Posterolateral | PIN Protection via Ulnar Flap | Monteggia and Radial Head Access
Surgical Imaging
The posterior interosseous nerve enters the supinator approximately 1 cm distal to the radial head. The Boyd approach protects it by subperiosteal elevation of the supinator origin from the ulna as part of a single flap with anconeus, reflecting the muscle mass radially with the nerve inside it. Never dissect within the supinator muscle itself.
Heterotopic ossification and radioulnar synostosis occur in up to 5-10 percent of cases if the interosseous membrane or periosteum is stripped from both the ulna and radius, creating a bridge for bone formation. Meticulous subperiosteal technique on the ulna only, avoidance of haematoma in the interosseous space, and early mobilisation reduce this risk.
Lateral decubitus with arm supported on a padded bolster or supine with arm across the chest are standard. Prone is rarely used. The C-arm must be positioned to obtain true AP and lateral views of the radial head and neck without obstruction from the table.
Primary uses include Monteggia fracture-dislocation fixation, radial head ORIF or arthroplasty, annular ligament reconstruction, and complex proximal ulna fractures with radial head involvement. It is the approach of choice when simultaneous access to the proximal radius and ulna is required.
Superficial: lateral cutaneous nerve of forearm branches. Deep: posterior interosseous nerve within supinator (protected by flap technique). Vessels: radial recurrent artery may require ligation. Joint: annular ligament must be preserved or repaired for stability.
Proximal extension along the lateral border of triceps allows olecranon visualisation. Distal extension follows the subcutaneous ulnar border for ulnar shaft access. Do not cross the interosseous space to the radius at any level to avoid synostosis.
At a Glance
The Boyd approach is a posterolateral exposure of the proximal radius and ulna that utilises a single subperiosteal flap containing anconeus and the origin of supinator to protect the posterior interosseous nerve while providing access to the radial head, radial neck, annular ligament and proximal ulna. It is the classic approach for Monteggia fracture-dislocations, radial head fractures requiring replacement or complex fixation, and annular ligament reconstruction. The key principle is staying strictly subperiosteal on the ulna and reflecting the entire muscle mass radially rather than dissecting within the supinator. This keeps the PIN safe within the reflected flap. The major long-term complication is radioulnar synostosis, which is minimised by avoiding any periosteal stripping on the radial side of the ulna or ulnar side of the radius and by meticulous haemostasis.
BOYD SAFEBOYD APPROACH - Surgical Steps
Hook:BOYD SAFE - subperiosteal flap protects the PIN every time!
PIN SAFEPIN Protection - Boyd Principles
Hook:PIN SAFE - the subperiosteal ulnar flap is the key to nerve protection!
NO BRIDGESynostosis Prevention
Hook:NO BRIDGE - never create a pathway for radioulnar synostosis!
Indications and Approach Selection
Primary Indications:
- Monteggia fracture-dislocations (Bado types I-IV) requiring ulnar fixation and radial head reduction
- Radial head fractures (Mason III/IV or comminuted) needing ORIF or arthroplasty
- Annular ligament reconstruction or repair in chronic Monteggia or radial head instability
- Complex proximal ulna fractures with associated radial head or neck injury
- Revision surgery for failed radial head fixation or malunited Monteggia
Why This Approach is Chosen:
The Boyd approach allows simultaneous exposure of the proximal ulna and the radial head/neck through a single posterolateral incision. The subperiosteal flap technique protects the posterior interosseous nerve without requiring its formal identification or dissection. It is the standard approach when both the ulna and radius proximally must be accessed, particularly in Monteggia injuries where the ulnar fracture and radial head dislocation are addressed together.
Contraindications:
- Active infection over the proposed incision
- Severe soft tissue compromise requiring alternative exposure
- Isolated radial head fracture accessible through Kocher or Kaplan approach (less invasive)
- Patient factors precluding lateral positioning
Alternative Approaches:
- Kocher approach: Between anconeus and ECU for isolated radial head exposure - less extensile
- Kaplan approach: Between EDC and ECU - more anterior, good for radial head but limited ulnar access
- Anterior (Henry) approach: For radial shaft or when PIN needs formal exploration
- Posterior (Thompson) approach: For radial shaft distal to supinator
Overview
Boyd Approach provides posterolateral access to the proximal radius and ulna by reflecting a single subperiosteal flap containing anconeus and supinator origin from the ulna, thereby protecting the posterior interosseous nerve within the reflected muscle mass.
Key Characteristics:
- Incision along subcutaneous border of ulna
- Subperiosteal elevation from ulna only
- PIN remains safe inside the flap
- Excellent exposure of radial head, neck and annular ligament
- Risk of radioulnar synostosis if technique violated
Why This Approach Matters:
- Gold standard for Monteggia fracture-dislocations
- Allows radial head arthroplasty or complex ORIF with ulnar fixation
- PIN injury rate less than 5 percent when subperiosteal technique followed
- Radioulnar synostosis is the most feared long-term complication
- Essential knowledge for upper limb trauma viva and operative surgery stations
Exam Relevance:
- High-yield surgical approach for FRCS/FRACS/EBOT/ABOS
- PIN protection mechanism is a classic examiner question
Anatomy
Bony Anatomy:
The proximal ulna forms the greater sigmoid notch articulating with the trochlea. The radial head is a cylindrical structure articulating with the capitellum and the proximal radioulnar joint. The annular ligament encircles the radial head and attaches to the anterior and posterior margins of the radial notch of the ulna. The radial neck lies distal to the head and is a common site of fracture in children and adults.
