Subchondral cystic lesions (SCLs) are also called subchondral bone cysts or osseous/subchondral cyst-like lesions (10). Although SCLs share some similarities with true bone cysts, such as aneurysmal bone cysts and unicameral bone cysts, they are not considered true cysts because of their lack of epithelial linings and frequent articular communications (2,10). In a previous report, postmortem gross examination revealed fibrous-looking tissue inside the cyst and peripheral sclerosis; and histologic examination identified sclerosis inside the cyst, osteochondral junction disruption, and increased peripheral bone remodeling activity (9). Additionally, von Rechenberg et al. (19) reported increased levels of prostaglandin E2, matrix metalloproteinase, and nitric oxide, indicating augmented osteoclastic activity inside an SCL. However, its etiology is not fully understood (10,12). Osteochondrosis and biomechanical stress have been suggested as causes of SCLs (9,17).

Patients with SCLs show varying degrees of lameness and joint effusion depending on the site and pathological changes in the lesions (7). SCLs generally occur in horses aged below 3 years; however, older horses may also be affected (4,16). Intra-articular anesthesia often minimally improves lameness in horses with SCL (7). Radiography can identify most SCLs; however, some lesions may not be clearly visible on radiography (7). Therefore, magnetic resonance imaging or computed tomography is beneficial in such cases; in particular, these imaging modalities can help detect lesions in the distal limb region (1,7).

Horses with SCLs are treated either medically or surgically (10). For asymptomatic horses, conservative therapy can be applied, such as stall rest, intra-articular or lesional steroid injection, or systemic non-steroidal anti-inflammatory drugs; however, medical treatments are usually unsuccessful in symptomatic horses (10,11,12). Therefore, surgical treatments are recommended for symptomatic animals, with a success rate of 50-85% (4,10,13,14,16,18). These surgical treatments include arthroscopic debridement, grafting of cancellous bone and allogenic chondrocyte, and cortical screw fixation (4,10,13,14,16,18). Bone screw fixation reportedly promotes bone healing (6), and Santschi et al. (16) reported increased bone density inside SCLs in horses with bone screw fixation. Bone screw fixation is a reasonable treatment for horses that do not show improvement after medical procedures (11,16).

This report describes the diagnosis and screw fixation procefure performed on a horse with SCLs in the medial femoral condyle in South Korea.

A 2-year-old male Thoroughbred was brought in for evaluation due to prolonged right hindlimb lameness. The horse had been lame in its right hindlimb for 3 months; however, no definitive diagnosis was made. On the presentation, the horse showed grade 3 lameness (American Association of Equine Practitioner Lameness scale) in the right hindlimb and effusions in the right stifle and left fetlock joints. Radiography revealed lucent bone lesion (26 × 22 mm) in the medial condyle of the right femur, indicating a grade 4 subchondral cystic lesion (grading system was based on Santschi et al. (16); Fig. 1B). The owner opted for surgical treatment because of the prolonged lameness.

Figure 1. Radiographs of the right stifle. (A) Latero-medial radiography of the right stifle before surgery. (B) Dorso-palmar radiography of the right stifle before surgery. Radiolucency is circled by a dashed yellow line. (C) Dorso-palmar radiography of the right stifle immediately after transcondylar screw fixation. Radiolucency is circled by a dashed yellow line. (D) Dorso-palmar radiography of the right stifle 7 months after transcondylar screw fixation, in which the reduced of radiolucency and increased radiodensity around the screw are visible; the lesion is circled by a dashed yellow line.

Preanesthetic blood analysis revealed no significant findings. The horse received penicillin G (12,500 IU/kg IM; PPS; Daesung Microbiological Laboratory Co., Ltd., Gyeonggi-do, Korea), gentamicin sulfate (6.6 mg/kg, IV; Samwoo Gentamicin; Samwoo median, Seoul, Korea), and phenylbutazone (4.4 mg/kg, IV; Arthridine; Virbac, Seoul, Korea) as pre-medication; as well as detomidine (0.02 mg/kg, IV; Detomidine; Provet Veterinary Products, Istanbul, Turkey), diazepam (0.03 mg/kg, IV; Diazepam Inj; Samjin Pharm. Co., Ltd., Gyeonggi- do, Korea), and ketamine (2 mg/kg, IV; Ketamine Inj; Yuhan, Cheongju, Korea) to induce general anesthesia. The horse was positioned in dorsal recumbency position under general anesthesia, which was maintained with isoflurane (Ifran; Hana Pharm. Co., Ltd., Gyeonggi-do, Korea) and 100% oxygen. Mechanical ventilation was provided and blood pressure was directly monitored through a catheter inserted in the facial artery.

