Abstract: Diaphyseal third metacarpal fractures are common injuries in horse. The purpose of this study was to determine the strain values from proximal to distal on the medial and lateral regions of Mc₃ diaphysis. Left Mc₃ 256 mm length was used. Strain gages were located on the lateral and medial regions of the bone diaphysis. Diaphyseal length was 204 mm and strain gauges were spaced at 4 cm intervals. Strain gauges were numbered from proximal to distal. The 500-1000-1200-1600 ve 1800 N forces were applied on the bone with using computer aided Instron 4301 instrument with 5 kN capacity. Strains on the lateral side were observed to be compressive and on the medial side they were tensile. Compressive strains on lateral regions were significantly higher than tensile strains on medial regions. Maximum strain values were found on the distal ends in both sides of the diaphysis and distal lateral end of the bone has maximum strain.

INTRODUCTION

Third Metacarpal (Mc₃) bone fractures in horses is one of the most common stress fractures (Mckee, 1995; Gibson et al., 2006). Diaphyseal fractures of the Mc₃ bone compose 22% of all horse limb fractures. Because of the limited soft tissue envelope in this area, open fractures are common, frequently the result of the sharp fracture fragments penetrating the skin (Mckee, 1995; Watkins, 2006). The 12% of those racehorses develop stress fractures at the dorsal or dorsolateral aspect of Mc₃ during the racing (Nunamaker, 1996). Pain and inflammation on the dorsal surface of the Mc₃ referred to as shin soreness is most common in racehorses. Shin soreness develops as a result of increased strain on the Mc₃ from training in high speed of young horses, concussions and contusions (Nunamaker, 2000). Fractures of the condyles are the most common long bone fractures of the Mc₃ and Mt₃ in horses in training (Ferraro, 1990; Ellis, 1994). For these reasons Mc₃ and Mt₃ bones of the horse is the most studied bone for its structural characteristics (Nunamaker et al., 1989; Stover et al., 1992), mechanical features (Turner et al., 1975; Biewener et al., 1983; Nunamaker et al., 1990; Les et al., 1997) and stiffness (Nunamaker et al., 1991; Gibson et al., 1995).

The goal of the present study were to determine the resistance from proximal to distal on the medial and lateral regions of Mc₃ diaphysis.

MATERIALS AND METHODS

In the study, left Mc₃ was obtained from a native horse of with the weight 350 kg that was brought to the Department of Anatomy as a course cadaver and having no problem related to locomotor system was used. The bone was removed from the body and stored in 0.9% saline solution at -20°C until required (Les et al., 2002).

This was fallowed by the attachment of strain gauges onto medial and lateral sides of bones. Five strain gauges were used for each sides of bone (Micro-Measurements EA-06-062AK-120 ohm resistance, gage factor of 2.035). The length of the bone was 256 mm and diaphyseal length was 204 mm and strain gauges were spaced at 4 cm intervals. Strain gauges were numbered from proximal to distal.

Placement sites were gently curreted before the strain gauges were attached. The surface of the bones were cleaned and degreased with ether. Gauges were glued to the bone with epoxy resin (M-Bond Adhesive Resin, Micro-Measurement) and kept in position by manual pressure for 1 min. Thereafter, the cables to be used for connecting the strain gauges and terminals with the measurement set were soldered to strain gauges and terminals. With this purpose, three cables with 1 m length were used for each strain gage-terminal pair. Strain gauges and terminals were covered with silicone for protection from external effects because of the sensitivity of all systems. After that the bone were fixed at their distal parts by polyester molding in order to provide the limit conditions (Fig. 1).

After this last stage, compression experiments were started on the bones. The 500-1000-1200-1600 ve 1800 N forces were applied on the bones. The trials were carried out using computer-aided Instron 4301 instrument with 5 kN capacity. Changes in the zone in which strain gage was attached as a result of the force applied were determined numerically using strain gage measurement set.

Statistical analysis: The statistical procedures were done by using the Least Square Means (LSM-PROG GLM) of the statistical analysis software program SAS. The p-values of 0.05 or less were considered significant.

Fig. 1:	Applied pressures to sample. Computer-aided Instron 4301 instrument and strain gage measurement set were used

RESULTS AND DISCUSSION

In this study left Mc₃ bone from one native horse were used. Exercise histories were not available. Strains recorded from the strain gauges placed to the diaphysis of the bone. Table 1 and 2 show strain results for the axial compression tests and strain changes are shown in Fig. 2 and 3.

The values of strain obtained from tests were negative and positive in lateral and medial sides, respectively suggesting that strains on the lateral side were observed to be compressive and on the medial side they were tensile.

