A novel treatment using extracted bone marrow has given new hope to a sport pony with a ligament injury, as Matt Smith MRCVS explains
The sesamoidean ligaments form part of the suspensory apparatus. Two bones at the back of the fetlock, known as the sesamoid bones, act as an anchor. Above them runs the suspensory ligament, from fetlock to hock/knee, and below them sit the distal sesamoidean ligaments.
The anatomy of the sesamoidean ligaments is complex. There are four ligaments in each leg, with fibres running in different directions and with different attachment points. Together, they act as a relatively tough and elastic “sling” that provides support to the fetlock and pastern region.
Due to their function, most injuries to these ligaments are thought to be caused by over-extension of the fetlock joint. While the specific event may be unknown, this type of acute injury is most likely the result of a one-off bad step or a steep landing over a fence, for example.
Injuries can also occur as a result of chronic repetitive strain, from regular work. Other factors are then relevant, including the horse’s conformation and genetics, the type of work he undertakes and the surface used. Horses with straight hocks and overextended fetlocks appear predisposed.
Since there are rarely any external signs, such as swelling or heat, diagnosis can be difficult. Nerve blocks may be required to localise the source of lameness. One clue is a positive response to a fetlock flexion test, but this is not specific to sesamoidean ligament injuries.
Diagnosis is generally confirmed by ultrasound, although the ligaments can be difficult to image — particularly in thick-skinned, cob-type horses. Magnetic resonance imaging (MRI) may be necessary.
Stimulating repair
Treatment is largely based around a period of rest and controlled exercise, in the form of a graduated walking programme, much like any other tendon or ligament injury. Success rates have typically been poor, however, which has led to the creation of new techniques aimed at stimulating repair.
One such treatment is shockwave therapy. While useful for proximal suspensory desmitis (PSD), the response with sesamoidean ligament injuries has been less positive.
Sport pony Cherryhills Arctic Kiss (known as Callie) had developed a hole in a hind sesamoidean ligament, an injury that does not tend to heal well. Callie had not come right with a few months of rest, so we knew that giving her more time was not the answer.
We decided to treat the injury with a form of stem cell therapy called bone marrow aspirate concentrate (BMAC). Still relatively untested for sesamoidean ligament injuries, BMAC offers a number of potential therapeutic benefits.
Unlike the more widely known stem cell treatments, where the cells are sent to a laboratory to be “cultured” to produce many millions more, those in BMAC remain unmanipulated. Not only can the bone marrow be aspirated (extracted) from the horse and administered back into the injury site shortly afterwards, but it is thought that the unmanipulated cells retain more of their potential therapeutic qualities.
The horse is given local anaesthetic so the bone marrow can be extracted from his sternum using a large needle. The aspirate is then placed in a centrifuge for 15 minutes. This concentrates the stem cells and growth factors naturally found in the marrow. It also filters out white cells and other tiny particles known as bone spicules.
The sample is then ready to be injected into the injured tissue. This is performed under ultrasound guidance and either standing sedation or general anaesthesia, depending on the location of the injury. Because the sesamoid ligaments are difficult to access and situated close to structures such as the tendon sheath, Callie was anaesthetised for the whole procedure.
What’s the prognosis?
Previously, it was suggested that stem cells regenerated injured tissue by forming new tendon, ligament or cartilage. We know now that they orchestrate healing by exerting a “paracrine effect”, secreting factors that signal the horse’s own cells to change their behaviour.
Whereas we would once inject as many stem cells as possible, the aim of BMAC is to guide healing of cells already there. In addition, the growth factors from the bone marrow combine to stimulate better-quality repair than the proliferation of new tendon or ligament fibres generated by the regenerative therapy platelet rich plasma (PRP).
The prognosis for sesamoidean ligament injuries remains guarded, however, and will relate to the cause — greater success is seen with acute rather than chronic injuries. A treatment such as BMAC is generally more effective the sooner it is given after the injury.
Most tendon and ligament injuries will heal, in a fashion, but the question is whether the repair will stand up to work. Follow-up scans revealed Callie’s injury had filled in significantly, so she was able to undergo a careful rehab programme and resume exercise.
Stem cell therapies are a tool we can reach for with tendon and ligament injuries in an attempt to bring about a good outcome. While refinements will no doubt be made to these techniques, they are a step forward in the treatment of these often challenging injuries.
The owner’s view
Hannah Freer describes the onset of Callie’s sesamoidean ligament injury in late 2017 as “insidious”.
“She has always had plenty of power, but when I was schooling her she felt sluggish — as if there was no engine,” says Hannah of the 14hh home-bred mare, now 11. “My vet looked at her and couldn’t see any mechanical lameness, but I felt that Callie wasn’t quite right.
“After a few weeks’ box rest, some new gait analysis software at the clinic revealed she was lame on her left hind,” continues Hannah, an accident and emergency nurse. “She seemed to react positively to a proximal suspensory nerve block and underwent a course of shock-wave therapy, but things still didn’t seem right.
“In January 2018, I took Callie to see Matt Smith at the Newmarket Equine Hospital. He went over her with a fine-tooth comb and finally discovered a hole in a sesamoidean ligament. It was awful news. Matt explained the injury probably hadn’t been caused by one incident, rather a combination of things.
“We discussed treatment options. An injection of steroids into the hole might just trigger a healing action, while PRP treatment offered a 50/50 chance of success. We felt that BMAC, while more expensive and relatively untried, was our best bet.
“Callie was in hospital for a couple of days for the treatment. Back home, my dad and I followed the rehab programme to the letter. We were out walking her around the arena that winter in wind, rain, snow and hail, not finishing a session until the timer went ‘beep’. I was back on her in March, hacking her in straight lines, before moving on to gentle hillwork. By June, she could start cantering.
“I haven’t really jumped Callie yet, as I’m frightened of undoing all the work, but this year she earned her first British Dressage points and qualified for the Area Festivals in four outings. I was so lucky to get my horse back.”
Matt Smith MRCVS is a surgeon and partner at Newmarket Equine Hospital. This purpose-built referral hospital is one of the largest in Europe, treating horses from throughout the UK ranging from leisure animals to elite athletes. Matt is a Royal College of Veterinary Surgeons (RCVS) specialist in equine surgery and has a particular interest in orthopaedic problems.
Ref Horse & Hound; 24 October 2019
