David R. Lionberger, MD

Patient Education: MIS Total Knee Replacement

Excerpts from University of Texas Grand Rounds – February 2006

Traditional knee replacement surgery and minimally-invasive total knee replacement surgery have the same endpoint—namely to provide the most amount of function and the least amount of recovery for the patient. They both involve an incision over the patella, into which the replacement is inserted and cemented or press-fit into place. However, many of the differences exist when technology takes yet another step forward by a technique to prevent tissue damage referred to as less invasive or minimally-invasive surgery (MIS).

MIS uses a small exposure to perform the surgery. Although this is more time consuming, the benefit to the patient may be less pain, less blood loss, better range, earlier hospital discharge, and faster recovery. By avoiding the quadriceps muscle through a “quad-sparing” incision (the front muscle of the knee), scar formation, bleeding, and postoperative pain can be reduced. MIS surgery also brings its own set of challenges due to the reduced visibility from a smaller incision.

A number of reasons are at work with regards to this type of system. Alignment bars or rods used to calculate positions of the traditional knee replacement systems allows for the replacement to be performed with fair accuracy. This was the older method of performing a total knee replacement surgery. Many of the complications from knee replacement surgery are an indirect result of this particular maneuver in that they increase the bone pressure producing microembolism of the bone marrow, forcing air and/or fat during the knee replacement surgery into the circulatory system. Microembolism is often a subtle problem that can cause temporary confusion, mild breathing trouble, and occasionally respiratory distress following surgery. If these symptoms can be eliminated or at least reduced by the use of the CAS system, while not sacrificing any type of accuracy, the computer provides the surgeon with precise updates on orientation and position without using these older invasive instruments.

Damage to the patella is also lessened by lateral movement rather than dislocation positioning done in the older style incision. Intra-articular pain blocks linked to pre and postoperative pain relievers complete the full circle of pain control, making possible same day ambulation by most patients and promoting discharge in approximately 1-2 days.

However, the greatest timesaver begins after discharge from the hospital. In the review of my knee replacements from the previous year (over 300), patients’ rehab times averaged a little over 5 weeks to completion, compared to the older-style knees during the previous years, which averaged over 10 weeks.

MIS in total knee replacements is not for everyone. Simply put, this technology is best used in patients with a high propensity for motivation in rehabilitation. Yet, what is also important to remember is if you wear a size 32 belt, you will get a size 32 incision. A small incision is not as important as lessening damage to muscles and soft tissue, which allows for a rapid, less painful rehabilitation. “Reduced body mass” is a term you may hear us use to rank a patient’s appropriateness for MIS knee replacement. To protect patients from infection, we may provide them with a goal weight to reach before scheduling surgery. In one series, patients who carried extra weight beyond a body mass of 35% had six times the incidence of infection! We want to make sure your replacement goes as smoothly as it can and the above measure will help you achieve that goal.

That is why our staff will send you through preconditioning exercises, have you take a special preoperative medication to pre-block pain as well as use analgesics during your stay in the hospital. We strive in our office to make all efforts to shorten recovery, yet ultimately this comes down to the patient's own enthusiasm. It is also important to be thin and not overweight.

Other technologies have the chance at improving outcome even more by computer navigation.

--Revised by David R. Lionberger, MD, 2006

Computer-assisted Surgical Navigation: The Definition

Computer-assisted surgery (CAS) of the knee is a new technique used to optimize accuracy. Although CAS joint replacement is identical to the traditional knee replacement, in many ways, exposure and time are fundamentally different. Computer systems are designed with a variety of different applications and qualities. Many of the older systems rely on infrared (IR) tracking, which like a GPS, calculates geometric position. Older IR systems require fixing frames around the knee to the lower femur (above the knee) and upper tibia (below the knee). These reference frames are applied to the bone by small pins. Knowing where the positions of the surgical instruments and anatomy during surgery allows the system to make an accurate calculation as well as predictions about all important aspects of the knee replacement surgery, including size, position, alignment, motion, and ligament balance.

A newer and more exciting variety of this is electromagnetic tracking systems. This has a distinct advantage in that it can look around a corner or through non-metallic objects. In a surgical application, this has definite advantages. When performing minimally-invasive surgery (MIS), the surgeon’s vision is significantly limited, making a navigation system extremely useful. That is why CAS and MIS belong together. With a small window to work through, CAS gives enhanced visibility and accuracy, which is arguably better than the older version of TKR that required long insicions. One of the most important is the fact that the trackers lie inside the incision, next to the bone rather than anchored through multiple skin incisions. This becomes the perfect CAS system for small incision surgery such as MIS since it does not require the surgeon to enlarge the incision as in older IR systems.

CAS provides numerous additional advantages beyond accuracy. For example, in older traditional knee replacements, alignment bars and rods are used to align the end of the thigh bone (femur) by pushing metal rods into the bone to gauge alignment. Many of the complications from knee replacement surgery are an indirect result of this maneuver in that they increase the bone pressure producing microembolism of the bone marrow, which may force air and/or fat into the circulatory system. Microembolism is often a subtle problem that can cause temporary confusion, mild breathing trouble, and occasionally respiratory distress following surgery. If these symptoms can be eliminated or at least reduced by the use of the CAS system, while not sacrificing accuracy, it is felt that this may lessen some postoperative complications.

Why is Accuracy Essential?

There are many reasons for why accuracy in performing total knee replacements (TKR) is important. First and foremost, the properly replaced knee when done precisely is more likely to perform better. To the owner, this has obvious ramifications which start the day of surgery. The knee replacement will function better, rehab better, and feel better. However, an even more important aspect of accuracy is the longevity of the implant. Simply speaking, inaccuracy begets failure. If a knee replacement is placed in proper alignment and orientation, it is likely that it will last longer. Even though a TKR may feel and function properly for many years, there is ample evidence in the medical literature of premature failure in improperly aligned TKR. For example, if an alignment is off by more than 3º, there is double the amount of load on the plastic insert. As time goes on, the history of revisions in TKR suggest that at least 25% of failures occur as a result of malalignment.

Surgeons are only human and as such, are subject to their own set of inaccuracies. Even though many orthopedic surgeons specializing in joint replacement perform hundreds of TKRs per year, 20% of the time, we miss the exact alignment despite our best efforts. This is where CAS comes to the rescue. It can reduce (but not eliminate) the inaccuracy to about 5-8% instead of 20%. It makes us better surgeons. Despite countless articles supporting CAS in reducing surgical errors, the insurance companies and Medicare still do not reimburse surgeons for either the extended time and effort or the cost of supplies. Herein lies the ethics lesson; the best time to get a total knee correctly is the first time. As such, I will always encourage (but not force) patients to allow me to use navigation in every total knee replacement because I believe it to be the best for my patients. As long as the research account and/or the hospital supports this, and as long as I can pay the $850.00 per tracker kit fee, future knee replacements will continue to be available for other patients. It is hoped that surgeons will someday be paid for using CAS, however, this is not likely until 2008. Until then, generosity and grants for research are what keep this technology available.

--Written by DRL, 2006

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