Hip Replacement - Term Paper Example

?Your Full Your The Hip Replacement Introduction Hip replacement, also described as hip arthroplasty, is a surgical procedure that employs the use of synthetic implants to replace the articulating surfaces of a worn-out hip joint. Hip replacement is one of the most successful and cost-effective interventions in medicine (Crawford and Murray). It effectively provides pain relief and improved functioning of the hip joint in patients suffering from inflammatory or degenerative arthritis (joint inflammation). The hip joint is a ball-and-socket joint made by the articulation (contact) of two bones – the femur and pelvic bone. The femur consists of a long shaft, and a ball-shaped end called the head of the femur. The head acts as the ball of the hip joint, it inserts into the cup of the pelvic bone. It is attached to the shaft by the neck of the femure. The pelvic bone has cup-shaped depressions on each side, called the acetabulum, which accommodates the femur head. The articulation of the femur head and acetabulum makes the hip joint (Morton, Foreman and Albertine). This paper shall discuss the details of the design and components of the hip arthroplasty, the important reasons for performing the procedure on patients, the physical and chemical properties that the implant must possess in order to satisfy functional requirements of the hip joint, and the future outlook for the procedure. A summary of the discussion will conclude the paper. 2. What is a Hip Replacement? A hip replacement refers to the use of a structural implant device to replace the normal bone surfaces that interact at the hip joint. The implant components include one shaped like the head of the femur, and another shaped like the acetabular cup. The implant device is made from biocompatible materials - biomaterials. Hip replacement is performed as a major orthopedic surgical procedure. The surgery became common in the 1960’s, and has since then revolutionized the outcome of patients with severe hip arthritis by providing pain relief and restoration of function of the joint. The synthetic hip replacement prosthesis consists of a femoral component and an acetabular component (Hickok, Purtill and Marcolongo). Hip replacements can be primary – when performed for the first time, or revision – when a replacement is done of a hip joint that was previously replaced but has suffered a complication. It can be total, where both acetabular and femoral components are implanted, or partial (also called hemiarthroplasty) where only the synthetic femoral head is implanted. The femoral component consists of the head, Morse taper (head-neck junction), neck, and stem that fixes into the femur shaft. The acetabular component includes a shell and liner that fits into the acetabular cup of the pelvic bone. The two components stay in contact and articulate in to restore function of the hip joint – the femoral component acts as the ball, and the acetabular component as the socket of the joint.The components have to be firmly fixed to the respective underlying bone to ensure joint stability; this is achieved by either using bone cement between the implant and bone, or including a porous coating on the implant surface to promote bone growth into the implant. 3. Why is it done? The primary goal of a hip replacement is to restore the normal, pain-free functional integrity of the hip joint (Hickok, Purtill and Marcolongo). The implanted devices restore smooth articulation at the ends of the bones – the femur and pelvic bone – thus allowing normal movement to take place at the joint. Joints of the body undergo wear, tear and damage with age. This damage is accelerated or exacerbated by trauma, infection, and joint inflammation. Joint damage can, in its early stages, be slowed by drugs such as anti-inflammatory agents. However, a joint exposed for a long time, or to an extreme intensity, of stress can degenerate to a state where it can no longer perform its normal function, and movement at the hip joint is reduced, lost, or made unbearably painful (Hickok, Purtill and Marcolongo). As joint mobility becomes restricted, patients are unable to engage in walking, stair climbing, and putting on shoes, and require walking aids. In some inflammatory conditions, the bones at the joint become fused together, resulting in such severe immobility that the patient becomes bed-bound. Medications can mollify the pain but cannot restore the lost mobility of the joint (Crawford and Murray). Patients who suffer falls or accidents may suffer a fracture at the neck of the femur bone that cuts off the blood supply to the femoral head, leading to avascular necrosis of the femur head. In this condition, the necrosed femur head makes the joint non-functional. Thus, total hip replacement is the definitive treatment for end-stage arthritis or severe abnormalities of the hip joint. It has been shown to improve patients’ quality of life in a highly cost-effective manner (Pruitt and Chakravartula). About 85% of patients who undergo hip arthroplasty suffer from osteoarthritis (degenerative joint disease). As joint degeneration is an age-related process, most patients undergoing hip arthroplasty are 65 years old or older (Crawford and Murray). Many younger individuals can also develop severe joint inflammation due to inflammatory diseases such as rheumatoid arthritis or traumatic joint injury. Hip arthroplasty offers the best intervention to restore normal daily functioning for these patients. 