Posterior Stabilized Total Knee Arthroplasty

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CHAPTER SYNOPSIS:

While at this time there is no consensus regarding the question whether to retain or resect the posterior cruciate ligament (PCL) in total knee arthroplasty, the elements surrounding the debate are addressed with a systematic review of the literature. Each issue is considered while placing special emphasis on recent kinematic studies and randomized, controlled trials. An understanding of the advantages and disadvantages of cruciate retention (CR) versus cruciate substitution (PS) helps to establish proper surgical indications. The surgical technique highlights the importance of establishing symmetric, rectangular extension and flexion gaps with appropriate soft tissue balancing and offers alternatives to ligament tensioning.

IMPORTANT POINTS:

Posterior Cruciate Retention Versus Cruciate Substitution

1

Clinical results are comparable.
2

Range of motion is slightly improved with PS designs.
3

Kinematics of the normal knee is not reproduced by either design but PS forces rollback on both the medial and lateral compartments.
4

Proprioception has not been clinically shown to be improved with CR designs.
5

Polyethylene wear/osteolysis
6

Cam and post impingement can lead to failure in PS designs.
7

Correction of deformities
8

Femoral bone loss with the PS box is greater.
9

Mobile-bearing designs eliminate some constraint.
10

High-flexion designs can improve motion.

INDICATIONS:

1

Severe deformities
2

Inflammatory arthritis
3

Postpatellectomy
4

Altered geometry of femur and/or tibia
5

PCL-deficient knee

SURGICAL TECHNIQUE:

1

Positioning
2

Medial parapatellar arthrotomy
3

Intramedullary guide for femur
4

Extramedullary guide for tibia
5

Extension space
6

Correction of varus deformity
7

Correction of valgus deformity
8

Femoral component rotation
9

Flexion space
10

Trial reduction
11

Patellar resection
12

Component Insertion

CLINICAL/SURGICAL PEARLS:

1

Preoperative template use can help to avoid unforeseen problems in the operating room.
2

Position to avoid need for multiple assistants
3

Distal skin incision and retinacular incision medial to tibial tubercle
4

Exposure of lateral tibial plateau
5

Osteophyte removal
6

Medial soft tissue release—PCL, superficial and deep medial collateral ligament, semimembranosus, posteromedial capsule, pes anserine tendons
7

Lateral soft tissue release—PCL, iliotibial band, posterolateral capsule, popliteus, lateral collateral ligament, lateral head of gastrocnemius
8

Transepicondylar axis for femoral component rotation
9

Protect collateral ligaments, especially during preparation of the femur.
10

Optimize proper patellar tracking—correct patellar resection, restore patellar thickness, external rotation of femoral and tibial components, lateralize femoral component, medialize patellar component
11

If necessary, inside-out lateral release preserving superior lateral geniculate artery
12

Importance of trial reduction
13

Cement application in doughy stage
14

Meticulous cement clean-up
15

Careful wound closure
16

Emphasize knee extension during postoperative rehabilitation.

CLINICAL/SURGICAL PITFALLS:

1

Distal incision directly over tibial tubercle
2

Avulsion of patellar tendon
3

Transection of deep medial collateral ligament during excision medial meniscus
4

Excessive removal of bone from distal femur and/or proximal tibia
5

Varus resection of proximal tibia
6

Excessive posterior slope of proximal tibial resection
7

Inadequate soft tissue releases
8

Failure to equalize extension and flexion gaps
9

Notching anterior cortex of distal femur
10

Internal rotation of femoral component, especially in valgus knees
11

Transsection of medial and/or lateral collateral ligament during posterior femoral resection
12

Overstuffing patellofemoral space: oversizing femoral component, flexing femoral component, inadequate patellar resection
13

Performing lateral release before assessing patellar tracking with tourniquet deflated
14

Inadequate removal of posterior femoral osteophytes and/or release of posterior capsule
15

Failure to achieve full knee extension with trial components in place
16

Malrotation of tibial component leading to cam and post impingement
17

Failure to remove extruded cement—source of third-body wear

VIDEO AVAILABLE:

None.

HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

Controversy persists regarding the role of the posterior cruciate ligament (PCL) in total knee arthroplasty (TKA). In the normal knee, the PCL prevents posterior translation of the tibia and allows the femur to glide posteriorly or rollback on the tibia during knee flexion to allow for maximum range of motion. A posterior cruciate retaining (CR) design TKA provides minimal constraint and relies on an intact, well-balanced PCL to create proper femoral rollback. Proponents of a CR design site potential advantages such as more normal knee kinematics, increased quadriceps strength due to the increased moment arm of the extensor mechanism, improved stair-climbing ability, preserved proprioception, decreased patellar complications, diminished shear forces at the tibial component–bone interface, and maintenance of distal femoral bone stock. However, all these claims are predicated on an intact and properly tensioned PCL. A posterior cruciate substituting or posterior stabilized (PS) design removes the PCL and relies on a more conforming articular surface, as well as a polyethylene tibial post and femoral cam to provide restraint against posterior translation of the tibia and proper femoral rollback. Potential advantages of a PS design include more predictable restoration of knee kinematics, improved range of motion, decreased polyethylene wear because of more congruent articular surfaces, easier correction of severe deformities, and easier ligament balancing. While eliminating the reliance on a well-functioning PCL, a PS design introduces the risk of component dislocation with flexion instability, tibial post and femoral cam impingement creating polyethylene wear, patellofemoral problems, and increased bone resection of the distal femur.

Despite these potential advantages and disadvantages, CR and PS designs demonstrate similar clinical, radiographic, and survivorship results at short-, mid-, and long-term follow-up. Several well-designed prospective, randomized studies failed to show any significant difference between CR and PS designs at short-term follow-up. Clark et al. randomized patients to receive either a CR or PS knee but excluded patients with a flexion contracture greater than 15 degrees, a varus deformity of greater than 20 degrees, a valgus deformity of greater than 15 degrees, and preoperative flexion of less than 90 degrees. Maruyama et al. randomized patients who underwent bilateral TKA and found no difference in clinical scores except for improved range of motion with the PS design. They theorized that the decreased flexion noted in CR knees was due to an unbalanced PCL creating abnormal knee kinematics in flexion. Retrospective reviews of CR and PS knees at mid- and long-term follow-up show survivorship of greater than 90% with both designs. Kelly and Clarke reviewed the long-term results of PS knees and stated that “use of posterior cruciate ligament-substituting prostheses in fixed-bearing TKA has resulted in excellent long-term clinical outcomes with increased knee flexion.” They emphasized the need for strict soft tissue balancing to create symmetric, rectangular extension and flexion gaps and found no evidence of the increased constraint of the PS design leading to aseptic loosening. The Cochrane Collaboration attempted to identify the difference in functional, clinical, and radiologic outcome between retention and sacrifice of the PCL in TKA. A search for randomized, controlled trials comparing PCL retention and sacrifice published between 1966 and 2004 revealed only eight studies. These studies demonstrated no difference between CR and PS designs except for a range of motion increase of 8.1 degrees with PS knees. This Cochrane review published in 2007 stated “there is, so far, no solid base for the decision to either retain or sacrifice the PCL with or without use of a posterior stabilized design during total knee arthroplasty.” This review also emphasized the need for more knowledge regarding the proper technique to balance the PCL during surgery.

While in general there appears to be no significant difference between CR and PS designs when considering clinical scores, radiographic criteria, and survivorship after TKA, Pagnano et al. explored individual parameters in an attempt to identify any advantage of one design over the other. Kinematics refers to motion both translational and rotational. In a fluoroscopic study of total knee patients performing a single-stance deep knee bend, PS knees consistently exhibited posterior femoral rollback with flexion that more closely replicated normal knee kinematics. By contrast, CR knees demonstrated a paradoxical anterior femoral translation during deep knee bend. This abnormal translation of the femur on the tibia during flexion could limit flexion by creating impingement of the posterior tibia on the posterior femoral metaphysis, decrease the quadriceps moment by moving the tibiofemoral contact point anteriorly, and accelerate polyethylene wear by increasing shear forces at the articular surface. In another comparative kinematic study assessing a stepup activity, CR knees demonstrated rotation and translation movements that more closely resembled physiologic values for normal knees. In addition, more normal stair-climbing ability has been observed in CR knees. A functional review of various activities revealed that patients with a PS knee reported more limitations with squatting, kneeling, and gardening, while patients with a CR knee noted improved overall satisfaction.

