Chapter 19B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
Knee Disarticulation: Prosthetic Management
John W. Michael, M.Ed., C.P.O.
Knee disarticulation is an uncommon level for amputation in North America and hence is seldom encountered by the physician or prosthetist. Most demographic surveys suggest that the frequency of knee disarticulation in the United States is between 1% and 2% of all major lower-limb amputations. The incidence appears to be similar in many other countries but gradually increases to about 20% in centers that advocate this level, such as those in Copenhagen and Toronto.
Batch and colleagues began their 1954 paper with a statement that aptly summarizes the contemporary situation, which has apparently changed very little over the intervening 38 years:
"Disarticulation at the knee joint has been periodically extolled as the procedure of choice over amputation through the thigh. Convincing arguments and statistics have been presented to support this view, but the procedure has not been generally accepted because of the belief that the stump is unsightly and that the prosthesis is difficult to fit, and because of lack of experience with the procedure."
Although immediate postoperative fitting with a prosthesis is technically feasible, it is rarely provided today. This may have more to do with the unfa-miliarity of this level of amputation to both prosthetist and surgeon than any other factor. Soft gauze dressings followed by elastic bandaging once the wound has healed is probably the most common approach. However, knee disarticulation can be managed by the same range of techniques as more familiar amputations. These include the use of an inflated air splint (with or without a walking apparatus over it), Unna's paste semirigid dressing, and a novel polyurethane foam dressing that is formed directly over the residual limb between layers of stockinette. This latter technique (Neofract) incorporates a full-length zipper for easy dressing removal. While immediate ambulation on a plaster of paris socket is seldom advocated for the dys-vascular individual today, some opt to use the postoperative plaster cast alone, preferably applied by the surgeon in the operating theater. Those who advocate such a rigid dressing believe that it enhances wound healing and facilitates rehabilitation.
Although the biomechanics of the transfemoral (above-knee) amputation are well known, the principles of knee disarticulation biomechanics are less commonly understood. The basic problem at midstance is similar: to stabilize the superincumbent body mass during single-limb support on the prosthesis (Fig 19B-1.). The resultant socket stresses, however, are significantly different from those at the transfemoral level. Successful knee disarticulation surgery is predicated on the ability to comfortably tolerate full end weight bearing on the residual limb. Once this is accomplished, there is no need for any proximal weight bearing, and ischial contact is superfluous. As is the case in all end weight-bearing amputation stumps (e.g., the Syme level), the effective center of rotation in the socket is at the distal-most aspect. Thus, during ambulation the femur will remain stationary while the proximal socket borders will press against the soft tissues of the upper part of the thigh (Fig 19B-2.,A). In contrast, the center of rotation for the transfemoral socket is located near the ischium, so ambulation causes rotary forces at the distal part of the socket (Fig 19B-2.,B). Since the transfemoral socket is primarily pelvic bearing, the femur is relatively unconstrained and tends to displace within the soft tissue distally. In the knee disarticulation socket with end weight bearing, the femur is effectively stabilized by body weight. In many respects the knee disarticulation socket is the inverse of the transfemoral type: the distal portion supplies precise weight bearing, while the proximal aspect provides prosthetic stability.
Suspension in the knee disarticulation socket is ideally provided by intimate fitting just proximal to the condyles. One approach is to fashion a removable plate that permits the bony condyles to pass during donning but locks them securely in place when fastened with a Velcro closure Fig 19B-3.). It should be noted that particularly for obese or muscular individuals, it may be extremely difficult to secure adequate supracondylar suspension. This is especially true for the initial fitting when postoperative edema may be present. In such cases, it may be necessary to augment suspension with suction or a Silesian belt variant (Fig 19B-4.).
The biomechanics of prosthetic knee function are directly analogous to transfemoral principles: the amputee must use hip musculature to compensate for the loss of active knee control. Because the center of rotation is just above the knee mechanism and because the bony lever arm is full length with undisturbed musculature, it is easier for the knee disarticulate to control the prosthetic knee mechanism than if he were a transfemoral amputee.
