# Shoulder Joint Anatomy: Articulation and Movement The shoulder joint (articulatio humeri, or glenohumeral joint) is a spheroidal synovial joint formed by the articulation of the head of the humerus with the glenoid cavity of the scapula. It is a multiaxial joint capable of flexion, extension, abduction, adduction, and rotation, and in domestic mammals it depends heavily on periarticular soft tissue and surrounding muscles for stability because the bony socket is shallow. The shoulder joint matters because it is one of the most common sources of forelimb lameness in small animal practice. Shoulder disease is a frequent cause of forelimb lameness in dogs, and determining the precise underlying cause can be challenging even with modern imaging [1]. Conditions such as osteochondrosis, bicipital and supraspinatus tendinopathy, infraspinatus contracture, medial shoulder syndrome, and glenohumeral luxation all arise from the specific anatomy described in this article [2]. A working knowledge of the articulation, its passive stabilizers, and the muscles that move it is the foundation for interpreting lameness examinations, radiographs, ultrasound, and surgical approaches. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## The Bony Articulation of the Shoulder Joint ### The glenoid cavity of the scapula The scapula of domestic mammals is a flat, roughly triangular bone that lies against the thoracic wall. Its distal end bears a shallow, oval articular surface called the glenoid cavity (cavitas glenoidalis). The glenoid is much smaller than the humeral head it receives, and this size mismatch is the central anatomical fact that shapes everything else about the joint. In dogs, the outline of the glenoid cavity is not a fixed shape. A dissection study of 23 canine forelimbs recorded circularity values between 0.73 and 0.83, meaning the socket is moderately oval rather than perfectly round, and the shape differed between individual dogs and between left and right limbs [3]. The same study found no clear relationship between a dog's size, body condition score, or muscle mass and the shape variants observed [3]. The practical takeaway is that normal canine glenoid geometry is variable, and a mildly irregular socket on imaging is not automatically pathological. ### The head of the humerus The proximal humerus carries a rounded articular head that faces caudomedially in the standing animal. In dogs, the head is separated from the greater (lateral) and lesser (medial) tubercles by the intertubercular groove, through which the biceps brachii tendon runs. The tubercles serve as muscle attachment sites and as bony landmarks for radiographic and arthroscopic orientation. In the southern giant pouched rat, a wild African rodent used as a model for comparative forelimb anatomy, the minor and major tubercles sit further distally relative to the head of the humerus than in dogs, and radiography of this species has been documented to allow evaluation of the bones forming the shoulder, elbow, and carpal joints [4]. Comparative details like this remind students that the basic spheroidal plan is conserved while proportions shift across species. ### The glenoid labrum The glenoid labrum is a fibrocartilaginous rim that deepens and extends the articular margin of the glenoid cavity. In dogs, the labrum is not a uniform ring. A protruding, fibrocartilaginous labrum was present caudolaterally in 45% of the limbs examined in one dissection series [3]. The labrum blends with the joint capsule and with the glenohumeral ligaments, and its relationship to the biceps brachii tendon varies between individuals. ### The joint capsule and synovial membrane The joint capsule attaches around the margins of the glenoid and the humeral head and encloses the synovial cavity. The synovial membrane lines the capsule and produces synovial fluid, which lubricates the articular cartilage and nourishes it. The capsule is thin and loose, particularly in its caudal and lateral aspects, which permits the wide range of motion characteristic of the shoulder but contributes little to stability on its own. In dogs, the relationship between the joint capsule and the biceps brachii tendon is variable. A synovium-lined space between the joint socket and the ligaments or the biceps tendon was found in 15% of limbs, while more frequently the gap between the tendon and the joint socket was filled with loose connective tissue [3]. This anatomical variability is relevant to surgeons and sonographers because