R. C. Whitton

The diagnosis of lameness associated with distal limbpathology in a horse: A comparison of radiography, computed tomography and magnetic resonance imaging

A cadaver limb from an eight-year-old horse with right forelimb lameness that was relieved with an intra-articular distal interphalangeal joint block was imaged with radiographs, spiral computed tomography (CT) and magnetic resonance imaging (MRI). Spiral CT demonstrated several lucencies within the deep digital flexor tendon immediately proximal to the navicular bone. On MRI these areas had increased signal and there was enlargement of the tendon at this site. Effusion in the proximal interphalangeal joint and navicular bursa and thinning of the fibrocartilage of the navicular bone were also observed on MRI images. These changes were not detected on radiographs. Histopathology confirmed that there were focal areas of collagen necrosis within the deep digital flexor tendon with thinning and degenerative changes in the fibrocartilage of the navicular bone.

Relationship between muscle forces, joint loading and utilization of elastic strain energy in equine locomotion.

SUMMARY Storage and utilization of strain energy in the elastic tissues of the distal forelimb of the horse is thought to contribute to the excellent locomotory efficiency of the animal. However, the structures that facilitate elastic energy storage may also be exposed to dangerously high forces, especially at the fastest galloping speeds. In the present study, experimental gait data were combined with a musculoskeletal model of the distal forelimb of the horse to determine muscle and joint contact loading and muscle–tendon work during the stance phase of walking, trotting and galloping. The flexor tendons spanning the metacarpophalangeal (MCP) joint – specifically, the superficial digital flexor (SDF), interosseus muscle (IM) and deep digital flexor (DDF) – experienced the highest forces. Peak forces normalized to body mass for the SDF were 7.3±2.1, 14.0±2.5 and 16.7±1.1 N kg–1 in walking, trotting and galloping, respectively. The contact forces transmitted by the MCP joint were higher than those acting at any other joint in the distal forelimb, reaching 20.6±2.8, 40.6±5.6 and 45.9±0.9 N kg–1 in walking, trotting and galloping, respectively. The tendons of the distal forelimb (primarily SDF and IM) contributed between 69 and 90% of the total work done by the muscles and tendons, depending on the type of gait. The tendons and joints that facilitate storage of elastic strain energy in the distal forelimb also experienced the highest loads, which may explain the high frequency of injuries observed at these sites.

Retrospective investigation of prognostic indicators for adult horses with infection of a synovial structure

OBJECTIVE To investigate predictors of survival and athletic function in adult horses with infection of a synovial structure. HYPOTHESES Increasing duration from contamination to referral, bone or tendon involvement and positive microbial culture decreases short-term survival. Synovitis and/or sepsis at 5 days post-admission and involvement of Staphylococcus spp. decreases long-term athletic function. DESIGN Retrospective study. METHODS Records over 4 years of adult horses with synovial sepsis were reviewed. A two-tailed Fishers exact test, Mann-Whitney U test or t-test was used to examine whether variables were predictive of short-term survival and long-term athletic function. RESULTS During the study period 75 horses underwent treatment for infection of 93 synovial structures. Short-term survival was 84% (63/75) and 54% (30/56) of horses returned to athletic function. Of the variables measured at admission, duration from contamination to referral did not affect survival, whereas evidence of bone or tendon involvement decreased survival and athletic function. Of the variables available during treatment, abnormal synovial fluid at 4-6 days post-admission and positive microbial culture reduced athletic function. Staphylococcal infection was associated with persistent sepsis. CONCLUSIONS Of the variables available at presentation, only evidence of bone or tendon involvement negatively affected survival and athletic function. During treatment of synovial sepsis, analysis of synovial fluid at 4-6 days and bacterial culture results have prognostic value.

