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Prosthetic design considerations
Unicompartmental components have been available for over 50 years, beginning with designs such as the Polycentric Knee, and the Marmor Knee prostheses. , Many of these early designs were also advocated for bicompartmental arthroplasty, but their success was hindered by the fact that instrumentation was poor and they did not provide for resurfacing of the patellofemoral compartment. As time passed, one-piece bicompartmental components became available on both the femoral and tibial sides and femoral components eventually had trochlear flanges to allow for resurfacing of the patellofemoral compartment. From that point, the use of individual components for each condyle and plateau has been mostly confined to unicompartmental arthroplasty of either the medial or lateral compartment. There is some limited experience combining medial or lateral unicompartmental components with modular patellofemoral components.
Experience has shown that certain design features of either the femoral or tibial component can favor ease of implantation, conservation of bone stock, and longevity of the arthroplasty. These features include size options, prosthetic geometry, symmetry versus asymmetry, primary fixation (cement versus ingrowth), and ancillary fixation (keels and lugs).
The femoral component
Sizing
Early designs often provided just two sizes: large and small. Since the early 2000s, size options for fixed-bearing femoral components have increased to as many as five. The advantage of multiple sizes is the ease of accommodating extremes of size variation, but disadvantages include inventory management problems. The advent of custom patient-specific components addresses both size variation and inventory issues , (see Chapter 11 ).
Prosthetic geometry
The sagittal shape of the femoral component should attempt to replicate the radius of curvature of the native distal femoral condyle. It must also allow for adequate capping of the resected surface of the posterior condyle. Most components call for a linear resection of the posterior condyle to facilitate bone preparation ( Fig. 3.1 ). Where “robotic” bone preparation is utilized, nonlinear resection can be appropriate. The sagittal femoral component shape must also allow adequate recession of the leading edge of the component in order to eliminate the potential for patellar impingement as the knee goes into flexion.
From a coronal perspective, the articulating surface of the femoral component should have a radius of curvature that is large but not infinite (flat) in order to allow for adequate contact between the femoral and tibial articular surfaces. Flat on round articulations are never appropriate. Flat on flat articulations are prone to edge-loading (see Fig. 1.4 ). Round on round articulations cannot be fully congruent unless they are mobile-bearing (see later). Round on relatively flat (large radius of curvature) articulations are appropriate as long as the polyethylene is of high quality and wear resistant. The coronal width of the femoral component should be proportional to the sagittal sizing and wide enough to adequately cap the prepared surface of the femur and resist subsidence (see Figs. 1.1 and 1.2 ).
For a cemented femoral component, the surface that contacts bone should promote cement fixation and containment, most often achieved via cement “pockets.” For cementless fixation, a surface that encourages bone ingrowth is required.
Symmetry versus asymmetry
Early designs were symmetric and could be used on either condyle and in both right and left knees. All modern designs, except for mobile-bearing articulations, are asymmetric to better conform to patient anatomy. Very few systems provide separate femoral component designs for the medial and lateral condyles and rely on the availability of multiple sizes to accommodate the slightly smaller lateral condyle. Custom components, of course, will provide the most accurate replication of a patient’s anatomy.
Primary fixation
Cement has been traditionally used for component fixation with great success. Components are available in some systems for cementless use with bone ingrowth potential and some encouraging early results. , Long-term longevity has yet to be determined. A possible argument against cementless fixation is the need for condylar resection to be deep enough to allow bone ingrowth. Furthermore, if fixation is dependent on ingrowth into deeply penetrating lugs or fins, extraction of well-fixed components could lead to significant loss of bone stock.
Ancillary fixation
Ancillary fixation, whether in cemented or cementless components, is achieved via lugs, keels, or a combination of both. When fins are utilized, they must be relatively narrow and not so deep as to cause loss of bone stock upon removal. Lugs can be round in cross section or square and also vary in length with the same caveat that longer lugs will risk loss of bone upon removal. They are often slightly conical in the sagittal view and can be tapered, smooth, rough, porous, or corrugated, depending on whether they are used with or without cement ( Fig. 3.2 ). Lugs should probably be angled away from and not parallel to a linear posterior condylar resection in order to achieve maximum prosthesis to bone contact upon insertion ( Fig. 3.3 ).
Keels can be of various depths and thicknesses as well as smooth or porous (when used cementless). The deeper they are, the more loss of bone stock can occur upon removal. They also increase the risk of femoral condylar fracture during bone preparation and component insertion or removal. For this reason, shallow keels are probably preferable.
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