Finding the Right Fit For Knee Replacements

Orthopedics

By: Richard Berger

Published: 10/7/2020

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Understand the keys to delivering successful outcomes with the newest generation of implants.


I replace about a thousand knees a year and treat each one as a unique case when restoring the joint's normal range of motion and stability. Soft tissues guide me to how the knee was positioned before the degenerative effects of osteoarthritis ?changed the bone alignment. Based upon how the knee moves with all the muscles, ligaments and tendons intact, I determine how the native knee was aligned, and adjust placement of the implant to match it. I've developed micro-shims to move the knee around by minute amounts — up and down, right and left, back and forth. After I place the implant and put the knee through an entire range of motion, the joint's surface areas remain in perfect contact throughout. This leads to a more stable joint, which results in less pain for the patient and better post-op function.

The key is getting knee replacements right the first time because fixing a poorly done knee never quite works out. Surgeons can improve how it feels and functions, but the joint will never be as good as it could have been. Achieving positive, patient-pleasing results demands understanding several factors that impact outcomes.

  • Cement vs. cementless fixation. There's a revival of cementless fixation in knee implants, which concerns me because I saw the problems of the first cementless knee wave decades ago. In certain areas of the body, cementless fixation works great. For example, most hip replacements involve cementless fixation. It's easier and faster, but more importantly, it's beneficial to the patient. Bone almost always ingrows into the implant, and remains ingrown for a long time. The technique retains bone stock better than cement does.

Cementless fixation of the knee, however, is associated with many problems. The blood supply around the knee isn't as good as it is around the hip, a factor that leads to a poorer ingrowth rate. Early after their surgeries, some total knee patients experience lack of ingrowth: 1% or 2% of patients on the femoral side of the knee and 3% to 5% of patients on the tibial side, which is a lot.

INDIVIDUAL APPROACH ?Dr. Berger customizes his standard multimodal pain management framework to optimize recovery for every patient.   |  Midwest Orthopaedics at Rush

Then there's the difficulty of diagnosing what's wrong if the patient isn't doing well and is unhappy after surgery. With cementless hips, it's easy; you take an X-ray, and because of the anatomy you can see if implant components are ingrown with bone. With cementless knees, it's exceedingly difficult, if not impossible, to know if the implant is ingrown. Maybe the patient is experiencing post-op discomfort because the surgeon didn't align the implant properly or its components are loose. Perhaps the patient developed scar tissue. There are countless possible reasons, but you can't see them on the X-ray. Surgeons are often forced to reopen the joint to assess the situation.

Another problem with cementless fixation: Wear from the implant pieces gets into the bone and causes destructive osteolysis. Cement acts as a barrier to those particles. With cementless fixation, that barrier is gone, so the patient is more likely to develop osteolytic lesions. If the patient ever needs a revision surgery, the bone behind the prosthesis remains stronger with cement fixation.

Cementless fixation for knees went away when these problems became evident. Its recent comeback is mostly about the push for "quicker, easier, more efficient." You don't need to mix the cement, wait for it to dry and clean it up, so it saves up to 30 minutes a case. However, patients pay the price. Cementless fixation on knees helps the surgeon more than the patient.

  • Preventing infections. Surgical site infections are a terrible problem, but fortunately they occur in less than 1% of cases. It's always top of mind for surgeons because when an infection occurs, it's a disaster for the patient and needs to be addressed quickly. The infection is sometimes introduced intraoperatively; sometimes it's a local invasion, meaning the incision didn't close quickly enough and bacteria entered; sometimes it's hematologic. Once bacteria get into a knee replacement, the body has a hard time fighting it. Best case, you reopen the knee, thoroughly wash it out, and administer IV antibiotics for four to six weeks. Sometimes, though, you must remove the prosthesis, so the patient has no knee function until the infection goes away, and before reinsertion of a new implant a few months later.
  • Custom implants. I helped design the gender knee implant a decade ago and use it for all my female patients. Its slightly different, narrower contour better matches the female anatomy. We did a great study looking at digitally mapped cadaveric femurs, and unsurprisingly, there's a significant difference between men and women.

Gender is a binary choice, though. What about fully customized implants? Well, they sound great, but in my experience they're associated with some challenges because they're customized based upon a scan of the arthritic, deformed knee. Another issue: You get one implant piece. If you don't like the fit, too bad. I find it much better to have hundreds of combinations of implants to fit each patient in the OR. You can't do that with custom implants.

  • Robotic assistance. You can use a robotic navigation platform with a cutting system to perform knee replacements. It's powerful stuff, but I don't use robots, because the robot's not perfect. It simply does what many surgeons do — put the knee back right where it was with arthritis. Maybe one day we'll have enough algorithms and data for a robot to match pre-arthritic knees, but we're not even close. Numerous prospective randomized studies show it performs to the average.

Some surgeons who perform knee replacements do less than 20 a year and never master the techniques and methods needed to become experts. A robot can help these low-volume surgeons. If their skills are below average, the technology will help make them better. In my opinion, accomplished knee surgeons who have mastered manual techniques don't benefit from using robotic assistance, and opt to stick with tried-and-true methods that they have confidence in and that they know work.

Clear expectations

SEE IT CLEARLY ?Because he uses cemented fixation for knee replacements, Dr. Berger can easily see in an X-ray if implant pieces are loose. That's not the case with cementless fixation.   |  Midwest Orthopaedics at Rush

Today's implants probably last about 25 million steps. An average person takes between one and two million steps a year — but not all steps are the same. A six-foot-eight, 400-pound man has a different step than a grandmom who's five feet, 100 pounds. There's not an exact answer. I tell patients that if you take two million steps a year, they'll hopefully get 12 years out of a new knee.

Some studies report up to 25% of patients are unhappy with their knee replacements to the point where they wish they never had the surgery. I'm pleased to report that 98% of my patients are happy with their knees and would have the operation again. With unsatisfied patients, the problem is often mismatched expectations. If a patient isn't very mobile but enjoyed running 20 years ago, and wants to run again, the chances of them running after surgery are almost zero. I talk to patients about expectation mismatches, but they don't always completely understand. That's why it's important to lay it all out for them in clear language by describing exactly how their new knee will function after surgery and the life activities in which they can expect to participate. OSM

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