it affects how the tendon sheath communicates with the joint cavity. ### The glenohumeral ligaments The glenohumeral ligaments are thickenings within the joint capsule that resist excessive translation of the humeral head. In dogs, the medial glenohumeral ligament was present in all limbs examined and occurred in three shape variants [3]. The lateral glenohumeral ligament was present in only a few dogs [3]. The medial ligament is the more consistent and clinically important passive stabilizer, and it is the structure most often implicated in medial shoulder syndrome. ## Summary Table: Key Facts About the Shoulder Joint | Feature | Detail | |--|--| | Joint type | Spheroidal (ball and socket) synovial joint | | Articulating surfaces | Glenoid cavity of the scapula and head of the humerus | | Glenoid labrum | Fibrocartilaginous rim, present caudolaterally in 45% of canine limbs studied [3] | | Medial glenohumeral ligament | Present in all dogs studied, three shape variants [3] | | Lateral glenohumeral ligament | Present in few dogs [3] | | Primary stabilizers in dogs and cats | Periarticular soft tissue and surrounding musculature | | Primary stabilizers in horses | Strong collateral ligaments limiting abduction | | Clavicle | Absent or vestigial in most domestic mammals | | Movements | Flexion, extension, abduction, adduction, rotation, circumduction | ## Movements of the Shoulder Joint ### Flexion and extension Flexion of the shoulder joint brings the humerus cranially relative to the scapula, decreasing the angle between the limb and the body. Extension moves the humerus caudally, increasing that angle. In the dog, the shoulder joint normally flexes and extends through a wide arc, and passive range of motion in this joint is routinely measured with a goniometer in rehabilitation settings. A pilot study of 50 dogs undergoing a 10-session underwater treadmill program found significant improvement in passive range of motion in all joints measured, including the shoulder, with median increases across joints ranging from 1.9% in the tarsus to 5.6% in the hip [5]. The study measured flexion and extension separately and found improvements in both directions across diagnostic groups [5]. ### Abduction and adduction Abduction moves the limb away from the midline, and adduction moves it toward the midline. In dogs, abduction is limited by the medial glenohumeral ligament and the joint capsule. When the medial ligament is torn or stretched, the humeral head can translate medially, producing medial shoulder instability. This is the anatomical basis of medial shoulder syndrome, a well-described cause of canine lameness [2]. ### Rotation and circumduction Rotation of the humerus around its long axis occurs at the shoulder joint and is produced by muscles that insert on the proximal humerus at an angle to the bone. Circumduction is the combination of flexion, extension, abduction, and adduction in sequence, producing a cone-shaped path of the distal limb. In quadrupeds, rotation and circumduction are less prominent than in humans because the limb is specialized for weight bearing and propulsion rather than overhead reaching. ### The scapulohumeral rhythm Movement of the forelimb is not confined to the glenohumeral joint alone. The scapula glides over the thoracic wall at the scapulothoracic articulation, a functional rather than a true synovial joint. In humans, the ratio of glenohumeral to scapulothoracic movement during arm elevation is called the scapulohumeral rhythm [6]. This concept is well established in human orthopedic surgery, where scapular kinematics change measurably after shoulder replacement [6]. In quadrupeds, the scapulothoracic interface is a muscular sling rather than a joint with a capsule, and the scapula has no bony attachment to the axial skeleton in dogs and cats. This arrangement allows the scapula to slide and rotate during the stride, effectively lengthening the functional reach of the limb. ## Muscles of the Shoulder Joint The muscles that cross the shoulder joint can be grouped by their primary action. Many shoulder muscles have more than one action depending on limb position, and the table below lists the predominant function for each. ### Flexors of the shoulder The flexors of the shoulder draw the humerus cranially relative to the scapula. The two most important flexors are the coracobrachialis and the biceps brachii. The coracobrachialis arises from the coracoid process of the scapula and inserts on the medial surface of the