Summary
Implant constraint failures are the consequence of in- adequate balance between the given, intrinsic stability of the implant replacing a joint and the extrinsic stabiliza- tion provided by the soft tissues enveloping the joint.
Achieving this balance is one of the central challenges in total knee arthroplasty (TKA). The success crucially de- pends on preoperative assessment of the deformity and the soft-tissue situation (extrinsic stability), the correct choice of implant (intrinsic stability),which also depends on the former, and the adequate intraoperative treatment of the soft-tissue stabilizers. Therefore, this chapter will focus on the aspects of intrinsic implant stability against the background of the functional interaction with the (often pathologically deformed) soft-tissue apparatus of the knee. Our guiding principle will be: “As little implant constraint as possible with the achievable soft-tissue sta- bility.” For this reason we start from a systematic classifi- cation of knee joint deformities, from which one can derive an algorithm that will facilitate the decision for a certain implant constraint combined with suitable soft- tissue treatment.
Introduction
Knee implants differ by, among other things, the degree of mobility in three-dimensional kinematic modes of movement - varus-valgus angulation (frontal plane), an- teroposterior translation (sagittal plane), mediolateral translation (frontal plane), rotation (transverse plane), and roll-and-glide (sagittal plane) – and by the extent to which the intrinsic stability of the implant can substitute or support the extrinsic soft-tissue stabilizers for these modes of movement ( ⊡ Table 12-1 ).
These properties are determined by the extent of im- plant constraint,in which the so-called kinematic conflict presents a fundamental problem. On the one hand, the implant should enable good mobility and kinematics as physiological as possible, with the soft-tissue envelope preserved. This requires an implant design with relative- ly little intrinsic constraint. In consequence, internal con-
straint forces transmitted to the implant-bone interface and thus the risk of implant-bone fixation failure are re- duced to a minimum in such a design. On the other hand, maximum congruency of the femoral and tibial joint sur- faces should be realized in order to increase the contact surfaces and thereby minimize the contact stresses, and thus wear, at the bearing surfaces. However, the increased congruency of the bearings restricts their relative mobil- ity and thus causes unfavorably high constraint forces, which might compromise the implant-bone fixation in designs with higher intrinsic constraint. Thus, every implant design aims to resolve this conflict by offering some suitable compromise.
12 Failure in Constraint: “Too Little”
F. Lampe, E. Hille
⊡ Table 12-1. Different implant designs with increasing constraint and corresponding level of intrinsic mobility
Implant A/P M/L Varus/ Rotation
constraint trans- trans- valgus lation lation angulation
Non-hinged Mobile bearings
Floating +/+ +/+ +/+ +
platform
Rotating +/+ +/+ +/+ +
platform Fixed bearings
PCL +/+ +/+ +/+ +
retaining (PR)
PCL +/- -/- +/+ (+)
substituting (PS)
Intercondylar +/- -/- (-/-) (-) stabilization
(ICS) Hinged
Rotating -/- -/- -/- +
hinge
Rigid -/- -/- -/- -
hinge
+ Unrestricted mobility; - restricted mobility
Despite the agreements on many aspects of total knee design, there is an impressive number of knee implants currently on the market. This reflects not only commer- cial interests but also design controversies. For decades there has been an ongoing discussion as to whether sta- bility should be provided by the soft tissues in conjunc- tion with low conforming prosthetic surfaces, by only the posterior cruciate ligament (PCL) in conjunction with shallow or moderately conforming (curved, dished) sur- faces, by ultra-conforming surfaces without the cruciate ligaments, or by conforming surfaces augmented by an intercondylar stabilizing arrangement or even a hinge.
A brief historical review reveals that total knee pros- theses first appeared in the 1950s, in the shape of simple hinges. These implants failed to account for the complex- ities of knee motion and suffered high failure rates due to aseptic loosening. They were also associated with unac- ceptably high rates of postoperative infection. In 1971, Gunston recognized that the knee does not rotate on a single axis like a hinge; rather, the femoral condyles roll and glide on the tibia with multiple, momentary centers of rotation [1]. His polycentric knee endoprosthesis en- joyed early successes with its improved kinematics but ul- timately failed because of inadequate alignment and fix- ation to the bone.The highly conforming and constrained Geomedic knee arthroplasty introduced in 1973 ignored Gunston’s principles,giving rise to the kinematic conflict.
