Elbow biomechanics

Elbow Anatomy and Biomechanics

  • Trochlea-Capitellar Axis: The axis is 6 degrees off a line perpendicular to the humerus, resulting in a valgus carrying angle of the forearm:
    • Men: Mean carrying angle of 7 degrees.
    • Women: Mean carrying angle of 13 degrees.
    • The carrying angle disappears with elbow flexion.
  • Joint Characteristics:
    • Radiohumeral Joint: Poorly congruent.
    • Ulnohumeral Joint: Highly congruent.
    • Capitellum and Radial Head: The capitellum has a smaller radius than the radial head, allowing rotation in any position of flexion.
    • Annular Ligament: Stabilizes the radial head to the radial notch on the ulna.

Range of Motion (ROM)

Movement Normal ROM Functional ROM
Flexion 0-140° 30-130° (100° arc)
Supination 85° 50°
Pronation 75° 50°
  • Supination is more critical as shoulder movement compensates poorly for supination loss.
  • Pronation is especially important in the writing hand.

Elbow Kinetics

  • Forces About the Elbow: Short lever arms result in inefficient force transmission, leading to high ulnohumeral joint reaction forces, which predisposes the joint to arthritis.

  • Elbow Free Body Diagram:

    • Extension moment equals flexion moment.
    • The joint reaction force (JRF) can be calculated by considering these forces.

Forces Acting on the Elbow

Forces in Extension Forces in Flexion
Load in hand Pull of elbow flexors
Weight of the forearm
Joint reaction force

Lever Arms (in metres)

Lever for Extension Lever for Flexion
Distance from elbow COR to hand carrying load Distance from elbow COR to biceps insertion
Distance from elbow COR to midpoint of forearm

Example Calculation

For a 25 N weight in a forearm weighing 10 N and measuring 30 cm in length, with a biceps insertion 5 cm from the elbow COR:

- Biceps Pull = (25 x 0.3) + (10 x 0.15) = (0.05 x Biceps pull) → Biceps Force = 180 N. - JRF Calculation: JRF + 25 + 10 = 180 → JRF = 145 N.

Elbow Instability

Static Stability

  • Primary Static Stabilizer: The anterior band of the medial collateral ligament (MCL), which primarily resists valgus forces and distraction.
  • Secondary Stabilizer: Radial head, which becomes the primary stabilizer if the MCL is damaged.
  • Ulnohumeral Articulation: Highly congruent, providing stability in full extension.
  • Coronoid Process: Prevents posterior translation of the ulna. Loss of more than 50% can cause posterior instability, but an intact radial head may compensate.
    • The anteromedial part is especially important as the MCL anterior band attaches here via the sublime tubercle.
Medial Collateral Ligament Function
Anterior band Most important, tight in extension, slack in flexion.
Posterior band Tight in flexion, slack in extension.
Transverse ligament (Cooper’s ligament) Role in stability is less significant.
  • Lateral Collateral Ligament:
    • Consists of the lateral radial collateral ligament, lateral ulnar collateral ligament, accessory lateral collateral ligament, and the annular ligament.
    • The ulnar collateral ligament is the most important for varus and posterolateral rotatory stability, and is usually the first ligament torn in a dislocation.
  • Anterior Capsule: Resists hyperextension and posterior translation.

Dynamic Stability

  • Provided by muscles crossing the joint, which offer limited stability themselves.

Elbow Replacement

Elbow replacements can be classified as linked or unlinked: - Unlinked: - Rely on intact collateral ligaments and articular congruency for stability. - Technically challenging to balance and thus falling out of favor.

  • Linked:
    • Not a fixed hinge, but a “sloppy hinge” with 6 degrees of freedom.
    • Allows for 7-10 degrees of varus-valgus laxity, reducing stress on the bone-implant interface and preventing premature loosening or fracture.
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