- Proximal Transverse Arch: Formed by the bases of the metacarpals and the carpal bones. The capitate serves as the keystone for this arch, and the transverse carpal ligament connects the two sides.
- Distal Transverse Arch: A shallower arch formed by the heads of the metacarpals. Here, the heads of the 2nd and 3rd metacarpals act as the keystone.
Longitudinal Arch
The longitudinal arch begins at the wrist and runs the length of the metacarpals and phalanges for each digit. Interestingly, the heads of the 2nd and 3rd metacarpals also serve as the keystone for this arch.
Kinematics of the Thumb
The thumb is anatomically unique due to its orientation, which allows it to perform motions that the other four fingers cannot.
The Carpometacarpal (CMC) Joint
The thumb CMC joint is a triaxial saddle joint. This shape allows for complex movement patterns. Its primary motions include:
- Flexion and Extension: Occur in the frontal plane about a sagittal axis.
- Abduction and Adduction: Occur in the sagittal plane about a frontal axis.
- Opposition and Reposition: Opposition is a complex combination of motions occurring in a horizontal plane about a vertical axis. Reposition is the movement used to return the thumb from an opposed position.
Regarding arthrokinematics, the thumb CMC joint exhibits roll and glide in the same direction.
The Metacarpophalangeal (MCP) and Interphalangeal (IP) Joints
The thumb's MCP joint is a synovial uniaxial joint, allowing for flexion and extension only. During flexion, the first phalange rolls and glides medially, while during extension, it rolls and glides laterally. The thumb also possesses a single IP joint.
In contrast, the MCP joints of the four fingers are synovial biaxial condyloid joints. They allow for:
- Flexion/Extension: In the sagittal plane.
- Abduction/Adduction: In the frontal plane.
The finger IP joints are divided into the Proximal Interphalangeal (PIP) and Distal Interphalangeal (DIP) joints.
Arthrokinematic Motions and Grasping Mechanics
Arthrokinematics refers to the movement of joint surfaces (roll and glide) during osteokinematic motion (the movement of the bones themselves).
For the finger MCP joints, flexion involves the phalanges rolling and gliding anteriorly, while extension involves rolling and gliding posteriorly. For abduction and adduction, the phalanges roll and glide in the same direction.
Power vs. Precision Grasp
The hand's anatomy allows for two primary types of functional grasp:
-
Power Grasp: Used for strength and stability.
- Cylindrical grip: Holding a handle.
- Spherical grip: Holding a ball.
- Hook grip: Using the fingers like a hook.
-
Precision Grasp: Used for manipulation and fine motor tasks.
- Pad-to-pad (Pinch) grip: Using the tips of the fingers.
- Tripod grasp: Using three digits for stability.
- Lateral prehension (Pad-to-side): Holding an object against the side of the index finger.
- Side-to-side grip: Using the thumb and index finger sides.
- Lumbrical grip: Utilizing the unique action of the lumbrical muscles.
Exam Tip: The LOAF Muscles
To remember which intrinsic muscles are innervated by the Median Nerve, use the mnemonic LOAF:
L - Lumbricals (lateral two)
O - Opponens pollicis
A - Abductor pollicis brevis
F - Flexor pollicis brevis
All other intrinsic hand muscles are generally innervated by the Ulnar Nerve.
Clinical Considerations: Insufficiency and Tenodesis
Understanding the relationship between the wrist and the fingers is vital for clinical assessment.
Tenodesis refers to the mechanical grasp achieved when wrist extension causes the adaptive shortening of finger flexors, allowing a person to grasp objects even if they lack active finger flexion.
Students must also distinguish between active and passive insufficiency:
- Active Insufficiency: Occurs when a muscle is shortened to a point where it can no longer generate effective tension. For example, full wrist flexion causes active insufficiency of the extrinsic finger flexors.
- Passive Insufficiency: Occurs when a structure is stretched to its limit, preventing further motion. For example, when the fingers are already flexed, the extrinsic finger extensors cannot lengthen sufficiently to allow for full wrist flexion.
