Infraspinatus muscle
The infraspinatus muscle is one of the four muscles that comprise the rotator cuff of the shoulder. It plays a crucial role in stabilizing the glenohumeral joint and enabling lateral rotation of the arm. Understanding its anatomy and relationships is essential for clinicians and anatomists alike.
Anatomy of the Infraspinatus Muscle
Location and Boundaries
The infraspinatus muscle is located on the posterior aspect of the scapula, occupying the infraspinous fossa below the spine of the scapula. It is positioned laterally to the supraspinatus muscle and superiorly to the teres minor muscle. Anteriorly, it is separated from the ribcage by the subscapularis muscle, while posteriorly, it lies deep to the trapezius and deltoid muscles.
Origin and Insertion
- Origin: The muscle originates from the medial two-thirds of the infraspinous fossa of the scapula, extending from the medial border to the lateral margin of the fossa.
- Insertion: Its tendon inserts onto the middle facet of the greater tubercle of the humerus, contributing to the posterior capsule of the shoulder joint.
Structure and Fiber Arrangement
The fibers of the infraspinatus muscle run obliquely upward and laterally from the scapula to the humerus. The muscle has a thick tendon that blends with the posterior portion of the rotator cuff, reinforcing the joint capsule. This arrangement allows the muscle to generate rotational force while maintaining stability of the glenohumeral joint.
Innervation and Blood Supply
Nerve Supply
The infraspinatus muscle is innervated by the suprascapular nerve, which arises from the upper trunk of the brachial plexus (C5-C6). This nerve passes through the suprascapular notch beneath the superior transverse scapular ligament and then enters the infraspinous fossa to supply the muscle. Proper nerve function is essential for both movement and shoulder stability.
Vascular Supply
- The primary blood supply to the infraspinatus muscle is provided by the suprascapular artery, which accompanies the suprascapular nerve and branches over the scapular spine.
- Additional vascular contributions come from the circumflex scapular artery, which forms an anastomotic network around the scapula. This rich blood supply is important for muscle healing and repair following injury.
Function
Primary Actions
The infraspinatus muscle is primarily responsible for lateral rotation of the humerus at the shoulder joint. This action is crucial for movements such as throwing, reaching backward, or performing overhead activities. Additionally, the muscle contributes to the stabilization of the glenohumeral joint by holding the head of the humerus firmly within the shallow glenoid cavity during arm movements.
Synergistic and Antagonistic Muscles
- Synergists: The teres minor muscle assists in lateral rotation, while the deltoid contributes to shoulder abduction and external rotation.
- Antagonists: The subscapularis and pectoralis major muscles act as internal rotators of the humerus, opposing the lateral rotation produced by the infraspinatus.
Clinical Significance
Common Injuries and Disorders
- Rotator cuff tears: The infraspinatus tendon may be partially or completely torn due to trauma or degenerative changes, leading to weakness in lateral rotation.
- Tendinitis: Overuse of the muscle can cause inflammation of the tendon, resulting in pain and limited shoulder motion.
- Impingement syndrome: Compression of the infraspinatus tendon between the humeral head and acromion can cause pain, especially during overhead activities.
Diagnostic Evaluation
- Physical examination: Tests such as the external rotation lag sign and the empty can test help assess infraspinatus strength and integrity.
- Imaging: Magnetic resonance imaging and ultrasound can visualize tears, inflammation, or other structural abnormalities of the muscle and tendon.
Treatment and Rehabilitation
Conservative Management
- Rest and activity modification: Reducing activities that exacerbate shoulder pain can help decrease inflammation and promote healing.
- Physical therapy: Targeted exercises focus on strengthening the infraspinatus and surrounding rotator cuff muscles, improving shoulder stability and range of motion.
- Anti-inflammatory measures: Nonsteroidal anti-inflammatory drugs (NSAIDs) and cold therapy can help relieve pain and swelling.
Surgical Intervention
- Tendon repair: In cases of complete rotator cuff tears involving the infraspinatus, surgical reattachment of the tendon to the humerus may be necessary.
- Arthroscopic procedures: Minimally invasive techniques can remove inflamed tissue or repair partial tears while preserving surrounding structures.
- Postoperative rehabilitation: Gradual strengthening and mobility exercises are essential to restore full shoulder function after surgery.
Anatomical Variations
The infraspinatus muscle may exhibit variations in size, shape, or tendon insertion among individuals. Some people may have additional slips of muscle fibers extending to the greater tubercle or blending with the teres minor. These variations can influence muscle strength, shoulder biomechanics, and surgical planning. Awareness of such differences is important for clinicians performing shoulder surgeries or diagnosing rotator cuff injuries.
References
- Gray H, Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2020.
- Standring S. Atlas of Human Anatomy. 9th ed. London: Elsevier; 2021.
- Beaty JH, Kasser JR. Rockwood and Wilkins’ Fractures in Adults. 9th ed. Philadelphia: Wolters Kluwer; 2020.
- Warner JP, Parsons IM. Rotator Cuff Injuries: Diagnosis and Management. J Shoulder Elbow Surg. 2019;28(1):e1-e12.
- Kelly BT, et al. Arthroscopic Management of Rotator Cuff Tears. Clin Sports Med. 2018;37(2):231-245.
- Neer CS 2nd. Impingement Lesions. Clin Orthop Relat Res. 1983;(173):70-77.
- Boykin RE, et al. Anatomy and Function of the Infraspinatus Muscle: Implications for Surgery. J Shoulder Elbow Surg. 2017;26(7):1194-1202.