Other designs followed, either following Gunston’s prin- ciple in attempting to reproduce normal knee kinematics or allowing a conforming articulation to govern knee mo- tion. Hinged implants are still used today, though largely in special cases or as revision components. If an artificial knee is hinged it is described as being maximally con- strained. Due to the problems of constrained compo- nents, new designs were introduced that were semi-con- strained or even unconstrained. For such knees to be ef- fective, the soft-tissue envelope had to be functionally intact. Stability following the knee arthroplasty was pro- vided by the patient’s own ligaments, rather than by the intrinsic stability of the implant itself.
Each of the various design concepts proved more or less successful in the past, and each has its individual strengths and limitations. Thus we started from the pub- lished data and our own experiences and developed an implant concept for primary knee joint replacement, which will be discussed below. At our hospital, we use, in descending order, implants retaining the PCL (~70%), implants replacing the PCL (~20%), implants with inter- condylar stabilization (~7%),and hinged implants (~3%).
Before we describe the indications for these implant types we will give a brief overview of the experiences published to date on different design variants with various levels of constraint. The results reported in the literature and our own experience formed the basis for our implant concept, which we present in this chapter.
Experiences with Different Implant Designs with Various Levels of Constraint
When considering the issue of implant constraints one has to take into account some fundamental principles.
One approach to enable free, multiaxial mobility, as far as possible, is to reduce the intrinsic constraint of the artifi- cial joint by using flat tibial glide surfaces and to ensure the stability of the joint by preserving the soft-tissue en- velope. Such implants are characterized by the attributes of “low congruency, low constraint, high mobility, high contact stress”. High contact stresses as a consequence of non-conforming surfaces may lead to increased wear of the polyethylene component in these implants [2, 3].
Theoretically,polyethylene damage can be reduced by us- ing highly congruent bearing surfaces with significantly larger contact areas. However, this increases the intrinsic constraint of the implant. Consequently, the mobility of the implant is reduced, which gives rise to increased in- ternal constraint forces with the risk of damage to the implant-bone fixation. The design principle in this case can be summarized as “high congruency,high constraint, low mobility, low contact stress”. A compromise is achieved in designs with components providing sufficient relative mobility to minimize the risk of loosening through constraint forces on the one hand. On the other hand, the contact areas of the conforming bearings are large enough to reduce contact stresses and thus polyeth- ylene wear. These designs also offer adequate intrinsic stability to withstand external forces in conjunction with the extrinsic soft-tissue stabilizers. There are numerous designs that have successfully applied this compromise.
The biomechanically promising functional principle,
“high congruency, low constraint, high mobility, low con- tact stress”, can be realized by mobile bearings. Depend- ing on the degrees of freedom of the mobile platform, in- ternal constraint forces can be avoided, to a large extent, while high mobility is maintained.At the same time, wear in the femorotibial articulation is minimized by using highly congruent bearing surfaces.Although congruency of the bearings is maximum in these designs, their in- trinsic stability is low due to the mobility of the bearings.
For this reason, implants with mobile bearings require a stable and perfectly balanced soft-tissue apparatus to function properly. Therefore, an advanced operating technique, especially with regard to the soft-tissue and gap-balancing procedures,is a prerequisite for the success of mobile-bearing designs.
The most common examples of the implant concepts
cited above with exclusive focus on the aspect of insuffi-
cient constraint will be discussed below. First we present
an overview of the problems that can arise when using
implants offering insufficient constraint, and of the argu-
ments for using implants with higher constraint in certain
situations.Other important arguments for or against cer-
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tain implant types are treated in other chapters of this book. At this point, we ought to highlight again our fun- damental principle, “as little constraint as possible (i.e., posterior cruciate-retaining or -substituting implants as first choice, if sufficient extrinsic stability can be provid- ed by the soft tissues), as much constraint as necessary (i.e.,intercondylar stabilized or hinged implants in special cases)”, in order to counteract the impression that using high-constraint implants were the preferable solution for reasons of principle.
