E1 3 Ce Credits the Equine Pastern Physical Examination Diagnostic Procedures Tendon and Ligament Injuries

Injuries involving the pastern region are a common source of lameness in many types of performance horses. Knowledge of the common breedand activity-associated injuries can aid veterinarians in making an accurate diagnosis because injuries of the pastern region are often breedor use-specific. The differential diagnosis for disorders of the pastern region includes osteoarthritis, osteochondrosis, fractures, infection, and soft tissue injuries. In general, soft tissue injuries of the pastern region can be difficult to diagnose, and affected horses have a reasonable chance of returning to their intended uses; osseous injuries are typically easier to diagnose and have a good prognosis if arthrodesis is an option. The Equine Pastern Anatomy The proximal interphalangeal joint (PIJ; pastern joint) is a diarthrodial joint formed from the distal aspect of the proximal phalanx (P1) and the proximal aspect of the middle phalanx (P2).1,2 The pastern region is bounded dorsally by the common or long digital extensor tendon. Palmar/plantar support structures of the pastern region are formed by the distal sesamoidean ligaments (DSLs; straight, oblique, and cruciate ligaments), digital flexor tendons (superficial digital flexor tendon [SDFT] and deep digital flexor tendon [DDFT]), and proximal and distal digital annular ligaments within the digital flexor tendon sheath (DFTS).1,2 The medial and lateral collateral ligaments provide support in the sagittal plane.1,2 Physical Examination The physical examination begins with visual inspection of the limb and evaluation of range of motion and response to hoof testers. The pastern region should be symmetric and free of swelling or bony enlargement. Injuries involving the pastern region are rarely bilateral, but osteoarthritis (OA) may be; therefore, comparing the right and left limb or medial and lateral aspect of the same limb is helpful when identifying abnormalities. Abnormalities in the pastern region, such as swelling or bony enlargement, are usually obvious because of minimal soft tissue in the area.1,2 The severity of lameness associated with the pastern region ranges from subtle to severe, depending on the injury.1 Generally, injuries involving the PIJ or DFTS cause obvious lameness, whereas lameness due to early OA or strains of the sesamoidean ligaments may be mild. Pain in the pastern region is often exacerbated by distal limb flexion or lunging the horse with the affected limb on the inside of the circle. The differential diagnosis for disorders of the pastern region includes OA, osteochondrosis (OC), fractures, infection, and soft tissue injuries; however, the types of injuries are often breedor use-specific.1,2 Diagnostic Procedures Regional nerve blocks are an important technique for isolating lameness in any horse. The PIJ is not always completely anesthetized by perineural anesthesia at the level of either the basisesamoid or the abaxial nerve block; thus, a low, four-point nerve block may be necessary.1,2 However, a palmar/plantar digital nerve block can desensitize the pastern region, depending on the location of the block and the amount of anesthetic used. Schumacher et al3 reported that the PIJ is unlikely to be anesthetized when the palmar digital nerves are anesthetized at the proximal margin of the collateral cartilages of the foot. However, pain within the PIJ cannot be excluded when the palmar digital nerves are anesthetized at any site proximal to the proximal margin of the collateral cartilages of the foot.3 Therefore, in a horse with a suspected stress fracture of P1 or P2, it is important to avoid performing a nerve block because perineural anesthesia would likely cause the horse to become more comfortable and displace the fracture. Response to intraarticular anesthesia varies depending on the injury, but improvement of lameness by 50% or more implicates the PIJ and is essential for accurate diagnosis.1,2 Several imaging modalities, such as radiography or ultrasonography, are important for initial characterization of the injury. Ultrasonographic evaluation of the pastern is an integral part of characterizing the extent of the soft tissue injury. Additionally, nuclear scintigraphy, computed tomography (CT), magnetic resonance imaging (MRI), or tenoscopy of the DFTS may help make a more accurate diagnosis and prognosis.1,2 In general, osseous and joint abnormalities are best evaluated using radiography or CT, whereas MRI offers optimal evaluation of soft tissue injuries. Tendon and Ligament Injuries In general, injuries to the SDFT occur most frequently in the forelimb, followed by injuries to the oblique DSL, DDFT, and straight DSL. Injuries to the DDFT in the hindlimb are usually