Technical Notes on Manufacture of Long Leg Braces to Assist the Quadriceps Femoris
Clarence Medcalf *
The knee joints of the brace are set posterior to the upper side bars to allow hyperextension to approximately 183°. The upper side bars are extended three to four inches beyond the center of the knee joint. This extended arm will have to be longer for large patients than for smaller patients in order to provide adequate torque. The extension has been limited to four inches for adults and can be reduced to two and one-half inches for small children. To each of these extensions is fastened a spindle mounting a ball bearing pulley ( Fig. 1). Attached to the lower side bars an equal distance from the center of the knee joint are two spindles to which the Hunter Negator spring is attached. Care should be taken when aligning the brace to be sure that the spindles on the upper and lower bars are parallel on the horizontal and perpendicular planes to diminish twist and stress on the springs. It has been found that a five-eighths inch diameter aluminum spindle on the lower side bars reduces the stress and the rate of breakage of the spring at the point of attachment. Nevertheless, since these springs have, a life of approximately 10,000 cycles, the rate of fatigue and fracture is relatively rapid and springs will have to be replaced every week or two.
The spring is punched with a sheet metal punch and attached with a screw to the spindle on the lower side bar. Since the spring usually cracks adjacent to the spindle on the lower side bar, it can be cut off. repunched and re-used several times before replacement with a new spring.
The torque provided by the springs should not exceed the torque due to the weight of the lower leg and foot with the brace on. Otherwise the patient cannot bend his knee when sitting. At this Rehabiliation Center an attempt has been made to achieve the torque of the lower leg through the combined use of the spring plus the residual strength of the quadriceps. However, it has been found in several cases that considerably less torque than this was needed to provide a stable knee which allowed nearly normal knee motion during walking. Springs larger than the 5.3 lb. spring (#12H3K) have not been used because of the bulk. However, these springs can be laminated by interwinding to double or treble the tension required.
This brace works most effectively when there is slight hyperextendability of the knee and will not work successfully if knee extension is limited to 175° or less. The patient, when walking, can bend his knee as he swings bis lower extremity forward but must lock his knee on heel strike rather than allowing knee action for shock absorbtion. He can sit and rise without inconvenience. The greatest asset of the brace is that it requires additional muscular participation in the normal manner for locking the knee and consequently assists in building muscular strength where possible.