Introduction

Proximal sesamoid bone fracture is the leading reason for euthanasia of Thoroughbred racehorses in the United States.^1,2 In racehorses, PSB fracture is believed to be the result of an interaction between fatigue damage and stress remodeling.³ Subchondral bone lesions consistent with fatigue-induced stress remodeling are associated with transverse midbody fracture of the medial PSB. Palmar lesions have also been described in association with midbody fracture of the PSBs.⁴ A subchondral lesion is often associated with the fracture line in the midbody of a fractured PSB. In horses with unilateral biaxial fracture of a medial PSB, a similar subchondral lesion has also been observed at the same midbody location of the nonfractured contralateral medial PSB. The focal subchondral lesion is generally located toward the medial (abaxial) aspect of a medial PSB; lateral PSBs have not been examined.^5,6,a,b The characteristic focal, abaxial, subchondral lesions found at the midbody of medial PSBs with transverse fractures and described in association with some abaxial avulsion fractures of lateral PSBs suggest that an anatomic mechanism may cause high subchondral bone and transverse stresses that promote lesion development and fracture in Thoroughbred racehorses.^4,6

In the forelimbs of horses, the PSBs, MC3, and P1 articulate in the metacarpophalangeal (fetlock) joint. The distal AS of the MC3 condyle has 2 distinct aspects: a dorsal articulation with P1 and a palmar articulation with the PSBs. The dorsal and palmar portions of the distal AS of the MC3 have different curvatures and are separated by the TR.^7,8,9 The dorsal portion of the distal AS of the MC3 condyle appears more round (ie, has a greater curvature) than the palmar portion.⁸ The motion of the MC3 relative to P1 is well documented at a walk, trot, canter, and gallop^{10,11,12,13,14}; however, the motion of the PSBs relative to the MC3 condyle is not well documented at any gait. At low angles of fetlock joint extension, the PSBs do not appear to move distally past the TR and articulate with their congruent ASs.¹⁵ However, when a horse gallops, the fetlock joint is extended to large angles,¹⁶ and motion of the PSBs during extreme extension of the fetlock joint is not well understood. During high-speed galloping, the proximodorsal aspect of P1 can impinge against the dorsal aspect of MC3 proximal to the condylar AS, and that movement is beyond the articulation of P1 on the MC3.¹⁷ Therefore, the PSBs are likewise expected to move beyond their normal AS with MC3 at high angles of fetlock joint extension (ie, when a horse is at a high-speed gallop). Movement of the PSBs beyond the opposing congruent AS of MC3 may alter stresses on the PSBs and cause an articular incongruity, which could contribute to the subchondral bone lesions and fracture patterns typically observed in the PSBs of Thoroughbred racehorses.

The axial borders of the medial and lateral PSBs are connected by the intersesamoidean ligament, but the 2 bones are kept separate by the wedge-shaped SR. Although not quantitatively documented, subjectively, the distal aspect of the SR appears to be wider than the proximal aspect on the palmar surface of the MC3 condyle. During extension of the fetlock joint, the asymmetry of the SR may induce relative motion (ie, distraction) between the medial and lateral PSBs. At extreme angles of fetlock joint extension, distraction between the medial and lateral PSBs might alter the stress distribution within the bones and contribute to the subchondral bone lesions observed with PSB fractures.

The purpose of the study reported here was to assess the motion of the PSBs relative to the MC3 condyle by means of in vitro biomechanical testing of cadaveric forelimbs of horses. Our goal was to elucidate a possible mechanism for the formation of subchondral bone lesions at the abaxial portion of the midbody of medial PSBs in Thoroughbred racehorses. We hypothesized that during simulation of a high-speed gallop, the midstance load sustained by the forelimb would cause the medial and lateral PSBs to move distally beyond the TR and induce mediolateral separation of those 2 bones during extension of the fetlock joint.

