Balancing the Revision Total Knee Arthroplasty: Restraint with Constraint

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CHAPTER SYNOPSIS:

Constrained knee prostheses are infrequently necessary in arthroplasty surgery. While providing essential mechanical stability in situations where a patient’s own ligaments have failed, the increased mechanical load on the device results in higher risk of failure by component loosening. These devices provide differing levels of constraint depending on their design and intended use. An articulate diagnosis of the instability plus a coherent surgical technique for revision surgery will ensure that constrained implants are used properly and only when necessary.

IMPORTANT POINTS:

1

Types of constrained implants available and degrees of constraint.
2

The purpose of constrained implants is to substitute for deficient soft tissues.
3

Instabilities may be (1) varus-valgus in extension, (2) recurvatum, or (3) anteroposterior in flexion.
4

Patellar instability is a different problem usually related to rotational positioning of tibial and femoral components.
5

Many unstable arthroplasties may be stabilized with revision to different-sized (unconstrained) components implanted in different orientations.
6

Successful use of constrained implants depends on understanding and reducing the “destructive” forces that are responsible for the instability. Failure to do so results in destruction of the constrained implant.
7

A standard surgical technique of (1) tibia platform, (2) knee in flexion (rotation, anteroposterior component size, and joint line established), and (3) knee in extension to match flexion gap will work for all revisions and all implants.
8

Alternatives to constrained implants in situations where the patient’s own ligaments have failed are collateral ligament advancement and allograft substitution.

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HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

The title of this chapter (and with it the tacit remonstrance to limit our use of constrained prostheses in revision total knee arthroplasty [TKA]) is apt. Mechanically constrained knee prostheses are essential in some clinical situations, but it is almost always preferable to stabilize the joint with the patients’ own soft tissue structures. The mechanical properties of ligaments, their ability to absorb load elastically without damage to the fixation interface is ideal. The long history of constrained implants has not been good—the high failure rates have produced many cases of destructive loosening or breakage that are very difficult to reconstruct.

The key to revision surgery is that the femoral component provides control of the soft tissue, not the tibia. Furthermore, the size of the femoral component controls the flexion gap and the proximal-distal position of the femoral component defines the extension gap. When stability cannot be attained in flexion or extension by size and positioning of components, the surgeon has defined the need for constrained implants.

GENERAL DEFINITION

“Constraint” as it pertains to knee prostheses describes mechanical substitution for the normal stabilizing function of soft tissues. In this respect, constrained implants severely restrict the “scope, extent, or activity” of pathologic knee motion, presumably still permitting flexion and extension. The energy absorbed by the mechanism is inevitably dissipated in the fixation interface or the articulation itself leading respectively to increased risks of loosening and breakage.

Constraint should be distinguished from “conformity,” the relative matching of articular geometry that is favored for distribution (and dissipation) of joint load to diminish wear as well as a means to enhance articular stability. When conforming surfaces begin to separate from each other (subluxation and impending dislocation), some load is transferred to the interface, but much is delivered to the normal ligamentous envelope. Walker described this mechanism as the “uphill principle” ( Fig. 19-1 ).

GENERIC TYPES OF CONSTRAINT

Constrained knee prostheses may be viewed generically as to (1) degree of constraint, (2) modularity and provisions for fixation, and (3) condylar or noncondylar weight bearing. Degree of constraint has traditionally been depicted as “semiconstrained” versus fully constrained, intended to distinguish between “posterior stabilized” prostheses and “varus-valgus” constrained implants. With long-term results of posterior stabilized implants demonstrating results comparable in durability to any implant designs, the designation of “semiconstrained” has largely been abandoned.

Degree of Constraint

Fully constrained devices may be either “linked” or “nonlinked”—that is, hinged (with central bolt) or nonhinged, as in the original “Total Condylar 3” prosthesis, which enhanced stability by way of a large tibial intercondylar eminence that articulated between the two femoral condyles. The precision of fit, of this tibial eminence may vary in different designs, conferring greater or lesser degrees of laxity to varus-valgus and rotational loads ( Fig. 19-2 ).

