Hip-joint replacement

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. 

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. 

Transition metals are often used to construct prosthetic devices, such as this hip joint replacement. 

Carbon fibers, which are relatively inert under physiological conditions, are selected particularly to enhance the mechanical properties of various biomedical materials, such as their incorporation in bone cement [127]. Metal implants for total hip joint replacements do not match the mechanical properties of human bone, and epoxy-graphite implants may have better properties [128]. 

There is more interest in the effect of marginal lubrication or spurious contamination than in full or intentional lubrication. The very indifferent performance of PTFE in the original human hip joint replacements emphasises the probelm faced with PTFE and even some of its composites. Providing that a full fluid film is maintained there is no serious problem. However, if direct contact between the polymer and the counterface occurs a transfer film is deposited. 

Ultra high molecular weight polyethylene Linear thermoplastics with very high molecular weight (>2 X 10 g/mol) used for orthopedic devices such as acetabular cup for hip joint replacement. Umbilical cord vein grafts Vascular graft made from umbdical cord veins. 

FIGURE 45.1 Radiographs of (a) tibial fracture fixed with four pins and an external bar (b) a total hip joint replacement in a patient who sustained a femoral fracture and was treated with double bone plates, screws, and surgical wire (arrows) (c) application of wires, screws, and plates in the spine. 

Recently, a cellular, structural biomaterial comprised of 15 to 25% tantalum (75 to 85% porous) has been developed. The average pore size is about 550 p,m, and the pores are fully interconnected. The porous tantalum is a bulk material (i.e., not a coating) and is fabricated via a proprietary chemical vapor infiltration process in which pure tantalum is uniformly precipitated onto a reticulated vitreous carbon skeleton. The porous tantalum possesses sufficient compressive strength for most physiological loads, and tantalum exhibits excellent biocompatibility [Black, 1994]. This porous tantalum can be mechanically attached or diffusion bonded to substrate materials such as Ti alloy. Current commercial applications included polyethylene-porous tantalum acetabular components for total hip joint replacement and repair of defects in the acetabulum. 

Hip joint replacements are the most common type of joint replacements, with 300 000 operations in the USA in 2000. The aim of this case study is to highlight the wear resistance and low friction properties of UHMWPE, to discuss the effects of sterilisation on the properties of plastics, and the effects of wear debris in the body. The order of presentation is that followed by the polymer, through fabrication, sterilisation, implantation and wear, to the reactions caused by wear debris. 

The Smith-Petersen implant is an early example of a hemiarthroplasty, in which only the femoral side of the hip joint is replaced. By contrast, in a total hip joint replacement, both the femoral and acetabular surfaces are replaced. The radiograph, shown in Figure 6.1, was obtained at the Chamley Museum at Wrightington Hospital (Wigan, England) and illustrates this early design. 

Loads of nontrivial magnitude, such as those described previously, moving across the surface of the glenoid component have the potential to cause stress-related damage and wear to tire polyethylene similar to that seen in UHMWPE knee and hip joint replacement components. Stresses in the UHMWPE of the... 

Weightman, B., Simon, S., Paul, L.I., Rose, R. and Radin, E.L. (1972) Lubrication mechanisms of hip joint replacement prostheses. Journal of Lubrication Technology (Trans. American Society of Mechanical Engineers) 94, 131. 

Observational studies The efficacy of pulse pamidronate therapy in NSAID-refractory active ankylosing spondylitis has been assessed in 52 patients using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) 47 completed the study [9 ]. They were given an NSAID and monthly intravenous infusions of pamidronate 60 mg for 6 months. The most common adverse reactions were arthralgia and myalgia, in 11 cases each, after the initial infusions. There were five withdrawals two defaults, one for a hip joint replacement, and two because of asymptomatic rises in serum creatinine. 

Following hip joint replacement surgery, radiographs are taken at followup over a number of years and wear is thus determined. Typical penetration rates for a Charnley metal-on-UHMWPE hip replacement are 0.04 to 0.38 mmyear (Dowson, 2001). The tunneling expression can be used to estimate the volume of wear debris ... 

Jalali-Vahid D, Jagatia M, Jin ZM, Dowson D. Prediction of lubricating flbn thickness in UHMWPE hip joint replacements. J Biomech 2001 34(2) 261-6. 

Scholes SC, Unsworth A, Goldsmith AAJ. A frictional study of total hip joint replacements. Phys Med Biol 2000 45 3721-3735. 26. 

Hip Replacements More than 750,000 surgeries involving replacement of natural joints with artificial joints are carried out each year. About 200,000 of these are total hip-joint replacements. Most of these replacements are driven by debilitating arthritis. A hip-joint replacement is designed to provide joint mobility and structural support. It must be stable under considerable load, have high fatigue resistance, abrasion resistance, and biocompatibility. 

Figure 12.22 The components of a modem hip-joint replacement. The metal ball is composed of a corrosion-resistant alloy. The cup, which receives the ball, is lined with high molecular weight polyethylene. 

Contraindication to sedation Obstructing colon mass Post-operative colorectal cancer surveillance High risk patients Scanner weight limitations Pregnancy Hip joint replacement Incompetent ileocecal valve Claustrophobia ... 

Patients with metallic hip joint replacements will have significant artifacts in the pelvis with limited evaluation of colonic segments in this region. This is a relative contraindication depending on the clinical question asked. Intravenous iodinated contrast allergy is also a relative contraindication as any patient with a history of a mild contrast allergy can be premedicated for the exam or not receive the injection. Claustrophobia is also a relative contraindication to the study. Patients can take an oral sedative prior to the study. An incompetent ileocecal valve is another relative contra-indication for CTC as distention of the colon may be suboptimal. 

The wear rates shown for UHMWPE used for hip joint replacement are somewhat in conflict. 

Bone cements are widely used in surgery. An important example involves hip joint replacement (see Section 11.7.2), where the artificial joint must be sealed to both the femur and the hip, and be able to withstand millions of loading cycles over many years. 

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