Osteochondral allograft transplantation for treating medial femoral condyle subchondral bone cyst in a 14-year-old standardbred horse: a case report

Abstract

Allograft arthroplasty is a promising cartilage-resurfacing technique. A 14-year-old horse was diagnosed with a medial femoral condyle subchondral bone cyst. Allografts were harvested from a young donor animal and implanted to fill the cyst cavity. A visual assessment of the surgical site was possible during follow-up arthroscopy. In addition to a desirable gliding surface and a good adaptation of the grafts, fibrillation of the cranial ligament of the medial meniscus was noted and debrided. The recipient horse became sound four months after follow-up surgery with a high level of owner satisfaction. In conclusion allograft transplantation is a promising technique for treating subchondral bone cysts.

INTRODUCTION

Subchondral bone cysts (SBCs) frequently cause lameness in equine athletes [1]. In horses, the medial femoral condyle is the most common location for SBCs, which may lead to persistent pain, meniscal and cartilage damage and eventually osteoarthritic changes in the medial femorotibial joint [2,3]. Managing these lesions still poses a challenge in equine surgery [1].

Reconstruction of the weight-bearing articular surface together with the adjacent subchondral bone can be achieved by autologous mosaic arthroplasty. Horses older than 11 years are not good candidates for harvesting autografts because of their rigidity and poor quality [4,5]. Although osteochondral allograft transplantation is applied routinely in human surgery [6], there is limited experience in horses. Experimental studies on resurfacing the equine metatarsophalangeal joint [7] or the radial facet of the third carpal bone [8] were carried out with variable outcomes. This article describes allograft arthroplasty in a clinical case.

CASE PRESENTATION

A 14-year-old Standardbred gelding was referred to the author’s clinic with a history of right hind limb lameness of several months duration. The horse raced successfully until the age of 10 years and was subsequently used for pleasure riding. At presentation, the horse showed grade 3/5 (American Association of Equine Practitioners [AAEP]) right hind limb lameness in a straight line at a trot and revealed marked distension of the medial femorotibial joint on physical examination. Slight asymmetry with atrophy of the gluteal muscles on the affected side was observed. The flexion test of the stifle revealed moderately exacerbated lameness. With intrasynovial anesthesia, the pain was localized to the medial femorotibial joint. Radiographs revealed a large SBC in the medial femoral condyle. Modelling of the proximomedial aspect of the tibial plateau and periarticular osteophyte formation were also noted (Fig. 1).

Fig. 1. Preoperative radiographs of the affected right stifle. In the caudocranial oblique (B) view, a radiolucent area is visible in the subchondral bone of the medial femoral condyle (black arrows). Marginal osteophytes on the proximomedial aspect of the tibia and elongation of the tibial plateau indicating osteoarthritic changes are also present.

Allografts were harvested from an 11-month-old colt belonging to the same owner. The colt underwent umbilical hernia repair and, with the owner’s written consent, was employed as the donor animal during the same general anesthesia. Two osteochondral grafts 8.5 mm in diameter and 30 mm in length and a single graft 6.5 mm in diameter and 30 mm in length were harvested under arthroscopic control from both medial femoral trochleae with a tubular chisel using a routine mosaicplasty technique [4,9]. The grafts were rinsed with a high-pressure lavage using sterile saline solution through an 18-gauge needle and were stored in isotonic saline-soaked swabs for two hours at room temperature in the sterile operating theatre until insertion.

Before implantation, the recipient animal received amoxicillin trihydrate and clavulanic acid (Synulox RTU, Norbrook Laboratories Ltd., Northern Ireland) (5.6 mg/kg and 1.4 mg/kg im), gentamycin sulfate (Gentaject 10% inj., Franklin Pharmaceuticals Ltd., Ireland) (6.6 mg/kg iv), and phenylbutazone (CP-Phenylbutazon inj., Medicus Partner Kft., Hungary) (2.2 mg/kg iv). The recipient horse was positioned in dorsal recumbency during general anesthesia with the right stifle in flexion and prepared aseptically. The medial femoral condyle was exposed through a 6-cm-long arthrotomy wound. An 8.5-mm hole was drilled into the center of the cyst, 6–8 mm deeper than the base of the structure. The drilled hole was examined using arthroscopic equipment, and the remaining inside lining of the cyst was removed by curettage followed by rigorous flushing. Two osteochondral grafts of 8.5 mm diameter and one implant of 6.5 mm diameter were transplanted, replacing the cyst cavity (Fig. 2). The wound was closed in five layers, as described previously [10].

Fig. 2. Intraoperative picture showing two osteochondral grafts of 8.5 mm and a single graft of 6.5 mm diameter transplanted to fill the cyst cavity (A). During follow-up arthroscopy, 10 months after transplantation, a smooth flexible gliding surface could be observed with good adaptation of the grafts (B).

The donor horse received phenylbutazone (CP-Phenylbutazon gel; Medicus Partner Kft.) (1.1 mg/kg po) for seven days postoperatively. Regular monitoring, lameness examinations and physical examinations were performed daily in the first two postoperative weeks. The horse’s condition was assessed once each week for up to four weeks postoperatively. The patient was sound at a walk in the postoperative period and showed mild (1/5) bilateral hindlimb lameness at a trot (AAEP) with moderate femoropatellar joint effusion, which symptoms dissolved completely four weeks postoperatively. After three weeks of box rest with daily hand-walking from the second week, the horse was turned out into a small (3 × 3 m) paddock. Twelve weeks after the operation, pasture turnout was introduced, and the owner reported the animal to be free of musculoskeletal complaints until being sold at the age of 2.