Posterior Cruciate-retaining (PR) Designs and Cruciate-substituting (PS) Designs
Posterior cruciate-retaining (PR) designs, which are used in the majority of cases, are characterized by rela- tively low intrinsic stability. Their longevity depends on the presence of a functionally intact soft-tissue envelope, more specifically of a well-balanced posterior cruciate ligament. To fulfill its kinematically important functions in TKA, the functionality of the posterior cruciate liga- ment needs to be restored during surgery,if necessary by release, which can present a major technical challenge [4]. Consequently, critics of the cruciate-retaining de- signs argue that the precise balancing of the PCL, which is usually pathologically deformed in patients suffering from osteoarthritis or rheumatoid arthritis, is technical- ly difficult or even impossible [5, 6]. In a cadaver study, Mahoney showed that retaining the correct length of the PCL when implanting a PCL-preserving joint is prob- lematic and that the ligament tension changes signifi- cantly even if the thickness of the tibia component is changed only very slightly [7]. A posterior cruciate liga- ment that is too tight narrows the flexion gap and can thus result in a painful restriction of flexion and in in- creased wear of the polyethylene component.In contrast, excessive laxity of the PCL can lead to clinically appar- ent flexion instability [8, 9].Waslewski reported instabil- ities suffered by patients with PCL-retaining designs, caused by the early occurrence of PCL insufficiencies.
His advice was to remember this problem in cases of clin- ical instability complaints but normal radiological find- ings. In a prospective randomized study Straw found no differences, either in the function score or in the range of movement, between cruciate-sacrificing, cruciate-sub- stituting, and cruciate-retaining designs after an average follow-up of 3.5 years. In the same study, significantly worse outcomes were found only with patients whose posterior cruciate ligaments required balancing by re- lease [10].Hence,advocates of cruciate substitution point out that this procedure is technically more forgiving, more reproducible, and therefore less fraught with com- plications. Gait analyses generally produced unphysio- logical findings in patients with TKA compared with
healthy volunteers. Several authors proved a kinematic advantage in favor of cruciate retention, although these studies used cruciate-sacrificing implants without sub- stitution as the control group, for which lack of femoral roll-back must be expected [11-13]. However, kinematic studies by Dennis and Stiehl have shown that femoral roll-back failure will also occur with cruciate-retaining designs. Instead, in many such cases a paradoxical, dis- continuous anterior femur movement is observed [14- 16]. This anterior slip is thought to be the cause for in- creased polyethylene damage, as well as for reduced ef- fectiveness of the extension apparatus, e.g., when climbing stairs [17].In contrast,cruciate-substituting im- plants help to achieve more reproducible kinematics, at least, which comes closer to the physiological situation, even if no implant can exactly reproduce the natural kinematics of the native knee joint [7, 14-16]. Despite good clinical results with cruciate-retaining implants, the use of these designs is questionable, at least in the presence of severe deformities in the frontal and sagittal planes, because achieving adequate extrinsic stability, i.e., a balance between intrinsic and extrinsic stability of the joint, with a pathologically changed soft-tissue ap- paratus (severely contracted and overstretched struc- tures) can be extremely difficult. As reported by Scott, even with careful soft-tissue release a contracted poste- rior cruciate can hamper the mediolateral balance (gap symmetry) [18]. In addition to this, balancing of the ex- tension and flexion gaps (gap congruency) can also be more difficult in such a case. In a study by Laskin, pa- tients with significant varus deformities of more than 15°
profited more from cruciate-substituting designs, in the
long run, at least if flat inlays were used in the cruciate-
retaining variants and if the PCL was not recessed regu-
larly [19]. For patients with rheumatoid arthritis, too, a
cruciate-retaining joint replacement should be consid-
ered with caution. Laskin found an increased revision
rate in such cases, due to flexion instabilities and sec-
ondary genu recurvatum [20]. In the same way, sec-
ondary cruciate ruptures must be taken into account in
the context of the arthritic condition. In conclusion, in
the presence of severe deformities in the frontal (varus,
valgus) and sagittal (flexion contracture, genu recurva-
tum) planes, in cases of global instability and in the pres-
ence of inflammatory joint diseases, the use of cruciate-
retaining implants is questionable, to say the least. Con-
ventional cruciate-retaining and cruciate-substituting
implants have in common that they cannot ensure any
intrinsic varus-valgus stability, and can provide only lit-
tle, if any, rotational stability. Whiteside showed in an in
vitro study that adequate varus-valgus stability in the
frontal plane, achieved by correct balancing of the col-
lateral ligaments, automatically resulted in sufficient ro-
tational stability, making additional intrinsic rotational
stability of the implant unnecessary [21].
