Artificial Limbs Prosthetic Biomechanics PDF

Artificial Limbs Dr. Hassanain Ali Lafta Prosthetic Biomechanics Assist. Prof. Dr. Hassanain Ali Lafta [email protected] Introduction An understanding of biomech...

Artificial Limbs Dr. Hassanain Ali Lafta Prosthetic Biomechanics Assist. Prof. Dr. Hassanain Ali Lafta [email protected] Introduction An understanding of biomechanics is important when working with amputees and people with prosthetic limbs. It is especially relevant to understanding how gait deviations and pressure problems occur and how they can be remedied. Forces are applied to the human body in prosthetics, this will invariably lead to pressure. Pressure can be managed by increasing the area it is applied over and distributing it thoughtfully to pressure tolerant areas. Alignment of a lower limb prosthesis can have an effect on socket pressures in a predictable way Changes to a prosthesis can alter the prevailing biomechanical situation through limiting ranges of motion or moving the GRF. An understanding of biomechanics is essential for prosthetists as it influences so many aspects of their work during prosthetic rehabilitation. Forces A force is any interaction which tends to change the motion of an object. Forces are important in Prosthetic devices. A prosthetic device applies forces to the human body and can change the way forces interact with the body. When force is applied to the human body it is done over an area of skin. This produces pressure with the following equation; Pressure equals force over Area. When the area over which a force is distributed is made larger the pressure is reduced. 1|Page  Artificial Limbs Dr. Hassanain Ali Lafta If someone steps on her foot in a stiletto (a fine point high heeled woman’s is sigo shoe) there is a lot of pressure (and pain). The same person standing on her foot in a heel that is larger it would cause less pressure (and pain) because the area is bigger. The fact that it is the same person means that the Force is constant in this scenario. The heel of a stiletto shoe would cause large pressure per unit area. When the area is increased, the pressure is decreased. In the below example, the pressure is reduced by demonstrating the effect of increasing the contact area under the foot. In this take on the classic “bed of nails” scenario, the pressure is reduced as the area is increased. The force again remains constant. This idea is used in prosthetic socket design to attempt to spread force over a large enough area to reduce pressure to an acceptable amount. 2|Page Artificial Limbs Dr. Hassanain Ali Lafta Often patients might ask for an uncomfortable part of a prosthesis to be removed. Sometimes when this is done the area that the force is spread over is reduced. Hence by the above equation P=F/A, the pressure is increased. This can lead to increased pain after some time. (gravity) Centre of Mass A general biomechanical principle is understanding the Centre of Mass, sometimes called the Centre of Gravity. It is the middle of the mass of an object and the force of gravity can be assumed to act on the object from this point. It is also the place that rotations will occur around. In an amputee because some mass is excised the center of the mass of the person changes. Ground Reaction Force The Ground Reaction Force (GRF) occurs when contact occurs with a support surface, it is equaling and opposing the force due to body mass passing through the foot to the ground surface. It should be noted that the ‘force due to body mass’ includes the momentum and accelerations of body mass as well as just the effect of gravity on it. Hence it is different and yet similar to the weight line. Prosthetic alignment alters gait by manipulating the position of the lower extremity and prosthetic joints with respect to ground reaction forces. 3|Page Artificial Limbs Dr. Hassanain Ali Lafta The GRF is not used clinically in many places due to difficulty obtaining it in real time. Clinicians can try to understand the movements they see by understanding the effect the GRF might be having on the human body and prosthesis. Moments or Torques If a force does not act directly on the center of mass of an object it will rotate the object. If a force does not act through the middle of a joint with another segment it will change the angle of the joint. In the below example the foot and the leg should be considered as two segments, joined at the Talo-Crural - Joint. As the GRF ‘pushes’ up on the foot it will tend to move the foot into - dorsiflexion. The force produces a torque that is the size of the force times the length of the lever arm (distance from the line of action of the force to the joint center). In normal human movement the moment