Knee Disarticulation Adds Prosthetic Advantages
Joint disarticulation at the knee, ankle and hip level ranks with the more controversial practices of amputation and prosthetic management. Knee disarticulation has been an option for more than 180 years and offers many benefits, but because the procedure also elicits design and cosmetic challenges, surgeons and patients more often opt for a higher level (and usually less-functional) transfemoral amputation. Thus, knee disarticulations have been estimated to account for only 2 percent of limb loss in America. However, recent studies suggest the inherent functional advantages of this amputation level are translating into growing acceptance among surgeons and prosthetists.
From a prosthetic standpoint, knee disarticulation may be the best choice for certain patient groups, notably children and trauma and cancer patients whose tibia cannot be saved but the femur is whole with good tissue for padding. Here are some key advantages:
• Because knee disarticulation leaves the femur intact with overlying soft tissue, the residual limb can usually tolerate distal (or end) weight-bearing, a key improvement over a transfemoral amputation in which pelvic structures must provide most of the support. When the femur can in fact accept weight-bearing, the prosthetist can design a lower profile socket with potentially greater comfort.
• No bones or muscles are cut in the surgery, so strength, muscle tone and balance are typically good. The intact femur provides a long mechanical lever powered by strong muscles for effective ambulation, better sitting balance and leverage.
• Growth plates at both ends of the femur are preserved, a particular advantage for child patients. Moreover, the bony overgrowth common in children with a transection is usually eliminated.
• By preserving the femoral condyles, a knee disarticulation provides a prominent base from which to suspend the prosthesis and help in controlling unwanted rotation.
• As compared with a transfemoral socket, which normally must extend up to the ischium for weight-bearing, the proximal end of a knee disarticulation socket fits much lower on the femur and can be made of softer material, providing substantially more comfort both standing and sitting.
…And the Drawbacks
On the other hand, the long residual limb with its condylar protuberances carries several disadvantages:
• The bulbous distal end of the residual limb typically requires a special socket design, sometimes including one or more cutout openings for donning.
• The socket with distal padding, attachment brackets and knee mechanism results in a long “prosthetic thigh,” which locates the prosthetic knee axis lower to the ground than that of the sound knee. (Little evidence exists, however, that this knee level difference is in fact physiologically or functionally harmful.)
• With the prosthesis applied, the residual limb may appear noticeably larger than the contralateral leg, presenting a self-image problem for some people. In fact, patients have been known to choose a higher-level amputation largely for cosmetic reasons.
The Socket — Because knee disarticulation patients can tolerate distal weight bearing to differing degrees and residual limb features can vary widely, socket design is highly individualized and depends on the knowledge and experience of the prosthetist.
Various approaches have been used to securely attach a knee disarticulation prosthesis to the intact femur. One popular design employs a flexible gel liner with air expulsion inside a rigid outer socket. The liner simplifies construction but precludes use of condylar suspension and minimizes the benefit of distal weight-bearing.
Some patients with a prominent medial condyle are candidates for a design incorporating a medial door and external strap without an inner liner, effectively reducing external distal socket bulk. Inclusion of inflatable pneumatic pads or silicone bladders can help overcome the difficulty of inserting condyles into a narrow socket but add to the wall thickness and socket complexity.
Knee components — Although relatively few prosthetic knee joints are built specifically for the knee disarticulation level, some of the many knees designed for transfemoral applications can be adapted for a K.D. limb. The problem with these knees is that they most always result in a lower knee center than the sound limb and thus exaggerate the apparent thigh length differential.
While single-axis knees incorporating friction, pneumatic and hydraulic control mechanisms can be used, polycentric designs provide a more proximal knee center and achieve better toe clearance in swing phase. Polycentric knees optimized for knee disarticulations minimize the attachment space beneath the socket and fold under during flexion to minimize the appearance of thigh length discrepancy.
In general, the knee disarticulation offers significant prosthetic advantages and thus is worthy of consideration for many patients facing a lower-limb amputation.
Please call LeTourneau Prosthetics today for your free consultation. Our offices are located in Beaumont, Nederland and Jasper. Many patients in the Southeast Texas area travel to see us from the surrounding areas including Port Arthur, Orange, Silsbee and Liberty, TX.