Asymmetric Patellar Component Shear Force Reduction
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Solution Overview
Problem
Current patellar components in knee replacements experience excessive wear and loosening due to shear forces, leading to premature failure and patient discomfort, particularly during deep flexion, and are sensitive to surgical mal-implantation errors.
Innovation Solution
Design of patellar components with a substantially axis-symmetric portion and facet surfaces that reduce shear forces by maintaining contact vectors close to normal to the patellar bone interface, accommodating mal-rotation, and featuring reduced portions for smooth transition, thereby minimizing interface shear forces and accommodating surgical inaccuracies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If traditional dome or button patellar components are used, then the implant structure is simple and easy to manufacture, but excessive shear forces occur during flexion causing peg failure and component loosening
Solution Approach 1:
The patent transitions from symmetric dome/button shapes to an asymmetric anatomical patellar design with a convex articular surface that conforms to the natural patella shape. This asymmetric geometry optimizes contact force distribution during knee flexion, reducing shear forces on fixation pegs while maintaining manufacturing feasibility through standardized anatomical contours.
Solution Approach 2:
The patent modifies the articular surface geometry parameters, specifically creating a convex surface with optimized radius of curvature that changes across the surface. This parameter optimization redirects contact forces to be more perpendicular to the bone interface, reducing shear forces that cause peg failure, while the overall component shape remains manufacturable using conventional processes.
2Reliability
If highly conforming anatomic patella designs are used, then shear forces are reduced, but the designs become sensitive to mal-rotation and surgical implantation errors
Solution Approach 1:
The patent employs moderate asymmetry in the anatomical design that provides shear force reduction without extreme conforming. The convex articular surface has optimized curvature that reduces shear forces while maintaining sufficient tolerance to rotational misalignment, balancing anatomical fidelity with surgical robustness.
Solution Approach 2:
The patent implements partial conforming rather than full anatomical replication. The convex surface provides sufficient anatomical alignment to reduce shear forces while deliberately avoiding excessive conforming that would amplify sensitivity to mal-rotation, achieving an optimal middle ground for surgical tolerance.
3Stress or pressure
If the contact surface area is increased to reduce stress, then the component becomes more conforming to the femoral groove, but sensitivity to mal-implantation increases
Solution Approach 1:
The patent optimizes the contact surface area parameters of the convex articular surface to achieve adequate stress distribution without excessive conforming. The radius of curvature and surface extent are carefully selected to provide sufficient contact area for stress reduction while maintaining tolerance to implantation variations.
Solution Approach 2:
The asymmetric distribution of contact area on the convex surface concentrates support in regions that are less sensitive to mal-rotation, providing robust stress distribution even with moderate implantation errors while maintaining adequate overall contact area.
4Strength
If pegs are made longer and larger to strengthen fixation, then component stability improves, but shear forces on pegs increase during deep flexion
Solution Approach 1:
The patent optimizes peg dimensions to provide adequate fixation strength with moderate size. The critical improvement comes from changing the contact force direction through convex surface geometry, which reduces shear forces on pegs during deep flexion, allowing smaller pegs to achieve equivalent or superior fixation strength.
Solution Approach 2:
The convex articular surface geometry is designed in advance to redirect contact forces before they reach the fixation pegs. This preliminary force redirection minimizes shear forces on the pegs throughout the range of motion, preventing overload that would require excessively large or long pegs.
Data Source
AI summary
Embodiments of the present invention provide patellar component designs that are optimally shaped to help reduce shear force and accommodate slight implantation error. Further, they help lessen anterior knee pain, particularly during deep-flexion activities and help ease the transition during the range of knee movement in a controlled way.