A number of socket variations are possible and differ primarily in their adjustability and cosmetic appearance. Since knee disarticulation in children preserves the distal femoral growth plate and eliminates the risk of bony overgrowth, the prosthetist will encounter a significant percentage of pediatric cases. Zettl has described a segmental socket design with independently adjustable proximal and distal portions to accommodate linear growth in children. Suspension is achieved by adjustable supracondylar straps (Fig 19B-5.). Because immature condyles are not very bulky, it is sometimes possible to create a self-suspending socket where flexible inner walls barely allow the condyles to pass (Fig 19B-6.). This approach may also be feasible when the condyles have been surgically trimmed. Because such sockets are extremely difficult to fit and allow no adjustments, they are only rarely encountered.
It is generally assumed that the affected femur will not grow as rapidly as the uninvolved one and therefore by adulthood the residual limb will retain all the positive aspects such as full end bearing and self-suspension but will terminate far enough above the anatomic knee center to present as a very long transfemoral amputation. This would obviate the prosthetic problems of limited cosmesis and restricted choice of knee mechanisms that are inherent in knee disarticulation. However, such differential growth is not inevitable. Weiner has reported three cases where the femurs remained identical in length (even though amputation occurred as early as 1 year of age) and suggested epiphysiodesis just prior to the end of growth.
Another socket option for this level is the traditional anterior lacing design. Although originally developed for molded leather sockets (Fig 19B-7.), it can be easily adapted to modern flexible plastics. Despite being somewhat cumbersome to don and doff, it has the advantage of accommodating moderate volume fluctuations in the residual limb. Botta of Switzerland is one of the leading prosthetic advocates of knee disarticulation fittings. With two decades of experience involving several hundred cases, he advocates a carefully molded distal liner to protect the condyles and provide suspension (Fig 19B-8.). When combined with a socket that is rigid distally but gradually changes to flexible at the proximal edges, he reports good success even with geriatric and bilateral amputees (Fig 19B-9.). Because the polyethylene foam inner liner can be readily adjusted to maintain snug pressure over the condyles, marked distal atrophy occurs over time. This ultimately improves cosmesis for fleshy individuals since the residual limb dimensions become smaller than the uninvolved thigh (Fig 19B-10.). Kristinsson of Iceland has reported success with a variation using a flexible rubber cup that terminates just above the condyles to provide suction suspension. Since the patient pushes his condyles into the suspension cup to don the prosthesis, this version is termed the Icelandic Push-on Suction Socket (ICEPOSS). The majority of the thigh is not covered by any socket materials; an adjustable circumferential band is attached to a medial strut to provide stability (Fig 19B-11.).
There are two primary approaches to taking the plaster of paris cast impression for knee disarticulation sockets. Lyquist suggests a weight-bearing procedure by placing a foam pad beneath the cast while the plaster is wet to form the transcondylar contours. This technique is difficult for some geriatric or bilateral amputees to manage. Botta and Baumgartner advocate a non-weight-bearing method with the amputee supine and stress meticulous hand molding of the femoral condyles. Their technique is applicable to geriatric as well as other knee disarticulation amputees. Modifications for both approaches are similar and consist primarily of establishing the supracondylar contours necessary for suspension and altering the proximal thigh region to provide stability during stance phase. Relief is usually provided for the lateral posterior femoral condyle. Most knee disarticulation sockets are laminated of reinforced plastic resins, but thermoplastic materials are also being used successfully, particularly for geriatric individuals.
Despite its functional advantages, knee disarticulation significantly restricts amputees' options in prosthetic knee mechanisms and results in cosmetic compromises in addition to reduced durability when compared with transfemoral levels. As a recent text notes, "Until about fifteen years ago none of the prosthetic knee mechanisms available could meet even reasonable cosmetic requirements, nor could they meet the functional demands of the young active amputee."Most available knee units are designed for transfemoral amputation; when used for knee disarticulation, they protrude as much as 2 in. beyond the anatomic knee center. Although this causes no significant gait deviations, it results in a decidedly bizarre appearance that most find objectionable. It also makes sitting in confined spaces such as automobile and theater seats difficult since the thigh segment juts out so far.
