Chapter 121 Kinematic Alignment in Total Knee Arthroplasty

The most important predictor of clinical outcome in total knee arthroplasty (TKA) is placement of the femoral and tibial components. Prior to the advent of kinematic alignment, the placement of components was based on the widely accepted principles of classic mechanical alignment, as follows: (1) aligning the femoral component perpendicular to the mechanical axis of the femur; (2) aligning the tibial component perpendicular to the mechanical axis of the tibia; (3) adjusting the anterior-posterior and internal-external rotation positions of the femoral component so that the extension and flexion gaps are equal; and (4) releasing ligaments when necessary to restore motion and balance the knee. In kinematic alignment, the principles for placing the components are different from those of classic mechanical alignment, as follows: (1) coaligning the transverse axis of the best-fitting femoral component with the primary transverse axis in the femur about which the tibia flexes and extends; (2) removing osteophytes to restore ligament length motion and stability; and (3) placing the tibial component so that the longitudinal axis of the tibia is perpendicular to the transverse axis in the femur, about which the tibia flexes and extends.

Kinematic alignment can prevent the loss of flexion and extension, stiffness, instability, pain, and prolonged recovery associated with mechanical alignment.^16,¹⁷ Kinematic alignment of the femoral component is confirmed intraoperatively by comparing the symmetry of the thickness of the distal medial, distal lateral, posterior medial, and posterior lateral femoral bone resections after measuring the thickness of the resections with calipers and after correcting for cartilage wear, bone wear, and kerf (i.e., the bone removed by the saw blade). Once kinematic alignment of the femoral component is confirmed, restoring motion and balancing the total knee arthroplasty is simplified by following a stepwise algorithm. The algorithm consists of four steps—removing osteophytes, adjusting the plane of the tibial cut, releasing the posterior capsule from the femur, and medializing or lateralizing the tibial component.

This chapter reviews the history and definition of kinematic alignment with patient-specific guides, highlights the inherent disadvantages of mechanical alignment that kinematic alignment strives to avoid, describes the planning, outlines the algorithm for restoring motion and balancing the kinematically aligned TKA, provides an overview of the surgical technique of kinematic alignment with patient-specific cutting guides and unconventional use of conventional instruments, addresses theoretical concerns of kinematic alignment, and reviews clinical studies showing the early benefits of kinematically aligning the TKA.

History and Definition of Kinematic Alignment

The biomechanical rationale for kinematic alignment is traced to Hollister and colleagues’ classic research on the kinematics of the knee.¹² Kinematics refers to the relative relationship of the femur, patella, and tibia at any angle of flexion, without force applied to the knee. The joint surface, menisci, and ligament structures determine the normal kinematic relationship among the femur, patella, and tibia. The center of the femoral head and center of the ankle, which are used by conventional and computer-assisted instruments to align a TKA mechanically, have no bearing on the kinematics of the knee.^10,^11,^14,¹⁵

Three axes govern the movement of the patella and tibia with respect to the femur, and understanding how the placement of the femoral and tibial components affects the interrelationship of these axes is the key to kinematically aligning a TKA (Fig. 121-1). The primary axis is a transverse axis in the femur about which the tibia flexes and extends. It passes through the center of a circle fit to the articular surface of the femoral condyles from 10 to 160 degrees of flexion.^7,¹⁰^–¹² There is a second transverse axis in the femur about which the patella flexes and extends that is parallel, proximal, and anterior to the transverse axis in the femur about which the tibia flexes and extends. The third axis is a longitudinal axis in the tibia about which the tibia internally and externally rotates on the femur that is perpendicular to each of the two transverse axes in the femur. Although each of the three axes is aligned parallel or perpendicular to one another, none are aligned orthogonally to the three anatomic planes, which means that the axis cannot be found with imaging studies performed in the sagittal, coronal, and axial planes.¹⁵

Figure 121-1 Interrelationship of the three kinematic axes in a right knee. A, Coronal projection with the knee in extension. B, Axial projection of the femur with the knee in 90 degrees of flexion. C, Lateral projection with the knee in extension. The primary transverse axis in the femur about which the tibia flexes and extends passes through the center point of the best-fit circles of the medial and lateral femoral condyles (green line and circle), which is equidistant from the articular surface of the condyles (double-headed black arrows). A second transverse axis in the femur about which the patella flexes and extends axis (magenta line and circle) is oriented parallel, proximally, and anteriorly to the transverse axis in the femur about which the tibia flexes and extends. A third longitudinal axis in the tibia (vertical orange line) about which the tibia rotates on the femur internally and externally is oriented perpendicular to both transverse axes in the femur.

