UC Davis

Background: Fetlock breakdown (FB) remains the leading cause of euthanasia among California racehorses. The most common type of catastrophic FB is biaxial or medial proximal sesamoid bone (PSB) fracture associated with a pre-existing lesion (PEL) in the subchondral bone of medial PSB. Little is known about complex fetlock breakdown (CFB), characterized by simultaneous metacarpal/tarsal lateral condylar fracture (LCF), lateral or biaxial PSB fracture, and proximal phalanx (P1) osteochondral chip or severely comminuted fracture.Objectives: To characterize complex fetlock breakdowns in racehorses and compare signalment and exercise histories of racehorses with other fatal musculoskeletal injuries. Study Design: Retrospective analysis of necropsy and past performance records. Methods: Necropsy records of racehorses that sustained CFB between August 2018 and February 2021 were retrospectively reviewed. Signalment and exercise histories of affected horses were compared to those of racehorses that died with PSB fracture(s) associated with a PEL in the medial PSB (PSB-fx) and racehorses with non-FB musculoskeletal injuries (non-FB MSI). Exercise histories were also compared to three age, sex, and event-matched control racehorses with each CFB Thoroughbred racehorse. Age, sex, and limb distributions were compared between affected and comparison groups (Chi-square, Fisher's Exact test). Exercise history data were reduced to counts and rates of official high-speed works, races, and layups (periods without an official high-speed work or race >60 days). Variables were compared among groups using matched logistic regression, Kruskal-Wallis test with Steel Dwass post hoc, Welch’s or Wilcoxon tests (P ≤ .05). Results: Horses that sustained CFB (n=25) had biaxial (19) or lateral (6) PSB fractures, lateral condylar fracture (25), and P1 chip (12) or highly comminuted (13) fractures. Most CFBs occurred while racing (60%, 40% training) and on dirt (48%) or turf (32%) surfaces. Other MSI (PSB-fx, non-FB MSI) were least common on turf (9%, 8%). Gender predilection was not apparent. Four-year-olds were most commonly affected by CFB (52%) followed by ≥5-year-olds (28%). Four and ≥5-year-olds were least frequently affected by non-FB MSIs (19%, 16%, respectively). Three-year-olds were most commonly affected by PSB-Fx (41%). Most CFB horses sustained unilateral fractures (96%), of mostly forelimbs (64%); however, the proportion of hindlimbs with CFB (36%) was higher compared to PSB-Fx and nonFB-MSI horses (0% and 10%, respectively) (P<0.05). In CFBs, longitudinal axial avulsion fracture was present in all lateral PSBs. For medial PSBs the most common fracture types were apical (63%) or mid-body (21%), and configurations were transverse (53%) or oblique (32%). In CFB horses, gross evidence of PELs preceding the complete fracture was suspected in only 1 of 13 P1 fractures, and none was recognized in PSBs. However, PELs were recognized in all lateral condylar fractures, and they were associated either with palmar/plantar osteochondral disease (POD, 44%) or with parasagittal groove remodeling (PGR, 56%). Most horses with POD associated LCF had a recorded layup (91%) in contrast to only 50% of horses with PGR. Career spent in layup was significantly higher for POD horses than for PGR group (29%, 12%, respectively). Main Limitations: Retrospective review of necropsy records conducted by one postdoctoral fellow. Lack of consistent photographic necropsy records prevented morphometric measurements. Conclusions: Racehorses ≥ 4-years of age with evidence of bone remodeling in the lateral condyle should be considered at risk for CFB. LCFs have two possible etiopathogeneses, namely they can be associated with POD or PGR and the differences between the two are reflected in exercise history.