For example, the anterior cruciate ligament ACL attaches the thighbone to the shinbone, stabilizing the knee joint. Tendons, located at each end of a muscle, attach muscle to bone.
Tendons are found throughout the body, from the head and neck all the way down to the feet. The Achilles tendon is the largest tendon in the body. It attaches the calf muscle to the heel bone. The rotator cuff tendons help your shoulder rotate forward and backward. You can think of ligaments as rope, with a series of tough, intertwined cords that bind bones.
Ligaments also have some elastic fibers that allow the joint to move, but not so much that it moves beyond its capacity. The knee joint, for instance, has four major ligaments, one on each side of the knee and two that run diagonally across the front and back of the kneecap.
These ligaments help stabilize the knee and keep it from moving too far to the left or right, forward or backward. Tendons are also tough cords, but they have a little more give than ligaments. As a muscle contracts, the attached tendon pulls the bone into movement. Think of what happens to your bicep when you bend your elbow.
Tendons also help absorb some of the impact muscles take as they spring into action. Many sprains happen suddenly, either from a fall, awkward movement, or blow. Sprains commonly happen in the ankle, knee, or wrist. For example, a misstep can cause you to twist your ankle in an awkward position, snapping a ligament and causing your ankle to be unstable or wobbly.
You might hear a pop or feel a tear when the injury occurs. A wrist is often sprained when you reach out your extended hand to break a fall, only to have the wrist hyperextend back. That hyperextension overstretches the ligament. Symptoms of a sprained ligament generally include pain, swelling, and bruising in the affected area. The joint may feel loose or weak and may not be able to bear weight. The intensity of your symptoms will vary depending on whether the ligament is overextended or actually torn.
Doctors classify sprains by grades, from grade 1 a mild sprain with slight stretching of the ligament to grade 3 a complete tear of the ligament that makes the joint unstable. Common areas affected by strains are the leg, foot, and back. Strains are often the result of habitual movements and athletics. When the tendon joins with the muscle, it continues as epimysium in the epitendon muscle. At this point, the muscle-tendon junction must transmit the muscle contraction to the tendon exactly.
The tendon adhesion of the muscle occurs when the fibrous tissue layers of the muscle enter the collagen fibers of the tendon into the collagen fibers. In a study conducted by electron microscopy, the position of the muscle cells and tendons is like the fingers of two hands that are locked together. Collagen fibers do not enter the muscle cells, but they bind tightly under the basal membrane. The movement of a normal tendon, the transfer of muscle power for the entire movement of the joints, and the feeding of tendons depend on peritendinous connective tissue.
This structure is called the peritendon. These structures form the sheaths, which are very finely organized structures from the loose connective tissue [ 3 , 6 ]. The cell and matrix compositions of tendons are similar to ligaments and capsules and contain only small differences.
In fact, they all have the same cell type and similar vascular and innervation sources. Collagen, elastin, proteoglycan, and noncollagenous proteins combine to form the macromolecular framework of dense fibrous tissues. In all of them, the dominant cell type is fibroblasts. In particular, the cells within the tendons are specific fibroblasts called tenocytes. The main role of these cells is to control cell metabolism production and degradation of extracellular matrix and to react to mechanical stimuli applied to the tendon.
Especially tensile loads act as a signal for collagen production, and this process is called mechanical transmission. These cells stretch along collagen fibrils in the form of longitudinal arrays where they have a tensile load [ 7 , 8 ]. The extracellular matrix of tendons is largely composed of collagen fiber network and less proteoglycans, elastin, and other proteins. The main task of these components is to maintain the structure of the tendon and facilitate the biomechanical reaction of the tissue against mechanical loads.
The main substance in tendons and ligaments is basically about 0. The most effective of inorganic substances are proteoglycans. In addition to prostaglandins with a small amount in the main substance, the most common biomechanical properties are the decorin and cartilage oligomeric matrix protein COMP [ 10 ]. The protein clusters in the structure are connected to a large portion of the extracellular matrix of tendons, making the matrix a structure similar to the gel.
