Canine Anatomy



Canine Anatomy



Cheryl Riegger-Krugh, Darryl L. Millis and Joseph P. Weigel


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This text is intended for people who already possess knowledge of either veterinary or human anatomy. To assist communication among human rehabilitation and veterinary colleagues, some anatomic terms used for dogs appear in regular print with the analogous terminology for humans in parentheses following the canine term. These comparisons have been minimized, as this is a chapter about canine anatomy and not a chapter about comparative anatomy. Comparative anatomy between dogs and humans has been described in other sources.13


We have chosen to use some terms consistently throughout the chapter, rather than use equally acceptable synonyms. The canine forelimb is known also as the thoracic limb and the pectoral limb, but we use the term forelimb. The canine hindlimb is known also as the pelvic limb or rear limb, but we use the term hindlimb. Because the term foot can be interpreted as a front foot or a hind foot, this term is clarified when used or specified as forepaw or manus, or hindpaw or pes. The terms trunk, neck, and head refer to the same body segments in dogs and humans. The word canine is an adjective and the word dog is a noun; these terms are used in this consistent grammatical form throughout the chapter.



Directional Terms and Anatomic Planes


Directional Terms from Normal Stance (Anatomic Position)


The dog stands upright on digits or phalanges of each forepaw or manus and each hindpaw or pes (Figure 5-1). This type of stance is termed a digitigrade stance. The human stands upright on the feet, with the plantar aspect of the feet contacting the floor and adjacent to each other. The upper limbs hang at the sides of the body, palms facing forward. This type of stance is called a plantigrade stance.



Directional terms from anatomic position in dogs are more directly compared with the directional terms in humans when the human is in a quadruped position or the dog is in an upright stance posture. Directional terms include cranial, caudal, rostral, dorsal, palmar, plantar, medial, and lateral. Other specific directional terms include (1) radial and ulnar to indicate toward the radius and ulna, respectively; (2) axial and abaxial to indicate toward or away from the axis of the digits, which is between the third and fourth digits of the forepaw, and the third and fourth digits of the hind paw, respectively; and (3) tibial and fibular to indicate toward the tibia and fibula, respectively.




Axes of Rotation


Motion may occur in any of three planes of motion or some combination. Joint motion within a plane usually occurs around an axis of rotation, which may be centered within the joint space or within the bone comprising the joint. Some joint motions are planar or gliding motions and do not occur around an axis of rotation.


An axis of rotation for a joint motion is a straight line or rod that is 90 degrees to the plane of motion. For each axis of rotation listed in the next section, the plane of motion around which joint motion occurs can be viewed from Figure 5-1.




Bones


The bones of the dog skeleton and limbs are illustrated in Figures 5-2, 5-3, and 5-4. Bony landmarks on the bones of the limbs are shown in Figures 5-5 through 5-10.












Forelimb


The forelimb skeleton consists of the thoracic or pectoral girdle and bones of the forelimb (see Figures 5-5 and 5-6). The size of forelimb bones varies a great deal, because of the greater variation in size for breeds of dogs. The forelimbs bear 60% of the dog’s weight.


The canine scapula is positioned close to the sagittal plane. Dogs have an abbreviated clavicle that does not articulate with the rest of the skeleton. It is a small oval plate often 1 cm or less in length and ⅓ cm wide, located at the tendinous intersection of the brachiocephalicus muscle. The adult canine clavicle is mostly cartilage and is usually not visible on radiographs.


The canine humeral head is less rounded compared with the human head, to assist with weight bearing. Distally, there is an olecranon fossa and supratrochlear foramen for the secure positioning of the protruding anconeal process of the ulna for more stability in weight bearing.


The radius is the medial forearm bone and is the main weight-bearing bone of the antebrachium distally. The proximal surface of the radius articulates with the humeral capitulum, which is not as prominent as in the human. The canine distal radius has distinct facets for articulation with carpal bones, providing stability in weight bearing.


The ulna is the lateral forearm bone and has a very prominent olecranon process, which allows secure attachment for the large triceps brachii muscle, needed as an antigravity muscle for weight bearing in dogs. The ulna is the longest bone of the canine body. It articulates distally with the ulnar carpal and accessory carpal bones by two distal facets and does not have an articular disk. The dog has an anconeal process, which is near the attachment site of the anconeus muscle. The anconeal process is needed for stability in weight bearing.


