# Scapula Bone: Anatomy, Landmarks, and Function

The scapula is a flat, triangular bone of the pectoral girdle that serves as the proximal attachment point for the forelimb musculature and forms the socket of the shoulder joint. In domestic mammals it has no bony articulation with the axial skeleton, relying instead on a sling of muscles (the synsarcosis) to connect the forelimb to the trunk.

Understanding scapula anatomy matters for every veterinary student because the bone sits at the center of forelimb locomotion, shoulder stability, and a surprising number of clinical conditions. Suprascapular nerve injury, scapular fractures, osteosarcoma of flat bones, and surgical approaches to the shoulder all depend on knowing the landmarks cold. This guide covers the bones of the scapula, their muscle attachments, and how the scapula differs across species you will encounter in practice.

## Osteology: The Bones of the Scapula

The scapula is classified as a flat bone, meaning it consists of two thin plates of compact bone separated by a layer of cancellous bone. It develops through endochondral ossification from a primary ossification center in the body and separate centers for the coracoid process and other prominences. In young animals, a prominent scapular cartilage (cartilago scapulae) caps the dorsal border and adds length to the bone as the animal grows.

The bone has three borders, two surfaces, and three angles.

### Borders

- **Dorsal border (margo dorsalis):** The long, straight edge that faces the vertebral column. In dogs, cats, horses, and ruminants, this border is capped by the scapular cartilage rather than bone. The cartilage extends the attachment surface for the trapezius, rhomboid, and serratus ventralis muscles.
- **Cranial border (margo cranialis):** The thinner, convex edge that runs from the dorsal border to the cranial angle near the shoulder joint. The suprascapular notch, a small indentation that transmits the suprascapular nerve, sits on this border near the cranial end.
- **Caudal border (margo caudalis):** The thicker, straighter edge facing caudally. This border provides attachment for the teres major and, in some species, part of the subscapularis.

### Surfaces

- **Lateral surface:** Divided by the scapular spine into the supraspinous fossa (cranial) and the infraspinous fossa (caudal).
- **Medial surface (facies serrata):** The broad, flat, slightly concave surface facing the ribcage. It carries the subscapular fossa, which is filled by the subscapularis muscle, and the serrated face where the serratus ventralis attaches.

### Angles

- **Cranial angle (angulus cranialis):** The thinnest part of the bone, located where the cranial and dorsal borders meet. It is a common fracture site.
- **Caudal angle (angulus caudalis):** The ventral end of the caudal border, near the glenoid. A metastatic lesion in this region has been documented in a dog with pilomatricoma [1].
- **Ventral angle (angulus ventralis):** The thickest, most complex region. It houses the glenoid cavity, the supraglenoid tubercle, and the coracoid process.

## Key Landmarks of the Scapula

### The Scapular Spine

The spine of the scapula is a prominent ridge of bone that runs obliquely across the lateral surface. It divides the lateral face into the supraspinous and infraspinous fossae. The spine's shape varies considerably across species and even between breeds. In horses, the spine is broad and smooth. In dogs and cats, it is thinner and more pronounced.

The spine of the scapula is a reliable surface landmark. In human medicine, it is used as a reference point for suprascapular nerve blocks, and a study of 92 dry scapulae found the distance from the midpoint of the spine to the base of the suprascapular notch averaged 3.32 ± 0.39 cm [2]. That level of detail matters when you are learning to locate the notch on a live animal.

### The Acromion

The acromion is the caudal projection of the spine at its ventral end. In dogs and cats, the acromion is well developed and hooks over the shoulder joint. In horses and ruminants, the acromion is either absent or reduced to a small tubercle. In humans, the acromion is large and forms a roof over the rotator cuff tendons, which is why subacromial impingement is a human clinical problem with no direct veterinary equivalent.

The acromion also serves as a landmark for measuring distances to the suprascapular notch. Research on human scapulae found that the distance from the medial border of the acromion to the notch base averaged 3.73 ± 0.42 cm, with less variation than measurements from the spine [2]. The same principle applies in veterinary anatomy: the acromion is a more consistent reference point than the spine when you need to find the notch.

### Supraspinous and Infraspinous Fossae

The supraspinous fossa is the depression cranial to the spine. It is filled by the supraspinatus muscle, which originates here and inserts on the greater tubercle of the humerus. The supraspinatus initiates shoulder extension and helps stabilize the joint.

The infraspinous fossa is the larger depression caudal to the spine. It houses the infraspinatus muscle, which originates here and inserts on the greater tubercle as well. The infraspinatus acts as a lateral collateral ligament of the shoulder and helps flex the joint.

In horses, the infraspinous fossa is caudally enlarged in larger draft breeds compared to smaller breeds like the Falabella, likely an adaptation to body weight [3]. This is a good example of how scapula anatomy reflects function across breeds.

