INTRODUCTION

Navicular bones or the distal sesamoids of the equine forelimbs are canoe-shaped small bones with a horizontally-oriented long axis (Daradka, 2000; Denoix, 2000; Waguespack and Hanson, 2010). The bone is supported in its position on the palmar aspect of the distal interphalangeal joint by several ligaments (Daradka, 2000; Denoix, 2000; Waguespack and Hanson, 2010). Firstly, the Navicular Bone Suspensory Ligament (NBSL) which is a thick horizontal band runs along the palmer edge of the navicular bone with medial and lateral branches that extends proximally on each side of the second phalanx and insert on the distal end of the first phalanx (Daradka, 2000; Denoix, 2000; Waguespack and Hanson, 2010). Secondly, the collateral ligaments of the distal interphalangeal joint (which includes, a medial and lateral ligament) and thirdly, the impar ligament (Daradka, 2000; Denoix, 2000; Waguespack and Hanson, 2010). Navicular bones provide constant insertion angle to the deep digital flexor tendon into the third phalanx and provide additional functional area to the articular surface of the distal interphalangeal joint (Daradka, 2000; Denoix, 2000; Waguespack and Hanson, 2010).

Navicular Bone Suspensory Ligament (NBSL) desmotomy is a common surgical procedure that is used to alleviate pain related to navicular diseases (Wright, 1993; Smith et al., 2007; Waguespack and Hanson, 2010; Auer and Stick, 2011). Wright (1993) reported results in 118 horses treated by NBSL desmotomy and found at 6-month follow up that 76% of the horses were sound. The aim of this study was to investigate the effects of desmotomy of the Navicular Bone Suspensory Ligament (NBSL) on radio-morphometric features and on remodeling of the navicular bone in normal donkeys.

MATERIAL AND METHOD

The experimental protocol used in this study was approved by Animal care and use committee at Jordan University of Science and Technology (No. 0043/2002).

Twenty normal adult, local breed donkeys (Equus asinus) aged between 5 and 7 years and weighing 120-150 kg were used in the experiment. The donkeys were housed in one group at the veterinary health center of the Faculty of Veterinary Medicine at Jordan University of Science and Technology in a fenced yard with sheltered area available. They were fed 3 kg barley/day/animal and chopped wheat hay adlib with free access to fresh water.

All donkeys were clinically healthy, free of lameness or any hoof wall or conformational abnormalities. All animals' hooves were trimmed, cleaned and their frog's central and collateral sulci packed using dough and covered by transparent adhesive tape prior to radiographic imaging. Three radiographic views (lateromedial, dorsopalmar and palmaroproximal- palmarodistal oblique) were obtained for all animals as described before Denoix (2000), Flavio et al. (2003). Experimental animals were divided randomly into two groups: Group 1 (n = 5) used as controls. Group 2 (n = 15), used for desmotomy, all animals were euthanized twelve months after surgery. Experimental animals were fasted for 12 h prior to surgery. The right foreleg was clipped from the carpal joint to the coronary band and hooves were trimmed and cleaned before surgery. Animals were premedicated using 1.1 mg kg^–1 xylazine hydrochloride (Xylaject, Egypt) intravenously. General anesthesia was then induced by intravenous injection of 2.2 mg kg^–1 ketamine (ketamine, Holland) and maintained by continuous intravenous injection of 10% solution of thiopental sodium at a dose of 1 g/90 kg h^–1 (Pentothal, Italy). The hooves of the right forelimb and the area distal to the carpal joint were prepared for routine aseptic surgery. A tourniquet was then applied at the distal end of metacarpus and medial and lateral desmotomy of the navicular bone suspensory ligaments was performed as described by Wright (1993). Briefly, a 2 cm straight vertical skin incision was made midway between the common digital extensor tendon and the alar cartilage. The incision was extended approximately 5 mm above the coronary band to the level of the eminence on the proximal phalanx. After correct identification of ligament, a small curved hemostat was passed in a dorsopalmar direction under the ligament and the ligament is severed using scalpel blade. The subcutaneous tissues and skin incision were then closed routinely. The second navicular suspensory ligament in the same limb was desmotomized in the same way. A sterile non adhesive was bandage was applied to the surgical site and changed every 48 h for 12 days. Sutures were removed after 21 days. Hand-walking exercise for 20 min was performed 24 h after surgery and gradually increased up to one hour for 3 weeks. Animals were treated postoperatively with penicillin/streptomycin (pen-strep, Norbrook Laboratories) at 10,000-30,000 IU kg^–1, IM, Bid and gentamycin (Alfamycin, AlfaMedic) at 3-6 mg kg^–1, IV, Bid for 5 days. Pain due to the surgery was managed by administering flunixin meglumine, (Finadyne, Schering Plough Animal Health) at 1.1 mg kg^–1, IV daily for 10 days. All animals were administered anti tetanus prophylaxis before surgery.

Three radiographic views (lateromedial, dorsopalmar and palmaroproximal- palmarodistal oblique) were obtained for all animals including the controls. Radiographs were taken while the animal is standing according to the following schedule: before surgery, post-operatively and then every 4 weeks for 12 months. Radiographic images of the navicular bone in experimental animals and controls were evaluated for the presence of abnormal bone remodeling at different time points following the surgery. In addition, the following radiometric measurements were obtained (Fig. 1): Articular surface width (D1), which was obtained from the dorsopalmar view, flexor surface width (D2), which was obtained from the palmaroproximal-palmarodistal oblique view, length of the bone (D3), which was obtained from the dorsopalmar view and the flexor surface thickness (D4), which was obtained from the lateromedial view.

Fig. 1:	A photograph showing the distal and palmer aspect of navicular bone, D1 = Articular surface width, D2 = Flexor surface width, D3 = Length of the bone, D4 = Flexor surface thickness

Radiographs were scanned into computer software and the measurements were obtained using digital imaging technique (Image J, National Institutes of Health, USA). Individual measurements were recorded in millimeters. At the end of the experiment (12 months after surgery), all animals were euthanized using an injectable anesthetic overdose. The forelimbs were collected and the hoof capsule was removed as described previously (Daradka, 2000). The Deep Digital Flexor Tendon (DDFT) was exposed by sagittal incision of the frog and digital cushion. Then DDFT was dissected away to expose the third phalanx. The navicular bone was then exposed entirely and the suspensory ligaments were identified and dissected. The Impar ligaments were incised from their third phalanx insertion and the collateral ligaments dissected from the second phalanx, then navicular bone was reflected proximally. The navicular bones were cleaned, dried and gross morphometric measurements of D1, D2, D3 and D4 were obtained (Fig. 1). Measurements were obtained using a caliper and recorded in millimeter.

Student t-test was performed to examine the mean difference of bone measurements before and after desmotomy. Statistical analysis was performed using SPSS version 10.0 computer program (SPSS, Chicago, IL). Differences were considered statistically significant at p value <0.05.