3D Knee Implant

The first step in the process of creation of a customized 3-D knee replacement is evaluation by a surgeon to understand the patient’s knee joint and to evaluate if the patient is a candidate to get a customized knee replacement.

If the patient is indeed a candidate to receive a customized knee replacement, the surgeon orders a CT scan of the affected knee. The CT scan is then transferred to the company that manufactures the customized knee. The manufacturing company then starts to make the implant. The implant takes about 6 weeks to be manufactured and to be delivered to the hospital.

The patients with significant deformities and patients with ligamentous injuries are not candidates for customized 3-D knee replacement surgery. Additional contraindications for customized knee replacements include all the contraindications for a traditional knee replacement, as example active infection in the knee joint.

I have had the following issues with customized 3-D knee replacement –

1. Some patients do not like to wait for long periods of time after their surgery is scheduled. As of 2018, the customized 3-D knee replacements take  about 6 weeks to manufacture and to deliver to the hospital. This is the major downside of using customized implants. The wait period is the major downside of using customized implants.
2. The second issue that I have had occasionally is patients go to a radiology facility with the script which clearly mentions that the CT scan is for a customized knee implant. However, the technician just does a plain CT scan.

   This plain CT scan is not enough for making a computerized model and to generate the prototype of the knee and to construct the customized implant. The patient therefore has to go again to get the correct CT scan so that the customized knee implant can be manufactured.

3. For preparation of the shin bone during a customized 3-D knee replacement, there is a jig which guides the drill into the deeper part of the shin bone. In my experience, this jig should be made stronger because on occasion (on preparation of the deep part of the shin bone) the jig sheared off prematurely.
4. Some patients like to have their customized instruments that were used to replace their knees. These customized instruments need to be washed and processed before delivery to the patient.

   I have found that this is somewhat of a logistical issue coordinating washing of implants and handing it over to the patient. I have had these custom instruments processed and then these were delivered to my office and patients received these instruments in the first postoperative visit.

The patient has to go to very specific radiology centers which follow the protocol for making a customized 3-D knee replacement. The CT scan for the knee replacement surgery is very specific and not all radiology centers are able to do that.

I recommend getting a CT scan within 4 months of the scheduled surgery. The reason for this time frame is because if the deformity in the knee increases or if there are additional defects that arise in the knee after the initial CT scan is done, then the customized implant will not be as accurate as we want it to be.

If the patient is considering surgery after 6 months, it is best to get the CT scan at a later date. Most patients that I see really want the surgery at the earliest available date and therefore get the CT scan as soon as possible.

The conversion of a failed partial knee replacement surgery to a full replacement surgery is called revision knee replacement. Custom knee replacements are not a good option for revision knee replacement surgery unless there are no revision knee systems in the market that can provide off the shelf options.

The customized 3-D knee replacement surgery is designed to match the natural knee joint. The CT scan provides accurate geometry for manufacturing this customized knee. The CT scan data is used to generate a computerized model of the custom knee by the custom knee manufacturing facility.

Additionally, the custom knee 3-D manufacturing facility also makes very specific instruments that are customized to the anatomy of the native knee for doing the customized knee replacement.

These instruments are made specifically for the patient and make the surgery less invasive. As an example drilling into the thigh bone and the shin bone is not needed because the instruments contour very well to the anatomy around the knee joint providing accurate alignment, rotation, offset for placement of the custom knee.

The earliest the surgery can be scheduled is 6 weeks from the time the CT scan is performed. The CT scan is performed immediately after seeing the surgeon. Six weeks is a reasonable time frame to schedule the surgery.

Additionally, it is important to ensure that the medical clearance for the patient is obtained prior to the surgery. I also get dental clearance prior to the surgery. It is important to note that the medical clearance as well as the blood investigations have to be done four weeks before the date of the surgery.

If additional tests, for example cardiac tests are mandated by the internist, the surgery may need to be postponed. All “I’s have to be dotted, T’s have to be crossed” before the patient is actually wheeled into the operating room.