Postoperative management of the recipient horse consisted of stall confinement for 14 days with the oral administration of non-steroidal anti-inflammatory drugs (2.2 mg/kg phenylbutazone po) and coverage of the recipient wound with adhesive dressing (Animal Polster; Kruuse UK Ltd., Denmark). Systemic antibiotic treatment was continued for six days after surgery. The patient was monitored for pain during the first two weeks, followed by daily hand-walking up to 12 weeks after intra-articular medication of the right medial femorotibial joint with 15 mg triamcinolone acetonide and hyaluronic acid was performed by the referring veterinarian. After this period, the horse was turned out into a small paddock. According to the owner, the horse's condition improved permanently during the first four months and was claimed to be more comfortable than before surgery. The surgical wounds healed by primary intention with an excellent cosmetic result. Slight asymmetry due to atrophy of the gluteal muscles of the affected side could no longer be observed.

The recipient horse showed moderate lameness (2/5) on the right hind limb six months after surgery. Lameness was localized to the medial femorotibial joint with intra-articular analgesia, and the horse was referred to the clinic for further evaluation. Radiographs taken 10 months after surgery showed that the remaining cyst had reduced to approximately 1/4 of its original size. No deterioration of the osteoarthritic changes could be detected (Fig. 3).

Fig. 3. Postoperative lateromedial (A) and caudocranial (B) oblique radiographs of the recipient's right stifle taken 10 months following surgery. A small isolated radiolucent area is still visible in the medial femoral condyle in the caudocranial view (B). No deterioration nor the osteoarthritic change is present.

Follow-up arthroscopy of the recipient medial femorotibial joint was performed 10 months following the first operation. Extensive fibrillation of the cranial ligament of the medial meniscus was observed. The implanted grafts resulted in an even, intact, gliding surface with minimal fibrillation of one host–graft interface on the craniomedial aspect of the medially implanted 8.5-mm graft. The medial aspect of this graft showed fibrillation at the interface between the host and transplanted cartilage, while the lateral side was slightly below the level of the articular surface. The area between the transplanted cartilage cups disappeared during the 10-month period. The transplanted cartilage was functional and congruent with the surrounding cartilage (Fig. 2). Debridement of the injured ligament and fibrillated interface was performed with an arthroshaver (Drillcut-X Arthro shaver, aggressive full radius resector blade 3.5 mm diameter, Karl Storz). After follow-up arthroscopy, routine two-month postoperative care consisted of oral phenylbutazone administration (CP-Phenylbutazon gel; Medicus Partner Kft.) (1.1 mg/kg po) during the first week. Box rest was allowed until suture removal at postoperative day 14 and then daily hand-walking until postoperative week 4 when paddock exercise was started. Four months later, the owner reported that the horse was comfortable and started regular exercise under saddle with a high level of owner satisfaction.

The donor horse was trained under saddle after the age of three years and since then has been ridden regularly by junior riders and competing on amateur levels of show jumping with no history of lameness.

DISCUSSION

Mosaic arthroplasty using osteochondral allografts is a promising surgical alternative for resurfacing full-thickness cartilage defects and filling SBCs in the horse. The importance of perfect host and donor cartilage matching, as well as gentle insertion, cannot be overemphasized. In the present case, a SBC was diagnosed in a 14-year-old animal, where autograft transplantation is not advised [5]. This technique was adapted to the equine joint based on the successful transplantation of allogenous osteochondral grafts in human orthopedic surgery [6,11].

Using a healthy animal as a donor for allograft harvesting might raise ethical issues. Fresh cadavers may serve as donors for allograft transplantation, providing a copious source of grafts with appropriate size and surface, posing no donor site morbidity. Because the distal physis of the femur closes at approximately 24–30 months of age [12], animals under 18 months may be ideal candidates for allograft harvest because of the improved repair and adaptation capacity of multipotent stem cells in the transplanted growing cartilage. The donor channels are filled completely with trabecular bone and covered partially or entirely with hyaline cartilage, hyaline-like cartilage or fibrocartilage 12 months after graft harvest [10]. In the present case, the donor animal showed mild femoropatellar joint effusion and slight stiffness in the first four weeks postoperatively, which was resolved completely after this period with no lameness present. This finding correlates with previous observations on more than 60 operated donor femoropatellar joints, where no remaining long-term complications, lameness, or cosmetic failure were present. General anesthesia of the donor animal was necessary for umbilical hernia repair.

At 10 months after surgery, recurring lameness was localized to the transplanted joint. Follow-up arthroscopy was recommended to evaluate the operated medial femorotibial joint. The interface between the implanted grafts was visible only in particular areas. This finding may be explained by hyaline cartilage flow, as described previously [13]. Recurrent lameness could be explained by fibrillation of the cranial meniscal ligament. Concurrent or sequential development of medial meniscal and subchondral cystic lesions affecting the weight-bearing articular surface of the medial femoral condyle has been reported [14].

The cyst decreased in size radiographically over 10 months. On the other hand, approximately one-fourth of the radiolucent area remained in the medial femoral condyle. The cyst was not filled completely, or a smaller-sized cyst could have developed at the interface between the host and the transplanted bone.

Good osteochondral graft adaptation, reduction in cyst size and resolution of lameness were experienced after the interventions, which enabled this horse to perform again under saddle with a high level of owner satisfaction. The use of juvenile osteochondral allografts for treating subchondral cystic lesions of the medial femorotibial joint in older horses can be a promising alternative for the future, even though more clinical cases are needed to obtain sound evidence of this surgical option. To the best of the authors’ knowledge, this is the first article reporting allograft transplantation to treat a subchondral cystic lesion in an equine patient.