34th Annual Scientific Meeting proceedings

The predominant surgery being considered during this presentation is neurectomy of the deep branch of the lateral plantar nerve and decompressive fasciotomy, as this is still the most commonly performed surgical procedure. There are a number of inclusion criteria that should be met for the horse to be a candidate for surgery which will be discussed first, and then we will discuss whether surgical management is most appropriate for the individual case. 

Obviously, a critical component of the inclusion criteria for being a surgical candidate is an accurate diagnosis. Logically the horse should show a positive response to blocking the deep branch of the lateral plantar nerve. If it does not then it should be considered unlikely that surgery will be beneficial, but the lack of specificity of this block must be taken into account. Because one is blocking the lateral plantar nerve at the same time, there could be desensitisation of the lateral side of the foot and fetlock and additionally there can be diffusion to desensitise the distal portion of the tarsus. Thus I use a diagnostic block sequence with blocking of the tarsometatarsal joint first, then a low six-point block. If these are negative then we perform the deep branch block, knowing that the sites of potential false positives have already been desensitised. I nearly always perform the deep branch blocks unilaterally as significant information can be missed by performing a bilateral block, as the condition is normally bilateral and a positive switch can be missed. Also assessing the horse’s way of going under saddle after blocking to see if the particular presenting problem has been improved a lot is critical in assessing the likely benefit of surgery. It should be borne in mind that there are often significant comorbidities such as sacroiliac region pain. I commonly see this as secondary to the proximal suspensory desmitis, but this may actually be the performance limiting problem. If blocking the sacroiliac region gives a greater improvement in the horse’s way of going we may still operate in cases of underlying PSD, as solving the primary problem makes treatment of sacroiliac pain more successful.

Once pain is localised to the proximal plantar metatarsal region, the area should have diagnostic imaging. Radiography is helpful to assess any sclerotic reaction at the origin of the suspensory ligament and rule out the presence of tarsal disease. Ultrasonography should be performed, carefully assessing the whole of the origin, with longitudinal scans being the most helpful in my experience. The wide variation in the appearance of this region should be appreciated. MRI or CT examination generally only needs to be performed in a small proportion of cases. It can provide helpful information, particularly in cases of bone pathology at. The suspensory origin or if there are adjacent pathologies. Thus, I tend to reserve its use for cases in which there is a not good correlation between the conventional imaging findings and the block pattern. 

Surgical treatment is not appropriate in acute, severe cases of proximal suspensory desmitis. The severely lame horse with apparent loss of suspensory infrastructure is treated with rest and intralesional orthobiologics. Surgery might be considered at 3-4 months down the line if there is persistent lameness.

However, the majority of cases present as insidious poor performance and lameness. There is a choice in these cases between managing them medically (intralesional medication and shockwave therapy together being most effective in most cases) or surgical management. Medical management is most effective in milder cases (less than 3/10 lameness) with just a relatively subtle loss of performance. The more severe cases and particularly in those where there are behavioural abnormalities under saddle, then medical management is usually ineffective and I am more likely to proceed straight to surgery. It does not adversely affect the prognosis to try more conservative therapy first but this can add additional expense and will take more time before the horse is then back in full work. The most important differentiators where I recommend surgical or medical management relates to the horse’s clinical presentation, rather than the severity of ultrasonographic findings. Negative indicators for surgery would be if the horse has a very upright hindlimb conformation or significant sinking of the hind fetlocks. However, we do not find that the surgery is ineffective in horses with a more upright conformation, but it can have a lower success rate. But given that these horses often have no other realistic treatment option, owners are often happy for surgery to be undertaken with an appropriate realism as to the likely level of work the horse is able to return to. Others have said that concurrent sacroiliac region pain offers a poor prognosis but the majority of our cases do have this and still respond appropriately to surgery. The post-operative rehab is critical and medication of the sacroiliac joints when the horse is back in work can lead to a high success rate in these cases.

There may be practical considerations influencing the choice of surgery such as the time of year relative to the competition season and insurance considerations. Working up and operating on a straightforward PSD case can fall within most UK insurance limits, but not treat to medically and then perform surgery if that fails. Many horses are managed medically during the competition season and then have surgery during the rest period before the next season if there has not been an excellent response to medical management. Regulatory concerns need to be taken into account as well.  In some countries neurectomy is illegal. Some discipline regulatory bodies prohibit neurectomy, such as for racehorses in the United Kingdom, and in those cases we would perform a fasciotomy alone, although this is associated with a poorer prognosis than the combined procedure. The procedure is still deemed acceptable in the majority of sport horse disciplines. We performed a welfare study assessing owner perception of the horse’s welfare after PSD surgery compared to after shockwave treatment (Bathe, Murphy, Verwijs, Pearson ACVS 2022), and showed a significant improvement in horse welfare in cases managed surgically. It is a low morbidity procedure with appropriate case selection and thus I would suggest that it is contrary to animal welfare to not perform surgery in many cases, since we often underestimate the effect of chronic low-grade pain on our patients. In cases where a neurectomy is declined, then I will perform an ultrasound guided desmoplasty procedure, although my results with this are poorer. 

Overall case selection is critical in achieving a good outcome in the management of PSD and ensuring that the majority of horses return to work successfully.