Anatomy of the Upper Extremity: Scapula, Wrist, and Hand

Scapulohumeral Rhythm and Clinical Considerations

The relationship between the humerus and the scapula is defined by the Scapulohumeral Rhythm. This describes the coordinated movement between the glenohumeral joint and the scapulothoracic joint during arm abduction.

The ratio of this rhythm is typically expressed as: $\frac{\text{Glenohumeral Motion}}{\text{Scapular Motion}} = \frac{2}{1}$ This means for every 2 degrees of humeral movement, there is approximately 1 degree of scapular rotation.

Clinically, deviations in this rhythm or scapular stability can lead to issues such as:

• Scapular Tilt: An anterior or posterior tilting of the scapula.
• Scapular Winging: A condition where the medial border of the scapula protrudes from the back, often due to muscle weakness.

Exam Tip: When asked about the prime movers of scapular elevation, always remember to include both the Upper Trapezius and the Levator Scapulae.

The Rotator Cuff and Shoulder Stability

To maintain the integrity of the shoulder joint during these complex motions, the Rotator Cuff muscles provide essential dynamic stability. These are often remembered by the acronym SITS:

1. Supraspinatus
2. Infraspinatus
3. Teres Minor
4. Subscapularis

These muscles work together to hold the head of the humerus in the glenoid fossa, preventing dislocation during the various scapular motions described above.

Wrist and Hand Anatomy

Moving distal to the shoulder, the wrist and hand present a highly complex arrangement of bones and ligaments designed for both strength and fine motor control.

Bony Architecture

The hand is composed of several distinct groups of bones:

• Carpal Bones: The small bones that make up the wrist.
• Metacarpals: The bones that form the palm of the hand.
• Phalanges: The bones of the fingers, categorized into Proximal (PP), Middle (MP), and Distal (DP) phalanges.

The forearm provides the foundation for the wrist via the Radial bone (radius) and the Ulna bone. The connection between these two bones at the wrist is known as the Distal Radioulnar Joint (DRUJ).

A unique feature of the hand is the Saddle Joint, located at the base of the thumb, which allows for the specialized range of motion required for opposition.

The Carpal Tunnel and Soft Tissue Structures

The Carpal Tunnel is a narrow passageway on the palmar side of the wrist. It is bounded by the carpal bones and is capped by the Transverse Carpal Ligament (also known as the Flexor Retinaculum).

This tunnel serves as a conduit for several structures, most notably the nerves and tendons. A critical component within this area is the Median Distribution (median nerve), which provides sensation and motor control to much of the hand.

Functional Organization

The hand is not just a collection of bones; it is organized into functional units:

• Arches of the Hand: These provide the structural flexibility needed to cup the hand or flatten it against a surface.
• Finger Mechanics: Movement is facilitated by complex pulley systems and ligaments, including the extensor hood and flexor pulleys, which allow for precise flexion and extension of the fingers.

Clinical Note: Be prepared to differentiate between finger deformities. For example, understanding the mechanics of the flexor tendons is key to distinguishing between a Boutonniere deformity and a Swan-neck deformity, or identifying the causes of Trigger Finger.