Metallic Alloys in Orthopedic Implants

Metal has been used extensively in the manufacturing of orthopedic implants in a multitude of different forms. Multiple different materials throughout history have been tested as replacements for bone. Materials as diverse as ivory, wood, rubber, acrylic, and Bakelite have been used in the manufacture of prosthetic implants.

The extensive use in modern times of metallic alloys is related to the availability and success at the beginning of the 20th century of several different alloys made of the noble metals. ^[1] Implants made from iron, cobalt, chromium, titanium, and tantalum are commonly used (see the images below). Magnesium and its alloys have been the subject of interest and appear promising as biodegradable implant materials, though their fast corrosion rate in biologic environments has limited their clinical application. ^{[2, 3]}

Clinical studies have demonstrated that such metallic alloys can be used safely and effectively in the manufacturing of orthopedic implants that are left in vivo for extended periods. The mechanical, biologic, and physical properties of these materials play significant roles in the longevity of these implants.

Implants are made in three basic ways:

• They can be machine milled or drilled into a desired shape
• They can be cast, which means that the implant is formed from molten metal that is poured into a mold
• They can be forged, which means that the implant is shaped into its final form with the use of forces such as bending or hammering

Alloys that provide for a long-term stable implant need to have a high level of corrosion resistance as well as certain mechanical properties (see Immune Response to Implants).

Outcomes

Dalury et al followed 96 patients for 5 years who had undergone total hip arthroplasty (THA) with single titanium stems. ^[4] The average Harris Hip score was 96 points (range, 73-100) at final follow-up, and radiographically, all stems were ingrown. No stem had more than 3 mm of subsidence, and there were no leg-length discrepancies greater than 5 mm. The authors concluded that the titanium stem is a versatile option for THA.

Grupp et al reported their experience regarding failed modular titanium neck adapters, in combination with a titanium alloy modular short hip stem, after hip arthroplasty, as a result of fretting or corrosion. ^[5] They were then replaced by cobalt-chromium adapters. The authors noted that by the end of 2008, 1.4% (68/~5000) of the implanted titanium alloy neck adapters failed at an average of 2 years (range, 0.7-4.0) postoperatively.

Grupp et al concluded that failure of modular titanium alloy neck adapters can be initiated by surface micromotions due to surface contamination or highly loaded implant components. ^[5] In the study, according to the authors, the patients at risk were men with an average weight over 100 kg. They added that with a cobalt-chromium neck, micromotions can be reduced by a factor of 3 and the incidence of fretting corrosion substantially lowered.

An element is considered metallic if a positive charge is demonstrated on an electrolysis test. ^[6] This test consists of dissolving the element in acid and running a current through the solution. When such elements are fully reduced, their metallic nature is recognized and they and their alloys are called metals; when oxidized, they can serve as ceramic materials. ^[7]

Metals have several properties that are specific to them. One is malleability, which allows the shaping of metal into implants; another is ductility, which refers to the ability to draw out metal in the shape of wire and is an important property in allowing the manufacture of intramedullary rods, screws, and long stems. By combining several metallic elements in alloys, improved properties can be achieved beyond those of a single element.

The alloys used in orthopedic surgery must have particular properties. For example, because the alloy of the implant is bathed in body fluid, a low rate of corrosion and relative inertness are imperative. Accordingly, much study has focused on modifications of the physical surfaces of allow implants. ^[1]

All metallic alloys have a modulus of elasticity significantly higher than that of bone. This mechanical incompatibility causes implants to be structurally stiffer than bones. Alloys with elastic moduli closer to that of bone may cause less stress shielding.

Different metals can form a battery effect when in solution in the body. The galvanic series provides electrochemical comparisons that allow prediction of corrosion between two different metals when they are in physical contact in saline solution. ^[8] Galvanic corrosion occurs if stainless steel surgical wire is wrapped over a cobalt- or titanium-based alloy femoral component or if a cobalt-chromium femoral head is placed on a titanium alloy femoral stem; consequently, this metal mismatch is not recommended. Cobalt- and titanium-based alloy components may be used in contact with each other, and stainless steel components, such as sutures, may be used with either if actual physical contact is avoided.

