Dr Hrishikesh Mahapatra, Dr Sumit Bedia
Post Graduate student,
Department of Prosthodontics and Crown & Bridge,
Bharati Vidyapeeth Deemed to be University Dental College and Hospital, Navi Mumbai,
In Prosthodontics, magnets have created great interest, and their applications are various, primarily as retentive aids. Magnets are common because their small size and their attractive or repulsive forces allow them to be positioned within the prosthesis without becoming obtrusive in the mouth. Conventionally magnets have been used as retentive devices for removable partial dentures, obturators, and maxillofacial prosthesis. The retentive properties and the rare earth magnets' compactness have contributed to their widespread use in the last few decades. This article reviews the types of magnetic devices available, their applications in various fields of Prosthodontics, advantages, and disadvantages followed by newer magnetic systems.
Keywords : Metals, Rare Earth, Overlay, Implant retained overdentures, Magnetic Phenomena, Maxillofacial prosthesis
Citations : Mahapatra H, Bedia S. Magnets in prosthodontics - An Overview. J Prosthodont Dent Mater 2020;1(1& 2): 42-50.
For several decades now, magnetic phenomena have been known, and their applications in diverse fields have been various. In dentistry, magnetism is not new, though it has not been closely investigated. Its most prevalent use has been in the field of prosthetic dentistry, where both its repulsive and attractive properties have been used.
Magnets that have been initially in use were bulky, and questions about their potential toxic effects were raised. The currently available literature, however, does not display such evidence. The cause for magnets' popularity is their small size and strong, attractive forces, which facilitates them to be positioned in the prosthesis without being obtrusive in the mouth. Despite many benefits, including ease of washing, positioning for both dentist and patient, automatic reseating, and continuous retention, magnets have a low resistance to corrosion within oral fluids and thus require encapsulation within a relatively inert alloy such as stainless steel or Titanium. Action with saliva quickly results in corrosion and loss of magnetism when such casings are breached.
The very first documentation of magnets was around 2500-3000 years BC. It was labelled as Load Stone by the ancients. Their origin is first noted in a large area in Asia Minor called Magnesia, where the locals called it 'Magnetite'. In 1953, Freedman initially used a magnet to improve retention and seating of complete denture against the alveolar ridges by using mutual repulsion at the time of closing the jaw. In 1956, Nadeau used magnets in combined extraoral and intraoral prosthesis. In 1960, Behrman used the technique of incorporating magnets in the jaw to increase the prosthesis's retention. In 1976, Federick used magnets in a sectional denture. Magnets were also used in the maxillofacial prosthesis in eyelid and lip closure at the fabrication of obturator.
A. Based on Alloys used
- Those comprising cobalt Examples are Alnico, Alnico V, Cobalt-Platinum (Co-Pt), Cobalt-Samarium (Co5Sm)
- Those not containing cobalt Examples are Neodymium-Iron-Boron (Nd-Fe-B), Samarium Iron Nitride.
B. Based on the type of magnetism
C. Based on Capability to retain magnetic properties (intrinsic coercivity or hardness)
- Soft (easy to magnetize or demagnetize but less permanent): Palladium-Cobalt-Nickel (Pd-Co-Ni) alloy, Palladium-Cobalt (Pd-Co) alloy, Palladium-Cobalt-Chromium (Pd-Co-Cr) alloy, Palladium-Cobalt-Platinum (Pd-Co-Pt) alloy, Magnetic stainless steels, Permendur (an alloy of Fe-Co), Chromium-Molybdenum (Cr-Mo) alloy.
- Hard (retain magnetism permanently): Alnico alloys, Co-Pt, Co5Sm, Nd-Fe-B.