At the carpus or wrist (see Figure 5-7), there are seven carpal bones. The radial carpal bone is analogous to the fused scaphoid and lunate. There are five metacarpal bones. The first metacarpal is short and nonfunctional. Dogs have many sesamoid bones that are embedded in tendons or near them. Sesamoid bones occur when there are significant changes in directions of pull on tendons in addition to the tensile forces produced during muscle contractions. They allow for constant, biomechanically advantageous alignment of angles of insertion of tendons at their attachment sites, which helps relieve stress on the tendinous insertions for animals that walk on their digits. Dogs are digitigrade animals and bear weight on digits II to V, with the main weight bearing occurring on digits III and IV. The sesamoid bones at the dorsal surface of each metacarpophalangeal joint align the extensor tendons for optimal muscle action. Those on the pad surface of the manus align the flexor tendons.



Hindlimb


The hindlimb skeleton includes the pelvic girdle, consisting of the fused ilium, ischium, and pubis, and the bones of the hindlimb (see Figures 5-8 and 5-9). The size of hindlimb bones varies a great deal, because of the great variation in size for breeds of dogs. The hindlimbs bear 40% of the dog’s weight.


The canine pelvis is positioned between the dorsal and transverse planes and closer to the dorsal plane. The canine pelvis is relatively small and narrow. The canine ischiatic or ischial tuberosities are wide and project caudally to form a broad ischiatic table. The canine pelvis shape from a ventral view resembles a rectangle. The symphysis pelvis is relatively long and has two portions, the symphysis ischii and symphysis pubis, compared with the relatively shorter joining of the anterior aspect of the human innominates at the symphysis pubis. The distinction of the shape of the male and female pelvic inlet and outlet in humans is not made in dogs.


The canine femur is the heaviest4 and largest5 canine bone. In most dogs, it is slightly shorter than the tibia and the ulna and approximately one-fifth longer than the humerus. The average canine angle of inclination or cervicofemoral angle is 144.7 degrees.5 Dogs have an average degree of anteversion or positive femoral torsion of +27 to 31 degrees, when measured from a direct radiograph or with a method using trigonometry and biplanar radiography, respectively.5 The canine femur has a relatively thick and short femoral neck, a caudomedially located lesser trochanter, a prominent lateral greater trochanter, and a relatively short and wide shaft with a narrow isthmus in the middle. The greater trochanter has a craniolateral prominence called the cervical tubercle. Dogs have a third trochanter, which is the attachment site of the superficial gluteal muscle. Canine medial and lateral femoral condyles are equally prominent, but the articular surface of the medial femoral condyle projects more cranially than that of the lateral femoral condyle.


There are three sesamoid bones in the caudal stifle joint region. Two are located in the heads of the gastrocnemius muscle caudal to the stifle joint and are called fabellae. The sesamoid in the lateral head is the largest, is palpable, and articulates with the lateral femoral condyle, whereas the one in the medial head is smaller and may not have a distinct facet on the medial femoral condyle. The third is the smallest, is located in the proximal attachment of the popliteus muscle, and articulates with the lateral tibial condyle.


The canine patella, or kneecap, is the largest sesamoid bone in the body. It is an ossification in the quadriceps femoris muscle. The patella alters the pull, increases the moment arm, and protects the quadriceps tendon, as well as provides a greater contact surface for the tendon on the trochlea of the femur than would exist without the patella. The canine patellar articular surface is mildly convex.


The canine tibia is the major bone in the crus. The triangular proximal tibia is wider than the distal cylindrical tibia. Medial and lateral tibial condyles, an intercondylar eminence, and a tibial tuberosity are on the proximal tibia. The tibial plateau slopes distally from cranial to caudal. The extensor groove, on the cranial tibia and lateral to the tibial tuberosity, provides a pathway for the long digital extensor muscle. There is a popliteal notch on the caudal tibia in the midline, where the popliteal vessels course. The tibia articulates with the fibula proximally, along the interosseous crest, and distally. The tibial cochlea articulate with the trochlea of the talus to form the talocrural joint.