Materials and Methods

Biomechanical testing

All biomechanical tests were performed with a servohydraulic material testing system^c equipped with an axial-torsional load transducer.^d A translation table (405 × 400 × 40 mm) on a linear-bearing system^e was mounted on the actuator so that, when the forelimb specimen was mounted in the machine and exposed to a mechanical load, the hoof could be moved in a dorsal direction while the radius and MC3 remained approximately vertical to simulate midstance conditions.¹⁷

The proximal end of each forelimb specimen was fixed (potted) in a cylinder with polymethyl methacrylate while the limb was in a standing position.^17,f After potting, the PSBs were instrumented with 2 LVDTs spaced 20 mm apart.^g The LVDTs were rigidly attached between the medial and lateral PSBs by means of 2 threaded rods, each of which was inserted to a standard depth into the palmar aspect of a PSB abaxial to the flexor tendons and palmar to the insertion of the suspensory ligament (Figure 1). The fixed ends of the LVDTs were attached to the rod in the lateral PSB, and the spring-loaded ends of the LVDTs were free to move against a smooth plate that was attached to the rod inserted in the medial PSB. Three 3-mm-diameter lead markers were inserted into the medial cortex of the MC3, P1, middle phalanx, and medial PSB to assist with radiographic measurements. A 20-gauge 1-inch needle was inserted through the palmar aspect of the limb at the base of the lateral PSB to facilitate identification of that PSB on radiographic images.

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Dorsopalmar radiographic image (A) and photograph showing a proximal view of the palmar aspect (B) of the metacarpophalangeal (fetlock) region of an equine cadaveric forelimb that depict placement of lead markers and instrumentation of the PSBs to facilitate data collection during biomechanical testing in an in vitro study conducted to assess the motion of the PSBs relative to the MC3 condyle under physiologic loads consistent with standing (1.8 kN) and midstance walking (3.6 kN), trotting (4.5 kN), and galloping (10.5 kN). A—Three 3-mm-diameter lead markers (small white dots) were inserted into the medial cortex of the MC3, P1, middle phalanx (P2), and medial PSB to assist with radiographic measurements. Two threaded rods that were inserted to a standard depth into the palmar aspect of the medial and lateral PSBs abaxial to the flexor tendons and palmar to the insertion of the suspensory ligament as a means to instrument those bones with 2 LVDTs are also evident. B—The PSBs were instrumented with 2 LVDTs to record movement between the 2 bones during biomechanical testing. The fixed ends of the LVDTs were attached to the rod in the lateral PSB, and the spring-loaded ends of the LVDTs were free to move against a smooth plate that was attached to the rod inserted in the medial PSB. The lateral side of the limb is to the right in both images.

Citation: American Journal of Veterinary Research 82, 3; 10.2460/ajvr.82.3.198

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The cylinder was then secured to the material testing machine, and the hoof was secured to the translation table such that the radius and MC3 were aligned parallel to the axis of loading under 500 N of axial compression.¹⁷ For testing, each limb was loaded from 500 N to the target load with a 5 mm/s displacement control, held at that load for 30 seconds while lateromedial radiographs^h,i were obtained, and then unloaded back to 500 N. The displacement of the LVDTs was recorded at 20 Hz for the entire loading profile. This process was sequentially repeated for target loads of 1.8, 3.6, 4.5, and 10.5 kN, which were selected to represent the physiologic loads when a horse was standing, walking, trotting, and galloping at a high speed, respectively, as estimated on the basis of known in vivo peak vertical ground reaction forces for equine forelimbs at those postures or midstance of those gaits.^{16,17,18,19,20}

Radiographic measurements

All radiographic measurements were obtained by the same investigator (NS) who was unaware of (blinded to) the load condition and horse identification. Before performing the measurements for this study, the investigator trained on 2 complete sets of radiographs. That training consisted of the investigator performing each measurement on 3 consecutive days. The coefficient of variation was less than 5% for all measurements. All radiographic measurements recorded for the study were obtained once within a 2-week period.

The fetlock angle was the palmar angle at the junction of a line through the proximodistal axis of MC3 and a line through the proximodistal axis of P1 (Figure 2). The MC3 and P1 axes were determined by a standard method on all radiographs.

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Lateromedial radiographic image of the fetlock region of an equine forelimb under 1.8 kN of axial compression (ie, physiologic load consistent with standing) with annotations depicting the measurements obtained during the study described in Figure 1. The fetlock angle was the palmar angle at the junction of a line through the proximodistal axis of MC3 and a line through the proximodistal axis of the P1. The AS angle was the angle at the intersection of a line through the TR (star) and proximal edge of the palmar AS of the MC3 and a line through the proximal and distal edges of the AS of the lateral PSB. The PSB-TR distance was the distance between the TR and distal edge of the AS of the lateral PSB. For both AS angle and PSB-TR distance, a positive value indicated that the distal edge of the lateral PSB was proximal to the TR and a negative value indicated that the distal edge of the lateral PSB was distal to the TR. The dorsal and palmar portions of the AS of the MC3 condyle are demarcated by solid red and blue lines, respectively. The dashed red and blue circles depict the difference in curvature between the dorsal and palmar portions of the AS of the MC3 condyle.

