Case reveal

Findings

• Within the palmaroproximal aspect of the third metacarpal bone, extending into the cortex from the periosteal margin, well-defined, sharply marginated and crescent-shaped fractures are identified (green outline/circle).
• Also seen in the palmaroproximal aspect of the at third metacarpal bone, a moderate amount of ill defined increase in mineral opacity is identified and noted to be most severe surrounding the fracture (yellow arrows). 

• Along the dorsal and dorsomedial margins of the antebrachiocarpal and middle carpal joints, minimal osteophytes formation is identified. 
• At the level of the middle carpal joint, the dorsal carpal fat pad is reduced in conspicuity.

Conclusions

• Avulsion fracture of the left proximal third metacarpal bone at the origin of the suspensory ligament.
• Minimal osteoarthrosis, left antebrachiocarpal and middle carpal joints.
• Possible joint effusion, left middle carpal joint.

A little bit more…

• Avulsion fractures at the origin of the suspensory ligament has been reported in a wide range of horses and should be considered in horses presenting with an acute lameness localizing to the proximal metacarpal/metatarsal region (Jones et a., Booth et al.).
• Lesions occur primarily in the thoracic limb in reported cases and is seen at a higher incidence within the left thoracic limb in racehorses (Jones et al.).
• Local anesthesia (direct infiltration versus lateral palmar/deep branch of the lateral plantar nerve block) is often used to help localize the source of the lameness (Booth et al.).
• Increased radiopharmaceutical uptake has been observed in all horses with avulsion fractures at the origin of the suspensory ligament but is poorly specific (41%) and needs to be followed with radiographs, noting that some lesions may be subtle and advanced imaging (CT and/or MRI) may be helpful for the identification of avulsion fractures (Edwards et al.)

• Radiographic features: Avulsion fractures form crescent-shaped lucencies (also described as “punched-out” lesions) within the palmar/plantar cortex of the third metacarpal/metatarsal bone on dorsopalmar/dorsoplanter projections. Those crescent-shaped lesions correlate to the attachment site of the suspensory ligament on the medial and lateral depressions within the palmar/plantar metacarpal/metatarsal bone cortex (Booth et al.).

• In a lateral radiographic projection, a separate bone fragment may be isolated (Butler et al., Booth et al.).
• Ultrasound may be more sensitive for the identification of small avulsion fragments and is recommended for evaluation of the proximal suspensory ligament; however, visualization of the suspensory ligament origin may be difficult in some cases (Butler et al., Booth et al.).
• A favorable outcome may be observed with rest alone (Bramlage et al., Butler et al., Booth et al.).
• Concurrent desmitis of the proximal suspensory ligament may carry a more guarded prognosis (Butler et al., Booth et al.).

References

• Jones, R. D., and J. F. Fessler. “Observations on small metacarpal and metatarsal fractures with or without associated suspensory desmitis in Standardbred horses.” The Canadian Veterinary Journal 18.2 (1977): 29.
• Booth, T. M. “Proximal suspensory ligament desmitis with suspensory ligament avulsion fractures.” Equine Veterinary Education 15.3 (2003): 132-133.
• Edwards, Ryland B., et al. “Scintigraphy for diagnosis of avulsions of the origin of the suspensory ligament in horses: 51 cases (1980–1993).” Journal of the American Veterinary Medical Association 207.5 (1995): 608-611.
• Bramlage, L. R., A. A. Gabel, and R. P. Hackett. “Avulsion fractures of the origin of the suspensory ligament of the horse.” Journal of the American Veterinary Medical Association 176.10 Pt 1 (1980): 1004-1010.