History
A 500-kg (1,100-lb) 10-year-old Quarter Horse gelding was admitted to the veterinary hospital for lameness examination. The horse was used for barrel racing. Five months earlier, the horse had chip fractures of the proximal aspect of the intermediate carpal bone and the distal aspect of the radiocarpal bone of the left forelimb, for which the referring veterinarian treated the horse with intra-articular carpal joint injections.
At the time of hospital admission, the left carpus was swollen with palpable carpal joint effusion. The horse had a left forelimb lameness and was mildly lame at the walk and severely lame at the trot (4/5 lameness score, on the basis of the American Association of Equine Practitioners scale).1 Left carpal joint flexion produced a substantial exacerbation of the lameness. Abaxial sesamoid nerve block of the left forelimb resulted in persistence of the lameness with no degree of improvement, ruling out the foot as a source of lameness. Radiographic images of the left carpus were obtained for further evaluation (Figure 1).
Lateromedial (A), dorsopalmar (B), dorsomedial-palmarolateral oblique (C), and dorsolateral-palmaromedial oblique (D) radiographic views of the left carpus of a 500-kg (1,100-lb) 10-year-old Quarter Horse gelding used for barrel racing that was admitted to the veterinary hospital for evaluation of left forelimb lameness.
Citation: Journal of the American Veterinary Medical Association 250, 7; 10.2460/javma.250.7.751
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page→
Radiographic Findings and Interpretation
A radiolucent ovoid, smoothly marginated, 8 × 13-mm, cyst-like structure is evident in the medial styloid process of the radius and is surrounded by mild sclerosis (Figure 2). A narrow linear radiolucent structure measuring 10 mm in length by 0.8 mm in width is seen on the dorsolateral-palmaromedial oblique view, and can be traced from the osseus cyst-like lesion to the articular surface. Periarticular osteophytic new bone formation is evident at the cranial aspect of the distal portion of the radius, the dorsal aspect of the intermediate and third carpal bones, and the proximal aspect of the accessory carpal bone. The dorsomedial aspect of the radiocarpal bone is irregular. The proximal carpal bones are irregular along their articular surfaces and mildly sclerotic. An approximately 4-mm linear to ovoid, sharply marginated, osseous fragment is evident along the dorsolateral aspect of the radiocarpal articulation. Moderate intra- and extracapsular soft tissue swelling is found especially surrounding the dorsomedial aspect of the carpal joint.
Same radiographic images as in Figure 1. Notice the ovoid, smoothly marginated, 8 × 13-mm cyst-like radiolucent lesion in the medial styloid process of the radius (B, C, and D; arrowheads). Notice the trochlear fossa proximal to the radius’ articulation with the intermediate carpal bone. The trochlear fossa is seen as a large radiolucent area between the medial and lateral styloid processes (A, B, and C; arrows), and should not be confused with the cyst-like lesion (B, C, and D; arrowheads). Notice the narrow, linear, 10-mm-long × 0.8-mm-wide radiolucent structure connecting the cyst to the articular surface of the medial styloid; this lesion was only seen on the dorsolateral-palmaromedial oblique projection (D). This structure was consistent with a probable communication to the synovium. Notice the small, focal radiolucent structure adjacent to the area of the lateral styloid process (C; asterisk), which represents a normal anatomic variation resulting from incomplete fusion between the lateral styloid of the radius and distal epiphysis of the ulna.
Citation: Journal of the American Veterinary Medical Association 250, 7; 10.2460/javma.250.7.751
Differential diagnoses for radiolucent structures along the articular and subchondral areas of the distal portion of the radius include osseous cyst-like lesions, osteochondritis dissecans, and septic arthritis or small bone abscess. Preferential consideration was given to the osseous cyst-like lesion of the distal portion of the radius. Additionally, concurrent osteoarthritis was evident at multiple sites of the left antebrachiocarpal articulation.
Treatment and Outcome
In the 2-week interim following the preliminary evaluation, the lameness had rapidly progressed (4/5 lameness score at a walk). Exploratory arthroscopic evaluations of the left antebrachiocarpal and middle carpal joints were performed to further evaluate the lesions found radiographically Two small bony fragments (1 of which was not visible on radiographs) were removed from the middle carpal joint. The described larger radiolucent lesion of the distal portion of the radius was evaluated and debrided, and the antebrachiocarpal and middle carpal joints were lavaged.
Perioperatively, the horse received penicillin G procaine (IM), gentamicin (IV), and flunixin meglumine (IV). Before closure of the arthroscopic window, intra-articular infusion of amikacin sulfate was performed.
Postoperative medications included a single dose of tiludronate (IV). Recommended outpatient treatment regimens included the use of the following: hyaluronate sodium (IV), polysulfated glycosaminoglycans (IM), oral chondroitin sulfate supplementation, and a course of interleukin-1 receptor antagonist protein (intra-articularly).