D. By the number of magnets in the system
E. Based on Type of surface coating (materials may be stainless steel, Titanium, or palladium)
F. Based on the type of magnetic field
- Open field
- Closed field — Rectangular closed-field sandwich design, — Circular closed-field sandwich design
G. Based on the number of magnets in the system:
- Duo-system open field
- Mono-system open field
- Mono-system closed field
H. Based on the arrangement of the poles
- Reversed poles
- Non reversed poles
COMMERCIALLY AVAILABLE MAGNETIC SYSTEMS
Table 1: Few commercially available magnetic systems for use in Prosthodontics
Types of Magnetic Materials Used in Prosthodontics:
1) Rare earth permanent magnets:
a) Samarium-Cobalt Magnets (Sm-Co)
b) Neodymium-Iron-Boron Magnets (Nd-Fe-B)
- Rare elements consist of seventeen elements with atomic numbers 21, 39, and from 57 to 71.
- These elements have a strong affinity for the non-metallic elements, due to which they are used for producing alloys, which are used in metallurgical industries.
- Made from alloys of rare earth elements, rare earth magnets are strong permanent magnets producing higher magnetic fields.
- Due to magneto crystalline anisotropy, rare earth magnets can generate high forces relative to their size. This property enables the preferential alignment of single crystals in one direction (along the C-axis), which increases the magnetism.
- Another beneficial aspect of rare earth magnet, relative to Alnico magnets, is their very strong coercivity. Strong coercivity means a superior capacity for these magnets to resist demagnetization.
- Sm-Co can be embedded in heat polymerizing resin when fabricating dentures as its curing temperature is above 7000C, and their magnetic properties are stable below 2000C.
- Nd-Fe-B has a curing temperature, which is low about 3000C and magnetic properties deteriorate above 800C. Therefore, care should be taken not to overheat the magnet.
2) Soft Ferromagnetic alloys3:
a) Palladium Cobalt Magnets (Pd-Co)
b) Palladium Cobalt-Chromium Magnets (Pd-Co-Cr)
c) Palladium Cobalt Nickel Magnets (Pd-Co-Ni)
- Materials of rare earth magnets are too hard to be shaped into a form suitable for each patient.
- To overcome this problem, soft magnets were introduced in 1984 for use in the root face.
Components of The Magnetic System:
A two-component device is the standard magnetic retention unit: the magnetic retention element and the keeper element. The magnetic retention element consists of paired magnets, an attached keeper, and two protective plates covering the paired magnets' faces. The oval shape of the magnetic retention element is 5 mm long, 3.2 mm wide, and 3 mm high. The keeper element is a detachable, oval-shaped, magnetizable preferred disk or a cast root cap. The keeper element is prepared with a magnetizable alloy, which is not a magnet. Still, it acts as a magnet (induced magnet) when it comes in contact with the element's magnetic retention. The alloy used is Pd-Co-Ni alloy or stainless steel alloy.
Selection of keeper
- Cement-in keeper: It should be used where available denture space is limited. It is not suitable for small roots because of the danger of lateral root perforation or for patients with high caries susceptibility. It is 5 mm long, 3 mm wide, and 1.2 mm thick, and the root face should be large enough to accommodate a cavity of this size. It is fitted in one appointment.
- Screw-on keeper: It can be used where ample denture space is available or where the root face is too small for a cement-in keeper to accommodate. As it can be easily removed and replaced, it is used where the root may require shortening. Its most extensive measurements are 6 mm long and 4 mm wide and typically covers much of the root face. It is fitted in a single appointment.
- Cement-on keeper: Other than its retention by a soldered-in wire loop, it is comparable to the screw-on holder. It's hard to remove, but the easiest one to match with all keepers. It is possible to finish the procedure in minutes. It is the most favored type.
- Cast root cap and dowel keeper: Due to the risk of dental caries, it should be used where complete root face coverage is needed. It must be cast in a magnetizable alloy, and techniques are required for cobalt-chromium casting.