The goal for kinematically aligning the femoral component is to coalign the transverse axis of a symmetrical femoral component with the primary transverse axis in the femur about which the tibia flexes and extends (Fig. 121-2).^7,¹⁰^–¹² ^,14 Because there is no clinically important asymmetry between the medial and lateral femoral condyles in the varus and valgus knee with end-stage osteoarthritis, a symmetrical, single-radius femoral component is an optimal design for replicating knee kinematics.

Figure 121-2 Single-radius femoral component of a right knee (silver) shape-matched to the femur of a normal knee model (pink). Shape matching the femoral component to the femur is the critical step in kinematically aligning the knee and in reestablishing the orthogonal interrelationships among the three kinematic axes. A kinematically aligned femoral component greatly simplifies the stepwise algorithm for identifying the correct options for restoring motion and balance to the knee before cementation of the components. The principle for kinematically aligning the tibial component to the femoral component is to align the anterior-posterior axis of the tibial component perpendicular to the transverse axis in the femur and femoral component. The principle for kinematically aligning the tibia to the tibial component is to accept the assumption that the internal-external rotational relationship between the femur and tibia is normal in a non–weight-bearing MRI scan and then center the tibia under the center of the tibial component.

The principle for kinematically aligning the femoral component to the femur is the simple step of shape-matching the femoral component to the articular surface of the femur on a three-dimensional model of the knee that has been restored to normal by filling in the worn articular surface. Shape matching the femoral component to the femur coaligns the transverse axis of the femoral component with the primary transverse axis in the femur about which the tibia flexes and extends, which is requisite to restoring the normal interrelationships among the three axes.^13,¹⁶

In contrast to the simplicity of kinematically aligning the femoral component, kinematically aligning the tibial component involves several steps. The first step is to align the anterior-posterior axis of the tibial component perpendicular to the transverse axis in the femoral component, which has previously been coaligned to the primary transverse axis in the femur about which the tibia flexes and extends by the shape-matching step.^1,⁷ The second step is to align the tibia to the tibial component kinematically, which is based on the assumption that the internal-external rotational relationship between the femur and tibia is normal in a magnetic resonance imaging (MRI) scan of the non–weight-bearing knee with end-stage osteoarthritis. The assumption that the internal-external rotational relationship is normal in the non–weight-bearing knee with end-stage osteoarthritis is inferred from the layer of joint fluid that is consistently seen separating the worn articular surface between the femur and tibia on the MRI scan of the knee. Both the layer of joint fluid and the image of a non–weight-bearing knee indicate that there is no contact between the worn femur and tibia and no transmission of force across the knee to malrotate the tibia on the femur. The final step is aligning the center of the tibia under the center of the tibial component.

With a goal of improving on the 20% prevalence of patient dissatisfaction from mechanically aligned TKA with conventional and computer-assisted instruments,^3,⁵ we began developing the method for performing kinematic alignment with patient-specific femoral and tibial cutting guides in 2005. We developed software that creates a three-dimensional model of the arthritic knee from a non–weight-bearing MRI or computed tomography (CT) arthrogram of the knee (OtisKnee, OtisMed, Alameda, Calif; http://www.otismed.com). Additional software transforms the arthritic knee model to a normal knee model and then kinematically aligns the components by shape-matching the best-fitting femoral and tibial components to the normal knee model. The three-dimensional position of each component is then transferred from the normal knee to the arthritic knee model. Patient-specific cutting guides that incorporate the cut planes of each component and that reference the arthritic knee model are made to fit the patient’s femur and tibia (Fig. 121-3). The cutting guides are used intraoperatively to transfer the six degrees of freedom (6DOF) positions of the femoral and tibial components from the computer to the patient—varus-valgus, internal-external rotation, flexion-extension, anterior-posterior, proximal-distal, and medial-lateral.^13,^14,^16,²⁵

Figure 121-3 Femoral (A) and tibial (B) patient-specific cutting guides (orange) on the arthritic knee model (right knee). The saw slot (black arrow) in each guide sets the proximal-distal, flexion-extension, and varus-valgus degrees of freedom of each component. The two holes (white arrows) in each guide set the internal-external rotation, anterior-posterior, and medial-lateral degrees of freedom of each component.

The first use of patient-specific cutting guides to align a TKA kinematically was in January 2006. As of August 2009, 23,000 kinematically aligned TKAs had been implanted with patient-specific cutting guides nationwide with the lead author (SMH) implanting over 700. From September through December 2009, he implanted 116 kinematically aligned TKAs with unconventional use of conventional instruments instead of patient-specific cutting guides. This 4-year developmental and clinical experience, which is admittedly limited in terms of long-term follow-up, forms the basis for the concepts shared in this chapter with the primary goals of stimulating debate and advancing the understanding of kinematic alignment of total knee arthroplasty.

