Five Quality Assurance Steps for Balancing a Kinematically Aligned Total Knee Arthroplasty



Fig. 8.1
A right femur (left) and kinematically aligned TKA (right) shows the relationships between the three kinematic axes of the knee and the joint lines of the distal and posterior femoral resections and the 6 degree-of-freedom position of the components [3]. 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 three axes are closely parallel or perpendicular to the joint lines. The flexion-extension plane of the extended knee lies perpendicular to the flexion axes of the tibia and patella and centered in the knee. Compensating for wear and kerf and resecting bone from the distal and posterior femur condyles equal in thickness to the condyles of the femoral component kinematically aligns the femoral component by co-aligning the axis of the femoral component with the flexion axis of the tibia assuming that the condyles of the femoral component are symmetric in the flexion-extension plane of the tibia





8.3 Goal Two: Restore the Native Knee and Limb Alignments


The second goal of kinematically aligned TKA is to restore the native knee and limb alignments [3, 4, 15, 17]. Several studies support correction to the native or “constitutional” alignment when performing TKA as opposed to creating mechanical alignment to neutral (Fig. 8.2) [15, 1820]. Creating mechanical alignment in patients with constitutional varus and valgus alignment is unnatural and causes greater strain deviations in the medial and lateral collateral ligaments from the native knee [15, 18, 21, 22]. Patients with preoperative varus have better clinical and functional outcome scores and the same implant survivorship at 7 years when the alignment is left in the native varus, as compared with patients overcorrected to neutral [19]. At a mean of 6 years after kinematically aligned TKA, restoration of the native alignments of the knee, limb, and tibia did not adversely affect implant survival and resulted in high function, which supports the consideration of kinematic alignment as an alternative to mechanical alignment when performing primary TKA [4].

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Fig. 8.2
Composite shows (1) the kinematically aligned TKA (left patient) restores the natural tibial-femoral joint surface (blue line) and the natural limb alignment (white line) and coaligns the axes of the femoral component with the flexion axes of the tibia (green line) and patella (magenta line), and (2) the mechanically aligned TKA (right patient) changes the natural tibial-femoral joint surface (red line), the natural limb alignment, and malaligns the axes of the femoral component oblique to the flexion axes of the tibia and patella. Studies have shown that kinematic alignment has less varus limb and varus knee outliers and has the same average limb and knee alignment as mechanical alignment [2, 34, 35]

Current evidence suggests that the native alignment of the limb does not cause osteoarthritis of the knee. The clinical findings of bilateral osteoarthritis with a varus deformity in one knee and a valgus deformity in the other (“wind swept”), and the lack of osteoarthritis in the majority of elderly Asian patients with severe constitutional varus suggest that native alignment plays little role in the development of osteoarthritis. Instead, the onset of osteoarthritis is associated with known changes in cartilage metabolism that occur with aging. Articular cartilage is a mechanosensitive tissue that, when healthy, increases anabolic activity and thickens when loaded. Chondrocytes experience age-related declines in their anabolic activity and thickening response which causes osteoarthritis as the ability to respond and compensate for high loads from activity and obesity is gradually lost [23].


8.4 Goal Three: Restore the Native Laxities of the Knee


The third goal of kinematically aligned TKA is to restore the native laxities of the knee, which are tighter at 0° of flexion than at 45° and 90° of flexion [16, 24] (Fig. 8.3). At 0° of flexion the native tibia-femoral joint behaves as a rigid body since the average varus (0.7°), valgus (0.5°), internal (4.6°), and external (4.4°) rotations of the tibia on the femur are negligible under applied loads that just engage the soft tissue restraints [16, 24, 25]. At 45° and 90° of flexion, the mean laxity is fivefold greater in varus (3.1°) rotation; fourfold greater in distraction; threefold greater in valgus (1.4°), internal (14.6°), and external (14.7°) rotation; and twofold greater in anterior translation than at 0° of flexion [16, 24]. The maintenance of these native differences in laxities between positions of knee flexion requires the maintenance of the native resting lengths of the collateral ligaments, posterior cruciate ligament, and retinacular ligaments. The alignment goal of gap balancing a TKA overtightens the laxities of the flexion gaps at 45° and 90° of flexion to match those at 0° of flexion, which patients may perceive as pain, stiffness, and/or limited flexion [6, 16].

