|Year : 2022 | Volume
| Issue : 3 | Page : 9-18
Pediatric elbow – Developmental and radiological anatomy
Sreekanth Kashayi-Chowdojirao1, Dinesh Ranjan Yadala2
1 Associate Professor, Department of Orthopaedics, Nizam's Institute of Medical Sciences (NIMS), Hyderabad, Telangana, India
2 Senior Resident, Department of Orthopaedics, Nizam's Institute of Medical Sciences (NIMS), Hyderabad, Telangana, India
|Date of Submission||29-Mar-2022|
|Date of Acceptance||04-Apr-2022|
|Date of Web Publication||25-May-2022|
Kashayi-Chowdojirao, NIMS, Hyderabad, Telangana
Source of Support: None, Conflict of Interest: None
Mismanagement of pediatric elbow injuries is not uncommon because of lack of knowledge of anatomy and development of elbow joint. This article describes postnatal development of elbow joint and radiological landmarks essential to differentiate normal and abnormal radiographs. Elbow joint is formed from six secondary ossification centres the order of appearance of which on radiographs can be remembered by the mnemonic “CRITOE”. The osteology, ligamentous and neurovascular anatomy of elbow have been described. Standard and special radiographic landmarks have been discussed while are invaluable to assess pediatric elbow radiographs especially in the setting of trauma.
Keywords: Pediatric elbow, Anatomy, Development, Radiology, Ossification center
|How to cite this article:|
Kashayi-Chowdojirao S, Yadala DR. Pediatric elbow – Developmental and radiological anatomy. J Orthop Assoc South Indian States 2022;19, Suppl S1:9-18
|How to cite this URL:|
Kashayi-Chowdojirao S, Yadala DR. Pediatric elbow – Developmental and radiological anatomy. J Orthop Assoc South Indian States [serial online] 2022 [cited 2022 Jul 6];19, Suppl S1:9-18. Available from: https://www.joasis.org/text.asp?2022/19/3/9/346027
| Introduction|| |
Elbow injuries constitute 5%–10% of total fractures of upper extremity fractures, of which distal humeral region is the most commonly involved one., Making an accurate diagnosis in case of pediatric elbow injuries is often challenging to a general orthopedician owing to the complex anatomy of the joint, which if missed may result in decreased function and deformities. A thorough knowledge and understanding of anatomy and development of the elbow joint are essential in the management of pediatric elbow injuries.
The fundamental anatomy of any long bone has three parts: (1) epiphysis which is the bulbous, articular cartilage covered end which tapers to the metaphysis, (2) metaphysis which is funnel shaped, adjacent to the physis and tapers toward the diaphysis and (3) diaphysis which is central and interposed between the metaphyses. In a growing child, the epiphyseal and metaphyseal regions are separated by the organized cartilaginous physis, which is the major contributor to longitudinal growth of the bone. However, the elbow does not contribute significantly to the longitudinal growth of the upper limb as the main contribution is from the proximal humerus and distal radius. For example, the distal humerus accounts for only one-fifth of the growth of the humerus [Figure 1]. Unlike proximal humeral and distal radial fractures, which can be managed nonoperatively to a large extent, especially in younger kids,, fractures around the elbow such as supracondylar humerus and lateral condyle humerus have lesser remodeling potential and have to be managed with caution.
The periosteum in a child is thick and loosely adherent as compared to that in adults where it is thin and firmly adherent. Hence, in general, any displaced fracture in a child lifts off the periosteum, while that in an adult periosteum is easily torn. Fractures in children heal faster and have greater remodeling potential when compared to adults., As a general rule, deformities in the plane of motion remodel well, i.e., flexion–extension deformities in the sagittal plane remodel well as compared to varus–valgus deformities in the coronal plane.
|Figure 1: Contribution of physes around elbow to the longitudinal growth of the long bones|
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| Embryological Anatomy|| |
Elbow being a synovial joint develops in three phases: homogeneous interzone, three-layered interzones, and cavitation at 6, 7, and 8 weeks of life, respectively. Primary ossification begins at 12 weeks, which is first seen in the lower epiphysis of the humerus and then in the trochlea followed by the head of the radius.
