Issue: May 2006
May 01, 2006
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TKR soft tissue balancing drives new interest in navigation

Morphing software now provides a way to assess ligament gaps and may help avoid knee instability.

Issue: May 2006
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Ask any surgeon who regularly does total knee replacement surgery and he will tell you that soft tissue balancing is half the procedure.

The other half is, of course, the bone cuts.

However, he or she will also say that soft tissue balancing during total knee replacement (TKR) comes with a unique set of problems — it is a subjective art that is hard to master and even more difficult to teach.

In response to such challenges, a growing number of doctors have turned to computer-assisted navigation technology. They seek systems with special software modules to help them do a better job at soft tissue balancing during TKR.

While several manufacturers now offer computer navigation systems for orthopedic surgery, not all currently provide the support for performing TKR soft tissue balancing. And among those that do, the options vary widely.

But, with the growing need surgeons have for help in this area, it is just a matter of time before systems featuring soft tissue balancing functions become widely available. “I’m sure everyone’s going to have it, because everyone’s realizing the same problem,” said Richard S. Laskin, MD, who presented information on taking a navigational approach to soft tissue balancing during TKR at the Knee Society’s Specialty Day Meeting in Chicago.

“I’m not the only one who’s realized that there’s a problem. A lot of people have looked at this,” he said.

photo
Using navigation during TKR has made soft tissue balancing an accurate science. Still needed: mid- and long-term outcome results in order to get a more definitive word on its effect on impacted implant wear and stability.

Courtesy of Stryker

Next wave

“I think it’s the wave of the future … to give us the most accurate insertion of total joints, which will help them last longer,” Joseph P. Pizzurro, MD, said of the new systems. “Most joints that fail, especially knees, do so because of malrotation or imbalance, or at least a significant percent. So this helps with those two aspects,” said Pizzurro, director of orthopedic surgery, Valley Hospital, Ridgewood, N.J.

Laskin, at the Hospital for Special Surgery (HSS), New York, told Orthopedics Today the need for longer lasting implants has drivena lot of this innovation. “At most of the meetings 80% to 90% of the talks are still on bony cuts,” and the published studies tend to follow suit, he added.

Nevertheless, the need to improve soft tissue balancing during TKR has long been recognized. Based on most revision data and those in the literature, “the common reason that implants failed is aseptic loosening from the bone or instability of the joint,” Laskin said.

Backing up that view: recently published findings from about 100 TKR revisions in Laskin’s patient database, where “the most common reason [for TKR revision] of patients I saw here was instability.” This failure mechanism might be among the reasons computer navigation is gaining in popularity as a soft tissue balancing tool. “The instability occurs probably about 30% of the time as the primary indication and a lot more times as a secondary indication,” Laskin said.

Subjective, not objective

The problem has never been that surgeons are not balancing the knees. They typically know how to do the balancing. They just needed a more exact way of doing it and knowing when to do it during TKR., Laskin said.

Today’s systems seem to help in both these areas. For example, a lot of the knee instability seen postoperatively and at follow-up likely results from the way orthopedic surgeons have traditionally determined whether the knee is stable during a TKR. “We do it really by feel,” Laskin said. That’s tough to reproduce consistently. Before computerized navigation, there had been no objective way to ensured the implant was truly stable and adequately balanced once the “feel” was right. Now, most systems available help the surgeons titrate the whole knee area a lot better, he said

Another area of controversy typically involved this question: At what point during surgery is the best time to balance the soft tissues and maybe do a ligament release? Many current systems have new software that helps with that, giving the surgeon on-screen cues about when the ligaments needs balancing. Some even offer suggested approaches.

How the systems work

Orthopedic navigation systems are now available from companies like BrainLab, Aesculap, OrthoSoft, Medtronic, Stryker and Smith & Nephew to name a few. They provide a variety of ways to more objectively check knee stability and balancing intraoperatively. Each offers different approaches, ranging from on-screen depictions of the tissues and ligament gaps to numeric readouts, of medial and lateral gap size.

At the most basic level, systems include tensioners or tools for measuring gaps.

Others go a step further and allow the surgeon to apply the intraoperative gap measurements in a way that is reflected in implant positioning, their resection, or both.

For example, in March, BrainLab introduced an integrated tensioner. The system feeds back to the VectorVision software the ligament force data obtained from a stress test done with the knee in flexion and extension. Any entensive valgus force is quantified and displayed on the computer monitor in the operating room so the surgeon can make needed adjustments.

