How to diagnose a retained fish bone in a puncture wound?

Diagnosing Retained Fish Bone in Puncture Wounds

Start with plain radiographs, but proceed immediately to ultrasound for superficial wounds or CT for deep wounds when radiographs are negative, because fish bones have extremely poor radiopacity and are frequently missed on x-rays. 1, 2

Initial Imaging Strategy

• Obtain plain radiographs first as the initial screening test, though fish bones are notoriously radiolucent and will be missed in most cases 1, 2
• Metal, stone, and graphite are radiodense and visible on x-rays, but fish bones, wood, plastic, and rubber are radiolucent 1
• Never rely on negative radiographs to exclude a fish bone foreign body—clinical suspicion should drive further imaging 2, 3

Next-Step Imaging Based on Wound Depth

For Superficial Puncture Wounds (No Deep Structure Involvement)

Ultrasound is the optimal next imaging modality for superficial fish bone foreign bodies 1, 2

• Ultrasound allows real-time visualization, precise localization, and assessment of adjacent tendons and vascular structures 2
• All foreign bodies appear hyperechoic with posterior acoustic shadowing on ultrasound 1
• Ultrasound can guide immediate removal at the bedside 2
• This modality is particularly valuable when the foreign body is within superficial soft tissues without surrounding bone 2

For Deep Puncture Wounds or When Ultrasound is Non-Diagnostic

CT without IV contrast is the definitive imaging study when fish bones penetrate deep tissues or bone 1, 2

• CT is 5-15 times more sensitive than radiography for detecting foreign bodies 1
• Use thin (1 mm) slice thickness because fish bones may be very small 1
• Fish bones appear moderately hyperdense on CT, making them distinguishable from soft tissue 1
• CT has 63% sensitivity and 98% specificity for foreign bodies in soft tissue sites 2
• CT is superior to MRI for water-rich organic material like fish bones (MRI sensitivity only 58%) 2

Critical Complications to Assess

Evaluate for both immediate mechanical complications and delayed infectious complications on imaging 1

• CT can identify cellulitis, muscle/fascial edema, abscesses, sinus tracts, vascular injuries, and tendon damage 1
• Consider IV contrast if vascular injury or pseudoaneurysm is suspected 1
• MRI is more sensitive than CT for detecting bone marrow edema if osteomyelitis is a concern, though less sensitive for the foreign body itself 1

Infection Risk Considerations

Fish bone injuries carry high risk for severe infections, including atypical mycobacterial infection 1, 2

• Mycobacterium marinum ("fish tank granuloma") should be considered, particularly in patients with fish-tank exposure 1, 2
• This organism causes chronic granulomatous soft tissue infections involving skin and bone 1
• Retained organic foreign material triggers granulomatous reactions that predispose to secondary soft tissue infections including cellulitis, abscess, myositis, septic arthritis, or sinus tracts 1
• If M. marinum infection is diagnosed, treat with two active agents (clarithromycin plus ethambutol or rifampin) for 3-4 months total 1, 2

Common Pitfalls to Avoid

• Do not discharge patients with persistent symptoms based on negative radiographs alone—fish bones are frequently radiolucent 2, 3
• Do not assume absence of foreign body on palpation—fish bones may be buried in muscle layers and impalpable 4, 3
• Do not delay imaging in high-risk scenarios—retained fish bones can cause disproportionate morbidity including flexor tendon sheath infections requiring ray amputation 3
• Always seek a foreign body even if not visible on radiograph, as residual fragments lead to troublesome secondary infections 3

Practical Diagnostic Algorithm

1. Obtain plain radiographs (multiple views) of the affected area 1, 2
2. If radiographs are negative but clinical suspicion remains:
   • For superficial wounds: Proceed to ultrasound immediately 1, 2
   • For deep wounds or bone involvement: Obtain CT without contrast (1 mm slices) 1, 2
3. If imaging confirms foreign body: Plan surgical removal with imaging guidance 1, 2
4. If imaging is negative but symptoms persist: Consider repeat imaging or surgical exploration, as some fish bones may still be missed 4, 3