Joint Hip

Hydroxyapatite (HA) coating on the surface of the hip stem and the acetabular cup is the most recent advancement in artificial hip joint implant technology. This substance is a form of calcium phosphate, which is sprayed onto the hip implant. It is a material found in combination with calcium carbonate in bone tissue, and bones can easily adapt to it. When bone tissue does grow into HA, the tissue then fixes the hip joint implant permanently in position. These HA coatings are only used in press-fit, noncemented implants. 

Over time a large variety of materials have been used, including ivory, stainless steel, chromium—cobalt, and ceramics for the acetabular component. None proved sufficient. The implant material composition must provide a smooth surface for joint articulation, withstand hip joint stresses from normal loads, and the substance must disperse stress evenly to the cement and surrounding bone. 

A significant aspect of hip joint biomechanics is that the stmctural components are not normally subjected to constant loads. Rather, this joint is subject to unique compressive, torsion, tensile, and shear stress, sometimes simultaneously. Maximum loading occurs when the heel strikes down and the toe pushes off in walking. When an implant is in place its abiUty to withstand this repetitive loading is called its fatigue strength. If an implant is placed properly, its load is shared in an anatomically correct fashion with the bone. 

Resorption of bone tissue occurs in total hip joint replacement patients if sufficient stresses are not adequately transmitted to the remaining bone in exactiy the same way that the bone transmitted those stresses originally. Therefore, the design and proper placement of the neck coUar and hip stem must be effective in recreating anatomical stmcture. 

As a last example we turn to the world of medicine. Osteo-arthritis is an illness that affects many people as they get older. The disease affects the joints between different bones in the body and makes it hard - and painful - to move them. The problem is caused by small lumps of bone which grow on the rubbing surfaces of the joints and which prevent them sliding properly. The problem can only be cured by removing the bad joints and putting artificial joints in their place. The first recorded hip-joint replacement was done as far back as 1897 - when it must have been a pretty hazardous business - but the operation is now a routine piece of orthopaedic surgery. In fact 30,000 hip joints are replaced in the UK every year world-wide the number must approach half a million. 

Figure 1.6 shows the implant for a replacement hip joint. In the operation, the head of the femur is cut off and the soft marrow is taken out to make a hole down the centre of the bone. Into the hole is glued a long metal shank which carries the artificial head. 

Fig. 1.6. The titanium alloy implant for a replacement hip joint. The long shank is glued into the top of the femur. The spherical head engages in o high-density polythene socket which is glued into the pelvic socket.

In the general area of medicine uses range from spare-part surgery, such as hip joints and heart valves, through catheters, injection syringes and other sterilisable equipment, to more mundane but nevertheless desirable uses such as quietrunning curtain rails. 

One of the main problems of corrosion testing in-vivo and the interpretation of the mechanisms of corrosion which have taken place in-vivo after the implant has been removed from a patient is well illustrated in Figure 2.35 used by Semlitsch and Willet to illustrate the types of corrosion which are associated with a total hip joint replacement. This figure shows that many corrosion mechanisms could be taking place simultaneously in a system of this nature. 

Charnley, J., Total Prosthetic Replacement of the Hip Joint Using a Socket of High Density Polyethylene , Wrightington Hospital Publication No. 1 Centre for Hip Surgery, (1966)... 

Semlitsch, M., and Willert, H. G., Properties of Implant alloys for Artificial Hip Joints , Medical and Biological Engineering and Computing, 18, 511-520 (1980)... 

In a typical hip replacement operation, the top of the thigh bone is removed and a cavity is drilled along the direction of the long axis of the remaining bone. A metal prosthesis is placed in this cavity and secured in place with PMMA cement. In the pelvic girdle a plastic cup is fitted to act as the seat of the new, smaller hip joint. This cup is made of ultra-high molar mass poly (ethylene) and is also secured in place with PMMA cement. The components of an artificial hip joint are shown in Figure 10.1. 

Once polymerisation is complete, the components of the new hip joint can be connected together and the operation completed. This surgical procedure has been very successful over the past 30 or so years and now an estimated 45 000 such operations are carried out each year in the UK alone. Similar procedures are used for the replacement of both arthritic knees and arthritic fingers, though these latter operations are less common than hip replacements. Overall considerable amounts of PMMA are used each year as bone cements for these surgical procedures. 

Titanium implants are widely used owing to its good mechanical properties and excellent compatibility. A hip joint ball is shown. 

As well as the obvious risks, there can be less obvious risks too. Heart pacemakers can be disrupted by strong magnetic fields so this needs to be pointed out to anyone who enters the area in case they are reliant on one. Another risk is for people who have certain metal prosthetics (e.g., hip joints) - you wouldn t want them stuck to the side of the magnet, would you Another example that we have had is with metal breathing apparatus - someone was pulled back to the magnet when wearing it during a fire drill. 

Spatial variation of structure in a natural sample or in a technical part (e.g., gradient materials for hip-joints) can be studied by means of microfocus [44] beams (microbeams). The size of the probing X-ray beam limits the spatial resolution. 

These include attempts by surgeon M.N. Smith-Petersen in 1925, Robert and Jean Judet in 1938, and Dr. Edward J. Haboush in 1953. However, the first successful hip prosthesis was not developed until 1961 when Dr. Chamley made a hip prosthesis out of a high molecular weight polyethylene cup. Today, artificial hip joints are implanted in over 200,000 people each year in the USA (Ratner, 2004). 

This photo shows the components of an artificial hip joint. 

In normal motion, the load exerted on the hip joint is 2.5 times body weight, (a) Estimate the corresponding stress (in MPa) on an artificial hip implant with a cross-sectional area of 5.64 cm in an average-sized patient, and (b) calcnlate the corresponding strain if the implant is made of Ti-6A1-4V alloy. Clearly state yonr assnmptions and sonrces of data. 

Enhancing the wear resistance of polymers (e.g., ultra-high-molecular-weight polyethylene-UHMWPE-used for in vivo implants, such as artificial hip joints). ... 

Imagine the possibilities, some of which are just now becoming reality Tools, knife blades, and scalpels coated with diamond remain forever sharp. Eyeglass lenses and wristwatches coated with diamond remain scratch-free. High-fidelity loudspeakers coated with diamond give a nearly perfect, undistorted sound at high frequencies. Hip joints and other biological implants coated with diamond are not rejected by the body s immune system. 

At about 1860 the first sterile surgical techniques were introduced by Lister. About 100 years ago, the research into the field of implants, notably of the hip joint, was started. In 1891 Znamensky first described the use of ceramic materials in the manufacture of implants. 

In the 1960s the first heart valve was replaced. At approximately the same time the Englishman Sir John Charnley developed a bone cement on the basis of PMMA. Nowadays an improved version of this cement is still used, e g. to secure an artificial hip joint to the upper leg. 

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