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Fig. 8.3
A composite shows column graphs of the natural varus (+), valgus (−), internal (+), and external (−) rotational laxities of the normal knee at 0° and 90° of flexion (a and b) and the natural gaps of a right knee at 0° and 90° of flexion after making the resections using kinematic alignment (c) [24, 36]. Those paired columns connected by a p-value less than 0.05 indicate the laxity at 90° is greater than at 0° of flexion. The resected right knee shows a symmetrically shaped gap that is equal medially and laterally at 0° of flexion and an asymmetrically shaped gap lesser medially than laterally at 90° of flexion. Therefore, the surgical goal of gap balancing a TKA over-tightens the flexion gap. Error bars show ±1 standard deviation

Restoring the native laxities of knee at 0° of flexion requires removal of all osteophytes, extending the knee to 0°, and adjusting the varus-valgus angle and thickness of the tibial component until the varus, valgus, internal, and external rotational laxities are negligible [3]. Flexing the knee to 90° and adjusting the anterior-posterior slope and thickness of the tibial component until the offset of the anterior tibia from the distal medial femoral condyle measured at the time of exposure matches the knee with the trial components, and the internal and external rotation of the tibia approximately 14° restores the native laxities of knee at 90° of flexion (Fig. 8.4) [3]. The ability of kinematically aligned TKA to restore the native knee and limb alignments and the laxities of the knee may explain the reports from a randomized clinical trial and a national, multicenter study that showed patients with a kinematically aligned TKA reported better pain relief, better function, better flexion, and a more normal-feeling knee than patients with a mechanically aligned TKA [2, 5].

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Fig. 8.4
Intraoperative photographs of a right knee with a varus deformity in 90° of flexion shows the measurement of the natural anterior offset of the tibia from the worn distal medial articular surface of the femur in a knee at the time of exposure (left) and at the time of reduction with the trial components (right). Compensating 2 mm for cartilage wear on the distal medial femur, adjusting the anterior-posterior slope and the thickness of the tibial component until the offset of the anterior tibia from the distal medial femoral condyle with the trial components matches that of the knee at the time of exposure, and setting the internal and external rotations of the tibia approximately 14° restores the laxities of the knee in 90° of flexion


8.5 Technique for Kinematically Aligning the Femoral Component to the Native Articular Surface


Kinematic alignment sets the femoral component at the native angle and level of the distal (0°) and posterior (90°) joint line. The surgical technique begins by using an offset caliper to measure the anterior-posterior offset of the anterior tibia from the distal medial femur with the knee in 90° of flexion (Fig. 8.4). Two millimeters is subtracted from the offset measurement if cartilage is missing on the distal medial femoral condyle. The measured offset is subsequently used as the reference for completing Quality Assurance Step 5 to restore the native F-E angle or slope of the tibial joint line. Once the knee is fully exposed, the locations of cartilage wear are assessed on the distal femur. A ring curette is used to remove any partially worn cartilage. The flexion-extension position of the femoral component is set by the insertion of a positioning rod 8–10 cm through a drill hole placed parallel to the anterior surface of the distal femur and perpendicular to the distal articular surface (Fig. 8.5). The use of a starting hole midway between the top of the intercondylar notch and the anterior cortex reduces the risk of flexing the femoral component more than 5° from the anatomic axis of the femur, which is associated with patellofemoral instability [26]. Minimizing flexion of the femoral component completes Quality Assurance Step 1.

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Fig. 8.5
Composite shows the method of setting the flexion extension and varus-valgus rotations and the proximal-distal translation of the kinematically aligned femoral component with disposable instruments (blue). The insertion of a positioning rod 8–10 cm through a hole drilled parallel to the anterior surface and perpendicular to the distal articular surface of the distal femur sets flexion-extension rotation of the femoral component. An assembly of the distal cutting block inserted into the offset distal femoral resection guide that compensates for 2 mm of cartilage wear on the worn condyle(s) is placed over the positioning rod in contact with the distal femur and sets varus-valgus rotation and proximal-distal translation of the femoral component