CR and PS knees have been shown to behave similarly at low flexion angles such as the swing phase of gait. In most PS designs, the cam and post engage at flexion angles between 60 degrees and 90 degrees. Therefore, knee stability at flexion less than 60 degrees relies on soft tissue balancing including that of the PCL. In a cadaveric study, CR knees demonstrated posterior translation of the femoral condyles beyond as little as 30 degrees of flexion, while PS knees showed a significant increase in posterior translation only after 90 degrees of flexion. However, even at low flexion angles between 30 degrees and 60 degrees, CR knees have demonstrated anterior femoral translation during weight-bearing activities. The anterior femoral translation of CR knees noted at both low and high flexion angles has led some authors to question whether the physiologic function of the PCL can be preserved during TKA.

In general, compared with the normal knee, total knee designs demonstrate kinematic abnormalities such as reduced posterior femoral rollback during flexion, anterior femoral translation, abnormal axial rotation between the femur and tibia throughout a range of motion, and femoral condylar lift-off. Clinical outcome scores, in most studies, show no difference between patients with a CR total knee and those with a PS total knee. However, kinematic studies using fluoroscopic data demonstrate more consistent posterior femoral rollback in PS knees. Differences noted in kinematic studies could be attributed to multiple factors such as variations in implant geometry, coronal and sagittal alignment, soft tissue balancing, joint line restoration, and joint loading conditions for each individual patient. Therefore, implant design, patient factors, and surgical technique play vital roles in the clinical and functional outcome after TKA. The importance of surgical technique was confirmed by Dennis et al. in a multicenter kinematic study that reported wide variations in results among surgeons using the same implant.

Proprioception refers to joint position and can diminish with age and degenerative arthritis. Proprioception has been found to improve after TKA. This improvement may be due to a restoration of joint space and soft tissue tension, as well as a reduction in pain and inflammation. The PCL contains mechanoreceptors, and proponents of a CR design state that more normal proprioception is possible after a CR knee. However, two studies comparing joint position sense demonstrated no significant difference between CR and PS knees except that patients with severe degenerative changes preoperatively scored better with a PS knee. In a prospective randomized study evaluating proprioception, kinesthesia, and balance after TKA, there was found to be no advantage to preserving the PCL. PS knees more accurately reproduced joint position when extended from a flexed position. Histologic evaluations of the PCL obtained at the time of surgery reveal extensive architectural damage throughout the ligament, especially when associated with severe degenerative changes. Therefore, the PCL may not be providing the normal proprioceptive feedback even if properly balanced. Furthermore, a comparison of isokinetic and isometric muscle testing 6 to 13 years after TKA failed to show any difference between sacrificing and preserving the PCL.

The durability of a TKA depends on a low polyethylene wear rate and, therefore, a low incidence of osteolysis and aseptic loosening. The increased articular congruity evident in PS knees offers the advantage of lower shear forces and reduced polyethylene wear. However, the increased constraint transfers higher forces to the fixation and implant interfaces, which can increase polyethylene wear on the backside of the tibial insert. Therefore, a CR knee has the potential for increased shear forces and polyethylene wear due to a paradoxical anterior translation of the femoral component during flexion, while a PS knee may increase the incidence of backside wear. In addition, the tibial post in a PS knee can be a source of polyethylene wear debris because of tibial post impingement on the femoral component at low flexion angles. Retrieval studies have demonstrated wear and deformation of the anterior side of the tibial post. A cadaveric study revealed that the anterior tibial post impinges upon the superior aspect of the box of the femoral component at low flexion angles and hyperextension. Impingement during hyperextension was found to diminish anterior translation of the tibial component and act as a substitute for the anterior cruciate ligament (ACL). The tibial post may also be at risk for impingement during the rotational movements and condylar lift-off noted during the kinematic studies. This tibial post impingement not only increases the risk of wear debris but also can increase the force through the tibial post and locking mechanism, which can further escalate backside wear. The findings of this study led the investigators to recommend an avoidance of hyperextension, flexion of the femoral component, excessive slope of the tibial resection, and anterior placement of the tibial component while performing a TKA.

