History
A 10-year-old Thoroughbred gelding, which was used as a pleasure horse, was referred for advanced imaging because of a hind limb lameness of 4 months' duration. At the time of referral, lameness in the right hind limb was graded 3 of 51 at a trot in a straight line and while circling to the left and right. Upper limb flexion of the right hind limb exacerbated the lameness. Moderate tarsocrural joint effusion was noted.
The referring veterinarian previously had observed a 50% improvement in lameness with a low 4-point nerve block (ie, anesthesia of the medial and lateral plantar nerves and medial and lateral plantar metatarsal nerves). As perineural anesthesia findings were inconclusive, nuclear scintigraphy of the right hind limb was performed (Figure 1) followed by radiographic evaluation of the right tarsus (Figure 2).
Plantar (A) scintigraphic image of both hind limbs and lateral (B) scintigraphic image of the right hind limb obtained at the level of the mid-tarsal region in a 10-year-old Thoroughbred gelding with a right hind limb lameness of 4 months' duration.
Citation: Journal of the American Veterinary Medical Association 238, 8; 10.2460/javma.238.8.977
Lateromedial (A), dorsoplantar (B), dorsomedial-plantarolateral oblique (C), and dorsolateral-plantaromedial oblique (D) radiographic views of the right tarsus of the same horse as in Figure 1.
Citation: Journal of the American Veterinary Medical Association 238, 8; 10.2460/javma.238.8.977
Determine whether additional imaging studies are required, or make your diagnosis from Figures 1 and 2—then turn the page →
Diagnostic Imaging Findings and Interpretation
Local, intense, increased radiopharmaceutical uptake (IRU) is evident in the mid-tarsal region of the right hind limb on scintigraphic images (Figure 3). No abnormalities are identifiable on the radiographic images that correlate with the region of IRU on scintigraphic images.
Same scintigraphic images as in Figure 1. Notice the local, intense IRU in the mid-tarsal region of the right hind limb (arrows).
Citation: Journal of the American Veterinary Medical Association 238, 8; 10.2460/javma.238.8.977
On the basis of the location of the IRU, a pathological condition of the central tarsal bone was suspected. Anesthesia of the tarsocrural joint was performed via intra-articular injection and resulted in a substantial improvement in lameness.
Magnetic resonance imaging (MRI) of the right tarsus was performed with the horse standing.a A transverse, T1-weighted gradient echo MRI image revealed a linear area of high signal intensity extending in a dorsomedial to plantarolateral direction through the central tarsal bone (Figure 4). The T1-weighted gradient echo images in the frontal, transverse, and sagittal planes revealed low signal in the central tarsal bone. Short-tau inversion recovery (STIR) images revealed high signal intensity in the central tarsal bone. These findings are consistent with a sagittal fracture of the central tarsal bone. Additional MRI findings included increased fluid in the tarsal sheath.
Transverse T1-weighted gradient echo MRI image of the right tarsus at the level of the central tarsal bone. A linear area of high signal intensity extends from dorsomedial to plantarolateral (arrow) through the central tarsal bone and is surrounded by low signal intensity. These findings are consistent with a sagittal fracture of the central tarsal bone. In addition to the fracture, excessive, high–signal intensity fluid is present in the tarsal sheath (arrowhead).
Citation: Journal of the American Veterinary Medical Association 238, 8; 10.2460/javma.238.8.977
Comments
Soft tissues, such as joint capsule and ligaments, adipose tissue, and fluid, have different signal intensity on MRI depending on the sequence used to acquire the images. On MRI, tissue that appears white is referred to as having high signal intensity while tissue that appears black is designated as having low signal intensity with multiple shades of gray in between. Normal trabecular bone is light gray on T1-weighted images because of adipose tissue and therefore has a high to intermediate signal intensity. Low signal intensity in trabecular bone on T1-weighted images indicates abnormal fluid, sclerosis, or both. T1-weighted images can be compared with STIR image sequences to differentiate fluid from sclerosis. On STIR image sequences, normal bone has low signal intensity but fluid in bone results in areas of high signal intensity. Areas that have low signal intensity in trabecular bone on T1-weighed images without a corresponding high signal on STIR images indicate sclerosis.
Because of the focal uptake identified on scintigraphic images and negative findings on radiographic evaluation, MRI was used for this horse. Low-field MRI often does not provide detailed images of structures proximal to the foot or proximal phalangeal (pastern) area in standing horses. When the location of injury is narrowed to an osseous structure by use of another imaging modality and a substantial lesion is suspected on the basis of degree of lameness, the low-field MRI unit can be used in a standing horse to define pathological changes. Advantages of the high-field magnet are image quality and reduced motion artifact. Thinner slices with higher resolution of smaller structures allow for more detailed information acquisition. Low-field magnets are capable of acquiring diagnostic images without the need for general anesthesia. Although the low-field MRI images obtained in this standing horse were not as detailed as those that could have been obtained with a high-field system,2 MRI was instrumental to an accurate diagnosis. After MRI, serial oblique radiography was performed to determine the fracture plane but was nondiagnostic.
Central tarsal bone fractures commonly occur in the sagittal plane and are usually associated with high-speed performance because of compression and torsion applied to the tarsal region. Typical history includes acute lameness associated with exercise. Soft tissue swelling is usually not seen if the fracture is not displaced or is only minimally displaced.3 Radiography is often unsuccessful in identifying sagittal fractures, especially in the acute phase, because the x-ray beam must exactly match the fracture plane to be visible. In cases where advanced imaging is unavailable, repeated radiography at intervals after the injury may be the key to identifying fractures in the remodeling stage. However, radiography may still prove unrewarding, as in this horse.
Conservative management of central tarsal bone fractures includes stall rest, anti-inflammatory therapy, and controlled exercise. Surgical management consists of lag screw fixation which may increase the chance for return to performance, compared with conservative management.3 Conservative management was initiated in this horse because of the cost associated with surgical management.
EQ2, Hallmarq Veterinary Imaging Inc, Acton, Mass.
- 1.↑
American Association of Equine Practitioners. Lameness scale. In: Definition and classification of lameness. Lexington, Ky: American Association of Equine Practitioners, 1991; 19.
- 2.↑
Werpy NM, Ho CP, McIlwraith CW. Review of magnetic resonance imaging systems available for use in equine patients and the implications of field strength on clinical imaging: comparison of high- and low-field systems, in Proceedings. 53rd Annu Am Assoc Equine Pract2007; S22–29.
- 3.↑
Sullins KE. The tarsus. In: Stashak TS, ed. Adam's lameness in the horse. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2002; 930–987.