Advantages of Kinematic Alignment Over Mechanical Alignment

Studies from the United Kingdom and Canada that reviewed more than 10,000 patients at 1 year following mechanically aligned TKA with conventional instruments and contemporary components have shown that one of five patients are not satisfied because of continued pain and poor function in activities of daily living (ADLs).^3,⁵ The use of computer-assisted surgery has improved the mechanical alignment compared with conventional surgery, but has not improved the clinical outcome (much to the dismay of proponents of computer-assisted surgery).^2,^9,^21,²⁶ Therefore, a mechanically aligned TKA, whether performed with conventional instruments or computer assistance, has an unacceptably high prevalence of continued pain, poor function in ADLs, and patient dissatisfaction, which means that there is ample room for improvement.

For the kinematics of a TKA to be the same as a normal knee, the three-dimensional placements of the femoral and tibial components have to be chosen so that the orientation of the three kinematic axes is unchanged from that of the normal knee. None of these axes can be found by referencing the transepicondylar axis, center of the femoral head, and center of the ankle with the use of conventional or computer-assisted instruments.^1,^10,¹⁵ There is a 5-degree average difference (range, 2 to 11 degrees) between the transepicondylar and primary transverse axes in the femur about which the tibia flexes and extends, which means that referencing the transepicondylar axis substantially changes the joint line from normal in the axial plane.¹¹ Of normal subjects, 98% do not have a neutral hip-knee-ankle axis because the longitudinal shape of the femur and tibia are unrelated and variable among subjects, which means that referencing the center of the femoral head and of the ankle changes the joint line in the coronal plane.¹² Changing the joint line of the femur from normal in the axial and/or coronal planes using instruments that align components to the transepicondylar axis, center of the femoral head, and center of the ankle kinematically malalign the knee and may explain the midrange instability reported in TKA (Fig. 121-4).⁷ Because mechanical alignment kinematically malaligns the knee, we hypothesized that kinematic alignment would reduce the high prevalence of persistent pain, poor function in ADLs, and patient dissatisfaction after mechanically aligned TKA with conventional and computer-assisted instruments.^16,²⁵

Planning Kinematic Alignment With Patient-Specific Cutting Guides

Protocol for Aligning and Performing Magnetic Resonance Imaging of the Knee

For kinematic alignment, the projection of the knee in the MRI scan has to be customized to the patient’s knee position in the MRI scanner so that the oblique sagittal image plane is perpendicular to the primary axis in the femur about which the tibia flexes and extends. A patient that has a painful osteoarthritic knee should be allowed to choose a position for the leg that is comfortable so that he or she does not inadvertently move the knee during image acquisition and cause a motion artifact. The knee with a severe varus or valgus deformity or flexion contracture can be successfully imaged without forcing the knee into extension or an uncomfortable rotation by customizing the projection of the knee in the coronal and axial planes.

The following are contraindications for MRI of the knee for patient-specific cutting guides: (1) presence of a pacemaker; (2) movement of the knee during image acquisition because of inability to follow instructions, tremor, and claustrophobia; (3) obese knee that prevents the use of a dedicated knee coil; (4) hardware about the knee that distorts the image and subsequent three-dimensional model; and (5) metal in the body that might move in the magnetic field (e.g., brain aneurysm clips, metal in the eye, shrapnel near vital structures).

The following is a description of the suggested MRI technique, which relies on the use of coronal and axial locator images to obtain nonorthogonal, oblique, sagittal images perpendicular to the primary transverse axis in the femur about which the tibia flexes and extends (Fig. 121-5).¹⁴ A nonorthogonal, oblique, sagittal MRI scan of the treated knee is obtained using a 1.5- or 3.0-T scanner and dedicated knee coil. The plane for the nonorthogonal, oblique, sagittal scan is based on the use of coronal and axial locator images. These images are used to align the image plane perpendicular to the primary transverse axis in the femur about which the tibia flexes and extends, which projects the femoral condyles approximately in the same plane that the tibia flexes and extends about the femur. Coronal, axial, and sagittal high-resolution locator images are obtained using a 4-mm slice thickness, 1-mm spacing/gap, 256 × 128 matrix, one number of excitations (NEX), and 24-cm field of view (FOV), which yield nine slices in all three planes.

