Lameness in the Pony

Chapter 126Lameness in the Pony



Andrew M. McDiarmid


Horses under the height of 14 hands 2 inches (148 cm) at the withers generally are recognized as ponies. Despite this classification, ponies of some breeds are referred to as horses: for example, Icelandic (toelter) horses. Many native pony breeds are found throughout the world—for example, Asturian (Spain), Connemara (Ireland), Fell (England), Gerrano (Portugal), Highland (Scotland), M’Bayar (Senegal), Merens (Pyrenees), Ob (Russia), Pindos (Greece), and Welsh ponies. Several small island breeds also exist—for example, Balearic, Eriskay, Faroe Island horse, and Chincoteague ponies. The size and conformation of most pony breeds have evolved over centuries because of specific work requirements and geographical isolation. Several breeds are endangered, including the Yonaguni and Noma in Japan and Sorraia in Spain, and some have become extinct—for example, the Fen, Galloway, and Tarpan. Attempts are actively being made to prevent extinction of several breeds—for example, the Kerry bog pony in Ireland and the Taishuh in Japan. In recent times new breeds of pony have evolved, including the Welara and Pony of the Americas. Despite this, most ponies are crossbred.


Ponies have a considerable range of height, weight, and conformation and are involved in most spheres of equine work, including show jumping, eventing, dressage, driving, and general pleasure activities. Many ponies show great athletic ability, and when used for show jumping, they often jump heights similar to their own height at the withers. Ponies have a long life expectancy and can remain in athletic work well into their twenties. Although a small number of ponies are used for high-level competition work such as eventing and show jumping, most are used for general purpose work, when they may be ridden only in the summer months. General purpose ponies tend to be much less valuable than horses and are ridden predominantly by children, which may create emotional and financial conflicts for owners (parents) when deciding the appropriate treatment of a seriously injured pony. Many general purpose ponies remain at grass all year round and do not have access to a stable. Successful competition ponies often change ownership, at high prices, every 2 or 3 years as children outgrow them.


Few reports in the literature describe orthopedic conditions specifically affecting the pony. In this chapter I hope to give an overview of some of the conditions that are recognized in the pony.



Lameness Affecting the Pony


Ponies develop orthopedic problems similar to those in horses, but the overall prevalence of lameness in the pony is less than in the horse. This may be because of differences in temperament and body weight. With a relatively older population the incidence of some specific orthopedic conditions affecting older ponies may appear higher than in horses, but the prevalence is often unclear and may, for some conditions, be similar (e.g., laminitis as a result of Cushing’s disease).



Lameness Examination


Gait assessment in ponies can be challenging because they can have a higher limb speed than horses, and the larger, heavier pony breeds (e.g., Highland) often have a base-wide gait behind, which may make assessment of hindlimb lameness difficult. Several breeds also have gaits additional to walk, trot, and canter; these include the South African, Basotho pony, and Icelandic horse. For small ponies the handler may have to walk when leading a trotting pony, rather than run, to keep the limb speed to a rate at which the observer can detect low-grade lameness.


The low height of some ponies compared with most veterinary surgeons may create difficulties with assessment of the lower limb, and excessive force or torsion can easily be applied to the distal joints when a flexion test is performed.


In small ponies, care should be taken not to apply excessive pressure with hoof testers and, if the veterinary surgeon is tall, not to flex or twist the lower limb excessively in an effort to raise the foot to a convenient height to use the testers. Paradoxically, in comparison with horses many ponies have harder hoof horn, making eliciting foot pain difficult, so care always should be taken when assessing the response to hoof testers. The use of small or adjustable hoof testers is advisable for ponies.


Ponies can have a positive response to lower limb flexion tests despite being sound and performing to the owner’s expectations.



Diagnostic Analgesia


Some ponies are strong-willed, which can limit a conventional lameness evaluation using perineural or intrasynovial analgesia. Using a nose twitch or a low dose of an α2-sedative may aid this (e.g., 40 to 60 mcg of romifidine per kilogram or 0.6 to 0.9 mg of xylazine per kilogram intravenously). In some ponies the lower limbs may have thick skin and be very hairy, making accurate palpation of the structures difficult, particularly finding the site for a palmar digital nerve block. I am conscious that because of the relatively shorter limbs in ponies, local anesthetic solution may diffuse further than in the horse, and a reduced amount of local anesthetic solution is used for perineural analgesia (e.g., 1 mL of mepivacaine hydrochloride for each nerve for palmar digital or palmar [abaxial sesamoid] analgesia).



Imaging Considerations


For most clinical situations a similar number of radiographic images are required to examine the lower limb of a pony as for a horse. Although obtaining good-quality radiographs of the lower limb is generally easier, some important lesions may be more subtle than in the horse, particularly those involving the distal interphalangeal (DIP) joint and navicular bone. Conversely, one should note that a substantial amount of joint degeneration can be present in some sound ponies. It is therefore important in the pony to undertake intrasynovial analgesia to confirm that any periarticular and intraarticular abnormalities are associated with joint pain.


