Knee mechanism
The knee is the joint between the hip and the toe it operates in a unique way hence resulting to the use of the term mechanism since it works in a machine like way. A machine is mostly made of moving parts and the correlation between parts brings about its movement so is the articulation of the knee in the human body. It constitutes of a number of parts where by some are movable and each aid in the articulation of the knee bringing about its mechanism. Each of these parts plays a vital role and they include hamstring, quadriceps, femur (thigh bone), tibia, ligament, meniscus and patella. The knee is considered as one of the largest joint in the human body and it consists of two articulations between the femur and the tibia and the other one between the femur and patella. The knee is a unique joint in the body which allows for articulation at 180̊, slight internal and external rotation concurrently, and extension. This paper will discuss the mechanism of the knee; all the parts involved in the articulation and finally give the opinion concerning the knee mechanism.
The knee
It is vital to understand the knee and all the parts involved in its mechanism before explaining the mechanism behind it movement and extension. The knee can be seen as a modification of a hinge joint which is made up of three functional compartments, the first is the patella commonly referred as the ‘knee cap’. The patella is a flat triangular bone which moves as the knee moves, its function is to relieve friction during movement as the knee is being bent and straightened and finally protection of the joint. Femur it is commonly referred as the thigh bone which is the largest, strongest and longest in the human body, its end consist of round knobs known as condoyle which have enable articulation to be much easier. Tibia which is commonly known as the shin bone and runs from the knee to the ankle, it also has condoyle at the end which enables the articulation of the knee. Fibula is a long narrow bone at the lateral side of the tibia and runs from the knee to the uncle.
The ligaments are found on top of the bone and their function ids to attach bone to bone thereby providing strength and stability. The ligaments are strong bands which are not flexible but have the ability to stretch but when stretched past the limit they snap resulting to injury.
For the knee it consists of two main ligaments medial collateral ligament and lateral collateral ligament: the medial collateral ligament attaches the medial side of the femur and medial side of the tibia thereby limiting the sideways movement therefore preventing disjointing of the knee sideways. The lateral collateral ligament attaches the lateral side of the femur to the lateral side of the fibula thereby reducing the sideways motion to a snapping point. Anterior cruciate ligament attaches the femur and the tibia at the centre it is located deep inside the knee and its function is to limit rotation and forward motion of the tibia. Posterior cruciate ligament it is located deep inside the knee behind the anterior cruciate ligament, it is considered as one of the strongest ligament within the human body and attaches tibia to femur. Its function is to limit the backward movement of the knee. Finally under the ligaments we have the patellar ligament which attaches the knee cap to the tibia.
Cartilages of the knee are found at the end of the bones whereby at the end of the joints there is a cover of white cartilage which enables the articulation of the knee to be smooth and non painful. Articular cartilage and its supporting bone functional conditions are tightly coupled as injuries of either adversely affects joint mechanical environment (Shirazi & Shirzi-Adl, 2009). These articular cartilages cover the end of the femur, top of the tibia and finally a back of the knee cap, in the middle of the knee there is the meniscus disc which acts as a shock absorber for the knee. The muscles give the knee a vintage in its articulation their contraction and relaxation facilitates the movement, the muscle consists of two main groups the quadriceps and the hamstrings. The joint capsule is a thick, fibrous structure that wraps around the knee and contains the synovial fluid which happens to produce the synovial fluid, his fluid acts as a lubricant during movement thereby reducing the wear and tear of the bones during articulation.
The biomechanics of the knee
The articulation of the knee consists of the load, pivot and effort these bring about the scenario of mechanism whereby the body acts as the load, joint as the picot and the muscles provide the effort. All these parts act as part of the mechanism whereby each part has a role to play as the load is lifted and movement takes place. Since the knee comprises of two main joint the tibiofemoral joint and patellofemerol most of the biomechanics will take place at these two joints.
Patellofemoral articulation
This articulation results to transmission of tensile forces generated by quadriceps to the patellar tendons, the force should not be big in order to prevent the knee from buckling up hence causing injuries. The quadriceps acts as the force whereby energy is transmitted through the muscles resulting to movement either by bending or straightening the knee, the quadriceps acts as effort providing the energy the forces the knee in to action. The force must be withheld by the patella to prevent over articulation or else the knee will disjoint forward or backwards, the patella reduces the extensor mechanism by 30%. This mechanism of reducing the extensor energy by the patella is referred as patelloctomy which is a reaction within the patellofemoral joint. During the daily activities the reactions depends on cause of the forces that the patella has to resist like squatting which has up to 7X the body weight and descending stairs bring about 3X of the body weight. Collagen fibrils networks in knee cartilage and menisci change in content and structure from a region to another. While resisting tension, they influence global joint response as well as local strains particularly at short-term periods (Shirazi, Shirazi-Adle & Hurtig, 2008). During motion in case of sliding the energy providence forces the patella to extend up to 7cm in full flexion while the maximum contact between the femur and patella is 45̊.