There are numerous radiology centers that do the CT scan and appointment for a CT scan is generally available within a couple of days.

The CT scan appointment generally takes less than an hour. The time varies according to the radiology facility, but generally a CT scan appointment takes up less than an hour.

A customized knee replacement surgery generally takes about one hour to one and half hour to perform. By the time the patient goes into the operating room and the nurse calls the relative to visit the patient, it is a lot longer than one and half hours. The additional time is because of the time required by the anesthesiologist to anesthetize the patient which may or may not include spinal anesthesia. There is also time that is needed to prep and drape the patient before the final surgery starts.

The customized knee replacement is manufactured from the CT scan. This CT scan is also used for manufacturing the custom knee instrumentation. This instrumentation assists in implantation of the custom knee into the patient. The customized jigs conform very well to the anatomy around the knee joint.

The traditional knee replacement uses an intramedullary guide for placement of jigs on to the thigh bone. This process involves drilling the hole into the thigh bone and placement of a rod into the bone to get an accurate alignment.

When a customized instrumentation is used, there is no need for placement of this intramedullary guide and there is no need to drill the bone, because the customized implants have already accounted for the alignment and geometry of the thigh bone and for the mechanical axis of the lower extremity. The surgery is therefore less invasive.

Similarly on the side of the shin bone (lower part of the knee joint), there are two ways to place the guide that is used for making the bone resections.

One way is to drill hole into the shin bone (“intramedullary” guide) and place a rod into it and attach a guide to it (similar to the thigh bone) or the surgeon may choose to use an “extramedullary” guide (which means that there is a rod on the outside of the shin bone) to accurately align the guide on the shin bone so that the knee implant on the shin side will be accurately aligned.

The custom instrumentation does not need an intramedullary guide. We use custom instrumentation for placement over the shin bone and the resection of the bone is made based on this less invasive guide.
In addition to using the customized instruments, I also use visual techniques to ensure that the cuts are accurate.

There are several avenues in which the current technology for customized knees can be improved. First and foremost the manufacturing process needs to be shortened. The current six weeks period as of 2018 is very long in my opinion.

The patients should have the option of having knee replacement within 6 weeks of seeing the surgeon if all other requirements like medical clearances are met.

The CT scan does involve some radiation to the patient. With improved techniques, I feel we can significantly decrease the amount of radiation involved with CT scanning of the knee.

There are some steps in the process which I feel are not as accurate and could be improved. As an example, during placement of the jigs for alignment of the guides on the shin side of the bone, cartilage, if any, has to be manually scraped. I feel this could create inaccuracies if additional cartilage is scraped by the surgeon or if less cartilage is scraped by a surgeon.

The inaccuracy is very small. It is in millimeters, however, for patients who are tall, a small change in millimeter at the knee joint can vary the mechanical axis to a greater extent than patients who have shorter bones.

The cutting guides on the thigh bone could be consolidated into fewer cutting guides. As example, it is possible to create one cutting guide for all the cuts that are done on the thigh bone. The advantage of a single cutting guide for the femur is that the process will be faster.

Additionally, the inaccuracies will be decreased. For example, one cutting guide is placed, the cut is made and then the second cutting guide is placed on top of the first bone cut. I feel that if the first cutting guide is placed inaccurately, the first bone cut will be incorrect.

The second cutting guide placed on the first cut just adds additional errors to the subsequent bone cuts. The use of a single cutting guide will decrease these inaccuracies.

Additionally, it is difficult for the manufacturing process to account for the flexion deformity, which is due to the soft tissues. The computerized scan is very good at evaluation of the bony anatomy, however, the knee may be bent due to fluid in the knee joint or due to contractures which are outside the knee joint and the current protocol does not account for these contractures.

The customized knee replacements are cemented into the shin bone and thigh bone. There is plastic in between the two metal parts. The underneath of the kneecap is also cemented into position.

The underneath of the kneecap is replaced by plastic, which is off the shelf.  The underneath of the kneecap is not customized.

Physical therapy is mandatory after any knee replacement surgery including customized knee replacement. I do not change my immediate postoperative protocol for customized knee replacement. Without physical therapy, the range of motion of the knee can suffer.

Whatever range the patient has at 3 months after the surgery is what remains for life. I try to achieve as much range as range as possible during the surgery. This range has to be maintained postoperatively and physical therapy is a critical part of the postoperative protocol.

After having customized knee replacement, if you have requested the surgeon may give you the mold from which your knee was manufactured. The patient’s implant was made from this mold. This mold is customized for you and is available for you to take home.

One of the issues that I have had is that there is some logistics involved in getting this mold and the custom instrumentation washed and processed and handing it over to the patients or relatives. Sometimes I have had the instruments washed, then delivered to my office and these instruments are then given to the patient in the postoperative visit to my office.

There are some patients who love having their own custom instruments. Doing a customized knee replacement allows them the options to have these taken home with them.

“PSI” or “patient specific instrumentation” is technology of one company and “custom knee replacement” is of another. There is a significant difference between the two. PSI involves the instrumentation being specific for the patient. For PSI, the knee undergoes an MRI and the jigs (instruments used to replace the knee) are individualized or customized for the patient. The actual knee joint is “off the shelf”.

No, the implant that caps the thigh bone and the shin bone is only one and is customized for the patient; however, the plastic between the two implants comes in numerous sizes. This allows intraoperative flexibility in resection of the bone as deemed best-fit by the surgeon.

Custom knee replacement is a relatively new procedure. We do not have long-term data on customized knee replacement surgery. The current data that is available for the past few years shows that the custom knee replacement surgery results are satisfactory. The results of the custom knee replacement are equivalent to traditional knee replacement surgery.

The routine 3D printed knee replacements manufactured as of 2018 are cleared by the FDA. On occasion if I am using a customized knee replacement for a significant defect in the knee joint or for treatment of knee joint after an infection or for revision, then I have had custom knee implants made by manufacturing companies which have not been cleared by the FDA.

However, I use it in my best judgement for these patients. These non-FDA approved implants are extremely far and few. The last one I did was several years before 2018. The current manufacturing company that routinely makes custom knee implants was not in existence at that time.

If a knee is necessitated by the patient which is custom made and which is not FDA approved, the patient will be counseled about that custom implant in advance.

This non-FDA approved custom implant will be used on patients who are not candidates to get routine off the shelf joint replacement implants and who are not candidates to get routine custom made knee implants. There is nothing in the market that can be used in these patients which is why I custom make these implants in the first place.

Custom knee implants are made of cobalt-chromium-molybdenum alloy and the plastic is medical grade plastic, which is ultra-high molecular weight polyethylene which is cross-linked and also can be vitamin D infused. This is also the standard material used in off-the-shelf implants.

 Custom 3-D knee replacements do not cost more than off-the-shelf implants. There should be no additional cost to the patient.

The customized knee replacements are customized for individual patients. The custom knee implants that I use are not available separately for women and men. There are no separate sizes for the implants that cap the thigh bone and the shin bone. It is just one customized implant for one patient.

As of 2018, there is no implant company that pays me for anything! I do not get paid by any hospital nor does any pharmaceutical company pay me. I work only for my patients.

Customized knee replacements are largely divided into the following:

1. Total knee replacement which is posterior cruciate “substituting”.
2. Posterior knee replacement which is posterior cruciate “retaining”.
3. Bicondylar replacement with replacement of only one compartment of the shin bone called as the “iDuo” and then there is
4. The unicompartmental customized knee replacement prosthesis.

A CT scan of the knee is needed to make a computerized model for the manufacturing company to make a customized knee implant. In addition the patient needs blood tests which are routine before any joint replacement surgery. The patient may need additional tests like cardiac stress tests that may be needed by the internist to clear the physician for surgery.

The time to recover from a customized knee replacement surgery is almost identical to traditional knee replacement surgery. The hope is that because the new prosthetic implant matches the anatomy of the native bone, the long-term satisfaction scores will be improved- this is the real advantage of customized knee replacements.