humerus. It flexes the shoulder and also helps adduct the limb. In dogs, the coracobrachialis is one of the three structures visualized in the medial compartment of the shoulder on diagnostic ultrasound, along with the medial glenohumeral ligament and the subscapularis [7]. This grouping is not arbitrary. The medial compartment is a functional unit, and disease in one structure frequently accompanies disease in the others. The biceps brachii arises from the supraglenoid tubercle of the scapula, and its tendon of origin passes through the intertubercular groove of the humerus. It is a strong flexor of the shoulder and an extensor of the elbow. The biceps tendon is enclosed in a synovial sheath that communicates with the shoulder joint cavity in some dogs, and the relationship between the tendon and the joint socket varies between individuals [3]. Bicipital tendinopathy is a well-recognized cause of shoulder lameness in dogs [2]. ### Extensors of the shoulder The extensors of the shoulder draw the humerus caudally relative to the scapula. The principal extensors are the infraspinatus, the teres minor, and the long head of the triceps brachii. The infraspinatus arises from the infraspinous fossa of the scapula and inserts on the greater tubercle of the humerus. It extends the shoulder and also acts as a lateral collateral stabilizer. Infraspinatus contracture, a fibrotic shortening of the muscle, is a recognized cause of shoulder lameness and produces a characteristic gait abnormality in dogs [2]. The teres minor arises from the caudal border of the scapula and inserts on the greater tubercle, just distal to the infraspinatus insertion. It extends the shoulder and helps stabilize the joint laterally. The long head of the triceps brachii arises from the caudal border of the scapula and inserts on the olecranon. It is primarily an extensor of the elbow but also extends the shoulder because it crosses the joint. ### Rotators and stabilizers The subscapularis arises from the subscapular fossa on the medial surface of the scapula and inserts on the lesser tubercle of the humerus. It adducts the limb and acts as a medial stabilizer of the shoulder. The subscapularis is one of the three structures evaluated in the medial compartment on ultrasound, and subscapularis tendinopathy is a recognized clinical entity in dogs [7]. The supraspinatus arises from the supraspinous fossa and inserts on the greater tubercle. It initiates abduction and stabilizes the humeral head within the glenoid during weight bearing. Supraspinatus tendinopathy is a common cause of shoulder lameness in dogs [2]. The deltoideus arises from the spine and acromion of the scapula and inserts on the deltoid tuberosity of the humerus. It flexes and abducts the shoulder. The teres major arises from the caudal border of the scapula and inserts on the medial surface of the humerus. It flexes the shoulder and rotates the limb medially. ### Summary Table: Muscles of the Shoulder Joint | Muscle | Origin | Insertion | Primary action | |--|--|--|--| | Coracobrachialis | Coracoid process of scapula | Medial surface of humerus | Flexion, adduction | | Biceps brachii | Supraglenoid tubercle of scapula | Radial tuberosity (via tendon) | Flexion of shoulder, extension of elbow | | Infraspinatus | Infraspinous fossa of scapula | Greater tubercle of humerus | Extension, lateral stabilization | | Teres minor | Caudal border of scapula | Greater tubercle of humerus | Extension, lateral stabilization | | Triceps brachii (long head) | Caudal border of scapula | Olecranon | Extension of shoulder and elbow | | Subscapularis | Subscapular fossa of scapula | Lesser tubercle of humerus | Adduction, medial stabilization | | Supraspinatus | Supraspinous fossa of scapula | Greater tubercle of humerus | Abduction, stabilization | | Deltoideus | Spine and acromion of scapula | Deltoid tuberosity of humerus | Flexion, abduction | | Teres major | Caudal border of scapula | Medial surface of humerus | Flexion, medial rotation | ## Comparative Anatomy Across Domestic Species ### Dogs and cats Dogs and cats have a shallow glenoid cavity and relatively weak periarticular ligaments. Stability depends more on the surrounding muscles and tendons than on bony congruence or ligamentous restraint. The medial glenohumeral ligament is the most consistent passive stabilizer in dogs, present in all limbs studied, while the lateral ligament is present in only a few [3]. The biceps brachii tendon functions as part of the passive stabilizing