Thanks to this compound, collagen provides spaces and lubrication between microfibrils, while cement-like material also makes the collagen structure of tendons stable and contributes to the resistance of the tissue [ 3 , 11 , 12 ]. Collagen Type-I fibers are capable of withstanding large tensile loads and are found in abundance from the tendon structure, allowing a certain degree of stretch and mechanical deformations of the tendons [ 13 ].
This synthesis process is similar to that of all connective tissue, although it may differ slightly depending on the type of complex collagen. Therefore, tendons, which contain Type-I collagen, have a process of synthesis and degradation similar to those in the ligaments and bones. From here, with a more detailed look, we can say that synthesizing for collagens in tendon structure begins in the cell membrane of the tenocytes. In other words, the integrins are like force sensors and, in particular, detect cell withdrawal, allowing the cell to react to these mechanical stimuli.
At the same time, various growth factors contribute to the regulation of this mechanical conversion process [ 14 ]. Cross-linkages form between collagen molecules, which are very important for clustering at the fibril level. The cross-links between the fibrils are more complex.
And this cross-link structure of collagen fibrils provides the strength of the tissue and thus ensures that it performs the task of the tissue under mechanical loads. In the newly formed collagen, these cross bonds are less in number, soluble in salt or acid solution, and can easily break with heat. As collagen matures, the number of cross bonds that can dissolve and break down decreases and decreases to the minimum level.
As a result, organized collagen molecules form microfibril, sub-fibrils, and fibrils. The fibrils are also clustered to form collagen fibers, collagen clusters or fascicles, and the tendon.
Tenocytes are arranged between these fascicles and aligned in the direction of the mechanical load [ 10 ]. In the cellular structures of tendons, as mentioned above, there is much less amount of elastin than collagen, because the mechanical properties of the tendons depend not only on the architecture and properties of collagen fibers but also on the extent to which this structure contains elastin.
Because the bond has a special function and the nerve roots of the spine, mechanical stresses, stresses, etc. Blood circulation in tendons is very important, because the current circulation of blood directly affects metabolic activity especially during healing. Therefore, they have a white color when compared to the muscles with a much higher blood vessel density.
However, there are a few factors such as the anatomical location, structure, previously damaged condition, and physical activity level of tendons that contribute to blood supply besides the small amount of vascular structure.
There are studies that show that blood flow increases in tendons in the case of increasing physical activity in the literature. There are more vascular tendons due to their anatomical position or shape and function. The flushing of tendons is primarily derived from the synovium at the point of attachment to the bone or paratenon. However, some tendons feed on the tendon like the Achilles tendon and the paratenon structure, and some tendons are fed by a true synovial sheath they are surrounded.
Bone and tendon adhesion is a layer of cartilage where blood flow cannot pass directly from the bone-tendon compound. Instead, they make anastomosis with the veins on the periosteum and make indirect connections [ 16 ]. In contrast, tendons have a very rich neural network and are often innervated from the muscles in which they are associated or from the local cuticle nerves. However, experimental studies on humans and animals have shown that tendons have different characteristics of nerve endings and mechanoreceptors.
They play an important role especially for proprioception position perception and nociception pain perception in joints. In fact, studies have shown that there is internal growth in the nervous and vascular systems during the healing of tendon, which causes chronic pain. Overview of overuse chronic tendinopathy. Overview of the management of overuse chronic tendinopathy.
Protect your tendons: Preventing the pain of tendinitis. National Institute of Health News in Health. Laskowski ER expert opinion. Mayo Clinic, Rochester, Minn.
Varshney A, et al. Autologous platelet-rich plasma versus corticosteroid in the management of elbow epicondylitis: A randomized study. Rubio-Azpeitia E, et al. Adult cells combined with platelet-rich plasma for tendon healing: Cell source options. Orthopaedic Journal of Sports Medicine. Langer PR. Two emerging technologies for Achilles tendinopathy and plantar fasciopathy.
Clinics in Podiatric Medicine and Surgery. Barnes DE, et al. Percutaneous ultrasonic tenotomy for chronic elbow tendinosis: A prospective study. Journal of Shoulder and Elbow Surgery. Related Shoulder joint Tendinitis pain: Should I apply ice or heat? Associated Procedures X-ray.
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