Muscular Layers:
|| Layer | Muscle | Nerve Supply | Action | ||-------|--------|--------------|--------| || Superficial | Anconeus | Radial nerve | Elbow extension, stabilises joint | || Superficial | Extensor carpi ulnaris | Posterior interosseous | Wrist extension, ulnar deviation | || Deep | Supinator | Posterior interosseous | Forearm supination | || Deep | Common extensor origin | Radial/PIN | Wrist and finger extension |
Neurovascular Anatomy:
|| Structure | Location | Clinical Significance | ||-----------|----------|----------------------| || Posterior interosseous nerve | Enters supinator 1 cm distal to radial head | MOST IMPORTANT - motor to extensors; protected by subperiosteal flap | || Radial nerve | Lies in spiral groove proximally | Not at risk in Boyd approach | || Radial recurrent artery | Arises just distal to elbow | May require ligation during deep dissection | || Ulnar nerve | Posterior to medial epicondyle | Not at risk unless medial extension | || Lateral cutaneous nerve of forearm | Superficial sensory branches | Risk of neuroma if superficial dissection injures branches |
Internervous Plane:
The classical interval is between anconeus (radial nerve) and extensor carpi ulnaris (posterior interosseous nerve). However, the safety of the Boyd approach relies on the subperiosteal elevation of both muscles' origin from the ulna as a single flap rather than true internervous dissection within the interval.
Positioning and Patient Setup
Position: Lateral Decubitus or Supine with Arm Across Chest
Pre-positioning Checklist:
- Confirm no contraindication to lateral positioning (shoulder pathology, spinal instability)
- Arm board or padded bolster positioned for elbow support
- C-arm available with unobstructed access to AP and lateral projections of radial head
- Tourniquet applied high on arm if planned
- Preparation of both upper limbs if bone graft or contralateral comparison required
Positioning Details:
- Lateral decubitus with affected arm supported on padded arm board or bolster, elbow flexed 90 degrees
- Supine alternative: arm placed across the chest with shoulder internally rotated and elbow flexed
- Tourniquet applied high on the arm, exsanguination before inflation
- Skin preparation from axilla to wrist, including hand for intraoperative rotation assessment
Lateral positioning risks include brachial plexus stretch, pressure injury to dependent arm, and compartment syndrome if tourniquet time is prolonged. Document all protective measures and limit tourniquet time to less than 120 minutes when possible.
Alternative Positioning:
- Supine with arm across chest is widely used and allows easy conversion to anterior approaches if needed
- Prone position is rarely required and limits C-arm access
Surface Anatomy and Landmarks
Key Bony Landmarks:
- Olecranon process - most prominent posterior landmark
- Lateral epicondyle - origin of common extensor tendon
- Radial head - palpable anterior to lateral epicondyle with forearm rotation
- Subcutaneous border of ulna - palpable along entire length
Key Soft Tissue Landmarks:
- Anconeus triangle - bounded by olecranon, lateral epicondyle and radial head
- Common extensor origin - palpable ridge on lateral epicondyle
- Ulnar nerve - palpable posterior to medial epicondyle (not directly in field)
Incision Planning:
- Longitudinal incision along the subcutaneous border of the ulna
- Start 2-3 cm proximal to olecranon tip if proximal extension needed
- Extend distally along ulnar border as far as required (typically 8-12 cm)
- Centre the incision over the radial head for isolated radial head work
Surgical Technique
Patient Positioning
Lateral decubitus with arm on padded bolster or supine with arm across chest. Elbow flexed 90 degrees. Tourniquet high on arm. C-arm positioned for true AP (forearm neutral) and lateral (forearm in neutral or slight supination) views of the radial head. Mark the olecranon, lateral epicondyle, radial head and subcutaneous ulnar border before skin preparation.
Surface Landmarks
The incision follows the subcutaneous ulnar border from just proximal to the olecranon distally. The anconeus triangle (olecranon-lateral epicondyle-radial head) is outlined. The interval between anconeus and ECU is palpated as a soft spot distal to the lateral epicondyle.
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“A 35-year-old falls from a height onto an outstretched hand and presents with a displaced proximal ulna fracture and radial head dislocation. CT confirms Bado type I Monteggia injury. Describe your surgical approach and key steps.”
“A 48-year-old sustains a comminuted radial head fracture (Mason type III) after a fall. CT shows greater than three fragments with significant articular comminution. Discuss your choice of approach and key operative considerations for radial head arthroplasty.”
“A 29-year-old labourer undergoes Boyd approach for a complex Monteggia injury with radial head fracture. Six months later he has complete loss of forearm rotation. CT confirms radioulnar synostosis. What went wrong and how could it have been prevented?”