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Chapter 19A - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles Knee Disarticulation: Surgical Procedures
Michael S. Pinzur, M.D.
When compared with transfemoral amputation, knee disarticulation (through-knee amputation) has the potential benefits of (1) durable end weight bearing (direct load transfer); (2) retention of a long, powerful, muscle-stabilized femoral lever arm; (3) ease of prosthetic socket suspension due to the bulbous end; (4) decreased surgical blood loss and; (5) resistance to infection by maintaining the cartilage barrier to infection.
Its use in growing children has generally been confined to the treatment of congenital anomalies, malignant tumors, nonsalvageable trauma, or infection. Knee disarticulation maintains femoral length in growing children by preserving the growth potential of the distal femoral epiphysis. It also avoids the risk of appo-sitional bony overgrowth inherent in pediatric transosseous amputation. The weight-bearing capacity of the distal end of the femur allows fabrication of a prosthetic socket with direct load transfer. The retained femur tends to grow at a slower rate than the contralateral femur, eventually allowing the prosthetic knee joint center to approach the same level as the normal knee.
This amputation level is infrequently used in adults for both cosmetic and functional reasons. The residual limb is perceived to be unsightly due to its length and distal bulbousness. If a standard transfemoral (above-knee) prosthetic knee joint is used in a knee disarticulation prosthesis, its knee center will be far distal to that of the contralateral normal knee center. External knee hinges improve sitting cosmesis somewhat but are cumbersome, unsightly, somewhat unstable, and often damage overlying clothing. Development of the polycentric prosthetic knee joint has allowed the prosthetic knee joint center to approach that of the normal knee, thereby smoothing out the gait pattern and making the procedure a more reasonable option in adults.
USE IN THE NONWALKER
Nonwalking patients often develop knee flexion contractures following transtibial (below-knee) amputation or hip flexion-abduction contractures following transfemoral amputation due to muscle imbalance. Knee flexion contracture may lead to distal stump ulcers in the transtibial amputee (Fig 19A-1.). The residual limb of the transfemoral amputee with hip joint contracture provides only a small posterior thigh platform for sitting and a short lever arm for transfer (Fig 19A-2.,A and B). The residual limb in the knee disarticulation amputee is muscle balanced, so these patients rarely develop early or late hip joint contracture. A large surface area for weight bearing and balance while sitting and for turning in bed is provided as well as a long lever arm for transfers (Fig 19A-3.).
PROSTHETIC CONSIDERATIONS FOR THE POTENTIAL WALKER
The active walker should benefit from the direct load transfer possible following knee disarticulation. This may be limited with some techniques for knee disarticulation in which the soft-tissue envelope covering the femoral condyles and interfacing with the prosthetic socket consists only of skin. The knee disarticulation technique described by Wagner provides a soft-tissue envelope over the femoral condyles that is composed of a mobile nonadherent gastrocnemius muscle flap and full-thickness skin. This cushioned, shear-absorbing, weight-bearing platform comfortably allows direct load transfer, which is preferable to the unphysiologic unloading of the terminal part of the residual limb and indirect load transfer of transfemoral and transtibial amputations. The metabolic cost of walking with knee disarticulation is less than with transfemoral amputation but somewhat greater than with transtibial amputation due to the walking propulsion provided by the quadriceps-tibia lever arm.
Wagner's technique allows retention of the entire expanded surface area of the distal portion of the femur to efficiently dissipate pressure. While this provides an excellent method of load transfer, the residual limb has a bulbous distal shape. The distal portion of the femur can be narrowed and/or shortened to cosmetically decrease the bulbous end, but at the expense of decreased surface area and increased pressure concentration for weight bearing. This should generally be reserved for young trauma patients and avoided in dys-vascular patients where the added dissection may well compromise surgical wound healing. The risk seen in severely dysvascular transtibial amputees for the development of pressure ulcers in weight-bearing areas with tenuous, nonresilient skin due to pistoning and shearing within the transtibial socket is minimized in knee disarticulation because an intimate total-contact prosthetic socket fit is not essential when the end bearing of direct load transfer is utilized. Residual-limb volume fluctuations (e.g., as in renal failure) are also better tolerated than in surgical levels, which require intimate prosthetic socket fit.
