Project Overview

This project, developed in collaboration with Medtronic, addresses a critical issue in anterior cervical discectomy and fusion (ACDF) surgeries: post-operative dysphagia caused by bulky implant profiles.

The Challenge: Existing cervical plates provide necessary stabilization but often impinge on the pharynx wall due to excessive thickness and mass.

The Solution: We developed a “NextGen” low-profile plate system (1-level and 2-level) that maintains FDA-specified load-bearing capabilities while significantly reducing implant volume.

Figure 1: (a) Front (b) Isometric (c) Side View of the 1 level Plate
Figure 2: (a) Front (b) Isometric (c) Side View of the 2 level Plate

The Result: A topology-optimized implant that achieves a 20% reduction in mass and lower profile compared to the predicate device, validated through ASTM-F1717 computational testing.

Figure 3: Comparison of the Final Prototype (right, top) and Medtronic’s Predicate Plate (left, bottom)

Mathematical Foundation

To reduce bulk without compromising safety, we utilized Topology Optimization.

1. Optimization Objective The goal was to minimize the mass ($M$) and compliance ($C$) of the plate to find the most efficient material layout:

$$\min_{x} \quad (1 - w) \frac{C(x)}{C_0} + w \frac{M(x)}{M_0}$$
  • $x$: Material density distribution variable.
  • $w$: Weighting factor.

2. Constraints (Safety Factors) The optimization is constrained by the Von-Mises yield criterion to ensure the device never fails under physiological loads:

$$\sigma_{vm} \leq \frac{\sigma_{yield}}{SF}$$
  • Constraint: Minimize mass by 90% while maintaining global stress limits.

3. Load Cases (ASTM-F1717) We modeled the FDA-standard testing setup mathematically as a boundary value problem under three distinct loading conditions:

  • Compression: Simulating head weight.
  • Tension: Simulating neck extension.
  • Torsion: Simulating neck rotation.

System Architecture (Design Process)

The development followed a rigorous V-Model engineering process:

  • Requirements Definition: Mapped qualitative surgeon feedback (visibility, locking mechanism) to quantitative engineering specifications (profile thickness $< 2.0$mm, safety factor $> 1.5$).
  • Topology Optimization: Used computational solvers to remove “dead mass” from non-load-bearing regions.
    Figure 3: Topology Optimization
  • FEA Validation:
    • Static Structural Analysis: Simulated the screw-plate interface and vertebral loads.
    • Result: Final prototype showed peak stresses within safe limits (e.g., -28% displacement in compression vs predicate).
  • Prototyping: 3D printed (SLA) and machined prototypes for fit-check validation with Medtronic surgical instrumentation.

Key Challenges & Resolutions

1. Geometric Constraints vs. Visibility

  • Challenge: Surgeons require a clear “viewing window” to monitor bone graft fusion, but reducing material in the center weakens the plate.
  • Resolution: We optimized the material layout specifically around the screw holes and locking mechanisms, creating a “truss-like” structure that preserved the central window while handling torsional loads.

2. Patent Landscape

  • Challenge: The orthopedic device market is saturated with patents, making unique locking mechanisms difficult to design.
  • Resolution: We adapted a standard “twist-lock” mechanism compatible with existing Medtronic drivers, ensuring the innovation focused on the plate topology rather than the commodity locking screw.

Future Work

  • Fatigue Testing: Physical cyclic loading (10 million cycles) to validate long-term durability.
  • Cadaver Labs: Surgical implantation trials to quantify the reduction in tissue retraction required.