Abstract
The goals of total knee arthroplasty (TKA) are to relieve pain, restore function, and provide a stable joint. In regard to types of implants, the workhorses are posterior cruciate retaining (CR), posterior stabilised (PS), and posterior stabilised constrained (PSC) designs. However, the continuum of constraint now ranges from standard cruciate retaining (CR-S) to CR lipped (CR-L), to anterior stabilised (CR-AS), to posterior stabilised, to a PS “plus” that fits with a PS femoral component but provides a small degree of varus-valgus constraint, to a PSC or constrained condylar type of device, to a rotating hinge. As the degree of deformity, bone loss, contracture, ligamentous instability and osteopenia increases, so does the demand for prosthetic constraint. When deformity is minimal and the posterior cruciate ligament (PCL) is intact and functional, a CR-S device is appropriate. For moderate deformity with deficiency or compromise of the PCL, a CR-AS or posterior stabilised device is warranted. In severe cases, with attenuation or absence of either of the collateral ligaments, a constrained condylar device, with options of stems, wedges and augments, is advisable. In salvage situations, when both collaterals are compromised, a rotating hinge should be utilised. Prerequisites for use of a CR-S device are an intact PCL, balanced medial and lateral collateral ligaments, and equal flexion and extension gaps. With a CR-L bearing, a slight posterior lip is incorporated into the sagittal profile of the component to provide a small amount of extra stability in the articulation. It is important for the surgeon to be aware of the design features of the implant system he or she is using. For example, in a system where the CR-S bearing has 3 degrees of posterior slope and the CR-L bearing has no slope, the thickness of a CR-L bearing posteriorly is approximately 2mm greater than the CR-S. A CR-L bearing is indicated for to provide stability where the flexion gap is just slightly looser than the extension gap and the PCL is intact. If the patient's knee is somewhat lax in flexion and stable in extension, a CR-L bearing may help to stabilise both the flexion and extension gaps yet still allow the knee to obtain full extension, whereas if a CR-S bearing in the next thicker size is used to stabilise the flexion gap, a flexion contracture may result. CR-AS bearings are required less frequently. They are indicated when the flexion and extension gaps are balanced, but the PCL is deficient, and the surgeon does not want to change to a PS design, which requires additional bony resection of intercondylar notch. The PCL is one of the strongest ligaments in the knee, and affords inherent stability to the TKA. In flexion, the PCL not only affords AP stability, but also imparts flexion gap stability, acting as a lateral stabiliser of the medial compartment and a medial stabiliser of the lateral compartment. The PCL has a crucial role with respect to femoral rollback, which imparts added efficiency to the extensor mechanism. PCL retention is a more biologically preserving operative intervention than PS-TKA.
