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How is Balance Achieved in the Varus Knee?

The degradation of intra-articular cartilage associated with knee osteoarthritis not only causes the patient discomfort, but it can permanently alter the alignment of the lower limbs. As the abnormal cartilage of the osteoarthritic knee breaks down, the femoral bone begins to settle into the joint space, usually with a tilt to the inside or the outside of the joint, resulting in visible bowlegged (varus) or knock-kneed (valgus) alignment. This unfavorable orientation malaligns the mechanical axis of the leg, and subsequently, can change the pattern of a healthy gait. Unfortunately, the progression of osteoarthritis and poor alignment work together in a positive-feedback loop – as malalignment worsens, knee osteoarthritis becomes more severe, and vice versa.1

In most cases of advanced osteoarthritis, patients present with a varus inclination of the knee.2 This clinical presentation may include a collapse of cartilage on the medial or inner side of the affected knee, with an associated overloading in tension of the ligaments on the opposite lateral side as well. In order to relieve tension in a varus knee, surgeons often elect to release the scarred and shortened surrounding medial soft tissue to open the collapsed joint space of the medial compartment and restore proper alignment. Release of these structures must be precise in order to ensure appropriate ligament balance through the full range of motion.

While there are several mechanisms that can be employed to release soft tissue, conservative and small corrections may avoid accidental ligament over-release and/or rupture. With that in mind, the “pie-crusting” technique utilizes an 18-gauge needle to make small interruptions to localized sections of ligament in a controlled manner. This technique, used in conjunction with feedback from intraoperative sensors, has been shown to be effective in the achievement of balance.3-5

Intraoperative sensors provide surgeons with quantitative feedback regarding soft tissue balance and intra-articular loading. The following techniques have been suggested to correct several states of imbalance in the varus knee:

1) Medial overload in extension only (Asymmetric Imbalance)

In this scenario, the surgeon may notice asymmetric loading during extension with excessive medial load present. The MCL may be the ligament responsible for this state. Additionally, the leg may be inhibited from reaching full extension. The posterior aspect of the MCL, the medial posterior capsule, or a combination of the two should be evaluated. The first indication for correction should be to palpate the posterior fibers of the MCL to assess tension and release such fibers in tension using a pie-crusting technique. Once the release(s) has been made, the surgeon should cycle the leg through the range of motion several times to encourage further lengthening of the ligament(s) addressed.

Upon re-evaluation, if the surgeon notices that the posterior MCL does not fully correct excess tension, release the medial posterior capsule and/or semimembranosus at the tibial attachment site. Following the release(s), the leg should be cycled again and loads checked for need of any further release to these structures.

1a. Medial tension in extension only (Symmetrical)
If symmetric loading is present in extension only with absolute mediolateral loading between 20 – 40lbf, release the posterior capsule. If absolute mediolateral loading is symmetric and greater than 40lbf, consider recutting the distal femur

2) Medial overload in flexion only (Asymmetric Imbalance)

In this scenario, the surgeon may notice asymmetric loading during flexion with excessive medial load. The first indication for correction should be to palpate the anterior fibers of the MCL to assess tension and release such fibers in tension using a pie-crusting technique. Once the release(s) has been made, the surgeon should cycle the leg through the range of motion several times to encourage further lengthening of the ligament(s) addressed.

If the surgeon notices that the medial femoral contact point exhibits excessive tension and posterior positioning, they may consider releasing the anterolateral bundle of the PCL fibers. Following this release, the leg should be cycled and loads checked for need of any further release to these structures.

2a.) Medial tension in flexion only (Symmetrical)
If symmetric loading is present in extension and flexion with excessive posterior rollback, visible anterior lift-off and absolute mediolateral loading is greater than 40lbf, the surgeon may consider adding more tibial slope. If absolute loading is less than 10lbf it may be necessary to increase the insert thickness.

3) Medial tension in both extension and flexion (Asymmetric Imbalance)

In this scenario, the surgeon may notice that the MCL feels contracted and under tension. The lateral side of the knee may exhibit gapping through the range of motion.

Extension balancing (loads 20 – 40lbf): The first indication for correction should be to release the posterior MCL fibers if in tension. After rechecking the loads, if needed, the posterior medial capsule should be checked for tension and released. The loads should be rechecked again, and if necessary, the semimembranosus can be released to relieve any residual tension.

Flexion balancing (loads 20 – 40lbf): Using the pie-crusting technique, the anterior fibers of the MCL should be released if in tension; loads rechecked.

If loading is beyond 40lbf on the medial side in both extension and flexion, consider recutting the tibia plateau to add more varus alignment.

If loading is symmetrical in both compartments the surgeon may increase tibia resection or decrease shim thickness.

References:

1 Cerejo R, Dunlop D, Cahue S, et al. The influence of alignment on risk of knee osteoarthritis progression according to baseline stage of disease. Arthri Rheum. 2002; 46(10): 2632-2636.

2 Sharma L, Song J, Dunlop D, et al. Varus and valgus alignment and incident and progressive knee osteoarthritis. Ann Rheum Dis. 2010; 69(11): 1940-1945.

3 Gustke KA, et al. Primary TKA patients with quantifiably balanced soft-tissue achieve significant clinical gains sooner than unbalanced patients. Adv Orthop. 2014:628695

4 Gustke K, et al. A new method for defining balance: Promising short-term outcomes of sensor-guided TKA. J Arthroplasty. 2014 May;29(5):955-60

5 Leone, WA. Using intraoperative sensing technology to guide revision in the chronically painful knee: A two-patient case study. ECOR. 2015; 2(2): 77-81.

 

Originally published on OrthoSensor.com.

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