Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Reexamination Certificate
2001-10-23
2004-02-03
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
C528S353000, C623S016110, C623S018110, C623S066100, C623S020230, C264S241000, C264S250000, C264S255000, C264S259000, C128S898000
Reexamination Certificate
active
06686437
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to medical implants made in part or in all of formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide, process for manufacturing the implants and method of implanting the implants in subjects in need. More particularly, the present invention relates to orthopedic implants made in part or in all of formable, pyromellitic, dianhydride (PMDA)-free, non-halogenated, aromatic polyimide, process for manufacturing the orthopedic implants and method of implanting the orthopedic implants in subjects in need.
BACKGROUND OF THE INVENTION
Synthetic bio-compatible materials have been used in a wide range of medical and dental applications. Since the earliest uses of gold strands as soft tissue sutures for hernia repairs (around 1000 B.C.), silver and gold artificial dental crowns, and gemstones as tooth replacements (inserted into bone and extending into the oral cavity), bio-compatible materials have evolved to standardized formulations.
Since the late 1930s, high-technology polymeric, ceramic, metallic and composite substrates have played a central role in expanding the application of biocompatible materials-made medical devices.
Plastics hold an important position in the field of medicine as structural materials implanted in the body and as surgical aids. Plastic materials are preferred over metals and ceramics due to their low specific weight, high mechanical strength (as evaluated on a strength-to-weight basis), toughness and chemical inertness and hence stability. Plastic materials are also readily shaped and machined and are commercially available in diverse forms and structures.
Structural plastics are used for permanent endo- or exo-prostheses. Other applications of plastic materials in medicine are blood tubing, heart valves, artificial cornea, artificial heart and kidney components, artificial joints and bone, encapsulants for implanted electrical devices such as pacemakers, flexible circuits, etc.
The advent of orthopedic implants is possibly one of the greatest advances of the past century in orthopedic surgery. The concept of introducing an artificial joint became possible only when new materials and fixation methods were developed and applied by successful collaboration of materials scientists, engineers and surgeons. A joint replacement should have the great advantage of providing pain-free and as smooth as possible movement for the patients, mimicking, as much as possible, the functionality and movements repertoire of the respective natural joint.
In the case of an endo-prosthesis, where two bearing surfaces are replaced by artificial prostheses (made of either similar or different materials), thus creating an artificial joint, high friction and wear problems may occur over a period of time, depending, to a great extent, on the patient's activity.
In the orthopedic joint endo-prosthesis
(1)
, three basic conditions are necessary for successful total arthroplasty: (i) bio-compatibility with the surrounding tissues; (ii) good adhesion and stable fixation of the endo-prosthesis to the bone; and (iii) negligible friction without formation of wear debris of the joint elements during service under dynamic load. Large frictional forces in the hinged joint and tendency of the implant material to spall and delaminate can cause loosening of the construction and creation of wear debris, which lead to acute or chronic inflammation.
A natural joint is a connection between two bones and is classified in two fundamental types
(16)
: (i) joints lacking a joint cavity, which allow little or no movement, for example the joint between adjacent vertebrae and the joint between the ribs and sternum; and (ii) joints having a joint cavity, which constitute the freely movable joints of the body, and are called the synovial joints. Depending on their position in the body, synovial joints have evolved to permit one or more of the following types of movements: flexion, extension, abduction, adduction, rotation and circumduction. In the human body, there are six kinds of synovial joints: ball and socket, hinge, pivot, ellipsoidal, saddle and gliding.
Continuous friction and accelerated wear of two contacted non-lubricated surfaces moving one with respect to the other are the result of the interaction of asperities or surface roughness. When two surfaces are rubbed together under load, asperities on the two surfaces may adhere, and relative sliding movement will then be possible only if the adhesive forces are overcomed by shear forces. Unless the shearing takes place exactly along the original interface, material will be removed, resulting in what is known as adhesive wear.
Cyclic variations under load, or cyclic movements of a bearing under constant load, impose dynamically varying stresses on the elements of the material. Such dynamically varying stresses may cause fatigue fractures at, or close to the surface, thereby promoting particle detachment. This process is known as fatigue wear.
Lubrication minimizes frictional resistance between bearing surfaces by keeping them apart. Fluid and boundary lubrication
(17)
are probably the most important mechanisms preventing cartilage wear under high loads in natural joints. It will be appreciated in this respect that, the coefficient of friction in a synovial joint is about 0.001-0.01, while typical dry coefficient of friction of plastic on plastic is about 0.1-0.3 and for metal on metal about 0.3-0.8, rendering these materials poor substitutes.
Several families of materials such as polymers, metals, ceramics and composite materials were tested as potential implant materials for hip and knee arthroplasty. Due to the harsh environment of the human body fluids and the frequent movement of these parts, the useful lifetime of these implants is about less than 10 years. Thus, a replacement implant is needed which involves repeated (once or more) surgery (depending on the age and activity of the patient). Fractures and wear of the implant are clearly observed. The wear debris produced by friction from the damaged implants circulate in the body or stay in the tissues and cause inflammation. In order to solve this problem a low friction, wear-resistant material is needed.
The majority of total hip prostheses currently implanted consists of a hard metal, or ceramic femoral head placed against a cross-linked or uncross-linked ultra-high-molecular weight-polyethylene (UHMWPE) acetabular cup with or without cement fixation. Currently, more than 500,000 artificial hips are implanted annually, worldwide. The low-friction, low-wear un-crosslinked UHMWPE has been considered for the last two decades as the best polymeric solution for total artificial hip implants. Notwithstanding the success, over the last 10 years these prostheses exhibited frequent failures, due to late aseptic loosening, creep, migration and inflammation, resulting in the need to be revised through surgery. No implant survived more than 25 years, while most of the implants lasted less than 10 years, typically 5 to 7 years.
The following materials have been used to form artificial medical implants over the years:
Teflon®
(2)
is a material known to cold flow, namely, irreversibly deform over time under loading. It failed due to mechanical wear (80%), infection (10%) and metal femoral head penetration, absorption of great masses of the surrounding bone and implanted material.
Silicone
(14)
prostheses are used in finger and toe joints. Fatigue fractures occurrence and the low durability of this material results in weak bonding which leads to particles release.
UHMWPE
(3-5)
promotes bone lysis (Periprosthetic Osteolysis) due to release of submicron size debris into the surrounding soft tissues as a result of wear. The body reacts by “activating” macrophages that attack the particles and release biochemical mediators in the bone implant interface, causing inflammation and loosening. The UHMWPE implant degrades in the body, chips off, exposes the base metal and becomes released from the cemented fixation. There are 20-30
Bryant Robert G.
Buchman Alisa
Mendes David G.
Payne Raymond G.
Sibony Simha
G.E. Ehrlich (1995) Ltd.
Hampton-Hightower P.
M.M.A. Tech Ltd.
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