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Differentiating Proximal Plantar Region Pain
W.R. Redding
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Proximal suspensory desmitis (PSD) of the hindlimb has become a frequently diagnosed cause of acute and chronic hind limb lameness. This condition occurs in many disciplines of equine athletes but appears to be more common in mature horses that perform dressage, eventing and general purpose work.1,2 PSD has been loosely defined as desmitis of the origin of the suspensory ligament (SL). All ages can be affected but PSD most often occurs in horses 4-10 years of age. The recent increase in the diagnosis of PSD may be due to improvement by veterinarians and equine professionals in recognizing this condition. Increasing the degree of difficulty of certain maneuvers during training/showing, the wide variability of the training/showing surfaces and certain conformations (straight hind limb/hock, hyperextended fetlock, and long toe-low heel with a negative palmar angle of the distal phalanx) may play a role in the increased incidence of PSD. Some conformations may be the result of moderate to severe SL injury. Unfortunately, irrespective of the cause, PSD carries a guarded to poor prognosis. Early diagnosis before significant damage has occurred in conjunction with a period of rest and rehabilitation may minimize further injury and may improve the chances of the horse to return to work. However, lameness referable to this region is often initially assumed to originate from the distal tarsal joints. Subsequent intra-articular injection of corticosteroids into the distal tarsal joints may provide a local anti-inflammatory effect which can minimize the pain associated with the active desmitis. This may allow the resumption of work before appropriate repair has occurred leading to further damage to the SL. An early accurate diagnosis is therefore critical to minimize further damage to the SL and justify aggressive and expensive treatment to salvage their career.
A clear understanding of the anatomy of the proximal plantar region is critical to appreciate the difficulties in blocking and imaging this area. The SL is a highly modified interosseous muscle and consists of collagenous tissue, striated muscle, adipose tissue, connective tissue and nerves and vessels. The SL travels in a deep groove formed by the metatarsal bones and divides distally into branches that insert on the proximal sesamoid bones. For purposes of this paper the proximal part of the SL is defined as the section extending from the origin (at the tarsometatarsal (TMT) joint) to approximately 10 cm distally. The SL attaches to the plantar aspect of the distal tarsus including the third tarsal bone, plantar cortex of the third metatarsal bone and a small band attaches to the axial aspect of the lateral splint bone. There is a distinct fascial band closely associated with the plantar aspect of the proximal part of the SL with transverse fibers extending from the medial to the lateral splint bone. This has been called the deep laminar plantar metatarsal fascia. Recently, MRI examination of the tarsus has helped elaborate the normal cross sectional anatomical appearance of the proximal plantar metatarsal region. The SL maintains a unique but consistent appearance with 2 distinct lobes that originate from the plantar cortex of the metatarsus. These lobes remain connected axially until the level of the SL branches. A third, much smaller collagenous lobe originates more proximally from the plantar aspect of the 4th tarsal bone, and joins the lateral lobe at its origin. Each lobe of the proximal SL contains a focal hyperintense signal consistent with fat and a small hypointense muscular core and these are surrounded by dense homogenous collagenous ligament tissue. The profile of these bundles is consistent and well delineated. Each muscle bundle starts immediately distal to the origin of the SL and continues distally to the level of the SL branches. The proximal aspect of the SL of the hind limb receives innervation from the tibial nerve.3 Just proximal to the calcaneus, the tibial nerve branches into lateral and medial plantar nerves. The lateral plantar nerve gives off a deep branch approximately 3 cm proximal to the head of the fourth metatarsal bone.4 The deep branch of the lateral plantar nerve branches into medial and lateral plantar metatarsal nerves, which provides sensory and motor innervation to the proximal SL. There is a close association of the TMT joint and the proximal SL and its innervations. A previous study outlined the plantar distribution of the TMT joint using radiographic analysis after intra-articular injection of contrast medium.5 Significant amounts of contrast was seen located plantar and distal to the TMT joint following injection. It was hypothesized that this may result in perineural anesthesia of the plantar metatarsal nerves, and the proximal aspect of the SL. Perhaps more importantly, these plantar outpouchings of the TMT joints may allow for the inadvertent intra-articular injection of local anesthetic following diagnostic anesthesia of the proximal metatarsus in an analogous situation in the forelimb.
Clinically, lameness associated with PSD of the hind limb is more variable than what occurs with this condition in the front limb. Lameness can be unilateral or bilateral and can range from barely discernable to severe. Horses with bilateral injuries may show a lack of performance more than overt lameness. Lack of performance may be manifested as loss of hind limb impulsion, unwillingness to work on the bit, resistance to lateral work, specific dressage maneuvers, loss of power when jumping, as well as refusal of jumps among other things. Overall there are few localizing clinical signs of PSD. Lameness can be more obvious when the horse is ridden, especially when the rider sits on the diagonal of the lame limb. Both proximal and distal limb flexion may accentuate lameness. Lameness may be worse on a circle, although this is not necessarily with the limb on the outside of the circle as is often the case in PSD of the forelimb. [...]
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