### Glenoid Cavity

The glenoid cavity is the shallow, oval articular surface at the ventral angle. It articulates with the head of the humerus to form the scapulohumeral (shoulder) joint. The cavity is deepened slightly by a fibrocartilaginous rim called the glenoid labrum.

The glenoid is much shallower than the acetabulum of the pelvis, which is why the shoulder joint relies heavily on muscles and tendons for stability rather than on bony congruence. In humans, the glenoid faces laterally, which allows a wide range of motion but also predisposes to instability. In dogs and cats, the glenoid faces more ventrally and caudally, reflecting the weight-bearing role of the forelimb.

A study of human scapulae identified two bony pillars that approach the glenoid: one directed inferiorly near the lateral border and one directed superiorly into the spine [4]. These pillars provide the strongest bone for implant fixation. While the surgical context is human, the principle that the scapula has thicker bone columns near the glenoid applies to veterinary species as well.

### Supraglenoid Tubercle

The supraglenoid tubercle is a bony prominence at the cranial margin of the glenoid. It is the origin of the biceps brachii muscle. In dogs, it is well developed and can be palpated during orthopedic examination of the shoulder.

### Coracoid Process

The coracoid process is a small, hook-like projection on the medial side of the supraglenoid tubercle. It is the origin of the coracobrachialis muscle. In dogs and cats, the coracoid is small but consistent. In humans, the coracoid is much larger and serves as an attachment for several muscles and ligaments. The coracoid process is also one of the landmarks used in 3D modeling studies of scapular morphology [5].

### Suprascapular Notch

The suprascapular notch is a notch on the cranial border near the ventral angle. The suprascapular nerve passes through this notch to reach the supraspinatus and infraspinatus muscles. The nerve is sensory to the shoulder joint and motor to those two muscles [2]. Because the notch is a bony tunnel, the nerve is vulnerable to compression here, a condition called suprascapular neuropathy.

## Scapular Musculature and Attachments

The scapula is unique among long bones because it has no true joint with the axial skeleton. Instead, it is suspended by a sling of muscles that connect it to the trunk, ribs, and vertebral column. This arrangement is called a synsarcosis. The scapular musculature can be divided into two groups: muscles that attach the scapula to the trunk (extrinsic) and muscles that attach the scapula to the humerus (intrinsic).

### Extrinsic Muscles (Scapula to Trunk)

**Trapezius:** The trapezius is a superficial muscle that originates from the dorsal border of the scapula and the scapular cartilage. It inserts on the spine of the scapula and the thoracic vertebrae. The trapezius has cervical and thoracic parts. Its main action is to elevate and rotate the scapula during the swing phase of the gait. It also helps advance the limb.

**Rhomboid:** The rhomboid lies deep to the trapezius. It originates from the dorsal border and medial surface of the scapula and inserts on the nuchal crest and thoracic vertebrae. The rhomboid elevates the scapula and pulls it dorsally and cranially.

**Serratus ventralis:** The serratus ventralis is the most important extrinsic muscle for weight-bearing. It originates from the serrated face of the medial scapula and inserts on the ribs and cervical vertebrae. This muscle forms the sling that supports the trunk between the forelimbs. When the serratus ventralis contracts, it can advance or retract the limb, and it is essential for stabilizing the scapula against the ribcage during locomotion.

**Latissimus dorsi:** The latissimus dorsi originates from the thoracolumbar fascia and inserts on the caudal border of the scapula and the humerus. It retracts the limb and helps flex the shoulder.

**Omotransversarius:** This muscle connects the scapula to the atlas and helps advance the limb.

### Intrinsic Muscles (Scapula to Humerus)

**Supraspinatus:** Originates from the supraspinous fossa and inserts on the greater tubercle of the humerus. It extends the shoulder joint and stabilizes it during weight-bearing. Supraspinatus injury or tendon mineralization is a common cause of lameness in dogs.

**Infraspinatus:** Originates from the infraspinous fossa and inserts on the greater tubercle. It flexes the shoulder and acts as a lateral stabilizer, preventing medial displacement of the humeral head.

**Subscapularis:** Originates from the subscapular fossa on the medial surface and inserts on the lesser tubercle of the humerus. It adducts the limb and acts as a medial stabilizer of the shoulder joint. The subscapularis is the only intrinsic muscle on the medial side of the scapula.

**Teres major:** Originates from the caudal border and caudal angle of the scapula and inserts on the teres major tuberosity of the humerus. It flexes the shoulder and rotates the limb medially.

**Teres minor:** Originates from the infraspinous fossa and inserts on the greater tubercle. It flexes the shoulder and helps stabilize the joint.

**Coracobrachialis:** Originates from the coracoid process and inserts on the humerus. It adducts the limb and helps stabilize the shoulder.

