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Findings

Within the plantaroproximomedial aspect of the third metatarsal bone, ~2 well-defined and linear lucencies with a parallel and proximal to distal orientation are identified (white arrows).

Within the plantaroproximal aspect of the third metatarsal bone, moderate surrounding sclerosis is noted (yellow arrows).

Along the periarticular aspect of the tarsometatarsal joint, mild osteophyte formation is noted dorsolaterally and dorsomedially with associated subchondral bone sclerosis.

Incomplete longitudinal fracture/repetitive stress-related bone injury of the plantaroproximomedial aspect of the third metatarsal bone with sclerosis.
Mild tarsometatarsal osteoarthrosis.

Incomplete longitudinal fractures with similar characteristics have been described in the palmaroproximomedial aspect of the third metacarpal bone and are more commonly a consequence to repetitive stress or acute trauma (Morgan et al.). Such fracture within the third metatarsal bone is unusual.
Excessive repetitive forces will result in remodeling and may exceed the body’s ability to adapt, resulting in fracture (Pleasant at al., Morgan et al.).
Higher prevalence in thoroughbred and standardbred, but seen in sport and leisure horses are well.
May not be associated with any evidence of pain or swelling (Pleasant et al.).
Regional analgesia may help localize the lameness but may also have limited effects, possibly due to the prevalence of bone pain.
Most often unilateral but may occur bilaterally (Butler et al.)
Lameness worsen with exercise in a continuous line, then improve after changing direction before worsening again as the horse goes back to trotting on a straight line (Butler et al., Morgan et al.)

Radiographic features:

Best seen on dorsopalmar projections (Thrall et al., Butler et al., Morgan et al.).
Fractures are incomplete and may extend from the carpometacarpal joint to level of the nutrient foramen in more severe cases (Butler et al.).
Fracture extends in a proximal to distal orientation, parallel to the long axis of the third metacarpal bone or at a mild oblique angle (Butler et al., Thrall et al., Morgan et al., Pleasant et al.).
Sclerosis is more commonly present but may also be absent (Butler et al., Thrall et al., Morgan et al.).
Occasionally, a fracture line may not be apparent radiographically and microfractures cannot be ruled out (Butler et al.).

Nuclear scintigraphy: Focal, intense radiopharmaceutical uptake within the palmaroproximomedial aspect of the third metacarpal bone.

Callus is reported to be absent at the time of diagnosis (Morgan et al.).
MRI is often times not necessary for a diagnosis (Thrall et al.).
Most horses respond well to rest. Morgan et al. reported a 98% return to full work (45 out of 46 horses) after a 1.5–7 month long recovery.
On follow-up radiographs, the fracture becomes less defined or shorter. Additionally, there is reduced sclerosis of the third metacarpal bone (Morgan et al.).

A few things to note:

Lameness associated with an avulsion fracture at the origin of the suspensory ligament is often more severe (Morgan et al.)
Horses with incomplete palmaroproximal fractures of the third metacarpal bone had worsening lameness scores on the straightaway, whereas horses with proximal suspensory desmitis are reported to show more severe lameness on a circle in soft ground (Morgan et al., Thrall et al.).
Flexion tests had little effect on the lameness grade in horses with incomplete palmaroproximal fractures of the third metacarpal bone. In contrast, flexion test will more commonly exacerbate the lameness score of horses with proximal suspensory desmitis (Morgan et al.).

Morgan, R., and S. Dyson. “Incomplete longitudinal fractures and fatigue injury of the proximopalmar medial aspect of the third metacarpal bone in 55 horses.” Equine veterinary journal44.1 (2012): 64-70.
Pleasant, R. Scott, et al. “Stress reactions and stress fractures of the proximal palmar aspect of the third metacarpal bone in horses: 58 cases (1980-1990).” Journal of the American Veterinary Medical Association 201.12 (1992): 1918-1923.
Thrall, Donald E. Textbook of veterinary diagnostic radiology-E-book. Elsevier Health Sciences, 2012.
Butler, Janet A., et al. Clinical radiology of the horse. John Wiley & Sons, 2017.