This article is being published in two parts. This is the 2nd of them.
3.3. Knee replacements:
Total Knee Replacement (TKR) is a surgical procedure where worn, diseased or damaged surfaces of the knee joint are removed and replaced with artificial surfaces. A round ended implant is used for the femur, mimicking the natural shape of the bone. On the tibia the component is flat, although it often has a stem which goes down inside the bone for further stability. A flattened or slightly dished high density polyethylene surface is then inserted onto the tibial component so that the weight is transferred metal to plastic not metal to metal.
UHMWPE has been used in knee replacements since the late 1960s this early knee replacement resurfaced the individual condyles of the femur and the tibia. Total knee arthroplasty (TKA), which replaces the articulation between the femur and tibia, as well as between the femur and the patella, was developed in the 1970s, primarily at surgical centers in North America.
Figure 9: (A) Anterior/posterior and (B) lateral radiographic views of an Insall/Burstein II total knee replacement, with associated anatomical landmarks and implant terminology.
Five fundamental adaptations of UHMWPE for knee replacement since the 1970s, These five evolutionary stages for UHMWPE in TKA include: (1) Gunston’s initial design concept for the Polycentric total knee replacement, which replaced both condyles of the femur individually; (2) the adaptation of Gunston’s design to unicondylar knee arthroplasty for carefully selected groups of patients; (3) the evolution to a bicondylar total knee, in which the tibial and femoral components were joined for ease of insertion and anatomical positioning; (4) the resurfacing of the patello-femoral joint; and (5) the incorporation of metal backing in the design of UHMWPE components. It should be emphasized that all of these major evolutionary steps in the clinical application of UHMWPE for TKA were initiated in the 1970s, even if the final embodiments of these design concepts did not reach fruition until the following decades. Even today, surgeons and biomechanical engineers continue to debate and refine their understanding of these fundamental adaptations of UHMWPE for knee arthroplasty.
3.4. Shoulder replacement
Shoulder replacement, although done much less frequently than hip and knee replacement, is the third most prevalent joint replacement procedure worldwide. Current shoulder replacement systems rely on ultrahigh molecular weight polyethylene (UHMWPE) components for motion and load bearing. Because of this critical role, the performance of UHMWPE components can determine the overall performance of the replacement system.
When considering the performance of UHMWPE in shoulder replacement components, one should have a basic understanding of the anatomical and biomechanical system into which they are placed, as well as the ways that system can be compromised by disease or trauma.
Figure 11: Diagram of conventional shoulder replacement and X-ray of a stemmed shoulder replacement
Figure 12: Radiographic images of frontal views of shoulders before and after total shoulder arthroplasty. The preoperative anatomy in (A) the placement of the humeral component in (B) can be clearly seen. The UHMWPE glenoid component in (B) is radiolucent and is evident only as a space between the humeral component and bone of the glenoid
3.5. Ankle replacement
Ankle replacement, or ankle arthroplasty, is a surgical procedure to replace the damaged articular surfaces of the human ankle joint with prosthetic components. Ankle system includes three functional components, and a set of customized accessory instruments that are used in the surgical procedure. The three principal components of the prosthesis are:
- A metal tibial component with titanium plasma spray coating;
- An ultra high molecular weight polyethylene mobile bearing; and
- A metal talar component with titanium plasma spray coating,
The mobile bearing articulates on the tibial and talar metal implant surfaces, as shown in the photograph of the three components of the Ankle system below:
Figure 13: Ankle is designed to replace a portion of the tibial and talar components of the normal ankle joint, while preserving range of motion as much as possible.
3.5.1 Mobile Bearing
The polyethylene mobile bearing is manufactured from Ultra High Molecular Weight Polyethylene (“UHMWPE”), which is machined from medical grade extruded plate stock and conforms to ASTM F-648. The proximal surface of the mobile bearing is flat. The distal or talar surface is concave and has a central radial groove running from anterior to posterior. The walls are straight, and a 0.5mm stainless steel x-ray marker wire is placed 2mm from the proximal surface. The wall height varies to provide spacer thicknesses of 6mm, 7mm, 8mm, 9mm, and 10mm. Revision mobile bearings are available in sizes of 11mm, 12mm, 13mm, and 14mm.
3.6. Elbow replacement
The common reason for doing an Elbow joint replacement is pain that affects the activities of daily living. This is followed by instability of the elbow and ankylosis (fusion) of the elbow. The prosthesis is metal with high molecular weight polyethylene bushing at the articulation. The components are cemented using antibiotic containing cement. Although total elbow arthroplasty has been successfully used for the treatment of relatively inactive patients with rheumatoid arthritis, implant survival has been limited when used for treatment of osteoarthritis or trauma. Like the knee, the axis of rotation of the elbow shifts in both position and orientation during flexion. This makes the use of a fixed-hinge device unacceptable for total elbow arthroplasty (TEA).
Figure 14: Elbow Replacement
3.7. Spinal disk replacement
Total disc replacement is indicated for spinal arthoplasty in skeletally mature patients with degenerative disc disease (DDD). DDD is defined as discogenic back pain with degeneration of the disc confirmed by patient history and radiographic studies. Spinal disc consists of two metal plates each made of cobalt chrome. Extending from the upper and lower part of each plate is a metallic fin. The two plates ride on a UHMWPE insert that is fixed to the lower plate. The endplates are sprayed with a titanium layer to promote bony in-growth, which helps the device stay firmly in place. UHMWPE has been used clinically in spinal arthoplasty devices since 1987 and has been determined to be safe and effective in total disc replacement through the studies of researchers.
Figure 15: spinal disc replacement, and flexion & radiograph series
A biomaterial is essentially a material that is used and adapted for a medical application. Here, over 50 years of experience has determined that, in the vast majority of circumstances, the sole requirement for biocompatibility in a medical device intended for sustained long-term contact with the tissues of the human body is that the material shall do no harm to those tissues, achieved through chemical and biological inertness. Rarely has an attempt to introduce biological activity into a biomaterial been clinically successful in these applications. A wide variety of studies on biocompatibility of UHMWPE showed that it is a potential material for biomedical applications. UHMWPE surface offers favorable condition for the growth of cells like fibroblasts, osteoblasts, and macrophages, without any inflammation and cytotoxicity.
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