Soon after a deadline has passed, I end up treating myself to a movie, and this time, it was Hot Shots Part Deux.
The only thing that made an impact on a brain fresh (or would that be
exhausted?) with materials design was the various implants in the body
of President Tug Benson. The president had ceramic eyes, asbestos skin,
magnetic skull plate, aluminum siding facial bones, and stainless steel
ear canals.
Artificial implants have been known for a long time [1]. My
grandmother tells me that my ancestors had golden tooth implants, to
showcase their wealth. I am told that after a certain age, they would
pull out their perfectly normal teeth and replace them with gold and
silver tooth (depending upon their financial position) and swore off manangombu and mysore paks for the rest of their lives. If I were given the choice between golden teeth and murukku , I would most definitely choose the latter. But I digress.
The first recorded case of implant was an accident. In 1952, Per
Ingvar Branemark, a Swedish orthopedic surgeon was following the
processes of bone healing by embedding microscope heads made of
titanium metal in holes drilled into thighbone of anesthetized rabbits.
A few months later, he could not retrieve the microscope heads as the
titanium metal had integrated into the bone. He called this phenomenon
"osseointegration" and so started the long journey with artificial hips
[2]. There are many types of orthopedic implants that have been used over the years [3]. Metal and Plastic Implants are
most commonly used (and most often turn on the alarm at the airport
security terminal, along with belt buckles) where both the ball and
socket are replaced with metal parts with a polyethylene spacer in
between. The common metals used for this are titanium alloys, stainless
steel, and cobalt chrome. Surgical stainless steel is an alloy of iron,
carbon, chromium (12–20%), molybdenum (0.2–3%), and nickel (8–12%).
Medical grade titanium alloys have a significantly higher strength to
weight ratio than competing stainless steels. The lower modulus of
titanium alloys compared to steel and resistance to crack propagation,
or fracture toughness makes it better than stainless steel, despite the
cost of SS being more favorable. Thus steel is usually used for
temporary implants and the more expensive titanium for permanent ones.
It has been reported that more than 2.2 million pounds of titanium
devices of every description and function are implanted in patients
worldwide every year [4].
Surface engineering plays in important role in improving the
performance of the alloy. For example conventional titanium alloys used
in hip and knee replacements are smooth in surface structure and the
defense mechanism of the body tries to cover it up with fibrous tissue
to enable removing this foreign body. Recently, Thomas Webster has
shown that by covering the implant materials with nanometer-scale bumps
(I can't resist bringing the nano into nono, can I?), not only prevents
the body from rejecting the implant, but actually stimulates the body
to re-grow bone and other types of tissue [5].
Shape memory alloys such as nitinol are being increasingly used for
prosthetic applications. However, nickel, one of the main constituents
of this alloy is known to cause allergies or poisoning in living
tissues and studies are being made to mitigate leaching of nickel by
surface modification [6]. Laser surface treatment [7], strain hardening
[8] and chemical passivation methods [9] have been investigated.
Ceramic-on-Ceramic Implant are made of alumina and are
resistant to wear and are more scratch resistant and smoother than any
of these other implant materials. The ceramic-on-ceramic implant is
reported to produce less than 1 micron of debris each year, while a
metal-on-plastic or metal-on-metal implant can create 20 to 200 microns
of debris each year [10]. Plastic implants made of high
density polyethylene are being used more recently, but there has been
no historic data as yet to assess their performance.
Artificial bones are usually made of hydroxyapatite ,
which has the same chemical formula as bone itself. Synthetic
hydroxyapatite, however, is neither as porous as real bone nor as
strong [11]. Exotic materials such as sea coral have been studied for
bone substitution, but they have not been found better than existing
material.
While gold and silver tooth have (thankfully) fallen out of fashion,
dental implants continue to torment medical insurers! Dental fillings
don't strictly come under implants, but can be considered here.
