Why Would Surgeons Want to Adopt Calipered Kinematic Alignment for TKA?

    Dr. Stephen Howell answers ICJR’s questions about calipered kinematic alignment total knee arthroplasty, including the learning curve, how to set the femoral and tibial components, and how to reduce the risk of complications.

    ICJR: Why do kinematic and mechanical alignment use different targets for setting the femoral and tibial components?

    Stephen M. Howell, MD: Although mechanically aligned total knee arthroplasty (TKA) has had great success in restoring function and relieving pain, a substantial portion of TKA patients – between 15% and 30% – remain dissatisfied. Calipered kinematic alignment (KA) reduces the number of unhappy patients by restoring the pre-arthritic joint lines and co-aligning the axes of the components with the 3 kinematic axes without releasing healthy ligaments.

    A significant limitation of mechanical alignment is that most patients’ pre-arthritic joint lines and limb alignment are changed to achieve a limb alignment of 0°. Hirschmann’s phenotype studies show that mechanical alignment places 84% of femoral components in more varus and 70% of tibial components in more valgus than the pre-arthritic joint line. [1]

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    Mechanical alignment also changes the limb alignment in 3° or more valgus for the 32% of the native population with constitutional varus and 3° or more varus for the 27% of the native population with constitutional valgus. [2] Changing the patient’s pre-arthritic joint lines and limb alignment creates instabilities not correctable with ligament release, which can result in a knee that feels unnatural to the patient. [3,4]

    Calipered kinematic alignment, conceived in 2006, uses the “kinematic” target of restoring the patient’s pre-arthritic joint lines, which co-aligns the rotational axes of the components with the 3 kinematic axes of the knee. This concept is based on the seminal findings in 3 studies:

    • In 1993, Hollister et al [5] discovered a single transverse axis in the femur connecting the center of the medial and the lateral femoral joint lines and parallel to the native joint line about which the tibia flexes and extends (Figure 1).
    • In 2003, Coughlin et al [6] identified a single transverse axis in the femur about which the patella flexes and extends that is parallel and approximately 1 cm proximal and anterior to the first axis.
    • In 2005, Freeman and Pinskerova [7] located a vertical axis in the knee slightly posterior to the center of the medial compartment and perpendicular to the native joint lines and 2 transverse axes, which the tibia internally and externally rotates under joint compression about the femur.

    Figure 1. Projections of a right femur (left) show the orthogonal relationships among the 3 kinematic axes of the native knee and the distal and posterior femoral joint lines, and kinematic alignment of the femoral and tibial components (right) coincident to the native joint lines. The flexion axis of the tibia is the green line, the flexion axis of the patella is the magenta line, and the longitudinal rotational axis of the tibia is the yellow line. All 3 axes are closely parallel or perpendicular to the joint lines. Compensating for wear and kerf and resecting bone from the distal parts of the femur and femoral condyles equal in thickness to the condyles of the femoral component kinematically co-aligns the axes of the components with those of the native knee.

    ICJR: What type of learning curve is involved with adopting calipered kinematic alignment?

    Dr. Howell: Kinematic alignment relies on caliper measurements of the bone resections and intraoperative recording of serial verifications checks to accurately quantify the accuracy of each surgical step. The importance of verification checks cannot be over-emphasized: They minimize mistakes while the surgeon is “assembling the TKA,” much like quality assurance steps improve the reliability of the process of manufacturing a high-quality car (Figure 2).

    Figure 2. This composite of images shows the verification worksheet (left) and the millimeter recordings of the thicknesses of the distal and posterior femoral bone resections compared with those of the condyles of the femoral component (right). The femoral component is kinematically aligned when the femoral resections match the thicknesses of the condyles of the femoral component within ± 0.5 mm after compensating for approximately 1 mm from the loss of bone from the kerf of the saw blade and 2 mm for cartilage loss.

    Open-minded surgeons proficient with a caliper quickly learn kinematic alignment when they place no limits on restoring the pre-arthritic joint lines and follow the verification checks. The process is slow when a surgeon converting from mechanical alignment tries to reconcile the differences between the kinematic and mechanical alignment targets and limits the correction from the patient’s pre-arthritic joint lines. This tactical error tarnishes the results.

    As novelist F. Scott Fitzgerald wrote: “The test of a first-rate intelligence is the ability to hold two opposed ideas in mind at the same time and still retain the ability to function.” Surgeons who simultaneously try to satisfy opposing mechanical and kinematic alignment targets may become confused and may not learn at all. Those who follow just the kinematic alignment targets and intraoperatively record bone resection thicknesses on a verification worksheet tend to quickly master the technique.

    ICJR: How is the femoral component set?