The introduction of steel plates for fracture treatment is credited to Sherman. ^[9] Surgical stainless steel alloys (316L) made with varying amounts of iron, chromium, and nickel are presently used in the manufacture of prostheses. The low carbon (L) in surgical stainless steel diminishes corrosion and decreases adverse tissue responses and metal allergies. Although many implants are still manufactured from this excellent material, its use is currently relegated mainly to plates, screws, and intramedullary devices that are not meant to be weightbearing for an extended period. Fatigue failure and relatively high corrosion rates make it a poor candidate for the manufacture of modern joint replacement implants. ^[10]

Chromium-containing iron (and cobalt base) alloys have a chromium oxide–based surface that is a result of passivation or oxidation of the surface. The chromium oxide forms a very thin invisible shield that provides resistance to biodegradation. Because this oxide layer slowly dissolves in vivo, these alloys have a relatively high rate of corrosion. This is evident as a propensity toward both fretting and crevice corrosion, which limits the possibility for biologic fixation or for the manufacture of modular implants.

In an in-vitro study, Heise et al determined that macrophages were able to affect the oxide surfaces of both stainless steel and titanium oxide disks. ^[11] The macrophage activity released a statistically significant amount of nitric oxide from the stainless steel disks and produced corrosive pits.

Venable and Stuck discovered the battery effects of metals in the body through their testing of the electrolytic effects of various metals on surrounding tissue and bone. ^[6] These tests demonstrated the low level of corrosion of the cobalt-based alloy vitallium.

Alloys made of cobalt, chromium, and molybdenum can be used in various different porous forms to allow for biologic fixation by ingrowth. These alloys are among the least ductile when compared to either iron- or titanium-based alloys, making manufacture of these intramedullary rods and spinal instrumentation more difficult. These alloys have some of the highest moduli of elasticity observed in orthopedic implants, and as a result, this was a factor in the stress shielding and thigh pain observed in the first generation of biologically fixed femoral hip implants made with cobalt alloys. ^[12]

These alloys are well suited for the production of implants that are designed to replace bone and to be loadbearing for an extended period, if not permanently.

The Austin Moore prosthesis and the Thompson prosthesis were manufactured from the cobalt-based alloys. The first-generation biologically fixed implants (ie, porous-coated anatomic [PCA] and anatomic medullary locking [AML] implants) were manufactured of this material. Numerous modern prostheses are still manufactured from this excellent alloy and are used in both cemented and porous forms for hip and knee replacement.

In 1951, Levanthal introduced titanium as a metal for surgery. ^[13] Titanium-based alloys have excellent properties for use in porous forms for biologic fixation of prostheses. The one most commonly used has been Ti-6 aluminum Ti-4 vanadium (Ti6Al4V), but other, more modern alloys are being employed increasingly often. Because titanium-based alloys have a lower moduli of elasticity than cobalt-based alloys or surgical stainless steel, they have not been as reliable when used as a cemented hip replacement. Moreover, their use in total knee replacements has been limited to the nonarticulating parts of the tibial component because of significant wear observed in femoral heads. ^[14]

Titanium's high level of biocompatibility, low level of corrosion, and modulus of elasticity closer to that of bone allow for its use in numerous porous implants that have yielded excellent long-term results. The low level of corrosion allows for the construction of modular implants that saves in inventory and allows for more precise implant fit. ^[15]

Current use of titanium in various forms is in the production of fracture plates and intramedullary rods and in the production of both femoral and acetabular implants designed for bone ingrowth. Fracture fixation components fabricated from titanium-based alloys are also used preferentially when the implant site is known to be infected or when postoperative shadow-free imaging is desired.

