Knee Arthoplasty

Total Knee Arthroplasty

Total knee arthroplasty (TKA), also called total knee replacement, is a widely performed procedure for advanced arthritis of the knee, primarily in older patients (≥ 70 years of age) with osteoarthritis. However, during the decade between 1990 and 2000, the proportion of younger patients undergoing TKA increased significantly. During this period the proportion of knee replacements performed in the 40- to 49-year-old age group increased by 95.2% and in the 50- to 59-year-old age group by 53.7%. This indicates the criteria for TKA, traditionally reserved for the patient over 65 years of age, are broadening. The primary goals of TKA are to relieve pain and improve a patient’s physical function and quality of life.

Knee Arthoplasty

 

Indications for Surgery

The following are common indications for TKA.Severe joint pain with weight bearing or motion that compromises functional abilities Extensive destruction of articular cartilage of the knee secondary to advanced arthritis. Marked deformity of the knee such as genu varum or valgum Gross instability or limitation of motion Failure of nonoperative management or a previous surgical procedure

Procedure

Background

Prosthetic replacement of one or more surfaces of the knee joint began to develop during the 1960s. Initially, only the tibial plateau was replaced (hemiarthroplasty). This was followed by the first generation of TKA, which involved a noncemented, double-stemmed, hinged, metal prosthesis that replaced the articulating surfaces of the distal femur and proximal tibia. This early design had a high failure rate the number of components implanted or articulating surfaces replaced. Another is based on the degree of constraint (i.e., the amount of inherent congruency/stability in the design). Most TKA procedures today involve a twocomponent (bicompartmental), semiconstrained prosthetic system to replace the proximal tibia and distal femur. These systems typically are composed of a modular or nonmodular femoral component with a metal articulating surface and a single all-polyethylene or metal-backed modular or nonmodular tibial component with a polyethylene articulating surface. TKA designs also are classified as mobile-bearing or fixed-bearing. The most recent development in the evolution of TKA is introduction of the mobile-bearing, bicompartmental prosthetic knee. A mobile-bearing knee has a rotating platform inserted between the femoral and tibial components whose top surface is congruent with the femoral implant (round-on-round articulation) but whose undersurface is flat for rotation and sliding of the tibial component (flat-on-flat articulation). A fixedbearing knee does not have such an insert. The purpose of the mobile-bearing insert is to decrease long-term wear of the polyethylene tibial component. A mobile-bearing knee design is recommended most often for the active patient, under 55 to 65 years of age. Another way to classify TKA design is based on the status of the posterior cruciate ligament (PCL). Although the ACL is routinely excised during knee replacement, except with UKA, the PCL can be preserved or excised. If the PCL is intact to provide posterior stability to the knee, one of several cruciate- retaining designs that require less congruency and allow some degree of AP glide can be used. If the PCL is irreparably deficient, a cruciate-substituting prosthesis is selected. This type of design has inherent posterior stability from the congruency of the components, a posterior prominence in the tibial component, or a cam-post mechanism built into the design. Cruciate-retaining and cruciate-substituting designs can have a fixed-bearing or mobilebearing design.

Surgical approach

 TKA and UKA procedures are also described in terms of the surgical approach employed. Since the inception of knee arthroplasty, an open approach requiring a relatively long anterior incision traditionally has been employed to provide sufficient exposure of the knee joint during the procedure Fixation. The method of fixation—cemented, uncemented, or “hybrid”—is another way to classify TKA procedures. That is, implants are held in place with acrylic cement, bone ingrowth (uncemented), or a combination of these two methods. Initially, almost all total knee replacements relied on cemented fixation. In fact, cemented fixation revolutionized knee arthroplasty.98,191 However, a long-term complication associated with early designs of cemented prostheses was biomechanical loosening, primarily of the tibial component at the bone-cement interface. Young, active patients were believed to be at highest risk for component loosening. To address the problem of loosening, cementless (biological) fixation relying on rapid growth of bone into the surfaces of a porous-coated or beaded prosthesis was introduced and recommended primarily for the young, active patient.

 Complications

Overall, the incidence of complications after TKA is low. Intraoperative complications during knee arthroplasty, such as intercondylar fracture or damage to a peripheral nerve (e.g., the peroneal nerve), are uncommon. Because minimally invasive TKA is considered more technically challenging than traditional TKA, early reports suggest that the rate of intraoperative complications, such as fracture or malpositioning of an implant, is higher with a minimally invasive than a standard approach. Early and late postoperative complications include infection, joint instability, polyethylene wear, and component loosening. As with arthroplasty of other joints, there is a risk of wound-healing problems and deep vein thrombosis (DVT) during the first few months after surgery. Although the incidence of deep periprosthetic infection is low, it is the most common reason for early failure and the need for revision arthroplasty. In contrast, polyethylene wear of the patellar and tibial components is the most common late complication requiring revision.

 

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