The day following surgery, the horse was no longer lame at the walk. No postoperative complications were noted. Follow-up radiography at 4 weeks after surgery revealed that the previously noted radiolucent lesion of the distal portion of the radius was static and the osteoarthritis had progressed mildly. The gelding was sound at the walk and trot at 6 weeks after surgery. In addition, extra- and intracapsular soft tissue swelling were substantially reduced, compared with the preoperative evaluation. At the 6-month follow-up examination, the gelding had returned to full athletic performance.
Comments
The lesion seen in the medial distal portion of the radius of this horse had a characteristic radiographic appearance of an osseous cyst-like lesion, consisting of a well-demarcated radiolucent area within the subchondral bone surrounded by a region of sclerosis. This lesion was also accompanied by a narrow, radiolucent, communicating channel that likely connected the radiolucent structure to the articular surface, another characteristic of osseous cyst-like lesions, thereby contributing to the lameness by inducing synovitis.2 Although previously thought to be a part of the osteochondritis complex, osseous cyst-like lesions have been experimentally induced by damaging the articular cartilage and subchondral bone in mature horses.3 With regard to cystic bone lesions associated with osteochondrosis, or developmental etiologies, lesions arise from collapse of abnormal cartilage into the subchondral bone with cessation of endochondral ossification. Nondevelopmental etiologies of osseous cyst-like lesions in mature adult horses include the following: invasion of the synovial fluid through an articular defect of the subchondral bone, traumatic bone necrosis, cartilage injury, regional overloading, and ischemia.4 The distal portion of the radius is a known site for osseous cyst-like lesion formation.5 Although mostly diagnosed in younger animals, osseous cyst-like lesions can arise in mature horses that were previously performing successfully.6 This lends further credence to the idea that osseous cyst-like lesions do not arise solely from developmental orthopedic issues and may be caused by trauma to the joint.
Subchondral or osseous cyst-like lesions in the antebrachiocarpal joint are more likely to occur in the ulnar carpal bone.7 Other predisposed carpal sites include the heads of the second and fourth metacarpal bones and first (if present) and second carpal bones.7 Other differential diagnoses for lameness localized to the carpus in a mature horse include condylar fracture, radiocarpal joint osteoarthritis, enthesis of the radial carpal extensor or long digital extensor tendons, osteochondroma of the distal caudal aspect of the radius, and radial physeal exostoses.
Radiography is the most common method of diagnosing osseous cyst-like lesions. However, in cases where the lesion cannot be detected radiographically, nuclear scintigraphy, ultrasonography, and MRI have been used to identify the lesion and can be considered if radiography is not diagnostic.2
The distal portion of the radius in horses has several structures that may be easily confused with pathological changes. The trochlear fossa proximal to the articulation with the intermediate carpal bone in the caudal distal portion of the radius can appear as a large radiolucent structure in the middle of the epiphyseal region. The horse of the present report also had incomplete ossification of the ulnar epiphysis, which appears as a small radiolucent structure in the lateral condyle with no connection to the articular surface.8
With regard to implemented therapeutic interventions, bisphosphonate drugs are a newly emerging treatment that inhibit the function (and decrease the lifespan) of equine osteoclasts in vitro.9 Thus, they may help with resorptive bone lesions, such as osseous cyst-like lesions. Joint supportive therapy (such as chondroitin sulfate and polysulfated glycosaminoglycans) is an important component of managing any lesions affecting articular cartilage and subchondral bone, and should be considered regardless of whether the lesion is managed conservatively or surgically.5
References
1. American Association of Equine Practitioners. Horse health. Lameness exams: evaluating the lame horse. Available at www.aaep.org/info/horse-health?publication=836. Accessed Feb, 15, 2016.
2. Little D, Redding WR, Gerard MP. Osseous cyst-like lesions of the lateral intertubercular groove of the proximal humerus. Equine Vet Educ 2009; 21: 60–66.
3. Ray CS, Baxter GM, McIlwraith CW, et al. Development of subchondral cystic lesions after articular cartilage and subchondral bone damage in young horses. Equine Vet J 1996; 28: 225–232.
4. Baxter GM. Subchondral cystic lesions in horses. In: McIlwraith CW, Trotter GW, eds. Joint disease in the horse. Phildelphia: WB Saunders Co, 1996;384–397.
5. Specht TE, Nixon AJ, Colahan PT, et al. Subchondral cyst-like lesions in the distal portion of the radius in four horses. J Am Vet Med Assoc 1988; 193: 949–952.
6. Hogan PM, McIlwrath CW, Honnas CM, et al. Surgical treatment of subchondral cystic lesions of the third metacarpal bone: results in 15 horses (1986–1994). Equine Vet J 1997; 29: 477–482.
7. Murray RC, Dyson SJ. The equine carpus. In: Thrall, DE, ed. Textbook of veterinary radiology. 6th ed. St Louis: Elsevier. 2013;374–394.
8. Butler JA, Colles CM, Dyson SJ, et al. The carpus. In: Butler JA, Colles CM, Dyson SJ, et al, eds. Clinical radiology of the horse. Oxford, England: Blackwell Science, 2000;171–205.
9. Gray AW, Davies ME, Jeffcott LB. Generation and activity of equine osetoclasts in vitro: effects of the bisphosphonate pamidronate (APD). Res Vet Sci 2002; 72: 105–113.