The canine fibula is a long, slender bone that articulates with the tibia and also serves as a site for muscle attachment. There is a distinctive groove in the lateral malleolus, the sulcus malleolaris lateralis, through which course the tendons of the lateral digital extensor and peroneus brevis muscles.


The tarsus, or hock, consists of the talus, calcaneus, a central tarsal bone, and tarsal bones I to IV (see Figure 5-10). The talus articulates with the distal tibia and has prominent ridges. At the talocrural joint, two convex ridges of the trochlea of the talus articulate with two reciprocal concave grooves of the cochlea of the tibia. The orientation of the grooves and ridges deviates laterally approximately 25 degrees from the sagittal plane. This deviation allows the hindpaws to pass lateral to the forepaws when dogs gallop.4 The calcaneus is large and serves as the insertion of the common calcaneal tendon. The central tarsal bone lies between the talus and the numbered tarsal bones I to III. Tarsal IV is large and articulates with the calcaneus and metatarsal bones, spanning this entire region.


The canine hindpaw has five metatarsal bones; however, the first metatarsal can be short or absent. Dogs have many sesamoid bones that are embedded in tendons where there are significant compressive and tensile forces produced during muscle contractions. The sesamoid bones at the dorsal surface of each metatarsophalangeal joint align the extensor tendons for optimal joint action. The sesamoid bones on the plantar surface of the hindpaw align flexor tendons.



Spine


The spine consists of five areas of the vertebral column: the cervical vertebrae and its articulation with the head, thoracic vertebrae, lumbar vertebrae, sacral vertebrae, and the coccygeal vertebrae (Figures 5-11 through 5-14). The number of vertebrae is listed in Box 5-1.



Box 5-1   Body Segments



Forelimb




• Forelimb: Arm, forearm, and forepaw


• Thoracic or pectoral girdle



• Arm or brachium: Shoulder to elbow


• Forearm or antebrachium: Elbow to carpal joint


• Forepaw or manus



• Sesamoid bones or cartilages



• One sesamoid bone in the tendon of the abductor pollicis longus


• Digits or phalanges I to V, numbered medial to lateral



• Dewclaw or pollex or digit I with 2 phalanges


• Pads on the paws or digital pads: Weight-bearing pads



• Ungual process: Extension of the phalanx into the claw


• Nails or claws



Hindlimb




• Hindlimb pelvic limb, or rear limb



• Hip bone or os coxae



• Pelvic girdle



• Thigh: Hip to stifle or knee


• Leg or crus: Stifle to talocrural joint


• Hindpaw or hind foot or pes



• Sesamoid bones or cartilages



• Digits or phalanges or toes



• Dewclaw or digit I or hallux—may be absent, fully developed and articulating with a metatarsal, or may be a vestigial, that is, a trace or rudimentary structure, with a terminal phalanx and no proximal phalanx or metatarsal bone


• Digital pads or pads on the hindpaws—weight-bearing pads



• Ungual process: Extension of the distal phalanx into the nail


• Nails or claws



Spine




C, Cervical; Cd, caudal; DIP, distal interphalangeal; L, lumbar; MCP, metacarpophalangeal; MTP, metatarsophalangeal; S, sacral; T, thoracic.






All vertebrae, except the sacral vertebrae, remain separate and form individual joints. Four sites with limited motion exist within the canine spine.6 These sites occur at areas where the cranial and caudal articular surfaces are inclined in a nonparallel manner and in different directions. The nonparallel alignment of the articular surfaces markedly restricts joint accessory motions, such as glides. The restricted joint motions and areas resulting from these joint alignments include atlantoaxial motion other than rotation, the cervical (C) 7-thoracic (T) 1 junction, the caudal thoracic region, and the sacrum.


Individual vertebral bone size and shape vary among breeds. For any one breed, canine cervical through lumbar vertebrae are fairly consistent in size. The consistent size in dogs reflects the relatively equivalent cranial-to-caudal compressive loading. Because dogs are quadruped, there is weight bearing on all four limbs. There is cervical spine compression as a result of the positioning of the dog’s head as a cantilever, which requires cervical extensor muscle activity to maintain head posture. The massive cervical extensor muscle activity requires relatively large and strong cervical vertebrae to support the muscle mass. Canine intervertebral disks likewise change little in size from the cervical through the lumbar vertebrae. The C5-C6 area is a site of relative hypermobility in large dogs. The spinal cord ends at lumbar (L) L6-L7.


