Magnesium Screws and Plates for Bone Augmentation: a New Concept in Dental Surgery
- 1. Department of Prosthodontics, Malmö University, Sweden
Abstract
Objectives: This short review gives an overview of magnesium as a potential material for temporary metal screws and plates for maxillofacial applications.
Materials and Methods: PubMed was searched for magnesium as a biomaterial. Only in vivo studies and reviews were included.
Results: Until now, titanium screws and plates have been a golden standard in fixing augmented bone and bone grafts. However, a second operation for screw removal causes further damage to the tissues, increases infection risk, and increases patient suffering and costs. Magnesium is a biocompatible metal that resorbs in tissues without causing tissue damage, and it has better mechanical properties than titanium. Magnesium alloys are commercially available for orthopedic and cardiovascular uses, but not for maxillofacial applications.
Conclusion: Magnesium alloys have better properties than titanium and are beneficial in fixing bone grafts. Further development and implication of magnesium in the maxillofacial field would be beneficial for patients undergoing dental surgery.
Keywords
• Bone augmentation
• Magnesium screws
• Magnesium implants
Citation
WetterlövCharyeva O (2017) Magnesium Screws and Plates for Bone Augmentation: a New Concept in Dental Surgery. JSM Dent Surg 2(1): 1011.
INTRODUCTION
Metal screws and plates that fix the bone and then dissolve upon healing is a new concept in dentistry. Until now, titanium has been the metal of choice for fixing augmented bone. Although titanium plates, screws, and nets provide good stability for tissues during the healing phase, these materials have to be removed before further treatment with dental implants [1,2]. The second operation on screw and plate removal leads to new tissue damage, increased infection risks, and high patient costs and suffering.
The concept of biodegradation is already known in medical practice resorbable sutures are successfully used in surgery. Polymers and metals are bioresorbable materials that are suitable for bone augmentation. Polymers have low impact strength; low wear resistance, low capacity to absorb high-strain energy compared to metals Moravej and [3], and might provoke foreign body reactions [4]. The idea behind magnesium screws is to combine the degradability similar to polymers with good mechanical properties similar to or better than titanium.
Historically, the first metals to be used for medical applications were Ag-, Fe-, Au-, and Pt-based alloys in the late 18th century [5]. The discovery of elemental magnesium by Sir Humphrey Davy in 1808 led to the design of metallic biodegradable implants. The first implantation of magnesium wires as ligatures to stop bleeding vessels of human patients was performed [6]. Today, magnesium has been rediscovered in the medical field due to its fantastic properties.
MATERIALS AND METHODS
The PubMed database was searched for magnesium as a biomaterial. The search words were “magnesium biomaterial,” “magnesium in vivo,” “magnesium biodegradable,” and “magnesium implants.” Only in vivo studies and reviews from 2005 were selected. The aim was to get insight into magnesium’s properties, problems, and medical application areas. In total, 12 articles met the search criteria.
RESULTS
Properties of magnesium
It is crucial for metal alloys that are to be used for surgical applications be biocompatible and provide enough strength to the tissues. Magnesium is considered a suitable material for medical uses for a number of reasons. First, it is highly biocompatible [7]. Secondly, magnesium occurs naturally in our bodies: 50–60 % of this element is found in bone, and we get about 380–850 mg of magnesium daily from [8]. Foods rich in magnesium include cabbage, spinach, nuts, and grains. Magnesium deficiency can lead to serious problems, such as migraines and cardiovascular disorders [8]. Excess magnesium is removed by the kidneys[8]. Thirdly, magnesium seems to have osseoinductive and osseoconductive properties [9]. Numerous publications show that magnesium stimulates bone attachment to implant surfaces compared to conventional materials [10,11]. Magnesium promotes formation of calcium phosphate, which is favorable for bone mineralization [12-14].
It has been shown that magnesium’s mechanical properties are better than those of titanium [15]. Magnesium-based alloys are typically very light, since they are 1/3 less dense than titanium-based alloys [15]. Titanium alloys typically have a modulus of elasticity of around 115 GPa. In contrast, magnesium alloys’ E-modulus is around 45 GPa, which is much closer to the bone’s 3-20 GPa. This is much better matched to the bone and lessens the likelihood of stress shielding [15].
By-products of degradation
A major problem of magnesium is hydrogen gas formation as a by-product of degradation [16]. Four types of degradable magnesium-based materials were tested in the in vivo study of [12]: magnesium-hydroxyapatite, magnesium-calcium phosphate cement, alloy of 96 % magnesium and 4 % yttrium (W4), and 99.95 % pure magnesium. It has been shown that hydrogen bubbles push away osteoclasts and osteoblasts from the healing site, making it difficult to form osteoid on the material’s surface [12]. However, hydrogen seems to appear within one week after implantation and then vanish after around two to three weeks, as shown in another in vivo experiment [13]. The rate of hydrogen gas production can be controlled by alloying magnesium with other metals. Alloying not only slows down the degradation rate but also increases mechanical properties. It is necessary for magnesium-based alloys to have a slow degradation rate so that the bone heals before the screw resorbs. Hard-tissue repair typically requires implantation of the fixture for a minimum of 12 weeks [17]. Thus, designing an alloy that has good mechanical properties over this critical 12-week period is of high importance.
Another method to slow down the degradation rate is purification. Studies have shown that purification of magnesium reduces the corrosion rate considerably; however, due to the low yield strength of pure magnesium [13], its application in medical appliances that require good load-bearing properties is rather limited.
Applications of magnesium
Two magnesium alloys are commercially available today for cardiovascular and orthopedic applications, WE43 and MgYREZr, respectively. However, no screws or plates are available for dentistry. MgYREZr is a modification of WE43, which, apart from magnesium, yttrium, and rare earth elements, also contains zirconium. The degradation time for MgYREZr is around 24 months [18]. These screws have shown positive results in the pilot study on 13 [2]. However, a limitation of this study is the relatively low statistical power.
DISCUSSION
Titanium-based implants are currently used in maxillofacial surgery for fixing augmented bone. Although they provide excellent stability, they may cause adverse tissue reactions, and additional surgery for implant removal may be necessary [1,2]. Alternatively, degradable polymers are mechanically weaker and might aggravate foreign body reactions [4]. The idea behind magnesium implants is to combine degradability similar to polymers with good mechanical properties similar to titanium.
The problem with magnesium is that it forms hydrogen gas in contact with fluids, which can interfere with tissue healing [17]. Thus, a slow degradation rate is required to achieve optimum healing [5]. The rate of hydrogen gas production can be controlled through several methods, the most common being alloying and purification [12,13]. Using these techniques can increase degradation times up to 24 months in vivo [16]. Even though slow degradation of magnesium is desired, degradation shorter than 24 months is essential for the dental and maxillofacial fields. For dental and maxillofacial medicine, six-month degradation would be optimum.
In our opinion, the potential application areas of magnesium in the maxillofacial field apart from bone augmentation procedures are screws and plates for fixation of traumatic orbital defects, treatment of zygomatic fractures, fixation of mandibular fractures, and fixation in orthognathic and pediatric craniofacial surgeries.
CONCLUSION
Although magnesium has excellent biomechanical properties, it is biocompatible and a perfect material for temporary bone fixation. It is currently used only in orthopedic and cardiovascular medicine. Development of optimal degradation times together with a screw and plate design specific for the dental field would begin a new era in maxillofacial surgery.