USE IN THE TRAUMA OR INFECTION PATIENT
Push-off at the terminal stance phase of gait is accomplished by advancement of the limb against a stable foot and ankle. In patients with nonsalvageable lower limbs secondary to trauma or infection, every effort should be made to retain the forward propulsive capacity of the knee joint and proximal end of the tibia. Transtibial amputation should be performed when the following structures can be retained: (1) a serviceable joint with no more than a 25-degree loss of full extension, (2) the proximal end of the tibia including the patellar tendon attachment, (3) an adequate soft-tissue envelope of mobile muscle to cover the end of the tibia and, (4) full-thickness skin in areas of load transfer. A transtibial amputation without an adequate soft-tissue envelope in an active post-traumatic patient will often lead to continued skin breakdown and residual-limb discomfort. If these problems cannot be overcome by plastic revision of the residual limb or by use of a weight-bearing thigh corset, these patients may be better served by provision of an adequate soft-tissue envelope at the knee disarticulation level (Fig 19A-4.).
USE IN THE DYSVASCULAR PATIENT
Viable tissue to provide an adequate soft-tissue envelope is the first consideration. Patients amputated at the level of the tibial tuberosity with retention of the patellar tendon insertion retain a functional knee joint and transtibial amputee gait. It is unusual to see a patient with the vascular capacity to heal a surgical wound at the knee disarticulation level not be able to heal at the high transtibial level. Viability of remaining tissue in the dysvascular patient is determined by preoperative vascular testing and intraoperatively by muscle color and consistency, skin and muscle bleeding, and muscle contractility with electrical stimulation.
When a knee flexion contracture approaches 50 degrees, knee joint excursion and hence forward propulsion following transtibial amputation will not be adequate to make functional use of the quadriceps-tibia lever arm due to the greatly increased energy cost of flexed-knee gait. Small knee flexion contractures can sometimes be improved following prosthetic limb fitting, but when the contracture approaches 50 degrees, the patient may be better served with knee disarticulation.
Prosthetic accommodation of a hip flexion contracture also becomes difficult when that contracture exceeds 30 degrees. Increased hip flexion at initial floor contact (heel strike) causes the dynamic weight-bearing line (hip-knee-ankle axis) to fall posterior to the knee center. When this happens, the knee will "buckle," and the patient will stumble or fall unless he can accomodate by forceful quadriceps contraction. Hip flexion contractures can frequently be corrected by having the patient lie prone for periods sufficient to stretch out the deformity. Surgical correction is rarely indicated.
SURGICAL PROCEDURES
Soft-Tissue Envelope
The soft-tissue envelope is the interface between the hard prosthetic socket and the hard bone of the residual limb. Most late breakdowns in residual limbs, however, are due to tissue shear, not direct pressure. To minimize late tissue breakdown, the soft-tissue envelope should be formed with a mobile, nonadherent muscle mass and full-thickness skin in the areas of load transfer (Fig 19A-5.).
Sagittal Flaps
Knee disarticulation, as advocated by Wagner and others, allows the patient to be operated upon in the supine position under regional anesthesia. This technique is well suited to the dysvascular patient since the skin flaps, being equal, each have minimal length (Fig 19A-6.,A). On closure, the surgical scar lies posteriorly, between the femoral condyles (Fig 19A-6.,B). The gastrocnemius is retained to provide a cushioned soft-tissue envelope that will allow comfortable direct load transfer.
Sagittal skin flaps equal in length to half of the transverse diameter of the limb at the midpatellar tendon level are created with their anterior junction midway between the distal pole of the patella and the tibial tuberosity and the posterior junction directly opposite unless the knee has a major flexion contracture. In this case, the posterior junction is placed more distally to achieve equal sagittal flaps. Each flap is mobilized proximal to the knee joint. The patellar ligament is isolated and skived off the tibial tubercle. The knee joint capsule is incised circumferentially at the level of the joint, and the cruciate ligaments are skived from their attachments on the tibia. The vascular bundle components are ligated at this level, and the tibial and peroneal nerves are transected proximally and allowed to retract. The gastrocnemius is divided distally to form a flap long enough to allow gastrocnemius myofascia to be sutured to the remaining knee joint capsule. The skived patellar ligament is sutured to the stumps of the cruciate ligaments, with care taken to ensure that the distal pole of the patella does not extend distally into the weight-bearing plane of the knee joint (Fig 19A-7.,A). The menisci can be removed because their shock-absorbing function will be replaced by the gastrocnemius muscle flap. The posterior fascia of the gastrocnemius is then sutured to the remaining knee joint capsule, and the skin is reapproximated (Fig 19A-7.,B). The suture line assumes a midline posterior position between the femoral condyles. A soft compression or rigid plaster or fiberglass dressing is applied.
