Hip replacements

Research based on Wolff s law of bone transformation has resulted in some other important observations. Fluctuating loads, such as those that occur in walking, are better for bone than consistentiy appUed loads, such as weight gain. However, if the effective appUed load becomes extreme, pressure necrosis, ie, bone death, occurs. Pressure necrosis is a significant concern in hip arthroplasty. Necrosis means the localized death of living tissue. Undue pressure on living cells causes death. Some total hip replacement failures are the direct result of pressure necrosis. 

Bechtol in reviewing his clinical experience with 1087 cemented Charnley and Bechtol total hip replacements concluded that prosthetic stem failure is actually the final sequence of previous failures of bony and cement support. His suggested sequence of events leading to prosthetic stem failure have been substantiated in the literature and are as follows ... 

Bechtol, C. O., Failure of Femoral Implant Components in Total Hip Replacement Operations , Orthopaedic Review, 4, 23-29 (1975)... 

Titanium and its compounds are nontoxic and immune to attack by body fluids. These features account for the use of the metal in knee and hip replacements as well as dental implants. 

The bisphosphonates are used to treat osteoporosis in postmenopausal women, Paget s disease of the bone, and postoperative treatment after total hip replacement (etidronate). 

An idea of the range of materials and applications for polymers in medicine can be gained from the information in Table 10.1. As can be seen from this table a number of polymers are used in medical applications. One particular such polymer is poly (methyl methacrylate), PMMA. Early on it was used as the material for fabricating dentures later other biomedical applications developed. For example, PMMA is now used as the cement in the majority of hip replacement operations worldwide. 

Hip replacement surgery is now routinely used to relieve pain and restore mobility in patients suffering from osteoarthritis. In this condition the surfaces of bone in contact with each other within the joint become worn and the layer of lubricating cartilage disappears. This makes movement of the joint both difficult and painful. By replacing the hip with an artificial joint patients stop experiencing pain and are once again able to move freely. 

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. 

Another widely used surgical procedure is knee replacement. In this operation the natural components of the knee joint are replaced with an artificial femoral component made of metal that interfaces with a tibial plateau made of ultra-high molar mass poly(ethylene). Implantation of these joints is becoming more widespread, though the demand for them is not as great as for hip replacement. 

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. 

Vascular inujury Major orthopedic surgery (e.g., knee and hip replacement) Trauma (esp. fractures of the pelvis, hip, or leg) Indwelling venous catheters... 

Desirudin is a SC administered DTI approved for VTE prevention after hip replacement surgery but is not yet commercially available in the United States. Desirudin has an elimination half-life of 2 to 3 hours and is typically dosed every 12 hours. It is primarily eliminated through the kidneys, so dose reduction is needed in patients with renal impairment. The aPTT should be used to measure desirudin s anticoagulant activity.29,38,41... 

Eriksson B. I., Kalebo P Anthmyr B. A., et al. Prevention of deep-vein thrombosis and pulmonary embolism after total hip replacement. Comparison of low molecular weight heparin and unfractionated heparin. J Bone Joint Surg [Am] 1991 73A, 484-93. 

The success of hip replacements has been greatly advanced by major development in the biomaterials for orthopedic devices. What aspects of chemistry must be considered in designing an effective artificial hip ... 

Case Study of Materials Selection for Total Hip Replacement. Materials Department, Queen Mary and Westfield College, University of London, http //www.materials.qmul.ac.uk/casestud/implants/... 

A Patient s Guide to Artificial Hip Replacement. Randale Sechrest, MD, Medical Multi-media Group. http //www.medicaImultimediagroup.com/pated/joints/hip/ hip replacement.html... 

Desirudin (Iprivask) is approved for prevention of DVT in patients undergoing elective hip replacement surgery it is expected to be available for sale in the United States in late 2008. The recommended dose is 15 mg subcutaneously every 12 hours beginning 5 to 15 minutes prior to surgery and for up to 12 days thereafter. Daily aPTT monitoring is recommended. 

Prophylaxis should be continued throughout the period of risk. For general surgical procedures and medical conditions, prophylaxis can be discontinued once the patient is able to ambulate regularly and other risk factors are no longer present. Most clinical trials support the use of antithrombotic therapy for 21 to 35 days after total hip replacement and hip fracture repair surgeries. 

The artificial hip has been used to replace the human hip because the hip is easily worn out over a lifetime of mechanical stress resulting from normal activity. The first artificial hip implant was made by Thermistokles Gluck in 1891 in Berlin. This implant made use of a femoral head of ivory fixed with plaster of paris and glue (Gluck, 1890, 1891). The results were not good due to severe infection problems and adverse foreign body tissue reactions. To develop better hip replacement prosthesis, many materials and procedures were examined between 1925 and 1953. 

Short-term use of glucocorticoids, even in massive dosages, is less likely to produce harmful reactions. They can, however, produce a variety of effects that are neither limited to high doses nor to long-term therapy. When a low dose of steroids (prednisone) was given for several months to a 38-year-old man to treat eczema of his hands and feet, he developed bilateral avascular necrosis (AVN) of the femur. He therefore had total bilateral hip replacement, and several experts have attributed his AVN to the steroid administration. The man sued his allergist, who settled the lawsuit shortly before trial for approximately 400,000. Most practitioners, however, are unaware of the risk of short-term or low-dose steroids. Yet many of these cases can be found in the courts, the literature, and the MedWatch databases. 

Hip replacement surgery The usual duration of administration is 5 to 10 days after surgery up to 14 days was well tolerated in controlled clinical trials. 

For hip replacement surgery, consider a dose of 40 mg once daily subcutaneously, given initially 9 to 15 hours prior to surgery. Continue prophylaxis for 3 weeks. 

Prophylaxis of deep vein thrombosis (DVT) For the prophylaxis of deep vein thrombosis, which may lead to pulmonary embolism (PE) in patients undergoing the following hip fracture surgery, including extended prophylaxis hip replacement surgery or knee replacement surgery. 

DVT prophylaxis In patients undergoing hip fracture surgery, hip replacement surgery, or knee replacement surgery, the recommended dose is 2.5 mg subcutaneously once daily. After hemostasis has been established, give the initial dose 6 to 8 hours after surgery. The usual duration of administration is 5 to 9 days. Admixture incompatibilities Do not mix fondaparinux with other injections or infusions. 

Severe renal impairment (Ccr less than 30 mL/min) fondaparinux as prophylactic therapy in patients with body weight less than 50 kg undergoing hip fracture, hip replacement, or knee replacement surgery active major bleeding bacterial endocarditis thrombocytopenia associated with a positive in vitro test for antiplatelet antibody in the presence of fondaparinux known hypersensitivity to fondaparinux. 

Heterotopic ossification (etidronate) Prevention and treatment of heterotopic ossification following total hip replacement or caused by spinal injury. 

Complicating total hip replacement 20 mg/kg/day for 1 month preoperatively, then 20 mg/kg/day for 3 months postoperatively total treatment period is 4 months. 

Ghck HA, Connell ML, Koffer H, Eisenberg JM, Kelley MA. The economic impact of prophylaxis against venous thromboembolism in patients undergoing total hip replacement. Chn Res 1988 36 337A. 

Heterotopic ossification complicating total hip replacement PO 20 mg/kg/day for 1 mo before surgery then 20 mg g/day for 3 mo after surgery. 

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