The canine atlas, or C1 vertebra (see Figure 5-12), has a transverse foramen in each transverse process, a craniodorsal arch, and right and left lateral vertebral foramina for the passage of cervical spinal nerve 1. The atlas has correspondingly shaped condyles for articulation with the occiput. The canine lateral wings or transverse processes are prominent and easily palpable from the skin surface. The canine axis or C2 has a large spinous process with an expanded arch, a wide body, and large transverse processes (see Figure 5-12). The spinous process is nonbifid. The canine axis is very large relative to the size of other canine cervical vertebrae. The axis has a dens, which projects cranially to allow pivotal motion between the atlas and axis. The condyles are oriented near the transverse plane to allow cervical spine rotation. The C3-C6 vertebrae have nonbifid spinous processes, large and flat spinous processes, caudal and cranial articular surface facets that are narrower than the transverse processes, large transverse processes, and transverse foramina for the passage of vertebral arteries. Caudal and cranial articular surfaces are oriented between the dorsal and transverse planes to facilitate cranial and caudal glides needed for cervical spine flexion and extension. The C7 vertebra has a similar shape, a large prominent nonbifid spinous process, and caudal and cranial articular surfaces, which are oriented nearly craniocaudally.


Thoracic vertebrae (see Figure 5-13) have small bodies relative to the size of the entire vertebrae. Canine spinous processes are relatively long. The spinous processes block excessive extension of the thoracic spine. At T10, the size of the body begins to increase and the length of spinous process decreases. The spinous processes are oriented close to the transverse plane. Cranial to T11, the spinous processes project caudally, but caudal to T11, they project cranially. Caudal and cranial articular surfaces are oriented close to the dorsal plane.


Lumbar vertebrae (see Figure 5-13) have bodies that are larger than thoracic vertebral bodies. Canine lumbar transverse processes are long and thin, and they project lateroventrocranially. In the cranial lumbar spine, cranial and caudal articular surfaces are oriented between the transverse and sagittal planes, which facilitate lumbar spine flexion and extension. The L7-S1 joint appears to orient between the sagittal and frontal planes to allow more rotation at this intervertebral level. The canine sacrum is relatively narrow and is linked to the pelvis with sacroiliac joints (see Figure 5-14).


Caudal (Cd) vertebrae (see Figure 5-14) have distinct bodies and transverse processes. The cranial articular surfaces are similar to those in more cranial vertebrae in shape and location; however, the caudal articular processes are bifid and are more centrally located, whereas articular processes in more cranial vertebrae are located more laterally. Hemal arches are separate bones that articulate with the ventral surfaces of the caudal ends of the bodies of Cd4-Cd6. The hemal arches provide protection for the median coccygeal artery, which is enclosed by the arches. In vertebrae caudal to Cd6 and in relatively the same position as the hemal arches are the paired hemal processes, which extend from Cd7-Cd17 or Cd18.


The ribs have vertebral attachments (see Figure 5-11). There are nine pairs of vertebrosternal, or true, ribs and four pairs of vertebrocostal, or false, ribs. The sternum is relatively long and has a manubrium and xiphoid process, with a prominent xiphoid cartilage. The ribs limit overall thoracic spine motion and protect internal organs.



Joint Motion


The body segments of the forelimb and hindlimb are illustrated in Figures 5-3 and 5-4, respectively, with the major joints and their flexor and extensor surfaces. Body segments are listed and defined in Box 5-1. Types of joints are listed in Box 5-2.