Patterns of scintigraphic uptake in the fetlock joint of Thoroughbred racehorses and the effect of increased radiopharmaceutical uptake in the distal metacarpal/tarsal condyle on performance

REASONS FOR PERFORMING STUDY The effect of palmar osteochondral disease lesions on performance of Thoroughbred (TB) racehorses is unclear. There is a need to describe patterns of radiopharmaceutical uptake (IRU) in fetlock joints of TB racehorses and to evaluate post scintigraphy performance. HYPOTHESES IRU in the metacarpal (MC) and metatarsal (MT) condyles is more common than IRU in the parasagittal grooves and is associated with poorer post diagnosis performance than controls. METHODS Location of IRU within the fetlock region was identified and graded subjectively in TB racehorses. Performance variables were determined from race records for horses with moderate/marked MC/MT condylar IRU (cases), other horses undergoing scintigraphy (scintigraphy controls) and age/sex matched controls from the last race in which a case participated (controls). Statistical analyses included quantile regression, Fishers exact test, Kaplan-Meier survival curves, log-rank test and Cox regression. RESULTS Metacarpal/MT condylar IRU was identified in 103/220 horses with only 3/220 having parasagittal IRU. Moderate/marked IRU was identified in the MC and MT condylar regions in 62 and 39 horses, respectively, with palmaromedial and plantarolateral IRU most common. Fore- and hindlimb cases had fewer starts, reduced earnings (P<0.001) and reduced earnings/start than controls respectively. Cases were more likely to return to racing later than controls following a rest period. Cases were older than scintigraphy controls. CONCLUSIONS In TB racehorses presenting for lameness MC/MT condylar IRU is the most common abnormality identified within the fetlock joint. Racehorses with moderate/marked condylar IRU have a shortened racing career, are less successful than age/sex matched controls and are older than other racehorses presented for scintigraphy. POTENTIAL RELEVANCE Overload of the MC/MT condyles is a common and significant problem in TB racehorses that is readily identified with scintigraphy. Scintigraphy of horses that are lame or performing poorly is less useful for screening for potential condylar fractures.

Thoroughbred horses in race training have lower levels of subchondral bone remodelling in highly loaded regions of the distal metacarpus compared to horses resting from training

Bone is repaired by remodelling, a process influenced by its loading environment. The aim of this study was to investigate the effect of a change in loading environment on bone remodelling by quantifying bone resorption and formation activity in the metacarpal subchondral bone in Thoroughbred racehorses. Sections of the palmar metacarpal condyles of horses in race training (n = 24) or resting from training (n = 24) were examined with light microscopy and back scattered scanning electron microscopy (BSEM). Bone area fraction, osteoid perimeter and eroded bone surface were measured within two regions of interest: (1) the lateral parasagittal groove (PS); (2) the lateral condylar subchondral bone (LC). BSEM variables were analysed for the effect of group, region and interaction with time since change in work status. The means ± SE are reported. For both regions of interest in the training compared to the resting group, eroded bone surface was lower (PS: 0.39 ± 0.06 vs. 0.65 ± 0.07 per mm, P = 0.010; LC: 0.24 ± 0.04 vs. 0.85 ± 0.10 per mm, P < 0.001) and in the parasagittal groove osteoid perimeter was higher (0.23 ± 0.04% vs. 0.12 ± 0.02%). Lower porosity was observed in the subchondral bone, reflected by a higher bone area fraction in the LC of the training group (90.8 ± 0.6%) compared to the resting group (85.3 ± 1.4%, P = 0.0010). Race training was associated with less bone resorption and more bone formation in the subchondral bone of highly loaded areas of the distal metacarpus limiting the replacement of fatigued bone. Periods of reduced intensity loading are important for facilitating subchondral bone repair in Thoroughbred racehorses.

Role of subchondral bone remodelling in collapse of the articular surface of Thoroughbred racehorses with palmar osteochondral disease