Citation: American Journal of Veterinary Research 82, 3; 10.2460/ajvr.82.3.198

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Two measurements (angle at the intersection of a line through the proximal edge of the palmar AS of the MC3 and TR and a line through the proximal and distal edges of the AS of the lateral PSB [AS angle] and distance between the TR and distal edge of the AS of the lateral PSB [PSB-TR distance]) were used to evaluate the position of the PSBs relative to the TR in the sagittal plane (Figure 2). For both of those measurements, the sign (positive or negative) was determined on the basis of the location of the distal edge of the lateral PSB in relation to the TR; a positive value indicated that the distal edge of the lateral PSB was proximal to the TR, and a negative value indicated that the distal edge of the lateral PSB was distal to the TR.

The PSBs did not undergo pure mediolateral distraction as was hypothesized, but instead underwent rotation in the transverse plane. Specifically, the palmar aspects of the PSBs rotated away from each other during extension of the fetlock joint. Recorded data could not distinguish pure PSB rotation from PSB rotation coupled with mediolateral translation because of mathematical coupling between rotation and translation inherent in use of 1-dimensional displacement sensors. So, the full 3-D nature of the rotation could not be determined from the given data. However, it was assumed that PSB rotation was confined to a transverse plane and both PSBs rotated the same amount; thus, rotation (α) could be extracted because the palmar LVDT always displaced further than the dorsal LVDT (Figure 3). Given those assumptions, LVDT displacements were used to calculate the rotation angle (α) for 1 PSB as follows: tan(α) = [(0.5 × palmar LVDT displacement −0.5 × dorsal LVDT displacement)/(the spacing between LVDTs)]. Hence, α was used as a proxy for true PSB rotation under the given assumptions.

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Schematic illustration of the transverse plane of the distal aspect of MC3 for an equine forelimb of the study described in Figure 1 that depicts the positions of the PSBs relative to MC3 and apparent movement of the 2 LVDTs when the limb was unloaded and loaded during biomechanical testing. The black lines indicate the positions of the bones and the red lines indicate the positions of the 2 rods securing the LVDTs (LVDT rods) when the limb was unloaded. The gray lines indicate the positions of the PSBs and LVDT rods when the limb was loaded. In this study, the displacement of the palmar LVDT was always greater than the displacement of the dorsal LVDT. The known distance between the LVDTs and their relative displacements between unloading and loading of the limb were used to calculate the α for a single PSB, which was used as a proxy for the rotation angle of the PSB assuming that rotation was confined to a transverse plane and that both PSBs rotated the same amount. It is possible that the PSBs also underwent mediolateral translation, but pure PSB rotation could not be distinguished from PSB rotation coupled with mediolateral translation owing to the mathematical coupling between rotation and translation inherent in the 1-dimensional LVDT data.

Citation: American Journal of Veterinary Research 82, 3; 10.2460/ajvr.82.3.198

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Results

Target load had a significant (P < 0.001) effect on fetlock angle, and the mean fetlock angle differed significantly among the 4 target loads (Table 1). There was a strong positive correlation (partial r_s = 0.98) and a strong linear relationship (R² = 0.90) between fetlock angle and target load (Figure 4).

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Plot of fetlock angle versus load for 8 equine forelimbs that underwent biomechanical testing as described in Figure 1. Each dot represents the fetlock angle for 1 limb at the given load. The linear regression line (dashed line) had the following equation: fetlock angle = (0.0051 × load) + 209.5. The coefficient of determination (R²) was 0.90; the partial Spearman correlation between fetlock load and angle also indicated strong positive correlation (partial r_s = 0.98). The red square indicates the point at which the distal edge of the lateral PSB moved distodorsally beyond the TR as estimated from the PSB-TR distance data.

Citation: American Journal of Veterinary Research 82, 3; 10.2460/ajvr.82.3.198

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The AS angle data indicated that the PSBs moved distally then distodorsally then dorsally around the palmar portion of the MC3 condyle during extension of the fetlock joint. The AS angle decreased as the target load increased from 1.8 kN (stand) to 4.5 kN (trot) and became negative at 10.5 kN (gallop; Table 1). Target load had a significant (P < 0.001) effect on AS angle, and the mean AS angle differed significantly among the 4 target loads. There was a strong negative correlation and negative linear relationship between AS angle and target load (partial r_s = −0.96; R² = 0.85) and between AS angle and fetlock angle (partial r_s = −0.98: R² = 0.88; Figure 5).