The varus-valgus rotation and proximal-distal translation of the femoral component are set by using a disposable distal referencing guide that compensates 2 mm when there is cartilage wear on the distal medial femoral condyle in the varus knee and 2 mm when there is cartilage wear on the distal lateral femoral condyle in the valgus knee. The distal resections are measured with a caliper. The anterior-posterior translation and internal-external rotation of the femoral component are set by placing a 0° rotation posterior referencing guide in contact with the posterior femoral condyles (Fig. 8.6). The positioning of the posterior referencing guide infrequently requires correction because complete cartilage loss on the posterior medial and posterior lateral femoral condyles is rare in most varus and valgus osteoarthritic knees [27]. The posterior resections are measured with a caliper. Correction for bone wear is rarely needed at 0° and 90° of flexion even in the most arthritic knees [3, 27]. Adjusting the thickness of the distal and posterior resections to match the thickness of the femoral component after compensating for cartilage wear and the kerf of the saw blade completes Quality Assurance Step 2.

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Fig. 8.6
Composite of a right varus osteoarthritic knee shows the steps for kinematically aligning the femoral component at 90° of flexion. A 0° rotation posterior referencing guide is inserted in contact with the posterior femoral condyles and pinned (a). The correct size chamfer guide is inserted into the pin holes (b). A caliper measures the thickness of the posterior medial femoral condyle (c) and posterior lateral femoral condyle (d). These steps set internal-external rotation and anterior-posterior translation of the femoral component to the natural articular surface of the posterior femur (e)


8.6 Technique for Kinematically Aligning the Tibial Component to the Native Articular Surface


Kinematically aligned TKA sets the tibial component at the native internal-external, varus-valgus, flexion-extension, and proximal-distal positions of the articular surface of the tibia with use of an extramedullary tibial guide (Figs. 8.7, 8.8, and 8.9) [3]. The internal-external rotation of the tibial component is set parallel to the flexion-extension plane of the knee with use of either the major axis of the lateral tibial condyle or a kinematic tibial baseplate method [3, 28, 29]. When the major axis of the lateral tibial condyle method is used, the elliptical-shaped boundary of the articular surface of the lateral tibial condyle is identified, and the major axis is drawn (Fig. 8.7) [3, 28, 29]. A guide is used to drill two holes into the medial articular surface parallel to the major axis drawn on the lateral tibial condyle. After the tibial resection is made, the anterior-posterior axis of the tibial component is aligned parallel to these two holes. This technique uses a rationale similar to Cobb’s method, which finds the flexion-extension plane of the knee by fitting circles to the medial and lateral tibial condyles [30]. In contrast to mechanically aligned TKA where the medial border and medial one-third of the tibial tubercle are considered useful landmarks, a study of a case series of kinematically aligned TKAs showed that aligning the tibial component to the medial border or medial one-third of the tibial tubercle would have malrotated the tibial component 5° or more from the flexion-extension plane of the knee in 70% and 86% of the knees, respectively [3, 31, 32]. The use of the major axis of the lateral tibial condyle is a reproducible method as shown by a negligible bias (−1° internal) and an acceptable precision (± 5.4°) between the anterior-posterior axis of the tibial component and the flexion-extension plane of the knee and minimal malrotation of the tibial component on the femoral component [28, 29]. Aligning the anterior-posterior axis of the tibial component parallel to the flexion-extension plane of the knee completes Quality Assurance Step 3 [28, 29, 32].

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Fig. 8.7
Composite of a right knee shows the major axis of the lateral tibial condyle method for kinematically aligning the internal-external rotation of the trial tibial component to the anterior-posterior axis (blue line) of the nearly elliptical-shaped boundary of the articular surface of the lateral tibial condyle (black dots) (a). A guide is used to drill two pins through the medial tibial articular surface and parallel to the major axis (b). The tibial articular surface is resected and removed, the two drill holes are identified (pins), and lines parallel to the drill holes are drawn (c). The score marks (green arrows) indicate that the anterior-posterior axis of the trial tibial baseplate is aligned parallel to these lines (d)

Sep 26, 2017 | Posted by in ORTHOPEDIC | Comments Off on Five Quality Assurance Steps for Balancing a Kinematically Aligned Total Knee Arthroplasty

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