Amalgams of silver, tin, zinc and copper were used extensively for
filling teeth, but are fast losing ground due to the presence of the
neurotoxin-mercury. Galloy, an alloy or silver, tin, copper, indium and
gallium is being considered as a safer substitute for amalgam in tooth
fillings. Dentures are made of acrylic, stainless steel, or
chromium-cobalt, but can also be made nylon, a gold alloy, and
titanium. Many acrylic dentures may contain toxic cadmium, and there
has been growing awareness of the toxicity associated with cadmium
leach. Most dental "true implants" are made of titanium alloys.
Eye
implants commonly include intraocular lens made of
polymethylmethacrylate (for "hard" lens), silicone and more recently
acrylic ("soft" foldable lens). A pea-sized miniature telescope [see
picture] inserted into the eye is showing promise in treating macular
degeneration [12]. Polyvinyl alcohol hydrogels have been used as
artificial cartilages.
Stents are tubes that have been used to treat diseases that involve obstruction of fluid flow in the body. Vascular
stents open up clogged arteries (probably resulting from not having a
gold tooth and therefore stocking up on the mm..s as described
earlier), urinary stents to open up clogged urethras, and stent grafts
to strengthen weak spots in blood vessels, oesophageal stents, rectal
stents, pancreatic stents and stents for other conduits in the body.
Various materials have been used for the construction of stents -
metals, alloys, synthetic polymers and natural materials. Biological
stent graft integrates a non-biological stent made of biocompatible
metals and/or polymers, with the tissue of a vessel (the graft) by the
natural ingrowth that occurs when the stent is implanted in a vessel,
such as an arterial vessel, of an animal for a period of time
sufficient to permit ingrowth. Stents typically have been made using
balloon-expandable stainless steel or cobalt-chromium alloy and more
recently shape memory alloys such as nitinol [13, 14]. For aneurysm
repair applications, the stents have been wrapped with a graft of
either PTFE or collagen and more recently Dacron (polyester). Nylon,
polyester, teflon, polypropylene, polyacrylonitrile, and silicone
rubber have also been studied for use as graft material. Polymer stents
have also been studied for specific applications. Percuflex® has been
used to construct urethral stents.
Cranioplasty, the oldest known neurosurgical procedure practiced by the
Paracas Indians in Peru around 3000 B.C. has, over the years, involved
the use of coconut shells, bones from both human and non-human donors,
metals including gold, silver, tantalum, and titanium and more
recently, biosynthetic materials such as resins and ceramics. Acrylic
cranioplasty is frequently used for patients who have a cranial defect
after trauma or an infected craniotomy or meningiomas [15]. Of the
several materials used in cranioplasty, methyl methacrylate and
titanium have remained today as viable alternatives for cranial
implants. Titanium alloy and carbon fiber reinforced polymer are being
used for spinal implants.
Implants have been used to deliver drugs to diseased parts of the body. Saltzman has
reported on the use of implants to deliver nerve growth factor (NGF) to
nerve cells in the brainto prevent or control Alzheimer's disease.
Walter et al. have incorporated taxol into polyanhydride matix to be
implanted intracranially to deliver the drug across the blood-brain
barrier [16].
In some cases implants contain electronics. The pacemaker was the
first microelectronic medical implant that was introduced over 45 years
ago. It consist of a pager-sized housing device which contains a
battery (usually lithium-iodide or lithium anode cells or in rare cases
even plutonium-238.) and the electronic circuitry that runs the
pacemaker, and one or two long thin wires that travel through a vein in
the chest to the heart. Cochlear implant is a surgically implanted
electronic device that provides a sense of sound to a person who is
profoundly deaf or severely hard of hearing [17]. Neurostimulators are
rapidly gaining importance for pain relief, and other neurological
conditions ranging from the treatment of urge incontinence to
preventing epileptic seizures.
