It Is Time to Consider a Philosophical Change From Mechanical to Kinematic Alignment

Overview

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