Biomechanical basis of choosing the rational mass and its distribution throughout the lower limb prosthesis segments.

A solution for finding a rational distribution of mass in lower limb prostheses has been considered based on the formal premise favoring the identification of the movements of a prosthetic and an intact leg. For the purpose of simplicity, and analysis has been carried out for only the swing phase, the data about the properties of moving segments being determined without integrating differential equations of motion. At the formation of equations of motion, an assumption that body segments are absolutely rigid and have constant moments of inertia and locations of the center of mass was taken into consideration. Based on independent proportions formed of combinations of the coefficients of equations of motion, a system of three equations has been formulated and solved in relation to the mass values sought: a static radius and a radius of inertia of the prosthesis complex link "shin + foot + footwear." From the six unknowns included in the equations, three values are chosen as mean values determined empirically. The solution of obtained equations results in the following conclusions: the parameters of the mass distribution in a "shin + foot + footwear" complex link depend on the amputation level and the patient's mass. These data, reported in appropriate tables, may be used in prosthetics practice. Recommendations have also been presented with regard to a prosthesic mass relative to the age of the person with amputation and a method of a balancing of prostheses aimed at the achievement of a rational distribution of masses. The analysis of obtained equations has also allowed us to make recommendations about the artificial foot mass. It has been concluded that a reasonable desire to reduce the mass of the prosthetic segments is not an end in itself, but is only the means of a rational distribution by means of balancing. It has been proved that rational prosthetic fitting results in decreased energy costs and overloads are decreased and a normalized gait.

An above-knee prosthesis with a system of energy recovery: a technical note.

Knee flexion to 24 degrees during early stance transforms kinetic energy into potential energy of a total center of mass (TCM) position. Flexion is controlled by the musculoligamentous apparatus. Reproduction of such flexion in a new single-axis prosthesis knee unit has minimized the metabolic energy cost to the patient by a more favorable use of gravity acting upon the prosthetic segments and the body as well as of inertia. Potential energy is stored in the spring shock absorber of the knee unit. The coefficient of energy recovery increased by 30% in comparison with a conventional above-knee prosthesis. Energy costs to the patient decrease an average of 35% during gait with the new prosthesis. The same amount of unloading during walking is typical of an intact limb. The knee unit mechanism has a link set on the axle, thus providing two joints with a common axis: a) the main joint for knee flexion to 70 degrees during swing phase and flexion to 135 degrees during sitting; b) the second joint for bending at the beginning of stance phase. Compared with conventional units, gait with the new unit displays several functional advantages: 1) normal knee kinematics with movement of a TCM along a trajectory that contributes to an easy rollover of the foot and smooth and continuous translation of the body; 2) shock absorption during early stance prevents impact from the anterior brim of the socket; 3) at mid-stance, the increase of the TCM position accumulates potential energy that results in a significant increase of the push-off force; 4) during rapid gait, the unit provides adequate resistance to knee flexion; 5) location of the joint axis in front of the line of gravity loads the prosthesis in standing, making possible unimpeded carrying of the prosthesis over the support, the lengths of the prosthetic and the intact limb being equal; in addition, it facilitates flexion before the beginning of the swing phase. Production of the units began in 1992.