PDN device
Fig. 3: Raymedica PDN device
(larger view)

Newcleus device
Fig. 4: Newcleus device
(larger view)

Newcleus device
Fig. 5: Newcleus device within the disc (larger view)

A disc nucleus replacement device is designed to replace only the inner portion of the disc (the nucleus).

A variety of disc nucleus replacement technologies are currently being investigated in the laboratory, and some have been implanted in countries outside the US. Various materials are utilized in these implants, including metals and ceramics, injectable fluids, hydrogels, inflatables, and elastic coils.

The following is a brief history of early evolution of artificial disc technologies:

  • Early attempts at disc replacement were all nuclear replacements, based on the need to prevent disc space collapse following discectomy.
  • David Cleveland published the first study in 1955, when he injected methyl-acrylic into the disc space at the time of discectomy in 14 patients.
  • James Gardner who had "replaced hundreds of damaged lumbar discs with 8x22 mm Lucite pegs, yielding excellent results."
  • Hamby reported his reproduction of Cleveland's work in another 14 patients, reported in 1957 at the American Association of Neurological Surgeons meeting.
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  • In 1959, Harmon implanted Vitallium Spheres, inserted through an anterior retroperitonael approach, but never reported his results.
  • Nicholson injected silicon rubber inserts into cadaveric discs in 1962. He never proceeded to clinical trials since the implants did poorly in mechanical testing.
  • In 1964, Fernstrom published his results of replacing lumbar discs with a metal ball bearing, under local anesthesia. At 30 months, he stated that his results were better than discectomy alone, and comparable to fusion.
  • Similarly, Reitz and Joubert implanted 19 steel balls into 12 patients, and at 8-month follow-up concluded that results were superior to fusion.

Presently, there are several disc nucleus replacement technologies in various stages of development. The following outlines a few of the better-known technologies.

Prosthetic Disc Nucleus (PDN)

The Raymedica PDN ("Prosthetic Disc Nucleus") has been implanted outside the US in several experimental series and just recently the PDN-SOLO device has been implanted in the US as part of a non-randomized, prospective clinical trial. The PDN is composed of a hydrogel core in a flexible, inelastic, woven polyethylene jacket. The hydrogel undergoes a cyclic swelling and shrinking, depending on load.

Four-year follow-up on 350 patients was presented at the Spinal Arthroplasty Society meeting in 2001 by Schromayer, reporting improvements in Oswestry pain measurement from >50 to <10, in VAS from 7.0 to 1.9, and in mean Prolo Score from 4.5 to 9.1. Increases in mean disc height and flexibility were reported. Some problems with device migration were initially reported, but became less common after the operative technique was modified.

Figure 3: Raymedica PDN device

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The Newcleus

The Newcleus utilizes an elongated elastic memory-coiling spiral made of polycarbonate urethane. It is inserted through a posterolateral annulotomy after discectomy, and then spirals around within the annulus to fill the nuclear cavity. This device has undergone laboratory and animal investigation, and has been implanted in a small number of patients in Europe.

Figure 4: Newcleus device
Figure 5: Newcleus device within the disc

Aquarelle Hydrogel Nucleus

The Aquarelle Hydrogel Nucleus (by Stryker Howmedica Osteonic) is composed of a polyvinyl alcohol material. Biomechanical testing indicates that it has a performance similar to the intact nucleus. Some implants have been implanted in humans in Europe, and baboon studies have been initiated in the US.

Other Implants:

  • Interpore Cross International is developing a one-piece convex surfaced ceramic or metal implant that anchors to the inferior vertebral body as a hemiarthroplasty. It has been implanted in 2 patients, and animal studies in the US have been initiated.
  • Disc Dynamics is testing a balloon-like implant made of polyurethane. It is implanted through a cannula (tube), and animal studies have been initiated.
  • Cryolife is testing the Biodisc, a protein hydrogel device that is injected into the nuclear cavity in a fluid form, but it rapidly hardens.
  • Disc Augmentation Technologies has initiated mechanical studies on a thermopolymer that also hardens inside the disc after being injected as a fluid.
  • Replication Medical's Aquacryl is a hydrogel undergoing mechanical testing.

Ultimately, the results of clinical trials will determine the efficacy of any of these nucleus replacements. Potential problems include migration of the device, end-plate changes, and vitality of the anulus following an anulotomy for insertion.

This technology is still in the early stages of investigation and needs to prove itself clinically.