Can any writeup on medical implants be complete without mention of
cosmetic implants? There are facial implants to correct or enhance
facial contours, breast implants for reconstructive or cosmetic
modifications, nasal and other oral/maxillofacial implants to correct
defects and deformities. Gore-Tex® ,
an expanded form of of polytetrafluoroethylene is rapidly becoming a
popular implant material in cosmetic surgery. Radiesse, composed of a
suspension of hydroxyapatite in carboxymethylcellulose, is a
long-lasting implant material that has been approved by the federal
Food and Drug Administration (FDA) for the treatment of oral and
maxillofacial defects [18].
Hyaluronic acids and collagen are used to treat nasal defects.
Silicone is used for more permanent treatment, but is associated with
risk of granuloma formation or a persistent foreign-body reaction [19].
Breast implants comprise a silicone shell that is either filled with
saline or silicone gel. There have been several alternative types of breast implants developed, such as polypropylene string or soy oil, but these are uncommon.
President Bensen may not be a figment of fertile imagination after all.
1. E. M. Hetrick, M.H. Schoenfisch, Chem Soc Rev. 2006 Sep;35(9):780-9.
2. Implants
3. Hip Replacement Implant Options
4. Ttitanium Aalloys in Medical Applications
5. Nanometer Sized Bumps Improve Prosthetic Implant Performance - New Technology
6. K.W.K. Yeung, R.W.Y. Poon, X.M. Liu, Paul K. Chu, C.Y. Chung, X.Y.
Liu, S. Chan, W.W. Lu, D. Chan, K.D.K. Luk and K.M.C. Cheung, Surface
and Coatings Technology, Proceedings of the Fifth Asian-European
International Conference on Plasma Surface Engineering - AEPSE 2005,
Proceedings of the Fifth Asian-European International Conference on
Plasma Surface Engineering, (2007), 201, 9-11, , 5607-5612.
7. F. Villermaux, M. Tabrizian, L’.H. Yahia, M. Meunier and D.L. Piron,
Excimer laser treatment of NiTi shape memory alloy biomaterials. Appl
Surf Sci 109/110 (1997), pp. 62–66.
8. C. Montero-Ocampo, H. Lopez and A. Salinas Rodriguez, Effect of
compressive straining on corrosion resistance of a shape memory Ni–Ti
alloy in ringer's solution. J Biomed Mater Res 32 (1996), pp. 583–591.
9. C. Trépanier, M. Tabrizian, L’.H. Yahia, L. Bilodeau and D.L. Piron,
Effect of modification of oxide layer on NiTi stent corrosion
resistance. J Biomed Mater Res (Appl Biomater) 43 (1998), pp. 433–440.
10. A hip alternative
11. NASA - Better Bone Implants
12. Miniature 'bionic' eye implant rescues vision
13. Z. J. Haskal, A. Davis, A. McAllister A, E. F. Furth, Radiology, 205, (1997) 682-688
14. D. F. Kallmes, H-B. Lin, N-H. Fujiwara, J-G. Short, K-D. Hagspiel,
S-T. Li, A-H. Matsumoto, J.Vasc. Interven. Radiology 12,(2001) 1127-1133
15. D. J. Prolo, S. A. Oklund. Clin Orthop Relat Res.268 (1991) 270-8
16. Walter, Kevin A., Cahan, Mitchell A., Gur, Aya, Tyler, Betty,
Hilton, John, Colvin, O. Michael, Burger, Peter C., Domb, Abraham,
Brem, HenryCancer Res 1994 54: 2207-2212
17. Cochlear implant
18. G. Lemperle, V. Morhenn, U. Charrier U. Aesthetic Plast Surg 27 (2003) 354-66.
19. A. K. Deva, S. Merten, L. Chang. Plast Reconstr Surg102 (1998)1230-7.
Comments @ nOnoscience:
December 10th, 2007 at 4:17 am e
What a wonderful post! I am impressed. However, there are spelling errors (cochlear, eg) that need to be edited.