BIOCOMPATIBILITY AND THEIR EFFECTS ON TISSUES
The magnetic intensity of the dental attachment on the magnetic assembly surface is about 0.8 Tesla. The leakage flux in contact with the keeper on the magnet assembly is around 0.01-0.03 Tesla and about 0.005 Tesla outside the abutment tooth. Thus, there is no influence of the static magnetic field generated by a dental magnetic attachment on the human body and tissues. However, the potential ways in which a magnet might result in tissue injury are:
- The physical effects triggered by the ambient steady magnetic fields (magnetism)
- Chemical effects of alloys and their corrosion materials
In 1960, Behran studied the physical effects of magnetism on both bone and soft tissues in 450 subjects and concluded that tissues were harmless. It is shown that, as compared to an open field system, the closed-field system has improved tissue compatibility.
Late back in 1979, Tsutsui and his colleagues stated that Cobalt-Samarium does not have harmful chemical effects. Cobalt has also been an essential dietary trace element in remnants. Samarium salts are not considered toxic.
Nevertheless, Walmsley suggests that there is a need to encapsulate the magnets. He also found that the magnet will contact saliva as the coating wears out, resulting in magnetic corrosion. It was also shown that the presence of bacteria such as Streptococcus sanguinus increases the corrosion rate. The lifetime of the magnet will thus be shortened. Coated magnets have also been shown to have little effect on human dental pulp, gingiva, osteoblasts, or blood flow. Only uncoated magnets have shown that cytotoxic effects on cells are generated. Oral mucosal fibroblasts are among the most susceptible to the impact of these rare earth magnets. It is concluded that the magnetic potential produced by intraoral magnets in the surrounding blood vessels is very negligible (2*10-5V) compared to the resting membrane potential of cell membranes (60-100V).
CLINICAL APPLICATIONS OF MAGNETS
1) Magnets in Complete dentures based on Magnetic Repulsion and Attraction
- Magnets have been used because they are small and are easy to incorporate into a denture.
- The first attempt to use magnets to hold dentures involved implanting them into the mouth; because of their large size and insufficient powers, issues ensued. Improvements in magnetic materials have allowed magnetic attachments to be smaller and stronger.
- The first recorded use of magnets in prosthetic dentistry involved using the repulsion of like poles of magnets to maintain and improve the seating of complete dentures. The magnetic material used was Alnico type that has been discontinued because of the large bulk necessary for magnetic strength.
- The use of attractive force retention was reported in the early 1960s.13 This first attempt was made with Al-Ni-Co V, which was surgically implanted in the mandible of edentulous patients. Because of the distance between the two magnets, they provided the inadequate force to aid in retention.
- After that, the smaller and stronger Co-Pt magnets were implanted. Several disadvantages, including high cost, limited availability, difficult fabrication was associated.
2) Tooth Supported Overdenture retained by magnetic attachments
- The first reported use of magnets for the retention of overdentures took place in the 1960s with a patient's rehabilitation with a cleft lip and palate.14 Magnetic Co-Pt alloy was used to produce crowns for three remaining teeth with cast Co-Pt also built into the denture.
- This was soon followed by cementing magnets within the retained roots for the retention of overdentures.15 A Sm-Co was cemented into a prepared cavity in the root surface, and a similar magnet was placed in the denture.
- Magnetically retained overdentures are virtually maintenance-free and inexpensive to fabricate, and the technique lies within every dentist's scope.
- The primary purpose is to preserve the alveolar bone by retaining teeth or/and roots underneath. Magnetically retained overdentures transfer no detrimental lateral forces to those supporting elements that help in maintaining a favorable clinical situation.
- An overdenture with a magnetic attachment is a useful choice for an abutment tooth with chronic periodontal disease because the magnetic attachment dissipates the lateral stress component on the abutment teeth and improves poor clinical crown-to-root ratios.
3) Implant-supported overdenture with Magnetic attachments
- The various problems reported by the complete denture wearers can be eliminated with implant-supported fixed prostheses or removable overdentures.
