EVE 08-095 Goodrich 17/9/08 08:53 Page 1 EQUINE VETERINARY EDUCATION / AE / october 2008 521 Clinical Commentary Subchondral bone cysts - not always an easy diagnosis L. R. GOODRICH* AND C. W. MCILWRAITH† James L. Voss Veterinary Teaching Hospital; and †Gail Holmes Equine Orthopaedic Research Center, Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523, USA. Arnold et al. (2008) recently described in this journal the dome shaped, conical or spherical, with a thinner radiolucent diagnosis and surgical management of a middle aged horse tract communicating with the articular surfaces (Fig 1). with a subchondral bone cyst located in the intermediate Subchondral sclerosis surrounding the cyst may range from tubercle of the humerus. This case is unique due to the mild to intense sclerosis depending on their stage of uncommon location of the cyst. The authors performed a thorough work-up of the lameness and, through their determination and use of multiple imaging modalities, were able to diagnose the cause of pain and successfully manage it surgically. In this issue, Del Chicca et al. (2008) report on a 4-year-old gelding that presented with acute lameness of 5 weeks’ duration. A subsequent diagnosis of an intra-articular type IV fracture with an osseous cyst-like lesion and mild degenerative joint disease of the DIP joint was made based on radiological examination of the pedal bone. Computed tomographic examination was also carried out post mortem. Subchondral bone cysts (SBC) in horses are most commonly found in the medial condyle of the femur, followed by phalanges, carpal bones, metacarpal and metatarsal bones, tibia, radius, talus, sesamoid bones, humerus, patella and tarsal bones (von Rechenberg et al. 1998). To date, apart from the case described by Arnold et al. (2008), there have not been any reports of a subchondral bone cyst in the humerus or specifically, the bicipital bursa. The aetiology of SBC is considered multifactorial and various pathogenic processes have been implicated. They include genetic causes (Hoppe and Phillipson 1985), diet (Glade and Belling 1986), growth rate, hormonal and mineral imbalances (Savage et al. 1993), osteochondrosis (Pool 1993), biomechanics and trauma (Jeffcott et al. 1983). A study by Ray et al. (1996) proved that damage to the articular cartilage plus subchondral bone, but not articular cartilage alone, of the distal medial femoral condyle may lead to the development of subchondral cystic lesions. Currently, the most well accepted aetiologies for the development of SBC remain trauma or osteochondrosis (Baxter 1996; Ray et al. 1996). Regardless of the cause, grossly and histologically, SBC have characteristic features. SBC can range in size from shallow (<10 mm) to much greater than 10 mm and their shape can vary from Fig 1: A diagrammatical (left) and corresponding radiographic (right) representation of the grades of bone cysts most recently described by Wallis et al. (2008). Reprinted from Wallis et al. *Author to whom correspondence should be addressed. (2008) with permission from Equine Veterinary Journal Ltd. EVE 08-095 Goodrich 17/9/08 08:53 Page 2 522 EQUINE VETERINARY EDUCATION / AE / october 2008 a) b) Fig 2: A gross sagittal section revealing the appearance of a subchondral cystic lesion. Sclerotic bone is present around the cyst (a), dense fibrous tissue lines the cyst (b) and the centre is often filled with gelatinous material (c). Reprinted from Wallis et al. (2008) with permission from Equine Veterinary Journal Ltd. development (von Rechenberg et al. 1998). Various classifications of radiographic appearance have been reported (Jeffcott and Kold 1982; Howard et al. 1995). Often, SBC have c) a fibrous lining that is filled with fibrous and/or synovial gelatinous fluid and in some instances fibrocartilage (Fig 2) (von Rechenberg et al. 1998). Molecular studies of the fibrous lining revealed that the inflammatory mediators interleukin-1β and interleukin-6 are upregulated and, in combination with prostaglandin E2, nitric oxide and neutral metalloproteinases, may be responsible for pain and continuing expansion of these cystic lesions (von Rechenberg et al. 2000). SBC can be an incidental finding but often are associated with lameness. The degree of lameness can vary from mild to severe and the onset can range from an insidious presentation of lameness to fairly acute. Often, the lameness ensues at the commencement of training in young horses and in middle aged to older horses the incident of a traumatic event or correlation to ongoing intra-articular inflammation may be Fig 3: CT images from a foal with a subchondral bone cyst of associated with the onset of the SBC (von Rechenberg et al. the caudal aspect of the third carpal bone. The cyst was not 1998). Synovial effusion may be present but is not always detected on radiographic examination. The cyst can be apparent