REASONS FOR PERFORMING STUDY To gain a better understanding of the aetiology of articular surface collapse in horses with palmar osteochondral disease. OBJECTIVES To determine whether acceleration of focal bone resorption associated with reduced physical activity contributes to articular surface collapse in racehorses with palmar osteochondral disease. STUDY DESIGN Cross-sectional study comparing metacarpal bones from horses at varying stages of race training. METHODS Metacarpal bones from 36 racing Thoroughbred horses were examined with high-resolution peripheral quantitative computed tomography to determine the proportion of the articular surface that had collapsed and with backscattered scanning electron microscopy to quantify porosity and eroded bone surface. Racing and training histories were obtained for comparison with imaging data. RESULTS In 21 cases, inward collapse of the calcified cartilage layer was observed on backscattered scanning electron microscopy. An increased extent of articular surface collapse was associated with greater numbers of microfractures in the calcified cartilage and superficial subchondral bone (Spearmans correlation [rs ] = 0.62, P<0.001). In the deeper bone (6-10 mm), porosity was lower with a greater extent of articular surface collapse (rs = -0.38, P = 0.023), whereas in the superficial bone (0-4 mm) there was no association between articular surface collapse and porosity (rs = 0.19, P = 0.26). Both porosity (median 14, range 3.8-26 vs. 3.8, 1.6-17%, P = 0.008) and eroded surface (1.1, 0.74-4.5 vs. 0.64, 0.11-4.7 mm(-1) , P = 0.016) of the superficial subchondral bone were higher in resting than in training horses, and in some resting horses subchondral bone voids were highly concentrated, resulting in an apparent loss of support for the overlying calcified cartilage layer. CONCLUSIONS Articular surface collapse is common in cases of palmar osteochondral disease and is likely to be a sequel to fatigue injury of subchondral bone. Focal subchondral bone resorption appears to contribute to collapse of the calcified cartilage and is potentiated by a reduced-intensity exercise regimen.

Muscle strain injuries of the hindlimb in eight horses: diagnostic imaging, management and outcomes

OBJECTIVE To describe the clinical presentation, ultrasound findings, management and outcome in horses with muscle tear injuries of the hindlimbs. DESIGN Retrospective case series Procedure Medical records of eight horses were reviewed and information on signalment, history, presenting complaint, physical examination findings and further diagnostic tests were recorded. Diagnosis of muscle injury was determined by the presence of abnormal ultrasound findings, compared with the contralateral limb, and, when required, nuclear scintigraphy. Follow-up information was obtained via telephone interviews with owners, trainers and referring veterinarians. RESULTS Muscle tears causing lameness were identified in the middle gluteal (3), semitendinosus (1), semimembranosus (2) and gracilis (2) muscles. Tears were classified by ultrasound imaging as partial (6) or complete (2). The degree of lameness did not appear to be indicative of the extent of injury or of completion of healing. Long-term follow-up was available for seven horses and the outcome was favourable in six cases. CONCLUSIONS AND CLINICAL RELEVANCE Ultrasonography is useful in the diagnosis and assessment of moderate to severe muscle strain injuries. The prognosis appears to be favourable in most cases, although recurrence of injury and lameness can delay the return to athletic activity and an inferior outcome with persistent gait abnormality may occur.

Musculoskeletal injury rates in Thoroughbred racehorses following local corticosteroid injection

A retrospective cohort study was performed to compare the rates of musculoskeletal injury (MSI) in horses receiving local corticosteroid injection (LCI) with those that were untreated and those prior to treatment. Of the 1911 study horses, 392 had been treated. A LCI was defined as any injection of corticosteroid into or adjacent to a synovial structure, muscle, or tendon/ligament. A MSI was defined as any limb injury identified by a veterinarian, following which the horse did not race for at least 6 months, or was retired. Hazard ratios (HR) comparing hazard of injury following injection to that in non-injected horses and prior to injection were calculated using Cox proportional hazards models. At least one LCI was administered to 392 horses (20.5%; median 2, range 1-16). Most LCIs were performed bilaterally (70.9%) and intra-articularly into the carpal (49.7%) or fore fetlock (29.3%) joints. There were 219 MSIs of which carpal injuries (47%), fore fetlock (22%) and forelimb tendon injuries (16%) were the most common. The incidence rate of MSI in untreated horses and those prior to injection was 1.22 (95% CI 1.04-1.44) injuries/100 horse-months, and following LCI the hazard of MSI was greater (HR 4.83, 3.54-6.61, P<0.001). The hazard ratio returned to levels indistinguishable from before treatment after 49 days. The hazard of MSI in horses following second and subsequent LCIs in the data collection period was greater than in horses following their first LCI (HR 2.10, 1.31-3.36, P=0.002). There was a positive association between LCI and subsequent musculoskeletal injury rates which was most likely due to progression of the musculoskeletal condition which prompted treatment. Assuming horses that received LCI were at increased risk of MSI subsequently, any beneficial effects of the LCI were insufficient to counter this increased risk for at least 49 days after the injection.

