After cancer and heart disease, back pain is the third most costly medical disorder in the U.S. The spinal-surgery segment of the orthopedic industry is already the fastest growing portion of the market and with changing demographics of the population, the spinal-devices segment is expected to continue to outpace other areas of the industry. Treatments for spine disorders have produced a dynamic marketplace for several years, which is expected to continue through the next decade.

U.S. demographics are in a state of flux. People are physically larger, more active, and living longer than past generations. They want immediate relief of symptoms and expect spinal implants that relieve pain, and allow active lifestyles. Individuals are receiving implants earlier in life and expect them to perform better and last longer. Clearly, the growing demand on implant performance requires higher-performance materials.

Recently there has been a dramatic increase in the number and types of devices designed for the treatment of the four major spinal disorders: deformity, degenerative, spinal trauma, and spinal tumors. Each of these presents unique challenges to the surgeon and device designer. However, regardless of the disorder being treated, two broad-based trends have been consistent: the need for higher performance and longer lasting implants, and the device industry's genuine desire to provide better clinical outcomes for the patient.

A review of current spinal surgery trends shows where high-performance alloys and products may be used to satisfy the increased expectations of today's device designers, surgeons, and patients.

Improvements in fusionand fixation

The most frequent procedures for many spinal disorders have been fusion and fixation treatments. These use devices such as plates, screws, rods, hooks, spacers and cages. Most of the metals used in this segment are titanium-based alloys such as Ti-6Al-4V/ELI, Ti-6Al-7Nb, and CP Ti, as well as stainless and cobalt alloys.

Components for fusion are typically either machined from round or flat bar stock or cut from flat plate. Hence, minimizing scrap during machining and providing near-net shapes has been an increasing trend. In the case of cervical plates, for example, titanium flat products from Dynamet Inc., a subsidiary of Carpenter Technology, provide consistently closer-to-finish tolerances, improved surface finishes, and better shape consistency than conventional titanium parts cut from plate. An added benefit includes improved mechanical properties and grain structure when compared to titanium products typically cut from plate.

For some fusion applications that use stainless rods and screws, medical-device designers need higher strength components with enhanced galling resistance. In these cases, Carpenter's BioDur 108 alloy has proven beneficial. The alloy is a high nitrogen, essentially nickel-free stainless capable of high strengths in excess of 200 ksi. The material also has improved galling resistance when compared to BioDur 316LS and 22Cr-13Ni-5Mn.

Preserving motion

Although traditional fusion and fixation devices often prove successful, they tend to limit patient mobility. In response, a trend has formed to move toward nonfusion alternatives, including dynamic stabilization, artificial discs, and other motion-preserving techniques. In these cases, higher performance alloys may be required that exhibit enhanced wear resistance, finer microstructure, or higher fatigue strength than most current alloys. These attributes can come from an alternative alloying technique such as Carpenter's proprietary Micro-Melt process. The technique melts the alloy and then atomizes it into a fine powder by introducing the molten metal to a high-pressure stream of gas. The powder is then blended and screened to a controlled diameter and finally consolidated into a solid ingot of material which is then processed to finished diameters.

Artificial discs

Many disc designs are on the market and in development. Some of these use titanium alloys, others use stainless, cobalt, and still others ceramics, PEEK, or elastomer components. But each shares a common attribute: A wear resistant and highly polishable bearing surface. BioDur CCM Plus alloy (F1537 Alloy 2) has proven beneficial for this type of application. The alloy is manufactured using the Micro — Melt process, which results in better microstructural uniformity than other high-carbon cobalt-chrome-moly grades on the market.

The microstructure of BioDur CCM Plus alloy also produces a higher fatigue strength and enhanced mechanical properties when compared to the more widely used cast or wrought version of ASTM F1537 Alloy1.

Dynamic stabilization

Another trend in the spine segment is the adoption of dynamic stabilization devices. Like artificial discs, many systems are in development, submitted for FDA approval, and commercially available. The use of higher-performance alloys and existing alloys manufactured to higher-strength conditions shows great promise. For example, using already approved titanium alloys such as Ti-6Al-4V/ELI, Ti-6Al-7Nb and Ti-15Mo in a strain-hardened condition, or an annealed then heat-treated condition, provides enhanced mechanical properties, which helps meet the needs of demanding applications.

Deformity treatment

When it comes to the effective treatment of spinal deformities such as scoliosis, kyphosis, and lordosis, how an implant feels in the hands of the surgeon often becomes a driving factor for the type and condition of the alloy selected for a device. Using high-performance alloys and alternative-strength conditions of approved alloys has been an increasing trend in order to satisfy surgeons. For example, spinal rods for treating spinal deformities typically used stainless steel and titanium. However, cobalt-based alloys are now used in some circumstances for their higher stiffness and modulus, and better mechanical properties than competing materials.

Micro-Melt BioDur Carpenter CCM cobalt chrome alloy is increasingly used in small diameter (under 8mm) spinal rods. The alloy has a higher strength, finer grain structure and increased fatigue strength over conventional cast and wrought ASTM F1537 Alloy 1, cobalt chrome alloys.

Designers should also keep in mind that customized, ultra-high-strength stainless alloys are also available. Whether a designer needs the properties of titanium, cobalt, or stainless, available options can provide a customized feel that some surgeons require.

Minimally invasive surgical techniques

In addition to longer lasting implants, there is a trend toward minimally invasive surgical (MIS) techniques. Higher-performance alloys produced to tighter tolerances can help facilitate MIS technology by allowing the design of smaller, more durable, more flexible, and higher-strength instruments and implants than previously possible. For example, Carpenter's Custom 465 stainless has been useful for instruments that require high strength and improved toughness. Also, Dynamet's Titanium Ultrabar provides the ultra-tight-dimensional tolerances and exceptional diameter uniformity required for some titanium MIS implants. For instance, the bar provides diameter tolerances down to 0.00015 in., bar sizes from 3 to 12 mm, and lengths to 14 feet in a variety of implantable titanium alloys.

Patients suffering from back pain demand faster recovery times and implants that last longer than previous designs. These demands require innovation in both device design and surgical technique. However, innovation does not end there; high performance alloys and products now available in the marketplace and those in current development are helping to satisfy the needs of the device designer, the surgeon and ultimately the patient.

How titanium alloys stack up

How ASTM F1537 compares to BioDur CCM Plus alloy

BioDur CCM Plus alloy has better room-temperature-mechanical and fatigue properties than conventionally produced ASTM F1537 Alloy 1 (tested at 37-mm diameter). UTS is the ultimate tensile strength, EL is an elongation figure, and RA is an area reduction.

Comparing ASTM F1537 Alloy 1 to a Micro-Melt CCM alloy

Improved strength properties and enhanced grain structures are possible by using Micro-Melt CCM alloy versus the conventional cast/wrought F1537 Alloy 1. Samples were tested at 0.3125-in. diameter.

A few alloys that make custom implants possible

Customized mechanical- strength conditions of titanium, stainless, and cobalt alloys provide surgeons with the characteristics needed to treat spinal deformities. The variety of materials options also makes many custom implants possible. CW refers to cold worked materials

Typical properties for MIS materials

High-performance instrument alloys, such as Custom 465 stainless (aged at 950F) allow design possibilities for applications in which fracturing and toughness are a concern.