Courtesy: Dr Makridis Konstantinos, Dr Ashok Shyam, Ortho TV

Robotic-Assisted Total Knee Arthroplasty: Kinematic and Restricted Kinematic Alignment (rKA)

Fundamental Biomechanics of Knee Kinematics

Primary Cylindrical Flexion Axis: The transverse axis of the distal and posterior femoral condyles guides the flexion-extension arc.
Asymmetric Native Rollback:
- During deep flexion, the medial femoral condyle exhibits minimal anteroposterior translation (approximately 2 mm translation).
- The lateral femoral condyle demonstrates extensive posterior translation and rollback (approximately 20 mm translation).
Evolution Beyond Equal Rectangular Gaps:
- Traditional mechanical alignment targeted identical, symmetrical extension and flexion gaps.
- Contemporary kinematic principles recognize that a physiological knee requires constitutional lateral laxity in flexion to accommodate lateral femoral rollback.
Restoration of the Three Kinematic Axes: Re-establishing the transverse flexion-extension axis, longitudinal tibial rotation axis, and patellofemoral tracking axis restores native joint balance without excessive soft-tissue releases.

Kinematic Alignment Premise: Aims to resurface the knee back to its constitutional pre-arthritic anatomy by matching bone resections directly to implant thickness (accounting for cartilage and bone wear).
The Clinical Dilemma with Pure KA: Patients with severe constitutional outliers (such as extreme varus or valgus) risk abnormal shear stress on the implant-bone interface and eccentric polyethylene wear.
Restricted Kinematic Alignment (rKA) Boundary Philosophy:
- Designed to safely restore patient-specific anatomy while imposing strict boundaries to protect component longevity.
- Adjusts extreme bony morphology to keep the overall mechanical alignment and individual bone cuts within a safe envelope.

Overall Hip-Knee-Ankle (HKA) Boundary: Maintain the final postoperative mechanical axis within $\pm 3^\circ$ of neutral (177° to 183°).
Individual Bone Cut Boundaries:
- Lateral Distal Femoral Angle (LDFA): Restricted within $\pm 5^\circ$ of perpendicular (typically between 85°–86° and 93°).
- Medial Proximal Tibial Angle (MPTA): Restricted within $\pm 5^\circ$ of perpendicular (typically between 85°–86° and 93°).
The “Unworn” vs. “Worn” Resection Principle:
- On the unworn compartment, resect bone exactly equal to the implant thickness (minus blade kerf).
- On the worn compartment, compensate for measured cartilage/bone loss to restore the pre-arthritic articular joint line.
- Prioritize the femoral anatomy first, preserving native femoral condylar geometry to maintain natural kinematics.
Boundary Adjustments: If initial planned cuts result in an overall limb alignment outside the $\pm 3^\circ$ safe zone, modify the femoral or tibial cuts inward until the construct falls within boundaries.

Arithmetic HKA (aHKA):
- Calculated as $\text{MPTA} - \text{LDFA}$ .
- Represents constitutional limb alignment prior to arthritic joint destruction.
- Independent of joint space narrowing, subluxation, and distal femoral or proximal tibial cartilage erosion.
CPAK (Coronal Plane Alignment of the Knee):
- Matrix incorporating both the constitutional arithmetic axis (varus, neutral, valgus) and joint line obliquity (apex distal, neutral, apex proximal).
- Guides the surgeon on how far the patient’s native joint deviates from standard neutral mechanical orientation.

Precision in Multiplanar Cuts: Manual instrumentation shows high outlier rates when attempting subtle 1° to 2° angle variations. Robotic platforms execute precise angular corrections reliably.
Dynamic Gap Balancing and Soft-Tissue Mapping:
- Real-time intraoperative tracking of extension and flexion gap laxity throughout the full arc of motion.
- Quantifies medial stability alongside physiological lateral laxity during flexion without relying solely on subjective manual feel.
Pre-Planning and Virtual Modification:
- Allows the surgeon to review bone cuts, component rotation, and predicted limb alignment prior to committing to resections.
- Enables seamless intraoperative switching between mechanical, kinematic, or restricted kinematic philosophies based on patient anatomy.

Preoperative Planning & Robotic Execution: Accurately plan and verify bone cuts to minimize unintended mechanical outliers.
Boundary Adherence: Keep individual femoral and tibial cuts within $\pm 5^\circ$ (85°–93°) and overall limb alignment within $\pm 3^\circ$ of neutral.
Soft-Tissue Preservation: Restores constitutional joint lines and physiological lateral flexion laxity, substantially decreasing the need for ligament releases.
Implant Protection: Protects modern polyethylene inserts and fixation interfaces by filtering out extreme constitutional deformities while preserving the advantages of patient-specific kinematics.