A carpal hygroma is a fluid filled swelling over the dorsal aspect of the joint which is caused by trauma Stashak Adams, 2002. Importantly, these swellings are generally not painful nor cause lameness thus we can use clinical signs to remove hygroma from our list of differentials.
Rupture of the extensor carpi radialis occurs rarely and is usually caused by repeated over flexion of the carpus. As an acute injury the horse will be have a characteristic gait; it may have toe dragging or a high arc of the foot as the horse walks (Ross Dyson, 2003).Through careful examination of our horse's gait at walking pace we can rule out this injury.
Acute traumatic tenosynovitis of the extensor tendon is associated with inflammation of the tendon sheath over the dorsal carpus. Clinical signs include effusion, heat, pain and possible lameness (Ross Dyson, 2003). Although these are similar to signs present in our horse, traumatic tenosynovitis will in most circumstances have a history of some type of trauma. We also can use ultrasonography to rule out inflammation of the tendon sheath.
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Osteochondral fragmentation, also known as carpal chip or slab fractures depending on their nature, are commonly seen in racehorses with horses aged from 2 to 4 years old most at risk (Stashak Adams, 2002). Immediate, post-fracture lameness is possible, the grade of which varies according to the type/number of facture (Ross Dyson, 2003). Horses with large slab fractures usually stand with carpus partially flexed (Stashak Adams, 2002). Distension of the joint will occur quickly, usually within 3-6 hours which is accompanied by heat and pain over the carpus (Pasquini, Jann, Pasquini, Bahr, 2005). These clinical signs are consistent to those in our case.
We can examine gait, synovial fluid, perform flexion tests and radiology to confirm carpal osteochondral fractures. If a fracture is suspected an intra-articular block is contraindicated (Pasquini, Jann, Pasquini, Bahr, 2005). At a walk, carpal fragmentation will cause a degree of shortening of cranial phase of stride with reduced height of foot flight in arc (Stashak Adams, 2002). A serosanguinous aspiration of synovial fluid denotes acute carpal trauma (Ross Dyson, 2003).
The carpal flexion test is the most specific of the forelimb flexion tests to localise lameness to that joint (Ross Dyson, 2003). This is performed by holding the carpus in flexion, without flexing the fetlock, for at least 1 minute to evaluate lameness (McIlwraith, 2000). A positive response is seen as an observable or increased lameness for more than 3-5 strides at a trot after the carpus is released (Ross Dyson, 2003). A positive result is expected if a carpal osteochondral fracture is present (Pasquini, Jann, Pasquini, Bahr, 2005). As bilateral chip fractures are common is it important to compare both limbs (Pasquini, Jann, Pasquini, Bahr, 2005).
Radiology in most instances will be diagnostic and can confirm the type of fracture (chip, slab or comminuted) that has occurred. A chip fragment is when a piece of bone and cartilage has been severed from an articular surface. Slab fractures involve 2 surfaces of the joint (differentiating them from small chip fragments which only involve one articular surface) and most commonly occur in the racehorse (Thrall, 2007). Multiple fractures in more than one carpal bone are referred to as comminuted, these are the least frequent type of carpal fracture and the horse usually presents non-weight bearing (Ross Dyson, 2003).
The distal radiocarpal is the most common location for small chip fractures in the carpus but they are also frequently seen on the distolateral radius, proximal intermediate carpus and the proximal radial carpus (Palmer, 1986). Large osteochondral fragments are most commonly slab fractures of the 3rd carpal bone in the dorsal plane (Ross Dyson, 2003). They may be displaced or non-displaced and may occur bilaterally. Clinical signs of displaced fractures such as lameness, soft tissue swelling and effusion are worse than non-displaced, incomplete fractures. (Thrall, 2007).
Accurate detection of carpal bone fragmentation requires standard radiographic views to be made (McIlwraith, 2000):
Lateromedial (fractures of the 3rd and the radiocarpal bone are seen well on this view)
Dorsomedial- palmarolateral oblique
Skyline view of the carpus (dorsoproximal-dorsodistal) of the distal radius and the proximal and distal row of carpal bones. This view is particularly important in diagnosing sagittal slab fractures of the 3rd carpal bone.
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Radiographs should be carefully evaluated for lucency, sclerosis, cartilage erosion and chip and slab fractures in all the potential sites (Thrall, 2007). Signs such as radial carpal bone lucency with collapse of subchondral bone can precede a chip fracture of that bone. Third carpal bone sclerosis and concurrent damage to the distal-dorsal aspect if the radiocarpal bone is common with 3rd carpal slab fractures (Thrall, 2007).
It is important we also radiograph and evaluate the left forelimb of our horse as chip fractures in the contralateral limb are seen in approximately half of horses (McIlwraith, Yovich, Martin, 1987).
I believe a slab fracture is the most likely diagnosis, evidence for this includes: immediate swelling, quite marked lameness (worse than would be expected for chip fracture (Ross Dyson, 2003)), effusion over the middle joint plus pain over dorsal aspect of the 3rd carpal bone (Thrall, 2007). This is also supported by the epidemiology whereby slab fractures are most commonly seen in race horses between 2-3 years old (Pasquini, Jann, Pasquini, Bahr, 2005). Although without radiographs we cannot rule out chip fractures completely.
It is difficult to truly differentiate the pathogenesis of osteochondral fragmentation in particular 3rd carpal slab fractures from osteoarthritis (Ross Dyson, 2003) (Thrall, 2007). Instead changes of articular cartilage and subchondral bone these should be viewed as continuum of changes present in chronic overload injuries.
