Total Knee Arthroplasty: Posterior Cruciate Retaining and Substituting Implants
Ashton H. Goldman
Robert T. Trousdale
Key Concepts
Both posterior stabilized (PS) and cruciate retaining (CR) knees can be successful when appropriately balanced.
With removal of the posterior cruciate ligament (PCL), the increased flexion gap must be managed by a combination of slightly increasing the distal femoral resection and decreasing posterior slope of the tibial resection.
Midflexion instability can occur if the joint line is markedly elevated. This can be seen with excessive distal femoral resection. Some argue this is more common with a PS design.
Flexion instability is different from midflexion instability and is a result of a flexion/extension mismatch. This can be a result of undersizing the femoral component, excessive tibial slope, or incompetence of the PCL. Some argue this is more common with a CR design.
Anteroposterior sizing tends toward undersizing with modern systems and must be avoided.
Patella resurfacing may decrease reoperation but can invite unintended risks (avascular necrosis, fracture, over/understuffing).
Sterile Instruments and Implants
Knee retractors
Implants: femoral PS or femoral CR component per surgeon preference
PS femur—small oscillating saw blade or reciprocating saw blade for box cut
Modular or monoblock tibial component that appropriately mates with the femoral component
Patella component if needed
Dilute betadine
Local joint anesthetic cocktail
Surgical Approaches
Median parapatellar approach—The standard extensile approach to the knee that can easily be augmented with a quadriceps snip or tibial tubercle osteotomy if required.
Midvastus approach—Splits the vastus medialis in line with its fibers starting at the top of the patella thereby sparing the quadriceps tendon. Because it is less extensile in the revision setting, it is more commonly used with medial unicompartmental knee arthroplasty.
Subvastus approach—minimally invasive quadriceps tendon sparing approach. Elevates the vastus medialis from the intramuscular septum and is less extensile than other approaches.
Preoperative Planning
A standing long limb alignment view
a. Shows the degree of deformity and other deformities/arthroses (i.e., hip, ankle)
b. Establishes entry point for the entry drill into the canal. This is especially important on the valgus/deformed knee
c. Can be used to determine the appropriate distal femur valgus resection angle if desired
A standing posteroanterior flexion (Rosenberg) view sometimes demonstrates significant arthrosis not seen on an anteroposterior (AP) view
The AP view is appropriate for tibial sizing for templating.
The lateral view aids with sizing the femur for templating.
Optimization in terms of smoking cessation, diabetes management, and nutrition may aid in postoperative wound complications
Previous scars—Blood supply is from medial to lateral. Using the most lateral scar that allows access to the knee is ideal. If a medial incision is used, a lateral flap with potentially compromised blood flow risks wound healing problems. Transverse incisions can be ignored.
Bone, Implant, and Soft Tissue Techniques
After the knee is exposed, an entry drill goes into the femur to identify the intramedullary cavity (Figure 50.1). Keep one hand on either condyle and ensure visualization of the distal anterior femoral cortex in order not to perforate. The entry spot can be determined by several means:
AP plane: Approximately 1 cm above the PCL insertion, which is approximately at the most anterior portion of the medial femoral condyle wear pattern.
ML plane: Just medial to the femoral AP axis (Whiteside line), or in a deformity case it can be found in line with the femoral canal on the standing long limb alignment view.
If a step drill (wider than the intramedullary alignment rod) is used, it is ok to enter in the deepest part of the trochlea; however, one must ensure the hole is large enough not to change the distal femoral valgus angle.
The intramedullary rod enters the canal. Be careful to not hit the anterior cortex with the intramedullary rod as this can cause the cutting jig to go into flexion. Allowing the rod to ride along the posterior cortex helps protect the guide from going into flexion.
The Distal Femoral Cut Sets the Joint Line in Extension
The goal is to reestablish the joint line by resecting the same amount of bone as the implant is thick: typically 8 to 9 mm.
Elevation of the joint line changes the axis of rotation of the medial collateral ligament (MCL) and lateral collateral ligament (LCL) and can cause these ligaments to become unbalanced in midflexion, thereby causing instability.
Figure 50.1 ▪ Placement of the entry drill. A, With your hand extended posterior femoral perforation and risk to neurovascular structures can occur. B, Correct alignment of the entry drill colinear with the femoral canal.
Figure 50.2 ▪ Excessive medial osteophytes can displace the distal femoral resection distally thereby underresecting the distal femur. Removal of excessive medial osteophytes allows the distal femoral resection guide to sit flush on the distal femur.
For significant preoperative flexion contracture, 1 to 2 mm additional can be taken off the distal femur to allow easier balancing.
Be careful not to flex the distal femoral cutting jig or have it resting on prominent osteophytes (Figures 50.2 and 50.3).
Figure 50.4: Ensure removal of enough bone from the trochlea to have a flat distal femoral cut.
 Figure 50.3 ▪ Distal femoral resection typically is done perpendicular to the femoral axis. A, Correct placement of distal femoral resection guide resting on the anterior femur. B, Flexion of the distal femoral resection guide: note how it hugs the end of the distal femur. |
 Figure 50.4 ▪ Inadequate level of distal femoral resection. Failure to remove trochlear cartilage is a sign of inadequate distal femoral resection. This leads to distalization of the component and overstuffing of the patellofemoral joint. Adequate resection of the trochlea will remove the trochlear cartilage and reveal a completely flat surface. |
Femoral Anterior, Posterior, Chamfer Cuts
Most current systems utilize a 4-in-1 femoral cutting jig.
Rotation/AP Sizing
Whiteside line, the epicondylar axis and posterior femoral condyles are all useful for setting rotation. Correct rotation is perpendicular to the Whiteside line, parallel to the epicondylar axis, and 3° to 5° of external rotation from the posterior condylar axis (Figure 50.5).
Internal rotation risks patellar maltracking. This may be more dramatic with older symmetric femoral component designs. Excessive external rotation will tighten the lateral joint space in flexion (Figure 50.6).
Figure 50.5 ▪ Demonstrates alignment markings for femoral rotation. The vertical line through the deepest portion of the trochlea extending to the center of the femoral head is Whiteside line. The line from the LCL origin to the deepest part of the MCL origin is the transepicondylar axis (perpendicular to Whiteside’ line). The posterior condylar axis often is internally rotated 3° to 5° relative to these lines.
Figure 50.6 ▪ When placing the AP sizing guide, abundant lateral cartilage can excessively externally rotate the femoral component (as compared with the epicondylar axis and Whiteside line) thereby overresecting the posterior medial condyle and elevating the medial joint line in flexion. This can cause instability.
Anterior referencing (Figure 50.7) uses the anterior cortex of the femur as the base for sizing, meaning the implants will grow in the flexion space with increasing size. This minimizes the risk for femoral notching but may increase the risk for flexion instability in the CR designs if undersized owing to inadequate restoration of the posterior condylar offset. If between sizes, use the larger size and plan to downsize if flexion space is too tight.
Anterior referencing systems allow one to set the rotation with the identified landmarks. Increasing the external rotation from 3° to 5° allows opening of the medial side, which can be useful in the extremely tight varus knee.Related posts:
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