4. Functional Requirements The hip replacement device must essentially maintain the functional requirements of the hip joint. The prosthesis must maintain articulation of the femur head within its socket with the pelvic bone. The functional requirements of the implant for hip replacement are based on the function of the joint and the biological environment around the implant (Pruitt and Furmanski). The implant has to bear the stress of the body without undergoing breakage, wear or molding. The biomaterials used in the synthesis of the implant must also be biologically non-reactive, thus they must not incite elicit cellular reactions or incite an inflammatory response by the immune system. The implant must withstand the corrosive action of enzymes and other physiological chemicals in the biological fluids surrounding the implant. Thus, the implant must hold the properties of corrosion resistance, wear resistance, fracture resistance, toughness, and compliance match to the surrounding tissue. Specifically, the acetabular cup must be resistant to inflammatory effects, the femoral neck must sustain bending stresses, the femoral stem must resist fatigue and corrosion. The implant must be firmly fixed to the underlying bone; porous coatings or bone cement are used for this purpose. The porous coatings may use titanium beads or bioactive glass to promote bone growth. Bone cement must provide interdigitation between the device and bone for optimal fixation (Hickok, Purtill and Marcolongo). Another option is locking mechanisms consisting of notches and grooves on the implant, that allow effective fitting into the bone (Pruitt and Furmanski). Metals, ceramics and polymers are the main engineering materials used in the making of synthetic implants. Among metals, cobalt and titanium alloys are commonly used. For synthesis of the acetabular cup, UMHWPE polymer is the biomaterial of choice used. The femoral head is made from ceramics e.g. zirconia or cobalt alloys (Pruitt and Chakravartula). The choice of biomaterial used for implant synthesis is based on the physical chemical properties of the materials, and the needs of the patient. Polymers are intrinsically resistant to biological attacks (Pruitt and Furmanski), thus they offer the best properties for the acetabular cup. The femoral head is made from ceramics e.g. zirconia in total hip replacement designs for a more active patient, as they offer exceptional wear resistance and low friction. However, the downside of polymers is that their strength is limited as load-bearing devices, thus they are more susceptible to fracture. One previous clinical concern was the phenomenon of wear-mediated autolysis, in which microscopic particles of debris flaking off the implant surface with wear and tear incite an inflammatory reaction that results in bone dissolution and eventual compromise of implant function. However, modern UHMWPE polymers have high wear resistance, minimizing the occurrence of bone lysis. The ultimate design of the total hip replacement prosthesis combines the prerequisites for functional requirements of the hip, the anatomical needs of the patient, selection of material, and manufacturability (Pruitt and Furmanski). 5. Future Outlook and Summary Medical devices are not immune to biological failures (Pruitt and Furmanski). The implant may incite an inflammatory reaction and become damaged due to biological attack, it may become loosened and dissociated from the bone, and it may undergo fracture. In all these cases, the implant would become non-functional. However, over the past 50 years, tremendous improvements have been made in the synthesis of biomaterials and the procedure itself. The best assessment of the success of the hip replacement procedure is the long-term outcome of patients, in terms of the percentage of implants that are functioning well after ten years or more since the procedure. Several studies have reported hip arthroplasties to be functioning well after 10-15 years in as many as 85-91% of patients. However, other authors contradict these findings, and report the incidence of joint loosening to be as high as 73% after 11 years (Hickok, Purtill and Marcolongo). Overall, however, most reports indicate that the hip arthroplasty is a highly successful procedure that dramatically improves patients’ quality of life. Indeed, within three months of surgery, patients report relief of pain, better walking ability, increased energy levels, and better sleep, social, and sexual function (Crawford and Murray). In summary, hip replacement is a procedure that has developed and improved over the last several decades and is now a highly effective treatment option for relieving pain and restoring joint function in patients with severe disease of the hip joint. The surgical procedure involves fixing synthetic implants, shaped like the acetabular cup and/or the femoral head, to the bones at the joint surfaces of the hip. Although challenges have been faced in creating biomaterials that satisfy functional requirements at the joint and restore joint function, metal alloys and ceramics have been designed to maximize fracture and wear resistance, and minimize bio-reactivity and friction. This has helped ensure maintenance of long-term functioning of the implant, and a vast majority of hip arthroplasties are found to be functioning well after ten years. References Crawford, R.W. and D.W. Murray. "Total hip replacement: indications for surgery and risk factors for failure." Ann Rheum Dis. (1997): 455-7. Dvorak, Marcel, C.P. Duncan and Brian. Day. "Arthroscopic anatomy of the hip." Arthroscopy: The Journal of Arthroscopic & Related Surgery (2005): 264–273. Hickok, Noreen J., et al. "Ch.5 Biological Fixation in Hip Replacement." Sinha, Raj K. Hip Replacement Current Trends and Controversies. New York: Marcel Dekker Inc., 2005. 137-140. Pruitt, Lisa A. and Ayyana M. Chakravartula. Mechanics of Biomaterials: Fundamental Principles for Implant Design. Cambridge: Cambridge University Press, 2011. Pruitt, Lisa and Jevan Furmanski. "Polymeric Biomaterials for Load-bearing Medical Devices ." JOM (2009): 14-20. Read More