The Basic Principles

Giles R. Scuderi

In primary total knee arthroplasty, whether a posterior cruciate-retaining or posterior cruciate-substituting design is implanted, the clinical results are influenced by the surgical technique. Adherence to the basic principles of the surgical technique ensures a successful outcome.

The goal of primary total knee arthroplasty is to reestablish the normal mechanical axis with a stable prosthesis that is well fixed (Fig. 1.1). This is achieved by both the bone resection and the soft tissue balance. The femoral component should be aligned with 5 to 10 degrees valgus angulation in the coronal plane and 0 to 10 degrees of flexion in the sagittal plane. The tibia should be resected at 90 ± 2 degrees to the long axis of the tibia in the coronal plane. In the sagittal plane, the posterior slope is dictated by the prosthetic design, but it appears preferable to recreate the posterior slope of the natural tibia.

Regardless of prosthetic design there are three basic bone cuts in primary total knee arthroplasty: the proximal tibia, the distal femur, and the posterior femur (Fig. 1.2). Each one influences the arthroplasty in a different manner (Table 1.1). Usually the amount of bone resected corresponds to the thickness of the component being implanted. Resection of the proximal tibia influences both the flexion and extension gaps and is replaced by the tibial component. The more tibial bone resected, the thicker the tibial component. Resection of the distal femur selectively influences the extension gap. Usually the distal femur is resected 9 to 10 mm from the unaffected or normal side, which in the case of a varus knee is the lateral femoral condyle. This concept of removing as much bone as being replaced by the femoral component helps to reassure reestablishment of the joint line. Over-resection of the distal femur creates an extension gap that is larger than the flexion gap resulting in recurvatum, whereas under-resection creates a flexion contracture. Resection of the posterior femur selectively influences the flexion space. If the flexion gap is larger than the extension gap,

Fixation t

Alignment ^ Total Knee Arthroplasty ^ Stability Kinematics

Figure 1.1. The goals of total knee arthroplasty.

then posterior flexion instability will occur. It is recommended that the amount of bone resected be replaced by the implant.

There is a fourth cut that seems to receive less attention. The anterior femoral resection influences both the flexion space and the patellofemoral joint. The amount of bone resected from the ante-

Flexion Extension Gap Balancing

arthroplasty

1. Restoration of the mechanical axis

2. Restoration of the joint line

3. Balance of the soft tissues

4. Equalize flexion and extension gaps

5. Restore patellofemoral alignment and mechanics rior femur is dependent upon sizing of the femur and position of the anteroposterior cutting guide. Under-resection of the anterior femur is caused by an inappropriately large femoral component or by anterior placement of the correct size component with excessive posterior resection. This leads to overstuffing of the patellofemoral joint, which may possibly lead to limited motion and patellofemoral dysfunction. Conversely, over-resection of the anterior condyles may result in notching of the distal femur.

The ligament releases, to correct fixed angular deformities, are discussed elsewhere in this text and should be reviewed, but the basic principles will be highlighted. For the fixed-varus deformity, the medial soft tissue release includes the deep medial collateral ligament, the posteriomedial corner (including the semitendi-nosus), and the superficial medial collateral ligament. Correction of a fixed-valgus deformity tends to be sequential with release of the posterolateral capsule, the iliotibial band, and the lateral collateral ligament. If possible, it is preferable to preserve the integrity of the popliteus tendon in order to maintain flexion stability. Whatever the fixed deformity, balancing of the tight contracted soft tissues is critical in reestablishing the normal mechanical axis of the knee.

Of prime importance is establishing equal flexion and extension gaps (Fig. 1.3). Anteroposterior stability depends on balanced flexion and extension gaps. These gaps are influenced by femoral component sizing, asymmetry of the flexion space, flexion contracture, and release of the posterior cruciate ligament. Each variable affects the knee in a different way. Failure to address these issues may result in posterior subluxation or dislocation, irrespective of prosthetic design. It is a misconception that proper soft tissue releases that restore the mechanical axis to neutral in extension will ensure stability in flexion. As each variable is reviewed, their influence will be better understood.

Matching the femoral component to the anteroposterior dimension of the femur has always been recommended. When the femur measures in-between sizes, it may be preferable to downsize the femoral component. In this situation, an anterior-referencing system will resect more bone from the posterior femur enlarging the flexion gap, whereas a posterior-referencing system will resect more bone from the anterior femur resulting in an anterior notch (Fig. 1.4). The ideal system should allow the additional bone resection to be divided between the anterior and posterior condyles. Slight flexion of the distal femoral resection avoids anterior notching and permits blending of the anterior femur. There may be situations in which upsizing of the femoral component is preferable,

Resection Measure Knee

this is usually the case with a wide distal femur whose anteroposterior measurement is within 1 to 2 mm of the next larger femoral component.

External rotation of the femoral component has always been advocated. Whether the rotation is set at a predetermined 3 degrees, referenced off the posterior condyles or set in line with the epicondylar axis, a certain amount of external rotation is desirable. The femoral epicondylar axis is a reliable and reproducible landmark for setting femoral component rotation. Following soft tissue balancing, setting the femoral component along the epicondylar axis creates a balanced rectangular space. In addition to its influence on patellar tracking, internal rotation of the femoral component must be avoided because this will cause asymmetry of the flexion space. This asymmetry results in a trapezoidal flexion space that would be tight on the medial side and loose on the lateral side.

Asymmetry of the flexion space can also be related to overrelease of a valgus deformity. As discussed elsewhere in this text, there are several techniques described for correction of a fixed-valgus deformity. Although complete release of the lateral supporting structures will correct the axial alignment in extension, over-release will result in an asymmetry of the flexion space. The resultant trapezoidal space would be larger on the lateral side than on the medial side. Correction of the valgus deformity should be sequential, lengthening the lateral soft tissues and attempting to maintain flexion stability.

Following standard resection of the femur and tibia, a knee with a preoperative flexion contracture will probably have a flexion-extension space imbalance. The flexion space would be larger than the extension space. Although it might be appealing to use a thinner tibial polyethylene component, this would cause

Figure 1.4. When sizing the femur the level of resection can be referenced from the posterior or anterior femur. The posterior reference point causes variation in the anterior resection when the femur measures in between sizes, while an anterior reference point causes variation in the posterior cuts.

Anterior Referencing Total KneeAnterior Referencing Total Knee

Figure 1.4. When sizing the femur the level of resection can be referenced from the posterior or anterior femur. The posterior reference point causes variation in the anterior resection when the femur measures in between sizes, while an anterior reference point causes variation in the posterior cuts.

flexion instability. The correct management of this situation should be a posterior capsule release and resection of additional bone from the distal femur so that the extension space equals the flexion space.

Finally, correct preparation of the patella ensures improved performance of the extensor mechanism and reduces the incidence of complications. The preparation of the patella includes a measured resection that is parallel to the anterior cortex. The bone-patellar component composite should be as thick as the original patella. Even though lateralization of the femoral and tibial components are advocated, the patellar component should be medialized. The assessment of patella tracking is judged by the rule of "no thumbs." Further details of patellar preparation will be discussed in later chapters.

Adhering to these basic principles in both the simple and complex cases ensures a successful outcome.

+1 0

Responses

  • FNAN
    How to measure the Flexion / Extension gap?
    6 years ago
  • geronimo mancini
    How to manage in between size of femur in tka?
    6 years ago

Post a comment