complex in functional terms, even though it is a muscle tendon [3]. This muscle-dependent strategy has clinical consequences. When the medial compartment structures fail, the humeral head translates medially and the joint can luxate. Traumatic glenohumeral luxation in dogs is treated surgically, and temporary transarticular stabilization with a locking compression plate has been described as a technique that restored stability and range of motion in a case series of 10 dogs [8]. In that series, 9 of 10 dogs returned to complete function at 6 months, and one dog required glenohumeral arthrodesis after recurrence [8]. ### Horses Horses have a stronger collateral ligament apparatus at the shoulder than dogs. The collateral ligaments limit abduction and confine the joint to a predominantly sagittal plane of motion, which suits the horse's cursorial lifestyle. The shoulder joint of the horse is also large and heavily muscled, and the articular surfaces are more congruent than in the dog. Arthroscopic approaches to the shoulder joint have been described in detail for horses, and these approaches have been adapted for use in cattle [9]. ### Ruminants Cattle and other ruminants have a shoulder joint arrangement similar to that of the horse, with strong collateral support and limited abduction. An anatomic and arthroscopic study of 34 cadaveric forelimbs from 20 cattle assessed arthroscopic approaches adapted from the horse and described the intraarticular structures of the bovine shoulder joint [9]. The arthroscope was inserted either immediately cranial or 1 cm caudal to the tendon of the infraspinatus muscle for the cranial and caudal approaches, respectively [9]. This level of anatomical detail matters for surgeons performing minimally invasive joint surgery in cattle. ### The clavicle in domestic mammals Most domestic mammals do not have a true clavicle. The clavicle is a membrane bone that in humans connects the sternum to the acromion and provides a bony strut between the axial skeleton and the forelimb. In dogs and cats, the clavicle is present only as a small vestigial structure embedded in the brachiocephalic muscle, and it does not articulate with either the sternum or the scapula. The absence of a functional clavicle means the forelimb has no bony connection to the axial skeleton. The scapula is suspended in a muscular sling, and the limb is attached to the trunk by muscles alone. This arrangement has two important consequences. First, it allows the scapula to slide and rotate over the thoracic wall during the stride, effectively increasing stride length. Second, it means that the shoulder joint and the scapulothoracic interface together provide the shock absorption and range of motion that in humans is distributed across the sternoclavicular, acromioclavicular, and glenohumeral joints. Some mammals do have a well-developed clavicle. The southern giant pouched rat, a large African rodent, has a well-developed clavicle, coracoid process, and acromion visible on radiography [4]. This species difference reflects different functional demands. Animals that climb or manipulate objects with their forelimbs tend to have more developed clavicles than animals specialized for running. ## How the Shoulder Joint Is Examined and Imaged ### Physical examination Palpation of the shoulder joint begins with the animal standing and then in lateral recumbency. The examiner palpates the greater and lesser tubercles of the humerus, the spine of the scapula, and the acromion. Passive range of motion is assessed by flexing and extending the joint through its available arc while watching for pain, crepitus, or a reduced range. Abduction stress testing can reveal medial instability when the medial glenohumeral ligament is compromised. ### Radiography Survey radiographs remain the first-line imaging modality for shoulder lameness. Standard views include the mediolateral and craniocaudal projections. Radiography allows evaluation of the bones forming the shoulder joint and can reveal osteochondrosis, osteoarthritis, or luxation. In the southern giant pouched rat, radiographic examination of the thoracic limb allowed evaluation of the bones forming the shoulder, elbow, and carpal joints, and shoulder joint dysplasia was identified in one female rat [4]. This illustrates that radiography is useful across a wide range of species. ### Ultrasound Diagnostic ultrasound is increasingly used to evaluate the soft tissue structures of the shoulder. A described technique for