What is a disc replacement?

Lumbar disc replacement technology (arthroplasty) has been developed over the last 40 years. Most devices theoretically exist as alternatives to spinal fusion and few of them have undergone clinical trials. There is a significant obstacle to the formation of an artificial intervertebral disc: the difficulty of creating a faithful copy of the intervertebral disc, both in terms of design and function, due to its complexity.

This replacement should also allow full mobility of the spine, as well as the support of the torso for the rest of the patient’s life. For mobility to return and for disc arthroplasty to succeed, the source of the pain must be limited to the intervertebral space, and the assessment of small joints should be an important parameter in the diagnosis of low persistent back pain.

Lumbar disc replacement methods

A patient with diseased or damaged small joints from vertebral arthritis is not suitable for disc arthroplasty. Modern technology in the field of the intervertebral disc has to propose the use of an artificial disc nucleus and total lumbar disc replacement, by inserting an implantable device of different degrees of movement’s freedom.

a) Replacement with artificial core

Initial efforts to design artificial nuclei focused on creating a faithful replica with the same mechanical function, as the body’s intervertebral disc nucleus. Then, with open access, the degenerated nucleus was removed and replaced with the artificial one. The goal is to restore normal tension in the fibrous ring around the disc nucleus and, of course, to return to normal height in the intervertebral space.

In 1988, scientists developed an artificial core made of PVA inside a polyethylene outer cover (Prosthetic Disc Nucleus, PDN). The cover was a semi-permeable mesh, that had the property of absorbing water and expanding. It was placed in the appropriate place, with rear or front side access. Due to the ability of the external part to absorb water, the device swelled and in a way “locked” in the place where it was placed. The result was incompressible and responded to the support of the load and its transport to the surrounding structures.

b) Total lumbar disc replacement

The use of devices for total replacement of the intervertebral disc allows the removal of the gel nucleus and the fibrous ring around it, elements that have degenerated and are a source of pain. Many types of devices have been described such as limited freedom devices, devices with a smooth flat very low friction surface, and devices with enclosed fluid compartments.

Nowadays, are developed designs that differ in the metal or plastic surfaces, the restriction or not of freedom of movement of the implant components, and the anchorage characteristics between the vertebrae.

Charité artificial disc

In 2004, such a device called the Charité artificial disk was tested. The study compared the effectiveness of the artificial disc with the anterior lumbar intervertebral disc, about the clinical and radiological outcome. The results showed that the improvement in mobility and the reduction of pain was immediate, lasted for two years, and were similar in both groups.

The Charité artificial disk has been released in several editions. In its modern form, it consists of two final cobalt-chrome plates with six protrusions for anchoring on the lower surface of each in varying sizes and curvatures. The upper and lower concave surfaces of the endplates are in contact with a smooth, very high molecular weight mobile polyethylene core. Such surfaces resemble the movement of a normal vertebral unit. This is an important advantage, as research in both the laboratory and humans has shown.

The movable nucleus of the prosthesis mimics the function of the normal nucleus of the disc and allows movement of the small joints very close to normal. The placement of the device requires great precision to achieve the desired result and is done with front access to the appropriate point.

Postoperative results

Patients who underwent total disc replacement were more satisfied and this is attributed to the absence of postoperative limitations, compared to patients who underwent spinal fusion, where these limitations are necessary. Cases of slight displacement of the artificial disc have been reported, but no cases of severe displacement have occurred. In addition, the radiological examination confirmed the maintenance of the height of the intervertebral space and the correct position of the vertebrae, as well as the implant that was associated with the satisfactory clinical result.

ProDisc

Several other similar devices are currently under investigation. They differ from the Charité disk in the absence of a mobile core. Instead, they have construction that functions as a ball and socket joint about the same as the hip joint. One such device is the ProDisc, which consists of three components. It has a spherical non-movable articular surface made of high molecular weight polyethylene that is connected with two endplates of cobalt-chromium.

The endplates are connected to the vertebral bodies and with the adjustment of the overlying plate, the correct curvature is achieved. ProDisc was first implanted in 1990, and its effectiveness is currently being investigated when used in one or two degenerative intervertebral discs.

Maverick Disc & Flexicore Disc

Some other devices, which are also still under research, are the Maverick Disc and Flexicore Disc designs. The Maverick artificial disc is anchored to the upper and lower vertebrae with protrusions that enter the vertebral bodies, and the Flexicore has a titanium coating on the contact surfaces with the upper and lower vertebrae and a layer of bioactive hydroxyapatite to integrate with them.

Spinal disc arthroplasty is still in its infancy and is expected to be highly developed in the coming decades. Intensive research is required to determine the usefulness of total disc replacement and to determine the criteria by which a patient will fit into this treatment regimen. The installation of these devices must also be accurate because the final result is directly affected and access techniques require improvements.

Spinal disc arthroplasty, like peripheral joint arthroplasty, has a definite lifespan, either due to material wear or due to the development of the disease of the small joints of each vertebral unit. Whether a repeat operation can be performed and what exactly it will involve (removing the device or adding an extra one) is a big topic of discussion, that is expected to be resolved in the future.