Exostoses on the palmar or plantar aspect of the diaphysis of the third metacarpal or metatarsal bone in horses: 16 cases (2001–2010)

Lélia Bertoni CIRALE, Université Paris-Est, Ecole Vétérinaire d'Alfort, 14430 Goustranville, France; and INRA, USC BPLC 957, 94704 Maisons-Alfort, France.

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Daniela Forresu CIRALE, Université Paris-Est, Ecole Vétérinaire d'Alfort, 14430 Goustranville, France; and INRA, USC BPLC 957, 94704 Maisons-Alfort, France.

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Virginie Coudry CIRALE, Université Paris-Est, Ecole Vétérinaire d'Alfort, 14430 Goustranville, France; and INRA, USC BPLC 957, 94704 Maisons-Alfort, France.

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Fabrice Audigie CIRALE, Université Paris-Est, Ecole Vétérinaire d'Alfort, 14430 Goustranville, France; and INRA, USC BPLC 957, 94704 Maisons-Alfort, France.

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Jean-Marie Denoix CIRALE, Université Paris-Est, Ecole Vétérinaire d'Alfort, 14430 Goustranville, France; and INRA, USC BPLC 957, 94704 Maisons-Alfort, France.

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Abstract

Objective—To characterize the clinical features, diagnostic procedures, treatment, and outcome for horses with an exostosis on the palmar or plantar cortex of the third metacarpal bone (MC3) or third metatarsal bone (MT3).

Design—Retrospective case series.

Animals—16 horses.

Procedures—Records from 2001 through 2010 were evaluated to identify horses with radiographic and ultrasonographic evidence of an exostosis on an MC3 or MT3. Signalment, history, lameness examination results, diagnostic imaging results, surgical and histopathologic findings, treatment, and outcome were evaluated.

Results—9 horses (group A) had unilateral lameness of the exostosis-affected limb that was alleviated with local or perineural analgesia. Seven horses (group B) had inconsistent lameness of the affected limb. The exostosis was located between the middle and distal third of the MC3 or MT3 in all horses. Desmopathy or peritendinous fibrosis (or both) of the suspensory ligament at the exostosis site was identified in 6 horses. All horses in group A returned to full function after conservative or surgical management. Lameness did not recur in any of the horses in group B despite no treatment or only conservative management.

Conclusions and Clinical Relevance—Exostosis of the palmar cortex of an MC3 or plantar cortex of an MT3 should be considered as a cause of lameness in horses. The diagnosis can be made by the use of radiography and ultrasonography combined with specific diagnostic analgesia. Prognosis for return to athletic activity can be good but should be modified contingent on concurrent desmopathy of the suspensory ligament.

Abstract

Objective—To characterize the clinical features, diagnostic procedures, treatment, and outcome for horses with an exostosis on the palmar or plantar cortex of the third metacarpal bone (MC3) or third metatarsal bone (MT3).

Design—Retrospective case series.

Animals—16 horses.

Procedures—Records from 2001 through 2010 were evaluated to identify horses with radiographic and ultrasonographic evidence of an exostosis on an MC3 or MT3. Signalment, history, lameness examination results, diagnostic imaging results, surgical and histopathologic findings, treatment, and outcome were evaluated.

Results—9 horses (group A) had unilateral lameness of the exostosis-affected limb that was alleviated with local or perineural analgesia. Seven horses (group B) had inconsistent lameness of the affected limb. The exostosis was located between the middle and distal third of the MC3 or MT3 in all horses. Desmopathy or peritendinous fibrosis (or both) of the suspensory ligament at the exostosis site was identified in 6 horses. All horses in group A returned to full function after conservative or surgical management. Lameness did not recur in any of the horses in group B despite no treatment or only conservative management.

Conclusions and Clinical Relevance—Exostosis of the palmar cortex of an MC3 or plantar cortex of an MT3 should be considered as a cause of lameness in horses. The diagnosis can be made by the use of radiography and ultrasonography combined with specific diagnostic analgesia. Prognosis for return to athletic activity can be good but should be modified contingent on concurrent desmopathy of the suspensory ligament.