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Scatterplots of AS angle (A) and PSB-TR distance (B) versus fetlock angle for 8 equine forelimbs when biomechanically tested at loads consistent with standing (diamonds), walking (squares), trotting (triangles), and galloping (circles) as described in Figure 1. Each panel is accompanied by representative lateromedial radiographs of the fetlock region of a forelimb obtained when the limb was loaded with 1.8 (standing) and 10.5 (galloping) kN of axial compression to demonstrate the difference between the 2 loads in regard to the position of the PSBs relative to the MC3 condyle. The radiographs have been annotated to depict measurement of the AS angle (A) and PSB-TR distance (B), respectively. For both AS angle and PSB-TR distance, negative values indicate that the distal edge of the lateral PSB had moved distal to the TR.

Citation: American Journal of Veterinary Research 82, 3; 10.2460/ajvr.82.3.198

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The PSB-TR distance data indicated that the distal edge of the lateral PSB moved dorsally past the TR at a target load consistent with a gallop. The distal edge of the lateral PSB approached the TR as the target load increased from 1.8 to 4.5 kN, which was reflected by a decrease in the PSB-TR distance. The distal edge of the lateral PSB moved distal to the TR as the target load increased from 4.5 to 10.5 kN, resulting in a negative value for PSB-TR distance at a target load consistent with a gallop Table 1). The distal edge of the lateral PSB was closest to the TR at a target load of 4.5 kN. Target load had a significant (P < 0.001) effect on PSB-TR distance, and the mean PSB-TR distance differed significantly among the 4 target loads. There was a strong negative correlation between PSB-TR distance and target load (partial r_s = −0.96) and between PSB-TR distance and fetlock angle (partial r_s = −0.98; Figure 5) and a positive correlation between PSB-TR distance and AS angle (partial r_s = 0.97).

Radiographic and load data were used to construct linear regression equations to estimate the fetlock angle and load at which the distal margin of the lateral PSB moved distal to the TR. The regression equations and model R² were as follows: PSB-TR distance = (−0.2692 × fetlock angle) + 66.568 with an R² = 0.93 and PSB-TR distance = (−0.0014 × load) + 10.365 with an R² = 0.88. From these equations, it was estimated that the distal edge of the lateral PSB moved distal to the TR at a fetlock angle of 247° or a load of 7.4 kN.

Target load had a significant (P < 0.001) effect on the displacement of both the dorsal and palmar LVDTs and α (rotational angle). For each of those 3 variables, the mean value increased as target load increased and differed significantly among the 4 target loads Table 1). Also, the displacement of the palmar LVDT was significantly greater than that of the dorsal LVDT by 46% to 56% at each target load. That finding indicated that the palmar aspects of the PSBs rotated away from each other as the limb was loaded (Figure 3). There were strong positive correlations and positive linear relationships between load and dorsal LVDT displacement (partial r_s = 0.97, R²= 0.86), palmar LVDT displacement (partial r_s = 0.96, R² = 0.91), and α (partial r_s = 0.97, R² = 0.90).

Discussion

The TR demarcates the separation of the dorsal and palmar portions of the MC3 condyle, which articulate with the P1 and PSBs, respectively. The arc of curvature differs between the dorsal and palmar portions of the MC3 condyle to correspond with the curvature of the opposing ASs of the P1 and PSBs. The present in vitro study determined the position of the PSBs relative to the MC3 condyle at biomechanical loads selected to simulate those on the forelimb when a horse is standing and at midstance when the horse is walking, trotting, or galloping at a high speed. Results indicated that the distal margin of the PSBs moved distal to the TR, and thus beyond the congruent AS of MC3, when a horse is at a high-speed gallop. Findings also indicated that the palmar surfaces of the PSBs rotate away from each other when the fetlock joint is extended, and that rotation is greatest when a horse is at a gallop.

