Dr. Bender, what is the key benefit of the 3D printed knee implants that you use?
Mathias Bender: Unlike standard implants, 3D printed knee replacements are custom- made for the respective patient. They look very similar to the original bone and therefore adapt perfectly to the joint.
After all, every person has a slightly different anatomy.
Bender: That’s correct. The interior and exterior of the thighbone, which forms the knee joint together with the shinbone and kneecap, has a different shape by nature in each of us. Compared to the inner bone, the outer bone is always somewhat smaller and narrower. Both of them are also curved differently. Oftentimes, these asymmetries and uneven curvatures are not factored in with standard implants.
What does this mean for patients?
Bender: For the individual patient it means that we need to take away far less of his/her own bone for the implant to align to the joint contours. Oftentimes the significantly less bone loss during the custom-made implant insertion is a big advantage particularly for younger patients. After all, the knee implants are replacement parts that have a certain life expectancy. If you need to insert a standard implant after ten or 15 years, there is still enough bone left to do it.
However, we still don’t have any long-term results on the 3D printed implants, since we have only been using them in our patients as partial replacements since 2007 and as total replacements since 2011. During this time, there haven’t been any patients, who needed a switch from a customized to a standard implant. This is why so far we are not able to prove the anticipated effect; we can only hypothesize.
What does the manufacturing process look like?
Bender: We take a look at the patient’s X-rays and ask about his/her ailments. Based on that, we decide whether the patient needs an implant. We subsequently present the options we can provide – whether that’s a partial or total implant, standard or custom-made implant from the 3D printer. If a custom-made implant is suitable for the patient and he/she chooses it, we decide on a surgery date together.
For the next step, we take 3D computer tomography images of the hip, knee and ankle joint regions. With the help of these sectional images, the leg axle, rotation and the shape and anatomy of the bone in the knee joint can be completely illustrated. We send this image data along with an order to Boston where the implant is being made. After 4-6 weeks for a partial implant and 6-8 weeks for a total implant, we then receive the product in sterile packaging along with the custom-made equipment for the replacement. We receive everything we need for the surgery in one package. This is a huge advantage for us surgeons.
Is this an advantage that made you feel confident about these implants?
Bender: The way I see it, this is above all an innovation that doesn’t have any drawbacks, just advantages for the patient. Especially the partial implants – that are only used when the inside or outside of the knee joint needs to be replaced – have one key advantage: thanks to CT planning, they cover the bone one hundred per cent.
What’s more, having less bone loss during the insertion of the custom-made implant absolutely convinced me. As a surgeon, this provides me with a certain level of security during surgery: along with the implant, I get a detailed plan on which bones I need to remove so the implant fits perfectly. With the help of individual patient data, I can tailor the surgery accordingly and check anytime whether I am still following the plan.
Why is a custom-made implant not suitable for every patient?
Bender: Health insurance companies pay for this relatively expensive implant. Not everybody can get this type of care. The implants should be for those patients who can also benefit from them. A 60-year-old patient for instance will live to see a knee implant switch. When I am able to save bone during the initial surgery, regardless of whether it is a partial or total implant, I have a better chance that there is still enough bone left for the second surgery to insert a second replacement. That said, some diseases or their consequences can simply not be adjusted with a customized implant; for instance if a patient after many years of suffering from osteoarthritis has a lack of leg extension of more than 15 degrees or knock-knees with loosening.
What materials are these implants made of?
Bender: As with standard implants, chrome, cobalt or molybdenum is also being used for 3D printed implants. The implant is printed, polished and blasted. It also undergoes a quality circle where it is being compared to the original design. Each implant is also subjected to an optical control – it is checked for lack of definition or edges and cracks.
How often are individualized knee implants used in your clinic?
Bender: We have implanted 220 knee replacements in the past year, of which 70 were manufactured using 3D print technology. This customized implant is a supplement to the standard knee joint endoprosthesis. Decisions on which patients should receive these implants must be made carefully. We check all options the industry offers us with every patient and then choose the best implant for him or her.