [7, 8]. A major part of the posterior stability generat- ed at the normal knee is attributed to the posterior cruci- ate ligament (PCL). In the absence of a PCL, the intrinsic stability of the tibial-femoral articulation must play a sig- nificantly more prominent role in resisting posterior dis-
location, particularly for the posteriorly unstable knee at 90° of flexion. For systems demonstrating intrinsic con- straint below the shear forces estimated for the normal knee [10], competent soft tissues are mandated for func- tional stability (
⊡ Fig. 58-3, 58-4).
58
⊡ Fig. 58-1. Rotational stability of contemporary primary modular total knee systems. The loading conditions are 15° of flexion with an applied com- pressive axial load of 1900 N. Three tib- ial inserts were evaluated for each de- sign (n=3), and the average at ±15° is presented. Advantim (Wright Medical Technology, Inc., Arlington, TN, USA), AMK (DePuy Orthopedics, Warsaw, IN, USA), Columbus (Aesculap, Center Val- ley, PA, USA), MG (Zimmer, Inc., War- saw, IN, USA), NK II (Sulzer Orthope- dics, Austin, TX, USA), PFC (Johnson &
Johnson Orthopedics, Raynham, MA, USA)
⊡ Fig. 58-2. Rotational stability of contemporary posterior-stabilized modular total knee systems. The load- ing conditions are 15° of flexion with an applied compressive axial load of 1900 N. Three tibial inserts were evalu- ated for each design (n=3), and the av- erage at ±15° is presented. Advantim (Wright Medical Technology, Inc., Ar- lington, TN, USA), AMK (DePuy Ortho- pedics, Warsaw, IN, USA), Columbus (Aesculap, Center Valley, PA, USA), IB II (Zimmer, Inc., Warsaw, IN, USA), NK II (Sulzer Orthopedics, Austin, TX, USA), PFC (Johnson & Johnson Orthopedics, Raynham, MA, USA)
⊡ Fig. 58-3. Posterior stability of con- temporary primary modular total knee systems. The loading conditions are 0° extension with an applied com- pressive axial load of 2900 N. Three tib- ial inserts were evaluated for each de- sign (n=3), and the average is present- ed. Advantim (Wright Medical Technology, Inc., Arlington, TN, USA), AMK (DePuy Orthopaedics, Warsaw, IN, USA), Columbus (Aesculap, Center Valley, PA, USA), MG (Zimmer, Inc., War- saw, IN, USA), NK II (Sulzer Orthope- dics, Austin, TX, USA), PFC (Johnson &
Johnson Orthopaedics, Raynham, MA, USA). Testing was stopped at 2700 N, as this force represents an excessive stability when compared with values reported by Seireg et al. [15]
Clinical longevity of primary and revision knee arthro- plasty is enhanced by attaining the correct balance be- tween the intrinsic stability provided by the tibial-femoral articulating interface and the patient’s presenting pathol- ogy. In general, soft-tissue involvement should be encour- aged to decrease dependency on the intrinsic constraints afforded by condylar geometry for a particular knee de- sign.This load sharing will reduce the stresses transferred to the implant-bone interface, which is important for pro- moting the longevity of the fixation surface.
Surface Stress Distributions
While modularity expanded the armamentarium of the orthopedic surgeon, it also increased the variables affect- ing the longevity of the tibial-femoral articulating inter- face, which was not fully appreciated at the outset [11].
Surface deterioration characterized by material removal as a result of the relative motion between opposing sur- faces defines wear. And while component wear is an in- evitable consequence of in vivo articulation, optimiza- tion of design and material variables should seek to min- imize regions of high surface stresses.
Condylar Conformity
One of the advantages of modularity in knee systems is that several UHMWPE tibial components with differing intrinsic stabilities articulate with a single femoral com- ponent and tibial tray. However,
⊡ Fig. 58-5demonstrates the influence of flexion on the surface stress distributions, which should be taken into consideration when evaluat- ing additional patient factors inclusive of body weight and activity level.
When comparing a tibial design with minimal constraint (CR) with one that provides joint stability through in- creased curvature in anterior and posterior elevations (CS), it is apparent that positional changes associated with gait dramatically affect the resulting surface stress distributions. At 0° extension, the more conforming de- sign has less potential for polymer damage due to the small amount of contact area in the surface stress ranges exceeding the UHMWPE yield strength. However, with increasing flexion, this same articulating geometry loos- es conformity more rapidly and results in a higher po- tential for polymer damage.