### Summary Table: Scapular Landmarks and Muscle Attachments

| Landmark | Muscle(s) Attached | Function |
|--|--|--|
| Supraspinous fossa | Supraspinatus | Shoulder extension, joint stabilization |
| Infraspinous fossa | Infraspinatus, teres minor | Shoulder flexion, lateral stabilization |
| Subscapular fossa (medial) | Subscapularis | Adduction, medial stabilization |
| Caudal border | Teres major, latissimus dorsi | Flexion, retraction, medial rotation |
| Dorsal border and cartilage | Trapezius, rhomboid | Elevation, rotation of scapula |
| Serrated face (medial) | Serratus ventralis | Weight-bearing sling, limb advancement |
| Coracoid process | Coracobrachialis | Adduction, stabilization |
| Supraglenoid tubercle | Biceps brachii | Elbow flexion, shoulder stabilization |
| Acromion | Deltoideus (partial) | Shoulder flexion, limb advancement |
| Glenoid cavity | None (articular surface) | Articulates with humeral head |

## Comparative Anatomy of the Scapula

### Dogs and Cats

Dogs and cats have a relatively mobile scapula. There is no clavicle articulation in cats, and in dogs the clavicle is a small, inconsistently ossified structure embedded in muscle. A study of 50 adult dogs found that the clavicle had ossified in 96% of them, but it does not form a true joint with the scapula or sternum [6]. Instead, the clavicle and its associated connective tissue bands stabilize the brachiocephalic muscle and protect the brachial plexus and axillary vessels [6].

The scapula in dogs and cats is oriented more sagittally than in humans, and the glenoid faces ventrally. This orientation allows the forelimb to swing forward and backward in a pendulum-like motion. The scapula glides over the ribcage during locomotion, a movement that has been recorded in dogs using magnetic tracking devices [7].

In dogs, the scapula is a common site for osteosarcoma of flat bones. A study of 46 dogs with osteosarcoma of flat and irregular bones found that dogs with scapular tumors had a significantly greater hazard for death (2.8 times) compared to dogs with tumors in other locations, controlling for [alkaline phosphatase](/knowledge/molecular-biology/alkaline-phosphatase) elevation [8]. This makes the scapula clinically important beyond its anatomical role.

### Horses and Ruminants

Horses and ruminants have a more upright scapula than dogs and cats. The bone is longer and narrower, and the spine is broader. In horses, the scapula is oriented nearly vertically, and the glenoid faces ventrally. The infraspinous fossa is caudally enlarged in larger draft breeds compared to smaller breeds [3].

In horses, the scapula slides cranially and rotates dorsocranially when the shoulder joint moves to its most cranial position. A 3D CT study of a Falabella horse carcass found that the rotation angle was approximately 10 degrees [3]. This movement is important for understanding gait mechanics and for diagnosing shoulder lameness.

Ruminants (cattle, sheep, goats) have a scapula that is similar to the horse but with a more prominent acromion in some species. The scapula in sheep has been used as a model for 3D printing studies, and the bone's shape is well documented [9].

### Humans

Humans have a more laterally oriented glenoid than any domestic mammal. This orientation allows a wide range of motion at the shoulder, including overhead reaching and throwing. The trade-off is reduced stability. The human scapula also has a larger acromion and coracoid process, and the spine is more horizontal.

A comparative study of great apes and humans found that gorillas and chimpanzees have scapular morphology geared toward stability and weight-bearing during knuckle-walking, while orangutans have morphology more similar to humans due to their suspensory lifestyle [5]. This study used 3D CT reconstructions to measure angles such as acromial overhang and subacromial space. The takeaway for veterinary students is that scapula shape correlates with forelimb use, and this principle applies across all species.

### Other Species

The scapula varies widely across mammals. The Mexican anteater (Tamandua mexicana) has two spines, a foramen for the suprascapular nerve, and a long acromion [10]. Temminck's ground pangolin has a broad, triangular scapula similar to domestic carnivores but with more robust bone and open epiphyseal lines [11]. These variations remind us that the "typical" scapula is a template, not a universal rule.

## How the Scapula Is Examined and Imaged

### Palpation

In dogs and cats, the spine of the scapula is easily palpated along the lateral surface. The acromion can be felt at the caudal end of the spine. The dorsal border and scapular cartilage are palpable in thin animals. The supraglenoid tubercle can be palpated cranially to the shoulder joint.

### Radiography

Standard lateral and craniocaudal radiographs of the shoulder region include the scapula. The scapula is superimposed on the ribcage in lateral views, which can make fractures difficult to see. Oblique views are often needed.

### Advanced Imaging

CT and MRI are used for detailed assessment of scapular fractures, bone tumors, and soft tissue injuries. A 3D CT study of a Falabella horse demonstrated the position and movability of the scapula in relation to the ribcage [3]. In dogs, CT is commonly used for surgical planning of scapular tumors.