created by an externally applied Ground Reaction Force is balanced (for the most part) by an internally generated muscle force which provides an opposing moment. In this example, the dorsiflexion moment created by the anterior GRF is balanced by the planter flexors through tension in the Achilles Tendon. This muscle has a fixed lever arm. 4|Page Artificial Limbs Dr. Hassanain Ali Lafta Distribution of Weight Loading The load line is important for the proper distribution of the user’s body weight. For the proper construction of the prosthesis, it is important to identify. the course of this line. It is a thought vertical line which in a healthy individual runs in the sagittal plane through the centre of gravity, then 2 mm posterior from the hip joint, 15 mm anterior from the knee joint, and 60 mm anterior from the ankle joint. In the frontal plane, the load line runs through the centre of the body; when the prosthesis is constructed, it should run through the centres of above mentioned joints of one limb. In such case it is a stabile stance when the foot should be able to compensate the horizontal movements of the centre of gravity by shifting the weight forward or backward, to the left and to the right side. In majority of individuals with both limbs, the weight is distributed in the 50:50 ratio which facilitates ideal symmetric loading of lower limb joints. In such distribution, energy consumption is not increased to maintain the balance and there no unnecessary compensation movements must be made in the area of ankle and foot. 5|Page Artificial Limbs Dr. Hassanain Ali Lafta With amputations, the load is often transferred through the tuberosity of ⑭ - the ischium which is unsuitable due to changed position of the centre of gravity in the frontal plane; the centre of gravity is moved laterally to the healthy limb side. By proper construction and a suitable selection of prosthesis components it can be arranged that the transfemoral prosthesis transfers at least 40% of the individual’s body weight. Construction of Prosthesis The prosthetic limbs that are delivered to the users are made up of parts ① ② which can be grouped into three broad categories: Prosthetic Socket, Prosthetic ③&S >dis - 0 % stump Si, Components, and Consumables. The prosthetic socket is the upper part of the & prosthesis which fits around the residual limb (AKA stump). This Socket is custom - made onsite within the APC prosthetic workshop by specialized technicians. - - Q Below the socket are the prosthetic components which include connective - ② components (made of aluminium, stainless steel or titanium), prosthetic joints - - ③ (such as prosthetic hips, knees, elbows and wrists), and prosthetic feet or - prosthetic hands (also known as a terminal device). These components are ordered from suppliers to meet each client’s specific requirements. 6|Page Artificial Limbs Dr. Hassanain Ali Lafta -sid Finally, the consumables include items such as liners, knee sleeves, and socks which allow the prosthesis to comfortably hold on to the residual limb. These Consumables, like the prosthetic components, are ordered specifically for - each client, featuring materials such as silicone, polymer, rubber and much more. Stabilizing activity of the limb/prosthesis depends on the amputation height, i.e. on residual muscles that remained on the residual limb. In the amputation intervention, the muscles are transacted in various heights, depending on the damage, and thus the muscular function is reduced (flexors/extensors, abductors/adductors). > There are important factors for the creation of properly functioning prosthesis which may include: ①  Selection of appropriate components that depends primarily on the physical ~ O ⑤ and mental condition of the user, user’s activity, and method of use. The - principal factors for the selection of prosthetic components are a patient’s weight and physical activity. Depending on the user’s weight, the material of - prosthetic parts is selected so that sufficient strength and average weight of transtibial prosthesis is ensured.  Residual limb’s conditions, amputation height, residual limb’s shape (conic – smaller circumference on the distal part than on the proximal part, pear- shaped, cylindrical – the same circumference on the distal and proximal end), amputation scar, as well as other problems or diseases. ①  Construction of prosthesis that can be divided into following steps: basic - ② ③ construction, static and dynamic correction. - Construction of the prosthesis is an empirical process that depends on the skills of an orthopaedic technician and on a patient’s feedback. 