IDENTIFICATION AND PREVALENCE OF ERRORS AFFECTING THE QUALITY OF RADIOGRAPHS SUBMITTED TO AUSTRALIAN THOROUGHBRED YEARLING SALE REPOSITORIES

We aimed to identify common mistakes made when radiographing yearling sale horses. Radiographic examinations from repositories at eight yearling sales held in Australia in 2003 were assessed by one of four veterinary radiology specialists. Each radiographic examination consisted of a maximum of 34 radiographs. Each radiograph was assessed for errors associated with movement, exposure, positioning, labeling or marker placement, and processing, and categorized as either ideal, less than ideal or nondiagnostic. In addition, from the first 800 sets catalogued, 167 were selected randomly and read twice by the four radiologists for agreement analysis. A total of 81,297 radiographs were examined for errors affecting quality. Positioning errors were the most common reason for radiographs to be considered nondiagnostic (2432/81,297; 3%), with the flexed lateromedial (LM) metacarpophalangeal joint, LM metatarsophalangeal joint, and the dorsomedial palmarolateral (DMPaLO) carpal views being the most frequently involved. Overexposure (14,357/81,297; 17.7%) was the most common reason for radiographs being categorized as less than ideal with the LM stifle view the most represented. Agreement within and between radiologists for reporting errors in positioning of the flexed LM metacarpophalangeal joint, LM metatarsophalangeal joint, and DMPaLO carpal views varied from slight to almost perfect. The low repeatability within radiologists on some views suggests that before declaring a radiograph nondiagnostic it is worth considering rereading it at another time. Care should be taken in positioning of the flexed LM metacarpophalangeal, LM metatarsophalangeal, and DMPaLO carpus views to maximize radiograph quality.

Can high-resolution peripheral quantitative computed tomography imaging of subchondral and cortical bone predict condylar fracture in Thoroughbred racehorses?

REASONS FOR PERFORMING STUDY High-resolution 3D imaging may improve the prediction and/or early identification of condylar fractures of the distal metacarpus/tarsus and reduce the frequency of breakdown injury in racehorses. OBJECTIVES To test the hypotheses that horses suffering condylar fractures have higher bone volume fraction (BV/TV) of the distal metacarpal epiphysis, greater subchondral bone thickness at the fracture site and higher second moment of inertia in the metacarpal midshaft as identified with high-resolution 3D imaging. STUDY DESIGN Cross-sectional study using cadaver material. METHODS Thoroughbreds that died on racetracks were grouped as: 1) horses with third metacarpal (McIII) fractures with a condylar component (cases, n = 13); 2) horses with no limb fracture (controls, n = 8); 3) horses with fractures in other bones or suspensory apparatus disruption (other fatal injuries, n = 16). The palmar condyles of McIII and the midshaft were examined with high resolution peripheral quantitative computed tomography (HR-pQCT). Statistical analysis included logistic regression and Spearmans correlation. RESULTS There were no significant differences in BV/TV of distal McIII and second moment of inertia of the midshaft between cases and controls. Epiphyseal bone BV/TV was greater in injured limbs of horses with any fatal limb injury (Groups 1 and 3 combined) compared with controls (odds ratio = 1.20, 95% confidence interval 1.01-1.42, P = 0.034). An epiphyseal BV/TV>0.742 resulted in a sensitivity of 82.8% and specificity of 62.5% in identifying horses with fatal limb injury. In horses without condylar fracture, increased subchondral bone thickness was associated with palmar osteochondral disease lesions in the adjacent condyle (rs = 0.65, P<0.001). CONCLUSIONS Increased BV/TV of the distal metacarpus may have some value for identifying horses at risk of any fatal breakdown injury but not metacarpal condylar fractures. Measurement of parasagittal groove subchondral bone thickness is complicated by adjacent palmar osteochondral disease lesions. Thus, high-resolution imaging of the distal metacarpus appears to have limited ability to identify horses at risk of condylar fractures.