IN-DEPTH: SPORT HORSE LAMENESS


the hock flexed at 90° and the third metatarsal bone positioned vertically. The superficial digital flexor tendon (SDFT) is deflected medially, and a 25-mm, 23-gauge needle inserted perpendicular to the skin surface, 15 mm distal to the head of the fourth metatarsal bone, on the plantarolateral surface of the metatarsal region. The needle is advanced in a slightly dorsomedial direction between the fourth metatarsal bone and the lateral border of the SDFT up to the hub, and 3 to 4 mL of mepivacaine is injected without resistance. Occasionally, blood is seen to flow freely from the needle, indicating punc- ture of the venous portion of the (proximal) deep plantar arch, in which case the needle should be re-directed slightly more dorsolaterally to avoid in- travascular injection. The lateral placement of the needle in this technique reduces the risk of inadver- tent penetration of the tarsometatarsal joint and the tarsal sheath when compared with other methods of subtarsal anesthesia. However, in up to 20% of horses in which 2.5 mL of mepivacaine was injected at this site, the lateral plantar nerve also appeared to have been desensitized.7 Therefore, it is always advisable to assess the effect of anesthesia of the distal limb with a low six-point nerve block first, before performing diagnostic anesthesia of the deep branch of the lateral plantar nerve. Lameness is assessed 10 to 15 minutes after in-


jection. Critical evaluation of the degree of im- provement can be performed accurately and objectively with a wireless, inertial, sensor-based system of lameness quantificationa. This is essen- tial when considering treatment by neurectomy. It is also important when comparing the degree of improvement with that seen after intra-articular anesthesia of the distal tarsal joints. Subtarsal an- esthesia may improve tarsometatarsal joint pain and vice versa, but most improvement in suspensory pain is usually seen after anesthesia of the deep branch of the lateral plantar nerve. It has been suggested that pain is less successfully alleviated by anesthesia of the deep branch if enthesopathy of the proximal plantar portion of the metatarsal cortex is present. In these cases, direct deeper infiltration of 2 to 4 mL of mepivacaine at the bone surface may be more effective in abolishing lameness. Anesthesia of the tibial nerve alone eliminates suspensory liga- ment pain without completely removing sensation from the distal tarsal joints.


5. Differential Diagnosis


Recent high-field MR imaging studies of horses with proximal plantar metatarsal pain have indicated that PSD and/or enthesopathy was identified as the cause of lameness in the majority of them (55–80%), whereas in 20% to 25% of horses, a pathologic pro- cess unrelated to the suspensory ligament was doc- umented, and in 10% to 20% of cases, no reason for the lameness could be found in the proximal meta- tarsal or distal tarsal regions. Lesions that were considered responsible for lameness but were unre-


252 2013  Vol. 59  AAEP PROCEEDINGS


lated to the suspensory ligament included osteoar- thritis of the distal tarsal joints, osseous cyst-like lesions of the tarsal bones, tarsal bone edema, en- thesopathy of the intertarsal ligaments, osseous injury of the third or fourth metatarsal bones, ten- dinopathy of the deep or superficial digital flexor tendon, and desmopathy of the plantar ligament.1,2 Other injuries that should be considered in the prox- imal plantar metatarsal region are stress fractures of the plantar metatarsal cortex and avulsion frac- tures of the origin of the suspensory ligament. Neuropathy of the deep branch of the lateral plantar nerve may be the cause of pain in horses without imaging abnormalities.


6. Imaging


An accurate imaging diagnosis of proximal metatar- sal pain is of great importance because recom- mended options for management are costly and time-demanding. This diagnosis can be based on radiographic, scintigraphic, sonographic, and MR imaging findings. Radiographic and scintigraphic findings are useful for the detection of bone injuries but frequently nonspecific for PSD. Sonographic assessment of the proximal portion of the suspen- sory ligament is difficult. High-field MR imaging was recently shown to be the most reliable technique for accurate diagnosis of the causes of proximal metatarsal pain.1,2 It is recommended that a complete radiographic examination of the tarsus and proximal metatarsal regions is always performed because distal hock joint pain and PSD may coexist in horses with prox- imal metatarsal pain. Accurate radiographic as- sessment of the proximal aspect of the third metatarsal bone for the presence of increased ra- diopacity or avulsion fractures requires that radio- graphic views be centered at this level. In the dorsoplantar image of the proximal aspect of the third metatarsal bones of sound horses, there can be a variable amount of increased radiopacity that should not be interpreted as a pathological stress reaction at the origin of the suspensory ligament. In the dorsoplantar image, increased opacity is fre- quently most obvious laterally. On lateromedial images, remodeling changes may include thickening of the plantar cortex, endosteal new bone, alteration of the trabecular pattern of the proximoplantar as- pect, and enthesophyte formation on the plantar aspect of the third metatarsal bone. Although in- creased radiopacity in this region may be more ex- tensive in horses with chronic PSD, these radiographic findings are frequently not specific. In a recent report, features of PSD in 155 horses showed that 21% of lame limbs had a spur on the dorsoproximal aspect of the third metatarsal bone; 30% had mild, diffusely increased radiopacity proxi- molaterally in the third metatarsal bone; 3% had focal areas of intensely increased radiopacity; and 6% had low-grade osteoarthritis of the distal tarsal joints.4


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