| Developmental Anatomy (Postnatal)|| |
The distal humerus is completely cartilaginous in the newborn. The supracondylar region along with olecranon and coronoid is present both in the epiphyseal and metaphyseal regions. The physeal contour is relatively transverse and the epiphyseal cartilage exhibits the exact contour of the eventual mature bone. In the first 6 months, there are hardly any changes in the cartilaginous anlage except for the endochondral ossification and elongation of the distal humeral physis and the migration of the supracondylar fossa into the metaphyseal region.
Elbow joint is formed from six secondary ossification centers. The order of appearance is important in the evaluation of the elbow radiographs. The acronym “CRITOE” serves as a mnemonic in remembering the order of appearance of ossification centers of the elbow i.e., capitellum, radial head, medial (Internal) epicondyle, trochlea, olecranon, and lateral (External) epicondyle [Figure 2]. However, these are only average values and there are a lot of variations in the values in the literature. There may be variations in the order of appearance due to genetic and racial factors which were studied by Cheng et al. In Chinese population, the order of trochlea and olecranon is reversed. The time of appearance and order of fusion are different in both the sexes occurring around 2 years earlier in girls. [Figure 3] summarizes the order of appearance and physeal closure.
|Figure 3: Age of appearance of secondary ossification centers (a) and age of fusion of ossification centers (b). Note that the lateral epicondyle does not fuse directly to the humeral metaphysis as does the medial epicondyle|
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Ossification center of capitellum appears between 6–8 months of age. It can appear as early as 1 month in rare cases and may not appear until as late as 2 years, but is present invariably by the age of 2 years. The capitellar ossification center is spherical initially and it elongates into an oval shape to approach and include the lateral part of trochlea with increasing age. This expansion is the reason behind lateral condyle fractures in children reaching the trochlear articular surface. Ossification begins eccentrically and then anteriorly and medially and finally proceeds posteriorly and laterally. The capitellum is situated anteriorly and is wider posteriorly than anteriorly.
The initial ossification of proximal radial epiphysis happens around the same time as the medial epicondyle, during the 5th–6th year. The radial head epiphysis is initially a thin ellipse, but gradually matures to flatten and develop a concavity for radiocapitellar articulation. In an infant, the radial neck typically appears medially angulated, which should not be mistaken for proximal radial dislocation without confirming other views. It is to be remembered that when the epiphysis first ossifies, the lateral physis is typically wider than the medial physis on AP view. It is also to be noted that proximal metaphyseal clefts and notches and irregular or bipartite radial head ossification are normal variants.
Ossification center of medial epicondyle appears relatively early at age of 5–7 years, but is the last to fuse with humerus shaft [Figure 4]. It does not fuse to trochlear center and remains as a separate functional center throughout the development and fuses with humerus at the age of 15–17 years. The medial epicondyle is present just anterior to the posterior humeral line on lateral radiographs and below the distal extent of the olecranon fossa. It is extra-articular as it is the flexor origin and the attachment for the ulnar collateral ligament, a valgus stabilizer.
|Figure 4: Radiograph of a 15-year-old boy showing that the medial epicondyle is the last center to unite with humeral shaft|
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The ossification center appears at around 9–10 years. It has a large degree of variability sometimes appearing even before the medial epicondyle. It may be irregular with multiple centers giving rise to a fragmented appearance [Figure 5].
|Figure 5: Radiograph of an 11-year-old boy showing multiple foci of trochlear ossification|
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The olecranon ossification center (s) typically forms at approximately 8–10 years of age, occurring before the lateral epicondylar epiphysis. The olecranon serves as an attachment site for the broad insertion of the triceps and is typically formed from two or more separate ossification centers that later coalesce before final physeal closure. The location of the proximal ulnar physis is variable within the elbow joint but will commonly migrate distal with age. As this physeal migration occurs, the apophysis starts to contribute an increasing percentage to the articular surface of the proximal olecranon. The secondary ossification center of the olecranon may persist late into adult life.
