There are six possible degrees of freedom at the knee, three rotational and three linear. The less inherently stable the knee being treated, the more degrees of freedom need to be controlled by the prosthesis. Levens identified a transverse rotation of at least 8 degrees that occurred during gait.1,18
Early total knee implant designs used single axis hinges that did not allow for the normal rotational motion of the knee. In addition, single axis hinges do not allow for the changing instant center of rotation of the normal knee. The failure of the early hinge pros-theses to mimic normal knee kinematics placed increased loads on the bone-cement interface leading to a high incidence of loosening. Abnormal motion, impingement on flexion, and distraction of the femur were identified on motion studies of hinge prostheses in vivo. The abnormal forces and high frictional forces contributed to wear of the bearing surfaces with metal fracture and creation of wear debris.18
The problems with the early hinge designs led to a new generation of constrained prostheses in an attempt to improve the results over the single axis early hinge designs. The total condylar III prosthesis provides a deepened femoral intercondylar recess into which an elongated polyethylene peg articulates—providing varusvalgus and anteroposterior stability (Figs. 10.1A-D). Extended femoral and tibial stems transfer the loads away from the condylar bone prosthesis interface.
Newer hinge prostheses allow flexion-extension, distraction, and some degrees of rotation. The kinematic rotating hinge prosthesis (Howmedica, Rutherford, NJ) is a constrained hinge that allows axial rotation and distraction between the inner tibial bearing and the outer sleeve7 (Figs. 10.2A-D). The Noiles knee prosthesis was introduced in the late 1970s as a modified constrained hinged prosthesis. The Noiles has an uncemented tibial stem, set within a cemented sleeve, and reported to allow a 20-degree arc of both medial and lateral rotation in flexion as well as reduced tensile loading3,6 (Figs. 10.3A-B).
In the revision knee arthroplasty the quality of bone is often inferior and deficient. Because of the poor bone stock, the tibial and femoral surfaces are inadequate because the main load-bearing surfaces for implant fixation and the loads must be transferred to the intramedullary area of the tibia and femur via stems. Custom implants have been utilized to assist in situations in which there is marked bone loss and/or instability.12 Custom implants are limited by their cost, inflexibility for unanticipated anatomy at the time of surgery, and by the length of time that it takes to manufacture the implant. Custom implants have been largely replaced by modular designs. Modular total knee systems allow for varying length of stems. In addition, modular implants allow for distal and/or posterior femoral wedge augmentation for asymmetric bone loss. Augmentation blocks or wedges may also
Figure 10.3. (A) Anteroposterior and (B) lateral photograph of Noiles rotating hinge prosthesis.
be added to the undersurface of the tibia for similar reasons (Figs. 10.4A-C).
The type of intra-articular constraint desired may also be varied with modular total knee systems depending upon the clinical setting. A posterior-stabilized or cruciate-retaining tibial polyethylene implant may be used, or, in situations in which instability is greater, a more constrained tibial polyethylene component may be used that provides a thick intercondylar peg similar to the total condylar III design.
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