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Knee Replacement
Types of Knee replacement?
Fixed bearing:
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Cruciate retaining: If the bone stock is good and the supporting ligaments give adequate mediolateral and anteroposterior stability a cruciate retaining prosthesis can be used.
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Cruciate substituting: If there is good bone stock and mediolateral stability the surgeon has the choice to substitute the cruciate ligament and use a prosthesis which has a mechanism to replace the posterior cruciate ligament function. It is indicated in knees with severe deformities, post patellectomy, post high tibial osteotomy and in rheumatoid arthritis with ligament degeneration.
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Constrained prosthesis: It is used in knees with mediolateral and anteroposterior instability and in patients who need metal auguments and wedges to compensate for the bone loss as in revision replacements and in primary knee replacements with gross deformities and bone defect.
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Hinged knee prosthesis: If the bone stock is poor with bone loss or expected large bone resection along with weak ligaments both in the anteroposterior and mediolateral plane this type of prosthesis is used. It is mainly in revision knee replacements.
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Unicompartmental knee replacement: Unicompartmental knee arthroplasty (UKA) may be appropriate if you are age 60 or older, not obese and relatively sedentary. The prerequites include,
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An intact anterior cruciate ligament (ACL).
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No significant inflammation.
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No damage to the other compartments, calcification of cartilage.
It is indicated in patients with arthritis involving only one compartment. Unlike total knee replacement involving removal of all the knee joint surfaces, a unicompartmental knee replacement replaces only one side of the knee joint. Knee osteoarthritis usually occurs first in the medial (inside) compartment as this side of the knee bears most of the weight. In knees that are otherwise healthy, a unicompartmental approach allows the outer compartment and all ligaments to remain intact. By retaining all of the undamaged parts, the joint may bend better and function more naturally. Contraindications for the procedure include inflammatory arthritis like rheumatoid arthritis, lupus arthritis, psoriatic arthritis, arthritis inflammatory bowel disease and patients with gross deformities.
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Stem extenders: In patients with severe osteoporosis, with bone defect and in who a constrained prosthesis is used stem extenders are used to enhance fixation and to provide stability for the prosthetic components.
Total knee replacement in special situations:
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Post high tibial osteotomy: There can be lateralization of the proximal tibia at the site of osteotomy or sometimes medial shift of the proximal tibia depending on the initial fixation. The tibial component need to be aligned to the mechanical axis of the tibia and excessive lateralization or medialisation of the component should be avoided. Alteration of slope of the proximal tibia can also be present.
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Primary total knee replacement in severe deformities: Attenuation of collateral ligaments or severe contractures requiring extensive release, severe bone defect requiring reconstruction, osteoporosis, malunited stress fractures and difficulty in restoring alignment within 180±3°. Use of constrained prosthesis with stem extension, bone grafting and computer navigation helps in achieving good results.
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Osteoporosis: Difficulty in fixing the jigs during bone cuts, avulsion of soft tissue and ligament attachment, chances of fracture during reduction or manipulation are the problems. Careful soft tissue release and bone cuts and use of stem extension to provide stable fixation are required.
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