### Nuclear Scintigraphy

Bone phase scintigraphy can detect increased bone turnover in the scapula. In one case report, a dog with metastatic pilomatricoma had increased uptake of 99mTc-MDP in the caudal angle of the left scapula [1]. This technique is useful for detecting early bone lesions that are not visible on radiographs.

## Clinical Relevance, Limitations and Common Mistakes

The scapula is involved in several clinical conditions that every veterinary student should know.

**Suprascapular neuropathy:** The suprascapular nerve passes through the suprascapular notch. Compression or trauma here causes atrophy of the supraspinatus and infraspinatus muscles, leading to shoulder instability and a characteristic "shoulder drop" gait. This is most common in dogs after shoulder trauma or in working dogs that repeatedly injure the area.

**Scapular fractures:** Fractures of the scapula are uncommon but can occur from trauma. The cranial angle and the body are the most common sites. Because the scapula is covered by heavy musculature, fractures may be stable and heal without surgery. However, fractures involving the glenoid require accurate reduction to prevent osteoarthritis.

**Osteosarcoma:** The scapula is a site for flat bone [osteosarcoma in dogs](/knowledge/veterinary-medicine/clinical-methods/osteosarcoma-in-dogs). As noted above, scapular location is associated with a worse prognosis compared to other flat bone sites [8]. Early detection and staging are critical.

**Brachial plexus injury:** The brachial plexus lies medial to the scapula. The suprascapular, subscapular, and other nerves can be injured by scapular fractures or during surgery. A study of 36 canine brachial plexuses found that all originated from the ventral rami of C6 to T1 spinal nerves, and that nerve root distances from the skin at the scapulohumeral joint varied with body size [12]. This anatomical detail matters for nerve blocks and surgical approaches.

**Common mistakes students make:**

1. Confusing the supraspinous and infraspinous fossae. Remember: supraspinous is cranial (toward the head), infraspinous is caudal (toward the tail).
2. Forgetting that the scapula has no bony joint with the trunk. The synsarcosis is a muscular sling, not a joint.
3. Assuming the clavicle is absent in dogs. It is present in most dogs but does not form a true joint [6].
4. Overlooking the coracoid process. It is small but is the origin of the coracobrachialis and a landmark for surgical approaches.
5. Confusing the glenoid cavity with the glenoid labrum. The cavity is bone, the labrum is fibrocartilage.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

## Quick Review

- The scapula is a flat, triangular bone with three borders, two surfaces, and three angles.
- The scapular spine divides the lateral surface into supraspinous and infraspinous fossae.
- The glenoid cavity articulates with the humeral head. It is shallow and relies on muscles for stability.
- The suprascapular notch transmits the suprascapular nerve and is a site of nerve compression.
- Extrinsic muscles (trapezius, rhomboid, serratus ventralis) connect the scapula to the trunk. Intrinsic muscles (supraspinatus, infraspinatus, subscapularis, teres major and minor) connect it to the humerus.
- Dogs and cats have a mobile scapula with no clavicle articulation. Horses and ruminants have a more upright scapula. Humans have a laterally oriented glenoid.
- Scapular osteosarcoma in dogs carries a worse prognosis than osteosarcoma in other flat bones [8].

## Frequently Asked Questions

### What is the scapula bone?

The scapula is a flat, triangular bone of the pectoral girdle that forms the socket of the shoulder joint and serves as the attachment point for the forelimb musculature. It has no bony articulation with the axial skeleton in domestic mammals.

### What are the main landmarks of the scapula?

The main landmarks are the spine, acromion, supraspinous fossa, infraspinous fossa, glenoid cavity, supraglenoid tubercle, coracoid process, suprascapular notch, and the three borders (dorsal, cranial, caudal).

### Which muscles attach to the scapula?

The supraspinatus, infraspinatus, subscapularis, teres major, teres minor, trapezius, rhomboid, serratus ventralis, latissimus dorsi, omotransversarius, coracobrachialis, and biceps brachii all attach to the scapula.

### How does the scapula differ between dogs and horses?

Dogs have a relatively mobile scapula with a well-developed acromion and no clavicle articulation. Horses have a more upright scapula with a broader spine and a reduced or absent acromion. The infraspinous fossa is larger in draft horses than in smaller breeds [3].

### What is the function of the scapula in forelimb movement?

The scapula acts as a pivot point for forelimb movement, provides attachment for muscles that stabilize the shoulder, and transmits forces from the limb to the trunk through the serratus ventralis sling.

### What clinical conditions affect the scapula?

Suprascapular neuropathy, scapular fractures, osteosarcoma of flat bones, and brachial plexus injuries can all involve the scapula. Scapular osteosarcoma in dogs is associated with a worse prognosis than osteosarcoma in other flat bone sites [8].

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