Ossification center of lateral epicondyle is the last to appear, at the age of 11–12 years. Ossification of the lateral epicondyle starts distally and laterally and is initially thin with a wide physis. Before the final fusion, it assumes a triangular shape. Outline of the lateral epicondyle could be irregular, but frank fragmentation should raise suspicion for avulsion injury.
| Fusion of Ossification Centers|| |
The capitellum and trochlea may fuse as early as the age of 10 years, but fusion usually begins by 12 years of age. This combined ossification center fuses to the lateral epicondyle at the same time to form the main body of the distal humeral epiphysis, which fuses to the metaphysis of the humerus as early as 12 to 13 years of age, which signals the end of longitudinal growth of the distal humeral physis [Figure 3]. Finally, the medial epicondyle fuses to the distal end of the humerus between 14 and 17 years of age. At around the same time that the common distal humeral epiphysis fuses with its metaphysis (14–16 years), the fusion of the proximal radial and olecranon epiphyseal centers occurs with their respective metaphyses.
| Surface Anatomy|| |
The contours of biceps and cubital fossa are well seen anteriorly. Laterally, the avascular interval between brachioradialis and triceps (so-called “column”) is an important landmark that can be palpated for surgical exposures. Laterally, the olecranon tip, lateral epicondyle, and radial head form an equilateral triangle, an important landmark for the soft spot for the lateral elbow used for joint aspiration or elbow arthrography. The flexion crease of the elbow is in line with the medial and lateral epicondyles and indicates the joint axis, which is 1-2 cm proximal to the joint line with the elbow extended. The tip of the olecranon and both the epicondyles are aligned in line with the elbow extended and they form an inverted triangle posteriorly with the elbow at 90° of flexion [Figure 6]. Dhillon et al. in their study concluded that this triangle is scalene (has unequal sides).
It is the clinical measurement of varus–valgus angulation of the arm with the elbow fully extended and the forearm fully supinated. The clinical carrying angle of the normal elbow is a slight valgus alignment averaging approximately 7° [Figure 7].
|Figure 7: Normal carrying angle has been demonstrated on the right elbow. The patient had valgus deformity on the left secondary to nonunion of lateral condyle humerus|
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| Relevant Osteology and Ligamentous Anatomy|| |
The strong medial and lateral columns of the distal humerus are connected by a wafer-thin bone that is only 1 mm thick in the central portion, which is produced by the olecranon fossa posteriorly and the coronoid fossa anteriorly [Figure 8]. This has been demonstrated by McCarthy and Ogden in their cadaveric studies.
|Figure 8: Cross section of distal end of humerus through the region of coronoid fossa. At this level, the midportion of the humerus is very thin, while the medial and lateral columns are thicker|
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The elbow is a hinged joint comprised of three articulations: the ulnohumeral, the radiocapitellar, and the proximal radioulnar (PRUJ) joints. The primary static stabilizers of the PRUJ are the annular and the quadrate ligaments. The annular ligament encircles the radial neck from its origin and insertion on the proximal ulna. It tightens in supination owing to the shape of the radial head. The annular ligament is confluent with the remainder of the lateral collateral ligamentous complex.
The other structures that have a secondary role in the stability of PRUJ include the oblique cord (or Weitbrecht ligament) and the interosseous ligament. The former is present in 52.6% of forearms, which originates just distal to the radial notch on the ulna and inserts distal to the bicipital tuberosity on the radius at an angle of 45°, and the latter is distal to the oblique ligament with its primary fibers in the direction opposite (radius proximally to ulna distally) to that of oblique cord [Figure 9]. These structures are most taut in supination because of the shape of the radial head and the radial bow. The radial head is elliptical in axial cross-section, with the long axis of the ellipse perpendicular to the radial notch of the ulna in supination., This helps maximally tension the annular ligament as well as the thick anterior portion of the quadrate ligament in supination.