- Magnetic attachments used to retain dentures are typically shorter than mechanical attachments, which is particularly useful for patients with restricted interocclusal space and challenging aesthetic demands. They can also accommodate a moderate divergence of alignment between 2 or more abutments since they do not depend on the path of insertion.
- Magnets in Implant retained overdentures are particularly useful for elderly or disabled patients who may have difficulty in inserting and removing removable prosthesis.
- Various types of attachments have been developed for the use of implant overdenture, including O-ring, Locator, magnetic attachment, and bar type. The choice of the higher abutment and self-adjusting type magnet provides significant higher stability and retention of implant overdenture. Patients' satisfaction is expected to be greater by increased retention and stability of implant overdenture.
4) Magnets in Maxillofacial Prosthesis
- Magnets have been effectively used for the retention, maintenance, and stabilization of combined maxillofacial prostheses, and they are effective for this purpose.
- Magnets in coin form have more advantages in maxillofacial prosthetics than the other forms; the magnet can be chosen according to the size of the defect and in any diameter that is needed.
- Cheek plumper, a single unit prosthesis that adds on to the weight of the denture, increases the mesiodistal width of the prosthesis, thereby making its insertion difficult in patients with limited mouth opening and can also cause muscle fatigue. Thus, to combat this situation, a detachable cheek plumper prosthesis using magnets was fabricated in a completely edentulous patient with sunken cheeks.
- Robinson used horseshoe magnets for the retention of an upper denture and obturator for a patient with a complete maxillectomy. A sectional prosthesis can be used for patients with large maxillary defects. Two magnetic pairs are commonly used for connecting the sections. The magnets are embedded in the respective contacting surfaces at a depth of 0.5 mm. Because Sm-Co magnets are small, the obturator can be made hollow to reduce its weight. The size of ferrite or alnico magnets often prevents the use of a hollow obturator.
- Magnets are also used in an orbital prosthesis, auricular prosthesis, large and small maxillary defects, and intra oral-extra oral combination prosthesis.
Failure of Dental Magnets
Corrosion is the key issue connected with the use of magnets as retentive devices. SmCo5 and Nd-Fe-B magnets are both extremely susceptible to corrosion, particularly in environments containing chloride. Therefore, before being used in dental applications, magnetic materials must be separated securely from oral fluids. While some existing magnet assemblies are encapsulated in stainless steel or Titanium, due to corrosion and lack of retention provided by the attachment, some devices fail in about 18 months of clinical use.
Advantages and disadvantages of magnets: (Table 2)
Table 2: Advantages and Disadvantages of using magnets in Prosthodontic applications
Newer Magnetic Systems:
MAGFIT - New Generation Magnetic Attachment: This is a revolutionary dental magnetic attachment device made up of a strong but ultra-compact embedded magnet that maintains a prosthesis on an attractive magnet keeper set on the abutment tooth. It has the following advantages-
- Protection of the abutment tooth from excess stress
- Easy instrumentation and easy maintenance
- Superior aesthetics
- Strongest Retention in its class with an ultra-compact size
- No Corrosion
- Recommended applications - 1. Natural tooth, 2. Implant
Dentistry is a discipline that is ever-changing. Changes in treatment are needed as new research and clinical practice expand our understanding. Some decades ago, magnets were only rarely used for dental purposes. Intraoral magnets have been influencing the course of aesthetics and retention for both complete and removable partial dentures since the advent of rare earth magnet alloys.
Their advantages include convenience, low cost, self-adjustment, inherent stress breaking, comparative lateral freedom of movement, and the minimum trauma to the retained root while eliminating the need for adjustment in services.
Magnetic attachments will offer new possibilities in the field of removable prosthesis retention. Over the last five years, magnet technology and engineering have improved significantly, and it is now possible to manufacture much smaller magnets that give the keeper a better position.
These enhancements need long-term clinical studies to evaluate the durability of the upcoming generation of clinically working magnets. There is no doubt, however, that if magnets are chosen for a specific clinical situation, they act as a good introduction and can prove to be extremely effective in many cases.
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