Wrist Complex and Anatomy Quiz

Wrist Complex

• The wrist complex consists of two compound joints: the radiocarpal and the midcarpal.
• The wrist complex works with the shoulder, elbow, and forearm to control the hand's position and movement.
• The wrist's primary function is to adjust length-tension relationships in hand muscles.
• It allows for fine adjustments in grip and maintains optimal hand positioning.

Wrist Bone Anatomy

• The carpal bones are organized in two rows:
  • Proximal row: scaphoid, lunate, triquetrum, and pisiform.
  • Distal row: trapezium, trapezoid, capitate, and hamate.
• Radiocarpal: The radius and radioulnar disc (part of the TFCC) articulate with the scaphoid, lunate, and triquetrum.
• Midcarpal: The scaphoid, lunate, and triquetrum articulate with the distal carpal row (trapezium, trapezoid, capitate, and hamate).
• Intercarpal joints exist between the carpal bones.

Wrist Muscle Anatomy

• Wrist muscles are designed for control and balance, not maximal torque production.
• Most wrist muscles pass beneath the flexor retinaculum, along with the median nerve, preventing tendon bowstringing.
• Palmaris Longus and Flexor Carpi Ulnaris muscles are the only volar wrist muscles not passing under the flexor retinaculum.

Wrist Muscle Attachments

• Muscles that originate:
  • Pronator quadratus (ulna)
• Muscles that insert:
  • Pronator quadratus (radius)
  • Bracioradialis (side of the radius)
  • Abductor pollicis longus
  • Flexor carpi radialis
  • Flexor carpi ulnaris

Radiocarpal Joint Surfaces

• Concave surfaces of the radius and articular disc (part of the distal radioulnar joint) form the proximal components.
• Concave radial facet: articulates with the scaphoid.
• Medial concave radial facet: articulates with the lunate.
• TFCC: articulates predominantly with the triquetrum and sometimes with the lunate in neutral wrist position.
• Convex proximal surfaces of the scaphoid, lunate, and triquetrum (across the articular disc) are the distal components.
• The overall shape is biconvex (anteroposteriorly and transversally) and ellipsoid.

Radiocarpal Joint Ligaments

• Maintain intercarpal alignment and transfer forces within and across the carpus.
• Ligaments store muscle-produced forces contributing to wrist arthrokinematics.
• Mechanoreceptors, primarily in dorsal ligaments, protect the wrist.
• Ligaments are classified as extrinsic or intrinsic:
  • Extrinsic: proximal attachments on the radius or ulna, distal attachments within the wrist.
  • Intrinsic: both attachments within the wrist.

Radiocarpal Joint Ligament Classifications

• Radial (lateral) collateral ligament:
  • Extends from the radial styloid process to the scaphoid.
  • Stretches during adduction, medial ligament relaxes.
  • Provides modest stability, extrinsic muscles provide the majority.
• Ulnar (medial) collateral ligament:
  • Extends from the ulnar styloid process to the triquetrum and pisiform.
  • Stretches during abduction, lateral ligament relaxes.
• Anterior radio-carpal ligament:
  • Lies close to the center of rotation and does not contribute significantly.
• Anterior radio-carpal ligamentous complex:
  • Attached to the anterior edge of the distal radius and the neck of the capitate.
  • Stretches during extension.
• Posterior radio-carpal ligamentous complex:
  • Forms a posterior strap for the joint.
  • Stretches during flexion.

Midcarpal Joint Surfaces

• Articulation between the proximal and distal rows of carpal bones.
• Covered by a capsule continuous with the intercarpal joints.
• Divided into medial and lateral compartments descriptively.
• Medial Compartment:
  • Convex head of the capitate and apex of the hamate.
  • Fits into a recess formed by the scaphoid, lunate, and triquetrum.
• Lateral Compartment:
  • Slightly convex distal end of the scaphoid meets with the slightly concave proximal surfaces of the trapezium and trapezoid.
  • Shows less movement than the medial compartment.

Midcarpal Joint Position

• Defined as:
  • Wrist extension (dorsiflexion) of 20° to 30°-45°.
  • Ulnar deviation (adduction) of 10°-15°.
• Optimized hand position for grasping objects.
• Forearm pronation axis aligns with the arm and hand axes, running through the radius near the ulnar notch, passing between the lunate and scaphoid, the capitate, and the middle finger.

Wrist Motion Kinematics

• Wrist movements are combined motions of the radiocarpal and midcarpal joints.
• Axis of rotation passes through the head of the capitate, not the radiocarpal joint.
• The head of the capitate is the "keystone" of the wrist, providing stability to the carpal arch.
• The axis is medial-lateral for flexion and extension (sagittal plane) and anterior-posterior for radial and ulnar deviation (coronal plane).
• During flexion/extension, the lunate moves minimally, so the axis is usually at the lunate-capitate joint line.
• Wrist circumduction is a combined motion with a conical hand movement in space.

Wrist Motion Descriptions

• Flexion (80-85°):
  • Hand moves towards the anterior forearm.
  • Maximal when the hand is neither abducted nor adducted.
  • Exceeds 90° passively during pronation.
• Extension (70-85°):
  • Hand moves towards the posterior forearm.
  • Maximal when the hand is neither abducted nor adducted.
  • Exceeds 90° passively in both pronation and supination.
• Adduction/ulnar deviation (30°-45°):
  • Hand moves towards the body's axis.
• Abduction/radial deviation (15°):
  • Hand moves away from the body's axis.

Wrist Motion Muscle Contributions

• Abductor pollicis longus (AbPL) and extensor pollicis brevis (EPB):
  • Provide stability to the radial side of the wrist.
  • Radially deviate the wrist, potentially detracting from thumb function.
  • May require synergistic contraction of Extensor Carpi Ulnaris (ECU) to counteract unwanted wrist motion.
• Extensor carpi ulnaris (ECU):
  • Ulnar deviator muscle.
  • Crucial for stabilizing the wrist during hammer strikes.
• Extrinsic finger flexors (FDP and FDS):
  • Have a significant moment arm as wrist flexors.
  • Require wrist extensors to counterbalance their force for grip.
  • In strong grips, wrist extensors maintain functional wrist position.
• Flexor carpi radialis:
  • Strong ulnar deviator muscle.

Grip Strength

• Grip strength is significantly reduced when the wrist is fully flexed due to:
  • Passive insufficiency: Finger flexors cannot generate adequate force because they are shortened beyond their optimal length-tension curve.
• Wrist extensors are crucial for optimizing length-tension relationships in finger flexors, maximizing grip.