CHAPTER SYNOPSIS
Instability is widely recognized as a common mode of failure after total knee arthroplasty. Implant designs vary in the level on inherent constraint. Knowledge of the design of these implants is crucial in order to select the appropriate level of constraint. Proper evaluation of the patient with an unstable total knee arthroplasty helps determine the cause of the instability and the required steps needed to correct the underlying problem.
CLINICAL/SURGICAL PEARLS:
- 1
Femoral sizing: appropriate sizing is necessary to correctly balance the flexion gap.
- 2
Femoral rotation: malrotation can result in excessive stress on the bearing surface.
- 3
Hinged components: should be available in the revision setting.
- 4
Level of constraint: choosing the appropriate level of constraint is critical.
CLINICAL/SURGICAL PITFALLS:
- 1
Complete clinical and radiographic assessment should be made of patients with instability.
- 2
Component malrotation affects both the flexion gap and patella-femoral tracking.
- 3
Unrecognized ligamentous deficiencies may lead to inadequate implant constraint.
- 4
Undersizing the femoral component can result in flexion instability.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
Motion and stability in total knee arthroplasty (TKA) are controlled by the congruity of the femoral and tibial articulating surfaces and the remaining competent soft tissues. TKA designs that rely on ligaments for constraint require a less constrained articulating geometry. The amount of constraint in a knee arthroplasty varies from the least constraint as seen in a unicompartmental design to the most constrained rotating hinge design. Posterior cruciate sparing, posterior cruciate substituting, and the revision stabilized designs lie between the above two designs regarding the amount of constraint. Implant constraint is defined by the effect of articulation design on implant stability and the ability to counteract forces about the knee after arthroplasty in the presence of a deficient soft tissue envelope.
Instability After Total Knee Arthroplasty
Tibiofemoral instability is widely recognized as a common mode of failure after TKA and often requires revision surgery. One study found instability it to be responsible for 22% of TKA revisions. Fehring et al. reported instability to be the second leading cause of early failure.
Restoration of stability is critically important for a functional and durable knee arthroplasty revision. Surgeons have choices of component design and level of constraint, and it is vital to select the optimum implant for a given patient.
Instability after TKA can be divided into two major types:
- •
Collateral ligament imbalance (i.e., gap asymmetry)
- •
Flexion-extension mismatch (i.e., gap inequality)
Instability may result from inherent soft-tissue laxity, inadequate flexion/extension gap balancing, improper component positioning or alignment, ligamentous insufficiency, failure to balance the collateral ligaments, or choosing the wrong level of constraint.
Instability can occur in the different planes of motion and they can be divided into:
- •
Varus-valgus (coronal plane) instability
- •
Recurvatum (hyperextension, sagittal plane) instability
- •
Anteroposterior (or flexion) instability
- •
Rotational (cross sectional) instability
- •
Global
Varus-Valgus Instability
Medial-lateral instability is the most common type of instability and may result from incompetent collateral ligaments, incomplete correction of a preoperative deformity, or incorrect bone cuts with malalignment.
Clinically, the patient usually presents with complaints of instability occasionally giving way, recurrent knee effusions, and instability in flexion making stair climbing difficult.
Recurvatum
Recurvatum deformity may result from a weak quadriceps muscle or an incompetent extensor mechanism. This creates the necessity of walking with compensatory hyperextension (polio). In this situation, the hamstrings substitute for a deficient extensor mechanism. Recurvatum can also be caused by an extension gap that either is or has become larger than the flexion gap, as seen with subsidence of a loose femoral component. Revision arthroplasties for recurvatum often fail. These patients should consider permanent postoperative bracing. A linked constrained prosthesis with a hyperextension stop is a clever but only temporarily successful maneuver. If the hyperextension is a compensatory mechanism for a weak extensor mechanism, over time the patient will hyperextend again in order to keep the knee from bucking during ambulation.
Anteroposterior (Flexion Instability)
Flexion instability results in excessive anterior-posterior translation of the tibia in flexion and usually is associated with a mismatch of the flexion and extension spaces. Flexion instability has been recognized as a source of postoperative pain. On examination, the medial tibial metaphysis or soft tissue envelope may be tender particularly in the pes anserine region. The patient may also describe a sense of instability and recurrent knee joint effusions.
Similarly, if the flexion gap has become relatively larger than the extension gap, the flexed tibia may dislocate posteriorly. This situation may occur after revision surgery if an undersized femoral component was selected because it fit the residual bone and posterior augments were not used.
Flexion instability can also be caused by over resection of the posterior femoral condyles, undersizing of the femoral component, and excessive tibial slope.
Frank dislocation of a posterior stabilized prosthesis can occur when there is a relatively tight extension space and a loose flexion space ( Fig. 21-1 ). Most posterior stabilized designs do not require a posterior tibial slope since the slope is usually inherent in the design. Excessive posterior tibial slope can produce a loose flexion space and laxity in flexion can allow the femoral component to jump the tibial spine.
Rotational Instability
Malrotation of the tibial and femoral components affects kinematics of the patellofemoral joint and the flexion gap. Isolated internal malrotation of the femoral component results in an asymmetric flexion gap. Clinically, the patient suffers from either lateral instability or medial tightness in flexion. Lateral flexion instability leads to medial tibial pain, difficulties standing up from a chair, or instability during descending stairs or walking downhill. The balanced flexion gap technique seeks to achieve a perfectly balanced extension gap first, and then aligns the femoral component parallel to the tibial resection plane when the knee is under symmetric distraction in 90 degrees of flexion. Rotational positioning of the tibial component referenced on the tibial tuberosity represents the most reliable method. Placing the tibial component according to the femoral component using the floating technique may lead to internal malrotation of the femur if the medical collateral ligament has not first been properly balanced.
