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Lameness of the hind limbs caused by proximal suspensory desmitis (Proceedings)

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Aug 01, 2009

Inflammation of the proximal aspect of the interosseus medius muscle, or suspensory ligament (i.e., proximal suspensory desmitis, PSD) of the hind limb, is a common cause of acute or chronic lameness of horses and is most commonly diagnosed in competition horses 4 to 10 years old.1,2 Horses that are excessively straight in the hocks or that have hyperextension of the fetlock may be predisposed to developing PSD of the hind limbs.3 Discerning whether hyperextension is a cause or a result of PSD may be difficult.

Diagnosis of PSD of the hind limbs is based on the results of physical examination, regional analgesia, and ultrasonographic and radiographic examination of the proximal plantar aspect of metatarsi.1-6 Nuclear scintigraphy is unreliable in establishing a diagnosis of PSD.3 Lameness of a hind limb caused by PSD is temporarily exacerbated by flexion of that limb in about 85% of affected horses.6 Pain causing lameness is determined to be localized to the proximal aspect of the suspensory ligament of one or both hind limbs by observing that lameness improves substantially after anesthetizing either the tibial nerve 8 to 10 cm proximal to the tuber calcis, the dorsal branch of the lateral plantar nerve (DBLPN) 2 or 3 cm distal to the tarsometatarsal joint, or the lateral and medial plantar metatarsal nerves 4 to 5 cm distal to the tarsometatarsal joint.1,3 These nerve blocks are administered after observing a negative response to desensitization of the fetlock and structures distal to it after anesthetizing the medial and lateral plantar nerves and the medial and lateral dorsal and plantar metatarsal nerves at the level of the distal end of the second and fourth metatarsal bones.

Ultrasonographic evidence of PSD includes abnormal fiber pattern, enlarged cross-sectional area (defined as a cross-sectional area greater than 1.5 cm2),7 and poor definition of the margins of the suspensory ligament. One study evaluating the value of ultrasonographic examination in the diagnosis of PSD of the hind limb found that ultrasonography correlated poorly with MRI and histology in establishing the cross-sectional area of the proximal aspect of the suspensory ligament.8

Radiographic abnormalities of the proximal aspect of the metatarsi associated with PSD of the hind limbs include new periosteal bone on the proximoplantar aspect of the 3rd metatarsal bone, most apparent on a lateromedial projection, and trabecular sclerosis, which is seen best on the dorsoplantar projection.2 Dyson1 found radiographic abnormalities of the proximoplantar aspect of the 3rd metatarsal bone in 55% of hind limbs affected with PSD, whereas Crowe et al.4 found radiographic abnormalities of the proximoplantar aspect of the 3rd metatarsal bone in only 24% of hind limbs affected with PSD.

Treatments of horses lame because of PSD of the hind limbs include prolonged confinement followed by a regimen of controlled, gradually increasing exercise.1,9 Although approximately 90% of horses with PSD of one or both fore limbs return to soundness with this treatment,10 horses with PSD of one or both hind limbs usually remain lame.1,9 In one study, confinement followed by a regime of controlled, gradually increasing exercise enabled only 14% of horses with PSD of one or both hind limbs to return to their previous level of activity, without recurrent lameness, for more than a year.1

Treating horses with PSD of the hind limbs with a combination of confinement and shock wave therapy improves the prognosis for return to soundness.4 Rapid improvement in lameness after treatment may be due to analgesic provided by the therapy,4 and sustained improvement may be the result of an increased rate of deposition of collagen fibrils and greater expression of transforming growth factor β-1.11 In one study, treatment with shock wave therapy, in addition to restricted exercise for 12 weeks allowed 41% of horses suffering from PSD of one or both hind limbs to return to their previous level of activity, without recurrent lameness, for more than 6 months.4

Hewes et al., reported that 85 % of horses lame because of PSD of the hind limbs characterized by a core lesion, were able to return to full work after being treated by a combination of desmotomy and fasciotomy, in addition to restriction of exercise for 30 days.9 These investigators speculated that horses improved because fasciotomy decompressed the enlarged suspensory ligament, and desmoplasty facilitated angiogenesis in the damaged portion of the ligament.9