Knee Replacement Design and Improving Results

Design

Several different designs of knee replacement are used in Australia and surgeons frequently debate the merits of each. I use a “medial pivot” design for reasons of better movement, increased stability and improved patient satisfaction. This is not the most commonly used design by other Australian surgeons. Sophisticated kinematic research shows that medial pivot knees move more like the normal (unreplaced) knee, particularly with activities such as stair climbing, lunging, pivoting and kneeling, without paradoxical forward movement of the femur on the tibia as occurs with other (older) designs^[1]. Clinical follow-up studies with patient reported outcomes show superior function with medial pivot knee designs. Studies in patients who have had both knees replaced (with a different type of knee replacement in each knee) have shown that patients prefer the medially pivoting design to other designs^[2].
They say things like: “feels more normal”; “stronger on stairs”; “superior single-leg weight bearing”; “feels more stable overall”.

Patient Specific Implantation Instruments

One of the most important aspects of the surgery is getting the perfect “fit” and alignment of the artificial knee. It is accepted that an accurate positioning results in increased survival of the artificial knee^[4].  To this end I utilise instruments specifically produced for each individual patient. This involves having the knee scanned prior to surgery, the scan is then used to 3D print instruments which accurately fit the bone and allow precise bone preparation. When compared to conventional techniques this has been shown to result in more accurate positioning, less bleeding^[5] and faster surgery (thus potentially decreasing risk of infection^[6]). The downside of this technique is that it takes four weeks to produce the 3D printed patient specific instruments, but for most this wait is insignificant.

Resurfacing the knee cap (patella)

Whether or not to resurface the back of the patella as part of a knee replacement is a contentious issue in orthopaedics. The rate that the patella is resurfaced during a knee replacement in Australia is about 50% overall. Whether yours is resurfaced depends upon your surgeon’s opinion – it is rarely discussed prior to the operation. Patella resurfacing rates vary widely from state to state and between nations. Thus whether a surgeon resurfaces the patella is influenced (at least in part) by where they were “apprenticed” rather than their interpretation of the scientific evidence.

I choose to resurface the patella almost always. Resurfacing the patella has been shown in the scientific literature to reduce the risk of pain at the front of the knee (where the patella is) following knee replacement and reduce the risk of further “revision” surgery^[7]

The main downside of resurfacing the patella is that this requires the removal of some bone – and a consequent risk of fracture. This risk is very low (well under 1%) and can be ameliorated by not resurfacing particularly thin or fragile patellas.