Global
Global instability is defined as combined laxity of both the flexion and extension gaps. It can occur when the flexion-extension spaces are balanced but a polyethylene insert of insufficient thickness is used. Global instability can also result from use of an underconstrained implant in the presence of gross ligamentous insufficiency.
Initial Evaluation
Initial evaluation of the patient with instability after TKA should include a comprehensive history and physical examination of the knee. Overall ligamentous quality should be assessed. Radiographic evaluation should include long-standing views and appropriate stress radiographs. It is important to assess the overall axial alignment as well as the sagittal alignment of the specific components. Also, evaluation of the rotational alignment of the components is useful. This can be done by obtaining a computed tomography (CT) scan to evaluate the position of the femoral component relative to the epicondylar axis.
Correlating the complaints and physical findings is imperative. When the principal complaints are specifically mechanical, such as a sensation of slipping, subluxation, or giving way, there very well may be an instability problem. However, if instability cannot be determined on physical examination, then revision surgery is unlikely to be successful. Buckling has many causes: pain, fixed flexion contracture of the knee, quadriceps weakness, and patellar pathology.
Pain after TKA must be carefully evaluated, and it is imperative to determine whether the pain is the result of instability or related to another problem.
Principles of Total Knee Arthroplasty to Ensure Stability
A key principle of TKA is to always choose an implant with as little constraint as needed. Excessive constraint transfers stress to the implant and bone–cement interface and can result in increased aseptic loosening. Conversely, failure to use and adequately constrained implant result in instability. The pathology in an arthritic knee relates not only to abnormalities of the cartilage but also of the surrounding soft tissue envelope. Alteration of the cruciate and collateral ligaments in addition to the posterior capsule occurs and must be dealt with properly.
A contracted anterior cruciate ligament (ACL)/posterior cruciate ligament (PCL) will result in poor range of motion of the knee. Contracted collateral ligaments result in deformity in the frontal plane. Tightness of the posterior capsule will lead to a flexion contracture. This ligamentous pathology must be addressed prior to choosing an implant.
There are multiple levels of constraint in TKA designs available ranging from the least constrained (unicondylar knees) to the most constrained (rotating hinge designs). It is prudent for the surgeon to have multiple constraint options available during knee arthroplasty.
Unicondylar Arthroplasty
Unicondylar knee arthroplasty (UKA) designs require an intact and normal ACL, PCL, and posterior capsule. A contracted medial collateral ligament (MCL) can be balanced to a certain degree. Unicondylar implants will not correct poor range of motion, flexion contractures, and incompetent collaterals. This design is not suitable for patients with poor range of motion, severe varus/valgus deformities, knees with more than 5-degree flexion contractures, obese patients, and those with poor bone quality. The last two factors will prohibit adequate tibial fixation. If a UKA is implanted in a patient with a deficient ACL, the femur will sublux posteriorly on the tibia resulting in excessive polyethylene wear or early loosening. Most UKA designs have anterior pegs to enhance fixation. Posterior stress transfer leads to implant subsidence and failure.
Posterior Cruciate Ligament Sparing Designs
This implant design requires intact collateral ligaments and posterior capsule. A normal or accurately balanced PCL is also necessary for good implant function. However, several studies have shown unpredictable kinematics with this design, especially when it relates to posterior rollback. Studies using videofluoroscopy to analyze femorotibial contact during flexion revealed a consistent pattern of anterior translation of the femur on the tibia in flexion in cruciate retaining designs. The critical issue remains of whether the PCL be accurately balanced when it is pathologically contracted. If it is deficient and this design is used, the implant will not function appropriately. In the setting of revision TKA, the use of this implant is limited to cases with good-quality bone with minimal defects, intact soft tissues, and a PCL that remains functional and balanced. For example, PCL sparing designs could be used for conversion of a failed UKA. In one series of revisions of failed unicompartmental knees, cruciate retaining femoral components were used in 80% of cases with good results.
Posterior Cruciate Ligament Sacrificing Designs
This design requires intact normal collateral ligaments and posterior capsule for optimal function. The condition of the PCL is not important as it is compensated for by the design of the implant. With PCL sacrificing TKAs, the contracted collaterals and posterior capsule can be aggressively released to correct the underlying deformity.
PCL substitution provides more predictable kinematics, improved range of motion, a more conforming surface, which leads to less polyethylene wear. Multiple studies have demonstrated long term clinical success with no increased rates of loosening compared with PCL retaining TKAs.
The use of this design has expanded over the past decade from approximately 8% of all arthroplasties in the United States in 1992 to about 50% of TKAs in 2001. In the setting of revision TKA, one must recognize that the use of this implant does not provide significant varus-valgus stability and only minimal rotational stability. Accurate adjustment of the collateral ligament and posterior capsule is important and results in balancing of the flexion-extension gap. A loose flexion gap can lead to posterior tibiofemoral dislocation secondary to the femoral component riding up and over the tibial post.
Constrained or Stabilized Knee Designs
This design is the workhorse for revision knee arthroplasty. Use of this implant is indicated in cases where a complete revision of the femoral and tibial components is required and there is attenuation of the medial or lateral collateral ligament ( Fig. 21-2 ).
Related posts:
Soft Tissue Balancing during Total Knee Arthroplasty
Intraoperative Complications during Total Knee Arthroplasty: How to Get Out of Trouble
Removal of Well-Fixed Components during Revision Total Knee Arthroplasty
Extensor Mechanism Rupture
Adjunctive Fixation in Total Knee Arthroplasty Revision: Stems and Sleeves
Surgical Exposure in Total Knee Arthroplasty
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