the medial compartment of the canine shoulder positions the limb in flexion with external rotation to allow transducer access, uses a 2- to 14-MHz linear array probe with a 50 mm footprint, and visualizes the medial glenohumeral ligament, subscapularis, and coracobrachialis [7]. The ultrasound findings correlate well with MRI for both normal and pathological anatomy [7]. Medial compartment disease is a well-described cause of canine lameness, and ultrasound offers a practical alternative to MRI or arthroscopy for diagnosis [7]. ### Advanced imaging MRI provides excellent soft tissue contrast and can identify the medial glenohumeral ligament, subscapularis, and coracobrachialis [7]. CT provides superior bone detail but uses ionizing radiation and has limited soft tissue contrast [1]. A technique called FRACTURE (Fast Field Echo Resembling a CT Using Restricted Echo-Spacing) has been evaluated in normal dogs to enhance cortical and trabecular bone contrast on MRI, combining some advantages of both modalities [1]. In that pilot study, five research beagles were imaged with FRACTURE, T2-weighted, T1-weighted, and proton density-weighted sequences, and CT, and various parameters including delineation of cortical bone and conspicuity of trabecular bone were measured [1]. ### Arthroscopy Arthroscopy allows direct visualization of intraarticular structures and is used both diagnostically and therapeutically. In cattle, arthroscopic approaches adapted from the horse have been described, with the arthroscope inserted immediately cranial or 1 cm caudal to the infraspinatus tendon [9]. Arthroscopy improves diagnostic capabilities and broadens the spectrum of surgical techniques feasible for treating articular pathology [9]. ## Clinical Relevance, Limitations and Common Mistakes ### Clinical relevance The anatomy described in this article directly informs the diagnosis and treatment of shoulder lameness. Medial shoulder syndrome results from failure of the medial glenohumeral ligament and subscapularis, the two structures that provide medial stability [7][2]. Bicipital tendinopathy involves the tendon of origin of the biceps brachii, which passes through the intertubercular groove and has a variable relationship with the joint capsule [3][2]. Supraspinatus and infraspinatus tendinopathies affect the muscles that stabilize the lateral aspect of the joint [2]. Osteochondrosis affects the articular cartilage of the humeral head. Glenohumeral luxation occurs when the passive and active stabilizers fail together, and surgical stabilization may be required [8]. ### Limitations Individual animals vary in the shape of the glenoid cavity, the presence and configuration of the glenohumeral ligaments, and the relationship between the biceps tendon and the joint capsule [3]. These variations mean that imaging findings must be interpreted in the context of the individual patient. The biomechanical implications of the individual anatomical variants still need to be studied [3]. A veterinarian examining a specific animal must integrate the history, physical examination, and imaging findings to reach a diagnosis. ### Common mistakes Students frequently assume that the shoulder joint is stable because of its bony congruence. In dogs and cats, the opposite is true. The glenoid is shallow, the ligaments are inconsistent, and stability comes primarily from muscles and tendons. This is why shoulder luxation occurs without fracture and why rehabilitation focuses on muscle strengthening. A second common mistake is to confuse the shoulder joint with the entire shoulder complex. The shoulder complex includes the glenohumeral joint, the scapulothoracic interface, and in species that have them, the acromioclavicular and sternoclavicular joints. In humans, the acromioclavicular joint is a planar diarthrodial articulation between the clavicle and the acromion, and it contains a meniscus-like fibrous disk prone to degeneration [10]. Dogs and cats do not have a functional acromioclavicular joint because they lack a true clavicle. The scapulothoracic articulation is a functional interface, not a synovial joint. A third mistake is to assume that all domestic mammals have the same shoulder anatomy. Horses and cattle have strong collateral ligaments and limited abduction, while dogs and cats rely on muscles. The arthroscopic approaches used in horses had to be adapted and validated for cattle because the anatomy differs [9]. A fourth mistake is to overlook the medial compartment. Medial shoulder disease is a well-described cause of canine lameness, but diagnosis has traditionally relied on MRI or arthroscopy [7]. Ultrasound now offers a practical way to visualize the medial glenohumeral ligament, subscapularis, and coracobrachialis, and clinicians should include these structures in their diagnostic plan [7]. ## Quick Review 1. The shoulder joint is a spheroidal synovial joint between the glenoid cavity of the scapula and the head of the humerus. 