The skin was prepared aseptically. The stifle was extended and flexed using a hoist. The medial collateral ligament and medial femoral cartilage were located using ultrasonography and marked with skin staples. Serial lateromedial and caudocranial radiographic examinations were performed to localize the incision site using an 18-G needle as a marker. A blade was used to create a skin incision cranial to the medial collateral ligament and proximal to the medial femoral cartilage. A universal aiming guide was positioned proximal to the medial femoral cartilage and was angled from the distal portion of the medial femoral condyle to the proximal lateral tibia. A 3.2-mm drill bit was inserted into the universal aiming guide and pass through the medial intercondylar eminence centrally across the cyst. Radiography confirmedthat the tract penetrated the cyst. A 4.5 mm-self tapping cortical screw (4.5 cortex screw, self-tapping 38 mm; SYNTHES, USA) was inserted and tightened (Fig. 1C). Arthroscopy revealed no femorotibial joint lesions (Fig. 2). Skin closure was performed using Nylon 2-0. The horse recovered uneventfully with assistance.

Figure 2. Arthroscopic views of the right stifle after transcondylar screw fixation. (A) Arthroscopy shows an intact cranial cruciate ligament. (B) Arthroscopy shows a normal meniscus and cranial meniocotibial ligament. MC, medial femoral condyle; T, proximal tibial eminence; Cr L, cranial cruciate ligament; Me L, cranial meniocotibial ligament.

The horse was treated with penicillin G (12,500 IU/kg IM; PPS; Daesung Microbiological Laboratory Co., Ltd., Gyeonggi-do, Korea), gentamicin sulfate (6.6 mg/kg, IV; Samwoo Gentamicin; Samwoo median, Seoul, Korea), and phenylbutazone (4.4 mg /kg, IV; Arthridine; Virbac, Seoul, Korea) for 5 days after surgery. Exercise restriction was as follows: stall rest for the initial 2 months; then, stall rest and hand-walking for 15 min twice a day for 1 month; then, access to a small paddock for 1 month. Seven months after surgery, the horse showed no lameness, and a radiograph revealed a reduced size of SCL lucency and increased bone density around the screw (Fig. 1D). The horse returned to the training program 11 months after surgery and began to race 16 months postoperatively.

This study reports the case of a 2-year-old male Thoroughbred that had been lame in the right hindlimb for the previous 3 months before presentation. Radiography revealed a radiolucency in the medial condyle of the right femur, indicative of a grade 4 SCL, reportedly the most common shape of medial femorocondylar SCL (4,16). A self-tapping cortical screw was placed using serial radiographic examinations. Arthroscopy revealed no concurrent injuries to the femorotibial joint. The horse showed no postoperative lameness or complications and started racing after surgery.

The prognosis of SCL varies (7,10,12). Age is reportedly a salient factor in the prognosis of surgical treatments such as arthroscopic debridement and screw fixation (16,18). Horses aged below 3 years have a better prognosis than older horses (16,18). The size and location of SCLs affect prognosis (12). Additionally, horses with concurrent lesions in the proximal tibia or meniscus have a poor prognosis (3,8). In the current case, the horse was less than 3 years old, and no other injuries were found in the femorotibial joint, contributing to the return to athletic activities.

Screw removal is typically not performed after transcondylar screw fixation because of the costs and risks associated with general anesthesia. To date, no complications associated with implanted screws have been reported in horses (16). In the current study, the horse showed no complications related to the implanted screw 16 months after surgery. Hardware removal remains debated in both humans and veterinary medicine. In humans, Reith et al. (15) reported that 96% of patients who underwent hardware removal responded positively to the procedure. In that report, the survey showed pain improvement in 52% of patients and subjective functional improvement in 72%. In horses that underwent plate implantation because of fractures, plate removal resulted in favorable outcomes for surgery-related lameness and infection (5). However, hardware removal is rarely reported in equine medicine. Considering the favorable response to removal procedures in humans, additional research on screw removal for transcondylar screw fixation in horses is required.

This study reports the diagnosis and surgery of a horse with an SCL in South Korea, indicating that transcondylar screw fixation is beneficial for young horses with SCLs.

We would like to thank Eun-bee Lee, Kyung-won Park, Hyohoon Jeong, Soyeon Lee, Chang-won Jeon, and Suyeon Kim in Jeju National University Equine Hospital for assisting the whole process of the case, and Editage (www.editage.co.kr) for English language editing.

The authors have no conflicting interests.

Seyoung Lee: Conceptualization, Data curation, Formal analysis, Resources, Writing - original draft, Writing - review & editing; Masaaki Tagami: Data curation, Writing - review & editing; Jong-pil Seo: Conceptualization, Data curation, Supervision, Writing - review & editing.