As expected, results of the present study indicated that extension of the fetlock joint increases as the axial load applied to the forelimb increases. Importantly, the mean fetlock angles measured in the present study for target loads consistent with a walk (228°), trot (234°), and gallop (264°) were similar to those reported for those gaits in in vivo kinematic studies.^{10,11,12,13,14} Those findings suggested that the target loads evaluated in the present study accurately simulated the anatomic alignment of and physiologic loads applied to equine forelimbs during the midstance phase of the stride at the selected gaits. Therefore, the motion of the PSBs observed in this study should closely mimic that in vivo. The target load selected to represent a trot (4.5 kN) in this study was lower than that used to simulate a trot in other studies.^17,20 It is estimated that the peak load sustained by a forelimb during a trot is 90% of the horse's body weight¹⁹; thus, for an average Thoroughbred racehorse (mean body weight, 483 kg^k), the estimated peak forelimb load during a trot would be 4.3 kN. Additionally, the least squares mean ± SE fetlock angle (234 ± 2.1°) at the target load for a trot measured in the present study was consistent with the fetlock angles (range, 225° to 241°) reported during a trot for horses of in vivo kinematic studies,^14,21 which further supported our supposition that the in vitro conditions of the present study accurately simulated in vivo kinematics of the equine forelimb.

The AS angle data obtained during the present study indicated that, as expected, the PSBs moved distodorsally around the palmar portion of the MC3 condyle during extension of the fetlock joint. The extent of the distodorsal movement of the PSBs increased as load increased and was greatest at the target load (10.5 kN) representative of a gallop. In the present study, the AS angle was assigned a negative value when the distal edge of the PSBs moved distal to the TR or the proximal edge of the PSBs became distracted from MC3. Distodorsal movement of the PSBs was apparent, but distraction of the proximal edges of the PSBs from MC3 was not evident during visual examination of the radiographic images obtained during this study.

The PSB-TR distance measurements obtained during the present study indicated that the distal margin of the lateral PSB moved distal to the TR, and thus beyond its congruent articulation with the palmar aspect of the MC3 condyle, at a load consistent with a high-speed gallop. The data suggested that the distal margin of the lateral PSB moves distal to the TR at approximately 7.4 kN (a load consistent with a fast trot or canter¹⁸) and that distal movement continued until a target load of 10.5 kN (a load consistent with peak high-speed gallop^16,18) was achieved. The proximodorsal motion of P1 is limited by contact of P1 with the dorsal aspect of MC3; however, distodorsal movement of the PSBs is limited by the extent to which the suspensory ligament can lengthen before rupture. Any movement of the PSBs distodorsal to the TR implies an articular incongruity exists between the PSBs and MC3. Joint incongruities affect the stress distribution in subchondral bone because subchondral bone stress patterns are directly related to the load applied to the AS.²² Joint incongruity could be a factor in the formation of stress remodeling–induced subchondral bone lesions that precede biaxial midbody PSB fracture in racehorses.^5,6,a,b Additionally, joint incongruity might induce a bending moment about a transverse axis located at the location where the PSBs move distal to the TR. Because the distance that the PSBs move distally beyond the TR appears to approximately correspond to the midbody to basilar aspect of the PSBs, it supports development of microdamage (leading to lesion formation) at a basilar to midbody location of PSBs. Thus, movement of the PSBs distal to the TR may contribute to the high prevalence of midbody and basilar PSB fractures reported in Thoroughbred racehorses.^23,24 Alternatively, extreme extension of the fetlock joint may lead to supraphysiologic stresses in subchondral bone on opposing ASs.

The MC3-PSB contact areas under physiologic loads have yet to be fully elucidated for horses. Results of an in vitro study²⁵ of cadaveric forelimbs indicate that the distal portion of the PSBs do not come into contact with MC3 when exposed to loads consistent with a gallop, which implies articular incongruity at those loads. However, the forelimbs evaluated in that study²⁵ were harvested distal to the carpometacarpal joint; therefore, the proximal and distal accessory (check) ligaments and their respective contributions to the superficial and deep digital flexor tendons did not remain intact to provide support to the fetlock region. In another in vitro study¹⁵ of equine forelimbs in which the supporting structures of the fetlock region remained intact, it was reported that the PSBs did not move distal to the TR when the fetlock joint was maximally extended. However, the loading conditions were not specified for the limbs of that study,¹⁵ and it is unknown whether loads consistent with a highspeed gallop were simulated. In the present study, known physiologic loads were applied to equine forelimbs in which the passive support structures of the fetlock region remained intact, and results indicated that the PSBs moved beyond their congruent ASs at loads consistent with a high-speed gallop.

