Computer-assisted knee replacement

Total knee replacement surgery isn’t new. It has been practiced worldwide for 40 years, and as might be imagined, the procedure has been refined to the point where hundreds of thousands of people every year are returning to a life of pain-free mobility. However, standard knee replacement surgery has its limitations-the laws of physics being chief among them. A surgeon must implant the orthopedic device in such a manner that its components-a metal and plastic platform atop the tibia and a metal surface on the bottom of the femur-rub together, or “articulate,” at precise angles in order to prevent premature or excessive wear of the implant.

As is commonly practiced today, a surgeon achieves proper alignment through “feel.” That is, he uses specialized cutting blocks combined with his years of operating room experience to determine where best to remove bone for the implant. Once the cut is made, the natural bone cannot be replaced. Thus, a carpenter’s advice to “measure twice, cut once” is especially vital in knee replacement surgery.

The new solution

Computer-assisted surgery addresses this issue of alignment with an advanced convergence of multiple medical technologies. Using infrared cameras, images and advanced tracking devices, Smith & Nephew’s computer-assisted knee replacement procedure achieves precise alignment.

How it works

The logic is simple: By combining digital images of the femur and tibia with an implant-specific software package, the computer hardware can track the precise position of the patient’s knee and the surgeon’s instruments at all times during the procedure. It is as if the patient’s leg has a satellite tracking system the computer uses to follow it during surgery.

Do not fear. The surgeon performs the surgery. The computer simply puts together all of the information coming in from the patient and the instruments and tells the surgeon where the precise cut should be made. Given that every patient’s knee geometry is different, this level of patient-specific, computer-guided accuracy is unprecedented in the history of knee replacement surgery.

Patient Benefits

As you might imagine, a surgeon armed with these tools has the potential to achieve better outcomes for the patient.

As the computer-assisted procedure evolves, it will become less and less invasive. It has already eliminated the use of an intramedullary (IM) rod; a device inserted up the length of the femur used for determining proper knee implant alignment in relation to the hip joint. Since the data generated by the computer replaces this device, patients undergoing computer-assisted knee surgery may have a reduced risk of fat embolism, caused when the IM rod forces body fat into the patient’s blood stream. If fat travels through the blood stream, it could become lodged in the heart or brain and cause heart failure, dementia or stroke.

Further, the quality and accuracy of the virtual image provided to the surgeon by the computer enables smaller incisions while achieving the same successful outcomes. Smaller incisions lead to faster surgeries, shorter hospital stays and shorter rehabilitation.

The benefits to the patient include:

  • The elimination of the IM rod reduces the risk of fat embolism.The increased “vision” the procedure provides facilitates minimally invasive surgery. This means a shorter scar, less physical therapy and a faster return to your normal life.
  • The accurate alignment and placement of the implant may extend its lifespan and prevent future corrective surgeries.

Since the computer accurately assesses “joint laxity,” or soft tissue balance, the surgeon may not need to disrupt as much soft tissue-such as muscle, ligaments and tendons-when determining how tightly the new implant fits in place. This also reduces rehabilitation time and returns you to your active lifestyle more quickly.

The Equipment

cas_kneeThe Arrays

“Arrays” are metal prongs with small reflective spheres at their extremities. These devices attach to the surgical instruments and to the patient’s tibia and femur.

Once they are fixed in place, their positions in space are tracked throughout the procedure. That way, the computer will know exactly where the instruments are

in relation to the patient’s bones, based on where the spheres are at any given second.

The Camera

The camera emits infrared light that reflects off the spheres connected to the arrays. It collects this reflected infrared light, and sends the information about the location of the source of the reflection (the spheres on the arrays) to the computer.

The Computer

The computer receives information about where the spheres are in space, and combines that data with three-dimensional virtual images of the orthopaedic implants and surgical instruments.

The Software

The software displays on the screen the virtual images of the instruments, implants and bones and guides the surgeon through each step of the procedure.

The software alerts the surgeon when the instrument is in the most accurate position to make the ideal cut. Also, it helps determine where to best place the knee implant against the bones.