When a knee joint becomes damaged or diseased, it is known to replace all or part of the knee joint with a prosthesis. A common form of prosthesis comprises a femoral component, which is attached to a distal end of a femur, and a tibial component, which is attached to a proximal end of a tibia. The femoral and tibial components may articulate directly or may be separated by a meniscal bearing component. The femoral component also articulates with a patella, which is secured in position by a quadriceps tendon and a patellar ligament.
The articulation of a natural knee joint is stabilised by the action of medial and lateral collateral ligaments and anterior and posterior cruciate ligaments. Where possible, all of these ligaments are retained when a prosthesis is implanted, although in practice it is often necessary to remove at least the posterior cruciate ligament. It is desirable for tension in the knee ligaments after surgery to be balanced throughout the range of motion of the knee.
The most complex component of a knee prosthesis is the femoral component, as it carries not only the condylar bearing surfaces, but also the patella bearing surface, which extends along an anterior face of the distal femur. Conventional femoral components require resection of the distal end surface of the femur and the anterior and posterior faces of the femur. They also usually require two chamfered cuts to be made at the distal end of the femur anteriorly and posteriorly. The positioning of the femoral cuts determines the position and orientation of the implanted femoral component, and hence the bearing surfaces which it carries. The correct positioning of the femoral cuts is vitally important therefore to ensuring equal tension in the ligaments after surgery.
It is known to use surgical jigs to guide the positioning of the femoral cuts. Correct placing of a jig to ensure balancing of the ligaments with the knee in flexion is complicated by the presence of the patella. The patella is conventionally either everted or subluxed in order to provide sufficient space for the surgical jig. However, this means that the substantial force exerted via the patella and patellar tendon by the quadriceps mechanism is acting out of its normal alignment. With the patella out of position, this force acts to skew the joint either laterally or medially, making balancing of the remaining ligaments extremely difficult.
Owing, at least in part, to the complicated nature of balancing ligaments with the knee in flexion, flexion balancing conventionally comprises attempting to replicate in flexion a previously measured and balanced extension gap.