Abstract: Fractures of metacarpal and metatarsal bones are very common in horses. The purpose of this study was to evaluate the cross-sections and the cross-sectional moment of inertia and to determine the resistance to bending moments from proximal to distal on diaphyseal regions of Mc₃ and Mt₃ bones. Computed tomography images were taken from 10 mm apart cross sections of the diaphyses of bones. Cross sectional surface area and moment of inertia were estimated and variations were observed from proximal to the distal of the diaphyses. The results indicate that Mt₃ has more strong construct than Mc₃ especially in dorsal and plantar bendings but has more resistance to mediolateral bendings than Mc₃.

INTRODUCTION

Bone Fractures, particularly the limb fractures, in horses usually occur either as a result of direct trauma from a fall, kick or knock or during strenuous exercise. Diaphyseal fractures of the third metacarpal bone (Mc₃) compose 22% of all horse limb fractures. This ratio increases 33% when the diaphyseal fractures of the third Metatarsal bone (Mt₃) are included (Nunamaker et al., 1989). 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 (McIlwraith, 1987; Nunamaker, 2000) 12% of those racehorses develop stress fractures at the dorsal or dorsolateral aspect of Mc₃ during the racing (Nunamaker, 1996).

Fractures of the condyles are the most common long bone fractures of the Mc₃ and Mt₃ in horses in training (Ellis, 1994; Ferraro, 1990). 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 (Les et al., 1997; Biewener et al., 1983; Nunamaker et al., 1990; Turner et al., 1975) and stiffness (Gibson et al., 1995; Nunamaker et al., 1991).

In the study, we aimed to evaluate the cross-sections and the cross-sectional moment of inertia and to determine the resistance to bending moments from proximal to distal on diaphyseal regions of Mc₃ and Mt₃ bones.

MATERIALS AND METHODS

In the study, Mc₃ and Mt₃ which were obtained from a native horse of with the weight of 300 kg that were brought to the Department of Anatomy as a course cadaver and having no problem related to locomotor system were used. The bones were removed from the body and metacarpal II and IV and metatarsal II and IV were also removed. The specimens were stored in 0.9% saline solution at -20°C until required (Les et al., 2002).

At first, computed tomography images were taken from Mc₃ and Mt₃. For this purpose computed tomography scans were taken 10 mm apart sections from the diaphyses of bones. The images obtained were transferred to the computer with the aid of a scanner. The edge contours were determined in Photo Editor 3.0 (Microsoft, Redmond, WA, USA) in all images. After that all images were converted to vector graphics and saved as a dxf file using with Kvec software (KK-Software, Weiden, Germany) and igs file using with DesignCad97 software. Transferring images to the ANSYS 5.5 finite element program (ANSYS Inc., Canonsburg, PA, USA) software were used for generating cross section areas from cross section images. Cross sectional surface area and moment of inertia were estimated and variations were observed from proximal to the distal of the diaphyses (Fig. 1-3).

Fig. 1:	Coordinate replacements of the bones XL: Lateral radius of the cross sectional area; XM: Medial radius of the cross sectional area; YD: Dorsal radius of the cross sectional area; YP: Palmar/Plantar radius of the cross sectional area

Fig. 2:	Computed tomography images of Mc3 and Mt3

Fig. 3:	(a) Cross sections of Mc3 from proximal to distal (Z: Distance from proximal ends of the bone), (b) Cross sections of Mt3 from proximal to distal (Z: Distance from proximal ends of the bone)

RESULTS AND DISCUSSION

The examination of the sections reveal that the cross section of Mc₃ is smaller in the proximal and larger in the distal regions when compared with Mt₃. (Fig. 4). Cross sectional areas in the proximal and distal regions were not shown differences in Mt₃. The mean diaphyseal area values were found as 493.141 mm², standart deviation were found as 53.40862 for Mc₃ whereas these values were found as 533.085 mm² and 18.67639, respectively for Mt₃ (Table 1 and 2). The mean cross sectional area values in the Mt₃ were greater and less variable than those in the Mc₃ (Fig. 4).

The cross sectional moment of inertia had smaller values on x-axis whereas showing greater values on y-axis in both bones (Fig. 4). In general, moment of inertia values were greater in Mt₃ compared to Mc₃. Moreover, moments of inertia on x-axis varied more for Mt₃ when compared with Mc₃ whereas a higher variation was observed for Mc₃ on y-axis.

