This chapter discusses the research that is driving the change in philosophy from mechanical alignment (MA) to kinematic alignment (KA) in total knee arthroplasty (TKA). The first section discusses the association between MA and patient dissatisfaction after TKA, and how improving satisfaction requires a change in alignment philosophy to one that does not require ligament release. The second section describes the recent discovery of three axes in the knee that determine kinematics and how resurfacing the knee is the only method for coaligning the three axes of the femoral and tibial components to those of the patient’s prearthritic knee. KA can be performed with manual, patient-specific, navigational, and robotic instrumentation; however, intraoperative caliper measurements of bone resections are required to verify the components are set coincident to the prearthritic joint lines. The third section shows that MA TKA does not restore knee kinematics, because the targets of alignment do not coalign the axes of the components to those of the patient’s prearthritic knee. The fourth section uses evidentiary research to show that calipered KA TKA optimizes implant survival, medial compartment loading, and patellofemoral kinematics, when compared with MA TKA. The educational objective is to encourage surgeons to examine their experience with MA TKA and learn about the three kinematic axes that determine knee function. For those interested in improving clinical outcomes, consider performing calipered KA and intraoperatively verifying that the femoral component resurfaces the patient’s prearthritic femur, with their preferred method of instrumentation.
Time for a change in alignment philosophy
The recent identification of three axes in the knee that determine motion and kinematics encouraged a fresh examination of the limitations of MA and implant design ( Fig. 3.1 ). It is well established that MA does not consider the orientation and position of the three kinematic axes when setting components with manual, patient-specific, navigational, and robotic instrumentation, and that not doing so requires an undesirable use of ligament releases. , Although the combination of MA philosophy and modern implants is very successful in improving the lives of patients, regrettably, most studies show that nearly 20% report some level of dissatisfaction. ,
Three observations suggest it is time for a change from MA philosophy with the intent of improving the rate of patient satisfaction. First, sophisticated computer navigational and robotic MA instrumentation that increased the reproducibility of achieving a 0-degree hip-knee-ankle (HKA) angle has not improved the incidence of patient satisfaction, which indicates that MA has a ceiling effect. Second, an alternative alignment philosophy that balances the TKA, leaving healthy ligaments untouched, would mitigate pain, retain proprioception, and provide a more normal-feeling knee. Third, as some patients complain of discomfort and others are relieved, perhaps a philosophy that individualizes component placement to the patient’s prearthritic phenotype of the knee should be investigated (see Chapter 2 by Hirschmann). ,
Three axes of the knee make the case for changing to kinematic alignment
KA TKA strives to provide high patient satisfaction and function and long-term implant survival. The transition to KA follows naturally from biomechanical studies by Hollister, Coughlin, Freeman, Eckhoff, and Iranpour. They described the three “kinematic” axes in the knee that holistically define the patterns of movement between the femur, tibia, and patella ( Fig. 3.1 ) :
Axis 1: the tibia flexes and extends (F-E) around an axis centered in a cylinder best fit to the articular surface of the posterior femoral condyles. ,
Axis 2: the patella F-E around an axis that is parallel and around 10 mm anterior and proximal to the F-E axis about which the tibia moves. ,
Axis 3: the tibia internally-externally (I-E) rotates around an axis perpendicular to the F-E axes about which the tibia and patella move. ,
The three axes maintain an orthogonal interrelationship throughout flexion and extension. The transverse F-E axes of the tibia and patella are anatomically fixed in the femur and defined in orientation and position by the shape of the posterior condyles and trochlea, respectively. Both transverse axes are parallel to each other and parallel to the distal and posterior prearthritic joint lines. The orientation of the I-E axis of the tibia is perpendicular to the transverse F-E axes and the prearthritic tibial joint line. , The orientation of the I-E axis is fixed; however, the anterior-posterior (A-P) and medial-lateral (M-L) positions may translate within the tibia, depending on how the knee is loaded. During weight-bearing activities, the ball-in-socket conformity of the medial femoral condyle, meniscus, and articular cartilage fix the position of the I-E axis of the tibia just posterior to the center of the medial compartment. Similarly, this kinematic pattern is closely reproduced by the medial ball-in-socket knee prosthesis design. However, without compression force and with gravity distraction, the A-P and M-L position of the I-E axis of the tibia varies within the tibia. Continuous X-ray imaging showed differences in knee kinematics and the position of the tibial axis between open-chain and closed-chain activities. ,
The three goals of calipered KA TKA are:
Goal 1: Restore the patient’s prearthritic tibiofemoral articular surface ( Fig. 3.2 )
The first goal of KA is to set the A-P, proximal-distal (P-D), and M-L translation and F-E, varus-valgus (V-V), I-E rotation (6 degrees of freedom) of the femoral and tibial components, to restore the patient’s prearthritic tibial-femoral articular surface of the knee. This principle automatically coaligns the axes in the femoral and tibial components with those of the patient’s prearthritic knee and closely restores native patellofemoral kinematics. ,
Goal 2: Restore the patient’s prearthritic joint line and limb alignment
The second goal is to restore the patient’s prearthritic joint line and limb alignments. Any osteoarthritic knee will present with loss of cartilage on the femur and cartilage and bone on the tibia. The technique of KA uses strategies and methods to compensate for destruction at the time of knee arthroplasty, which automatically restores the patient’s prearthritic joint line and limb alignment.