2. The glenoid labrum, joint capsule, and glenohumeral ligaments are the passive stabilizers. The medial glenohumeral ligament is present in all dogs studied and is the most consistent passive stabilizer [3]. 3. The flexors of the shoulder include the coracobrachialis and biceps brachii. The extensors include the infraspinatus, teres minor, and long head of the triceps brachii. 4. Dogs and cats have a shallow glenoid and rely on muscles more than ligaments for stability. Horses and ruminants have strong collateral ligaments and limited abduction. 5. Most domestic mammals lack a true clavicle, so the forelimb has no bony connection to the axial skeleton. 6. The medial compartment of the canine shoulder contains the medial glenohumeral ligament, subscapularis, and coracobrachialis, and can be evaluated with ultrasound [7]. 7. Shoulder disease is a common cause of forelimb lameness in dogs, and accurate diagnosis requires a thorough examination and appropriate imaging [1][2]. ## Frequently Asked Questions ### What type of joint is the shoulder joint in animals? The shoulder joint is a spheroidal synovial joint, also called a ball and socket joint. It is formed by the head of the humerus articulating with the glenoid cavity of the scapula. ### What are the flexors of the shoulder? The primary flexors of the shoulder are the coracobrachialis and the biceps brachii. The deltoideus and teres major also contribute to flexion. ### Why do dogs and cats dislocate their shoulders more easily than horses? Dogs and cats have a shallow glenoid cavity and inconsistent collateral ligaments, so they rely on muscles and tendons for stability. Horses have strong collateral ligaments that hold the joint together and limit abduction. ### Do dogs have a clavicle? Dogs have only a small vestigial clavicle embedded in muscle. It does not connect the forelimb to the axial skeleton, so the scapula is attached to the trunk by muscles alone. ### What is the medial glenohumeral ligament? The medial glenohumeral ligament is a thickening in the joint capsule on the medial side of the shoulder. It is the most consistent passive stabilizer of the canine shoulder and is present in all dogs studied [3]. ### How is the shoulder joint imaged in veterinary practice? Radiography is the first-line imaging modality. Ultrasound can visualize the medial compartment structures, and MRI provides detailed soft tissue contrast [7][1]. Arthroscopy allows direct visualization of intraarticular structures [9]. ## Related Articles - [Equine Hindlimb Anatomy: Bones, Joints, and Ligaments](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/equine-hindlimb-anatomy-bones-joints-ligaments) - [Bovine Musculoskeletal Anatomy: Axial Skeleton and Joints](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/bovine-musculoskeletal-anatomy-axial-skeleton-joints) - [Canine Stifle Joint Anatomy and Cranial Cruciate Ligament Rupture](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-stifle-joint-anatomy-cranial-cruciate-ligament-rupture) - [Neer's Test for Shoulder Impingement: How to Perform and Interpret It](/blog/guides/neer-s-test-for-shoulder-impingement-how-to-perform-and-interpret-it) - [Surgical Approaches to the Canine Shoulder: Cranial and Caudal](/knowledge/veterinary-medicine/veterinary-surgery/surgical-approaches-to-the-canine-shoulder-cranial-and-caudal) - [Surgical Approaches to the Scapula and Shoulder](/knowledge/veterinary-medicine/veterinary-surgery/surgical-approaches-scapula-shoulder) - [Knee Joint and Muscles: Anatomy, Movement, and Species Notes](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/knee-joint-and-muscles-anatomy-movement-and-species-notes) - [Condyloid Joint: Structure and Movement](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/condyloid-joint-structure-and-movement) - [Ball and Socket Joints: Examples, Movement, and Anatomy](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/ball-and-socket-joints-examples-movement-and-anatomy) ## Sources 1. 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