In some ponies, particularly those kept at grass, a substantially increased thickness of skin in the distal limbs, pelvis, and thoracolumbar areas can be present and reduces skin penetration by ultrasound waves, resulting in poor ultrasonographic images. I have found that shaving the hair and hosing the area to be scanned with warm water for 15 minutes often substantially improves the quality of the image.



Ten Most Common Conditions Affecting Competition Ponies


Competition ponies appear to have a reduced prevalence of lameness in comparison with horses; however, the common conditions they develop mimic those found in competition horses. The following list contains the 10 most common lameness conditions in the ponies treated in my practice:


1 Laminitis and other foot-related injuries

2 Distal hock joint pain, osteoarthritis of the distal tarsal joints (bone spavin)

3 Desmitis of the accessory ligament of the deep digital flexor tendon

4 Conditions of the digital flexor tendon sheath

5 Osteoarthritis of the metacarpophalangeal joint

6 Superficial digital flexor tendonitis

7 Exertional rhabdomyolysis

8 Osteoarthritis of the carpus

9 Osteoarthritis of the femorotibial joint

10 Injuries of the palmar or plantar annular ligament


Limb Deformities


Mild angular limb deformities in ponies are common, but few affect performance or require medical or surgical correction. In many native breeds in the United Kingdom a small degree of carpal and fetlock valgus deviation is common. The persistence of a full-length ulna and fibula has been associated with the development of severe angular limb deformities in Shetland ponies.1 This may be a form of atavism, the inheritance of a characteristic from remote, rather than recent, ancestors.


Congenital laxity of the digital flexor tendons in the hindlimb of Shetland ponies is not uncommon and can be associated with hyperextension of the DIP joint. Treatment is similar to that in the horse.


Adactyly (absence of all or part of a normal digit) and polydactyly (duplication of all or part of a digit beyond the normal number) and other congenital musculoskeletal defects have been recorded in ponies.2



Joint Disease



Osteochondrosis


Rarely do ponies have lameness associated with osteochondritis dissecans (OCD). Histopathological evidence of osteochondrosis specifically in ponies has been reported only in the lateral trochlear ridge of the femur.3 Based on a limited number of ponies, in my opinion the most commonly affected joints are the tarsocrural and femoropatellar joints, and lesions are found at the same recognized sites as in the horse—namely, the cranial aspect of the intermediate ridge of the tibia, trochlear ridges of the talus, the medial and lateral malleoli of the tibia, and the lateral trochlear ridge of the femur. Osteochondrosis has been seen particularly in ponies bred for showing,4 perhaps because these ponies were receiving high feed intakes in an effort to improve show ring appearance.


Lameness associated with a subchondral bone cyst in the medial femoral condyle in the stifle is not uncommon in the pony.



Osteoarthritis


Ponies develop osteoarthritis (OA) in similar joints as horses. However, many ponies have mild-to-moderate joint degeneration, are clinically sound, and perform to the level of the owner’s expectation (Figure 126-1). OA in ponies tends to be primary rather than secondary to a previous intraarticular condition, such as intraarticular chip fractures, OCD, osseous cystlike lesions, or ligamentous damage.


image

Fig. 126-1 Lateromedial radiographic image of the metacarpophalangeal joint of a 12-year-old jumping pony. Periarticular osteophytes on the articular margins of the proximal and distal aspect of the proximal sesamoid bones reflect osteoarthritis. There is also evidence of supracondylar lysis (A) on the palmar aspect of the third metacarpal bone and villonodular (proliferative) synovitis (B) on the dorsal border. This pony had met the owners’ expectation until 5 weeks before this examination.



Scapulohumeral Joint


Shetland and Miniature Horses have a higher prevalence of OA of the scapulohumeral joint than do other equine breeds.5-7 This typically occurs in young ponies; the mean age in one survey was 5.2 years. The lameness is usually unilateral but can be bilateral and is sudden in onset and severe (grade 4 to 5 of 5) in degree. Lameness is characterized by a reduction in the cranial phase of the stride and a low arc of foot flight. Radiological abnormalities may not be present in the acute stage.5 Ultrasonography may be used to assist in the diagnosis.8 No reported treatment has resulted in successful resolution of the lameness. Some Shetland ponies have a flatter and more shallow glenoid cavity of the scapula than other equine breeds, and this may be caused by a primary joint dysplasia that could predispose Shetland ponies to OA of the scapulohumeral joint.6 In Miniature Horses scapulohumeral joint OA can be associated with an inability of the owner or veterinarian to pick up either forelimb.7 Ponies with severe bilateral OA of the scapulohumeral joint may exhibit an unusual, compensatory hindlimb gait deficit in an attempt to redistribute weight to the hindlimbs (Editors).



Carpus


When compared to horses older ponies appear to have an increased incidence of OA of the carpometacarpal joint, often referred to as carpal spavin. The radiological abnormalities found within the carpometacarpal joint are similar to those of OA of the distal tarsal joints, and periosteal new bone and enthesophyte formation on the abaxial margins of the second and fourth metacarpal bones may also be present.9

Related posts:

  1. Lameness in Horses: Basic Facts Before Starting
  2. Thermography: Use in Equine Lameness
  3. Counterirritation
  4. The Carpal Canal and Carpal Synovial Sheath

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Jun 4, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Lameness in the Pony

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