Diagnostic Evaluation of the Tarsus

In horses with obvious visible abnormalities of the tarsal region, identification of the tarsus as the  source of lameness may be simple. In other patients, a complete lameness examination including  diagnostic analgesia must be performed in order to localize the lameness to the tarsal region.  Regardless of the method used to identify the tarsus as the site of interest, results of diagnostic imaging should always be evaluated in light of the clinical presentation of the patient.  

During the physical examination, effusion of the tarsocrural joint is simple to recognize, but  effusion of the tarsometatarsal and distal intertarsal joints can be difficult or impossible to appreciate on physical examination alone. The tarsal sheath, gastrocnemius bursa, and calcaneal  bursa should also be assessed for the presence of effusion. Differentiation of these structures  from one another can be challenging, especially if severe effusion or edema are present. The  tarsal region should be palpated for any areas of tenderness and the horse’s response to flexion  and manipulation should be recorded.  

Lameness can be localized to the tarsal region by a positive response to perineural anesthesia of the tibial and peroneal nerves after a negative response to anesthesia of the metacarpal region  and distal limb. Alternatively, intra-synovial anesthesia of the tarsometatarsal, distal intertarsal,  and/or tarsocrural joints can be performed. If the tarsal sheath or other synovial compartment is  suspected as the source of the lameness, these areas can also be selectively anesthetized. In some  horses, distal tarsal pain can be eliminated or improved after anesthesia of the proximal metatarsal region due to inadvertent entry into the tarsometatarsal joint.¹ Conversely, anesthesia  of the distal tarsal joints can reduce lameness associated with the proximal suspensory ligament.² Additionally, mepivacaine may diffuse between synovial structures in the tarsus.³ These  confounding factors should be borne in mind when interpreting results of intra-synovial or regional anesthesia.  

Once the source of the lameness has been isolated to the tarsal region, diagnostic imaging can be  used to formulate a more specific diagnosis. Radiography and ultrasonography are usually chosen for an initial assessment of the area, as they are widely available, relatively inexpensive, and do not require general anesthesia or specialized equipment.  

Radiography provides an excellent overview of the status of the bony structures. However,  radiography is not particularly useful to evaluate soft tissue structures and can be insensitive to bony inflammation initially. Standard views include a dorsoplantar, dorsolateral-plantaromedial oblique, dorsomedial-plantarolateral oblique, and lateral-medial. Additional views may be  obtained to highlight specific areas of the joint. A flexed lateral-medial highlights the plantar  aspect of the trochlear ridges. A dorsoplantar view obtained 10 degrees from the dorsal plane  towards the lateral plane allows for improved assessment of the medial malleolus. A skyline  view of the tuber calcaneus enables assessment of the sustentaculum tali and the plantar aspect of  the tuber calcanei.  

Radiographs are often sufficient to diagnose many conditions of the tarsus, including  osteoarthritis, osteochondrosis, and fractures of the talus, lateral malleolus of the tibia, and cuboidal bones. In foals, radiographic signs of physitis and osteomyelitis may not be apparent at  the time of onset of lameness, but serial radiography may reveal these lesions.  

If radiography is unsuccessful at identifying the specific problem or a soft tissue injury is  suspected, ultrasonography is often employed. Although ultrasonography of the complex tarsal  region is challenging, ultrasonography allows evaluation of soft tissue structures as well as  external bony contours. Excellent reviews of ultrasonographic anatomy and technique have been  published.⁴ Briefly, a complete examination involves assessment of the synovial structures, bone  surfaces, and supporting soft tissue structures.  

The amount and character of the synovial fluid and synovial membrane should be assessed in the  tarsal joints, the tarsal sheath, and the cunean, calcaneal, and gastrocnemius bursae. However,  ultrasonographic evaluation does not replace synoviocentesis and fluid analysis for investigation  of a possibly septic synovial structure. When effusion and/or edema are present in the plantar  portion of the tarsus, identification of the exact synovial structure involved can be made easier  with ultrasonography, as the plantar pouches of the tarsocrural joint, the tarsal sheath, and the  gastrocnemius and calcanean bursae are in close proximity to one another.  

The bony surfaces are evaluated for the presence of any irregularity that may indicate the  presence of osteochondrosis, fracture, insertional desmopathy, osteomyelitis, or physitis. In  many cases, subtle bony or physeal margin irregularities as seen in cases of osteomyelitis or  physitis can be appreciated using ultrasonography sooner than they can be identified using  radiography. The sonographer should evaluate the echogenicity, fiber pattern, and size of the  tendons and ligaments of the tarsus, including the collateral ligaments, the long plantar ligament,  the peroneus tertius, the cranial tibial tendons, the common calcanean tendon, the superficial  digital flexor tendon, the deep digital flexor tendon, and the long and common digital extensor tendons. Comparison to the opposite limb is extremely useful to differentiate a subtle lesion  from a variation of normal.  

Nuclear scintigraphy may be used once the lameness has been localized to the tarsus or as a more  generalized screening tool to pinpoint potential regions of interest. Scintigraphy allows identification of areas of increased bony turnover and is particularly useful to diagnose occult  fractures, stress remodeling, or injuries at ligament-bone interfaces. In the tarsal region, this may  include areas of osteoarthritis, collateral ligament desmitis, and fracture of any of the tarsal  bones. Additionally, it should be borne in mind that areas of increased radiopharmaceutical uptake do not necessarily correspond to areas of lameness, and if scintigraphy is performed too  soon after an injury occurs, false negative results may occur.  

In many cases, radiography and ultrasonography are sufficient to formulate a diagnosis of  lameness originating from the tarsus. Due to the complexity of this anatomic region, the  availability and increased usage of three-dimensional imaging techniques are affording us new  insights into the sources of tarsal lameness. The use of computed tomography (CT) and  magnetic resonance imaging (MRI) has added greatly to our diagnostic abilities in the tarsal region. For example, the small intertarsal ligaments are inaccessible to ultrasonographic  evaluation. Subtle abnormalities of soft tissue structures may not be apparent when assessed  ultrasonographically. Superimposition of the bony structures can make radiographic identification of small lesions difficult, and early stages of bony lysis, sclerosis, or inflammation  may not be apparent even with digital radiography.

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