How to Perform Ultrasound-Guided Injection of Corticosteroids Into Subchondral Bone Cysts of the Medial Femoral Condyle in the Standing Horse

1. Introduction

The most common location for subchondral bone cysts (SBCs) in the horse is the medial femoral condyle (MFC).¹  SBCs are often bilateral² and are thought to arise either as a manifestation of osteochondrosis or as a consequence of local trauma.³  Horses are generally presented for evaluation and treatment of MFC SBCs in one of two scenarios:

1. The horse is lame and the lameness has been localized to the medial femorotibial joint by intra-articular analgesia, with subsequent radiologic identification of a SBC
2. A young horse is presented following prepurchase or survey radiographs, which identified a SBC in a horse, which may not be lame

In either situation, radiographic confirmation typically remains the mainstay for diagnosis of MFC SBCs.⁴ The use of ultrasound for the diagnosis of medial condyle SBCs has, however, been described in the standing horse.⁵ This modality has the advantage of allowing rapid assessment of joint, ligament, meniscus, and other soft tissues, which, in addition to evaluation of the SBC, are prognostically important.^5–7 Ultrasound of the equine stifle can be easily performed using widely available portable ultrasound equipment and a linear probe.^6,8 Concurrent radiographic and ultrasonographic examination in cases of MFC SBCs is often recommended.⁵ 

Several conservative treatment options have been proposed for treating MFC SBCs including restricted exercise and intra-articular steroid injection. Other techniques, which are more invasive and require general anesthesia and surgical facilities include: arthroscopic enucleation and forage^9,10, and cancellous bone grafting.¹¹ Such procedures can, however, lead to medial meniscal injuries,¹² possible postoperative enlargement of the cyst, and disruption to the articulator surface.¹³ They may also require a long convalescence.⁹ Additionally, debridement of lesions greater than 15 mm wide is associated with a poor prognosis.¹⁰

Other studies have focused on reconstruction of the SBC using platelet-rich-plasma (PRP) and/or cultured stem cells¹⁴ or the use of transcondylar lag screws.¹⁵ The aforementioned techniques require general anesthesia in a surgical facility.

Arthroscopic-guided injection of corticosteroids directly into the fibrous cyst lining has been recommended¹³ to attenuate the inflammatory process associated with cytokines within the cystic lining.¹⁶ The cytokine production is thought to contribute to lameness and bone resorption. It is hypothesized that reduction in the production of such inflammatory mediators may aid resolution of the SBC. Treatment via direct infiltration with corticosteroids yielded a success rate of 77%.¹ The procedure resulted in faster recovery times and similar or improved results over surgical intervention. However, arthroscopic-guided injection of SBCs requires a surgical facility, general anesthesia, expensive arthroscopic equipment, and may represent considerable expense to the client.

A less invasive ultrasound-guided injection of corticosteroids directly into the fibrous cyst lining under general anesthesia has been used with success to alleviate lameness arising from SBCs.^1,14 However, the materials and methods of the technique have not been reported. Additionally, application of this technique in standing horses has not been described in the literature. Ultrasound-guided injection of corticosteroid into MFC SBCs in the standing patient may represent advantages over existing treatment methods including: rapid treatment time, avoidance of general anesthesia, reduced cost, and ability to treat patients on the farm. The widespread availability of ultrasound equipment and the relatively simple ultrasonographic anatomy make this technique a useful alternative for treatment of SBCs in the MFC in appropriately selected cases.

The purpose of this paper is to describe the technique used to identify and treat femoral SBCs in the standing horse using ultrasound guidance.

2. Materials and Methods

Evaluation of the MFC and SBC

A standard series of stifle radiographs should be obtained¹⁷ prior to ultrasonographic evaluation and treatment, with the caudo 20 lateral-craniomedial oblique, flexed lateromedial and caudocranial views being of particular importance (Fig. 1). These lesions are often bilateral, so images of both stifles should be obtained.

Following sedation, a wide area around the medial femorotibial joint should be clipped with a #40 blade as the overlying skin will move considerably when the limb is flexed. A routine ultrasonographic examination of the stifle should be performed to identify associated soft tissue injuries.⁶ Most SBCs are present on the weight-bearing surface of the cranial aspect of the MFC.¹⁸ Ultrasonographic evaluation and treatment of the MFC lesion must be performed with the stifle flexed. Care should be taken when imaging the MFC with the limb flexed in fractious horses; appropriate sedation is recommended as is the help of a capable assistant to maintain the limb in the flexed position. To evaluate the MFC SBC, the hindlimb is lifted to maintain stifle flexion at 90°. The foot can either be placed on a wedge block or placed on a hoof jack (Fig. 2).
 

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