Functional Alignment Philosophy in Total Knee Arthroplasty - Rationale and technique for the varus morphotype using a CT based robotic platform and individualized planning

Abstract

Introduction: Alignment techniques in total knee arthroplasty (TKA) continue to evolve. Functional alignment (FA) is a novel technique that utilizes robotic tools to deliver TKA with the aim of respecting individual anatomical variations. The purpose of this paper is to describe the rationale and technique of FA in the varus morphotype with the use of a robotic platform.

Rationale: FA reproduces constitutional knee anatomy within quantifiable target ranges. The principles are founded on a comprehensive assessment and understanding of individual anatomical variations with the aim of delivering personalized TKA. The principles are functional pre-operative planning, reconstitution of native coronal alignment, restoration of dynamic sagittal alignment within 5° of neutral, maintenance of joint-line-obliquity and height, implant sizing to match anatomy and a joint that is balanced in flexion and extension through manipulation of implant positioning rather than soft tissue releases.

Technique: An individualized plan is created from pre-operative imaging. Next, a reproducible and quantifiable method of soft tissue laxity assessment is performed in extension and flexion that accounts for individual variation in soft tissue laxity. A dynamic virtual 3D model of the joint and implant position that can be manipulated in all three planes is modified to achieve target gap measurements while maintaining the joint line phenotype and a final limb position within a defined coronal and sagittal range.

Conclusion: Functional alignment is a novel knee arthroplasty technique that aims to restore constitutional bony alignment and balance the laxity of the soft tissues by placing and sizing implants in a manner that it respects the variations in individual anatomy. This paper presents the approach for the varus morphotype.

Keywords: Alignment philosophy; Functional alignment; Knee alignment; Robotically assisted arthroplasty; Total knee arthroplasty.

Figure 1

Functional alignment surgical workflow.

Figure 2

Functional Alignment Principals.

Figure 3

Typical plan for a varus morphotype knee.

Figure 4

Intra-operative assessment of the extension space. The limb is placed in a corrected position, and the robot “captures” the pose. The personalized plan will deliver extension gaps of 18 mm laterally and 17 mm medially. In order to achieve balanced compartments, the plan is modified in this case by decreasing the femoral valgus.

Figure 5

The flexion space before correction. The flexion space is assessed using sized spacer spoons until the corrected position is achieved. The personalized plan will deliver a medial space of 17 mm and lateral space of 18 mm. In order to balance the flexion space, the implant is externally rotated until the compartments are balanced.

Figure 6

The final intra-operative position. Limb alignment, resections depth, and tibiofemoral gaps in flexion and extension are shown. The achieved coronal alignment is 177° or 3° varus, and the sagittal alignment is 0° of flexion. The post-operative X-ray at 3 months follow-up of the same patient is shown.