In general, diaphyseal cross sectional area values were greater in Mt₃ compared to Mc₃ (Fig. 5). Cortical cross-sectional area affected to bone stiffness in long bones (Hanson et al., 1995). From this viewpoint Mt₃ has more strong construct than Mc₃ (Fig. 6).

Maximum tensile strains occurr on the palmar region of the proximal diaphysis in Mc₃ at dorsal bending. This region has lower fracture toughness (Les et al., 1997).

Overtensile strains may cause fractures at this region. Tensile strains occuring on the dorsal region of the distal diaphysis in Mt₃ at palmar bending. Dorsal region indurable to tensile strain, overstrain may cause fractures at this region (Turner et al., 1975; Martin et al., 1996).

The compressive strains occur on the dorsal region greater than the tensile strains occuring on the plantar region of the Mt₃ at dorsal bending. The bone have a high resistance to dorsal bending (Banks, 1986). Dorsal and lateral regions have more tensile strain than other regions in Mt₃ (Turner et al., 1975). The tensile strains occuring on the dorsal region greater than the compressive strains occuring on the plantar region of the Mt₃ at plantar bending. Dorsal region indurable to tensile strain, overstrain may cause fractures at this region (Turner et al., 1975; Martin et al., 1996). This bending may cause fractures at dorsal regions of proximal diaphysis of Mt₃.

Feature of the strain in medial bending was compressive and tensile on medial and lateral regions, respectively. There was converse situation in lateral bending. The finding showed that Mt₃ has more resistance to mediolateral bendings than Mc₃ (Fig. 7) (Hanson et al., 1995).

Tensile strains on lateral regions was greater than compressive strains on medial regions of the palmar surface of the proximal diaphysis in Mc₃ at medial bending. This region is the critacal region for fractures due to the bones had low resistance to tensile strain (Banks, 1986).

Fig. 4:	Cross sectional area and moment of the inertia changes of the Mc3 and Mt3. Z: Distance of the cross section from proximal ends of the bone, L: Length of the bones. (a) Cross sectional area changes of the Mc3, (b) Cross sectional area changes of the Mt3, (c) Moment of the inertia changes of the Mc3 and (d) Moment of the inertia changes of the Mt3

Table 1:	Moment of inertia and moment of inertia/cross sectional area ratio of the Mc3

Table 2:	Moment of inertia and moment of inertia/cross sectional area ratio of the Mt3
Z: Distance of the cross section from proximal ends of the bone L: Length of the third metatarsal bone, XL: Lateral radius of the cross sectional area; XM: Medial radius of the cross sectional area; YD: Dorsal radius of the cross sectional area; YP: Palmar/Plantar radius of the cross sectional area; KXL, KXM, KYD, KYP: Moment of inertia/cross sectional area ratio of the XL, XM, YD,YP

Fig. 5:	Cross sectional area changes of the Mc3 and Mt3. Z: Distance of the cross section from proximal ends of the bone, L: Length of the bones

Tensile strains on medial surface of the distal diaphysis had extreme value in Mc₃ at lateral bending.

The medial and dorsal regions have less tensile strain resistance than the other regions in Mc₃ (Turner et al., 1975). Resistance to lateral bending was greater than palmar bending of Mc₃ (Hanson et al., 1995).

Tensile strain at the lateral aspect of the bone, compressive and tensile strain dorsally and almost purely compressive strain at the medial and plantar sides (Turner et al., 1975).

Fig. 6:	Moment of resistance/cross sectional area ratio changes of the Mc3 and Mt3 with respect to x axis. Z: Distance of the cross section from proximal ends of the bone, L: Length of the bones

Tensile strains on lateral regions was greater than compressive strains on medial regions of the distal diaphysis in Mt₃ conversely Mc₃ at medial bending.

Tensile strains on medial surface was greater than compressive strains on lateral regions of the proximal diaphysis in Mt₃ at lateral bending.

Fig. 7:	Moment of resistance/cross sectional area ratio changes of the Mc3 and Mt3with respect to y axis. Z: Distance of the cross section from proximal ends of the bone, L: Length of the bones

CONCLUSION

The finding showed that Mt₃ has more strong construct than Mc₃ especially in dorsal and plantar bendings but has more resistance to mediolateral bendings than Mc₃.