with SBC. Intra-articular anaesthesia will often detected in the dorsal plane without flexion (a), in the saggital improve the lameness, although it may not abolish it and can plane with the carpus in a flexed position (b), and in the dorsal plane image with the carpus in a flexed position (c). occasionally only minimally improve the grade. When suspected, SBC are most often confirmed radiographically (McIlwraith 1990). However, occasionally (2008), use of CT to reveal bone abnormalities was the logical cysts may not be obvious using conventional radiography and choice for further diagnostics. imaging modalities such as nuclear scintigraphy, ultrasound, Management strategies for SBC include debridement MRI or CT scan may be helpful in confirming the diagnosis of (White and McIlwraith 1985; White et al. 1988), SBC (Barrett and Zubrod 2008; Mair and Sherlock 2008). debridement with cancellous bone graft (Jackson et al. Figure 3 shows an example of a CT scan confirming a SBC in 2000), debridement with bone substitute and chondrocyte a carpus of a filly that did not have radiographic signs of SBC or progenitor cell graft in fibrin glue (Fortier and Nixon formation. In the case of the SBC of the humerus (Arnold 2005), or steroid injection into the fibrous lining by either et al. 2008), MRI and CT would have been difficult due to ultrasound guided intra-articular injection (Foerner et al. limitations of imaging the upper extremities with current 2006) or arthroscopically (Wallis et al. 2008). The goal of equipment; however, the ultrasound was a useful tool (Barrett management is to eliminate the fibrous lining or control the and Zubrod 2008). In the case report of Del Chicca et al. inflammation associated with cytokine release within this EVE 08-095 Goodrich 17/9/08 08:53 Page 3 EQUINE VETERINARY EDUCATION / AE / october 2008 523 lining through local steroid administration (von Rechenberg also still differ in various reports with Quarter Horses and et al. 1998). Paints tending to have less success than Thoroughbreds. Case Reported prognoses for return to previous level of activity reports as well as large clinical studies reporting results with has ranged from 30–90% depending on procedure, age, current developing therapies will enhance our current breed, surface area of weightbearing cartilage affected and knowledge of appropriate management of subchondral bone the occurrence of co-inciting osteoarthritis (White and cysts in horses. McIlwraith 1985; White et al. 1988; Sandler et al. 2002; Hendrix et al. 2005; Smith et al. 2005). Early reports of References debridement ranged from a prognosis as high as 90% (White et al. 1988) to less than 50% (Howard et al. 1995). Quarter Arnold, C.E., Chaffin, M.K., Honnas, C.M., Walker, M.A. and Heite, Horses were found to have a poorer prognosis compared to W.K. (2008) Diagnosis and surgical management of a subchondral bone cyst within the intermediate tubercle of the humerus in a Thoroughbreds (Ray et al. 1996; Wallis et al. 2008). horse. Equine vet. Educ. 20, 310-315. Subchondral bone forage as part of cyst enucleation was Barrett, M.F. and Zubrod, C.J. (2008) Use of magnetic resonance associated with cyst expansion and was determined to be imaging to detect and direct therapy of an osseous cystic lesion at detrimental (Howard et al. 1995). Furthermore, packing the the solar surface of the third phalanx of a horse. Equine vet. Educ. SBC with cancellous bone graft following curettage did not 20, 19-23. improve cartilage healing (Jackson et al. 2000). More recently, Baxter, G.M. (1996) Subchondral cystic lesions in horses. In: Joint Smith et al. (2005) reported 64% of horses aged Disease in the Horse, Eds: C.W. McIlwraith and G.W. Trotter, W.B. 0–3 years with medial femoral condylar SBC returning to Saunders, Philadelphia. pp 384-397. soundness following debridement but only 35% of horses Del Chicca, F., Kuemmerle, J.M., Ossent, P., Nitzl, D., Fuerst, A. and Ohlerth, S. (2008) Use of computed tomography to evaluate a older than 3 years returning to soundness following fracture associated with a subchondral pedal bone cyst in a horse. debridement. Sandler et al. (2002) reported 70% return to Equine vet. Educ. 20, 515-519. soundness if the surface area of cartilage affected on the Glade, M.J. and Belling, T.H. (1986) A dietary etiology for medial femoral condyle was <15 mm. However, in that same osteochondrotic cartilage. J. equine vet. Sci. 6, 151-155. report, if the surface area was >15 mm only 30% of the Foerner, J.J., Rick, M.C., Juzwiak, J.S., Watt, B.C., Smalley, L.R., Keuler, horses returned to soundness (Sandler et al. 2002). Recently, N.S. and Santschi, E.M. (2006) Injection of equine subchondral Wallis et al. (2008) reported a 67% success rate (return to bone cysts with triamcinolone: 73 horses (1999-2005). Proc. Am. soundness) with injection of corticosteroids into the fibrous Ass.