Understanding "Bone Fragment Greater Than 33% of the Articular Surface"
When a bone fragment involves more than 33% (one-third) of the articular surface at the middle phalanx base, this refers to the percentage of the joint surface area that has been fractured off, typically measured on radiographs, and represents a critical threshold that determines whether the injury requires surgical intervention versus conservative splinting. 1
How the 33% Threshold is Measured
The percentage is calculated by measuring the fractured fragment's articular surface area relative to the total articular surface of the middle phalanx base on lateral radiographs. 1
In practical terms, if you divide the joint surface into thirds on X-ray, a fragment involving more than one complete third crosses this surgical threshold. 1, 2
Standard three-view radiographs of the injured finger are sufficient to make this determination—advanced imaging like CT or MRI is not routinely needed for measurement. 1
Why the 33% Threshold Matters Clinically
Fractures involving ≥33% of the articular surface create joint instability and risk of palmar (volar) subluxation of the distal phalanx, which cannot be adequately managed with splinting alone. 1, 2
The American College of Radiology designates this as an absolute indication for surgical referral because conservative treatment leads to poor functional outcomes including persistent extensor lag, joint instability, and chronic pain. 1
Research demonstrates that approximately 50% of patients with fractures exceeding one-third of the articular surface develop subluxation of the joint, particularly when the fragment size reaches 48% or greater. 2
Clinical Context for Middle Phalanx Base Fractures
For volar plate avulsion fractures at the middle phalanx base (PIP joint injuries), fragments involving approximately 30% of the articular surface typically show loss of volar stability with hyperextension stress testing. 3
These injuries often result from PIP joint subluxation mechanisms and may demonstrate rotational displacement of the fragment (up to 90 degrees in severe cases). 3
Surgical reconstruction with open reduction and internal fixation, or hemihamate arthroplasty for comminuted fractures, is indicated when >30-50% of the volar articular surface is involved. 4, 5
Additional Surgical Indications Beyond Fragment Size
Even if the fragment is smaller than 33%, surgery is still required if there is palmar subluxation of the distal phalanx on lateral radiographs, an interfragmentary gap >3mm, or an open injury. 1
Volar subluxation visible on lateral X-rays represents an absolute surgical indication regardless of fragment size. 1
Common Pitfalls to Avoid
Delaying radiographs can lead to unreliable assessment and missed surgical indications—obtain three-view films immediately upon presentation. 1
Do not rely solely on fragment size percentage; always assess for subluxation on lateral views, as this changes management even with smaller fragments. 1, 2
Time to immobilization matters: delays beyond 12.5 days significantly increase the risk of joint subluxation developing, even with appropriate fragment size. 2