Fractures of Shaft of Humerus – Relevant Anatomy and Biomechanics

The shaft of the humerus lies between the upper border of the pectoralis major insertion proximally and the supracondylar ridge distally. This constitutes the middle three fifths of the entire humerus. The anterior portion of the greater tuberosity extends into an anterior ridge that ends at the coronoid fossa distally. The posterior aspect of the greater tuberosity continues distally as a lateral ridge that ends in the lateral supracondylar ridge. The lesser tuberosity melds into a medially located ridge that forms the medial supracondylar ridge distally.

The deltoid tubercle forms a lateral prominence just proximal to the midshaft. The shaft of humerus has a posterior, an anterolateral, and an anteromedial surface. The medullary canal of the humerus ends proximal to the olecranon fossa.

The anatomy of humerus has important implications for internal and external fixation

The arm is divided into anterior and posterior compartments by fascial septae. Posterior compartmentcontains triceps muscle, radial nerve beteen the long and lateral heads of triceps. The anterior or flexorcompartment contains the flexors of the elbow, biceps brachii and brachialis, and the coracobrachialis. The brachialis has got two nerve supplies- one from musculocutaneous and other from radial nerve.

Dissection through the mid-muscle belly  helps to kee the nerve supllies intact.

Neurovascular Bundles

• The brachial artery and musculocutaneous and median nerves areprsent in the anterior compartment throughout the length of arm.

• The ulnar nerve takes origin from the anterior compartment but then passes into the posterior compartment in the distal arm.
• The radial nerve enters the posterior compartment proximally but comes to  anterior compartment in the distal portion.

• The axillary nerve lies near the posterior humerus and exists as one main trunk as it exits the quadrilateral space in the posterior upper brachium.

Complete paralysis of the deltoid may occur if the nerve is injured in this region.

Biomechanics of Fractures

Force may be applied to humerus bone

• Directly
• Indirectly
• Through muscular forces.

Direct force

An impact between the arm and an object, creating a three-point bending moment. This occurs when the patient falls against or is thrown against a fixed object or when a blunt object strikes the arm. This  mechanism produces a transverse type fracture line, occasionally with a nondisplaced butterfly fragment

Indirect force

The energy absorbed by the humerus is through the distal portion of the limb. It occurs in situations like  twisting of the arm behind the back or during arm wrestling. These injuries create a spiral fracture.

Muscular Forces

Strong muscle contractions like throwing can lead to spiral fractures.

Related posts:

1. Fractures of Shaft of Humerus- Clinical Presentation
2. Xray of Uniting Fracture of Shaft of Humerus Treated Nonoperatively
3. Xray of Fracture Shaft of Humerus With Failed Implant
4. Operative Treatment of Fracture of Shaft of Humerus
5. Fracture of Shaft of Humerus- When To Operate?

Tags: anatomy of humerus, fractures of shaft of humerus, mechanisms of fractures, shaft fractures

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• Xray of Uniting Fracture of Shaft of Humerus Treated Nonoperatively
• Fractures of Shaft of Humerus- Clinical Presentation