BrainLab’s system also offers a kinematic analysis capability that tracks the femur on the tibial component.

Pizzurro said the Stryker knee navigation system software he uses provides posterior gap analysis and related readouts that help with soft tissue balancing. “It morphs [your patient’s knee] to models of knees that are in the computer and picks out a diagram of your knee, so you are seeing a knee on the screen also.”

The latest version displays how the knee looks after bone cuts.

Morphing approach

Laskin has used several types of computer navigation for his total joint replacement procedures over the last four years. They include systems requiring: fluoroscopic images to be taken in the operating room; separate computed tomography scans; and other approaches where line-of-site issues affect ease of surgery.

He prefers systems that include morphing programs, like the OrthoSoft system, which has an optional soft tissue balancing software package. They are helpful, particularly when putting a patient’s knee through a range of motion with the trial implant in place, Laskin said.

Lawrence G. Morawa, MD, said with a trial implant in place the Stryker system helps him clearly see whether it fills 20 mm or 21 mm. Then, he can also tell exactly what the corresponding measurements are at 90°, 60° or 30° of flexion and extension.

As for the growing numbers of users, manufactures confirm they are training more orthopedic surgeons in soft tissue balancing.

diagram
This diagram of a computer navigation screen shows a knee is in 30° of flexion and is stressed medially and laterally. Users can see that the lateral side opens up an additional 5 mm than does the medial side.

Courtesy of Richard S. Laskin

Aesculap officials said investigator Michael J. Fracchia, MD, Port Jefferson, N.Y., has used the gap-balancing feature of its OrthoPilot navigation system in more than 400 cases.

Morawa, with Oakwood Health Care System, Dearborn, Mich., has used Stryker’s knee navigation system to complete more than 600 TKR cases in the last four years. He told Orthopedics Today he appreciated the user-friendly changes the company recently made with version 3.1 of its software and the fact they paid attention to the areas of soft tissues.

They’ve “looked at the balancing part of it; not only getting the bone cuts correct, but to make sure that before you put your implant in or when your trials are in, that your cuts are parallel and that your gaps — your flexion gap and your extension gap — are balanced. … Because if you have the cuts right and the ligaments aren’t balanced properly, you’re going to end up being too tight or too loose on one side or the other.”

Without an objective and reliable way to balance the knee’s soft tissues during TKR, “another problem arises, and that involves the surgeon with 20 years of experience teaching the resident how to soft tissue balance by feel. There’s no way to teach them,” Laskin pointed out.

These new systems help because they provide on-screen confirmation for each step of the TKR surgery and give immediate feedback as to why you’re doing what you’re doing. According to Morawa, it’s a lot better than saying, “Well, that feels good,” when training a resident.

When doing the intraoperative kinematic studies and bringing the knee through the range of motion, “If you feel it’s lax, on the computer you can see the differences and you will know to change it to a different size,” Pizzurro explained. It’s now easy to show those subtle nuances to someone else.

Balance, release guidance

But surgeons still debate when it is necessary to perform a soft tissue release and to what tolerances. To many, 0° to 5° of varus/valgus in full extension is acceptable. To solve that dilemma, Laskin is working with developers at OrthoSoft to fine tune that even further based on results of a Belgian cadaver study. Researchers found that in 65-year-olds — the average age at which most TKRs are done — the knee opens up about 5° in full extension, but the medial and lateral sides don’t open up the same amount.

He expects OrthoSoft will soon incorporate more normative data, like this, into its navigation system via built-in help screens. That should ultimately translate into even better long-term results. “My gut feeling is that is what’s going to convince people to use computer navigation,” particularly surgeons who do not perform large numbers of knee replacements, Laskin said.

Other enhancements are in the works. “We’re advancing now and looking at actually putting a sensor [in the knee joint for] … a pressure readout to balance that even more accurately than what we’re currently doing,” Morawa said. Though it is still being worked on in the laboratory, in clinical use such an approach might one day help ensure there is appropriate pressure across the joint.

Laskin is a paid consultant to Smith & Nephew.

For more information:
  • Laskin RS. Computerized navigation in total knee replacement past, present and future. Presented at the Knee Society Specialty Day. March 25, 2006. Chicago.
  • VanDamme G, Defoort K, Ducoulombier Y, et al. What should the surgeon aim for when performing computer-assisted total knee arthroplasty? J Bone Joint Surg Am. 2005;87:52-58.