How to Remove an Over-Countersunk or Buried Single Transphyseal Screw

1. Introduction

Angular limb disease is a common limb deformity in young horses resulting from an incongruity in growth between the medial and lateral sides of the bone. These deformities can be congenital or developmental. They are named using both the location of the origination of the deformity and the direction of the limb from that point distal (either valgus or varus). A valgus deformity is defined as a deformity in which the leg distal to the point of interest is abaxial to a line bisecting the bone proximal to the site of interest. For example, a carpal valgus is a deformity in which the leg distal to the carpus is abaxial from a line bisecting the radius. In comparison, a varus deformity is one in which the limb distal to the point of interest is axial to a line drawn through the bone proximal to the site of interest. For example, a fetlock varus is a deformity in which the pastern is axial to a line drawn through the metacarpus/metatarsus. The most common angular limb deformities are carpal valgus and fetlock varus. 

In the past, multiple procedures have been performed to correct these angular limb deformities, including hoof trimming and the use of hoof extensions. Growth acceleration procedures such as periosteal stripping and the use of iodine injected periosteally have also been used. Growth retardation surgeries such as the use of screws and wires, screws and plates, and, most recently, the use of a single transphyseal screw have also been used. The placement of a single transphyseal screw has been shown to be as effective as a screw-and-wire technique. This procedure has a more cosmetic result and has therefore become more commonly performed.

The single transphyseal screw technique involves placing a screw obliquely across the physis. This involves placing the screw at an angle into the bone. In placing a 4.5-mm transphyseal screw, the first step is to drill a bed for the screw head with a 4.5-mm drill bit. Then, using a drill guide, the operator drills across the physis with a 3.2-mm drill. A 4.5-mm screw is placed, engaging 50% to 75% of the epiphysis.

2. Methods

As with any other screw removal, both anteroposterior and lateral radiographs should be taken before surgery to assess the location of the screw head. A single skin staple can be placed in the skin in the suspected position to use as a marker to aid in locating the initial skin incision. Evaluation of these radiographs may reveal overgrowth of bone over the screw head and a lack of the screw profile protruding from the bone. In these cases, the operator should be prepared to use this technique to extract the screw (Figs. 1–5).
 

The foal is treated with broad-spectrum antibiotics and anesthetized for surgery. The foal is placed in lateral recumbence with the affected leg placed on a Mayo stand or a table leg extension with the screw head positioned upward. The sensory plate for a DR digital radiograph system is placed under the leg so that intra-operative radiographs may be taken easily. The leg is clipped, scrubbed for aseptic surgery, and draped.

A 2-cm skin incision is made in the approximate location of the screw head, based on the previously taken preoperative radiographs. The incision is continued through the subcutaneous tissue to expose the bone. A self-retaining retractor (Weitlander) is placed in the incision. 
 

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