The hinged devices were among the very earliest knee prosthesis, beginning in some accounts with the Walldius in 1953. All of these early hinges had fixed (nonmodular) stem extensions that extended into the medullary canals, initially without cement. Later hinged devices, such as the GUEPAR implant, experimented with cemented and uncemented stem extensions as well as central and then posteriorly offset axes of rotation. Nonetheless, loosening and breakage were common problems. Rotational constraint was eliminated from some designs—the Kinematic Rotating Hinge —but it seems that failure rates were only marginally improved, implying that some aspect of constraint, perhaps the hyperextension stop, is the common feature in all designs leading to failure. All early hinged designs also bore weight directly through the axle. In an attempt to improve function and durability, more recent designs have provided for condylar load bearing in addition to the linked, constrained mechanism.

Nonlinked constrained devices were first embodied in the Total Condylar 3, from the Hospital for Special Surgery and provided by more than one manufacturer. This implant progressed to the “Constrained Condylar Knee” (CCK) and then the “Legacy Constrained Condylar Knee” (LCCK) prosthesis. Working on similar design concepts, other iterations of this concept have been described as “varus-valgus constrained” (VVC). The height of the tibial spine and the degree to which it fits between the femoral condyles will determine “jump height” (the separation of tibia and femur necessary to result in posterior dislocation of the flexed tibia) and rotational constraint. This device can be expected to resist surprisingly high loads if the spine remains in the intercondylar housing. The magnitude of varus or valgus moments that can be resisted increases with most of these designs as compressive, joint reaction force increases. Pure varus valgus moments are more difficult to resist.

Modularity and Provisions for Fixation

The original devices were “nonmodular” in the sense that stem extensions were narrow diameter, part of the whole casting. These stems fit easily into the medullary canal, and were fully cemented. Implants were soon developed that had the potential for modular articular spacers and modular assembly of the hinge axles to facilitate implantation. Later versions feature modular stem extensions that enable the surgeon to apply stem extension for cemented or uncemented use, of a wide variety of diameters, lengths, surface coatings, and cross sections. The introduction of asymmetric or offset stems facilitates fitting of the stem into the wide variety of canal shapes, as well as enabling surgeons to manipulate alignment.

PURPOSE OF CONSTRAINT IN TOTAL KNEE ARTHROPLASTY

Constrained implants are used, in general, to substitute for failed ligaments and specifically, to protect the patient against varus-valgus instability in extension and anterior-posterior instability in flexion. Recurvatum or hyperextension instability is very difficult to prevent mechanically and durably over an extended period of service, although devices with mechanical limits to extension are available. The forces generated may be very destructive.

Surgeons resort to constraint when the size and position of components have not reestablished functional tension in existing ligaments. This may ensue from plastic failure of the ligaments or from a surgical technique that does not acknowledge and evaluate the altered status of the soft tissues as a result of the antecedent failure. For example, if a surgeon reconstructs bone defects and establishes fixation of the femoral and tibial components independent of each other, but perhaps with respect to some anatomic landmark, say the distal level of articulation, the joint may or may not articulate with motion and stability. Whereas if the tibia and femur are linked by the technique, kinematics will be preserved if the ligaments are present, even if they are altered.

Constraint, especially the (linked constrained) hinges, are sometimes (perhaps inappropriately) implanted in the face of extensive bone loss. When the bone loss itself has resulted in detachment or destruction of the stabilizing envelope, this is clearly appropriate. However, intact soft tissue sleeves often confer highly functional stability to significantly deranged knees.

WHEN TO USE CONSTRAINT APPROPRIATELY

Diagnosis

Instability in a knee arthroplasty does not necessarily accrue from ligament failure. Accordingly, a constrained implant may not be required in the face of instability. As ever, a diagnosis is required. A patient who reports “instability” or “giving way” must be questioned carefully. Coincident pain suggests a different differential diagnosis than painless instability. The concept of “pain inhibition,” that pain itself causes muscles to relent, leading to spontaneous buckling is always a consideration. The pain may emanate from the spine, hip, or knee. Instability in the plane of motion may result from any extensor mechanism problem but most commonly pain or maltracking. Flexion contractures, exhausting as they are to the quadriceps, will occasionally result in buckling. A weak quadriceps muscle, especially if due to neurologic compromise (poliomyelitis, spinal stenosis), is difficult to rectify. These patients must hyperextend the knee to function; when the flexed knee is loaded, the joint collapses. This situation has been described as a (relative) contraindication to knee arthroplasty ( Fig. 19-3 ).