In horses, the digital flexor tendons, ligamentous suspensory apparatus, and geometry of the MC3 and P1 limit motion of the fetlock joint primarily to the sagittal plane. Bone-fixed in vivo and in vitro kinematic markers indicate only small amounts of internal-external rotation and adduction-abduction exist between MC3 and P1.^12,17 Results of the present study indicated that rotation between the lateral and medial PSBs occurs as the limb is loaded. The cause of that rotation is unknown and could be related to the geometry of the interdigitating ASs or tension from soft tissue attachments (eg, branches of the suspensory or extensor ligaments) during load application. In Thoroughbred racehorses, subchondral bone lesions are frequently identified on the abaxial portion of the AS of the medial PSB and are believed to contribute to biaxial midbody PSB fracture.^5,6,a,b Rotation of the PSBs relative to each other likely changes the joint contact pattern and enhances stresses on the abaxial side of the PSBs. That rotation may contribute to the formation of subchondral bone lesions that are commonly observed abaxial to the median sagittal plane of the PSBs.^5,6,a,b

In the present study, the relative rotation of the PSBs was determined under the assumption that the rotation was confined to the transverse plane. One-dimensional kinematic measures (eg, LVDT displacements) cannot distinguish between rotation and rotation combined with distraction. If medial-to-lateral distraction of the PSBs occurs in combination with the observed rotation of the palmar surfaces of those bones away from one another, it may be a factor in axial avulsion fractures of the PSBs. An axial avulsion fracture of the lateral PSB in combination with a transverse fracture of the medial PSB is a commonly observed fetlock breakdown pattern for Thoroughbred racehorses.²⁴

Finite element analysis indicates that the SR angle affects stress distribution on MC3 and P1. Specifically, more shallow SR angles result in higher stress at the condylar groove of P1, which is believed to contribute to MC3 and P1 fractures in racehorses.²⁶ Because MC3 articulates with the PSBs, it is likely the SR angle also affects stresses on the PSBs. Further investigation of the relationship between SR angle and PSB rotation is necessary and may help elucidate the specific cause of subchondral bone lesions on the abaxial surface of the PSBs.

The primary limitation of the present study was the fact that it was performed in vitro rather than in vivo. However, tracking PSB location with kinematic skin or embedded bone markers is not practical in live horses. Skin markers would likely have become dislodged during extension of the fetlock joint because the palmar aspect of the joint commonly impacts the ground surface at a high-speed gallop. Additionally, the amount of PSB movement beneath the skin would severely limit the accuracy of skin markers. Implants embedded in the PSBs would weaken the bone and increase the risk of PSB fracture during a high-speed gallop. Another limitation associated with the in vitro nature of the study was the fact that the forelimb muscles were not active during biomechanical testing; however, the passive support structures of the forelimb remained intact. The passive support structures are considered the main stabilizers of the forelimb during midstance,²⁷ so the lack of muscle activation was unlikely to have any clinically relevant effect on our results. Also, fetlock suspensory apparatus strains and translations should be directly related to fetlock angle independent of digital flexor muscle activation, and the fetlock angles observed in this study were consistent with those reported in in vivo kinematic studies.^{10,11,12,13,14,21} The in vitro nature of the present study also resulted in the loading rates for the forelimbs to be slower than the physiologic loading rates of forelimbs in vivo. This may have allowed stress relaxation of the limb's passive support structures. However, given that the observed fetlock angles were consistent with those observed during in vivo kinematic studies,^{10,11,12,13,14,21} it appeared that any error associated with nonphysiologic loading rates was minimal. Finally, the position of the PSBs relative to the condyle of MC3 was measured on lateromedial radiographic images by use of the lateral PSB as a landmark. It is possible that the distance of movement beyond the TR differs between the lateral and medial PSBs, although that seems unlikely given the extent of binding between the lateral and medial PSBs provided by the intersesamoidean ligament.²³

Results of the present in vitro study indicated that distal translation of the PSBs beyond their congruent AS with the condyle of MC3 and PSB rotation occurred during biomechanical testing at loads consistent with a high-speed gallop. Preexisting lesions of the subchondral bone of PSBs have been identified in Thoroughbred racehorses with PSB fracture.^4,5,6 The findings of the present study suggested a potential anatomic cause for the observed abaxial midbody location of subchondral bone lesions and midbody-to-basilar location of most transverse PSB fractures. Additionally, rotation of the PSBs away from each other during extension of the fetlock joint at loads consistent with a high-speed gallop might contribute to intersesamoidean ligament rupture and axial avulsion fracture of the lateral PSB.