Exostoses are benign bony proliferations that develop on the surface of a bone, and they differ with regard to etiologic origins and histologic lesions. The most common exostoses observed on equine limbs are the result of a periosteal response to external trauma or to cyclic strains associated with exercise, and most commonly involve the second and fourth metacarpal and metatarsal bones.1,2 Although uncommon, exostoses on equine limbs may also be benign bony neoplasms, with osteochondromas, osteomas, and ossifying fibromas most frequently reported.3 Osteochondromas generally involve long bones, with the radius being the bone most commonly affected.4,5 Osteomas and ossifying fibromas form at different stages of the same process3 and generally affect the bones of the skull, mandible, and paranasal sinuses,6–10 with only 1 reported in the appendicular skeleton (proximal portion of the tibia) of a horse.11 Exostoses generally do not cause clinical problems, but they can cause lameness if they impinge on surrounding tendons, ligaments, synovial sheaths, or periarticular structures.2 To our knowledge, exostoses of the palmar and plantar diaphyses of MC3s and MT3s have not been reported as a cause of lameness in horses.

The primary objective of the study reported here was to characterize the clinical, radiographic, ultrasonographic, scintigraphic, and MRI results for horses with an exostosis on the palmar or plantar diaphysis of MC3 or MT3. A secondary objective was to determine the clinical resolution without treatment or after surgical management of these lesions in horses.

Materials and Methods

Case selection—Medical records of horses admitted to the Center of Imaging and Research on the Equine Locomotor Affections between 2001 and 2010 were reviewed. Horses with radiographic and ultrasonographic evidence of an exostosis on the palmar cortex of an MC3 or plantar cortex of an MT3 were included in the study.

Medical records review—Data collected from the records included age, breed, sex, use, history, results of physical and lameness examinations, diagnostic imaging results, response to diagnostic analgesia, treatment, and outcome. The horses were allocated into 2 groups. Group A included horses with unilateral lameness of the affected limb that was alleviated with local analgesia, and group B included horses with inconsistent lameness at the time of examination, which made it impracticable to use diagnostic analgesia.

Examination protocol—Lameness examinations were performed for each horse. Degree of lameness and response to flexion tests were graded on a scale of 0 to 5 in accordance with the lameness scale of the American Association of Equine Practitioners.12 Radiographic examinations were generally performed before diagnostic analgesia when the lameness was not severe enough to localize the pain and to rule out fatigue fractures. Ultrasonographic examination was always performed before diagnostic analgesia to avoid any artifacts (air or fluid) created by injections.

Lateromedial and dorsopalmar radiographs of the MC3 or MT3 of the affected and contralateral limbs were obtained for all horses. The palmar regions of the MC3 or plantar regions of the MT3 of the affected and contralateral limbs were then examined ultrasonographically in transverse and longitudinal scans by the use of a 7.5-MHz linear transducer. Diagnosis of an exostosis was made on the basis of extraneous bony proliferation detected during radiographic and ultrasonographic evaluations.

For horses in which the source of the lameness could be localized, diagnostic analgesia was performed by the use of various local anesthetic techniques.13 Each horse's locomotion was videotaped before and after diagnostic analgesia to allow for an objective assessment of the efficacy of the analgesia. Lameness was reevaluated 10 minutes after injection of the local analgesic. If the severity of the lameness was decreased, the lameness was evaluated again at 15 and 20 minutes after injection of the local analgesic. Substantial improvement was defined as a reduction in lameness > 70%.

Nuclear scintigraphy or MRI was performed when necessary for diagnostic purposes. For nuclear scintigraphy, bone-phase images were obtained 3 hours after injection of technetium-99-DP (10 MBq/kg [4.5 MBq/lb], IV) in horses sedated with detomidine hydrochloride (0.1 mg/kg [0.045 mg/lb], IV) and morphine chlorhydrate (0.1 mg/kg, IV).

When necessary, MRI was performed on both fore-limbs or hind limbs of anesthetized horses by the use of a human surface coil and a 0.2-T magnet.a Images were acquired in transverse and sagittal planes with T1-weighted, T2-weighted, both T1- and T2-weighted, and short tau inversion recovery sequences.

