Knee replacements are carried out through one of several different surgical approaches. In practice, however, there is not as much difference between knee surgical approaches as there is between hip surgery approaches. There are advantages and disadvantages to each approach, and there is some controversy among knee surgeons as to which is the best. All surgeons have a favored surgical approach, and while there are often heated and spirited debates at academic conferences and meetings, it is a testament to the success of the procedure that all of them generally produce good results.
Recently there has been much interest in minimally invasive surgery for knee replacement, as there has been with hip replacement surgery. There is no widespread agreement as to exactly what constitutes "minimally invasive," but most surgeons would agree that the general principal is to have less soft tissue disruption and dissection. As with the hip, this often may translate into a smaller skin incision, but it is what goes on under the skin that is far more important for speed of recovery. The single most important difference for minimally invasive knee surgery appears to be decreasing the amount of muscle and tendon that is disrupted.
The muscles of the knee are shown, and the medial peripatellar
arthrotomy most commonly used is marked by the dotted line.
Note how it goes just along the patella and quadriceps.
Anterior Surgical Approach
Most of the surgical approaches for the knee have a similar skin incision, going vertically over the front of the knee. The reason for this is primarily that most of the important blood vessels and nerves are in the back of the knee and are avoided. It is also important to have an extensile approach, meaning that the incision can be extended upwards or downwards as needed.
The traditional anterior surgical approach goes straight down the middle of the knee. The tight capsule around the knee, called the retinaculum, is usually opened along the inner (or medial) side of the patella (knee cap). This is often called a "medial peripatellar arthrotomy," meaning an opening into the knee joint is made just along the inside of the patella. The patella is then moved out of the way to expose the knee joint. Traditionally, this involved everting (flipping) the knee cap, but recent studies have shown that patients rehabilitate faster if the surgeon takes care to simply slide the knee cap to the side. It does not offer as much visualization, but we find in our practice it is worth doing it this way in order to promote a faster recovery, and most surgeons who perform many knee replacements a year have a good understanding of the anatomy that facilitates such less invasive techniques.
On the surface, this incision also typically goes vertically down the front and center of the knee, and from the outside skin incision it is difficult to tell a difference between the various approaches described here. However, the subvastus approach, first described in 1929, employs a slightly different approach once under the skin layer. Instead of making an incision along the patella, this approach lifts the vastus muscle up and over the knee. It requires starting far along the inner aspect (medial side) of the knee and elevating the entire front muscle mass up and over to expose the joint. Advantages include less muscle disruption and possibly better patella tracking by leaving the extensor mechanism intact. However, disadvantages include less visualization and access to the joint, and because of the nature of the approach it is not well-suited for patients who are not thin. Access to the joint can be problematic if the patient is at all obese, has a tight knee or contractures, has significant bowing or deformity, or has had any previous surgery. For these reasons, it is not used as commonly as the anterior/medial peripatellar approach described above.
The midvastus approach is very similar to the anterior approach described above, except that in the muscle layer (the skin incision again is the same) the incision turns away from the center of the knee and avoids cutting into the quadriceps tendon. This approach gained popularity in the 1990's because it offers some of the advantages of both the anterior and subvastus approaches, with good visualization and access but better preservation of the extensor mechanism. Advocates of this approach believe that avoiding cutting into the quadriceps tendon leads to a rapid restoration of post-operative extensor mechanism function and knee range of motion. This is the technique most often used in our own practice, and we are advocates of its use in evolving minimally invasive techniques.
Previous Incisions and Scars
Previous incisions and significant scars have a significant impact on the surgeon's decision of exactly how to make the incision and approach. The primary reason for this is the blood supply to the skin and soft tissues around the knee. Although an old surgical scar may be long since healed, that previous scar has disrupted the blood supply. As a result, if a parallel incision is made too closely, there may be significant wound healing problems because of a diminished blood supply. In general, surgeons will typically either try to incorporate an old incision if possible, or make a new incision at a right angle to it to minimize the effect of the old scar.
Another consideration is scar tissue that may make the knee very tight. The surgeon needs to be able to get within the knee joint to work on the bone surfaces and put in a new knee replacement (and possibly remove an old one in revision surgeries). If the knee is too tight to allow this, then several options exist. The surgeon may disconnect the area of bone below the knee cap where the tendon attaches (called the tibial tubercle) and re-attach it at the end of the case, or alternatively the quadriceps tendon may be cut at the upper end of the knee and re-attached afterwards. While these techniques make the surgery possible, they do add significantly to the recovery time and often require that the knee be immobilized in a brace for weeks or months while the repair heals.
As noted at the introduction to this chapter, there is not as much variability in surgical approaches to the knee as there are to the hip. In fact, even among different surgical approaches, the outside skin incision typically will look the same (e.g., a vertical line down the front of the knee). The differences arise among the way that the underlying muscles are split.
As greater interest has evolved in minimally invasive surgery, a major factor has been changes in the instrumentation actually used during the surgery (retractors, insertion devices, jigs, etc.) that allow significantly shorter incisions today (and more importantly, less disruption to underlying muscle and tendons). Other changes with less invasive techniques have included not everting the patella throughout the case and minimizing damage to the quadriceps tendon.
Advances in knee surgery in recent years have mostly been evolutionary rather than revolutionary, given that the original designs over 30 years ago were functionally quite good, and refinements have been gradual. Materials science and engineering have advanced, and the knee replacement prostheses used today have a typical lifespan measured in decades.
Industry and implant companies have developed a number of different technologies in the past 10 years that allow the surgeon to use a computer system during surgery that is coupled with some sort of positioning system. Commonly, a set of video cameras are positioned around the operating table as the computer system's "eyes," and it monitors the position of instruments and the patient's bones by using some sort of landmarks (often markers that are fixed to the bone temporarily during surgery).
The value of computer navigation during surgery has been debated extensively in recent years among surgeons. Many surgeons and centers specializing in joint replacements have found that the added operating time for setting up the cameras, "registering" the patient's bones and anatomical landmarks, and using the software and navigation system adds significant operating time without clearly improved outcomes. There is concern that this may translate into increased complication rates from increased time on the operating table (e.g., infection, blood clots, etc.). At the time of this writing, some studies show it to be helpful while others do not.
Navigation systems may prove helpful to surgeons who do not frequently perform joint replacement surgery and assist in accurate placement of the components. However, it has been our experience and that of many other large joint centers that outcomes are not significantly improved with navigation. This technology will very likely mature in the years ahead to the point at which it is not as cumbersome, and computer navigation may become more commonplace. For now, some surgeons find it useful and others find it unnecessary.
Conclusion - Multiple Surgical Approaches
In summary, there are multiple surgical approaches for knee surgery just as there are for hip surgery, and there are also multiple surgeons who advocate one particular approach over others. However, surgical approaches to the knee actually differ very little from each other, without the significant differences seen between hip surgery approaches. At our center, we do a great deal of research and publication regarding minimally invasive surgical techniques and feel that the anterior approach has a strongly proven record of good outcomes over the past three decades, and we are currently using the midvastus technique for many minimally invasive surgeries, but there are proponents of all surgical approaches at various centers.
As with hip surgery, the best advice for the patient is to find a surgeon whom you like and feel comfortable with, be sure that he has good surgical outcomes and a significant volume of knee replacements (preferably multiple knee replacements each week, rather than several per year), and allow the surgeon to use the surgical approach and technique that he is most accustomed to.
Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.