Managing Severe Deformities With Calipered Kinematic Alignment

Overview

Currently, there is lack of consensus about the universal definition of severe deformity for the arthritic knee. Some consider a deviation of more than 10 degrees for a varus knee as a significant deformity. Others consider a severe deformity as more than 15 degrees for the varus knee and more than 20 degrees for a valgus knee. ^, These angles are relative to a hip-knee-ankle (HKA) angle of 180 degrees, which has been believed traditionally to be neutral for every patient, but has recently been proven to be different and highly variable in the population. Only recently has a universal classification system been proposed for varus knees that considers the amount of deformity and its origin of location, and whether it is metaphyseal or diaphyseal.

Whatever the numerical definition will be, there is consensus among orthopedic surgeons that total knee arthroplasty (TKA) for severely deformed knees poses a challenge. We believe that this challenge has been aggravated for many years by an incorrect approach to TKA that tries to reproduce a neutral alignment, thereby creating a bucket of iatrogenic difficulties, each of which had to be solved. We will propose a different approach for treating complex knees without the creation of nonanatomic complexity, by applying the kinematic alignment (KA) technique.

The etiology of severe deformities has been considered for many years as the result of either long-standing neglected arthritis that deteriorated to a point of extensive bone loss and secondary ligament insufficiency, or of congenital pathologic states such as Blount’s disease or rickets. Both of the latter are rare in the general arthroplasty practice in developed countries, although they may still be found in less developed parts of the world.

Today, the etiology of those deformities as seen from the perspective of constitutional alignment ^, might best be categorized into two separate groups that will be further discussed in this chapter:

• 1

  Severe constitutional varus or valgus (single-level or multi-level) coupled with cartilage wear and occasionally bone loss to create severe deformity.

• 2

  Posttraumatic extra/intraarticular deformities.

Today, the most common treatment workflows for cases with severe deformity include extensive soft tissue releases to attain balance and neutral mechanical alignment (MA), in addition to the use of stems, augments, and constrained or hinged implants. The addition of constrained or stemmed implants increases the cost burden two- to threefold. Extensive releases, reduction osteotomies, and even epicondylar transfers have been described as means to achieve stability. ^, Failure to manage these cases using the aforementioned techniques and restoring mechanically neutral alignment is widely believed to lead to premature loosening and failure. Several methods have been described to address severe deformities using these techniques, with the main emphasis being on short-term survivorship with minimal attention toward functional outcomes.

During the last decade, a better understanding and definition of the phenotypic presentation of constitutional alignment, coupled with a pronounced paradigm shift toward a more natural and anatomic reconstruction of the knee joint, have been gaining traction. At the same time, a changing population demographic skewing toward younger patients with higher expectations for clinical function of their artificial joints has challenged the surgical community to strive for better clinical results. These trends have encouraged alternative solutions and the exploration and optimization of kinematic knee alignment philosophies, even in the context of considerable deformity.

The concept of KA has been extensively studied and is a hot topic of debate in the field of total knee arthroplasty. In KA, the goals of the reconstruction are to restore the native tibial-femoral articular surfaces to their original alignment relative to the long bone anatomic axes, restore the native prearthritic limb alignment, and restore the native soft tissue laxities of the knee.

Unlike traditional mechanical alignment techniques, which focus on making the tibia orthogonal to the floor, KA techniques focus on placing the femoral component to match the native three-dimensional (3D) position of the distal femur as the primary goal of the reconstruction process. Next, by compensating for tibial wear (cartilage and bone) and placing the tibial component in its prearthritic anatomic position, there is no need for ligament releases or other soft tissue interventions to restore normal motion around the knee’s native rotational axes. The result is a well-balanced and more natural-feeling knee, with multiple studies showing favorable patient-related outcome measurements.

With respect to restoring native soft tissue laxity, we note that MA TKA for severe deformity has traditionally emphasized the need for ligamentous release to balance the knee perpendicular to the mechanical axis of the entire limb. The goal is to create “balanced gaps” where the same tension is created between the medial and lateral ligaments both in full extension to 90 degrees of flexion, even though in a normal knee there is more laxity in flexion on the lateral side. We believe there is a price to be paid for every ligament release or soft tissue manipulation, reflected in the way patients perceive their new knee as not natural.

From our perspective, one of the primary benefits of KA TKA is, therefore, its inherent ability to address severe deformity using the same general principles as are used for common minor deformity, with only a few specific considerations.

The aim of this chapter, therefore, is to briefly summarize the following:

• The caliper-based KA TKA technique applied to severe deformities.

• The application of KA TKA in six examples, including severe varus deformity, severe valgus deformity, and intra- and extraarticular posttraumatic deformities.

• Special considerations relative to applying KA TKA to severe deformity.

Calipered Kinematically Aligned Total Knee Arthroplasty (Linked) Technique Principles and Application to Treating Severe Deformities

The key step of KA reconstruction is to replicate the distal femur’s 3D position, as it is the primary driver of knee kinematics, not the tibia. In this context, it has been noted that there is seldom much bone loss from the femur, whereas most of the intraarticular deformity originates from the proximal tibia. Therefore, once the femoral component is reconstructed as close as possible to its prearthritic state (within 0.5–1 mm), all that is necessary is to match the tibial tray position to that of the reconstructed femur. This can be done through the principles of a “linked technique” (described herein) or according to the basics of KA as discussed in previous chapters.

The femur

Step 1: Distal cut

After exposure of the knee through a standard medial parapatellar arthrotomy for both severe varus and severe valgus knees, we assess cartilage wear on the distal aspect of the femur. From the eroded condyle we remove any partially worn cartilage to reference bare subchondral bone with our distal femoral cutting blocks. The flexion-extension axis of the femoral component is set parallel to the anterior surface of the distal femur and perpendicular to the distal articular surface ( Fig. 14.1 ). The V-V angulation and proximal distal position of the femoral component are set using a 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 anterior-posterior (AP) translation and internal-external rotation of the femoral component are set as described in previous chapters, by placing a posterior referencing guide in contact with the posterior femoral condyles and in 0 degrees of relative rotation ( Fig. 14.2 ).

We assess cartilage thickness with a scalpel blade at the most posterior aspect of the posterior condyles; this requires deep flexion for proper assessment. We aim for the same cartilage thickness at both posterior condyles. If one is eroded, it is compensated for by applying a 1- or 2-mm shim between the posterior lateral condyle and the sizer.

The positioning of the posterior referencing guide rarely requires correction, because complete cartilage loss is uncommon on the posterior medial femoral condyles in varus osteoarthritic knees, although it may be found quite often in valgus knees, as they commonly present a flexion disease with posterior erosion pattern. Correction for bone erosion is rarely needed at 0 and 90 degrees of flexion, even in the most arthritic knees.

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