7|Page Artificial Limbs Dr. Hassanain Ali Lafta Al The main objectives of the good construction of the lower limb prosthesis is Ans : Fig is his to provide and ensure for the user sufficient certainty, stability, balance, and s comfort during the stance and during the walk to minimize the energy cost and - gait asymmetry. In the first phase, it is crucial to determine the construction line - which is an arbitrary vertical line towards which individual prosthesis components are positioned according to certain rules. People with amputations have missing body parts, so there are cases where the muscle action is not available to act as a counter to the GRF as the muscle and joint are no longer present. If the GRF passes anterior to the ankle in a Transtibial amputee, then the ankle is permanently stiff enough to resist deforming into dorsiflexion. In the case of a person with a Transfemoral amputation with free knee (with no locking mechanism), the GRF should be placed anterior to the knee for the duration of single limb support. This is done in the following manner through static alignment using the more identifiable Trochanter Hip and Ankle (TKA) construction line. 8|Page Artificial Limbs Dr. Hassanain Ali Lafta In this scenario, the amputatee continues to have indirect control of the knee though the ability to extend the hip joint. When unwanted biomechanical situations present themselves, prosthetist may have the opportunity to alter the biomechanical situation, by fixing a joint to movement or by manipulating and moving the GRF to a more advantageous position. The second step is the static adjustment of the prosthesis that is carried out together with the patient. By turning and shifting the components, required adjustments to the prosthesis construction are made, until the stability is achieved in the stance. In the static adjustment, it is important to adjust the correct prosthesis length so that both limbs are evenly loaded and the pelvis is leveled. Negative effect on the stance with prosthesis is influenced by the shift of the load line caused by the plantar flexion of the foot or moving the foot forward. 9|Page Artificial Limbs Dr. Hassanain Ali Lafta Other changes in the adjustment are carried out during the dynamic adjustment of the prosthesis when a patient’s gait is assessed in the sagittal and frontal planes, deviations from the normal step cycle are examined. The deviations can be caused by improper construction of the prosthesis or by physical deficiencies, as well as a patient’s mental condition. During the gait with a prosthesis, the first contact of the foot and the - ground is important, as well as transfer of load on the foot. The foot contact is carried out through the heel so that the walking is as natural as possible, and subsequently the entire sole surface contacts the ground and the load is transferred to the foot. It is followed by the foot rolling away from the ground and the push-off through the toe, when the energy is required for the swing phase. In this phase, the important role is played by the proper selection of a foot (foot roll away from the ground, adaptation to the surface, compensation movements, energy accumulation and expenditure) and a proper position of the knee joint. 10 | P a g e Artificial Limbs Dr. Hassanain Ali Lafta Particularly these components and their proper assembly affect the final function of the prosthesis and thus influence the user’s activity. In the swing phase, the knee function is important, as it is necessary to ensure the movement from flexion to extension (extension moment of the knee) which facilitates the foot transition from plantar flexion to dorsal flexion, i.e. the toe elevation, in order to avoid stumbling and subsequent fall of the user. Biomechanics of the Socket The residual limb is placed in the socket that provides rigid and stable attachment to the limb, which is important for the control thereof. The prosthesis socket can be divided into 3 parts that have different functions. The top part is the so-called seating face, the central part is actually the controlling socket area with the function to ensure correct movement and restrain it in the P-A direction, which is important during the gait. The last part is the distal socket end which, in an ideal case, should transfer only 10% of an individual’s weight to avoid inappropriate load transfer and subsequent damage to soft tissues. Is 2 palis socket 3 11 | P a g e Artificial Limbs Dr. Hassanain Ali Lafta When we think of forces in a prosthesis, the prosthetic socket provides the - medium for forces to pass from the ground to person, and from the person to the ground. The prosthetist loads skin with pressure in order to transmit forces from the prosthesis to the skeletal system of the amputee. If an area is ideal for axial loading then a large