Below-knee amputation - "Any amputation above the Syme level and below the knee joint is known as a below-knee amputation. Because circulatory troubles have often developed in long below-knee stumps, and because the muscles that activate the shank are attached at a level close to the knee joint, the below-knee amputation is usually performed at the junction of the upper and middle third sections. Thus nearly full use of knee is retained- an important factor in obtaining a gait of nearly normal appearance. However, it is rare for a below-knee amputee to bear a significant amount of weight on the end of the stump; thus the design of the prostheses must provide weight-bearing through other areas. Several types of surgical procedures have been employed to obtain weight-bearing through the end of the below-knee stump, but none has found widespread use." (Limb Prosthetics Services and Devices: Critical Unmet Need: Market Analysis)
Below-Knee Prostheses - "Until recently most below-knee amputees were fitted with wooden prostheses carved out by hand. A good portion of the body weight was carried on a leather thigh corset, or lacer, attached to the shank and socket by means of steel hinges. The shape of corset and upper hinges also held the prosthesis to the stump. The distal, or lower, end of the socket was invariably left open. Other versions of this prosthesis used aluminum, fibre or molded leather, as the materials for construction of the shank and socket, but the basic principle was the same. Many thousands of below-knee amputees have gotten along well with this type of prosthesis, but there are many disadvantages. Because the human knee joint is not a simple, single-axis hinge joint, relative motion is bound to occur between the prosthesis and the stump and thigh during knee motion when single-jointed side hinges are used, resulting in some chafing and irritation. To date it has not been possible to devise a hinge to overcome this difficulty. Edema, or accumulation of body fluids, was often present at the lower end of the stump. Most of these prostheses were exceedingly heavy, especially those made of wood.
"In an attempt to overcome these difficulties, the Biomechanics Laboratory of the University of California, in 1958, designed what is known as the patellar-tendon-bearing (PTB) below-knee prosthesis. In the PTB prosthesis no lacer and side hinges are used, all of the weight being taken through the stump by making the socket high enough to cover all the tendon below the patella, or kneecap. The patellar tendon is an unusually inelastic tissue which is not unduly affected by pressure. The sides of the socket are also made much higher than has usually been the practice in the past in order to give stability against side loads. The socket is made of molded plastic laminate that provides an intimate fit over the entire area of the socket, and is lined with a thin layer of sponge rubber and leather. Because it is rare for a below-knee stump to bear much pressure on its lower end, care is taken to see that only a very slight amount is present in that area. This feature has been a big factor in eliminating the edema problem in many instances. The PTB prosthesis is generally suspended by means of a simple cuff, or strap, around the thigh just above the kneecap, but sometimes a strap from the prosthesis to a belt around the waist is used.
"After the socket has been made, it is installed on a special adjustable leg so that the prosthetist can try various alignment combinations with ease. When both prosthetist and patient are satisfied, the leg is completed utilizing the alignment determined with the adjustable unit.
"The shank recommended is of plastic laminate and the foot prescribed is usually the SACH (solid-ankle, cushion-heel) design but other types can be used. It is now general practice in many areas to prescribe the PTB prosthesis in most new cases and in many old ones, and if side hinges and a corset are indicated later, these can be added.
"Stumps as short as 2-1/2 in. have been fitted successfully with the PTB prosthesis. In special cases, such as extreme flexion contracture, the so-called kneeling-knee, or bent-knee, prosthesis may be indicated. The prosthesis used is similar to that used for the knee-disarticulation case." (Limb Prosthetics Services and Devices: Critical Unmet Need: Market Analysis)
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