It was also observed that under all loading conditions strain values on lateral side were higher than strain values on medial side for all strain gages. Maximum strain values were found on the distal ends in both sides of the diaphysis (p = 0.0001) and distal lateral end of the bone has maximum strain.

Strain values were increasing from proximal to distal. Surface bone strains increased with the increasing load and strain values on the distal area were found significantly (p<0.001) higher than on the proximal area. There were no statistically differences between strain values from SG #1 and SG #3 for 500, 1000, 1400, 1600 and 1800 and from SG #3 and SG #4 for 1200 N on lateral side. There were significant differences between other strain gages in all lateral side (p = 0.0001) (Table 3). There were no statistically differences between strain values from SG#2 and SG#3 for 500 N on medial side. There were significant differences between other strain gages in all medial side (p = 0.0001) (Table 4).

There were no meaningful changes between strain values obtained from strain gages from proximal to distal on the medial side.

Table 1:	Strain values on medial side of the diaphysis of Mc3 bone
*Exp: Experiment, N: Newton. The negative sign (-) signifies compressive loads

Table 2:	Strain values on lateral side of the diaphysis of Mc3 bone
*Exp: Experiment, N: Newton

Table 3:	Mean strain values and standard deviations (mean±SD) for medial diphyseal region of Mc3
*a-d: Different letters within the same column indicate significant differences among the groups (p = 0.0001) **** p = 0.0001

Table 4:	Mean strain values and standard deviations (mean±SD) for lateral diphyseal region of Mc3
*a-d: Different letters within the same column indicate significant differences among the groups (p = 0.0001); ****p = 0.0001

Fig. 2:	Changing strain values on lateral side of the diaphysis of Mc3 bone. *SG1-2-3-4-5: Strain gage 1-2-3-4 and 5

Fig. 3:	Changing strain values on medial side of the diaphysis of Mc3 bone. *SG1-2-3-4-5: Strain gage 1-2-3-4 and 5

Compressive strains on lateral regions were significantly (p<0.001) higher than tensile strains on medial regions. Diaphyseal Mc₃ bone fractures are common injuries in horse. Epidemiological studies have shown that about 70% of young Thoroughbreds in race training bucked their shins and that about 12% of those racehorses develop stress fractures at the dorsal or dorsolateral aspect of Mc₃ during the following racing season (Norwood, 1978; Nunamaker, 1996; Carstanjen et al., 2003).

Strains on the lateral side were observed to be compressive and on the medial side they were tensile. Mc₃ bone is loaded axially (Watkins, 2006; Merritt et al., 2010). The medial and dorsal regions have less tensile strain resistance than the other regions in Mc₃ (Turner et al., 1975; Ozyigit and Ozcan, 2010). Lateral region has the greatest elastic modulus and static strength than medial region (Gibson et al., 1995, 2006). The lateral cortex was strongest and stiffest (Martin et al., 1997). Lateral region also has the highest longitudinal fiber values than medial region (Martin et al., 1996). The dorsomedial aspect is also the most common site for stress fractures in Thoroughbred racehorses (Nunamaker et al., 1991). Differences in crack growth initiation toughness demonstrate that the lateral bone tissue is more resistant to crack growth initiation than is tissue in the dorsal region (Malik et al., 2003). Strains on the medial side were observed to be tensile. This region is the critacal region for fractures due to the bones had low resistance to tensile strain. Bone is strongest in compression and weakest in tension (Martin et al., 1996; Nunamaker, 2002).

Third metacarpal compact bone shows variation in thickness from proximal to distal. Compact region of Mc₃ is observed to be thicker in the middle of the body but thinner in the terminals (Carter and Hayes, 1976). Local variation in equine metacarpal cortical material has also been demonstrated, both along the proximodistal axis and regionally within a proximodistal level. Areas of higher bone mineral densities and lower fracture toughness have been found in the proximal diaphysis The bone stiffness increases with the load rate (Hanson et al., 1995; Les et al., 2002). The researchers observed that surface bone strains increased with the increasing load and strain values on the distal area were found significantly higher than on the proximal area.

In this study maximum strain values were found on the distal ends in both sides of the diaphysis and distal lateral end of the bone has maximum strain. This strain may extend distally into the lateral condyle. Fractures of the lateral condyle are most common than fractures of the medial condyle of Mc₃ bone in horses (Mckee, 1995; Parkin et al., 2004, 2006).

CONCLUSION

In the study, tensile strains on medial regions were significantly higher than compressive strains on lateral regions and maximum strain values were found on the distal lateral end of the bone. Mc₃ bone is loaded mainly in axial compression and as a results of the present study medial diaphyseal side and lateral distal diaphyseal end of a Mc₃ bone can be critical sides for diaphyseal deformation.