|Figure 9: Annular and quadrate ligaments, oblique cord, and interosseous membrane|
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The medial collateral ligament is the major constraint to valgus instability of the elbow. Its origin is from anteroinferior surface of the medial epicondyle. It is comprised of 3 parts: anterior, posterior, and transverse bundle [Figure 10]. The most important valgus stabilizer is the anterior bundle and the transverse bundle does not play much role in stability, while the posterior bundle is a thickening of the posterior capsule. Anterior bundle is the primary stabilizer to valgus stress at 90° of flexion, and in extension, it provides 30% of the restraint. The posterior bundle is tight in elbow flexion and should be released in case of stiff elbow to gain some flexion.
| Muscles|| |
There are four compartments/groups: anterior, posterior, medial and lateral [Table 1].
| Range of Motion|| |
The primary stabilizer of the elbow is the ulnohumeral joint and it allows up to 150° of flexion. The other two joints are trochoid joints allowing forearm rotation up to 75° in pronation and 85° in supination.
| Vascular Anatomy|| |
Yamaguchi et al. described in great detail the vascular anatomy of the elbow. Elbow has highly consistent and extensive collateral circulation.
The blood supply of elbow is comprised of extraosseous and intraosseous supply, which are organized into three vascular arcades viz., lateral, medial and posterior arcades [Table 2]. The extraosseous blood supply is from the main vessel, the brachial artery in the cubital fossa anteriorly, and the anastomosis around the elbow joint [Figure 11]. The brachial artery lies medial to the biceps tendon and lateral to the median nerve. At the level of the radial head, it gives off its terminal branches radial and ulnar arteries. The major branches of the brachial artery are the superior and inferior ulnar collateral arteries. The intraosseous circulation is derived mainly from perforating vessels rising from nearby extraosseous arteries.
|Figure 11: Arterial anastomosis around the elbow joint. R: Radial artery, B: Brachial artery, RR: Radial recurrent artery, IR: Interosseous recurrent artery, SUCL: Superior ulnar collateral artery, IUC: Inferior ulnar collateral artery, RC: Radial collateral artery, MC: Middle collateral artery|
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The posterior arcade supplies supracondylar part of the humerus through olecranon fossa. This has a clinical bearing that the least opted approach for open reduction of supracondylar humeral fracture is posterior approach and cases of trochlear osteonecrosis have been reported. Haraldsson described that lateral epicondyle, capitellum, and trochlea [Figure 12] are supplied by the posterior condylar perforating vessels and so posterior subperiosteal dissection in this area should be strictly minimized. After any such dissection, circulation from the lateral epicondyle would be the only source left. On the other hand, the medial aspect of the distal humerus receives blood supply both anteriorly and posteriorly. There is a watershed area between the medial and lateral vascular contributions to the trochlea corresponding to the trochlear groove. Occasional avascular necrosis or nonunion in this area may be explained by the damage to the blood supply.
|Figure 12: Lateral condyle of the humerus derives its blood supply from the posterior aspect. (Redrawn from Beaty JH, Kassert JR: The elbow region: General concepts in the pediatric patient. In Beaty JH, and Kassert JR, editors: Rockwood and Wilkins' fractures in children, ed 7, Philadelphia, 2010, Lippincott Williams and Wilkins)|
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| Nerves|| |
It supplies biceps and brachialis, the elbow flexors, and continues through the brachial fascia lateral to the biceps tendon and terminates as lateral cutaneous nerve of the forearm.
It passes through the anterior aspect of the arm where it crosses the brachial artery as it passes across the intermuscular septum, then into the cubital fossa, medial to biceps tendon and the brachial artery. It then passes under the bicipital aponeurosis, then under pronator teres and then it enters the forearm and continues distally under the flexor digitorum superficialis (FDS) within its fascial sheath. Along with a few articular branches it supplies pronator teres, flexor carpi radialis, palmaris longus, and FDS. As these branches arise medially, medial retraction of the nerve is safe during anterior exposure. The anterior interosseous nerve innervates the flexor pollicis longus and the lateral portion of the flexor digitorum profundus (FDP). It arises from the median nerve near the inferior border of the pronator teres and courses along the anterior aspect of the interosseous membrane accompanying the anterior interosseous artery.
The radial nerve passes the distal humerus in the brachialis–brachioradialis interval. As the nerve descends into the forearm, it divides into the superficial radial sensory branch and the posterior interosseous motor branch. The PIN passes under the arcade of Frohse and between the two heads of the supinator. The PIN supplies the abductor pollicis longus, the extensor pollicis longus, the extensor pollicis brevis, and the extensor indicis on the dorsum of the forearm.
The ulnar nerve passes posterior to the medial intermuscular septum of the arm, through the cubital tunnel behind the medial epicondyle of the distal humerus, and then enters the forearm between the two heads of the flexor carpi ulnaris. Distally, it supplies the ulnar half of the FDP. Two cutaneous nerves arising in the distal half of the forearm innervate the skin of the wrist and the two ulnar digits of the hand.
| Radiological Anatomy|| |
It is said that man has two upper limbs and lower limbs for orthopedic surgeons to compare! This “adage” is useful not only in clinical examination but also in the radiological examination when there is doubt between a fracture and secondary ossification center. Having said that, it is to be remembered that comparison views may not always be helpful, as a certain degree of asymmetry exists for many of the ossification centers, especially during the initial stages of formation. For example, the ossification center for the lateral epicondyle may normally appear several months before its contralateral counterpart, and so may be mistaken for an avulsed fragment in an injured elbow.
| Standard Views|| |
True anteroposterior (AP) radiograph can be obtained only by placing a fully extended elbow on the X-ray cassette [Figure 13]a. However, in the setting of trauma, when the child cannot fully extend the elbow and a distal humeral fracture like a supracondylar fracture is suspected, the AP view of the distal humerus is obtained keeping the distal humerus parallel to and abutting the cassette with the elbow extended as much as possible [Figure 13]c. True lateral radiograph is obtained with the elbow at 90° of flexion and the medial/inner aspect of the arm, elbow, and the fully supinated forearm abutting the cassette [Figure 13]d. Parents' help may be sought in assisting the positioning (to alleviate pain and anxiety of the child) and the treating doctor should explain the importance of obtaining proper radiographs without which decision-making would be difficult and the quality of care would be compromised.
|Figure 13: True anteroposterior view (a). When the elbow cannot be fully extended, AP view should not be obtained in this manner (b), rather distal humerus or arm should be parallel to the cassette (c). True lateral view (d), with the elbow flexed and forearm supinated. AP: Anteroposterior|
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| Other Views|| |
Some views are very useful in special situations. Internal oblique view-in cases of suspected lateral condyle humerus fracture; Jones view-during intraoperative assessment of supracondylar fracture of humerus to look at the medial and lateral condyles (columns) with the elbow in a flexed position; distal humeral axial view-for medial epicondyle fracture.
| Radiographic Lines/Signs on Anteroposterior View|| |
Baumann's angle is the angle between the line through the physis of the lateral condyle of distal humerus and the long axis of the humerus. There is confusion in the literature regarding the second line which is taken by some authors as the line perpendicular to the long axis of the humerus. Acton and McNally opined that it is better to use the term shaft-physeal angle (the angle between the long axis of the humerus and the inclination of the capitellar physis). The normal angle is 75°–80°. Whether it is α or 90-α [Figure 14] that we call Baumann's angle, it should not be of concern practically.
The humeral–ulnar angle is between the lines longitudinally bisecting the shaft of the humerus with the shaft of the ulna on an AP view [Figure 15]. It is the most accurate in determining the true carrying angle of the elbow. In very young children, where almost the entire distal humerus is cartilaginous, medial and lateral humeral lines [Figure 16] may be used along with the ulnar axis to determine the alignment as described by Chou et al.
|Figure 14: Baumann's angle or shaft-physeal angle. (Redrawn from Beaty JH, Kassert JR: The elbow region: general concepts in the pediatric patient. In Beaty JH, and Kassert JR, editors: Rockwood and Wilkins' fractures in children, ed 7, Philadelphia, 2010, Lippincott Williams and Wilkins)|
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|Figure 15: Radiograph of a 9-year-old boy. Normal humeroulnar angle is depicted|
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|Figure 16: (a) Ulnar axis (red) falls within the boundaries of the medial humeral line and lateral humeral line in a normal elbow (left), while it is outside the confines of the two lines (right) in a case of transphyseal separation. (b) Radiograph of an 18-month-old girl with transphyseal separation (Salter Harris type I) distal humerus. Note that the ulnar axis (red line) is not within the confines of MHL and LHL (yellow). Also note that the radiocapitellar line (blue) is preserved which is a clue that the distal forearm is in continuity with distal humeral epiphysis. LHL: Lateral humeral line, MHL: Medial humeral line|
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| Radiographic Lines/Signs on Lateral View|| |
Anterior and posterior fat pads are normally present in the pericapsular region and can be seen on lateral radiographs. The anterior fat pad is present in the coronoid fossa and extends anteriorly out of the margins of the fossa; the posterior fat pad in the olecranon fossa lies totally within the depths of the fossa when the elbow is flexed [Figure 17]. The posterior fat pad is usually not visible on radiographs and the presence of the posterior fat pad sign points toward an occult fracture if the fracture line is not clear; it has 75% sensitivity for fracture. Most children who do not have an injury have an anterior fat pad sign; this is simply the anterior elbow capsule displacing the brachialis. If this fat pad is enlarged, the so-called “sail sign,” the child usually has an osseous injury.
Anterior humeral line
A line along the anterior border of the distal humeral shaft should pass through the mid-third of the capitellar ossification center [Figure 18].
|Figure 18: Radiograph of a 9-year-old boy showing anterior humeral line and radiocapitellar line. RCL: Radiocapitellar line, AHL: Anterior humeral line|
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Shaft condylar line
There is an angulation of 40° between the long axis of the humerus and the long axis of the lateral condyle [Figure 19].
|Figure 19: Shaft condylar angle: There is an angulation of 30°–40° on a lateral radiograph between long axis of humerus and long axis of the lateral condyle|
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Silberstein et al. noted that the physis of the capitellum is wider posteriorly than anteriorly on a [Figure 20] lateral view. It is important not to mistake it as physeal injury in a very young kid.
|Figure 20: Radiograph of an 18-month-old girl showing that the physis of the capitellum is wider posteriorly than anteriorly|
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The traditional radiocapitellar line (RCL), also called Storen line eponymously, is a line drawn along the radius that is expected to intersect the capitellum on normal lateral elbow radiographs [Figure 18]. Ramirez et al. studied 116 normal pediatric elbow radiographs and concluded that the traditional RCL misses the capitellum in 16% of the elbows. Fader et al. concluded that eccentric ossification of the capitellum explains RCL variability in young children. Wang et al. proposed a radiocapitellar P-line (passing through the midpoints of proximal and distal radial physes) as an alternative to the traditional RCL.
Lincoln and Mubarak described “ulnar bow sign” according to which tangent drawn to the posterior border of ulna on a true lateral radiograph is usually straight and should not deviate more than 0.01 mm; otherwise, a radial head dislocation or subluxation should be suspected [Figure 21].
|Figure 21: (a) Radiograph of a 9-year-old boy showing a normal straight ulnar border. (b) Radiograph showing bowing of ulna (“ulnar bow sign”) in a case of chronic Monteggia fracture|
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| Additional Imaging|| |
Additional imaging such as arthrography, ultrasonography, computed tomography, and magnetic resonance imaging have not been dealt in this article.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21]
[Table 1], [Table 2]