Ligamentous Balancing in Rotating-platform Knees

Abstract

The primary objectives of total knee arthroplasty are to reduce pain, maximize range of motion (ROM), and provide stability through the gait cycle. Pain relief depends on proper ligamentous balancing and solid implantation. Solid implantation is typically assured through cementation, whereas soft-tissue balancing is vital to ensure maximum ROM and stability. Special consideration should be given to obtaining proper axial alignment, symmetric tension on the ligaments in both flexion and extension, and equalizing the flexion extension gaps. Strategies to ensure proper alignment and ligamentous balancing intraoperatively are discussed.

The goal of total knee arthroplasty (TKA) is to reduce pain, maximize range of motion (ROM), and provide stability through the gait cycle. For pain relief to occur, the implant(s) must be solidly fixed and the ligaments properly balanced. Although solid fixation of the components is usually assured through cementation, maximizing ROM and providing stability through the gait cycle are determined by the surgeon’s skill in properly balancing the knee. The surgeon must recognize that TKA is a soft-tissue operation and that although bony carpentry remains important, the difference between good results and great results is proper soft-tissue balance.

Figure 1: Ligamentous tensioning device.

Ligamentous Balancing

Ligamentous balancing in rotating-platform knees is no different from balancing in fixed-bearing knees with regard to achieving predetermined goals. The surgical goals of TKA have not changed since its inception. The surgeon must obtain proper axial alignment as well as symmetric tension on the ligaments in both flexion and extension. Finally, the surgeon must equalize the flexion extension gaps. If the surgeon is able to achieve these three goals, the chances for patient satisfaction are great.

A stepwise surgical strategy focusing on each of these goals is important. Axial alignment is initially ensured by proper distal femoral and proximal tibial bone cuts, which are made using intramedullary alignment guides or extramedullary guides or with computer assistance. Preoperative hip-knee-ankle radiography is useful in obtaining proper axial alignment. A vertical line from the center of the femoral head to the middle of the knee is made. A transverse perpendicular line taken from this vertical line at the level of the distal femur ensures a cut parallel to the mechanical axis. The relative difference between the amount of bone taken off the medial and lateral distal femur is noted and should be reproduced at the time of surgery.

On the tibial side, a vertical line from the center of the knee to the center of the talus is made. A transverse perpendicular line is taken from this vertical line at the level of the proximal tibia, ensuring a tibial cut that is parallel to the mechanical axis. The relative difference between the amount of bone taken off the medial and lateral tibial plateaus is noted and should be reproduced at the time of surgery.

Because both cuts have been made parallel to the mechanical axis, axial alignment is ensured and the surgeon can move on to the second goal: obtaining symmetric tension on the ligaments in flexion and extension. To accomplish this, perfect rectangles must be established both in extension and in flexion. The rectangular extension space is achieved by releasing the tight concave side of the joint to “catch up” with the stretched convex side. This release is performed in a stepwise manner until a perfect rectangle is achieved with the ligaments equally tensed. The adequacy of this release can be tested by using a spacer block to determine how much the joint opens to slight varus stress compared with slight valgus stress.

Alternatively, a tensor can be used to ensure equal ligamentous tension on both sides of the joint in extension (Figure 1). The device shown in Figure 1 can deliver similar amounts of tension on each side of the joint by equalizing the joint force scale. If a release is complete and a rectangle has been established, the same millimeter reading will be noted on the gap scale. If the millimeter reading is less on one side, further release on that side of the joint is mandatory. Failure to fully release the concave side leads to unequal tension in the joint, and a trapezoidal rather than a rectangular extension gap will be made. A trapezoidal gap may limit ROM and put uneven stress on the polyethylene.

Varus and Valgus Knees

In a varus knee, the stepwise release in extension begins with the knee flexed 90°. Peripheral osteophytes are removed initially. A release of the deep medial collateral ligaments on the anteromedial aspect of the tibia is then performed. The next step is further release of the posterior medial corner of the proximal tibia continuing around to the midline if necessary. The depth of this release down the tibia is 1.5-2.0 cm from the cut tibial surface depending on the size of the patient. The insertion of the semimembranosus is protected in this release. If this release is insufficient to catch up with the stretched convex side, the superficial collateral is released, and a stripping of the structures down the posteromedial corner of the tibia underneath the pes tendons is performed. This is accomplished with a ½ inch osteotome sliding down the posteromedial aspect of the tibia. The length of the stripping ranges from 2-10 cm, depending on the degree of contracture. The surgeon should check the adequacy of the release every few centimeters of stripping to avoid over-releasing.

The pes tendons are important structures that help internally rotate the tibia and should not be released routinely. If a complete subperiosteal stripping still is not sufficient, a laminar spreader can be used to tense the medial side of the joint and residual tight medial structures piecrusted with a knife until equal tension is obtained.

