Wooden Foreign Body Injuries of the Foot: MR Evaluation

by John D. Reeder, M.D., Director of Imaging, Proscan Imaging Maryland

Introduction

Penetrating injuries involving the plantar soft tissues of the foot are commonplace and the presence of retained foreign matter impedes and complicates healing. Persistent pain and physical findings suggestive of increasing or chronic localized inflammation develop when a foreign body has not been successfully extracted. Cellulitis, focal abscess formation and osteomyelitis may develop as a consequence. Imaging evaluation maintains a vital role in identifying soft tissue foreign bodies, thereby, directing appropriate management.

Metallic fragments are typically evident on conventional radiographs. These foreign bodies are also conspicuous on CT and MRI, particularly considering the associated respective beam-hardening and magnetic susceptibility artifacts. However, the detection of retained glass, plastic, or organic foreign matter such as wood, creates a greater challenge. CT imaging may reveal wooden matter as hyperdense relative to soft tissue and, on ultrasound images, it may be echogenic with acoustic shadowing.(1) Because it provides direct image acquisition in all three anatomic planes, localization of a foreign body with determination of its relationship to adjacent osseous and myotendinous structures represents an advantage of MRI. Additionally, in detecting marrow edema as an early manifestation of osteomyelitis, MRI is superior to other imaging choices in recognizing subacute or late complications associated with retained foreign matter.

MRI Findings

Retained fragments of wood often exhibit a low signal intensity or signal void pattern on MRI. (2,3)

(Figure 1)
On a sagittal fat-saturated T2-weighted image (1A), a splinter is identified, just plantar to the flexor tendon, surrounded by fluid and soft tissue edema. A coronal (short-axis) fat-saturated T2-weighted image (1B) and a T1-weighted image in the same plane (1C) demonstrate the foreign body end on, adjacent to the flexor tendon complex of the second toe. A soft tissue abscess extends plantar to the splinter.

A surrounding zone of soft tissue edema or granulation tissue, appearing particularly hyperintense on T2-weighted fat-saturated or STIR imaging series, may be present. This reactive tissue typically exhibits enhancement following Gd-contrast administration.

(Figure 2)
Coronal T1-weighted (2A) and fat-saturated T2-weighted (2B) images reveal the presence of a wooden foreign body fragment, end on, within the subcutaneous soft tissue plantar to the first metatarsophalangeal joint. On a fat-saturated T2-weighted axial (long-axis) image (2C) a rim of increased signal intensity surrounds the splinter. In the same plane, a fat-saturated T1-weighted image, obtained following the intravenous administration of Gadolinium contrast material, (2D) reveals enhancement of the surrounding reactive, hyperemic tissue, rendering the foreign body particularly conspicuous.

If the foreign body extends into a joint, an effusion is typically present and, if the injury is subacute or chronic, capsular thickening and increased synovial enhancement are often noted.(1)

MRI also contributes to the evaluation of secondary complications of retained foreign matter. Extensive edema of the subcutaneous fat with adjacent skin thickening, conspicuous of STIR and fat-saturated T2-weighted imaging, suggests the presence of cellulitis. Although fluid accumulation surrounding retained foreign matter may simply reflect the presence of a sterile seroma, abscess must be suspected. Migration of a foreign body may occur and, if the fragment is located remote from the cutaneous entry wound, MRI permits anatomic localization, critical to surgical planning.

(Figure 3)
On a sagittal fat-saturated T2-weighted image (3A), a small linear foreign body is identified, plantar to the flexor hallucis longus tendon at the vertical level of the talonavicular joint, surround by fluid and/or granulation tissue. A larger fluid collection or sinus tract extends plantar and distal to the foreign body, towards the entry site (skin marker). An axial (long-axis) fat-saturated T2-weighted image (3B) demonstrates the long-axis of the splinter, located medial to the anterior aspect of the calcaneus. On a coronal (short-axis) fat-saturated T2-weighted image (3C), the splinter is identified end on, adjacent to the flexor tendons, remote (proximal and dorsal) to the skin entry site.

Summary

Detection of retained organic foreign matter within soft tissue following a penetrating injury may prove difficult with any imaging approach, depending upon the size of the foreign body, its structural integrity, its relative porosity, and the length of time since the injury.(1) However, MRI offers several advantages when compared to CT and sonography. Wooden foreign bodies often exhibit a conspicuous signal void pattern on MRI. On T2-weighted fat-saturated imaging and STIR imaging, the retained fragment may be encased within a zone exhibiting hyperintense signal intensity, reflecting the presence of adjacent reactive edema and/or granulation tissue. Because this reactive process tends to be hyperemic, a rim exhibiting prominent enhancement may be observed surrounding the foreign body on fat-saturated T1-weighted images obtained following Gd-contrast administration. Relative to CT and sonography, MRI offers superior sensitivity in detecting osteomyelitis. The presence of bone marrow edema and increased osseous enhancement, identified with MRI, suggest secondary osseous infection.

References
1. Peterson JJ, Bancroft LW, Kransdorf MJ. Wooden foreign bodies: imaging appearance. AJR 2002;178:557-562.
2. Bodne D, Quinn SF, Cochran CF. Imaging foreign glass and wooden bodies of the extremities with CT and MR. J Comput Assist Tomogr 1988;12:608-611.
3. Monu JUV, McManus CM, Ward WG, et al. Soft-tissue masses caused by long-standing foreign bodies in the extremities: MR imaging findings. AJR 1995;165:395-397.

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