Normally, articular cartilage has a shock absorbing role and maintains the normal joint environment whilst the subchondral bone provides much cushioning and maintains the joint's shape. As an adaptive response to loading (training and exercise) bones undergo sclerosis and remodelling. The increase bone formation is an attempt to enhance bone strength to tolerate greater compressive forces (Kawcak, McIlwraith, Park, James, 2001). This occurs and has been extensively studied in the 3rd carpal bones of the thoroughbred racehorse.
The carpus is the most frequently fractured joint in the horse; it is particular vulnerable to concussive forces explained by a number of factors. The carpus normally locks into perfect congruity during full weight bearing, however towards the end of a race, the extensor muscles fatigue causing the carpus to sharply slam into place. At the same time the flexor muscles, which normally support the carpus caudally also tire thus allowing overextension. This increases the load and trauma on the dorsal aspect of the carpus (Stashak Adams, 2002). Foot imbalances caused by inappropriate shoeing or trimming will cause unequal loading of the carpus and predispose to this carpal remodelling. Finally, calf-kneed horses (conformation where the dorsal aspect of the limb is concave) are prone to carpal fracture too (Stashak Adams, 2002).
During early remodelling bone resorption precedes bone deposition this is a weak period where the overlying cartilage is susceptible to injury (Stashak Adams, 2002). However, advanced sclerosis and loss of trabecular pattern causes erosion of the overlying cartilage leading to areas of resorption, ischemia then necrosis and collapse of the articular cartilage (Pool Meagher, 1990). Research suggests that accumulated micro-fractures in the subchondral bone also increases bone matrix fragility (Norrdin, Kawcak, Capwell, McIlwraith, 1998).
Osteochondral fragmentation and clinical signs of osteoarthritis are frequently seen in 2 year olds or when training begins in young horses (Kawcak, McIlwraith, Park, James, 2001). Fractures result when the stressed joint structures cannot withstand the biomechanical forces placed upon them. Ninety percent of horses which have a complete osteochondral fracture of the 3rd carpal bone also had severe sclerosis of that carpal (DeHaan, O'Brien, Koblick, 1987) thus slab fractures are the final event in a cascade of non-adaptive remodelling change (Ross Dyson, 2003) (Thrall, 2007) (McGavin Zachary, 2007).
Rarely osteochondral fragmentation may be caused by an acute external blow however this is usually noted in the patient history.
Treatment and Prognosis
Carpal slab fractures should be treated as quickly as possible after the injury to stabilise the fracture thus preventing further articular damage (Stashak Adams, 2002). However the treatment and management will vary according to the horse's age, value, type of fracture (incomplete or displaced) and the presence of other fractures or lameness issues.
Arthroscopic repair is indicated. Although arthotomy may be quicker and more precise, the incision must be larger making the wound more prone to complications. In comparison to arthrotomy, arthroscopy offers better diagnostic accuracy, less tissue damage, more thorough joint irrigation, better cosmesis,, reduced post-operative pain and improved post-operative performance (McIlwraith, 2000).
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For slab fractures where the fragment is small (<5mm or in multiple pieces) it can be removed completely. However larger acute slabs can be repaired using lag screw compression techniques. The screw should be placed in the middle of the fragment and parallel to the joint surfaces. Caution must be made to avoid the screw exciting the palmar surface of the 3rd carpal bone as it could cause damage to structures within the carpal canal (Stashak Adams, 2002). This requires the horse to be under general anaesthesia in dorsal recumbency with the limb moderately flexed (Stashak Adams, 2002). An arthroscope can be introduced on the lateral aspect with instruments brought through a medial side portal (McIlwraith, 2000).
After screwing the fragment, any sharp edges particularly the dorsal surface above the fracture should be smoothed, loose cartilage pieces removed and the joint thoroughly lavaged and closed (Stashak Adams, 2002). Phenylbutazone is given post-operatively (McIlwraith, 2000). Intra-articular corticosteroids are not recommended as they delay fracture healing and support the development of osteoarthritis.
Sterile bandages are used for 14 days however if bandages are too tight ulcers on the accessory carpal bone and medial tuberosity of the radius can develop quickly. Thus it is recommended that light sterile bandages are used for the 2nd week (Stashak Adams, 2002). Two months of stall rest is required after surgery followed by 1-2 months of hand walking and 2-3 months of turnout before training can recommence. Although longer healing times of 8-9 months are recommended (Ross Dyson, 2003).Radiographs to monitor fracture healing can be taken at 6, 12 and 24 weeks by which time it should be healed (Stashak Adams, 2002).
As slab fractures are closely related to osteoarthritis more broad treatments for osteoarthritis can be investigated to slow down the injury to subchondral bone. These include but not limited to, sodium hyaluronate (modest analgesic effects, and anti-inflammatory), shock wave therapy to decrease inflammation, polysulfated glycoaminoglycan (PSGAG) and pentosan polysulfate (McIlwraith, 2010). However horses in continued training and racing will have accelerated progression of osteoarthritis.
When treated by surgical excision or lag screw repair the prognosis for thoroughbreds retuning to race after 3rd carpal slab fracture is fair to good with about 65% of thoroughbreds racing again although race success was generally lower than pre-injury (Ross Dyson, 2003) (Rutherford, Bladon, Rogers, 2007). However we must remember that osteochondral fragmentation is intrinsically linked to osteoarthritis and is often bilateral. Consequently future lameness caused by this pathology cannot be ruled out.
Horses which are to be used for pleasure riding or breeding have a good prognosis (Stashak Adams, 2002) .