Artificial Discs: Types, Materials, and Spinal Applications

What are the different types of artificial discs?

Artificial discs are prosthetic devices surgically implanted into the spine to replace damaged or degenerated intervertebral discs, primarily aiming to alleviate pain while preserving motion at the affected spinal segment.

Understanding Artificial Disc Replacement

Artificial disc replacement (ADR), also known as total disc arthroplasty, is a surgical procedure designed to restore spinal function and reduce pain caused by degenerative disc disease or disc herniation. Unlike spinal fusion, which permanently joins two or more vertebrae, ADR aims to maintain the natural motion of the spine, potentially reducing stress on adjacent spinal segments.

The Purpose and Biomechanics of Artificial Discs

The natural intervertebral disc acts as a shock absorber and allows for flexibility and movement between vertebrae. When a disc degenerates, it can lead to pain, nerve compression, and instability. Artificial discs are engineered to mimic the biomechanical properties of a healthy disc, providing:

• Load Bearing: Supporting the compressive forces of the body.
• Flexibility and Motion: Allowing for bending, twisting, and side-to-side movements.
• Stability: Preventing excessive or uncontrolled movement.
• Durability: Designed to withstand the rigors of spinal motion over many years.

These devices are typically composed of biocompatible materials that resist wear and corrosion within the body.

Common Materials Used in Artificial Discs

The choice of materials is critical for the long-term success and biocompatibility of artificial discs. Common materials include:

• Metal Alloys:
  • Cobalt-Chromium (CoCr): Highly resistant to wear and corrosion, providing excellent strength.
  • Titanium Alloys: Known for their biocompatibility and ability to integrate with bone, often used for endplates.
• Polyethylene:
  • Ultra-High Molecular Weight Polyethylene (UHMWPE): Used as a bearing surface in many designs due to its low friction and wear properties, similar to materials used in hip and knee replacements.
• Ceramics: Less common for articulating surfaces but may be used in specific designs or coatings for their hardness and wear resistance.

Types of Artificial Discs by Spinal Region

Artificial discs are specifically designed for the region of the spine where they will be implanted, primarily the cervical (neck) and lumbar (lower back) spine, due to the distinct biomechanical demands of each area.

Cervical Artificial Discs

Cervical artificial discs are designed to replace degenerated discs in the neck, which are crucial for head movement and protecting nerves to the arms. They typically feature smaller profiles to fit the anatomy of the cervical spine.

• Common Designs:

  • Metal-on-Polymer (MOP): Consists of two metal endplates that articulate with a polyethylene core. This is a widely adopted design, leveraging the low-friction properties of UHMWPE.
  • Metal-on-Metal (MOM): Features two metal endplates articulating directly against each other. While offering high wear resistance, concerns about metal ion release have led to a decrease in their use in some applications.
  • Two-Piece Articulating Designs: Most cervical discs fall into this category, comprising upper and lower metal endplates that interface with an articulating core, which might be fixed or mobile within the endplates.
  • Single-Piece Flexible Designs: Less common, these designs aim to mimic the natural disc's elasticity more closely, often involving a flexible polymer or elastomeric core.

• Examples of FDA-Approved Cervical Discs (illustrative, not exhaustive):

  • Mobi-C® (Zimmer Biomet): A three-piece design with a mobile polyethylene core that allows for self-adjustment to the endplates.
  • ProDisc®-C (Centinel Spine): A two-piece implant with a highly congruent polyethylene insert that articulates between two cobalt-chromium endplates.
  • Prestige® LP (Medtronic): A two-piece, metal-on-metal design with a ball-and-trough articulation.
  • Bryan® Cervical Disc (Medtronic): Features a flexible polymer core encased by two titanium endplates.

Lumbar Artificial Discs

Lumbar artificial discs are designed for the lower back, which bears significant weight and stress. They are typically larger and more robust than cervical discs to accommodate the higher loads.

• Common Designs:

  • Metal-on-Polymer (MOP): Similar to cervical designs, these consist of two metal endplates that articulate with a polyethylene core. This is the most prevalent design for lumbar ADR. The core can be fixed or mobile.
  • Articulating Core Designs: These designs allow the core to move freely between the endplates, mimicking the natural disc's center of rotation.
  • Fixed Core Designs: The core is secured to one of the endplates, with articulation occurring between the core and the opposing endplate.

• Examples of FDA-Approved Lumbar Discs (illustrative, not exhaustive):

  • Charité® Artificial Disc (DePuy Synthes): One of the earliest FDA-approved lumbar discs, featuring a mobile polyethylene core between two cobalt-chromium endplates.
  • ProDisc®-L (Centinel Spine): A two-piece design with a highly congruent polyethylene insert articulating between two cobalt-chromium endplates, allowing for a controlled range of motion.
  • activL® Artificial Disc (Aesculap Implant Systems): A two-piece design with a viscoelastic polymer core that aims to provide both motion and shock absorption.
  • Maverick™ Disc (Medtronic): A metal-on-metal design, similar to the Prestige LP, but for the lumbar spine.

Considerations for Disc Selection

The choice of artificial disc type and design is a complex decision made by the surgeon based on several factors, including:

• Patient Anatomy and Biomechanics: The unique curvature and motion characteristics of the individual's spine.
• Degeneration Severity: The extent of disc damage and presence of osteophytes (bone spurs).
• Surgeon's Experience and Preference: Familiarity with specific devices and techniques.
• Clinical Evidence: Long-term outcomes and safety data for different disc types.

Conclusion

Artificial discs represent a significant advancement in spine surgery, offering an alternative to fusion for select patients suffering from debilitating disc degeneration. While the fundamental goal of restoring motion and relieving pain remains consistent, the specific designs, materials, and biomechanical principles vary significantly between cervical and lumbar discs, as well as among different manufacturers. Understanding these distinctions is crucial for appreciating the nuanced approach to modern spinal arthroplasty.