A New Generation of Implants to Treat Cervical Myelopathy and Radiculopathy
A recently published review article outlines the various types of viscoelastic cervical disc replacement implants that are intended to better mimic the native anatomy of the cervical spine than first-generation, ball-and-socket implants.
Laviel Fernandez, MD, and Peter Passias, MD
Jacobs CAM, Siepe CJ, Ito K. Viscoelastic cervical total disc replacement devices: design concepts. Spine J. 2020 Dec;20(12):1911-1924. doi: 10.1016/j.spinee.2020.08.007. Epub 2020 Aug 15.
The current gold standard for the management of cervical myelopathy and radiculopathy is anterior cervical discectomy and fusion (ACDF). However, this procedure results in loss of motion and development of sagittal imbalance, as well as adjacent segment degeneration. Cervical disc replacement (CDR) has been proposed as an alternative treatment to maintain cervical motion and reduce the risk of adjacent segment disease while addressing the cervical myelopathy/radiculopathy present.
First-generation CDR implants were based on the ball-and-socket design of knee and hip implants, and they used metal-on-metal or metal-on-polymer bearing surfaces. Mid-term results have demonstrated equivalent outcomes between the use of these first-generation implants and ACDF.
However, the sliding motion that occurs with the first-general CDR implants simplifies the complex anatomy and motion of the natural disc, creating an unstable environment in which the ligaments, muscles, uncovertebral joints, and facet joints experience abnormal loading. This can lead to uncovertebral joint degeneration and degeneration of adjacent segments.
A new generation of viscoelastic CDR implants is now available, and these implants are designed to more closely mirror the viscoelastic behavior of the native disc. This allows for more motion in all 6 degrees of freedom in addition to combined motion.
In a recently published review article, Jacobs et al provide an overview of the 11 current viscoelastic CRD implant, describing the advantages and disadvantages of each. These implant designs are categorized as endplates sandwiched or without endplates. The endplates sandwiched implants are further divided into a monoblock and mobile bumper designs.
In the monoblock design, the elastomeric core is firmly bonded to the adjacent endplate, whereas in the mobile bumper design, there is a polymer sandwiched between the endplates. The advantage of the of monoblock design is that it allows for motion only by deformation, which reduces the risk of wear particles compared with current CDR implants that use sliding motion.
However, there are several disadvantages with the monoblock design, including the risk of delamination between the metal and polymer due to the dissimilar properties of the 2 materials. Another potential pitfall is the transfer of shear stress between the interface of the implant and vertebrae, which can lead to implant loosening and subsidence.
Current monoblock viscoelastic implants include the Freedom, CP-ESP, Rhine, D-flex Carbon, NeoPhytos, and UFO.
The mobile bumper design offers the advantage of avoiding peak stress at the interface between the core and endplates because the stresses are distributed between multiple elements of the prosthesis. A potential disadvantage is the generation of more wear particles. In addition, the 2 currently available mobile bumper designs – the M6-C and Bryan disc – do not provide physiologic disc height and, therefore, have the potential for over-distracting the disc space. This can exert increased forces on the uncovertebral joints.
The CDR implant designs without endplates offer the advantage of avoiding metal endplates, which makes MRI imaging easier. Another advantage is that endplates could limit extremes of motion due to impingement at a particular degree of flexion/extension. Thus, the cervical motion is more limited by the viscoelasticity of the device, which more closely resembles the native disc.
A disadvantage of the design without endplates is the potential for reduced osseous integration between implant and endplate. Therefore, these designs often require polymers with surfaces that facilitate osseous integration, including plasma etching or hydroxyapatite coating. These polymers are softer and can experience increased creep and settling in the long term.
The currently available implants without endplates are the NeoDisc, CaDisc-C, and the 3D-F.
First-generation CDR implants offer an alternative to cervical myelopathy and radiculopathy that better maintains cervical motion and decreases adjacent segment disease. However, these implants do not appropriately replicate the native motion of the cervical disc, which can have negative biomechanical implications in the long term.
The review by Jacobs et al highlights the potential benefits of using the newer viscoelastic designs, which more closely mimic the complex kinematic motion of the cervical discs by reducing the compensation of adjacent soft tissue structures required by ball-and-socket implant designs. A hypothetical advantage is longer implant longevity compared with first-generation models due to less adjacent degeneration of ligaments, uncovertebral, and facet joints. Another potential advantage of viscoelastic CDR implants is the variable center of rotation, which improves the margin of error for implant positioning intraoperatively.
Potential disadvantages of these implants include a possible weak point at the interface between the viscoelastic material and metal, as well as potential generation of wear particles, which could lead to implant loosening.
Initial data on the use of newer-generation implants to manage cervical myelopathy/radiculopathy are promising, but longer-term studies are needed to determine their longevity and provide insight into their true replication of native disc kinematic motion.
Laviel Fernandez, MD, is an orthopaedic surgery resident at NYU Langone Medical Center – Langone Orthopedic Hospital, New York, New York. Peter Passias, MD, is a Clinical Associate Professor of Orthopaedic Surgery, Division of Spine Surgery, Department of Orthopaedic Surgery, at NYU Langone Medical Center – Langone Orthopedic Hospital, New York, New York.
Disclosures: The authors have no disclosures relevant to this article.