A Randomized, Self-Controlled Case Series Evaluating Core Osteostixis of Osseous Cyst-Like Lesions of the Navicular Bone to Improve Lameness in Horses with Podotrochlear Syndrome

**Background:** Podotrochlear syndrome (navicular syndrome) is a common, chronic, progressive cause of forelimb lameness in Quarter Horses, involving both soft tissue and bony pathology of the podotrochlear apparatus. MRI frequently identifies bone edema-like signal and osseous cyst-like lesions in the navicular bone, which are thought to contribute to pain via increased intraosseous pressure. Standard surgical treatments (palmar digital neurectomy, navicular bursoscopy) address soft tissue lesions but not these bony changes. Core osteostixis has been used in human osteonecrosis to reduce intraosseous pressure and promote neovascularization, and a prior equine study showed it transiently reduces navicular intraosseous pressure with drill tracts ~90% filled with woven bone by 12 weeks. However, its clinical efficacy in diseased horses was unknown.

**Methods:** This was a randomized, blinded, self-controlled trial conducted from July 8 to August 22, 2019, at a single private practice. Seven Quarter Horse geldings (age range 9–13 years, all used for Western events) met inclusion criteria: bilateral forelimb lameness ≥2/5 (modified AAEP scale), positive palmar digital nerve block, >4 months duration, failure of medical management, and bilateral MRI evidence of navicular bone edema and/or osseous cyst-like lesions on STIR sequences using a 0.27T standing MRI. Each horse had one forelimb randomly assigned (via simulated coin flip in R) to navicular bursoscopy plus core osteostixis (2.5-mm drill bit into the medullary cavity, depth guided by cyst location or ~75% of bone depth if no cyst), and the contralateral limb to bursoscopy only. Video recordings of trotting (straight line and circles) were graded by a single blinded observer (CMH) at baseline and 24 weeks post-operatively using a modified 5-point AAEP scale with half-grades. The primary outcome was the paired difference in lameness score change between limbs, analyzed by Wilcoxon sign-rank test (significance P < 0.05). MRI was repeated at 24 weeks to assess changes in bone fluid signal, cyst-like lesions, and DDFT tears.

**Key Results:** At baseline, there was no significant difference in lameness severity between limbs assigned to core osteostixis versus bursoscopy only (P = 0.4593). The primary outcome — the difference in improvement from baseline between core osteostixis and bursoscopy-only limbs — was significantly less than 0 (P = 0.0254), indicating greater lameness reduction in osteostixis-treated limbs. Median lameness score at 24 weeks was 0 (range 0–2) for osteostixis limbs versus 1 (range 0–2.5) for bursoscopy-only limbs (P = 0.0848). Five of 7 (71%) osteostixis limbs were sound at 24 weeks versus 1 of 7 (14%) bursoscopy-only limbs; this difference in paired soundness was not significant (P = 0.1250). All horses returned to intended use by 6 months. MRI findings: navicular bone fluid signal decreased in 5/7 osteostixis limbs versus 3/7 bursoscopy-only limbs. Cyst-like lesion sizes remained unchanged in both groups. New DDFT tears were identified post-operatively in 3/7 osteostixis limbs and 1/7 bursoscopy-only limbs. One horse (Horse 2) had a convex proximal navicular border making drill placement difficult, with inadvertent entry into the navicular bursa and distal interphalangeal joint, yet still showed greater improvement in the osteostixis limb.

**Clinical Implications:** Core osteostixis combined with navicular bursoscopy significantly improved lameness in Quarter Horses with podotrochlear syndrome and osseous cyst-like lesions that had failed medical management. The procedure appears feasible and safe, though new DDFT tears occurred in 3 of 7 osteostixis limbs (possibly iatrogenic), and careful surgical technique (complete soft tissue removal from the proximal navicular border) is essential. The study is limited by small sample size (n=7), single breed, single observer for lameness scoring, and lack of blinding of radiologists. Larger-scale studies in other breeds and with longer follow-up are warranted. The findings suggest that addressing bony pathology via osteostixis may be an important adjunct to soft tissue debridement in podotrochlear syndrome.