Anterior and Posterior Skin Flaps
The incision for the anterior flap begins posteromedially just proximal to the joint line and extends convexly, anteriorly, and distally to a point approximately 2.5 cm distal to the tibial tuberosity. It then curves proximally and posterolaterally to end just proximal to the joint line. The posterior flap is also convex but somewhat shorter than the anterior flap. The patellar ligament is incised at its insertion, and the knee joint capsule is incised circumferentially. The cruciate ligaments are severed from their attachment on the tibia. The vascular bundle is ligated at the joint level, and the sciatic nerve is severed proximally. The gastrocnemius is removed from its origin on the femur. The semimembranosus, semitendinosus, and biceps femoris muscles are divided at a level leaving adequate length for suturing to the patellar tendon without undue tension. The ili-otibial band and pes anserinus are sutured to the remaining anterior retinaculum. The skin is reapproximated and an appropriate dressing applied (Fig 19A-8.).
Circumferential Incision
The main value of this technique is that no flaps are produced; however, the operation must be performed with the patient in the prone position. The knee is flexed to 90 degrees, and a circumferential skin incision is performed approximately 1.3 cm (½ in.) distal to the tibial tuberosity. Anteriorly and medially, the incision is carried down to bone, with the patellar tendon and pes anserinus elevated before the knee joint is entered. The capsule and ligaments are incised circumferentially at the joint level. The cruciate ligaments are skived from the tibia, the origins of the gastrocnemius from the femur, and the biceps femoris from the fibular head. The patellar tendon and biceps femoris are sutured to the stumps of the cruciate ligaments. The an-teromedial portion of the retinaculum is sutured to the posterior part of the capsule and semimembranosus. The skin is closed longitudinally, and an appropriate dressing is applied (Fig 19A-9.).
Reduction Osteoplasty
Reduction osteoplasty decreases the bulk of the distal end of the residual limb to permit fabrication of a more cosmetic prosthetic socket. This is accomplished at the cost of decreased suspension from the expanded femoral condyles, so auxiliary suspension might be required. The distal articular surface can be retained, as advocated by Mazet and Hennessy, simply by trimming the medial, lateral, and posterior protruberances. Burgessadvocates shortening the femur by removing the distal portion of the condyles in order to keep the knee centers level (Fig 19A-10.). By maintaining the expanded metaphyseal region of the distal end of the femur, direct load transfer can still be accomplished. Any of the described surgical approaches can be modified to incorporate these options. Reduction osteoplasty, however, should generally be reserved for traumatic and tumor patients with normal vasculature in whom the extra surgical dissection will not compromise wound healing.
SUMMARY
Knee disarticulation allows the direct transfer of body weight from the end of the residual femur to the prosthesis. This restores useful proprioception as well as the ability to take advantage of the intrinsically stable, poly-centric four-bar-linkage prosthetic knee joint. Weight bearing and shear stress dissipation are enhanced when the distal end of the femur is covered with a mobile, nonadherent cushion fashioned from the gastrocnemius muscle belly.
In children, knee disarticulation has the added advantages of preservation of distal femoral growth potential and the elimination of appositional bony overgrowth. In adults, it is primarily used for patients with peripheral vascular disease who have the biological capacity for healing an amputation wound at the transtibial level but will be unable to functionally utilize a prosthesis. Another relative indication for knee disarticulation is in patients with large residual-limb volume fluctuations as seen in severe renal failure or congestive heart failure. In addition, traumatic transtibial amputees left with an inadequate soft-tissue envelope or with a nonfunctional tibial segment due to severe loss of knee mobility or knee motor strength will not be able to utilize the transtibial level. In these cases, knee disarticulation rather than transfemoral amputation is recommended for the reasons mentioned above.
References:
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- Pinzur MS, Gold J, Schwartz D, et al: Energy demands for walking in dysvascular amputees as related to the level of amputation. Orthopedics 1992, in press.
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Chapter 19A - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
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