Infraspinatus

Introduction

Introduction to the Infraspinatus Muscle

The infraspinatus is a thick, triangular muscle that occupies the majority of the dorsal surface of the scapula. It is a critical component of the rotator cuff, a group of four distinct muscles and their tendons that provide strength and stability during motion of the shoulder complex.

Clinically, it is highly significant due to its role in shoulder stabilization and its susceptibility to injury in athletes, particularly those involved in repetitive overhead throwing motions, as well as in the aging population due to degenerative rotator cuff disease.

Study

Origin and Insertion

The infraspinatus originates from the medial two-thirds of the infraspinous fossa of the scapula and the thick infraspinous fascia that covers it. The muscle fibers converge laterally to form a tendon that glides over the lateral border of the scapular spine.

Its tendon inserts onto the middle facet of the greater tubercle of the humerus, sitting just inferior to the insertion of the supraspinatus and superior to the insertion of the teres minor.

Innervation and Blood Supply

The infraspinatus is innervated by the suprascapular nerve, which arises from the superior trunk of the brachial plexus, carrying fibers primarily from spinal nerves C5 and C6. After supplying the supraspinatus, the nerve passes through the spinoglenoid notch to reach the infraspinatus.

Its arterial blood supply is derived primarily from the suprascapular artery (a branch of the thyrocervical trunk) and the circumflex scapular artery (a branch of the subscapular artery), which form a rich anastomotic network around the scapula.

Action and Function

The primary action of the infraspinatus is the lateral (external) rotation of the humerus at the glenohumeral joint. It works synergistically with the teres minor to perform this movement.

Furthermore, as a rotator cuff muscle, it plays a vital biomechanical role in dynamically stabilizing the glenohumeral joint. During movements of the arm, particularly elevation, it helps to depress and centralize the head of the humerus within the shallow glenoid cavity, preventing superior subluxation.

Clinical Anatomy

The infraspinatus is frequently implicated in rotator cuff tears, usually secondary to a massive supraspinatus tear extending posteriorly. In overhead athletes (like baseball pitchers), the muscle is heavily relied upon during the deceleration phase of throwing, leading to eccentric overload and potential tendinopathy.

Suprascapular nerve entrapment can occur at the spinoglenoid notch (often due to a paralabral cyst). Because the nerve has already supplied the supraspinatus before reaching this notch, entrapment here results in isolated weakness and atrophy of the infraspinatus, presenting as painless weakness in external rotation.

Summary

Summary of the Infraspinatus

• Origin: Infraspinous fossa of the scapula.

• Insertion: Middle facet of the greater tubercle of the humerus.

• Innervation: Suprascapular nerve (C5, C6).

• Blood Supply: Suprascapular artery and circumflex scapular artery.

• Main Action: Lateral (external) rotation of the arm; stabilizes the glenohumeral joint.

• Clinical: Susceptible to tears, tendinopathy from deceleration forces, and isolated atrophy from suprascapular nerve compression at the spinoglenoid notch.