Goal 3: Restore the patient’s prearthritic tibial compartment forces and laxities
The third goal of kinematically aligned TKA is to restore the medial and lateral tibial compartment forces and laxities to those of the native knee, without ligament release. Native tibial compartment forces and ligaments are higher and tighter at 0 degrees than at 45 degrees and 90 degrees of flexion. The calipered KA technique restores a tight rectangular extension space and a trapezoidal flexion space with the laxity of the prearthritic knee that varies widely between individuals. Chapters 11 (Hull) and 16 (Howell) comprehensively discuss tibial compartment forces and laxity in the calipered KA TKA.
Mechanical alignment is incompatible with restoring the three axes and normal kinematics
The early concept of a variable flexion-extension axis in the femur
Early studies promoted the idea of a variable orientation and position of the F-E axis in the femur, about which the tibia moves. For over 100 years, researchers studied the knee in an anatomic and not carefully defined sagittal plane that was not perpendicular to the F-E axis and concluded that the posterior femoral condyles are oval and not circular. The oval shape suggested multiple centers of rotation in the femur during flexion, which led to the design of the “J-Curve” knee prostheses with a plurality of distinct radii, such as five or more. However, more advanced analysis of knee biomechanics showed the variable F-E axis in the femur is not accurate, which made the J-Curve knee prostheses outdated. ,
Contemporary concept of a fixed flexion-extension or cylindrical axis in the femur
The introduction of the sagittal “kinematic” instead of the anatomic F-E plane, which is perpendicular to the distal and posterior joint lines and F-E axes in the femur, projected the posterior femoral condyles as circular and not oval. , , In 1993, Hollister described an F-E axis about which the tibia moves with a fixed orientation and position and a constant center of rotation, in a cadaveric study. In 2005, Eckhoff used computed tomography (CT) scans and confirmed the F-E axis is fixed by best-fitting two coaxial cylinders to the subchondral bone of the posterior condyles from 15 degrees to 115 degrees, and coined the term “cylindrical axis.” CT scans of the hip, knee, and ankle ( n = 90) confirmed that the mechanical axis of the leg is not a straight line and that the MA effect of changing the prearthritic limb alignment to neutral changes the orientation of the F-E or cylindrical axis in the femur. Not coaligning the rotational axes of the component with those of the prearthritic knee alters the soft tissue balance, which can manifest as mechanical imbalance and altered kinesthetics or motor function. , , ,
The transepicondylar line is not the flexion-extension axis in the femur
Some MA surgeons still follow the tenet that the F-E axis, about which the tibia moves, is coaligned with the transepicondylar line and the transepicondylar line through the medial sulcus. Anatomic studies showed no association between the fixed F-E axis in the femur and the transepicondylar lines, with deviations of up to 29 degrees. The average deviation between the fixed F-E axis and the transepicondylar line in three-dimensional space was 5 degrees, with a wide range from 2 degrees to 11 degrees. Setting the femoral component to the transepicondylar axis too often places the prosthesis outside the patient’s prearthritic joint line, which explains the frequent release of healthy ligaments with MA that is unnecessary with KA. ,
The valgus knee does not have a hypoplastic lateral femoral condyle
The concept of a circular condylar shape changed the understanding of the anatomy of the valgus knee. Magnetic resonance imaging (MRI) studies showed that the medial and lateral posterior condyles in osteoarthritic knees with varus and valgus knees treated with primary TKA had similar radii that were within ±0.2 mm. This observation dispelled the MA myth that the valgus knee has a “hypoplastic” or “laterally deformed” femoral condyle. In reality, the cranial translation of the lateral condyle relative to the medial condyle produces the valgus knee. , In the valgus osteoarthritic knee, MA sets the femoral component in varus and external rotation, which overtightens the distal and posterior gaps in the lateral compartment ( Fig. 3.3 ). A release of the lateral ligaments is required to enable the extension, flexion, and internal rotation of the tibia. Calipered KA TKA restores the patient’s prearthritic distal and posterior femoral joint lines because the radii of the medial and lateral femoral condyles are the same without the release of ligaments.