What is the anatomy and function of the scapula?

Anatomy and Function of the Scapula

Structural Overview

The scapula is a large, flat, triangular bone that serves as a critical component of the shoulder girdle, forming part of the complex shoulder joint mechanism that balances mobility with stability 1. The scapula functions as the essential platform or base for the rotator cuff muscles, directly determining their functional capacity through its positioning and stability 2.

Key Anatomical Components

Bony Landmarks and Surfaces

• Glenoid cavity: The shallow socket that articulates with the humeral head to form the glenohumeral joint 1
• Scapular body: Divided into upper and lower portions with distinct angular relationships to each other, showing significant correlation in their inclinations on the XZ plane 3
• Scapular spine: A prominent ridge running across the posterior surface that divides the scapula into supraspinous and infraspinous fossae, with its inclination significantly correlated to the upper and lower body portions 3, 4
• Acromion: The lateral extension of the scapular spine that forms the acromioclavicular joint; its shape varies (flat 10.2%, curved 89.8%) and can develop as a separate os acromiale in 7-15% of cases 4
• Coracoid process: An anterior projection whose central axis inclination correlates significantly with the acromion on the XY plane 3

Important Soft Tissue Structures

• Glenoid labrum: Dense connective tissue surrounding the glenoid cavity margin, with specialized anatomy at the supraglenoid tubercle where it intimately relates to the long biceps tendon, and at the ventral side where it stretches over the glenoid rim in approximately 55% of cases 4
• Glenohumeral ligaments: Located in the ventral articular capsule, these provide stabilizing function for the ventral glenoid labrum through a "labrum-lift effect" where they cross the glenoid notch 4
• Scapular ligaments: Include the coracoacromial ligament (forming an osteofibrous arch above the shoulder joint involved in impingement), superior transverse scapular ligament (which can cause suprascapular nerve compression), and inferior transverse scapular ligament 4

Functional Roles

Mobility and Positioning

The scapula must be mobile enough to place the glenoid in optimal relation to the humerus to facilitate concavity/compression mechanics, while simultaneously serving as a stable base for coordinated muscle activation 5. This dual requirement allows the shoulder to achieve the greatest freedom of motion of any joint in the body 1.

Rotator Cuff Platform

• When the scapula is well-stabilized, optimal positioning in both static and dynamic tasks allows the rotator cuff to function at peak efficiency 5, 2
• Scapular dysfunction produces an unstable base that negatively affects the rotator cuff's biomechanical contributions to arm motion 2
• The scapula provides attachment sites for the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis), which have a poorly vascularized area approximately 1.5 cm from the greater tubercle predisposing to degenerative changes 4

Closed-Chain Mechanism

The scapula plays key roles in the shoulder's closed-chain mechanism by balancing mobility requirements with stability needs, acting as a stable ball-and-socket base for upper extremity activities 5.

Clinical Significance

Imaging Considerations

• Complex osteology: The scapula's intricate three-dimensional structure and overlying ribs make fractures easily missed or underappreciated on conventional radiography 1
• CT superiority for fractures: CT is the best imaging modality for identifying and characterizing scapula fracture patterns, including intra-articular extension, glenopolar angulation, AP angulation, and lateral border offset 1
• Standard radiographic views: Should include anteroposterior projections and either axillary lateral or scapular Y views to properly profile the glenohumeral joint and scapula 1, 6

Pathological Implications

• Scapular dyskinesis: Frequently present in glenohumeral instability and may create or exacerbate abnormal glenohumeral kinematics; can be causative in rotator cuff dysfunction or result from rotator cuff injury 5, 2
• Fracture patterns: Isolated scapula body fractures generally heal well without surgical fixation, though glenoid neck fractures may require surgical intervention 1
• Scapular notching: Presence on postoperative radiographs of reverse total shoulder prostheses has been associated with poor clinical outcomes 1

Developmental Origin

The scapula blade originates from the dermomyotome (dorsal compartment of somites) in the hypaxial domain, representing a unique case of dermomyotomal chondrogenesis controlled by BMP signals from the somatopleure and ectoderm-derived signals 7. The cranial part articulating with the upper limb derives from the somatopleure of the forelimb field 7.