In the knee are found the most complicated joints of our body. It is composed of two joints between the leg’s bones.
- The prominent joint is located between femur and tibia while the smaller one is between the kneecap (patella) and the femur.
- A smooth and tough articular cartilage that covers the ends of the bones and facilitates sliding over each other.
- A lubricating synovial fluid, produced by the synovial membrane over the other surfaces of the knee joint to reduce friction.
Anatomy of the human knee.
Many factors can aggravate the wear rate of the bearing surface and increase the likelihood of osteolysis (active resorption of bone matrix). Femoral and tibial components loosening, torn ligaments and mobile bearing components are examples.
Indeed, the flexibility and articulation of the joint is jeopardized if articular cartilage is damaged or worn to the extent that the ends of the bones rub or grind against each other. Ultimately, it will lead to severe pain, swelling and stiffness. To alleviate the effects of severe knee damage, the patient may be oriented to the knee replacement surgery during which damaged cartilage and bone from the surface of the knee joint are removed and replaced with artificial surfaces.
In the long run, an implant’s stability depends on how well bone grows into it (screws can be used to stabilize the implant too).
Conventional knee systems are standardized and approximated to patients’ knees, based on anatomy data collected in a specific region. A standard model used to be selected first by surgeons, based on a patient’s knee shape and size, and necessary adjustments were performed during the surgical procedure.
Illustration of a traditional knee implant.
The typical causes of complications associated with traditional knee systems are:
- An uneven stress distribution in the femur that may lead to loosening of the joint
- The use of an imperfect size of tibial implant when the surgeon is unable to find a perfect one in the product portfolio. An oversized implant might damage the soft tissues and ligaments around the knee joint while an undersized implant might protrude into the cancellous bone. In both case, post-surgical pain can be expected.
- The Ultra High Molecular-Weight Polyethylene (UHMWPE) placed over the tibial tray can wear-out. Wear particles of UHMWPE induce osteolysis, a lack of mobility and pain. Eventually, a revision surgery is required. UHMWPE serves to avoid metal-to-metal contact between the femoral and tibial implants but this problem occurs if there is unintended micro motion between the tibial tray and the UHMWPE. In the worst cases, the bearing surface completely wears out until the metal-to-metal contact occurs.
- The insertion of the femoral implant might require resurfacing the patella to fit, which in many cases causes post-operative pain.
Beside this, traditional manufacturing methods like CNC machining, investment casting or plastic molding have a long lead time because they need specific tool’s design, development and production.
In recent times, the appeal of personalized knee systems has increased among surgeons and clinicians because they are designed on a principle that factors in the patient’s unique anatomy and lifestyle.
With a just in-time response to a surgeon’s request, metal additive manufacturing is a technique that allows the fabrication of personalized knee system that exhibit an almost perfect fit, uniform stress distribution and thus a higher load bearing capacity.
The better alignment allows the surgeon to perform cuts in the bone and soft tissue exactly where they need to be which reduces the risk of fat embolism and intraoperative bleeding due to minimal bone removal. There is also less tissue loss, a shorter recovery time and less postoperative pain or infection.
Finally, less instrumentation is needed and fewer efforts are required for the orthopedic surgery to position adequately 3d-printed implants.