How to Inject the Navicular Bursa with Ultrasonographic Control

Ultrasonography is a convenient and efficient way to control navicular bursa injection. It presents several advantages compared with radiographic control.

1. Introduction

Injection of the navicular bursa in the horse has been reported to be useful either as a diagnostic [1,2] or as a therapeutic tool [3] for chronic palmar foot pain. Radiographic control of needle positioning is recommended to assess the site of injection [2]. Unfortunately, this requires leaving the needle in place while the radiographs are acquired and processed. Because ultrasonography through the frog allows good visualization of the palmar aspect of the foot, it can be used as an effective control for needle positioning and injection in the navicular bursa.

2. Materials and Methods

Foot Preparation

The first step consists of preparing the foot as for a standard ultrasonographic examination through the frog. The frog needs to be trimmed to provide a flat and maximally wide area of contact for the ultrasonographic probe and to eliminate the dry superficial horn layers that prevent good penetration of the ultrasound. The foot is then soaked in warm water to provide a good hydration of the horn to facilitate ultrasound penetration. Soaking for 20 - 40 min is usually adequate depending on the initial horn quality. The most convenient method of soaking is, in our experience, to use a strong plastic bag (such as an IV fluid bag) filled with water and taped around the pastern. This allows the horse to be left alone in a stall during the soaking process.

The foot is cleaned, and the palmar aspect of the distal pastern down to the bulbs of the heels is clipped and surgically prepared.

Ultrasonographic Evaluation

To perform the ultrasonographic examination, the authors usually use a 7- to 10-MHz linear probe. A convex probe can be used if only a small area of contact is available. The ideal view for ultrasonographic guidance of navicular bursa injection is obtained with the probe applied in the sagittal plane on the frog. This view, described as the sagittal transcuneal scan [4], allows the veterinarian to identify the flexor surface of the navicular bone as a curved hyperechoic interface. The deep digital flexor tendon (DDFT) appears as an hypoechoic linear structure following the curve of the navicular bone and inserting on the flexor surface of the distal phalanx that appears as a rectilinear hyperechoic interface. The navicular bursa sits as a virtual space between the navicular bone and the DDFT, and it is usually not seen in normal horses. It is recommended that the image quality be checked before performing the ultrasonographic guidance, and it is particularly important to obtain a good visualization of the area palmar to the navicular bone, because this will be the path of the needle. If the image quality is not optimal, it can be improved by better trimming or further soaking.

Sedation and Restraint

Sedation is recommended to perform the procedure in quiet and safe conditions. The authors have obtained good results using acepromazine [a] (0.05 mg/kg body weight [bwt], IV), a twitch to improve restraint, and local analgesia with the use of 1 ml of analgesic solution [b] subcutaneously between the bulbs of the heels at the site of insertion of the needle.

Procedure

The authors usually perform this procedure with two operators: one holding the foot and performing the ultrasonographic examination and the other, wearing sterile gloves, inserting the needle and making the injection. A 3.5-in (8.9-cm), 20-g spinal needle is inserted between the bulbs of the heels in the sagittal plane 1 cm above the coronary band; it is directed toward the "navicular position" described as the projection in the sagittal plane of a point 1 cm distal to the coronary band and halfway between dorsal and palmar aspect of the foot [5]. For safety reasons, the authors prefer not having the probe on the foot at the moment of needle insertion in case the horse reacts violently. The ultrasonographic examination can begin as soon as the needle has gone through the skin (Fig. 1). The needle is seen as a hyperechoic line entering the palmar aspect of the digital cushion. An accurate position is obtained when the tip of the needle has crossed the DDFT and contacts the flexor surface of the navicular bone (Fig. 2). The bursa can then be injected. During adequate injection, flow can be seen at the tip of the needle, and progressive dilation of the bursa is visualized as an anechoic area that appears between the DDFT and the flexor surface of the navicular bone with the injection of 3 - 5 ml. It is sometimes necessary to release the pressure applied with the probe on the frog to decrease resistance to injection.

Figure 1. Ultrasonographic examination of the foot through the frog to confirm the needle positioning.

Figure 2. Sagittal ultrasonographic image of the foot through the frog. The needle (arrow) is seen as a hyperechoic line. It crosses the DDFT and contacts the flexor surface of the navicular bone, which shows adequate positioning for navicular bursa injection. Left, dorsal; right, palmar; top, distal; 1, navicular bone; 2, distal phalanx; 3, deep digital flexor tendon; 4, digital cushion; 5, frog.

This technique was used on 10 Standardbred mares from an experimental herd [6]. Both front feet were injected with radiographic contrast medium [c] according to the described technique. A lateromedial radiograph of the foot was then obtained to assess adequate injection of the navicular bursa.

3. Results

All 20 navicular bursae were successfully injected. No complications were encountered during or after the procedure [6].

Injection of the navicular bursa with ultrasonographic control is now routinely performed with success on clinical cases at the authors' institution; it is used for navicular bursa blocks, collection of synovial fluid, and therapeutic injections. The technique has also been used successfully on hind feet.

4. Discussion

Ultrasonography of the foot through the frog is an efficient technique for control of navicular bursa injection. Ultrasonography allows real time control of needle position and assessment of the site of injection, because flow at the tip of the needle and distension of the bursa can be visualized during the injection.

The authors achieved 100% success with good visualization of the area of interest [6]. The success of the technique may depend on the quality of the ultrasonographic examination. With some horses, the image quality may be sub-optimal, especially with dry horn and/or narrow frog. In these cases, foot preparation becomes critically important.

The drawback of the technique is the time needed, ~30 min, for foot preparation (trimming and soaking). However, this preparation is the standard preparation for foot ultrasonographic examination and does not necessarily add time if ultrasonographic examination is already included in the lameness evaluation. The technique was described using two operators, but if a single operator is available, ultrasonography can still be used after needle positioning to check needle placement. In this case, the advantage of real time positioning is not obtained, but ultrasonography remains an efficient tool to assess needle positioning before injection.

Compared with radiographic control, ultrasonography allows real time control of needle positioning and site of injection. Additionally, it avoids leaving the needle in place for the long time needed to obtain a radiograph, and it can be used in field practice when radiographic processing equipment is not available onsite.

Footnotes

1. Atravet, Ayerst, Guelph, Ontario, Canada.
2. Lidocaine 2%, Vetoquinol, Lavaltrie, Quebec, Canada.
3. Hypaque-M, Nycomed, Brampton, Ontario, Canada.