How to Inject the Synovial Cavities of the Digit

1. Introduction

Arthrocentesis of the equine interphalangeal joints and synoviocentesis of the digital synovial sheath is commonly performed for diagnostic analgesia as part of a lameness examination and to medicate these structures. The techniques for arthrocentesis are the same for the forelimb and the hind limb. Restraint is achieved by applying a lip twitch to the horse. Tranquilization or sedation is rarely used for restraint when centesis of these structures is part of a lameness examination, but in some cases tranquilizing or sedating the horse may be necessary to increase the safety of the procedure. Administration of a low dose of xylazine, detomidine, or acepromazine is unlikely to interfere with gait evaluation^1–4  and in some cases may even accentuate lameness. If centesis is performed with the foot bearing weight, the contralateral limb can be held by an assistant. Some horses, however, may buckle at the carpus when the needle is inserted, causing injury if the carpus strikes the ground. 

2. Materials and Methods

Arthrocentesis of the Distal Interphalangeal Joint At least 6 approaches to the distal interphalangeal (DIP) joint have been described: the dorsal perpendicular, the dorsal parallel, the dorsal inclined, the dorsolateral, the lateral, and the palmar approaches. A 20- or 21-gauge, 1- to 1.5-inch (2.54- to 3.8-cm) needle is commonly used for arthrocentesis of the DIP joint. A 20-gauge, 3.5-inch (9-cm) spinal needle is used for the palmar approach. When using a dorsal approach to the DIP joint, the limb can be held or bearing weight (Fig 1).
 

Dorsal Perpendicular and Dorsolateral Approaches to the DIP Joint

For the dorsal perpendicular approach, the needle is inserted at the proximal edge of the coronet, approximately 0.75 inch (̴ 2 cm) lateral or medial to the midpoint of the coronet (ie, at the edge of the extensor tendon) (Fig. 2). The needle is directed distally, perpendicular to the bearing surface of the hoof.⁵ A slight variation of the dorsal perpendicular approach is the dorsolateral approach, in which the needle is inserted at the same site and directed beneath the extensor tendon, aiming for the middle sagittal plane of the foot behind the extensor process of the distal phalanx (Fig. 3).^5–9 Depth of penetration is about 1 inch (2.54 cm). Synovial fluid usually appears in the needle hub, but accuracy of needle placement can also be determined by ease of injection. After injection, the syringe may refill when pressure on the plunger is released.

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