From the time knee disarticulation was first reported by Fabricius Hildanus in 1581 until the early 1970s, the only available knee alternative was external hinges similar to those used on knee-ankle-foot orthoses. Because these hinges transmit weight-bearing forces in the knee disarticulation socket (in contrast to their orthotic application), lack of durability has been a chronic problem. In addition, external hinges offer no swing-phase control. Since disarticulation retains full femoral leverage and thigh musculature, this is a significant disadvantage. One manufacturer provides a yoke attachment permitting use of a fluid-controlled cylinder with these hinges, but durability remains a concern (Fig 19B-12.). Although external hinges result in the least possible protrusion of the thigh segment when sitting, a somewhat wider mediolateral dimension is the inevitable result (Fig 19B-13.). Many find the bulky appearance objectionable despite specialized finishing techniques to minimize the discrepancy.
The only other available alternative for knee disarticulation is the polycentric knee. Greene has published an excellent paper discussing the four-bar class of poly-centric mechanisms, including those designed for knee disarticulation. As he notes, it is possible to analyze the function of four-bar polycentric knees through geometric analysis. The intersection of the anterior and posterior knee links define the instantaneous center of rotation: the effective point of rotation of the knee mechanism (Fig 19B-14.). At heel strike, the four-bar knee behaves as if it were a single-axis knee articulated at the instantaneous center of rotation. Because the instantaneous center is both proximal and posterior to the anatomic knee center, such mechanisms are very stable (Fig 19B-15.). Like the human knee, the locus of rotation of the four-bar polycentric knee changes with the flexion angle (Fig 19B-16.). This has two major effects on amputee gait. One is that the effective length of the shin shortens with increasing knee flexion. The second is that the shin automatically decelerates late in stance phase as the instantaneous center of rotation moves proximally back to its original location.
The first polycentric knee designed for knee disarticulation applications was developed at the Orthopedic Hospital Copenhagan (OHC) in 1969. Careful design of the linkage arms results in a mechanism that appears to fold back under the thigh when sitting, thus minimizing the protrusion of the knee (Fig 19B-17.). It is available with either mechanical swing-phase friction control or hydraulic swing-phase control to allow a varying cadence for more active individuals (Fig 19B-18.).
Several European and Asian manufacturers have developed similar polycentric designs during the past decade. Some are available in lightweight versions of carbon fiber or titanium. Locking modules are also available for those who unable to manage a free knee (Fig 19B-19.). Due to the inherent stability of polycentric knees, manual locking is only rarely necessary.
With the widespread availability of polycentric mechanisms, the cosmetic liability of knee disarticulation has been reduced, although not completely eliminated (Fig 19B-20.). Durability has also improved over that of external hinges, but specialized knee disarticulation joints are not as rugged as conventional transfemoral mechanisms. Foot and ankle mechanisms are selected for knee disarticulation prostheses by using the same criteria that apply to transfemoral and higher levels (see Chapter 18B, Chapter 20B, and Chapter 21B). Some advocate consideration of feet with elastic keels and/or tranverse rotation units to absorb some of the stress of ambulation since this is believed to reduce stress on the somewhat fragile knee mechanisms.
Although knee disarticulation remains uncommon in most of North America, it has strong proponents in many quarters. There is general consensus that it is functionally superior to higher-level amputation provided that full end weight bearing is achieved. It is undoubtedly the transfemoral level of choice for children since it preserves the distal epiphysis and avoids bony overgrowth. The cosmetic liabilities and knee mechanism shortcomings have been significantly reduced through the development of specialized four-bar polycentric designs, but prosthetic options, appearance, and durability are still somewhat compromised when compared with the transfemoral levels.
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Chapter 19B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles