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    Component Alignment in TKA: Mechanical or Kinematic?

    In a debate from the Pan Pacific Orthopaedic Congress, Dr. Norman Scott and Dr. Stephen Howell square off on the issue of whether mechanical alignment or kinematic alignment is preferable for patients undergoing total knee arthroplasty.

    I Align Along the Mechanical Axis
    W. Norman Scott, MD, FACS, Insall Scott Kelly Institute for Orthopaedics & Sports Medicine, New York, New York

    Mechanical alignment techniques include:

    • An overall assessment of the hip-knee-ankle alignment
    • Flexion/extension gap techniques
    • Measured resection techniques
    • Ligament releases

    These techniques are a composite of visually appropriate landmarks that allow the surgeon to build a checks-and-balances surgical technique. The anatomic landmarks are readily assessed at time of surgery, allowing the procedure to be reproducible.

    This is in contrast to the concept of kinematic alignment, which depends on the concept of 2 non-visualized transverse axes of the femur: The primary transverse axis, about which the tibia flexes and extends, and a secondary transverse axis about which the patella flexes and extends. The third axis is the longitudinal axis through the tibia, which is perpendicular to the transverse axis of the femur about which the tibia flexes and extends.

    Kinematic alignment is confirmed by calculating the asymmetry of the thickness or the distal and posterior femoral articulating surfaces. The whole technique centers around reestablishing the bone, cartilage, and kerf surfaces to recreate an un-visualized transverse axis of the femur. The surgical technique consists of 4 steps: (1) removing osteophytes; (2) adjusting the plane of the tibial cut; (3) releasing the posterior capsule; and (4) medializing or lateralizing the tibial component.

    This technique is done either via shape-matching the femoral component to the theoretical articular surface of the femur on a 3-D model, which has been created by filling in the worn surfaces with the use of proprietary software, or an unconventional use of a conventional system of instruments can be used to produce the kinematic alignment, as I am sure Dr. Howell will discuss.

    This theory, unlike mechanical alignment, allows for many hypothetical assumptions that are difficult to identify at time of surgery.

    Mechanical alignment is preferred over kinematic alignment because it:

    • Is based on decades of experience
    • Is reproducible
    • Does not require an adherence to non-visualized surgical landmarks

    Dr. Scott’s presentation can be found here.

    Kinematically Aligned TKA is Better
    Stephen M Howell, MD, University of California, Davis, Sacramento, California

    An accepted principle in total knee arthroplasty (TKA) is to restore normal kinematics, which is described by 3 axes in the knee. [2,4,5,8,9]

    In mechanically aligned TKA, the surgeon cuts the distal femur and proximal tibia perpendicular to the femoral and tibial mechanical axes. Insall called these cuts a compromise because they change the angle and level of the natural joint line. [10]

    Surgeons should be aware when mechanically aligning a TKA that they will frequently have to manage a wide range of collateral ligament imbalances that are complex, cumulative, and uncorrectable by collateral ligament release, and a wide range of changes in limb and knee alignment from normal. Patients who perceive these changes in stability, limb alignment, and knee alignment may be dissatisfied and require counseling. [4]

    In kinematically aligned TKA, the surgeon cuts the distal femur and proximal tibia to restore the natural angle and level of the joint lines, thereby minimizing these undesirable consequences. [2,4-6]

    A Level 1 randomized controlled trial (RCT) showed that kinematic alignment provided better pain relief and restored better function and range of movement compared with mechanical alignment. [1]

    A prospective study of 203 patients (208 knees) treated with a primary kinematically aligned TKA and evaluated at a median follow-up of 6.3 years (range, 5.8-7.2 years) showed that the revision-rate /100 component years for kinematically aligned TKA (0.40, 95% confidence interval (CI) 0.18 to 0.93) was no different from reports of mechanically aligned TKA (0.64, 95% CI 0.44 to 1.19). The mean Oxford Knee Score (42.7, 95% CI 41.6 to 43.7) was 10 points higher than reports of mechanically aligned TKA (32.6, 95% CI 31.9 to 33.4).

    Kinematically aligned TKA does not negatively affect 6-year implant survival and provides better function than mechanically aligned TKA, even though 80% of tibial components are aligned in varus. [7]

    Dr. Howell’s presentation can be found here.

    References

    1. Dossett Bone Joint J, In Press.
    2. Eckhoff J Bone Joint Surg Am, 2005.
    3. Eckhoff Bone Joint Surg Am, 2003.
    4. Gu Y J Bone Joint Surg Am, 2014.
    5. Hollister Clin Orthop Relat Res, 1993.
    6. Howell Knee Surg Sports Traumatol Arthrosc, 2013.
    7. Howell J Bone Joint Surg Am, Submitted
    8. Hungerford Orthop Clin North Am, 1982.
    9. Hungerford In Total Knee Arthroplasty, 1984.
    10. Insall JI Clin Orthop Relat Res, 1988.
    11. Iranpour J Orthop Res, 2010.