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Anatomy, histology and biomechanics of a tendon

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Anatomy, histology and biomechanics of a tendon
 
Example of the patellar tendon
 
Introduction
The challenges are to understand the operation of a knee from the anatomical analysis of the various constituent elements. The patellar tendon is only part of the knee extensor apparatus. The extensor mechanism is essential to the proper functioning of the knee joint. It consists, from proximal to distal, the quadriceps tendon from the four heads of the quadriceps muscle to the patella and patellar ligament, which ends on the anterior tibial tuberosity.
 
 
ANATOMY OF A TENDON
 
Structure and function of a tendon
The function of a tendon is to mobilize a joint. That is to transmit the force created by the contraction of this muscle.
The structure of a tendon is composed of collagen and elastic fibers in an aqueous matrix and proteo-glycans. These products are synthesized by tenocytes and t├ęnoblastes.
The tendon in a fibro-elastic texture, the shape can vary (flat, cylindrical or ribbon).
The patellar tendon runs from the tip of the patella to the tibial tubercle in front and at the top of the tibia.
 
From the micro to the macro-structure
The internal structure of the patellar tendon experiencing a hierarchical organization that is a microscopic element which are the collagen fibers, which measure between 20 and 150 nanometer until tendon that has him a circumference of 2 to 12 mm. From the smallest to the largest element which is the tendon, an organization we note: fibers made ??by a primary beam and number of primary beams that form a secondary beam is called booklet, all these papers is the tertiary fiber bundle , all of these tertiary fibers therefore the tendon.
The envelope of the primary, secondary and tertiary beams is called endotendon, the envelope of the tendon itself is called ├ępitendon. The electron microscope with a magnification of 4000 is used to display collagen structures. An electron microscope with 100x magnification can identify the beam side of collagen fibers.
At the cellular level
The cells are observed under the electron microscope from a blood eosin staining with a magnification of x200. They are called t├ęnoblastes. They are broad with an ovoid nucleus, the cell structure with a rough endoplasmic retinaculum very rich and developed, which synthesizes proteins intensely tendon tissue.
Surrounding structures
The tendon knows two important envelopes
- The paratenon which corresponds to a sliding membrane which is richly vascularized.
- The ├ępitendon which is a fibrillar network located in the para tendon around the tendon with a thickness of 10 nanometers. During the stretching of the tendon, the frame of the ├ępitendon modifies its obliquity is 60 to 30 ┬░ which also increases the rigidity of the tendon.
 
BIOMECHANICAL
The biomechanical characteristics of a tendon are:
- Contractile force due to the molecular and supramolecular organization of collagen
- Flexibility due to the presence of elastin fibers
- Inextensibility due to the transmission capacity of the muscle force to bone.
- Resistance to stretching and compressive forces.
The adaptation of the tendon to tension in all directions is done through a network rather disorganized while a resistance to tension in an axis is done through a parallel fiber organization.
By modifying the dimensions of a tendon subjected to a tensile force, one obtains:
- The increase of the section of the tendon increases the resistance of the tendon as well as its stiffness.
- Increasing the length equal force to a tendon reduces its stiffness but preserve the same force.
During exercise stress on a tendon, that is, by applying force to the two segments of it, a biomechanical behavior is observed:
1- a linear increase area of the tendon elongation depending on the force exerted so-called elastic behavior of the tendon. There is no molecular macro deformation of the tendon but a progressive stretching. The deformation is reversible after released from the force.
2- a so-called plastic deformation of the tendon if the voltage increases again. There is an alteration of microscopic structures, the tissue is altered in its on disposal stress structure, there is irreversible damage. Beyond the tendon resilience, he knows a breaking point.
 
The fibers constituting the tendon have different properties that characterize this tendon. First, the collagen that makes the very high voltage resistance.
Note that collagen fibers 1cm2 section can help withstand a 1000 Newton force.
Collagen can deform 8 to 10%, however it is very low torsional and lateral inclination. On the other hand, elastin known high scalability with an ability to increase in size of 200%.
By comparison, the biomechanical behavior of a tendon or ligament depends on the architecture, the tendon to a unidirectional organization which allows constraint in very large voltage while the ligament to a multidirectional organization which allows to lower loads in one direction but rather in different directions.
 
Role of the patella
The presence of the ball in the stent apparatus is a major element biomechanical:
it increases the lever arm that is to say that the rotation of the knee center in the femur in the femoral condyle is eccentric with this lever arm of the patella situated a few centimeters in front of this one. The forces exerted on the knee and are expensed in part by the quadriceps tendon. The stresses exerted on the cartilage between the patella and the femur are such that the thickness of the cartilage is the largest of the whole organism to withstand it.
From an anatomical point of view, during the contraction of the quadriceps there is a natural tendency toward eccentricity to the side ie the side of the patella, the V-shape of the patella that the wife of one called the anterior trochlea femur preserves sliding the extensor mechanism in the knee axis. There is therefore a pulley part of the anterior part of the femur trochlea called slides wherein the extensor mechanism of the knee, the patella first.
In vivo applications and pathological
The forces on the patellar tendon:
-          a shot a ball of 5200 Newton
-          receiving a jump 8000 Newton
-          a sprint 9000 Newton
-          deadlift (weightlifting) 14 500 Newton
 
The tendon stretching injuries occur during:
- Direct trauma
- A fast muscle contraction, an excessive tendon is in an abnormal direction is with abnormal speed. There was no activation of this pre tendon ( "heating").
Note that in the age of cell capacity tendon regeneration and distention of the collagen and elastin fibers capacity decreases, which is an increase of the tear of the Achilles tendon in particular during the fourth decade.
 
The junction bone tendon
There are two types of junction:
- Junction Direct: a junction at right angles to the bone resulting in four distinct transitional zones that become progressively go from the bone a fibro mineralized cartilaginous area to area with a fibro cartilage chondrocytes then a similar area to the tendon.
- Indirect junction: The fibers of the tendon to the bone with a transition zone which is called the fiber "sharpey" where the bone is continuous with a periosteum which binds the particular collagen fibers.
 
PHYSIOPATHOLOGY TIP OF THE BALL
This element describes a mechanism phenomenon implicated in the tendon of the tip of the patella:
- 1 mechanism: even conflict between the bone and the patella tendon during the flexion extension.
- 2nd mechanism: lesion of the deep surface of the patella tendon avulsion type on the deep side in fiber sharpey.
Both mechanisms theoretically explain the clinical aspect of the patella advanced pain in sports like jumping or reception like Volleyball or Basketball and tennis. This explains why this tendinopathy is also called "jumper's knee" in English.
We know that the compliance of the bone tendon junction, myo and tendon stress equal upper zone myo tendon, above the tendon and bone area above the stress resistance of a single tendon. This is due to the elastic property of muscle that is transmitted to the complex myo tendon.
 
CONCLUSION
The anatomical knowledge, histological and biomechanical patellar tendon allows us to explain the pathologies encountered. In sport, stressful situations are by repeating micro trauma or during a violent trauma beyond the anatomical and biomechanical capabilities of these structures.

Doctor Yoann BOHU, Doctor Nicolas LEFEVRE, Doctor Serge HERMAN. - 24 octobre 2013.

Conflicts of interest: the author or authors have no conflicts of interest concerning the data published in this article.

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