Potential of Using Mg Alloys in Medical Implants

Mg has a density of 1.74 g/cm3, which is much lower than stainless steel and titanium alloys. Mg has a good damping capacity compared to widely used polymers in medical devices and a good fracture toughness compared to widely used bioceramics, such as hydroxyapatite (HAp) (Haferkamp et al. 2000). Table 1

Table 1 Mechanical properties of various implant materials in comparison to natural bone and

ligaments

Bone

ACL

Mg

Ti

Stainless

Synthetic

Properties

tissue

tissue

alloy

alloy

steel

CoCrMo

PLLA

HAp

Density

1.8-2.0

n/a

1.74-2.0

4.4-4.5

7.9-8.1

8.3-9.2

1.23-1.25

3.1

(g/cm2)

Elastic

3-20

0.1-0.5

41-45

110-117

189-205

220-230

2.2-9.5

73-117

modulus

(GPa)

Tensile

100-200

13-46

230-250

830-950

~485

~655

16-69

n/a

strength

(MPa)

Ultimate

~1.5%

15-20%

6-16%

~14%

~40

~8

1-8%

n/a

strain %

Fracture

3-6

15-40

55-115

50-200

0.7

toughness

(MPa X m1/2)

highlights some physical and mechanical properties of bone and ligament tissue as well as representative biomaterials which are currently used for orthopedic applications. Obviously, most nondegradable metals (Ti alloys, stainless steel, and CoCr alloys) have much higher density and mechanical properties than actual bone and ligament tissues. All these conditions generate clinical complications and necessitate additional revision surgery. In addition, metallic implants made out of Ti alloys, stainless steel, and CoCr alloys are permanent and, thus, cannot be remodeled or replaced with time with healthy bone; this results in chronic clinical problems (such as possible consistent inflammation and malnutrition of surrounding bone tissue).

Although biodegradable polymers and resorbable ceramics have been used or explored for tissue engineering and regenerative therapies, metallic biomaterials are more suitable for load-bearing applications due to their combination of strength and toughness. One limitation of current metallic biomaterials is the possible release of toxic metallic ions and/or particles through corrosion or wear (Puleo and Huh 1995; Cohen 1998; Jacobs et al. 1991, 1998a, b, 2003; Lhotka et al. 2003; Sampath 1992) that lead to inflammatory cascades that reduce biocompatibility causing metallosis and ultimately resulting in tissue loss (Granchi et al. 1999; Niki et al. 2003; Bi et al. 2001; Wang et al. 2002). Moreover, the elastic moduli are not matched with that of natural bone tissue, resulting in stress shielding effects which decreases implant stability (Nagels et al. 2003). Current metallic biomaterials are essentially neutral in vivo, remaining as permanent fixtures, which in the case of plates, screws, and pins used to secure serious fractures must be removed by a second invasive surgical procedure after sufficient healing. Recent work has shown the promise of using magnesium (Mg) alloys as a new class of biodegradable metals for use in stents as well as for orthopedic applications (Staiger et al. 2006). Mg is exceptionally light weight with a density of 1.74 g/cm3, 2.6 and 4.5 times less dense than titanium and steel which are commonly used in current medical implants and devices. Mg has greater fracture toughness than ceramic biomaterials such as hydroxyapatite (HAp), while its elastic modulus and strength are closer to those of natural tissue than other commonly used metallic implants (Table 1). These attributes make it an ideal candidate for bone scaffolds, fixation devices, and implant applications.

The low corrosion resistance of Mg, especially in electrolytic and aqueous environments, while a major drawback in engineering applications, becomes useful for biomedical applications, where the in vivo corrosion of the Mg-based implant involves the formation of a soluble, nontoxic oxide that is harmlessly excreted in the urine. Research has shown that Mg alloys can truly degrade in vivo (Staiger et al. 2006; Hort et al. 2010). For example, it has been reported that all magnesium alloys (AZ31, AZ91, WE43, LAE442) implanted into the femur of guinea pigs degraded while the polymer (SR-PLA96) showed very little degradation 18 weeks post surgery. Theoretically, the rate of degradation of Mg alloys can be controlled without compromising their initial mechanical properties. Mg alloys have also been reported to produce less artifacts than titanium in MRI scans and to have almost identical artifacting behavior as polymeric materials (Ernstberger et al. 2009, 2010).

Moreover, due to its functional roles and presence in bone tissue, Mg may actually have stimulatory effects on the growth of new bone tissue (Witte et al. 2005; Revell et al. 2004; Zreiqat et al. 2001, 2002). Mg can remain in the body and maintain mechanical integrity over a time scale of 12-18 weeks while the bone tissue heals, eventually being replaced by natural tissue (Witte et al. 2005). Mg alloys have been proven to be osteoinductive (Witte et al. 2005, 2006; Waselau et al. 2007; Pietak et al. 2008). Mg-based substrates (AZ21) support the adhesion, differentiation, and growth of stromal cells toward an osteoblast-like phenotype with the subsequent production of a bone-like matrix under in vitro conditions (Pietak et al. 2008). It has also been shown that even fast degrading magnesium scaffolds (AZ91D) have good biocompatibility properties and react in vivo with an appropriate inflammatory host response (Witte et al. 2007a). Enhanced formation of unmin-eralized ECM and an enhanced mineral apposition rate adjacent to the degrading magnesium scaffolds (AZ91D) were accompanied by an increased osteoclastic bone surface, which resulted in higher bone mass and a tendency for a more mature trabecular bone structure around the Mg scaffolds in a rabbit model (Witte et al. 2007b). After 6 and 18 weeks of implantation of magnesium alloys (AZ31, AZ91, WE43, and LAE442) and a degradable polymer (SR-PLA96) intramedullary into guinea pig femora, the mineralized area and trabecular apposition rate on uncalci-fied sections were significantly higher around Mg alloys (AZ31, AZ91, WE43, and LAE442) compared to a polymer (SR-PLA96) (Witte et al. 2005). No statistical difference in bone formation was found among these Mg alloys (AZ31, AZ91, WE43, and LAE442).

Thus, the use of Mg alloys as biomaterials in orthopedic/craniofacial implants would overcome several of the current limitations of metallic and polyester implants, namely reducing the chance of implant breakage during implantation compared to degradable polymers, promoting bone tissue regeneration, limiting MRI interference, and degrading at a controlled rate.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment