Give an Example of a Hinge Joint

Hinge Joint

KNEE PAIN

Michael A. Krieves , ... Elizabeth Demers Lavelle , in Current Therapy in Pain, 2009

ANATOMY

The knee is a hinge joint between the femur and the tibia consisting of three compartments, medial tibiofemoral, lateral tibiofemoral, and patellofemoral. The three compartments share a common synovial cavity. The patella is a sesamoid bone in the quadriceps tendon that acts as a pulley to increase the mechanical advantage of the quadriceps as it articulates with the trochlear groove of the femur. The head of the fibula is within the knee capsule but is not a weight-bearing surface. The femoral condyles and tibial plateaus form the weight-bearing surfaces. The meniscal cartilage provides shock absorption in the space between the tibial plateaus and the femoral condyles. The collateral ligaments provide stabilization of the joint. The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) stabilize the knee in flexion and extension. The medial collateral ligament (MCL) stabilizes the knee against a valgus stress, and the lateral collateral ligament (LCL) stabilizes the knee against a varus stress.

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Orthopaedics 3. The proximal limbs

Marcus J. Head , Alistair Barr , in Equine Medicine, Surgery and Reproduction (Second Edition), 2012

Anatomy

The elbow is a ginglymus or hinge joint formed between the distal humerus and the proximal end of the radius and ulna and is restrained by collateral ligaments medially and laterally.

In the foal, the distal humerus comprises separate ossification centres for the distal epiphysis and the medial epicondyle. The distal humeral epiphysis fuses to the metaphysis at 11–24 months. Both the radius and ulna each have a single proximal epiphysis. The proximal radial epiphysis fuses to the metaphysis at 11–24 months. The proximal epiphysis of the ulna initially appears quite small radiographically and well separated from the rest of the ulna. It fuses to the rest of the ulna at 24–36 months.

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The Musculature of the Rat

Robert Lewis Maynard , Noel Downes , in Anatomy and Histology of the Laboratory Rat in Toxicology and Biomedical Research, 2019

Terminology of Movements

Flexion and extension: typically described at hinge joints such as the elbow, flexion refers to the reduction of the angle at the joint and extension is the opening of the angle. Flexion is also defined as the approximation of volar surfaces. The volar surface of the arm includes the palmar surface of the fingers, the palm (volar means the hollow of the palm or foot), and the inner surface of the wrist, forearm and upper arm. Thus flexion at the elbow and knee approximates the volar surfaces of the arm and leg, respectively. At the hip flexion means the forwards movement of the leg (kicking a ball involves flexion at the hip) and extension means swinging the leg back as if getting ready to kick a ball.

Dorsi-Flexion and Plantar-Flexion: these are special terms used by human anatomists for the action that lifts the toes towards (dorsi) and moves them away (plantar) from the shin.

Abduction: movement away from the midline of the body, adduction is the converse. For instance, at the hip adduction means swinging the leg out sideways,

Protraction: swinging the limb forwards at the shoulder or hip, retraction is the converse, both of which are the essential movements when a four-footed animal runs.

Pronation: the turning of the hand so that the palm faces downwards, supination is the converse.

The shoulder and hip joints present special difficulties, because they are ball and socket joints, not hinge joints, so movement in a number of planes can occur. In human anatomy, flexion of the shoulder is defined as swinging the arm forwards and across the trunk, extension is the converse. Abduction means swinging the arm outwards from the body at 90 degrees to the plane of flexion and extension, adduction is the converse. Rotation can, of course, also occur at ball and socket joints.

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Mechanical Work and Energy

Mark L. Latash , Vladimir M. Zatsiorsky , in Biomechanics and Motor Control, 2016

4.4.2.1 Energy transfer between adjacent segments

Consider a two-link chain with a hinge joint ( Figure 4.4). The joint is served by a single-joint muscle. Apart from a trivial case, when only one segment is rotating (and hence the second segment neither gives away nor acquires energy), there are two distinct situations. The adjacent segments may rotate either in opposite directions or in the same direction. When the adjacent links rotate in the opposite directions, energy is not transferred between the links. When they rotate in the same direction, the transfer of energy occurs.

Figure 4.4. A two-link system with a hinge joint. A planar movement. $\dot{\alpha }$ is joint angular velocity, ${\dot{\theta }}_p$ and ${\dot{\theta }}_d$ are angular velocities of the proximal and distal links, respectively. The angular velocities of the links are in the external coordinates.

When the adjacent segments rotate in the same direction with equal angular velocities $\left({\dot{\theta }}_d={\dot{\theta }}_p\right)$ , the joint angular velocity is zero. Hence, the joint power is also zero. The absolute values of power of the moments of force acting on the adjacent segments are equal, $\left|T{\dot{\theta }}_d\right|=\left|-T{\dot{\theta }}_p\right|$ . One segment loses energy and the second gains energy by the same amount. As a result, "pure" energy transfer from one segment to an adjacent segment occurs. Energy is transferred between the segments while the total energy of the system is conserved. The muscles that cross the joint act isometrically and do not do any work. They behave like nonextensible struts.

When the adjacent segments rotate in the same direction with unequal angular velocities $\left({\dot{\theta }}_d\ne {\dot{\theta }}_p\right)$ , the joint angular velocity is $\dot{\alpha }={\dot{\theta }}_d-{\dot{\theta }}_p$ . The power flow to/from the individual segments is $P_d=T{\dot{\theta }}_d$ and $P_p=-T{\dot{\theta }}_p$ , respectively, whereT is a joint torque. One segment gives away the energy to the other segment; the second segment acquires the energy. Because ${\dot{\theta }}_d\ne {\dot{\theta }}_p$ , the energy gain of one segment and the energy loss of the other segment are not equal in magnitude. The difference equals the joint power, $P=T·({\dot{\theta }}_d-{\dot{\theta }}_p)$ or $P=T·\dot{\alpha }$ . Two flows of energy exist: (1) from a segment to a segment and (2) between the segment(s) and the joint actuators (the muscles and tendons). The joint actuators transfer the energy from one segment to the other, and, in addition, they either provide mechanical energy to the system (when the joint power is positive) or absorb the energy (when the joint power is negative). The entire situation is summarized in Figure 4.5.

Figure 4.5. Energy transfer for a two-link system. The curved arrows indicate the direction of the link rotation. Transfer occurs when the angular velocities of the links ${\dot{\theta }}_i$ are in the same direction, in sectors 1 and 3. There is no energy transfer in sectors 2 and 4, where the links are rotating in opposite directions. Link i acquires energy when $T{\dot{\theta }}_i\>0$ and it gives away energy when $T{\dot{\theta }}_i\<0$ .

Adapted by permission from Zatsiorsky (2002), © Human Kinetics.

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Common Conditions and Physical Rehabilitation of the Athletic Patient

Sherman O. CanappJr., Deborah Gross Saunders , in Canine Rehabilitation and Physical Therapy (Second Edition), 2014

Anatomy

The elbow joint as a whole is a trochlea arthrosis or ginglymus (hinge) joint, allowing flexion and extension of the humerus over the radius and ulna. In dogs, the caudal aspect of the head of the radius rotates in the radial notch of the ulna during pronation and supination. ²² The normal elbow joint is characterized by a smooth transition from the ulnar articular surface to the radial surface. The medial coronoid process of the ulna is level with or slightly below the surface of the radius (Figure 33-17).

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Locomotor system

H.A.W. Hazewinkel , ... B. van Rijssen , in Medical History and Physical Examination in Companion Animals (Second Edition), 2009

Passive movements

Passive movements of the skull are confined to the temporomandibular joint, which functions as a hinge joint but also allows some lateral movement of the mandibles. The joint is examined for moveability (opening and closing the mouth), crepitation, and pain. Both joints are evaluated simultaneously by standing behind or beside the patient and placing the fingers at the base of the zygomatic arch on each side. The assistant or owner opens and closes the dog's mouth and then moves the mandible from side to side. Sedation or anesthesia is needed for further examination, and even for this part of the examination if the animal is excited or aggressive.

Passive movements of the vertebral column are only carried out if careful palpation and percussion have revealed no abnormalities. Passive movements have the danger of causing lasting damage to the spinal cord. The animal's resistance to passive movements sometimes make the response difficult to interpret. Attention is given to moveability, crepitation, and/or pain.

The examiner stands on the left side of the animal and uses the left hand to grasp the muzzle (first tied securely if necessary), while the right hand fixes the neck just caudal to the second cervical vertebra (the axis). The head is now moved in the dorsoventral direction (nodding 'yes'). The head can also be moved laterally (shaking the head 'no'), or rotated.

The examiner now places the right hand on the spine at the level of the scapulas and then moves the head downward (flexion) and upwards (extension), slowly and carefully! Then the head and neck are moved laterally until each cheek touches the corresponding thoracic wall.

The thoracic vertebral column is rigid, primarily because of its position within the rib cage. This part of the vertebral column cannot be examined by passive movements.

Passive movements of the lumbar vertebral column and the lumbosacral area can be performed in small and medium-sized dogs and in cats on the examination table, while large dogs are examined standing on the floor. In both cases the examiner stands behind the animal and raises it by grasping the proximal femurs (small animals) or the distal femurs (large animals). Then the hind legs are slowly raised to a horizontal position, thus forcing a lordosis (Fig. 17.29c). This movement extends first the hip joints and then the lumbosacral area. An assistant presses down first on the lumbosacral transition and then on the thoracolumbar transition. Attention is given to resistance and any pain reaction. Animals with a painful process in the caudal part of the back will not allow this type of extension and will already resist when it is begun. The lateralization of the pain (left or right) can be examined by alternately extending the legs (Fig. 17.29d). Painful processes in the hip joints and pelvis may obviously hamper interpretation of the observation.

Next in the standing animal the vertebral examination is extended again and the examiner rotates the dog on the axis of the spinal column, first right and then left. The rear limbs are also moved in both lateral directions. In these movements it is helpful if an assistant supports the dog beneath the thoracolumbar area. The dog is turned to the left and to the right.

Finally, the caudal part of the spinal column must still be bent in the dorsoventral direction (flexion). Only in small animals is this carried out with the animal standing. The examiner brings the left hand under the abdomen, just caudal to the costal arch, and places the right hand over the base of the tail. Now the dog is lifted with the left arm and kyphosis is produced by pushing the pelvis ventrally.

In heavy dogs the flexion and extension of the spinal column are performed while the dog is in left lateral recumbency, with the examiner standing beside its back. The examiner places the palm of the left hand on the last lumbar vertebra while the right hand curves around both of the animal's stifles and moves them caudally. The pelvis now tilts at the lumbosacral junction. The left hand is then moved cranially one vertebra at a time and the stretching of the stifles caudally is repeated (Fig. 17.30a). To hyperflex the spinal column, the left hand is held against the animal's abdomen while the right hand, placed over the base of the tail, tilts the pelvis ventrally (Fig. 17.30b).

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Treatment and Rehabilitation of Elbow Dislocations

Michael J. O'Brien MD , Felix H. SavoieIII MD , in Clinical Orthopaedic Rehabilitation: a Team Approach (Fourth Edition), 2018

Anatomy and Biomechanics

The elbow joint consists of two types of articulations and thus allows two types of motion. The ulnohumeral articulation resembles a hinge joint, allowing flexion and extension, whereas the radiohumeral and proximal radioulnar joint allows axial rotation ( Morrey 1986). Stability of the elbow joint is provided by the osseous articulations, medial and lateral collateral ligaments, and traversing muscles.

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The medial collateral ligament (see Fig. 14.1), or ulnar collateral ligament, consists of three parts: anterior, posterior, and transverse segments. The anterior bundle is the strongest and most distinct component, whereas the posterior bundle exists as a thickening of the posterior capsule and provides stability at 90 degrees of flexion.

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The lateral ligament complex (Fig. 17.1) consists of the radial collateral ligament, the annular ligament, and the lateral ulnar collateral ligament. The lateral ulnar collateral ligament contributes the most to stability on the lateral side of the elbow. Injury to this structure can lead to posterolateral rotatory instability.

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Primary stabilizers of the elbow joint include the ulnohumeral articulation, the anterior band of the medial collateral ligament, and the lateral ulnar collateral ligament.

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Secondary stabilizers include the radial head, the coronoid, and the anterior joint capsule (Fig. 17.2). Dynamic stability is provided by the muscles traversing the joint, including the brachialis and the triceps (An and Morrey 2000, Funk et al. 1987), the common extensor musculature origin, and the flexor–pronator musculature origin.

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Tissue and Organ Engineering

D.M. Elliott , ... S.A. Maher , in Comprehensive Biomaterials, 2011

5.524.3 Structure and Function of the Knee Meniscus

The tibiofemoral joint represents a significant engineering challenge, where stability and the need for significant range of motion countermand one another. Unlike hinge joints (e.g., in the digits), multiple degrees of freedom are required to allow mobility. Anatomically, the knee consists of the interaction between the rounded femoral condyles and the relatively flat tibial plateau. The two boney portions are held together via the intraarticular (i.e., anterior cruciate and posterior cruciate) and extra-articular (i.e., medial and lateral collateral) ligaments, with contributions via the patellar tendon and the flexors and extensors of the thigh. The knee menisci, of which there is a medial and a lateral in each knee, are positioned such as to add congruity to the joint and act as a bushing to complete the curvature of the femoral condyle as it impinges on the tibial plateau.

In the adult meniscus, the inner avascular region is more like hyaline or articular cartilage, with a higher concentration of PGs, while the outer vascular region is more fibrous, with a higher collagen content. This is particularly the case in the body of the meniscus as compared to the horns. ⁵ The meniscus extracellular matrix (ECM) contains 85–95% dry weight collagen, of which greater than 90% is type I, ⁶ with the remaining collagen consisting mostly of types II, III, V, and VI ⁷ (see Table 2 ). These collagens of the meniscus are of the most highly cross-linked in the body, providing the tissue with a tough fibrous character. ⁸ PGs makes up less than 2–3% of the dry weight, eight times less than is seen in articular cartilage. ^7–9 The meniscus, like articular cartilage, is highly hydrated, with 72–77% of the wet weight comprised of water. ⁹

In terms of vascularity, the peripheral 10–25% of the meniscus is supplied by a perimeniscal capillary plexus, while the inner stroma is largely avascular. ^10,11 During development, the meniscus is vascular throughout, though these vessels regress with time. ^11–13 Similarly, the cell type of the meniscus (i.e., the meniscus fibrochondrocyte) is a heterogeneous population – in the inner avascular region these cells have a chondrocyte-like phenotype, while in the outer rim these cells are fibroblast-like in appearance and matrix deposition. ^14,15 In both regions, a significant decrease in cell density is observed with skeletal maturation.

As may be expected for such a fiber-reinforced matrix, the mechanical properties of this tissue are highly anisotropic and strongly dependent on the prevailing fiber direction ^16,17 (see Figure 1 ). The tensile properties of the meniscus range from 48–259   MPa in the circumferential direction and 3–70   MPa in the radial direction. ^2,8,16–21 The compressive properties of the meniscus varies between the species and with location (e.g., it is highest anteriorly) ^8,12,22 but it is approximately one-half that of articular cartilage (100–400   kPa, aggregate modulus). The meniscus, while having a lower modulus, is also much less permeable than articular cartilage, ⁸ suggesting that the tissue is optimized to enhance congruency, load distribution, and shock absorption across the joint. ¹⁷

The main functions of the meniscus are to transmit and distribute load over the tibial plateau, increase joint congruency, stabilize the joint, and improve articular cartilage nutrition and lubrication. ²³ These functions are achieved by the unique load shifting that occurs between the more hyaline inner region and the more fibrous outer region of the meniscus. When the joint is loaded, axial loads impinging on the wedge-shaped portion of the meniscus are redirected laterally, causing lateral extrusion. This extrusion is resisted by the osseous anchors originating from the anterior and posterior horns, ¹⁷ generating hoop stresses within the dense network of circumferentially oriented collagen fibers. Lateral extrusion is thereby resisted and the joint stabilized. ^2,12,23,24 The secondary stability offered by the meniscus is particularly important in the anterior cruciate ligament (ACL) deficient knee. ²⁵ With normal knee kinematics, forces that are many times the body weight may be generated within the joint. ^26–30 The menisci transmit 50–100% of joint load in a healthy knee, ^24,27,31 which results in deformations within the tissue on the order of 2–6% with normal activity. ^32,33

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The Carpus

Mike W. Ross , in Diagnosis and Management of Lameness in the Horse (Second Edition), 2011

Anatomy

The carpus consists of the antebrachiocarpal, middle carpal, and carpometacarpal joints. The antebrachiocarpal and middle carpal joints are considered ginglymi, but they are not typical of hinge joints; the carpometacarpal joint is arthrodial. ¹ Arthrodial joints also exist between carpal bones in each respective row. Effective movement of the carpus originates from the antebrachiocarpal and middle carpal joints. The carpometacarpal joint does not open, but it is subject to shear stress. The antebrachiocarpal joint lies between the distal aspect of the radius and the proximal row of carpal bones. The distal, dorsal aspect of the radius has deep grooves in which run the tendons of the extensor carpi radialis and common digital extensor muscles. In flexion the tendons compress the dorsal aspect of the antebrachiocarpal joint, limiting visibility when arthroscopic examination is performed. The proximal row of carpal bones includes the accessory carpal bone, which articulates with the distal aspect of the radius and the ulnar carpal bone. The accessory carpal bone forms the lateral border of the carpal canal. From lateral to medial, the ulnar carpal, the intermediate carpal, and the radial carpal bones complete the proximal row.

The middle carpal joint lies between the proximal and distal rows of carpal bones. The number of bones in the distal row varies but always includes, from medial to lateral, the second, third, and fourth carpal bones. A first carpal bone is present unilaterally or bilaterally in approximately 50% of horses¹ and should not be mistaken on radiographs for an osteochondral fragment. The first carpal bone articulates with the second metacarpal bone (McII) and the second carpal bone, and its presence is often associated with radiolucent areas in the McII. A fifth carpal bone is rare, but if present is small, articulates with the fourth carpal bone and the proximal aspect of the fourth metacarpal bone (McIV), and can be confused with an osteochondral fragment. The second, third, and fourth carpal bones articulate with the McII, the third metacarpal bone (McIII), and the McIV, respectively. The articulation of the second carpal bone and the McII is broader than is that of the fourth carpal bone and the McIV, and hence the McII receives greater load, an important fact to consider with fractures of the McII and the McIV. The third carpal bone, the largest bone in the distal row, has two fossae separated by a distinct ridge, the intermediate (lateral) and radial (medial). The radial fossa is largest, receives greater load, and is more commonly injured. The third carpal bone is L shaped and has a large, dense palmar portion that is rarely injured.

The carpal bones are held together by intercarpal ligaments including the dense palmar carpal ligament from which the accessory ligament of the deep digital flexor tendon arises. The strong intercarpal ligaments play a major role in stability, and the palmar intercarpal ligaments have been shown to provide more resistance to extension of the carpus than does the palmar carpal ligament.² When large medial and lateral corner osteochondral fragments of the third carpal bone are removed, the intercarpal ligaments and capsular attachments must be incised. These dense attachments provide stability, which can be advantageous when slab fractures are repaired. The dorsomedial intercarpal ligament courses between the medial aspect of the second carpal bone and the dorsomedial aspect of the radial carpal bone,³ but during arthroscopic examination it appears to blend with the joint capsule. A theory was proposed that the dorsomedial intercarpal ligament became hypertrophied and impinged on the articular surface of the radial carpal bone, causing secondary modeling in young racehorses and lameness.⁴ Recent studies of normal carpi found that the dorsomedial intercarpal ligament was neither hypertrophied nor impinging on the radial carpal bone. A definite relationship exists between the development of pathological conditions on the distal aspect of the radial carpal bone and the attachment of the dorsomedial intercarpal ligament, but I have not observed hypertrophy or impingement. The majority of radial carpal bone osteochondral fragments occur within or just lateral to the attachment site of the dorsomedial intercarpal ligament. Because the dorsomedial intercarpal ligament resists dorsomedial displacement of the radial carpal bone,³ this site is prone to develop osteochondral fragments. In abnormal carpi, hypertrophy of the dorsomedial intercarpal ligament has been found to be apparent, but no correlation existed between hypertrophy and cartilage or subchondral bone damage.⁵

The medial and lateral palmar intercarpal ligaments resist displacement and dissipate axial forces by allowing abaxial translation of carpal bones.^6,7 The long and short medial and lateral collateral ligaments originate on the radius and attach to the proximal aspects of the McII and the McIV, and the abaxial surface of the carpal bones, respectively. The collateral ligaments provided the major resistance to dorsal displacement of the proximal row of carpal bones during experimental loading, but the small but important palmar intercarpal ligaments contributed 23% resistance.² The lateral palmar intercarpal ligament mostly attaches proximally on the ulnar carpal bone and distally on the third carpal bone and may be divided,³ findings different from those previously reported—that the distal attachment was mostly on the fourth carpal bone.⁸ The medial palmar intercarpal ligament has four bundles that vary in size, and it courses between the radial carpal bone proximally and the palmaromedial surface of the third carpal bone and palmarolateral surface of the second carpal bone distally.³ Tearing of the medial palmar intercarpal ligament and to a lesser extent the lateral palmar intercarpal ligament was observed in horses with carpal disease and was recently proposed to be associated with cartilage and subchondral bone damage (see the following discussion).^8,9

The carpus has a dense joint capsule dorsally that blends with the overlying fascia and retinaculum. Synovium in young horses is often thickened or folded dorsally in the middle carpal joint and can interfere with visibility during arthroscopic surgery. This fold appears to smooth as horses age or as osteoarthritis develops. The antebrachial fascia blends with the retinaculum that functions to restrain extensor tendons. Retinaculum thickens and forms the medial and palmar borders of the carpal canal. The palmar retinaculum is sometimes severed in horses with carpal tenosynovitis and tendonitis (see Chapter 75). Anatomical considerations and flexor and extensor tendon injuries are discussed elsewhere (see Chapters 69 and 77). The sheathed extensor carpi radialis and common digital extensor tendons, located dorsally and dorsolaterally, respectively, limit carpal palpation and restrict access. Cul-de-sacs of distended antebrachiocarpal and middle carpal joint capsules can be palpated medial to the extensor carpi radialis tendon or between the extensor carpi radialis and common digital extensor tendons in a standing horse. Arthrocentesis and arthroscopic examination require careful placement of needles and instruments in these portals to avoid injury to tendons and sheaths. These portals can be easily felt as distinct depressions when the carpus is flexed. The sheathed lateral digital extensor tendon, located on the lateral aspect, should be avoided during arthrocentesis of the palmarolateral pouches. The sheathed extensor carpi obliquus tendon is small and passes obliquely over the antebrachiocarpal joint from lateral to medial to attach to the McII. This tendon can readily be seen medially during arthroscopic examination of the antebrachiocarpal joint. Extensor tenosynovitis must be differentiated from middle carpal and antebrachiocarpal joint effusion and hygroma. The antebrachiocarpal and middle carpal joints each have a palmarolateral and a palmaromedial outpouching through which arthrocentesis and arthroscopic evaluation can be performed. Unless greatly distended, the palmarolateral outpouchings are larger than the corresponding palmaromedial outpouchings. The palmarolateral outpouching of the antebrachiocarpal joint is in close proximity to the carpal sheath, and inadvertent penetration of the carpal sheath can occur during arthrocentesis or arthroscopic examination even when the palmarolateral outpouching is distended.

Knowledge of the communications and boundaries of the carpal joints is important in understanding the extent of disease processes and the results of diagnostic analgesia (see Chapter 10). The antebrachiocarpal joint is considered solitary, although in a single specimen in a cadaver study the joint communicated with the middle carpal and carpometacarpal joints.¹⁰ In some horses a communication appears between the antebrachiocarpal joint and the carpal sheath. The middle carpal and carpometacarpal joints always communicate (see Figures 10-8 through 10-10 Figure 10-8 Figure 10-9 Figure 10-10 ). Communication between the middle carpal and carpometacarpal joints and the carpal sheath is rare. The carpometacarpal joint has distinct distopalmar outpouchings located axial to the McII and the McIV that have secondary pouches interdigitating within the proximal aspect of the suspensory ligament (SL). These outpouchings explain inadvertent analgesia of the carpometacarpal and middle carpal joint while performing high palmar analgesia and possibly why lameness abates during middle carpal analgesia in horses with avulsion fractures of the proximopalmar aspect of the McIII or proximal suspensory desmitis.¹¹

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The Head

Charles S. Farrow D.V.M. , in Veterinary Diagnostic Imaging: Birds, Exotic Pets and Wildlife, 2009

Jaws, Nasal Cavities, and Paranasal Sinuses

Unlike mammals, many birds have dynamic jaws that achieve maximal opening either by a system of adjustable bony levers and pivot points, or in the case of psittacines, by a jointed beak, termed the craniofacial hinge joint . The latter can be readily appreciated on a radiograph.

The maxilla and mandible are encased within the upper and lower elements of the beak. The maxilla contains the nasal cavity, which is composed of a system of longitudinally stacked conchae divided along the midline by a septum that can only be appreciated in the ventrodorsal (VD) projection (Figure 18-15). The conchae are not clearly discernible in either standard projection, other than with computed tomography (Figure 18-16). As with mammals, these elements warm, humidify, and filter the incoming air.

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Give an Example of a Hinge Joint

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