The patella offers stability to the knee but it does these by regulating the amount of extension and flexation in order to allow mobility the knee parts without jeopardizing articulation. The stability is also brought about by resistance whereby the first form of resistance that brings biomechanics of the knee is the passive restraints that brings about lateral subluxation. Medial patellofemoral ligament results to primary passive restraint to lateral translation in 20̊ of flexion. These resistance offer stability to the knee movement sideways because any further flexion to the side means disjointing of the knee hence the need to maintain the degree during the sideways articulation. This resistance withholds up to 60% of the total force placed upon by the body which acts as the load during the articulation process. The medial patellomeniscal ligament offers 13% of resistance of the force exerted on the knee while the lateral retinaculum offers 10% of the resisting force.
The tibiofemoral articulation
These is the articulation that takes place between the femur and the tibia, it responsible for the transmission of the body weight between the two bones. The articulation results to force which is dispensed by the body which acts as load during movement; the force is usually 3X the body weight during walking and increases to 4X during climbing. During these activities there is motion of which takes place at an angle whereby the knee can act as a plane (sagittal plane). There is a 3 degree of hyper extension to 155 degrees of flexion these is brought about during movement as the knee articulates by bending and straightening. According to Masouros, McDermott, Amis & Bull (2008), the menisci of the knee act primarily to redistribute contact force across the tibio-femoral articulation. For tibia and femur to provide articulation to the knee rotation has to take place though it happens at an angel because any have angulations may result to injury. There is the instant centre of rotation which is the point where the joint is in contact with the two bones. The tibia rotates 5 degrees in the last 15 degrees of extension this is because the medial tibular plateau is longer than the lateral tibular plateau. Anteromedial bundle brings about tight flexion while the posterolateral bundle which brings about tight extension.
Opinion on biomechanics
Integrity of the knee joint which brings about knee biomechanics depends upon the muscles and tendons about the knee, the articular capsule, the intrinsic ligaments of the joint, and the bone architecture of the tibia and femur. Lateral motion of the knee joint in extension is controlled by the capsule, collateral ligaments, and cruciate ligaments; in flexion, by the same structures minus the fibular collateral ligament. Rotary motion of the knee joint in extension is controlled by capsule, collateral ligaments, and cruciate ligaments; in flexion, by the same structures minus the fibular collateral ligament. Forward movement of the tibia on the femur is controlled by the anterior cruciate ligament. Backward gliding of the tibia on the femur is controlled by the posterior cruciate ligament. Lateral gliding of the tibia on the femur is controlled by the tibial intercondyloid eminence and the femoral condyles with the aid of all the ligaments. Flexion is brought about by cruciate ligaments, both menisci, the femoral attachment of the posterior aspect of the capsule, the femoral attachment of both heads of the gastrocnemius muscle, and the bone structure of the condyles of the femur and the tibia. The menisci cushion hyperextension and hyperflexion. The tibial collateral ligament is closely related to the medial meniscus, thereby bringing articulation of the knee this forms a mechanism which is biological oriented.
Reference
Masouros, S. D., McDermott, I. D., Amis, A. A., & Bull, A. M. J. (2008). Biomechanics of the meniscus-meniscal ligament construct of the knee. Knee surgery, sports traumatology, arthroscopy, 16(12), 1121-1132.<https://link.springer.com/article/10.1007/s00167-008- 0616-9>
Shirazi, R., & Shirazi-Adl, A. (2009). Computational biomechanics of articular cartilage of human knee joint: effect of osteochondral defects. Journal of biomechanics, 42(15), 2458-2465.<http://www.sciencedirect.com/science/article/pii/S0021929009004266>
Shirazi, R., Shirazi-Adl, A., & Hurtig, M. (2008). Role of cartilage collagen fibrils networks in knee joint biomechanics under compression. Journal of biomechanics, 41(16), 3340- 3348.<http://www.sciencedirect.com/science/article/pii/S0021929008005010>