Treatment—Treatment in the form of conservative management (stall rest, hand walking, and trotting exercise for 2 to 6 months) was proposed for all horses initially, but surgery was recommended if lameness recurred when a horse was returned to its regular activity. Surgery was performed with the horse anesthetized and positioned in lateral recumbency. A tourniquet was placed proximal to the exostosis, and the affected limb was surgically prepared. A longitudinal incision was made palmar or plantar to the fourth metacarpal or metatarsal bone and centered over the lesion. After sharp and blunt dissection was performed, soft tissues were reflected, and the integrity of the suspensory ligament was evaluated. A 12-mm osteotome and mallet were used to separate the exostosis from the affected bone. All overlying periosteum was removed with the exostosis. The affected area was then smoothed with a bone rasp, and the surgical field was lavaged to remove bone fragments. The wound was routinely closed in multiple layers. After surgery, horses were discharged to the owner with the recommendation that the horse be restricted to light exercise for 8 to 12 weeks before returning to full athletic activity.

An exostosis surgically removed from 1 horse was submitted for histologic examination. The bone sample was decalcified and processed by use of a standard histologic technique.

Long-term follow-up information was obtained via telephone communication with owners and referring veterinarians. The shortest follow-up period was 3 months, and the longest was 9 years. A positive outcome was defined as the absence of lameness and return of the horse to athletic activity similar to its activity prior to diagnosis of the exostosis.

Results

Sixteen horses were identified with exostoses on the palmar cortex of MC3 or plantar cortex of MT3. The affected horses included 6 mares, 7 geldings, and 3 stallions, and the mean age was 6.7 years (range, 2 to 15 years). Horses were used for racing (n = 5), show jumping (9), or 3-day eventing (2). Breeds affected included Thoroughbred (n = 2), French Trotter (3), and warm-blood (11). Twelve horses were examined because of lameness; 9 had lameness of ≤ 4 months' duration, and 3 had lameness of > 4 months' duration. Other reasons horses were examined included poor performance (n = 1), gait irregularity at the end of trotting races (1), and unrelated orthopedic issues (2).

Nine horses (group A) had unilateral lameness of the affected limb that became undetectable after administration of local analgesia and had radiographic and ultrasonographic abnormalities consistent with an exostosis. The diagnosis of an exostosis was made in 7 horses (group B) on the basis of results of radiographic and ultrasonographic evaluations only. The exostoses were located on MC3 of 15 horses, 6 of which had bilateral abnormalities of MC3. Exostosis on MT3 was diagnosed in only 1 horse. Lameness became exacerbated in 1 horse 1 month after the initial examination, and that horse was reevaluated.

Physical examination results—Abnormal physical examination findings were recorded for 7 horses (4 horses each had 1 abnormality, and 3 horses each had > 1 abnormality). The abnormal findings included mild to moderate soft tissue swelling on the palmar region of mid-MC3 (n = 3), moderate soft tissue swelling on the plantar region of mid-MT3 (1), sensitivity to focal pressure (2), a palpable mass (1), atrophy of the foot on the affected limb (3), and protraction of the affected limb (2). No physical examination abnormality was found in 9 horses.

Lameness examination results—Horses in group A had mild to moderate (grade 1 or 2) lameness with a lack of extension of the metacarpophalangeal or meta-tarsophalangeal joint (fetlock joint) when examined while trotting in a straight line. One horse had lameness in a hind limb, whereas the other 8 horses in the group had lameness in a forelimb. Lameness was exacerbated for some horses after warming up (n = 3), walking on soft ground (2), and lunging with the lame limb on the outside of the circle (4). Horses in group B had mild (grade 1) forelimb lameness while trotting in a straight line (n = 3), no lameness (1), and only mild forelimb lameness when examined while running at high speed on a racetrack (3). Flexion tests were performed on every horse, which resulted in mild to moderate exacerbation of the lameness for 12 horses.

Diagnostic imaging results—Radiographic examinations of all 16 horses revealed that the mean size of the exostoses was 30.9 × 7.3 mm (range, 10.1 to 48.9 mm in length and 3.6 to 12.4 mm in width), and all lesions were localized between the middle and the distal third of the palmar cortex of MC3 or plantar cortex of MT3 (Figure 1). Radiographically, the exostoses appeared sclerotic or had a combination of osteolytic and sclerotic changes. The surface of the lesions was smooth or irregular. Bony changes were identified in both fore-limbs of 6 horses.

