Creative geometry solutions for spinal column reconstruction

Creative geometry solutions for spinal column reconstruction

Additive manufacturing techniques make possible innovations in implant technology using titanium

Elastic disc implants from Tsunami can be additively manufactured "in a single-shot process"

Lichtenfels, 15 December 2014: Having a "strong backbone" is not only figuratively speaking a positive personal characteristic. Physically speaking too, a healthy spine is very important for quality of life and personal mobility. One way to alleviate spinal issues is by using minimally invasive technical components made of plastic or metal that are implanted in the body. For spinal surgery patients and surgeons, these implants can be a blessing: with just minor surgical intervention, they help patients become able-bodied again by regaining full confidence in the supportive functions of their spines. Behind these implants lies a combination of design, functionality and a great deal of manufacturing expertise. We spoke to Stefano Caselli, CEO of Tsunami Medical Srl based in Modena, Italy, about trends and product development in spinal implants.

Tsunami sees itself as a system developer for spinal implants. Thus it offers implants that restore the functionality of a damaged spine as well as tools or instruments that allow surgeons to conduct a successful operation. The aim is to strengthen the spine and thus ensure the mobility of the patient.

Conventional spine cages

Spine cages are implants for levelling and relieving the pressure on the spine. These implants are minimally invasive normally used in conjunction with screws made of Titanium in patients with problems caused spinal instability stenosis or many others spinal degenerative dieses. The cages themselves are made of PEEK (polyetheretherketone), a highly biocompatible material, even for allergy sufferers. In the past, these cages were manufactured conventionally in titanium, with the result being that, in some cases, the titanium component had a negative impact on the bone structure due to the rigidity of the cage realized by drilling with a very hard structure. This was caused by the implant's significantly lower elasticity versus the bone. One alternative is PEEK, a high-performance plastic with compression properties or elasticity (E-module) rivalling that of bone. In addition, PEEK components cause no artefacts in MRI (magnetic resonance imaging) and are thus easy to locate by the surgeon through imaging. One of the disadvantages of PEEK spine cages is their inability to be integrated into the bone structure and their eventual should slip off from the original position also after longer periods of implant. Polyetheretherketone is a high temperature resistant thermoplastic and belongs to the group of polyaryletherketones. Its melting temperature is 335°C.

Modern, laser-melted spine cages

The LaserCUSING additive laser melting method combines the strengths – and eliminates the weaknesses – of traditional titanium or PEEK components for spine cage applications. A laser-melted component can now offer the biocompatibility of titanium with the desired elasticity of a plastic material in a single product. Laser-melted spine cages have a very complicate geometry and not require to be re-treated (images 1a, b and c) to ensure an optimal surface structure. The highlight of this solution: its geometry, with partially different density distributions (embedded web-like structures), now gives titanium parts the same elasticity as a PEEK solution. Laser-melted cages can also be affordably manufactured in various dimensions depending on the anatomy of the patient, allowing for customized manufacturing. According to CEO Stefano Caselli, laser-melted cages represent "a true innovation that combines material-specific advantages such as biocompatibility, i.e., the ability to embed the material into the bone structure, and elasticity that is customized for the human body in a single product" (image 2). Surgeons can easily locate the cages using CT or MRI imaging. Laser melting also allows custom or small-series manufacturing, including "made-to-order" cages for patients with special anatomical conditions or standard solutions for other patients. A clinical study conducted in March 2014 in cooperation with Carl von Ossietzky University in Oldenburg, Germany, confirmed an excellent healing prognosis with these implants.  (*Oldenburg study)

"Lobster spacers" as mobile functional elements in spinal primary instability

Caselli described another Tsunami innovation – so-called "lobster spacers" (image 3 and 4) – self-spreading spacers that can be used between intervertebral discs in surgical spinal column reconstruction (image 5). The spreading mechanism developed by Tsunami is something of a miracle of manufacturing: it consists of a gearbox with a central screw and side gears that spreads apart two wings (images 6a and b). Creating lobster spacers with perfectly aligned surfaces is possible thanks to the LaserCUSING process, using an Mlab cusing R system from Concept Laser. Its highly smooth – nearly polished – surface ensures significantly less excrescence in the tissue. A remarkable aspect of the manufacturing technology, according to Caselli, is the ability to produce multiple copies of this complex, movable part at the same time. "LaserCUSING gives us time advantages, advantages in cost structure and is also much easier, under cleanroom conditions, than conventional manufacturing strategies. In addition, machine solutions from Concept Laser provide excellent performance and a high level of safety when working with reactive materials such as titanium or titanium alloys."

New development: disc prosthesis as a single component

When it comes to component design, additive manufacturing can open up entirely new directions. Another innovation that has resulted from the opportunities offered by LaserCUSING at Tsunami is the development of a customized disc prosthesis that acts as a vertebral spacer to strengthen the spine (image 7). It consists of an upper shell and a lower shell connected by a double spring. The shells' surface is designed to fit perfectly into the vertebral plate. The double spring is made of titanium with a silicone core for damping the movement of the spring. The design and spacing dimensions can be adjusted precisely to the patient's anatomy in accordance with the surgeon's specifications. The highlight of this innovative solution: the disc prosthesis is manufactured in a "one-shot" process using laser melting technology, a solution that eliminates the need for downstream assembly processes.

Functions of the disc prosthesis in detail

Tsunami's competencies were put to the test early on. First, it had to translate its experience in spinal implants into a product solution that would increase patient mobility. The disc prosthesis ensures 360-degree freedom of movement, providing enormous support to the patient's mobility (image 8a and b). Optimum elasticity is ensured by the double spring and enhanced through the silicone core. The company also needed a engineering solution that made the most of the geometric possibilities offered by the LaserCUSING method. Stefano Caselli: "From a design standpoint, the disc prosthesis as a product idea is the direct result of the geometric freedom and functional integration offered by the LaserCUSING method. Conventional manufacturing methods don't work for this kind of solution. Additive manufacturing complements our expertise in spinal column reconstruction by offering a multitude of new options. The solution is also very flexible: we can adjust the dimensions to a patient's specific anatomical profile."