⊡ Fig. 58-4. Posterior stability of contemporary posterior stabilized modular total knee systems. The two loading conditions are 0° ex- tension with an applied compres- sive axial load of 2900 N and 90° of flexion with an applied compressive axial load of 1780 N. Three tibial in- serts were evaluated for each de- sign (n=3) at each loading condition and the averages are presented. Ad- vantim (Wright Medical Technolo- gy, Inc., Arlington, TN, USA), AMK (DePuy Orthopaedics, Warsaw, IN, USA), Columbus (Aesculap, Center Valley, PA, USA), IB II (Zimmer, Inc., Warsaw, IN, USA), NK II (Sulzer Or- thopedics, Austin, TX, USA), PFC (Johnson & Johnson Orthopaedics,
Raynham, MA, USA). Testing was stopped at 2700 N, as this force represents an excessive stability when compared with values reported by Mor- rison [10]
⊡ Fig. 58-5. Contact areas by surface stress range for comparison of two contemporary tibial-femoral conformal geometries (n=3 for each condi- tion). CR, Cruciate retaining; CS, cruciate substituting
CR CS CR CS 0 – 20 MPa 20 – 40 MPa 40+ MPa
UHMWPE Yield Strength = 20 – 23 MPa 400
300
200
100
0
Contact Area [mm2] (n = 3)
0 Degrees Extension 60 Degrees Flexion
Tibial Plateau Thickness
Figure 58-6 presents surface stress distributions for two insert designs mated with a common femoral component and tibial tray at varying UHMWPE thicknesses under a single loading condition (10° of flexion and 2900 N). The cruciate-retaining tibial insert consistently produced higher surface stresses than did the cruciate-substituting design for all of the insert thicknesses evaluated. Howev- er, within each design, thickness did not significantly (p>0.05) influence the potential for polymer damage as measured by peak surface stress for the thicknesses mea- sured, which is in contrast to an earlier analytical paper [16]. Therefore, clinical decisions pertaining to joint line restoration and preservation of bone stock should be the
primary considerations in determining the thickness of the UHMWPE tibial components utilized in knee arthro- plasty procedures.Patient age and anticipated activity lev- el are further considerations that should be evaluated by the reconstructive surgeon.
The process of UHMWPE damage is multifactorial, and an appreciation of all variables is necessary to meet the increasing service life requirements of contemporary knee designs (
⊡ Fig. 58-6a, b).
Tibial Tray Design
Modularity in knee arthroplasty was introduced through the application of metal-backed UHMWPE tibial compo-
58
a b
⊡ Fig. 58-6a, b. Contact areas by surface stress range for comparison of two contemporary tibial-femoral conformal geometries (n=3 for each condition). (a) Cruciate retaining, (b) cruciate substituting
b a
⊡ Fig. 58-7a, b. Retrieved Synatomic modular knee replacement. (a) Distal UHMWPE surface indicates failure of the capture mechanism and wear.
(b) Mating tibial tray demonstrates UHMWPE film transfer indicative of component motion 6.7 mm 8.7 mm 10.7 mm 12.7 mm 15.7 mm
0 – 20 MPa 20 – 40 MPa 40+ MPa UHMWPE Yield Strength = 20 – 23 MPa 350
300
250
200
150
100
50
0
Contact Area [mm2] (n = 5)
6.7 mm 8.7 mm 10.7 mm 12.7 mm 15.7 mm 0 – 20 MPa 20 – 40 MPa 40+ MPa
UHMWPE Yield Strength = 20 – 23 MPa 350
300
250
200
150
100
50
0
Contact Area [mm2] (n = 5)
nents.This stiffer substrate was designed to attenuate and distribute the stresses transferred to the implant-bone in- terface while providing off-the-shelf flexibility to address a variety of knee pathologies including tissue incompe- tence, skeletal deformity, and bone loss.
There are an increasing number of citations in the clin- ical literature pertaining to the onset of tibial osteolysis as a result of UHMWPE wear debris coincident with tibial tray design in modular knee systems [12, 13]. One of the most influential variables in this equation is the UHMW- PE capture mechanism utilized on the tibial tray. It has been well documented that most modular tibial inserts displace during in vivo articulation (
⊡ Fig. 58-7).Therefore, all efforts should be applied to decreasing the potential for backside wear of these components, including polishing the proximal metal surface of the tibial tray, fully seating the screws (if utilized),and filling unused screw holes with a spacer. In this regard, cobalt-chrome-molybdenum as a tray metal has distinct advantages over titanium alloy.
Discussion
The evolution of total knee systems over the past two decades has resulted in contemporary design configura- tions which have been influenced by mid- to long-term clinical reports as well as an appreciation of component retrieval. Material factors which contribute to long-term in vivo durability include polymer selection, the steril- ization process,and the articulating counterface.Ongoing optimization of design variables such as tibial plateau capture mechanisms, articulation conformities, and in- trinsic stability will continue to help shape future knee systems. In the years ahead, with a growing interest in small-incision surgery, both knee instrumentation and design alteration of proven knee systems will offer a chal- lenge to the designer and reconstructive surgeon. Final- ly,improved technical surgical proficiency in conjunction
with appropriate patient and knee system selection define a triad which assures clinical in vivo longevity. In the for- mer regard, the emerging interest in computer-assisted knee surgery will play a role.
References
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