Figure 1—
Figure 1—

Dorsopalmar (A) and lateromedial (B) radiographic views of an exostosis in the diaphysis of the left MC3 of a horse. The exostosis (arrowhead) appears as a clearly delineated bony proliferation with an irregular surface located between the middle and the distal third of the palmar cortex of MC3.

Citation: Journal of the American Veterinary Medical Association 240, 6; 10.2460/javma.240.6.740

Ultrasonographic examination revealed abnormalities in 16 horses (13 horses each had 1 abnormality, and 3 horses each had > 1 abnormalities). There was sharpened to smooth remodeling of the palmar aspect of the affected MC3 or plantar aspect of the affected MT3 (Figure 2). The exostoses were localized at the bifurcation of the suspensory ligament (n = 9), at the axial part of the medial branch of the suspensory ligament (5), or over the distal suspensory ligament at the middle third of an MC3 (3). Two horses had concurrent enlargement and hypoechogenicity of the suspensory ligament close to the area of bone remodeling.

Figure 2—
Figure 2—

Combined adjacent longitudinal ultrasonographic images of the palmar aspect of a left MC3 bone (A; medial is to the left) and transverse ultrasonographic images of the palmar aspects of left (LF) and right (RF) MC3s (B; proximal is to the left) of a representative horse in the present study. There is extensive bone remodeling on the palmar cortex of the bone (arrowheads) located at the bifurcation of the suspensory ligament. The exostoses are impinging on the body of the respective suspensory ligaments, which has resulted in thickening and focal hypoechogenicity of the suspensory ligaments (arrow).

Citation: Journal of the American Veterinary Medical Association 240, 6; 10.2460/javma.240.6.740

Scintigraphic examination was performed on 4 horses (3 horses in group A and 1 horse in group B). The 3 horses in group A had no change in RU at the distal third of the affected MC3. The horse in group B had a moderate increase in RU in a focal area at the affected region of MC3, compared with that of the adjacent bone and the MC3 of the contralateral limb.

Magnetic resonance imaging was performed on 1 horse in group A. The MRI revealed similar abnormalities on both metacarpal bones, but the limb on which the horse was clinically lame was more severely affected. There was an irregular bony proliferation on the palmar cortex, but no abnormality was seen on the endosteal surface or in adjacent cancellous bone of the MC3 of the clinically lame limb. This proliferation was located sagittally, was approximately 45 mm in length proximodistally, and had a slightly increased signal intensity, compared with the signal intensity of adjacent cortical bone on all sequences (Figure 3). The MRI also revealed that the suspensory ligament was enlarged, and its dorsal border had an abnormal concave deviation adjacent to the exostosis. Signal intensity was increased at the proximal portion and axial branches of the suspensory ligament in T1-weighted images.

Figure 3—
Figure 3—

Transverse T1-weighted spin echo (A; medial is to the left) and sagittal mixed T1- and T2-weighted gradient echo (B; proximal is to the left) magnetic resonance images of an exostosis on the palmar aspect of the left MC3 of a horse. There is irregular bony proliferation on the surface of the palmar cortex of MC3 with slightly increased signal intensity (arrowheads), compared with the signal intensity of adjacent cortical bone. The suspensory ligament is enlarged and has an abnormal concavity in its dorsal border adjacent to the exostosis.

Citation: Journal of the American Veterinary Medical Association 240, 6; 10.2460/javma.240.6.740

Diagnostic analgesia results—Diagnostic analgesia was performed only on horses in group A. A distal palmar metacarpal nerve block (low 4-point block) was performed by the use of 9 mL of a 2% solution of lidocaine hydrochloride in 1 horse. Direct infiltration of anesthetic at the proximal insertion of the suspensory ligament was performed by the use of 4 mL of a 2% solution of lidocaine in 2 horses, and ultrasound-guided infiltration of anesthetic around the exostosis between the suspensory ligament and MC3 or MT3 was performed by the use of 2 to 4 mL of a 2% solution of lidocaine in 6 horses. One horse was examined a second time, and an ultrasound-guided block of the lateral palmar nerve distal to the accessory carpal bone was performed by the use of 7 mL of a 2% solution of lidocaine.

