Proximal suspensory desmitis

What it is

The suspensory ligament of the fetlock is the interosseus medius muscle, which in the horse has become almost entirely a ligament with few muscle fibres. Proximal desmitis (or proximal suspensory desmopathy) is an injury of its origin and proximal part: fibre tearing, enlargement, degenerative change and often injury of the bone where it attaches (enthesopathy). Because the ligament prevents hyperextension of the fetlock at every stride, it is subjected to very high repeated loading.

The thoracic and pelvic limb forms behave differently:

• Thoracic limb: usually an acute or subacute injury that responds well to rest and controlled exercise.
• Pelvic limb: often chronic, bilateral and recurrent. The origin of the ligament is enclosed between the third metatarsal bone, the splint bones and the deep plantar fascia, so its enlargement can compress the deep branch of the lateral plantar nerve, as in a compartment syndrome. A straight hock or a hyperextended fetlock increases the load and worsens the prognosis.

Who it affects

Sport horses of any discipline, above all dressage horses, in which the pelvic limb form is especially common; also showjumpers, event horses, Standardbreds and racehorses. The risk of suspensory desmitis increases with age.

Anatomy involved

• Interosseus medius muscle (suspensory ligament): it originates on the palmar (or plantar) aspect of the proximal end of the third metacarpal (or metatarsal) bone and, in the thoracic limb, also from the distal row of carpal bones. Its body runs down between the splint bones and divides into two branches that insert on the proximal sesamoid bones; from these, the distal sesamoidean ligaments continue the system to the phalanges, and extensor branches join the common digital extensor tendon. It is an essential part of the fetlock stay apparatus in all four limbs.
• Metacarpus: proximal desmitis of the thoracic limb lies on the palmar aspect of the proximal third of the third metacarpal bone, where sclerosis, cortical irregularity or avulsion fragments may be seen.
• Metatarsus: in the pelvic limb, the origin of the ligament is on the proximal plantar aspect of the third metatarsal bone, between metatarsal bones II and IV, with the same bone changes.

Clinical signs

Lameness of variable degree, often mild to moderate, which worsens on a circle with the affected limb on the outside and on soft ground. In the pelvic limb lameness is usually bilateral and shows more as lack of impulsion, poor performance or reluctance to work in collection than as obvious lameness. Visible swelling is rare; deep palpation of the origin of the ligament with the limb lifted may be painful, although this is an unreliable sign. Distal or proximal limb flexion may worsen the lameness.

Diagnosis

• Diagnostic analgesia: lateral palmar nerve block or local infiltration of the origin of the ligament in the thoracic limb; in the pelvic limb, block of the deep branch of the lateral plantar nerve. These blocks are not fully specific because they can desensitise neighbouring structures, such as the carpometacarpal or tarsometatarsal joint.
• Ultrasonography of the origin of the ligament, compared with the contralateral limb: enlargement, hypoechoic areas, poorly defined margins and irregularity of the bone surface. Interpretation is difficult in the pelvic limb.
• Radiographs: sclerosis of the trabecular bone of the third metacarpal or metatarsal bone, periosteal reaction and avulsion fragments.
• Scintigraphy and magnetic resonance imaging (including standing MRI under sedation) when ultrasonography and radiographs are inconclusive; MRI shows ligament and bone lesions together.

Treatment and prevention

• Thoracic limb: rest with controlled walking and a progressive return to work over a few months, with ultrasonographic checks.
• Pelvic limb: rest alone gives poorer results. Shock wave therapy, intralesional treatments (platelet-rich plasma, mesenchymal stem cells, corticosteroids) and, in chronic cases, neurectomy of the deep branch of the lateral plantar nerve with plantar fasciotomy are used. The evidence for all of them comes from case series and comparisons with historical controls, without randomised trials.
• Limb conformation (straight hock, hyperextended fetlock) and concurrent problems, such as back pain or sacroiliac joint pain, determine the prognosis and should be assessed before any treatment.
• Shoeing that supports the heels and progressive work adapted to the discipline.
• The horse is a food-producing animal in many countries and drug use is subject to national regulations and withdrawal periods. In the EU, the equine identification document (passport) states whether the horse is excluded from slaughter for human consumption, and this decides which medicines may be given.

Key studies

• Kasashima et al. (2004), retrospective study of 10,262 flat-racing Thoroughbreds in Japan. Forelimb suspensory desmitis affected 3.61% (370 cases), and in the hindlimb there were only 14 cases. Risk was 2.23 and 2.11 times higher at 3 and 4 years than at 2 years, and 5.07 times higher from 5 years, which supports the accumulation of degeneration with age. It does not distinguish proximal lesions from those of the body or branches.
• Murray et al. (2006), retrospective study of 1,069 horses undergoing orthopaedic examination at a referral centre, comparing injury site by discipline and level. Dressage horses had a significantly higher risk of hindlimb suspensory ligament injury, both at elite and non-elite level, than general-purpose riding horses. It is a referral population and the abstract does not give the odds ratios.
• Murray et al. (2020), retrospective study of standing magnetic resonance images of the proximal region of 359 forelimbs and 64 hindlimbs with lameness localised to that area. Suspensory ligament lesions were reported in 83.8% of forelimbs and 64.1% of hindlimbs, often together with cortical bone lesions of the third metacarpal or metatarsal bone; hindlimb suspensory origin lesions were overrepresented in dressage horses. It is descriptive and has no healthy controls.
• Dyson and Murray (2012), series of 155 horses with hindlimb proximal suspensory desmopathy treated by neurectomy of the deep branch of the lateral plantar nerve and plantar fasciotomy. 77.8% of horses with a primary lesion and no other problems were in full work for more than a year, compared with 44.2% of those with other causes of lameness; all horses with straight hocks or hyperextended fetlocks remained lame. There is no control group.
• Crowe et al. (2004), series of 65 horses with chronic or recurrent proximal suspensory desmitis treated with radial pressure wave therapy and controlled exercise. At 6 months, 41% of horses with hindlimb lesions and 53% with forelimb lesions were sound and back in full work, and the initial ultrasonographic grade influenced the prognosis. The authors considered the result better than that published with exercise alone, but the comparison was with historical controls.