34th Annual Scientific Meeting proceedings

Introduction
Proximal suspensory desmitis (PSD) refers to pathological changes of the M. interosseus medius (suspensory ligament) in the proximal metacarpal or metatarsal region (Dyson et al. 2007). This diagnosis includes lesions of the proximal portion of the suspensory ligament, adjacent bone structures, or both (Dyson 2007). A lasting source of pain may be an increased cross-sectional area of the suspensory ligament due to inflammation (Bathe 2012, Dyson 2007). Because of the limited space between the 2nd–4th metacarpal/metatarsal bones, the suspensory ligament, and the palmar/plantar fascia, this can lead to a compartment syndrome compressing the deep branch of the lateral palmar/plantar nerve (DBLPN) (Toth et al. 2008).

Surgical Treatment Options
Surgical interventions for proximal suspensory desmitis can generally be categorized into three types:

1. Neurectomy of the deep branch of the lateral palmar/plantar nerve
2. Desmoplasty of the origin of the suspensory ligament
3. Osteostixis of the palmar/plantar cortex of the proximal metacarpus/metatarsus
   In the hindlimb, these are often combined with a fasciotomy of the proximal metatarsal fascia.

Neurectomy and Fasciotomy
The technique of combining a neurectomy of the DBLPN with fasciotomy of the proximal metatarsal fascia was first described by Bathe in 2001. It enabled 19 out of 20 horses with chronic hindlimb PSD to return to previous performance levels (Bathe 2001). Later studies reported a success rate of 80% in over 600 horses (Bathe 2012). Bathe hypothesized that the low success rate of conservative treatments in chronic PSD was linked to compartment syndrome and neuropathy of the DBLPN.

Compartment Syndrome and Neuropathy
Compartment syndrome is a pathological condition involving increased pressure within a muscle compartment, which can lead to ischemia, necrosis, and peripheral nerve damage. It is theorized that compression of the DBLPN plays a key role in chronic PSD, especially when the suspensory ligament is enlarged proximally due to inflammation.

Tóth (2008) analysed DBLPN tissue in 16 horses with PSD and found histological changes including axonal swelling, myelin degeneration, and Renaut bodies, suggesting chronic nerve compression. Of the horses studied, 63% returned to previous performance levels within 6 months post-surgery.

Evidence of Neurogenic Atrophy
Several studies (Pauwels et al. 2009; Lopez-Navarro et al. 2017; Scharf et al. 2021) investigated whether DBLPN neurectomy causes neurogenic atrophy of the suspensory ligament. Histological evaluations showed muscle fibre reduction, fatty tissue replacement, and axonal degeneration. While some studies confirmed post-operative reduction in ligament size, others (e.g., Scharf et al. 2022) found no significant changes.

Long-Term Outcomes and Prognostic Factors
Dyson and Murray (2012) conducted a long-term study on 155 horses and found that 77.8% of horses with isolated PSD returned to full performance. In contrast, horses with additional orthopaedic issues or anatomical risk factors (e.g., straight hock conformation, fetlock hyperextension) had lower success rates. Neurectomy was associated with ligament size reduction in 95% of cases, likely due to neurogenic atrophy.

In contrast, Tóth (2009) reported an 89% success rate using neurectomy without fasciotomy. Tatarniuk et al. (2020) observed that 18 of 21 Western performance horses returned to sport post-surgery, though not all reached previous performance levels. The authors emphasized that neurectomy relieves pain but does not directly address healing of the damaged ligament.

Conclusion
In summary, neurectomy of the deep branch of the lateral palmar/plantar nerve combined with fasciotomy appears to be a favourable long-term treatment for PSD in both fore- and hindlimbs, provided there is accurate diagnosis and appropriate patient selection. Risk factors for poorer outcomes include straight hock conformation, fetlock hyperextension, and other pain sources. Despite promising results, the risk of neurogenic atrophy and potential breakdown of the suspensory ligament must be carefully weighed.

Moreover, according to current guidelines from the German Federal Ministry of Agriculture (BMEL), the German Equestrian Federation (FN), and the International Equestrian Federation (FEI), horses that have undergone neurectomy are ineligible for competition to ensure animal welfare (BMEL 2020, FEI 2024, FN 2024).