Ten minutes after the injection of lidocaine, 8 of 9 horses had a reduction in lameness of at least 80%, with 3 horses having complete remission of their lameness. Substantial improvement was recorded for 5 horses at 15 or 20 minutes after the lidocaine injection. One horse had only intermittent substantial improvement of lameness at 10 and 20 minutes after the lidocaine injection. One horse had no reduction in the lameness after infiltration of lidocaine around the exostosis, but a lateral palmar nerve block resulted in complete remission of the lameness 15 minutes after the injection of lidocaine.

Treatment and outcome—For 6 horses in group A, conservative management was initially attempted; this included stall rest and controlled hand walking for 1 to 2 months, then trotting exercise for 2 to 6 months. In addition to conservative management, 1 horse had light-weight aluminum shoes with wide toes and narrow beveled branches placed on both feet of the forelimbs to reduce extension of the fetlock joint in an attempt to alleviate irritation to the suspensory ligament. No treatment was administered to 1 horse, and it was kept working at full capacity with progressive reduction of the lameness.

Surgical removal of the exostosis was performed immediately after it was diagnosed in 2 horses in an attempt to return the horses to competition sooner than would have been achieved with conservative management. Surgical removal of the exostosis was performed in 4 horses when their lameness failed to resolve after at least 3 months (range, 3 to 6 months) of conservative management.

Soft tissue abnormalities were found during surgery in 5 of 6 horses. Lesions included thickening with hyperemia and mild tears of the suspensory ligament at the point where it came into contact with the exostosis (n = 2), peritendinous fibrosis with mild hyperemia (2), and palpable stiffening with roughness of the suspensory ligament's dorsal aspect (1). No soft tissue abnormality was identified in 1 horse. Histologic examination of the exostosis removed from 1 horse revealed a clearly delineated nodular mass of trabecular bone surrounded by a layer of dense connective tissue (periosteum), which included osseous spicules with osteocytes embedded in an osteoid surrounded by osteoblasts and yellow bone marrow (adipose connective tissue; Figure 4).

Figure 4—
Figure 4—

Photomicrograph of a section of an exostosis that was surgically removed from the palmar aspect of an MC3 of a horse. Notice the clearly delineated nodular mass of trabecular bone bordered by a layer of dense connective tissue (fibrous periosteum; 1), osseous spicules with osteocytes embedded in the osteoid (2) rimmed with osteoblasts (arrowhead), and yellow bone marrow (adipose connective tissue; 3). H&E stain; bar = 100 μm.

Citation: Journal of the American Veterinary Medical Association 240, 6; 10.2460/javma.240.6.740

Lameness did not recur following surgery in any of the horses. Follow-up information was available for all 9 horses in group A. All horses had a positive outcome and returned to their regular athletic activity within 6 months after the diagnosis or surgical removal of the exostosis. Follow-up radiographs were obtained for 2 horses (6 months and 4 years after surgery), and evaluation revealed mild sclerosis of the bone at the location where the exostosis had been removed. Four years after surgical removal of the exostosis, the horse that had an exostosis on MT3 developed degenerative disease of the proximal interphalangeal joint on the contralateral limb after competing in a 3-day event. One horse was retired from athletic competition 1 year after diagnosis of an exostosis for reasons unrelated to the lesion. One horse had a slight, persistent lameness in the affected limb when the horse was moved in a circle with that limb on the outside, but the horse was allowed to continue in competition.

Of the horses in group B, 1 was treated via conservative management, and the remaining 6 received no treatment. All 7 horses had positive outcomes, and 5 of 7 horses performed athletic activities similar to those they had performed prior to the diagnosis. One horse performed in a lower class of competition for reasons unrelated to the exostosis, and 1 horse did not return to racing for other medical reasons.

On the basis of the findings in the present report, an illustration of the suspected pathogenesis of an exostosis on the distal portion of MC3 or MT3 was created (Figure 5). Hyperextension of the fetlock joint creates tension on the metacarpal or metatarsal attachment of the intersesamoidean ligament, which may result in partial tearing and the formation of an exostosis at the ligament's insertion on MC3 or MT3.

