Case reveal

Findings:

• Along the periarticular margins of the distal interphalangeal joint, mild to moderate osteophyte formation is identified (blue arrows). The synovium of the distal interphalangeal joint is markedly thickened with finger-like projections (yellow arrows). Following contrast administration, the synovial capsule is moderately contrast enhancing, especially at the level of the palmar pouch. Additionally, there is extensive articular cartilage irregularity and thinning (pink arrows). The extracapsular soft tissues are mildly thickened and hyperintense.

• The palmar aspect of the distal phalanx has extensive mild to moderate fluid with a prominent vascular pattern. The middle phalanx has mild to moderate fluid at the periarticular margin that is affecting the distal interphalangeal joint capsule attachment with a prominent vascular pattern and mild palmar fluid.
• The navicular bone has extensive moderate fluid with shallow flexor surface defects (2 to 3 mm in diameter) at the proximal to mid aspect of the bone affecting the palmar and medial sagittal ridge margins. In addition, the navicular bone has mild to moderate synovial invagination enlargement.

• The proximal and distal recesses of the navicular bursa are compressed due to the synovial proliferation within the distal interphalangeal joint.

Additional subtle finding: The proximal extent of the distal phalanx extensor process has focal articular cartilage loss. There is susceptibility artifact in the distal interphalangeal joint dorsal recess with tracts extending through the dorsal subcutaneous tissues and dorsal digital extensor tendon located medial and lateral of midline consistent with prior arthroscopy (green line outlines tract).

Conclusions:

• Marked distal interphalangeal joint proliferative synovitis and mild to moderate osteoarthrosis with extensive articular cartilage irregularity, thinning and loss, compatible with septic arthritis. Involvement of the navicular bursa cannot be excluded on this study.
• Extensive moderate fluid with shallow flexor surface defects and mild to moderate synovial invagination enlargement, navicular bone 
• Extensive mild to moderate fluid with a prominent vascular pattern, distal phalanx 
• Mild to moderate fluid with a prominent vascular pattern and mild palmar fluid, middle phalanx 
• Compressed proximal and distal recesses, navicular bursa 

More information/case follow-up:

• The patient was diagnosed with a septic coffin joint prior to the MRI. The joint was flushed arthroscopically and treated with regional antibiotics. The patient became uncomfortable again a few days later. The synovial fluid was improved but the lameness persisted. The MRI was done to assess for other/contributing causes for the lameness.

• Following MRI, arthroscopic debridement with high volume lavage and regional antibtioics was performed. At surgery there was marked fibrin within the coffin joint which was collected and submitted for microbial and fungal culture. There was complete loss of cartilage with exposed bone on the extensor process of the coffin bone and moderate synovial proliferation and hyperemia. After samples for culture were collected, regional limb perfusion with imepenam was performed. Amikacin was injected into the joint at the end of surgery.
• Fungal culture was negative. A specific agent was not isolated (prior therapy may have affected the culture).

A little bit more…

• Septic arthritis has a better prognosis in mature horses when compared to foals. A good prognosis may ensure immediate aggressive therapy; therefore, an early diagnosis is important (Easley et al.).
• In mature horses, septic arthritis most often result from trauma, intraarticular injections, or postoperatively. Some cases without identifiable cause are classified as idiopathic (Easley et al.).
• A diagnosis of septic arthritis rely on the patient’s history, physical exam findings, and arthrocentesis for synovial fluid analysis (Easley et al.).

Reported prevalence of clinical findings by Easley et al.:

• Severe joint swelling, heat, and pain on flexion (86%)
• Severe lameness (4 or 5 out of 5 on AAEP Lameness scale) without evidence of a fracture (79%)
• Wounds with communication to a joint (21%)
• Hematogenous spread as a result of bacteremia (8.0%)

• Radiographs are important to acquire in all horses with suspected septic arthritis (Easley et al.).
• MRI is the gold standard for the diagnosis of human septic arthritis, acute osteomyelitis, and soft tissue infection (Easley et al.). Allows earlier detection of septic arthritis.

Most common MRI findings (from most to least common):

• Extracapsular and bone hyperintensity
• Joint effusion, marked synovial proliferation, and capsular thickening
• Disruptions/erosions of the articular cartilage (irregularity, cartilage thinning and/or heterogeneous signal)
• Subchondral bone damage and/or loss (~50% of cases)
• Contrast enhancement of the synovium (occasionally seen)
• Bone sequestration (rare)

References:

• Easley, J. T., Brokken, M. T., Zubrod, C. J., Morton, A. J., Garrett, K. S., & Holmes, S. P. (2011). Magnetic resonance imaging findings in horses with septic arthritis. Veterinary Radiology & Ultrasound, 52(4), 402-408.
• Murray, Rachel C., ed. Equine MRI. John Wiley & Sons, 2010.