Implants with Intercondylar Stabilizing (ICS) Arrangements
Implants with intercondylar stabilizing arrangements provide a significant degree of rotational and varus-val- gus stability due to their central cam-post design. How- ever, Auley pointed out that it appears questionable whether these implants can provide sufficient long-term stability in the frontal plane without any ligament sup- port. Hence, there is a risk of recurring instabilities [22].
Nevertheless,the post can provide short-term support for healing collateral structures or in association with collat- eral reconstruction. Severe flexion instability is another limitation for intercondylar stabilized implants. Despite the taller post, the implant can still dislocate posteriorly in case of a severe laxity in flexion. Other authors report- ed good clinical results with low complication rates (peroneal nerve palsy, flexion instability) in the medium- to long-term outcome for the primary implantation of the intercondylar stabilized CCK knee (Zimmer) in old- er, low-demand patients with severe valgus deformity.
The authors consider intercondylar stabilized implants for such cases as a suitable therapy option in primary knee arthroplasty, which helps to avoid instability prob- lems due to insufficient implant constraint [23].
Implants with Mobile Components
Implants with mobile components represent a special cat- egory. With them, the principle of “high congruency, low constraint, high mobility, low contact stress” can be real- ized in order to improve kinematics, reduce polyethylene wear, and allow implant self-alignment. However, as a large part of the forces in the knee have to be carried off through soft-tissue stabilizers (i.e.; extrinsically), this clearly requires particularly precise reconstruction of the soft-tissue balance, putting high demands on the skills and experience of the surgeon. Critics of mobile compo- nents point to polyethylene wear arising from the addi- tional articulation at the underside of the mobile poly- ethylene components. A recent knee simulator study by Bourne, in particular, caused some concern. The worst gravimetric polyethylene wear was found with mobile components that enable rotation and translation, fol- lowed by reduced wear in pure rotation components and the lowest wear in fixed bearings (Genesis II, Smith and Nephew) [24]. This is attributed to the additional articu- lating surface of mobile components. Comparative wear tests under standard (ISO) conditions are required to gain further insights into these aspects.Dislocations of mobile components are rare and caused mostly by mistakes made during implantation [25]. However, incongruence of the extension and flexion gaps, especially, can lead to an increased incidence of component dislocations, too
[26]. Dislocations of rotating platforms following prima- ry implantation are even rarer in comparison to the meniscal bearings, although their incidence is generally higher after revision operations.