Figure 5—
Figure 5—

An illustration depicting the suggested etiopathogenesis of an exostosis on the palmar aspect of MC3 or the plantar aspect of MT3 in a horse. Hyperextension of the metacarpophalangeal or metatarsophalangeal joint (fetlock joint) creates tension on the metacarpal or metatarsal attachment of the intersesamoidean ligament, which may result in partial tearing and the formation of an exostosis at the ligament's insertion on MC3 or MT3. Hyperextension of the fetlock joint also causes distal and dorsal displacement of the proximal sesamoid bones, which induces tension and dorsal shifting of the suspensory ligament. There is interference between the exostosis and the suspensory ligament (black arrowheads), and contact between the exostosis and suspensory ligament may cause pain and subsequent ligament tearing. The illustration was created by one of the authors (JMD). 1 = MC3 or MT3. 2 = Proximal phalanx. 3 = Proximal sesamoid bone. 4 = Suspensory ligament. 4a = Extensor branches of the suspensory ligament. 5 = Straight sesamoidean ligament. 6 = Intersesamoidean ligament. 7 = Metacarpal or metatarsal attachment of the intersesamoidean ligament.

Citation: Journal of the American Veterinary Medical Association 240, 6; 10.2460/javma.240.6.740

Discussion

Sixteen horses with exostoses on the palmar cortex of MC3 or plantar cortex of MT3 were identified in a 9-year period. The exostoses in the 7 horses in group B were identified on the basis of diagnostic imaging techniques alone because the horses were not consistently lame. This suggests that these lesions are uncommon, underdiagnosed, or often not clinically apparent. Most of the horses in the present report were young, mature horses and were engaged in strenuous activities. Exostoses appear to be more common in forelimbs as evidenced by the fact that exostosis was diagnosed in a forelimb in 15 of 16 horses in the present report. Many of the horses in the present report had no physical abnormality, making diagnosis difficult. The lesion was bilateral in 6 horses, which suggests routine radiographic and ultrasonographic examination of the contralateral limb should be performed when an exostosis is identified. Because the lameness could not be consistently localized in the 7 horses in group B, diagnostic analgesia was not performed in those horses, and we were not able to establish the respective causes of their lameness or determine the clinical importance of other lesions found on the same limb. Thus, we propose that exostoses may not cause clinical signs.

In horses that were clinically lame (group A), diagnostic analgesia was performed to determine whether the exostosis was the source of the lameness. Various techniques were used to desensitize the palmar aspect of the affected MC3 or plantar aspect of the affected MT3. Diagnostic analgesia resulted in a reduction in lameness of ≥ 80% for 9 of the 10 times it was performed, but the lameness was not completely resolved for the majority (6/9) of horses. The reason for this may have been because only partial analgesia of the nerve supply to the region was achieved, there was persistence of deep pain originating from the cortex of the damaged bone,14 or there were concurrent causes of pain originating in the limb proximal to the injection site of the diagnostic analgesia.15 The distal palmar metacarpal nerve block, performed with the needle inserted at the level of the distal extremity of the second and fourth metacarpal bones,13 resulted in a reduction in the lameness of 85% in 1 horse. In that horse, the exostosis was localized to the distal third of MC3, and the area may have been partially desensitized by local or proximal diffusion of the lidocaine.14 Local infiltration of lidocaine with ultrasonographic guidance was a more precise technique and was useful for desensitizing the palmar or plantar cortex of the affected MC3 or MT3, respectively.13,14 After this technique was performed, the sensitivity of the bulb of the heels and lateral aspect of the fetlock joint was maintained when the horse was evaluated while trotting, which suggests that the respective common palmar or plantar digital nerves and palmar metacarpal or plantar metatarsal nerves were not anesthetized. Similarly, a reduction in the lameness in 2 horses following direct infiltration with a small volume of lidocaine at the origin of the suspensory ligament was likely not related to alleviation of pain in the fetlock joint of the forelimb. Diagnostic analgesia of the distal portions of the limbs was not performed to avoid a reduction in lameness that may be caused by proximal diffusion of the anesthetic after abaxial sesamoid and low palmar (low 4-point) blocks are performed.14 The horse that was examined twice within 1 month had complete remission of lameness after infiltration of lidocaine around the exostosis during its first examination. However, during the second examination, the same analgesic technique failed to reduce the lameness, but a block of the lateral palmar nerve distal to the accessory carpal bone resulted in complete remission of the lameness, which suggested that desmopathy of the suspensory ligament was the major source of the pain.