Correction of preoperative deformities is possible with both a CR and PS total knee. While a CR design mandates optimal balance of the PCL, a PS design offers possible easier correction of coronal deformities, especially when combined with a flexion contracture. A flexion contracture often requires a larger distal femoral resection so as to restore the extension space. Although both CR and PS knees require creation of symmetric extension and flexion gaps, the joint line must be restored in a CR knee to properly balance the PCL. Since a larger distal femoral resection will raise the joint line, a CR knee relies on an increased tibial resection to address a flexion contracture. This increased tibial resection may place the tibial component on weaker metaphyseal bone. A PS knee provides more freedom to raise the joint line, which aids in correction of a flexion contracture. However, improper gap balancing places the PS knee at risk for dislocation. The PCL often contributes to severe deformities, and its function can be significantly diminished if it requires an extensive release for balancing. An extensive PCL release also introduces the risk of late instability or rupture. Removal of the PCL creates at least a 1.0- to 1.3-mm increase in both extension and flexion gaps. Therefore, PCL resection offers the advantage of more straightforward gap balancing, improved access to the posterior aspect of the knee, and improved exposure to the proximal tibia while avoiding the need for subjective release of the PCL, especially with severe deformities.

A PS design requires removal of bone from the distal femur to accommodate the intercondylar box of the femoral component. Bone loss from the distal femur may increase the risk of periprosthetic supracondylar fractures. Pronounced bone loss due to stress shielding has been seen beneath the anterior flange of the femoral component with all knee designs. While increased bone resection from the distal femur is necessary with a PS knee, it appears that even a CR design develops bone loss secondary to stress shielding over time. However, dual energy x-ray absorptiometry (DEXA) of CR and PS knees at 5-year follow-up revealed increased bone loss of the distal femur in PS knees. Although DEXA measurements at 5 years were compared with those obtained within 1 week after surgery, patients with less than 5 degrees of varus or valgus alignment and a 10-degree flexion contracture received a CR knee while more pronounced deformities underwent a PS knee.

To address some of these issues, CR and PS knees are available with mobile-bearing or rotating platforms and high-flexion designs. A mobile-bearing or rotating platform knee allows rotational freedom at all angles of flexion. The rotating platform design is highly conforming at the articular surface and confers polyethylene bearing rotation to the polyethylene–tibial junction. This rotational freedom makes component placement, especially of the tibia, less critical. In addition, this high conformity of the femorotibial bearing surface minimizes contact stresses which may decrease polyethylene wear. Kinematic studies have revealed that fixed and mobile-bearing PS knees behave similarly except that a fixed bearing design provides greater posterior femoral rollback during flexion. This enhanced posterior femoral rollback may improve the potential range of motion of a fixed bearing knee. Fixed and mobile-bearing CR knees both demonstrate a paradoxical anterior femoral translation during flexion. By maintaining conformity at the femorotibial surface, the rotating platform design for PS knee may reduce the incidence of cam and post impingement seen at low flexion angles and with component rotation and condylar lift-off. At this time, there has been no clinical advantage of the rotating platform design compared with a fixed bearing surface. Most current total knee designs allow for flexion up to 130 degrees. A high-flexion knee is designed to attain 135 to 155 degrees of flexion. Modifications of a high-flexion knee include thickening and extension of the posterior condylar surface of the femoral component proximally, recession of the anterior aspect of the tibial polyethylene, and, in a PS knee, movement of the tibial post posteriorly. Enhancement of the femoral posterior condylar surface increases both the posterior femoral condylar offset and the articular contact against the tibial insert in an attempt to prevent posterior impingement. Posterior femoral condylar offset is defined as the distance between the posterior femoral condyle and a tangent drawn from the posterior cortex of the femoral shaft on a lateral radiograph. In a prospective, randomized study comparing standard and high-flexion PS knees, there was no difference in range of motion at a mean 2-year follow-up. Radiographic evaluation revealed increased posterior femoral condylar offset in high-flexion knees. Recession of the anterior aspect of the tibial polyethylene insert decreases the potential for patellar-polyethylene impingement during deep flexion. A modification of the cam and post mechanism allows increased contact between the cam and post throughout flexion. Another prospective, randomized study comparing standard and high-flexion PS knees demonstrated improved range of motion with the latter design. Since kinematic and clinical studies have revealed more consistent femoral rollback and greater range of motion in PS knees, use of a high-flexion knee may be more important with a CR design that has the potential for paradoxical anterior femoral translation and, thus, impingement of the posterior aspect of the tibia against the posterior femoral metaphysis during flexion.