Box 5-2   Types of Joints




Hindlimb




• Synovial and fibrous: Sacroiliac


• Symphysis: Symphysis pelvis


• Ball and socket: Hip or coxofemoral


• Complex condylar: Stifle (the term knee is used commonly with an animal’s owner)


• Plane



• Tibiofibular



• Hinge: Talocrural, tarsocrural, tibiotarsal (the tarsocrural has been referred to as the talocrural and the talocalcaneal joints combined) or ankle joint (the term ankle is commonly used with an animal’s owner)



• Condylar: MT II to V with the same numbered digit


• Saddle





Joint Motion and Shape of Articular Surfaces


The shape of articular surfaces of bones helps define the motions available for a joint. Articular surfaces of two bones forming a joint are usually concave on one bone and convex on the other bone. Some articular surfaces are flat. Occasionally adjacent bones are convex on both joint surfaces. Intraarticular structures, such as the medial and lateral menisci in the stifle joint, may modify adjacent surfaces. Understanding the concave-convex relationships as a guiding principle in determining joint motion allows prediction of possible joint motions based on articular surface shape. Ligamentous and other soft tissue around the joint guide and restrict the motion that would be possible based on articular surface shape alone.


Joint motions are named, most commonly, by movement of the distal bone relative to the proximal bone. For example, cranial movement of the tibia on a stable femur is named stifle joint extension. The major direction of motion, such as flexion of the stifle, is physiologic or osteokinematic motion.


Accessory, or arthrokinematic, motion is smaller in magnitude and less observable. Examples of accessory motions are glide or slide, rotary motion, distraction or traction, and compression or approximation. A normal amount of glide occurs in normal functioning joints. Glides are shear type or sliding motions of opposing articular surfaces. Rolls involve one bone rolling on another. Gliding motion in combination with rolling is needed for normal physiologic joint motion. Spins are joint surface motions that result in continual contact of articular cartilage areas on opposite sides of a joint. Distraction or traction accessory motions are tensile or pulling-apart movements between bones. Compressive or approximation accessory motions are compressive or pushing-together movements between bones.


Normal joint motion involves both physiologic motion and accessory motion. Physiologic motion in joints with opposing concave and convex articular surfaces involves both roll and glide. Roll occurs in the same direction as the movement of the moving segment of the bone, but glide directions differ based on whether the moving articular surface is concave or convex. A glide is described by identifying the joint motion, the direction of the glide, and which bone is moving. For example, stifle flexion involving the tibia and femur is termed caudal glide of the tibia on the femur.



Joint Motion in the Limbs and Spine


Joint motions are named by one body segment approaching or moving away from another body segment or movement of some referenced body landmark. Joint motions are named in the following sections and described (see Figures 5-3 and 5-4) as they refer to the limbs, starting from normal stance. Limb motion is usually described by motion of the joint rather than a body segment. For example, elbow flexion is recommended rather than forearm flexion. Occasionally, body segment motion is used to describe limb motion when motion does not involve axial motion with a joint as a pivot point. For example, rotation of the forelimb might be observable when pronation at the radioulnar joint would be difficult to observe clinically.



Flexion


During flexion, a limb is retracted or folded, a digit is bent, and the back or neck is arched dorsally (i.e., the convex portion of the arch is directed dorsally). In the limbs, flexion motion occurs as the bones on either side of a joint move closer together and the joint angle becomes more acute. Flexion may also be referenced to limb motions involving closing angles during the swing phase of gait. Flexion motions of the limb joints are noted in Figures 5-3 and 5-4. In the spine, flexion occurs as the back or neck arches dorsally (i.e., the convex portion of the arch is directed dorsally).


A notable difference between dogs and humans is the meaning of shoulder flexion. In dogs, caudal retraction of the humerus in relation to the scapula is shoulder flexion, whereas cranial motion of the humerus in relation to the scapula is shoulder extension. The direction of shoulder flexion motion is opposite to this in humans. The terminology used in dogs is consistent with naming flexion as described previously.


In normal stance, as shown in Figure 5-2, a dog’s spine is flexed at the atlantooccipital and atlantoaxial joints, straight (neither flexed nor extended) in the remainder of the cervical spine, extended at the cervicothoracic junction, slightly lordotic in the thoracic spine, and flexed or normally kyphotic in the lumbar spine. There is either a slightly flexed or extended sacrum on the lumbar spine, depending on the tail posture. The flexed canine lumbar spine is beneficial to running speed. During running, the lumbar spine moves through varying degrees of flexion as running speed changes.


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Jul 8, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Canine Anatomy
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