Radiography provides information about the exostosis, such as its size, precise location in the bone, patterns of architectural alterations, amount of periosteal response, and structural alterations at its margins.3 Ultrasonography is easy to perform and appears to be an extremely sensitive technique for detecting early bone remodeling that cannot be detected on radiographs.16 Ultrasonography allows evaluation of the suspensory ligament close to the lesion; however, mild changes involving the dorsal aspect of the suspensory ligament can remain undetected. For example, of the 16 horses in the present report, concurrent desmopathy of the suspensory ligament was diagnosed by the use of ultrasonography in only 2 horses, but abnormalities of the dorsal aspect of the suspensory ligament were palpable or visible in 5 of the horses on which surgery was performed.

Magnetic resonance imaging of the metacarpal and metatarsal regions has been reported to be a valuable diagnostic modality for identifying injuries in horses with lameness localized to the proximal portion of the metacarpal and metatarsal regions when results from ultrasonography have been inconclusive.1,16,17 The MRI results provide additional information about the nature of the bone and its surrounding soft tissues. Comparisons of changes in the signals among the T1-weighted, T2-weighted, and fat-suppressed images can identify the presence of abnormal fluid, tissue, or mineralization in the tissues of interest. In the present report, MRI was performed on 1 horse, and the results revealed signal abnormalities that were compatible with abnormal fibrotic and tissue mineralization, which resulted in a diagnosis of an exostosis and concurrent desmopathy of the suspensory ligament. The MRI also provided detailed information about the exostosis in terms of its extent over the cortical surface of the affected MC3, its architecture and that of the adjacent bone, and its relationship with the surrounding soft tissues. Thus, MRI is valuable for making a diagnosis as well as providing detailed information about associated injuries to the middle and distal portions of the metacarpal and metatarsal regions that are not easily detected by use of radiography and ultrasonography. This in turn aids in making a more precise prognosis and developing a treatment plan.

Scintigraphy was performed on 4 horses in the present report. The scintigraphic results revealed no abnormalities in the affected MC3 of the 3 clinically lame horses in group A but did reveal an abnormal RU in the affected MC3 of an intermittently lame horse in group B. Thus, scintigraphy does not provide reliable information on the association of pain with a lesion. This is consistent with results of other reports18,19 in which exostoses on the second or fourth metacarpal and metatarsal bones and on the proximolateral aspect of MC3 and MT3 frequently had an abnormally increased RU at the site of the lesion, even when the affected horses were not clinically lame. Bone-phase scintigraphy is generally more sensitive for identifying active bone remodeling than for identifying chronic osteoarticular lesions.15,20 Thus, it is likely that the exostoses of the horses in group A were at the end stage of the remodeling process, whereas that of the horse in group B was still actively undergoing osteogenesis. Also, RU during scintigraphy is more likely to increase as the size of the lesion increases.18 This may help explain the absence of abnormal RU for the horses in group A because the size (< 28 mm in length) of the exostoses for those horses was much smaller than the size (47.5 mm in length) of the exostosis for the horse in group B. Furthermore, in our experience, absence of abnormal RU is not uncommon in chronic enthesopathies (such as enthesopathy of the proximal portion of the suspensory ligament on the palmar aspect of MC3 or plantar aspect of MT3), even when lameness is alleviated by focal diagnostic analgesia.