INDICATIONS/CONTRAINDICATIONS

TKA is indicated for end-stage degenerative arthritis that no longer responds to nonoperative management. Specific indications for a CR versus a PS knee remain a topic of controversy. Proponents of PCL retention claim that correction of almost all deformities except a severe flexion contracture is possible with a CR knee. However, the exact definition of a severe flexion contracture is not well defined. In one comparative randomized study, patients with flexion contractures greater than 15 degrees were excluded. The function of the PCL may be compromised if it requires extensive release for proper soft tissue balancing. Supporters of PCL substitution note easier extension and flexion gap balancing even with severe deformities. However, success of a CR knee depends on a well-tensioned PCL, while that of a PS knee relies on equivalent extension and flexion spaces. Several preoperative conditions that may be more appropriate for PCL substitution include rheumatoid arthritis, previous patellectomy, prior proximal tibial or distal femoral osteotomy, or post-traumatic arthritis with disruption of the PCL. The synovitis associated with rheumatoid arthritis can lead to weakening of the PCL, which could result in instability or rupture after a CR knee. Laskin and O’Flynn reported on an increased incidence of late instability and recurvatum with CR knees in patients with rheumatoid arthritis. However, another comparative study of CR and PS knees in patients with rheumatoid arthritis revealed a 6.5% (2 of 31) incidence of late dislocation in PS knees, while no CR knees required revision for instability. A patellectomy places increased loads on the PCL by disrupting the normal four-bar linkage of the knee. Since these abnormal forces could result in late PCL attenuation and instability, some investigators recommend a PS knee in patients with prior patellectomy. Previous osteotomies of the proximal tibia or distal femur often mandate bony resections or augmentations that affect the position of the joint line. In these situations, a PS knee provides more flexibility for soft tissue balancing. A CR knee is contraindicated in cases where the PCL is found to be torn or incompetent such as with post-traumatic arthritis. A PS knee is contraindicated when one or both of the collateral ligaments are significantly lax or disrupted. Failure to obtain balanced extension and flexion gaps after PCL resection necessitates conversion to a varus-valgus constrained implant.

SURGICAL TECHNIQUE

Preoperative templating is performed for all cases prior to the procedure. A standing hip-knee-ankle radiograph is used to determine the proper valgus distal femoral resection angle that is required to create a resection perpendicular to the mechanical axis ( Figs. 11-1 and 11-2 ). The hip-knee-ankle radiograph is also helpful to identify any abnormality in femoral and/or tibial geometry, to plan the tibial resection, and to confirm the absence of any hip pathology ( Fig. 11-3 ). Routine standing anteroposterior (AP), lateral, and Merchant x-ray views are used to determine component sizing, presence of posterior femoral osteophytes, and proper patellar resection ( Figs. 11-4 through 11-7 ).