    Dr. Howell: The calipered technique accurately sets the femoral component coincident to the patient’s pre-arthritic joint lines by recording the measurement and adjusting the thickness (when necessary) of bone resections and following a series of verification checks (see Figure 2, above). The distal femoral condyles are examined intraoperatively for cartilage wear. An offset distal referencing guide is selected to compensate for cartilage wear, which averages 2 mm. [8] A caliper measures the thickness of the distal femoral resections to ± 0.5 mm.

    For a femoral component with 9 mm-thick distal condyles, the target bone resection is 6 mm on the worn side, including 2 mm to compensate for cartilage wear and 1 mm for the kerf of the blade, and 8 mm on the unworn side, including 1 mm to compensate for the kerf. This step accurately restores the proximal-distal position and varus-valgus orientation within 0 ± 0.5 mm of the pre-arthritic joint line. [9,10]

    The anterior-posterior position and internal-external orientation of the femoral component is set coincident with the patient’s pre-arthritic joint line with a 0° posterior referencing guide. In most varus knees, cartilage is present at 90° and compensation for wear is not needed. In about 30% of valgus knees, the guide is rotated 2 mm posterior on the lateral condyle to compensate for cartilage loss.

    For a femoral component with 8 mm-thick posterior condyles, the target bone resection is 5 mm on the worn side, including 2 mm to compensate for cartilage wear and 1 mm for the kerf of the blade, and 7 mm on the unworn side, including 1 mm to compensate for the kerf. This step accurately restores the anterior-posterior position and internal-external orientation within 0 ± 0.5 mm of the pre-arthritic joint line. [11]

    ICJR: How is the tibial component set?

    Dr. Howell: The TKA is balanced by following 6 corrective measures, outlined in a decision tree, that adjust the V-V and slope of the tibial resection to the pre-arthritic knee and the thickness of the insert (Figure 3). Because the femoral component restored the patient’s pre-arthritic joint line, the surgeon does not need to release any ligament, including the posterior cruciate ligament, or recut the femur.

    Figure 3. This 6-step decision tree assists the surgeon in balancing a calipered kinematically aligned TKA when a posterior cruciate ligament-retaining insert is used. Fine-tuning the varus-valgus inclination and posterior slope of the plane of the tibial resection and adjusting the insert thickness restores the native laxities of the extension and trapezoidal flexion space and the native tibial compartment forces without ligament releases.

    In knees with severe flexion contracture, manipulating the knee into hyperextension with the trial component will stretch the contracted posterior capsule. One trick especially useful in the valgus knee is to insert the femoral trial component before preparing the tibia and then to stretch the posterolateral capsule by manipulating the knee into hyperextension. The complete correction of the valgus deformity with this step can be surprising and gratifying, as it simplifies the treatment of the severe valgus deformity without ligament release. [12]

    Tightness in the medial or lateral gap in full extension is corrected by fine-tuning the tibial resection in 1° or 2° increments of varus or valgus, respectively, with a recut guide. The kinematic target is a rectangular extension space with negligible varus and valgus laxities, like the native knee. [13]

    Restoration of the patient’s pre-arthritic slope is the target when the insert retains the posterior cruciate ligament (PCL). The tibial slope should be reduced to tighten the flexion space if the PCL is incompetent and the knee is tight in extension and loose in flexion. If reducing the slope is not an option, the surgeon can reduce the slope and tighten the flexion space by grafting a 4 mm- to 5 mm-thick posterior wedge of bone from the tibial resection to the posterior tibia.

    Experimental and surgical experience suggests that tibial bone correction in increments of 1 mm to 2 mm or 1° to 2° restores the native tibial compartment forces without the need for a costly 1-time use of the intraoperative force sensor built into a disposable tibial insert. [14-16]

    ICJR: How do patient-reported outcomes after calipered kinematically aligned TKA compare with those of patients with a mechanically aligned TKA?

    Dr. Howell: There is general agreement that outcomes of TKA after kinematic alignment are better than after mechanical alignment. In 2014, a multicenter national study reported that kinematic alignment restored a more normal-feeling knee than mechanical alignment. [17] Between 2012 and 2019, 7 of 8 randomized or case-controlled trials comparing kinematic and mechanical alignment found that kinematic alignment was associated with better motion, better alignment, better clinical outcomes, better balancing, less soft-tissue release, and comparable tibial component migration (Figure 4). [18-24]

    Figure 4. Table summarizing the 8 randomized and case-controlled trials published between 2012 and 2019 that compared kinematic and mechanical alignment.

    In the randomized trial showing similar results for kinematic and mechanical alignment, the authors had only included patients with small deformities and had limited postoperative correction within bounds recommended for mechanical alignment. [25] Both limitations biased the results in favor of mechanical alignment and against kinematic alignment.

    One way to assess the value of calipered kinematic alignment is to use the technique in the contralateral knee in patients with a previous mechanically aligned TKA. In a study of 78 patients, calipered kinematically aligned TKAs had a 15-point higher median Forgotten Joint Score and were more likely to recover faster and be the favorite knee. [26]

    ICJR: What is the long-term implant survivorship of calipered kinematically aligned TKA?