Industry has been modifying the surface area of titanium implants with many proprietary coatings. ^{[16, 17, 18, 19]} Attempts at mimicking the microscopic structure of cancellous bone have been extremely effective in increasing the scratch fit noted with these implants. ^[20] There remains the question of whether the longevity of these coatings will result in improved long-term ingrowth of components.

Magnesium demonstrates superb biocompatibility and load-bearing capability; however, it is also biodegradable and subject to relatively rapid corrosion in vivo. These latter properties make magnesium-based alloys largely unsuitable for orthopedic implant construction, in that such implants would likely suffer degradation before long-term healing can occur.

Despite these drawbacks, magnesium-based alloys have been suggested as temporary orthopedic implants. These implants could potentially be suitable for short-term use and would then dissolve, eliminating the need for explantation. ^[21]

Tantalum is also remarkably resistant to corrosion and has been used as an ingredient in super alloys, principally in aircraft engines and spacecraft, though 50% of current use is in the form of powder metal for the manufacture of transistors and capacitors. Tantalum can be fabricated in a highly porous form, which has a modulus of elasticity closer to that of bone than stainless steel or the cobalt-based alloys. Tantalum balls have been used in studies that have required bone markers; however, it was not used in the manufacture of implants until relatively recently. Because of its remarkable resistance to corrosion, tantalum is well suited to a biologic ingrowth setting.

Current use of tantalum has been in the form of a honeycombed structure that is extremely porous and conducive to bone ingrowth. It is available in several forms for bridging bone defects, but its use in the manufacture of femoral stems has yet to be established. Tantalum appears to be a promising metal for use in acetabular reconstruction, but there remains a need for data from long-term studies. ^{[22, 23, 15, 24, 25]}

The combination of metallic alloys with other biomaterials can result in implants with improved mechanical and physical properties. Although a number of attempts to design composite implants have not yielded highly successful results, the possibilities for future improvements are promising.

Different alloys demonstrate different rates of wear. The hardness of an alloy and the smoothness of the bearing surfaces determine its relative rate of wear. Cobalt-chromium-molybdenum alloys and alloys made of stainless steel are more wear-resistant than titanium or titanium-based alloys. When breakdown with titanium-based alloys occurs, it is often observed as black areas within the tissues.

Metallic ion release occurs in vivo, and numerous studies demonstrate soluble and precipitated corrosion products, as well as metallic wear debris, in the liver, spleen, lungs, and even remote bone marrow of the iliac crest. The constant motion of the metal-on-metal prosthesis causes a wearing away of the passivated surface and an increase in metallic ion release. The widespread interest in metal-on-metal prostheses has raised questions of biocompatibility and possible carcinogenic effects that these metallic ions can cause. ^{[26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38]}

Several metal-on-metal prostheses have been recalled, and concerns have been expressed about the long-term safety of metal-on-metal prostheses. In April 2010, the United Kingdom's Medicines and Healthcare Products Regulatory Agency issued a medical device alert on metal-on-metal hip replacements. Recommendations have included specific blood tests and imaging for patients with painful metal-on-metal hip replacements. Metal ion testing and evaluation for effects of metal debris such as possible local nerve palsy, local swelling, and joint dislocation or subluxation should be considered by orthopedic surgeons treating patients with metal-on-metal prostheses.

The consensus of the sixth advanced hip resurfacing course (Ghent, Belgium, May 2014), formulated by an international faculty of expert metal-on-metal hip resurfacing surgeons, was that hip resurfacing should be limited to high-volume hip surgeons who are experienced in hip resurfacing or have been trained to perform hip resurfacing in a specialist center. ^[39]

It is to be hoped that further developments in metallurgy will allow the development of new alloys that will have better mechanical and physical properties than currently available alloys and thus will yield better long-term results with implants.

Concurrent developments in other biomaterials, including ceramics and modified polyethylenes (eg, cross-linked polyethylene) may yield improvements in the longevity of total joint replacements either with the success of alternative bearing surfaces or with the use of composite materials. The total joint replacement that will last the life of the patient may be a reality one day. ^{[40, 41]}