What are the key considerations for radial tuberosity tunnel angulation and its relation with a pin in distal biceps repair?

Radial Tuberosity Tunnel Angulation and Pin Placement in Distal Biceps Repair

When drilling the radial tuberosity for distal biceps repair, the drill should be angled proximally and ulnarly (toward the ulna) from the bicipital tuberosity starting point to avoid posterior interosseous nerve (PIN) injury, with the forearm maintained in maximal supination throughout the procedure. 1, 2

Critical Anatomic Considerations

Posterior Interosseous Nerve Position

• The PIN sits directly opposite the biceps tuberosity prominence, often directly on the radial cortex when the forearm is in full supination 1
• The nerve crosses from anterior to posterior at approximately 46 mm from the lateral epicondyle, lying nearly at the same level as the bicipital tuberosity 1
• Perpendicular bicortical drilling starting at the bicipital tuberosity must be avoided as it places the PIN at unacceptable risk 1

Safe Drilling Technique

Forearm Position:

• Maintain maximal forearm supination during all drilling and pin placement 1, 2
• Supination increases the guidewire entry point to PIN distance (mean 16.01 mm) compared to pronation (13.66 mm) 2
• The perpendicular plane trajectory distance from bicipital tuberosity to PIN is significantly greater in supination (9.00 mm) versus pronation (1.96 mm) 2

Drill Angulation:

• Direct the drill proximally and ulnarly from the bicipital tuberosity starting point 1
• The drill exit point should be posterior to the lateral midline of the radius 1
• Avoid perpendicular drilling trajectories that would place the PIN at risk 1

Anterior Safe Zone for Exposure

• The anterior safe zone extends approximately 15-20 mm from the bicipital tuberosity prominence (mean 20.7 mm, range 16.0-24.1 mm) 1
• The PIN travels from an anterior position 1 cm proximal to the tuberosity, to a lateral position at the tuberosity level, to a slightly posterior position 1 cm distal 1
• High anatomical variability exists for PIN position, making strict adherence to safe drilling angles more important than relying on fixed anatomic landmarks 1

Pin Placement Relative to Anatomic Footprint

Anatomic Insertion Site

• The center of the native biceps tendon inserts 6.7 mm posterior to the apex of the radial tuberosity 3
• This posterior location allows the tendon to wrap around the radial protuberance, utilizing it as a mechanical cam during supination 3
• The biceps comprises medial (short head) and lateral (long head) components, with the short head inserting distal to the long head 3

Optimal Pin/Button Placement

• Guidewire placement should be as close as possible to the anatomic footprint posterior to the radial protuberance 2
• Ulnar and proximal guidewire trajectory is the safest technique when using single-incision bicortical suspensory button fixation 2
• The two-incision biceps button technique allows safe and accurate repair to the anatomic insertion site on the tuberosity 4
• Average distance from guide pin tip to PIN in cadaveric studies was 11.4 mm (range 8-14 mm) 4

Common Pitfalls and How to Avoid Them

Critical Errors:

• Never use deep retractors beneath the radial neck - PIN injuries during anterior repair are more likely from aberrant retractor placement than guidewire injury when proper technique is used 2
• Avoid anterior placement of the repair site (12.9 mm anterior to apex), which results in nonanatomic positioning and 10-33% loss of supination strength 3
• Do not drill perpendicular to the tuberosity surface 1

Technical Safeguards:

• Confirm maximal supination before any drilling 1, 2
• Use proximal and ulnar angulation consistently 1
• Consider the two-incision technique for anatomic repair when restoring full supination strength is critical 4, 3
• The average distance from guide pin to recurrent radial artery is 12.5 mm (range 8-19 mm), providing additional margin of safety 4