Hinged Implants
The limited mobility of hinged implants,which should be employed only for certain indications, gives rise to inter- nal constraint forces, which must be transferred though appropriate anchoring elements to the load-bearing bone in order to avoid implant fixation failure.This leads to de- cisive drawbacks of such implants (larger primary bone loss, secondary bone loss due to stress shielding, risk of infections with primarily diaphyseal involvement, risk of periprosthetic fractures, and increased stress for the ex- tension apparatus due to lack of femoral roll-back). Still, the hinged systems offer the advantage that the load- transferring surfaces can be completely congruent,which reduces wear. Another advantage is that these systems do not require any technically demanding soft-tissue balancing procedures and thus help to avoid potential faults that can lead to clinically manifest complications [27].When marked deformities in the frontal and sagittal planes require correcting,the contracted soft-tissue parts can be sacrificed without risking any instability. Even if the collateral ligaments are completely insufficient, or in cases of neuropathic joints, these systems can be im- planted successfully. More recent designs with rotating hinges generally have produced more encouraging clinical and radiographic outcomes than the earlier uni- axially hinged designs [28, 29].
Choosing the Correct Implant Constraint According to the Classification of the Knee Joint Deformities
Ultimately; every implant design can be only a more or less successful compromise regarding the sometimes mutually exclusive biomechanical requirements, result- ing in advantages in some respects and disadvantages in others. There is no such thing as the ideal implant that meets the requirements of every patient and every situa- tion. Therefore, to reduce trouble caused by insufficient constraint, it is important to arrive at a patient-adapted decision for a certain implant in every individual case.
When the surgical technique and the implant design are
decided on, the extent of the deformity with its osseous
and soft-tissue components is of crucial importance, in
our opinion.The bone inventory,the soft-tissue situation,
and the implant constraint must be assessed as interde-
terminative components of a complex system. Therefore,
we use a systematic classification system for knee joint
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Deformity Clas I-III
Weight Bearing
Axial
Pull Correction Alternative Correction
Implant Constraint
Class I
Characteristics:
Mild deformity Intra-articular defect Ligaments balanced Correction by:
· Resection planes
Class II
Characteristics:
Advanced deformity Pronounced i.a. defect Medially tight Laterally normal Correction by:
· Resection planes
· Medial release
Class III
Characteristics:
Severe deformity Severe i.a. defect Medially tight Laterally stretched Correction by:
· Resection planes
· Extend. med. release or LCL reconstruct. + limited med. release
- PR = posterior cruciate retaining, PS = posterior cruciate substituting; ICS = intercondylar stabilized - - * alternative implant, ** in case of instability, *** in case of severe instability -
- ligaments balanced, tight, stretched - favored choice, alternative choice, to be avoided
PR PS*
Hinged
PR PS*
Hinged
PR ICS**
Hinged***
⊡ Fig. 12-1a . Algorithm for choosing the implant constraint depending on the deformities of classes I-III, illustrated by the example of varus defor- mities.
⊡ Table 12-2. Catalog of measures for balancing the extrinsic soft-tissue stabilizers for a fixed varus deformity
When? What?
During approach Medial meniscectomy
Excision of the meniscotibial ligament Removal of medial osteophytes
Intra-articular subperiostal shifting-off of the medial and posteromedial capsule from the tibia After the bone resections at the distal Extensive subperiostal exposure of the medial proximal tibia
femur and proximal tibia Release of the anterior portion of the medial collateral ligament at the tibia Release of the posterior portion of the medial collateral ligament at the proximal tibia Detaching the semimembranosus insertion at the tibia
Release of the posteromedial capsule and the gastrocnemius insertion at the femur Release of the pes anserinus at the tibia
Reconstruction of the lateral collateral ligament Change of implant type, i.e. increasing Substitution of the posterior cruciate ligament (PS) the level of constraint Intercondylar stabilized implant (ICS)
Hinged implant
Deformity Clas IV-VI
Weight Bearing
Axial
Pull Correction Alternative Correction
Implant Constraint
Class IV
Characteristics:
Extra-articular defect Combined with I-III, VI Correction by:
· Extra-articular osteotomy
· Addit. see I-III, VI
Class V
Characteristics:
Intra-articular deformity (i.e. HTO) combined with I-III, VI Correction by:
· See I-III
· If nec. tibial augmentation
Class VI
Characteristics:
Global instability Severe intra-articular defects
Correction by:
· Stabilizing by thicker intercond. stab.
