Clinical Biomechanics: The Upper Extremity

Clinical Biomechanics: The Upper Extremity

The Chiro.Org Blog

We would all like to thank Dr. Richard C. Schafer, DC, PhD, FICC for his lifetime commitment to the profession. In the future we will continue to add materials from RC’s copyrighted books for your use.

This is Chapter 10 from RC’s best-selling book:

“Clinical Biomechanics: Musculoskeletal Actions and Reactions”

Second Edition ~ Wiliams & Wilkins

These materials are provided as a service to our profession. There is no charge for individuals to copy and file these materials. However, they cannot be sold or used in any group or commercial venture without written permission from ACAPress.

CHAPTER 10: THE UPPER EXTREMITY

This chapter considers forces acting within and upon the shoulder girdle, arm, forearm, and hand, and their related clinical problems, with emphasis on the related musculoskeletal disorders. Therapy can be directed efficiently when the mechanisms of injury are appreciated and correction is applied in harmony with proven biomechanical principles.

The Shoulder Girdle and Arm

The structure of the upper extremity is composed of the shoulder girdle and the upper limb. The shoulder girdle consists of the scapula and clavicle, and the upper limb is composed of the arm, forearm, wrist, hand, and fingers. However, a kinematic chain extends from the cervical and upper thoracic spine to the fingertips. Only when certain multiple segments are completely fixed can these parts possibly function independently in mechanical roles.

Basic Functional Anatomy

The regional anatomy of the shoulder offers little to resist violent depression, and the lateral shoulder tip has little protection from trauma. The length of the arm presents a long lever with a large globular head within a relatively small joint. This allows a great range of motion with little stability. The stability of the shoulder is derived entirely from its surrounding soft tissues, capsule, muscles, and ligaments.

THE SHOULDER GIRDLE

The scapula, clavicle, and the humerus function as a biomechanical unit. Forces generated from or upon one of the three segments will affect the other two segments. Thus, in this chapter, they will be discussed as a functional unit.

The Scapula.   The scapula is frequently underrated clinically. Morehouse calls it one of the most outstanding engineering miracles of the human body. The blade of the shoulder, resembling an inverted right triangle, lies at the posterior T2–T7 rib level and attaches to the thorax only by muscles and fascia. Its medial border is about 2 inches lateral from the spinous processes, and its laterosuperior aspect articulates with the clavicle and the humerus in a rocking and gliding action. Its design allows it to function as a base for humeral motion and at the same time to move independently of arm action.

The Clavicle.   This bony brace offers mechanical support to the anterosuperior thorax and cervical spine. It forms a mechanical link with the scapula and protects the vital vessels at the anterolateral aspect of the base of the neck. The clavicle is necessary in all animals that either climb, swim, or fly. It serves as a strut to hold the shoulder joint outward so that the range of motion of the upper extremity is increased and the pull of the brachiating muscles is mechanically improved. The clavicle articulates medially with the manubrium of the sternum and cartilage of the 1st rib (sliding joint), and laterally with the acromion (sliding joint) and coracoid process (syndesmoses)

The Sternoclavicular Joint.   This joint is supported by a capsule and reinforcing anterior and posterior ligaments. It is the sole point where the shoulder girdle articulates with the thorax. An interclavicular ligament crosses the jugular notch, connects the manubrium with both clavicles, and tends to restrict sternoclavicular motion. A costoclavicular ligament straps the inferomedial clavicle to the 1st rib below (Fig. 10.1).

The Acromioclavicular Joint.   The articulation between the acromion and the lateral clavicle is supported by a capsule that is reinforced superiorly by the acromioclavicular ligament. An incomplete articular disc is sometimes found at the upper aspect of the joint.

The Coracoclavicular and Coracoacromion Unions.   The clavicle is strapped to the coracoid process of the scapula by the conoid and trapezoid ligaments. The coracoacromial ligament extends between the coracoid process and acromion (Fig. 10.2).

The Shoulder Joint.   This highly mobile joint is aided somewhat by a narrow fibrocartilaginous rim around the glenoid fossa. A surrounding joint capsule extends loosely from the lateral scapula and clavicle to attach firmly at the anatomic neck and shaft of the humerus. A series of anterior glenohumeral ligament bands and tendon fibers reinforce the articular capsule, and the transverse humeral ligament forms a canal for the long tendon of the biceps brachii within the bicipital groove. The arm rotates about a point that is considered to be in the center of the head of the humerus.

Table 10.1. Shoulder Girdle and Shoulder Motion

Shoulder Movements

The versatile shoulder girdle consists of the sternoclavicular, acromioclavicular, and glenohumeral joints, along with the scapulothoracic union. These allow, as a whole, universal mobility by way of the shallow glenoid fossa, the joint capsule, and the suspension muscles and ligaments. The shoulder, a ball and-socket joint, lacks the close connection between its articular surfaces as seen in weight-bearing joints.