In this article, Dr. Lawrie discusses the general surgical principles of total knee replacement, and why each is important for achieving a successful outcome.
Total knee replacement is a common operation that provides reliable pain relief and restoration of function to hundreds of thousands of patients suffering from end-stage degenerative disease of the knee every year. There are a several different techniques, technologies and implants that may be utilized to perform the procedure. These details can lead to confusion amongst patients. There is no one “best” way for the procedure to be performed and no matter how it is performed, the overarching goals and outcomes remain the same. In this article, we will discuss the general surgical principles of total knee replacement, and why each is important for achieving a successful outcome.
Patients with arthritis often develop either a “knock-kneed” or “bow-legged” deformity. Knee replacement has the power to correct this deformity and give the patient a “straight” leg again. In the past, surgeons aimed to create a “straight” leg alignment, with the hip centered over the knee and the knee centered over the ankle. More recently, empowered by more accurate surgical instruments and reliable implants, surgeons have increasingly personalized their target for limb alignment. As most patients have some natural bow or bend to their knees, surgeons now aim to restore the patient’s native anatomy instead of an arbitrary target of a “straight” leg. The end of the femur and top of the tibia are cut at specific angles to achieve the surgeons goal for limb alignment.
Getting the limb alignment correct is important both for cosmetic and functional reasons. Even if pain-free, a crooked knee is likely to cause psychological distress for a patient after knee replacement. Functionally, errors in knee alignment can cause ligaments to be too loose or tight after surgery, can cause differences in leg length, and can lead to early wearing out or loosening of the implants from the bone.1
Being able to fully bend and straighten the knee is important for achieving a good functional outcome after knee replacement. Patients with severe arthritis often lose their ability to fully bend or straighten their knee due to tightening of their joint capsule or mechanical blocks to motion from torn cartilage, meniscus or bone spurs. One of the goals of the procedure is to restore the motion in the knee so patients can perform normal daily activities. In surgery, the amount of bone cut from the tibia, femur and kneecap, as well as careful stripping of the joint capsule and some ligaments, can be fine-tuned to restore full knee motion. Unfortunately, despite surgeon best efforts, a stiff knee before surgery is at risk for being stiff afterwards regardless of how well the surgery was performed.
The knee is often thought of as a simple hinge joint that bends and straightens. In reality, the knee is a very complex joint with complex motion. It bends and straightens, slides forward and backward, wobbles side to side, and rotates inward and outward. In the healthy, well-functioning knee, the anterior cruciate ligament, posterior cruciate ligament, medial collateral ligament, lateral collateral ligament, joint capsule and several other structures help keep the knee stable through all of these complex motions. Arthritic knees often lose the normal “balance” between these stabilizing structures. Furthermore, the anterior cruciate and sometimes the posterior cruciate ligament are removed as a part of routine knee replacement surgery. During surgery, ideal stability is achieved through bone cuts, careful selective stripping of ligaments, implant size and position, as well as implant design. However, getting the stability of the knee replacement perfect is a fine balance. If the knee is left “too tight”, the patient may have difficulty getting their full motion. If the knee is “too loose,” it may feel unstable. Therefore, surgeons must take great care to restore the normal stability of the knee throughout its motion.
While discussed individually, each of these principles is linked with the others. There are numerous techniques and implants that can achieve these goals. Talk to your surgeon to discuss the best option for you and your knee.