Lameness can be caused by injuries to soft tissues adjacent to bony lesions and has been described for exostoses involving the second and fourth metacarpal and metatarsal bones1 and for osteochondromas of the distal portion of the radius,4 distal portion of the tibia,5,21 and calcaneus.22,23 Frequently, ultrasonographic examination reveals no abnormality of the soft tissues surrounding an exostosis.4,5,24 For the horses in the present report, lameness may have been induced by contact between the exostosis and the distal body of the suspensory ligament during full extension of the fetlock joint, which resulted in desmopathy of the suspensory ligament (Figure 5). In the present report, surgical removal of the exostosis eliminated the cause of irritation on the suspensory ligament, and lameness resolved. Lameness may also develop because of direct pressure applied by the exostosis on adjacent palmar metacarpal or plantar metatarsal arteries and veins.25 Another cause of lameness is enthesopathy of the metacarpal or metatarsal attachment of the intersesamoidean ligament on the distal two-fifths of the palmar or plantar cortex of MC3 or MT3, respectively.25 The MRI performed on 1 horse in the present study did not reveal any changes in the bone adjacent to the exostosis; therefore, it is unlikely that the lameness in that horse was caused by pain arising from the deep part of MC3 or from bone remodeling associated with stress.

The exostoses in the present report had similar radiographic and ultrasonographic characteristics and were located in the same region of the affected MC3 or MT3, which suggests a common etiopathogenesis. Differential diagnoses for exostoses on horse limbs should include benign osteochondromas and osteomas. Osteochondromas are composed of trabecular bone continuous with normal bone and are distinguished from other benign bony lesions by a covering of cartilage.4,5,24,26 Osteochondromas are formed by endochondral ossification originating adjacent to the physis of a long bone. It is unlikely any of the exostoses described in the present study were osteochondromas because the distal physis of the affected MC3 or MT3 was not adjacent to any of the exostoses. Histologic examination of a surgically removed exostosis revealed characteristics similar to those reported3 for osteomas and included a benign, smoothly contoured bony growth that protruded from the cortex, was contiguous with normal adjacent bone, and was composed primarily of trabecular bone bordered by a layer of connective tissue resembling actively growing periosteum. The structure of an osteoma is the same as that of new periosteal bone formation, and the 2 conditions cannot be differentiated by microscopic evaluation.3 So although the histologic results for the exostosis examined in the present report were consistent with an osteoma, we cannot confirm that diagnosis with certainty. However, histologic examination of an exostosis is required to determine the precise nature of the lesion.

Similar to exostoses of the fourth and second metacarpal and metatarsal bones,1,27 exostoses of the palmar cortex of MC3 or plantar cortex of MT3 may develop secondary to periostitis, which results from direct trauma to the bone or cyclic strains during exercise. An exostosis may also develop as a result of chronic tearing of the metacarpal or metatarsal attachment of the intersesamoidean ligament (Figure 5).

Follow-up monitoring of horses in the present report suggested that the prognosis for return to athletic activity is favorable regardless of the method of treatment. Rest followed by controlled exercise should be recommended initially. Corrective shoeing to reduce extension of the fetlock joint to help alleviate irritation of the suspensory ligament may be recommended for horses with concurrent desmopathy of the suspensory ligament. Because lameness resolved without treatment in 6 horses, it appears exostosis-associated clinical signs can resolve even without rest. If owners desire a quick return of a horse to competition, or if conservative management fails, surgical excision of the exostosis with debridement of the underlying bone is advised to resolve clinical signs; this treatment is similar to that described for the treatment of osteochondromas and exostoses of the second and fourth metacarpal or metatarsal bones.1,5,27 In the present report, radiographs obtained for 2 horses, 6 months and 4 years following surgical removal of the exostosis, did not have evidence of recurrence. Nevertheless, periodic radiographic examination of the affected limb after surgical removal of an exostosis is advised.

Exostosis of the palmar cortex of MC3 or the plantar cortex of MT3 should be considered as a potential cause of lameness in horses. Accurate diagnosis of an exostosis can be made by the use of radiography and ultrasonography, in conjunction with a reduction in lameness in response to diagnostic analgesia. Magnetic resonance imaging can provide additional information regarding damage to the suspensory ligament and adjacent bone. Prognosis for return of a horse with an exostosis to its normal athletic activity is good but should be modified contingent on the extent of desmopathy to the suspensory ligament.

ABBREVIATIONS

MC3

Third metacarpal bone

MRI

Magnetic resonance imaging

MT3

Third metatarsal bone

RU

Radiopharmaceutical uptake

a.

Open Magnet, Siemens, Saint Denis, France.

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