Figure 121-5 Illustration of the coronal (A) and axial (B) localizers, femoral joint line (thick white line), transverse axis in the femur about which the tibia flexes and extends (green line), and plane for the nonorthogonal, oblique, sagittal scan (thin parallel lines). Aligning the nonorthogonal, oblique, sagittal scan perpendicular to the distal femoral joint line (thick white line) in the coronal localizer and the posterior femoral joint line (thick white line) in the axial localizer aligns the nonorthogonal, oblique, sagittal scan perpendicular to the transverse axis in the femur about which the tibia flexes and extends (green line), which projects the femoral condyles as a circle in the same plane that the tibia flexes and extends about the femur.

The locator image in the coronal plane that shows the largest projection of the distal femoral condyles is used to adjust the varus-valgus orientation of the plane of the nonorthogonal, oblique, sagittal scan. The nonorthogonal, oblique, sagittal scan plane is aligned perpendicular to a line connecting the cortical-cancellous bone interface of the distal femoral condyles on the locator image in the coronal plane. The locator image in the axial plane that shows the largest projection of the posterior femoral condyles is used to adjust the axial rotation of the plane of the nonorthogonal, oblique, sagittal scan. The nonorthogonal, oblique, sagittal scan plane is aligned perpendicular to a line connecting the cortical-cancellous bone interface of the posterior femoral condyles in the locator image in the axial plane. Because the contour of the posterior femoral condyles from 10 to 160 degrees forms a single radius of curvature, and because the primary transverse axis in the femur about which the tibia flexes and extends is equidistant from the distal and posterior articular surfaces of the femoral condyles, the femoral condyles are projected as circular in the nonorthogonal, oblique, sagittal imaging plane and perpendicular to the primary transverse axis in the femur about which the tibia flexes and extends.^10,^11,¹⁴

A nonorthogonal, oblique, sagittal scan is then acquired of the knee, which is subsequently processed with software to generate a three-dimensional model of the knee. The scanning parameters are selected to provide contrast among fat, joint fluid, cartilage, degenerative and normal menisci, and subchondral, cancellous, and cortical bone. For a 1.5-T scanner and a dedicated knee coil (General Electric Medical Systems, Milwaukee, Wisc), we use these parameters: FRFSE PD, 30 to 35 TE, 2800 to 3400 TR, 31.25-Hz bandwidth, and minimum of two excitations using a 16-cm field of view centered at the joint line of the knee, 512 × 512 matrix, 2-mm slice thickness, with no spacing or gap. The length of each side of a pixel in the oblique sagittal image is 0.31 mm.¹⁴

Generation of Three-Dimensional Arthritic and Normal Knee Models

Kinematic alignment of the femoral and tibial components begins with a three-dimensional arthritic knee model generated from the MRI scans, from 44 to 60 slices, depending on the width of the knee (Fig. 121-6). Proprietary software segments the femur, tibia, articular cartilage, and osteophytes from each image and meshes the images together to form a three-dimensional model of the arthritic knee (OtisMed). A series of steps is applied to the arthritic knee model to create a normal knee model. The articular surface of the arthritic knee model is transformed into a knee with a normal articular surface by filling articular defects. Osteophytes are removed to restore ligament length and restore a normal shape to the knee. The normal knee model is then aligned in the coronal plane by adjusting the varus-valgus rotation and proximal-distal position of the tibia until the distance between the femoral and tibial articular surfaces is equal medially and laterally.^13,¹⁶ This process of creating a normal knee from an arthritic knee by filling defects, removing osteophytes to restore ligament length, and reestablishing a symmetrical medial-lateral joint space borrows from the well-established principles used to align the mobile-bearing unicompartmental knee replacement (i.e., Oxford Knee).

Figure 121-6 Illustration of the arthritic knee model (A) and normal knee model (B) created with software from an MRI scan of the knee. The software creates the normal knee model from the arthritic knee model by removing osteophytes, filling in worn joint surfaces, centering the tibia under the femur, and reestablishing an equal medial and lateral joint space (white arrows).

Shape-Matching Femoral and Tibial Components

The three-dimensional model of the femoral and tibial component that best fits the normal knee model is selected by proprietary software (see Fig. 121-2). Algorithms shape-match the femoral component to the restored articular surface of the femur in the normal knee model from 10 to 160 degrees, which kinematically aligns the femoral component by coaligning the transverse axis of the femoral component with the primary transverse axis in the femur about which the tibia flexes and extends. The internal-external rotation of the anterior-posterior axis of the tibial component is set perpendicular to the transverse axis of the femur and femoral component, which kinematically aligns the tibial component to the femoral component. The tibia is centered under the tibial component, which kinematically aligns the tibia to the tibial component.^1,^7,^10,¹² In theory, kinematic alignment restores the normal parallel and perpendicular interrelationship among the three kinematic axes of the prearthritic knee.¹⁴

Only gold members can continue reading. Log In or Register to continue