    Dr. Howell: Calipered kinematic alignment provides similar or better long-term implant survival compared with mechanical alignment, which is explained by kinematic alignment’s lower knee adduction moment and medial stress during gait. [27]

    A 10-year follow-up study evaluated 222 kinematically aligned TKAs that were performed without inclusion restrictions, that were based on the preoperative deformity, and that that were done without limiting the degree of postoperative correction. The researchers reported a 98.4% rate of aseptic implant survivorship, despite alignment of a high proportion of knees and limbs outside the recommended limits according to mechanical alignment criteria. [28] High implant survival occurred even though 27% of the limbs were outside of ± 3° from the mechanical axis (mostly in varus), with up to 6° of varus inclination of the tibial component from the tibial mechanical axis. [28]

    These findings, which do not support categorizing kinematic alignment as an outlier using mechanical alignment criterion, are comparable to those of a combined New Zealand and Australian registry study that reported a survival rate of 96.9% after kinematically aligned TKA and 97% after mechanically aligned TKA at a mean of 7 years. [29]

    ICJR: What are the potential complications of kinematic alignment and how can the risk be reduced?

    Dr. Howell: All surgical techniques are associated with postoperative complications. In the case of calipered kinematic alignment, it is essential for surgeons to learn the validated intraoperative verification checks that help to reduce the risk of complications.

    Patellofemoral instability is a potential complication after calipered kinematic alignment, primarily caused by excessive flexion of the femoral component. The incidence is 0.4%, which is comparable to the incidence with mechanical alignment. The verification check is to maintain a 5-mm to 10-mm bone bridge between the posterior aspect of the positioning hole and the apex of the notch when setting the positioning hole for the intramedullary rod that sets the flexion of the distal femoral cutting block. Maintaining this bridge limits flexion of the femoral component, reducing the risk of patellofemoral instability. [30-32]

    Another complication after calipered kinematic alignment is the failure of the tibial component, which occurs from posterior (and not varus) subsidence of the baseplate or posterior insert wear. The primary cause is setting the tibial component in more posterior slope than the patient’s pre-arthritic slope, in conjunction with a PCL-retaining insert. The incidence is 0.3%, which is lower than the 1.2% failure rate after mechanical alignment reported by Ritter et al. [33,34]

    Excessive posterior slope slackens the flexion space, enabling excessive anterior translation of the tibial component and posterior tibial overload that manifests as posterior baseplate subsidence or posterior rim wear of the insert. [33,35] The verification check includes matching the pre-arthritic slope, retaining the PCL, using a medial ball and socket insert to restore anteroposterior stability, and using a flat lateral insert without a posterolateral rim to reduce the risk of posterior rim wear of the insert (Figure 5).

    Figure 5. The images on the left show the tibiofemoral relationships in the medial and lateral compartments of the native knee in full extension and full extension, with the medial femoral condyle hardly moving (orange square) and the lateral femoral condyle rolling posterior in full flexion (orange rectangle). The right images show the design of the cruciate retaining (CR) insert, which features a medial ball-and-socket and a lateral flat surface without a posterior rim. The use of this medial-stabilized implant design is a promising strategy for promoting anteroposterior stability and reducing the risk of late tibial component failure resulting from posterior rim wear of the insert.

    Extension and flexion space tibiofemoral instability can occur after calipered kinematic alignment. Extension instability results from failing to restore a tight rectangular extension space like the native knee. The primary cause is surgeon error, which occurs from incorrectly gap-balancing the tibial resection’s varus-valgus plane to match the distal femoral resection.

    The verification check is to insert the spacer block between the distal femoral and tibial resection with the knee in full extension and confirm negligible varus-valgus laxity. The check is repeated with trial components. When there is a 1mm to 3mm medial or lateral compartment gap with the knee in extension, use a recut guide to perform a 1° to 2° valgus or varus adjustment and eliminate the compartment gap and establish a tight rectangular extension space.

    Flexion space instability occurs when the flexion space is loose from removing the PCL. Reducing the slope to less than the patient’s pre-arthritic posterior slope reduces the risk. Either recutting the tibia with less slope or bone grafting the posterior tibial resection with a wedge removed from the tibial resection reduces the posterior slope and the risk.

    The verification checks, performed with the knee in 90° of flexion and the leg suspended by lifting the thigh, are a negligible posterior drawer when using a cruciate substituting medial stabilized insert and a negligible distal distraction when using a posterior stabilized (PS) insert.

    Author Information

    Stephen M. Howell, MD, is a Professor of Biomedical Engineering at the University California at Davis in Sacramento. He practices at Adventist Health/Lodi Memorial Hospital in Lodi, California.

    Disclosure: Dr. Howell has disclosed that he receives royalties from and is a consultant for Medacta.


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