Pretest

Introduction

A successful total knee arthroplasty (TKA) is a function of proper implant selection, proper alignment, and especially achieving proper soft-tissue balance. Establishing posterior stability (flexion balance) is a critical factor that is frequently overlooked during TKA. A common misconception is that a knee can be left loose in flexion as long as extension balance is obtained. In fact, it has been suggested that a knee somewhat loose in flexion would achieve rehabilitation more easily and have an improved overall flexion. Although tightness in flexion leads to pain, limited motion, and excessive polyethylene wear, flexion instability can also lead to excessive wear, pain, and the need for revision.

Posterior stability can be achieved through the soft tissues, the implant, or both. While the controversy between posterior cruciate retention and cruciate substitution continues, the published clinical results are the same.¹ Proponents of cruciate substitution believe it is easier to correct deformity and prefer this technique in most cases. Proponents of cruciate retention point to the importance of the posterior cruciate as a critical stabilizer of the flexion space and the fact that cruciate retention arthroplasty is more bone sparing and allows for an easier revision.

This author is an advocate of cruciate retention and cruciate substitution. In most primary cases, cruciate retention is preferred, as it is bone sparing and easier to balance the flexion space (Slide 1). The flexion space is determined medially by the strong medial collateral ligament that courses from femur to tibia, whereas the lateral space is determined by the short round lateral collateral ligament that goes from the femur to the fibula. The posterior cruciate ligament (PCL) balances this disparity and has been called the lateral ligament of the medial side of the knee. As most rotation during flexion occurs about a medial pivot point in the knee, the PCL serves an important role in achieving flexion or posterior stability.

Slide 1

Achieving proper balance is the difficulty reported in PCL retention. Residual cruciate tightness will limit flexion, cause abnormal "roll back," and increase wear. An excessively loose PCL will cause flexion instability.

In the author’s opinion, flat-on-flat articulations in PCL retention require too precise a balance to be predictably successful. A more conforming tibiofemoral articulation with an anterior "upsweep" that will give appropriate conformity during the healing phase following TKA is preferred. It is the extent of this conformity that differentiates the various schools of achieving posterior stability through use of implants.

Some advocate use of a central peg to provide additional support and state that it is predictable, reproducible, and consistent. Others believe the central peg is susceptible to wear from rotation, hyperextension, or excessive posterior load.² Moreover, linking the femur and tibia in a modular design increases "back sided" wear in modular tibial designs leading to pain, swelling, and osteolysis.³ Others advocate an ultracongruent anterior upsweep that obviates the need for a central peg.⁴ In a fixed bearing ultracongruent knee, however, rotational constraint is possible in these designs.

It is the author’s opinion that in most cases, cruciate preservation and use of a more conforming insert is preferred. These designs are available in a fixed bearing and mobile bearing option and the test for proper cruciate tension is different in the two designs. In a fixed bearing knee, a tight PCL will cause excessive medial roll back and tibial trial lift off (Slide 2). In a rotating platform cruciate sparing knee, a tight PCL will cause lateral "spinout" and reveal excessive anteromedial base plate metal ("too much metal sign") (Slide 3). In either case, proper soft-tissue balance can be achieved by recessing the PCL along its tibial insertion or releasing tight anterolateral fiber bundles from the femur. Following cruciate recession, the knee is again tested during trial reduction (Slide 4).

SLIDE 2		SLIDE 3		SLIDE 4

In summary, the author advocates posterior cruciate retention with a more conforming articulation in most primary TKAs and stresses the importance of assessing proper cruciate tension in fixed bearing and mobile bearing designs.

References

1. Schai PA, Scott RD, Thornhill TS. Total knee arthroplasty with posterior cruciate retention in patients with rheumatoid arthritis. Clin Orthop. 1999; 367:96-106.



2. Engh GA, Lounici S, Rao AR, Collier MB. In vivo deterioration of tibial baseplate locking mechanisms in contemporary modular total knee components. J Bone Joint Surg Am. 2001; 83:1660-1665.



3. Puloski SK, McCalden RW, MacDonald SJ, Rorabeck CH, Bourne RB. Tibial post wear in posterior stabilized total knee arthroplasty. An unrecognized source of polyethylene debris. J Bone Joint Surg Am. 2001; 83:390-397.



4. Hofmann AA. No post or tether required: A deep dish will do. Orthopedics. 2001; 24:889-890.