How can prosthetic limbs save physical disability?
For these patients, if they want to move freely again, they have to rely on all kinds of prosthetic limbs. How do these prostheses work? How to make the prosthetic limbs look more natural?
Today's prostheses are generally of skeletal structure, consisting of the receiving cavity, joint components, connecting tubes and feet. If you feel too naked, you can wrap the foam around the outside to simulate the sensation of meat.
The top receiving cavity is connected with the body to transmit the force, which needs to be customized according to the shape of the body.
During the production, first remove the negative mold of the limb with gypsum bandage and pour it into the gypsum to get the male mold; cover the PVA film and vacuum with a negative pressure of 30KPa; then cover the high-strength fabric such as carbon fiber cloth layer by layer and finish with PVA film; finally pour acrylic resin under negative pressure and trim after curing.
The shape of the receptive cavity is very important. Before 1985, quadrilateral receiving cavity dominated the industry. The ischium under the pelvis plays a load-bearing role. The short front and rear diameter of the mouth design makes the ischium nodule fall on the upper edge of the posterior wall of the receiving cavity, but the long internal and external diameter leads to insufficient side support, the ischium is easy to move inward, and the activity is not natural.
Nowadays, the more biomechanical ischial inclusion receiving cavity contains the ischial tubercle, and uses the femur and its surrounding soft tissue to share the weight. The lateral part is higher than the greater trochanter to keep the femur adduction, and the force distribution of the limbs is more uniform.
However, some people may be allergic to the synthetic resin in the recipient cavity, and there is another option - implantable bone integrated prosthetic limbs (OPRA), similar to implant teeth. After six weeks, the new bone has been attached to the surface of the titanium tube. Next, just install the base and screws to connect the prosthetic limbs.
However, due to the risk of implant loosening, fracture and wound infection, since 1990, only more than 500 people in 14 countries have performed the operation.
Compared with the receptive cavity, the joint component is the core of the prosthetic limbs, which determines whether the human can walk and run freely.
This is a normal gait period, divided into support period and swing period. The joint components of prosthesis knee must provide assistance in these two periods, which can be divided into single axis joint and multi axis joint.
Taking the multi axis joint as an example, the four-bar structure is usually adopted, and the rotation center moves in a certain track, similar to the movement state of the knee joint, which is enough to provide stability for the support period.
At the end of the swing, the prosthetic limb is impacted greatly, which needs to be buffered by air pressure or hydraulic device. It is mainly composed of cylinder, piston and piston rod. When the knee is bent, the gas or silicone oil in the cylinder is squeezed to store energy, which is released during stretching, producing a constantly changing nonlinear moment similar to normal muscle movement.
When a healthy pair of legs walk normally, the motor cortex of the brain sends signals to the spinal cord, and the nerve cells in it activate the motor neurons responsible for stretching and flexing the legs, and then the leg nerves feed back to the brain. Such a reflex circuit occurs about 20 times per second.
After putting on the prosthesis, the sensors of knee joint and ankle joint act as leg nerves, transmitting the information of angle, position, acceleration and so on to the microprocessor, and the "brain" adjusts the hydraulic system in real time. Only when the legs are fully extended and the front foot board carries 70% of the body weight, can the swing state be switched and the response be more timely.
In addition, there is a more advanced bionic foot equipped with a series elastic driver composed of a DC motor and a spring, which can actively provide power. At work, it can collect the contractile or diastolic EMG signals of the remaining limbs through the electrode patch attached to the skin, and convert them into the motion signals of the prosthesis, which is closer to the natural gait of the healthy limbs, and can be easily completed even in Latin dance.