In a valgus knee, a posterolateral corner release is made as described by Ranawat et al¹ to achieve a rectangle in extension. This release is performed intraarticularly with the knee in extension. A laminar spreader is placed on the lateral side of the joint, and the posterolateral capsular structures are released intraarticularly from just lateral to the popliteus posteriorly to the midcoronal plane laterally in line with the proximal tibia cut. This can be done safely with a cautery while an assistant positions his or her hands on the patient’s foot. Stimulation of the peroneal nerve will alert the surgeon that the release is too deep in the posterolateral corner.

If this release is insufficient, piecrusting of the iliotibial band a few centimeters above the joint can be performed. If this is still insufficient, release of the fibular collateral ligaments from the femur can be performed, although it is rarely necessary. Finally the popliteus can be released in extremely tight situations; however, we caution against routine release of this structure because this causes extreme lateral laxity in flexion.


Figure 2: Bony landmark methods to set rotation can lead to a trapezoidal flexion gap.	Figure 3: Trapezoidal flexion gap causing “nutcracker” pain on one side of the joint.

Flexion Balance

Once adequacy of the releases in extension has been assured by checking with a block or a tensor, a rectangular space in flexion is created. Obtaining a perfect rectangle in flexion is dependent on obtaining proper femoral rotation. Although a variety of methods based on bony landmarks have been described, it is counter-intuitive to believe that the same bony landmarks can be used to set rotation for every patient. Each knee requires a variety of different releases to balance that particular knee in extension. The surgeon must understand that previously performed releases used to recreate a rectangular extension gap affect how the ligaments respond in flexion. A minor release to create a rectangle in extension may have little effect on the ligaments in flexion. However, significant release to create a rectangular space in extension can have a marked effect on ligamentous tension in flexion (Figure 2).

At this center, the author described the inherent errors in using bony landmark methods to set femoral rotation due to the variability of extension releases.² It was found that if posterior condylar referencing is used, errors in rotation greater than 3º occurred 45% of the time (Figure 2). Such errors result in trapezoidal flexion gaps that can limit ROM and cause “nutcracker pain” as the patient tries to bend against an overly tightened ligament on the narrow side of the trapezoid (Figure 3). The author prefers the classic method described by Insall et al^3,4 in the 1970s. It is the preferred technique to achieve a perfect rectangle in flexion while obtaining proper femoral rotation. The knee is bent to 90º, and a laminar spreader or another distracting device is placed between the cut tibia and the uncut posterior condyles. Ligaments are tensed similarly on both sides of the joint, and an anteroposterior femoral cut parallel to the tibia is made (Figure 4). This ensures a rectangular flexion gap and symmetric contact of each condyle (Figure 5).


Figure 4: Classic method of setting femoral rotation.	Figure 5: Symmetric rectangular flexion gap.

Gap Balance

Once the anterior and posterior cuts are made, our final surgical goal can be addressed, which is equalization of the flexion and extension gaps. The gaps are usually checked with spacer blocks in both extension and 90º of flexion. If the gaps are unequal, an algorithmic approach to equalizing the gaps is undertaken. For example, if the extension gap is 2 mm smaller than the flexion gap, an additional 2 mm is resected from the distal femur creating equal flexion and extension gaps. Similarly if the flexion gap is smaller than the extension gap, the femoral component can be downsized to increase the flexion gap. In a mobile-bearing knee, it is critical to equalize these gaps to reduce the potential for bearing spin-out. The principle of leaving the knee loose in flexion to encourage ROM can lead to flexion instability or bearing spin-out and should be avoided. Solutions to different gap-balancing discrepancies are described in Tables 1-3.

Conclusion

Ligamentous balancing in a rotating-platform knee is no different from proper ligamentous balancing in a fixed-bearing knee. In our first 500 rotating-platform mobile-bearing knees, we have experienced no bearing spin-outs using the classic balanced-gap method of ligamentous balancing. Through careful attention to the details of ligamentous balancing outlined here, a surgeon can take advantage of the potential wear benefits inherent in a mobile-bearing design.

table 1

table 2

table 3

References

Ranawat AS, Ranawat CS, Elkus M et al. Total knee arthroplasty for severe valgus deformity. J Bone Joint Surg Am. 2005; 7(suppl 1):271-284.
Fehring TK. Rotational malalignment of the femoral component in total knee arthroplasty. Clin Orthop Relat Res. 2000; 380:72-79.
Insall J, Ranawat CS, Scott WN, Walker P. Total condylar knee replacement: preliminary report. Clin Orthop Relat Res. 1976; 120:149-154.
Insall J. Total knee replacement. In: Insall J, ed. Surgery of the Knee. New York, NY: Churchill Livingstone; 1984:587-695.

Author

Dr Fehring is from the OrthoCarolina Hip and Knee Center, Charlotte, NC.