Healio
Healio
Issue: February 2019
ByDavid F. Scott, MD
ByStephen M. Howell, MD
February 14, 2019
7 min read
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Kinematic alignment possible with manual instrumentation, medially stabilized implants

Issue: February 2019
ByDavid F. Scott, MD
ByStephen M. Howell, MD
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Total knee arthroplasty is a successful surgical procedure that relieves pain and restores function. However, TKA has not had the same reported patient satisfaction rates as total hip arthroplasty. Mechanical alignment was first described by John Insall in 1985 and has become the most common surgical approach for TKA, used worldwide by most surgeons. Mechanical alignment places the implant in an average position for all patients based on a predetermined definition of acceptable alignment and has been especially successful when coupled with posterior-stabilized devices. Most implant systems and instrumentation are optimized for use with mechanical alignment.

Kinematic alignment is an innovative TKA paradigm associated with long-term implant survival and high patient function that is an alternative to mechanical alignment. The kinematic alignment technique for TKA was developed by Howell in 2006 and first described in 2008. In contrast to mechanical alignment, kinematic alignment places the implant in a custom position for each patient, so the native, or pre-arthritic, femoral and tibial articular surfaces, limb and knee alignment unique to the individual, are restored. Kinematic alignment co-aligns the axes and joint lines of the components with the three kinematic axes and joint lines of the native knee, and balances the knee through bone resection and by minimizing the need for ligament release.

The first kinematic axis passes through the centers of the femoral condyles in the femur and delineates the arc of flexion-extension of the tibia with respect to the femur. The second axis, which is parallel, proximal and anterior to the first axis, is in the femur and delineates the arc of motion of the patella with respect to the femur. The third axis is in the tibia and passes perpendicular to the other axes through the center of the medial femoral condyle, and controls internal-external rotation of the tibia with respect to the femur. Although kinematic alignment has not yet been widely adopted, four meta-analyses, three randomized trials and a national multicenter study have found that patients who underwent kinematic alignment TKA had significantly better pain relief, function, flexion, and a more “normal-feeling” knee than those who underwent mechanical alignment TKA.

Calipered kinematic alignment

The articular surfaces are resurfaced by resecting an amount of bone, that, when cartilage loss and kerf of the saw blade are included, equals the thickness of the implant. Recording the caliper measurements on a verification worksheet during the procedure confirms proper positioning of the components before cementing (Figure 1). Decision trees are used to aid balancing with either PCL-retaining or PCL-substituting medially stabilized devices.

Decision Trees

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The first surgical goal achieved by the kinematic alignment technique is to restore the native joint lines, Q-angle, and limb alignment unique to each patient. The second surgical goal is to restore the laxities, tibiofemoral compartment forces and knee adduction moment of the native knee without ligament releases.

Alignment: Femoral component

Measure the offset of the distal femoral medial condyle to the anterior tibia with the knee in 90º flexion. This step is used when preserving the PCL, and quality assurance (QA) check #1: Reference this measurement during trialing.

TKA with manual instrumentatiom
Figure 1. The quality assurance record or intraoperative worksheet is shown.
Figure 2. Shown is the starting distal femoral drill hole position, as well as complete medial cartilage loss that is typically seen with varus osteoarthritis.
Figure 3. This shows use of the offset distal femoral guide that is marked “worn/unworn” and lays flat on the distal femoral articular surface.
Figure 4. Shown is the distal femoral guide with 2-mm build-up on the worn side that substitutes for the missing cartilage.
Figure 5. The EM tibial guide is shown.
Figure 6. The position of the EM guide with respect to the ankle is usually offset from center.
Figure 7. Shown is the tibial cut guide positioned to perform a symmetric resection.
Figure 8. Match of the tibial slope using the guide is shown.

Source: David F. Scott, MD

Set the flexion-extension orientation of the femoral component by starting the drill hole for the positioning rod midway between the top of the intercondylar notch and the anterior cortex (Figure 2). QA check #2: Confirm there is a 5 mm to 10 mm bridge of bone between the posterior rim of the hole and the top of the intercondylar notch, which reduces the risk of flexing the femoral component and of patella instability.

Set the proximal-distal position and varus-valgus orientation of the femoral component by using an offset distal referencing guide with a 2-mm offset to compensate for complete cartilage loss (Figures 3 and 4). QA check #3: Measure the thickness of the medial and lateral distal femoral resections and confirm that when 2 mm are added for cartilage loss and 1 mm is added for the kerf of the saw blade, they are within 0 ± 0.5 mm of the thickness of the femoral implant.

