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    PRACTICE PEARLS: Achieving Alignment in TKA

    Total knee arthroplasty (TKA) is one of the most successful operations in orthopaedic surgery. Although surgeons can utilize a variety of techniques and technologies when performing a TKA, the basic principles remain the same, making it a highly reproducible procedure with reliably good outcomes in most patients.

    During ICJR’s Instructional Course at the 9th Congress of the Chinese Association of Orthopaedic Surgeons, Donald M. Kastenbaum, MD, from Mount Sinai Health System in New York, shared the the basic principles and technical aspects of achieving coronal and rotational alignment of the TKA components.

    He also addressed emerging techniques and technologies for improving alignment, including kinematically aligned TKA, patient-specific instruments (PSI), and computer-aided surgery.

    To assist in the discussion of basic principles, Dr. Kastenbaum reviewed the axes of the knee:

    • Mechanical axis of the femur (MAF): a line from the center of the femoral head to the center of the distal femur
    • Anatomic axis of the femur (AAF): a line bisecting the medullary canal (it’s in 9° of valgus from the MAF)
    • Mechanical axis of the tibia (MAT): a line from the center of the proximal tibia to the center of the ankle
    • Anatomic axis of the tibia (AAT): a line bisecting the medullary canal (it’s in 3° of varus from the MAT)

    The net tibiofemoral angle is 6° of valgus.

    The goals of the proximal and distal bony cuts in TKA, Dr. Kastenbaum said, are:

    • To restore neutral mechanical alignment, as postoperative alignment more than 2.5° of varus/valgus has been shown to result in worse survivorship [1,2]
    • To maintain the level of the joint line to preserve ligament tension and function
      • Elevating the joint line more than 8 mm can lead to patella baja, mid-flexion instability, and patellar maltracking.
      • Excessive lowering of the joint line can lead to loss of full extension and flexion instability.

    Coronal alignment of the tibial component is set with the proximal tibial bone cut. This bone cut should be perpendicular to the MAT. The AAT can be established using either intra- or extramedullary guides. If the AAT and MAT are not coincident due to tibial deformity, Dr. Kastenbaum will use an extramedullary guide, centering the guide over the medial one-third of the tibial tubercle and the center of the ankle.

    Coronal alignment of the femoral component is set with the distal femoral bone cut. This bone cut should be perpendicular to the MAF. Dr. Kastenbaum uses an intramedullary guide to establish the AAF, and he recommends setting the cutting guide to 5° to 7° of valgus from the AAF.

    Patellar maltracking is one of the most common complaints after TKA, and it can lead to pain, increased component wear, possible dislocation, and, ultimately, failure of the TKA. To prevent patellar maltracking, the surgeon should aim for a normal Q-angle (11°±7°), proper component rotation, and normal patellofemoral tension, Dr. Kastenbaum said.

    Component positioning is important for optimal patellar tracking and the success of a TKA, Dr. Kastenbaum said. He emphasized that it’s critical to avoid these situations:

    • Internal rotation and medialization of the femoral component
    • Internal rotation and medialization of the tibial component
    • Lateralization of the patella button

    The native tibial plateau is in 3° of varus, but the proximal tibial bone cut is made perpendicular to the MAT. To avoid a trapezoidal flexion gap, Dr. Kastenbaum recommends externally rotating the femoral component by 3°.

    As long as the basic principles of femoral rotation are followed, either measured resection or gap balancing should lead to a successful operation, Dr. Kastenbaum said. Several landmarks can help the surgeon establish proper femoral rotation:

    • The anteroposterior (AP) axis (Whitesides Line) is a line from the intercondylar notch to the center of the trochlear groove and defines neutral rotation.
    • The trans-epicondylar axis is a line drawn from the center of the medial epicondyle to the center of the lateral epicondyle. Aligning the femoral component along the epicondylar axis provides appropriate femoral rotation for a rectangular flexion gap.
    • The posterior condylar axis is a line across the apexes of the medial and lateral posterior condyles. This line is in 3° of internal rotation, thus the femoral component must be externally rotated 3° to achieve a rectangular flexion gap.

    Which alignment method should be used? Dr. Kastenbaum reviewed a 1999 study by Olcott and Scott [3], which found that using the trans-epicondylar line is the most reliable method, with 90% accuracy.

    To set the correct tibial rotation, Dr. Kastenbaum recommends aligning the tibial component with the junction between the medial and central thirds of the tibial tubercle. The tibial component should rotate into alignment with the femoral component when the knee is brought into extension. Dr. Kastenbaum prefers using an asymmetrical tibial tray placed anatomically onto the cut tibial surface.

    Dr. Kastenbaum then addressed the role of technology in improving accuracy of component placement in TKA. He said PSI and computer-assisted navigation show promise. He cited a 2014 meta-analysis by Voleti et al [4] demonstrating that PSI may improve accuracy in femorotibial and hip-knee-ankle angle.

    In addition, a 2014 study by Cip et al [5] found that computer-assisted navigation resulted in a higher rate of alignment within 3° (90%) than conventional navigation. The computer-assisted navigation group also had higher Insall and HSS scores at 5 years of follow-up. [5]

    Dr. Kastenbaum noted that kinematic alignment in TKA is a new technique, in which 3D modelling based on preoperative magnetic resonance imaging fills the articular gaps and equalizes the gaps between medial and lateral compartments using PSI. The implant replicates the native knee motion on 3 axes, and the angle and level of the joint line match the natural alignment.

    Dr. Kastenbaum concluded his presentation by saying that every surgeon should strive for perfection – the patient deserves it. The surgeon must understand the extension and flexion gaps, and must continually evaluate these gaps during the procedure – neither gaps nor alignment will get any better once the patient leaves the operating room.

    Click the image above to watch Dr. Kastenbaum’s presentation from ICJR’s Instructional Course at the 9th Congress of the Chinese Association of Orthopaedic Surgeons.

    References

    1. Fang DM, Ritter MA, Davis KE. Coronal alignment in total knee arthroplasty: just how important is it? J Arthroplasty. 2009 Sep;24(6 Suppl):39-43
    2. Ritter MA, Davis KE, Meding JB, Pierson JL, Berend ME, Malinzak RA. The effect of alignment and BMI on failure of total knee replacement. J Bone Joint Surg Am. 2011 Sep 7;93(17):1588-96
    3. Olcott CW, Scott RD.. The Ranawat Award. Femoral component rotation during total knee arthroplasty. Clin Orthop Relat Res 1999;(367):39-42.
    4. Voleti PB, Hamula MJ, Baldwin KD, Lee GC. Current data do not support routine use of patient-specific instrumentation in total knee arthroplasty. J Arthroplasty. 2014;29:1709–1712.
    5. Cip J, Widemschek M, Luegmair M, Sheinkop MB, Benesch T, Martin A. Conventional versus computer-assisted technique for total knee arthroplasty: a minimum of 5-year follow-up of 200 patients in a prospective randomized comparative trial. J Arthroplasty. 2014;29:1795–802