You missed out on hernia implants, which are far commoner in practice. For hernias that are treted by the laparoscopic (keyhole) approach, it is necessary the the mesh material be biocompatible (of course), inert (prevent the intestines from getting stuck to it) ad flexible. GoreTex, as you mentioned is a proven material that is used, but has the problem that it is expensive and cannot resist infections at all. Any infection results in explantation: a disaster. Modern meshes are Composite meshes, with a background of polypropylene covered by a single layer of non-recative material that prevents the gut from adhesions: a polyglycolic acid in one brand, and a hydrocolloid in another. These meshes are also expensive.
Often, a woman or man who needs a lap incisional hernia repair with these meshes (called tissue separating meshes) ends up spending as much as for a bypass surgery! But worth it, IMO!
December 10th, 2007 at 4:18 am e
Spelling errors are also found in comments, especially mine! Would appreciate correction!
December 10th, 2007 at 7:16 am e
Rambodoc, thank you for your very informative comment. I should not have forgotten hernial implants considering half the men in my family over fifty have them !
I shall correct the spelling mistakes as soon as I can even spot them
December 10th, 2007 at 1:55 pm e
I second Rambodoc. Impressive indeed!!
I learnt a lot from this article - but then I’m just a radiologist who is happy sitting in a dark room looking at black & white images.
That apart, I think there is a lot of information in this post of yours that most doctors have no idea about.
Lakshmi, I wonder if the chance to study at IITM lured you away from what looks like it could have been a very promising career in medicine!
PS-1: One other spelling mistake that seems to have escaped Rambodoc’s eagle eye: It’s aneurysm not -ism.
PS-2: Rambodoc, after the last line in your first comment, I guess I shouldn’t ask which side of the Open Vs Lap Hernia surgery debate you are on
December 10th, 2007 at 6:53 pm e
I would like to direct you to a wonderful post on medical implants,Spare Parts. It is written by Lakshmi Gopal over at Nonoscience. She starts off: “Soon after a deadline has passed, I end up treating myself to a movie, and this time, it was Hot Shots Part Deux. The only thing
December 10th, 2007 at 10:53 pm e
Vijay, I have no doubt that we are still far away from the best treatment for hernias, but as of date, lap hernias are far better than open surgery. Less trauma, no infections, no restriction in movement after operation, discharge from hospital 24 hours of surgery, etc. You only spend more money, but what’s money? Just give it to me, it won’t bother you no more!
December 10th, 2007 at 10:57 pm e
Lakshmi:
“half the men in my family over fifty have them !”
Wait, wait, wait! When can I come over and operate on them? I need to invest in this new Mutual Fund that gives great returns!
December 11th, 2007 at 12:09 am e
Lakshmi: A good overview of implant materials. Mimicking biomaterials is such a challenge! I have a comment to add on your, “Artificial bones are usually made of hydroxyapatite, which has the same chemical formula as bone itself.”
The bone mineral is called hydroxyapatite (HAP) in the bone biology field. However, looking at the chemistry of the bone mineral, carbonated apatite better describes it. HAP is usually uncarbonated (pure) mineral. Bone mineral is similar to HAP, but not identical to it.
So, I think that bone implant materials should take into account the high substitution and poor crystallinity of bone mineral. Also, the protein matrix in the bone (collagen and others) play an important role in mineralization and affect its mechanical properties too!
Ref: N. J. Crane, V. Popescu, M. D. Morris, P. Steenhuis and M. A. Ignelzi, Jr., Bone, Volume 39, Issue 3, September 2006, Pages 434-442.
December 11th, 2007 at 7:22 am e
Vijay, thanks ! If I had become a doctor, I am sure the population in India would not be what it is today !!!!
Rlbates, thank you for the trackback.
Rambodoc, we already funded another local doctor’s retirement plan. If we run excess moolah and have some groins to spare, rest assured, you will be the first person we would think of !
Mekhala, thank you for the informative add-on and the reference. I did not write in detail about bone substitutes because the post would have gotten too long to sustain attention.
December 22nd, 2007 at 4:38 pm e
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