Set the anterior-posterior position and internal-external orientation of the femoral component by selecting a posterior referencing guide set in 0° rotation and position the feet of the guide in contact with the posterior femoral condyles. Set the stylus on the anterior femur and size the femoral component. QA check #4: Measure the posterior femoral resections. Confirm that when adding 2 mm for cartilage loss and 1 mm for the kerf of the saw blade, the resections are within 0 ± 0.5 mm of the thickness of the femoral implant. Fine-tune the thickness of these resections when needed, before performing the anterior and chamfer resections.

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Alignment: Tibial component

Resect the proximal tibia using an extramedullary (EM) tibial resection guide (Figures 5 and 6), matching the anatomy in the coronal (Figure 7) and sagittal planes (Figure 8), compensating for cartilage loss, as well as any bone loss. Natural varus, which is present in many cases (Figure 9), and posterior tibial slope are restored (Figures 10-12).

Pre and postop radiographs
Figure 9. The patient’s preoperative radiograph shows the varus tibia.
Figure 10. A long, standing radiograph view demonstrates bilateral varus alignment after the right knee is replaced according to kinematic alignment principles.
Figure 11. This shows appropriate selection of posterior slope that matches the 10° native slope.
Figure 12. A post-surgical radiograph illustrates correct placement of the tibial component in the desired varus position.

QA check #5: Measure the thickness of the medial and lateral tibial condyles by referencing the base of the tibial spine in an area that has intact cartilage.

QA check #6: Insert the tightest fitting spacer block between the femur and tibia. Like the native knee, the extension space should have a rectangular shape with tight medial and lateral gaps and negligible varus-valgus laxity. The flexion space should be trapezoidal with a tighter medial gap and looser lateral gap. When the extension space is 1 mm to 2 mm loose on one side (medial or lateral), re-cut the tibia with a 1° to 2° valgus or varus re-cut guide following the logic outlined in the decision trees.

Visually size and position the anatomic tibial baseplate, selecting the size and adjusting rotation to best cover the bone surface without overhang. The position of the tibial tubercle (TT) with respect to the baseplate is irrelevant; use of a medially stabilized device allows for anatomic rotational freedom.

Insert trial components and assess varus-valgus laxities through full range of motion while referring to the corrective measures in the GMK Sphere CR and Sphere CS decision trees (Medacta). The anterior tibial offset measurement is used to adjust tibial slope and depth of resection as required in PCL-retaining cases; it is not used with PCL-sacrificing cases.

Note that the tibial mechanical axis, intramedullary canal and position of the TT are not referenced with kinematic alignment.

Kinematic alignment implant survival and function were assessed in 222 knees (217 patients) at 10 years in a single-surgeon series without restricting preoperative varus-valgus and flexion deformity, and without restricting postoperative correction (Howell 2018). Using aseptic revision at 10 years as the endpoint, the 98.4% implant survival rate was 5.5% higher than the implant survival of about 93% after 398 mechanical alignment TKAs performed in the United States and 4.5% higher than the implant survival of about 94% after 270 mechanical alignment TKAs performed in the United Kingdom. The estimated number of revisions for 1,000 patients is 15 for kinematic alignment, and 70 and 60, respectively, for the U.S. and U.K. studies of mechanical alignment TKAs. In the kinematic alignment 10-year study, four revisions were for excessive flexion of the femoral component (n = 3) and reverse slope of the tibial component (n = 1) in the sagittal plane. The postoperative alignment of the tibial component, knee, and limb in varus and valgus outlier ranges according to mechanical alignment criteria does not adversely affect the 10-year implant survival, yearly revision rate, and function as measured by the Oxford Knee Score (OKS) and the WOMAC score. Restoring the native joint lines, Q-angle and limb alignments unique to each patient results in high function as measured by a 45-point median OKS and long-term implant survival regardless of the degree of preoperative varus-valgus and flexion deformity and the postoperative alignment.

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Key points

  • First surgical goal: Restore the patient’s native joint lines, Q-angle and limb alignments;
  • Second surgical goal: Restore laxities, tibiofemoral compartment forces and knee adduction moment without ligament release;
  • Individual native limb alignment and ligament balance are restored by appropriate bone resections without ligament releases, with the possible exception of an extreme valgus deformity with excessive medial laxity; and
  • The use of kinematic alignment is increasing. Among several studies, patients who underwent kinematic alignment TKA had better pain relief, function, flexion and a more “normal-feeling” knee than those who underwent mechanical alignment TKA.
  • References:
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Disclosures: Howell reports he receives institutional support and consulting fees from Medacta USA and ThinkSurgical. Scott reports he receives institutional support from Medacta USA, Stryker, OMNI, MicroPort; and receives consulting fees from Medacta USA.

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