component
· Alt. hinged implant
- PR = posterior cruciate retaining, PS = posterior cruciate substituting; ICS = intercondylar stabilized - - * alternative implant, ** in case of instability, *** in case of severe instability -
- ligaments balanced, tight, stretched - favored choice, alternative choice, to be avoided
Osteotomy + See I-III, VI
See I-III/VI
ICS Hinged***
⊡ Fig. 12-1b. Algorithm for choosing the implant constraint depending on the deformities of classes IV-VI, illustrated by the example of varus defor- mities
⊡ Table 12-3. Catalog of measures for balancing the extrinsic soft-tissue stabilizers in case of a fixed valgus deformity
When? What?
During approach Lateral approach if necessary
Lateral meniscectomy Removal of lateral osteophytes
Circumferential shift-off of the capsule from the posterolateral tibia After the bone resections at the distal femur and Lateral retinaculum release
proximal tibia Detaching the popliteus tendon from the femoral insertion
Successive detachment of the lateral collateral ligament at the femur or recessing it Release of the posterolateral capsule and the gastrocnemius insertion at the femur Detaching the iliotibial band from
Gerdi’s tubercle or recessing it
Reconstruction of the medial collateral ligament Change of implant type, i.e., increasing the level of Substitution of the posterior cruciate ligament (PS)
constraint Intercondylar stabilized implant (ICS)
Hinged implant
deformities, from which one can derive an algorithm that facilitates the decision for a certain implant constraint and the appropriate soft-tissue treatment ( ⊡ Fig. 12-1a, b ).
The required soft-tissue release must be carried out in a dosed manner, adjusted to the individual situation, to prevent excessive release and the resulting instability and to avert the necessity of using an implant with a higher degree of constraint.Hence,the soft-tissue balancing pro- cedures listed in ⊡ Tables 12-2 and 12-3 are meant as sug- gestions, which can be varied, e.g., regarding their se- quential order and timing. The principles involved in soft-tissue balancing are illustrated by means of a strong- ly simplified model ( ⊡ Fig. 12-2 ).
The stabilizers represented by symbols in this model (medial and lateral collateral stabilizers, posterior cruci- ate ligament, and posterior capsule) can be either con- tracted or lax.As a rule,contracted structures are released until a balanced situation is obtained; lax structures can be tightened and reconstructed in certain situations. The medial and lateral structures determine stability in both flexion and extension, or in flexion or extension only ( ⊡ Table 12-4 ). The posterior capsule is tight only in ex-
tension and thus defines among other stabilizers the width of the extension gap. The tension of the posterior cruciate ligament controls mainly the flexion gap.
Following our principle of “as little constraint as pos- sible,”and as the final decision about the actual procedure is made intraoperatively, according to the individual situation, an implant of the next higher constraint level than that determined preoperatively on the basis of cer- tain deformities should at least be available during the op- eration. In this way the risk of insufficient constraint can be minimized. Therefore, at our hospital we have modu- lar systems available which allow us to choose between cruciate-retaining,cruciate-substituting,and intercondy- lar stabilized components, depending on the pre- and in- traoperative decision. For certain cases hinged implants are considered, too. However, such implants are not kept on hand permanently, but are made available according to preoperative planning.Especially when using implants with low constraint (for example, mobile-bearing de- signs) a stable and balanced soft-tissue situation is cru- cial along with optimum component alignment. Since 1999 we have been gaining experience with the comput-
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●
Femur
●
Tibia
●
Medial collateral stabilizers
●
Medial menisci
●
Meniscotibial band
●
Medial and posteromedial capsule
●
Posterior deep portion of MCL
●
Anterior superficial portion of MCL
●
Medial gastrocnemius tendon
●
Semimembanosus tendon
●
Pes anserinus tendon
●
Lateral collateral stabilizers
●
Lateral menisci
●
Lateral and posterolateral capsule
●
Lateral patellar retinaculum
●
Iliotibial band
●
Popliteus tendon
●
LCL
●
Lateral gastrocnemius tendon
●
Posterior cruciate ligament
●
