EVE Clinical Commentary

The case report by describes an interesting case of rib fracture in a race horse with an attributable hindlimb lameness. This case is the first to suggest that caudal rib fractures are a cause of hindlimb lameness and peri regional diagnostic analgesia of a rib fracture can be used to alleviate hindlimb lameness.

A recent retrospective study reported 73 horses diagnosed with a rib fracture of which 56% (41/73) presented due to poor performance with a fewer number of horses presenting with lameness as a primary complaint (21/73). Undoubtedly rib fractures are painful in the acute stages of injury, with the most common site for rib fracture in a horse being dorsally (5-15cm from the costovertebral junction) on the 18^th rib (Hall et. al. 2022). In this study, not all horses with caudal rib fractures were found to have lameness those that were lame had a variable pattern of lameness. All of the horses which underwent diagnostic analgesia were found to have lameness independent to the rib fracture. However only a small percentage of horses underwent diagnostic analgesia, a limitation of a retrospective study.

An alternative retrospective case series of 50 rib fractures reported 5 fractures of the first rib which all occurred in racing thoroughbreds (age 2-7 years with a median age of 3), all of which had ipsilateral forelimb lameness. In the absence of a traumatic incident the authors suggested that fractures of the 1^st rib may represent fatigue fracture pathology associated with training. A similar aetiology is possible for caudal rib fractures but is considered unlikely as this injury is not over represented in racing Thoroughbreds compared with a general population of horses and trauma remains the most likely cause.

Ribs can be imaged in detail with nuclear scintigraphy given the relatively small amount of overlying soft tissue. Orthogonal images (lateral, dorsal and oblique) should be used to localise region, extent, pattern and origin of increased radiopharmaceutical uptake. Nuclear scintigraphy is highly sensitive in identifying osteoblastic activity but has low specificity for identifying the nature of the pathological process. Fractures show increased radiopharmaceutical uptake within 24-72 hours post injury, making nuclear scintigraphy highly sensitive in the acute stage of disease verses radiographic signs which may not be seen for around 7-10 days unless the fracture is displaced. The appearance of a fracture on nuclear scintigraphy includes 5-7 days of diffuse intense uptake, 1-4 weeks of focal intense uptake (figure 1.) and then a gradual decrease of radiopharmaceutical uptake over the next 6-12 months. This prolonged visibility on scintigraphy means that is difficult to age a fracture based on scintigraphy alone. Ultrasonography is potentially more suitable to monitor fracture healing and identify those cases which are non-healing and requiring intervention. Costochondral junctions normally have a moderate increased radiopharmaceutical uptake on nuclear scintigraphy and should not be mistaken for a rib fracture (figure 1.). The caudal ribs overly the kidney (figure 1.) but in most instances have a mild to moderate diffuse region of increased radiopharmaceutical uptake. The use of furosemide one hour before imaging has been advocated to improve soft tissue clearance and improve image count to background ratio. This was evaluated in a recent study where 1mg/kg furosemides was administered intravenously 1 h post 99m Tc-HDP administration and the image quality was assessed subjectively and semi-quantitively. There was no significant difference in image quality or radiation dose rate to handler, with a minimum distance of 30cm distance having the most effect on reducing handler dose rate by 65% .

Ultrasonographic examination is highly sensitive and specific in identifying rib fractures (Hall et. al. 2022). If a rib fracture is suspected in the initial stages of examination, then survey ultrasonographic examination of the ribs is recommended, especially the caudal ribs. However, rib fractures rarely result in focal localising pain on palpation and so even a detailed clinical examination may be unrewarding. Ultrasonography of the ribs is a simple technique; clipping isn’t required and discontinuity and callus within the lateral cortex of the ribs is easy to identify (Figure 2) but should not be confused with the costochondral junction (figures 3 and 4). The costochondral junction is identified by the presence of hypoechoic cartilage and it should be noted that the margins of adjacent ribs are normally irregular at this site. In contrast, a rib fracture does not contain hypoechoic cartilage, though the rib margins are likely to be irregular and commonly periosteal new bone may be seen.

In this case of hindlimb lameness due to fracture of the 18^th rib , the horse took 12 months to recover and resume ridden exercise with conservative therapy. Ultrasonographic examination can monitor fracture healing and is useful in identifying non-healing fractures which may represent surgical candidates. Hall and colleagues (2022) reported six horses which underwent surgery due to failed initial conservative management which resulted in continued fracture instability, callus at the fracture site or fracture displacement causing impingement on adjacent ribs. One horse which underwent wedge ostectomy and internal fixation with a locking compression plate returned to its previous level of exercise. All other surgically managed cases (5/73) underwent partial rib resection. Most horses in this study were managed conservatively (67/73). Given the very small number of surgically managed cases it is not possible to determine if fracture healing or outcome would have been improved if surgical intervention had been performed in more cases.

Peri-regional analgesia of a rib fracture is commonly used in human medicine as a therapeutic aid and is easy to perform in the horse. In this instance the ipsilateral hindlimb lameness was partially alleviated by this technique in the acute stage of lameness and completely abolished in the chronic stage of lameness. Perineural analgesia can result in increased radiopharmaceutical uptake within the soft tissues on bone phase scintigraphy, up to 7 days post blocking , with 50% of horses having uptake at the site of a tibial perineural injection one day post injection, 25% of horses at the block site 3 days post injection and 1 horse with increased radiopharmaceutical uptake 7 days post injection. An appropriate washout period is recommended prior to scintigraphy if perineural analgesia has been performed to prevent false positive results.

Although rib fractures are largely under reported in adult horses this case report and recent retrospective studies confirm they should be considered a differential diagnosis for cases of poor performance and both fore and hind limb lameness. Diagnostic analgesia could be used to determine significance of the rib fracture in cases of hind limb lameness.