Single Screw Transphyseal Bridging for Correction of Forelimb Angular Limb Deviation

Transphyseal bridging of the distal radius and distal metacarpus using a single-position screw is an efficient and effective surgical technique for the correction of angular limb deformities. Clinically, this procedure gives the impression of superior cosmetic results when compared to other transphyseal bridging techniques.

1. Introduction

In equine practice, transphyseal bridging of the equine limb is common for the correction of angular limb deviation. Historically, transphyseal screws and wires or transphyseal staples have been the most commonly employed techniques [1-5]. More recently, a technique using a single-position screw traversing the physis of the distal radius or of the distal metacarpus has been performed. The purpose of this paper is to describe the surgical technique as well as the complications, effectiveness, and duration required for the implant to achieve correction at a given age.

2. Materials and Methods

Cases were selected from the surgical records of Hagyard Davidson McGee, DVM and Paul Thorpe, DVM from January 23, 2004 to December 28, 2004. All horses undergoing transphyseal screw placement for correction of angular limb deformities of the carpus or fetlock were included in the study. A questionnaire was designed to gather pertinent information by both telephone or mail. Additional information was gathered from medical records or follow-up communications with clients.

Criteria Evaluated

Criteria evaluated included

1. severity of deviation
2. surgical site
3. age at implant placement and age at removal
4. intra-operative and post-operative complications
5. success of correction, assessed clinically and by owners satisfaction
6. cosmetics of the surgery site

Surgical Technique

Horses were anesthetized and positioned in lateral recumbency. The surgical site was clipped and prepped for aseptic surgery. A stab incision was made through the skin, SC tissues, and periosteum 1.5 cm proximal to the distal radial growth plate or 1 - 1.3 cm proximal to the distal metacarpal growth plate. The incision was placed palmar or caudal to the craniocaudal midpoint of the bone. For the distal radius, a 4.5-mm drill hole was made at the incision perpendicular to the axis of the bone and 2 - 3 mm deep. This created a shelf for the 3.2-mm drill to be directed distally at ~20° relative to the long axis of the bone; it could then cross through 2.0 cm of the physis and epiphysis. A 4.5 self-tapping cortical bone screw was inserted and tightened, and the position was confirmed radiographically. Average screw length was 48 - 56 mm (Fig. 1).

Figure 1. Screw placement to bridge the distal radius.

In younger foals (<2.5 - 3 mo), screw placement at the distal metacarpus was performed with a 3.5-mm self-tapping screw. In larger foals, 4.5-mm screws were used. The technique for screw placement was the same, except that a 4.0-mm drill was used for the shelf and a 2.5-mm drill was used for the screw channel (Fig. 2 and Fig. 3).

Figure 2. Drill orientation before screw placement.

Figure 3. Screw placement to bridge in the distal third metacarpus/metatarsus.

Carpi were bandaged with Elastikon [a] over a gauze and changed every 3 - 4 days for 2 wk. Horses were maintained in a stall for 2 wk before paddock turnout.

Fetlocks were bandaged either with Elastikon [a] or vet wrap [b] over combined dressings and the horses were kept in a stall for at least 2 wk. Duration of bandaging varied depending on the amount of reaction at the surgery site, but it generally required 3 - 4 wk.

Screw removal was accomplished through a stab incision performed while the patient was either recumbent under general anesthesia or standing using sedation and local anesthesia. The stab incisions were generally left open, and bandages were applied in the same method that the screws were placed.

3. Results

A total of 438 screws were placed in 350 horses. There were 310 screws placed in the distal radius, and 128 screws were placed in the distal metacarpus: 29.68% of the distal radial screws were placed medially, and 70.32% were placed laterally. There were 128 screws placed in the distal cannon bone; 99.22% were placed laterally, whereas 0.78% were placed medially.

The age range for implantation of screws in the distal cannon was 13 - 216 days, and the mean age was 94.09 days. Duration of time required for the implants to remain in the leg averaged 55.81 days with a range of 21 - 112 days and a median of 54 days.

The age range for implantation of screws in the distal radius was 25 - 563 days, and the mean age was 377.24 days. Duration of time required for the implants to remain in the leg averaged 38.83 days with a range of 10 - 143 days and a median of 35 days.

Criteria evaluated in the survey included severity of deviation (1, mild [1 - 2°]; 2, moderate [2 - 4°]; 3, severe [>4°]), effectiveness of the screw at correcting the deviation (1, good; 2, fair; 3, poor), and cosmetic appearance of the limb (1, good; 2, fair; 3, poor).

Severity of deviation was graded as 1 in 10.84% of patients, 2 in 62.65%, and 3 in 26.51%. Effectiveness of correction was 1 in 79.52% of patients, 2 in 16.87%, and 3 in 3.61%. Cosmetics were graded as 1 in 77.10% of patients, 2 in 14.46%, and 3 in 10.84%.

A complication encountered during surgery was difficulty in screw removal; this occurred in eight patients with 3.5-mm screws used in the distal cannon. Breakage of the screw on attempt at removal occurred in six patients; it is hypothesized that this occurred because of over-tightening of the screw at placement and/or repeated cycling effects that weakened the head-shaft interface. The broken shafts were removed using a screw extraction set provided by Synthes [c].

4. Discussion

This report documents the use of single screw transphyseal bridging in a large group of horses for correction of angular limb deviations. Although we were not able to compare cosmetic results with other techniques of transphyseal bridging, clinical impression is that this technique results in superior cosmetics. This is supported by the client observation of optimal cosmetics in 77.10% of the horses.

Surgical complications include difficulty locating the screw head, stripping of the head, and breakage of the screw. Locating the screw head may be made easier by not countersinking the head during screw placement and leaving at least one-half of the screw head above the level of bone. Stripping of the head is prevented by properly seating the screwdriver in the head and tapping with a hammer.

Screw breakage may occur if there is a callus at the perimeter of the screw head that resists removal. Also, as the bone lengthens with growth, there may be more tension on the threads of the screw. In the event of screw breakage, removal is facilitated by the use of the Synthes extraction set.

Age at the time of implant in the distal cannon averaged 94 days. Reportedly, maximum age for the transphyseal bridging of the metacarpus is 4 mo [6]. The ideal age for transphyseal bridging of the cannon seems to be ~2 mo, although some correction is achieved up to an age of 4 mo. Earlier intervention may be required if the deviation is severe [7].

The time required for limb correction varies with the severity of the deviation, age at implantation, growth potential at the physis, and underlying bone disease [3].

The ideal time for placement of screws in the distal radius has not been determined [8]. Acquired angular limb deformities at the carpus may not be evident until the animal is a late weanling or early yearling. Many are self-limiting and require no surgical intervention; therefore, it is important to monitor the patient continually to accurately decide if surgery is necessary. In our study, the age at the time of bridging of the distal radius ranged from 25 days to 18 mo. The older end of this range is greater than reported values for transphyseal bridging.

It is imperative to avoid over-correction with any implant. We have not experienced any higher incidence of over-correction with the single screw versus the other techniques of bridging. It is our clinical impression that any degree of over-compression of the physis or subsequent physeal dysplasia after implant removal will result in cessation of growth at the physis and continued deviation from collapse of the growth plate.

Footnotes

1. Elastikon, 3M Animal Care Products, Division 3M Center Bldg. 2251, St. Paul, MN 55144.
2. 3M Vet Wrap, 3M Animal Care Products, Division 3M Center Bldg. 2251, St. Paul, MN 55144.
3. Syntheses, Inc. , 1302 Wrights Lane East, West Chester, PA 19380.