amount of force can be transmitted through it. Forces are generally provided parallel to the skin, reducing the amount of sheer forces. Supporting forces of a prosthesis on a stump are always parallel to the ground reaction force. Changing the alignment (into some flexion) of the limb can affect the available area for vertical force transmission. The green area underneath the sockets in the below figure shows the available area for vertical force transmission. The more flexion that is added the more area is made available for vertical force transmission. Putting more flexion in the socket can have long term effects on joint range of motion and should be carefully considered as a solution to axial loading difficulties. A socket must be able to transfer the load, ensure stability, and provide efficient control during the mobility. In a standing position, the m. gluteus medius is stretched; it ensures that the pelvis is maintained in a balanced position. 12 | P a g e Artificial Limbs Dr. Hassanain Ali Lafta In a healthy individual, this process is ensured by attaching the femur to the ground by a lower limb; in case of the lower limb amputation, this function is taken over by the prosthetic socket. Therefore, proper socket shape is important, as well as its ML and AP dimensions, so that the femur can be attached. In a transverse oval socket of transfemoral prostheses, the pressure on the distal femur end increases and the body is excessively bending aside to reduce the pressure (left and middle figures). It is a non-physiological load transfer, as the load is transferred through the tuberosity of the ischium, which reduces the arm of the exerted force and the overturning moments are increased (right figure). On the contrary, the longitudinal oval socket facilitates the physiological transfer, as the rotation centre is in the hip joint and pelvis does not turn over (the pelvis is in a balanced position) and no unnatural stabilisation body movements are required. 13 | P a g e Artificial Limbs Dr. Hassanain Ali Lafta Prosthetic Coupling The coupling between the prosthetic socket and the amputee’s residual - limb is not completely stiff. This coupling can be thought of as being ‘a bit like a - joint’ between two body segments. As illustrated below, the GRF of the amputee - does not ‘push’ directly up on the prosthesis. Rather it `pushes` up more medially. This creates a torque or moment around the coupling between the socket and the - a amputee. Soft tissue is compressed by the rotation, leading to pressure inside the prosthetic socket. The pressure that is developed as a result of the torque is shown by the red arrows below, proximal medially and distal laterally. - dist : - B ② This torque is resisted by soft tissue compression, in other words the socket torque causes pressure in specific parts of the prosthetic socket in a predictable way. Socket pressure issues should always be dealt with by a prosthetist who understands the importance of alignment. Moving too quickly to altering the socket is not always a good solution in the clinic. Alignment changes are normally rapid and readily reversible with modern components. 14 | P a g e  Artificial Limbs · Dr. Hassanain Ali Lafta - Gait Deviations Se If the prosthesis construction procedure and principles are not thoroughly complied with and the prosthesis is not properly aligned, undesired deviations from the physiological gait develop during walking. These deviations result in the increased energy cost during the motion, overloading of certain groups of muscles, and can cause damage to joint structures and skin. Another aspect is the gait aesthetic discomfort (step cycle asymmetry, body inclination, etc.). Proper adjustment of the prosthesis is affected by a natural patient‘s walking stereotypes, function of the prosthetic foot and pressure on the stump, has a significant impact on comfort and energy consumption in the prosthesis use. Wrong prosthetic fitting can cause pain to users during the execution of daily activities. Manifestation of pain can correspond to lateral asymmetry of the body caused by incorrect length of the prosthesis or incorrectly selected components. O ② Wrong construction of the prosthesis can lead to forces imbalance, overload ③ muscle groups, risk of tripping and also to damage of the stump soft tissues. The prosthetist is the key professional tasked with managing the prosthesis - and its effects on the human body. The presence of the prosthetist as part of the - multidisciplinary team is essential for successful prosthetic rehabilitation. Use of knowledge of biomechanics by the prosthetist adds significant scope for - optimizing the biomechanical circumstances for the prosthetic user. 15 | P a g e