IDENTIFICATION OF AN OSSICLE ASSOCIATED WITH THE PALMAR ASPECT OF THE CARPUS IN THE HORSE

PER MARTENS, DVM

An ossicle associated with the palmar aspect of the carpus is described in five horses. The size of the ossicle varied from 3 x 4 mm to approximately 12 x 10 mm. The ossicles were located at the junction of the proximal and distal rows of carpal bones, between the palmaromedial aspect of the fourth carpal bone and the ulnar carpal bone. In 3 horses they were bilateral, in one horse unilateral, and only one limb was examined in the remaining horse. In all horses it was an incidental finding and not regarded to be a pathologic entity. Veterinary Radiology & Ultrasound, Vol. 40, No. 4, 1999, p p 342-345.

Key words: “accessory ossification center,” carpus, computed tomography, equine, horse.

Introduction

RRORS IN DEVELOPMENT of accessory ossification centers E discovered on radiographs of animals have been de- scribed to have two patterns of irregularity: 1) a supernu- merary ossicle not ordinarily found in the skeleton or, 2) an ossification center that has not fused, leaving a small bone as a separate structure adjacent to the remaining part of the ossification center.’ Bipartite sesamoid bones are examples that have been described in dogs and horses.’ They are differentiated from fractures by their smooth margins, rounding of the edges and a line of cortex along the entire surface.’

In horses bony fragments seen palinar to the palmar pro- cess of the distal phalanx and fragments found in the ex- tensor tendon proximal to the extensor process of the distal phalanx have been described as possible accessory ossifi- cation centers.223 Also in horses accessory ossification cen- ter has been suggested as a possible explanation for osteo- chondral fragments found distal to the intermediate ridge of the distal in the tarsocrural joint and osteochondral frag- ments found at the palmar/plantar aspect in the metacarpo4 metatarsophalangeal joint:

A secondary ossification center at the palmar border of the accessory carpal bone has been described in foals.235 A nonfused lateral styloid process has been described in an adult horse6 as well as in foals5 A separate ossification center in the proximal row of carpal bones has been de- scribed as an incidental finding in radiographs. The size,

From the Norwegian College of Veterinary Medicine, Department of

Address correspondence and reprint requests to Dr Martens. Received February 19, 1998; accepted for publication November 18,

Large Animal Clinical Sciences, Box 8146 Dep, N-0033 Oslo, Norway.

1998.

shape and articulations of these bones are described to be variable, but they are usually uniformly o p a q ~ e . ~

In humans, separate ossification centers have been de- scribed at numerous locations.’ They are considered to be normal anatomic variants.’ They are usually asymptotic,” but in some instances and at some locations, they are found in conjunction with clinical symptom^.^-'^ Well-defined bones appear as separate accessory carpal ossification cen- ters at approximately 25 locations in the wrist of normal persons. l 3

The purpose of this report is to describe ossicles in asso- ciation with the palmar aspect of the carpus in horses, seen as incidental findings on routine examinations.

Material and Methods

Twenty six standard bred racehorses and one cold blood trotter, including 13 mares and 14 stallions with an aver- age age of 3.7 years were examined both with computed tomography (CT) and conventional radiography. Eleven of the horses were clinical accessions with lameness related to the intercarpal joint and the other 16 were three- year-old half-siblings that were owned by the teaching hos- pital.

The radiographic examination included dorsopalmar and lateromedial as well as dorsoproximal-dorsodistal projec- tions of the proximal and distal row of carpal bones.

CT was performed under general anesthesia using a third- generation scanner.” Transverse images were obtained from the distal radius (about 2 cm above the accessory carpal bone) distally to the proximal aspect of the third metacarpal

*Siemens Soniatom 2, Siemens Aktiengesselschaft, Berlin und Miinchen, Germany.

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Voi,. 40, No. 4 IDENTIFICATION OF AN OSS~CLE

TABLE 1. Age and Gender of Standardbred Horses and Location, Size and Shape of Ossicles

343

Horse Gender Age Limb Place Form Size

2 3

Mare Mare

3 3

1 Stallion 6 RF Mediopalmar distal Cu Round opaque S x S m m LF Mediopalmar distal Cu Round opaque 6 x S m m

with a bony bridge to Cu

RF Mediopalmar proximal C4 Round opaque 3 x 4 m m RF Mediopalmar proximal C4 Opaque with a 1 1 x 6 m m

11 x 7 m m LF Mediopalmar proximal C4 Opaque with a

4 Stallion 4 RF Mediopalmar proximal C4 Round opaque 6 x 7 m m LF Mediopalmar proximal C4

Round opaque 9 x 8 m m

bony bridge to Cu

bony bridge to C4

5 Stallion 10 RF From Cu distal to C4 at Round opaque 12 x 10 mm their mediopalmar aspect

- ~ LF = left fore limb, mm = millimeter, C4 = fourth carpal bone, Cu

bone. Contiguous slices of 4 mm thickness were acquired. The image size was 256 x 256 pixels. The horses were in right lateral recumbency with both front limbs in the gantry. The limbs were secured in an extended position using a pneumatic splint and imaged separately using a 4.0 magni- fication factor. The gantry was tilted so the scan plane was perpendicular to the axis of the joint examined.

Results

Ossicles were found in the palmar aspect of the carpus in 5 horses and all were standardbreds (Figs. 1-5). There were two mares and three stallions. In three horses the ossicles were bilateral; thus a total of 8 ossicles were found (Table 1). In all horses the accessory ossification centers were found at the palmar aspect of the carpus on the palmarome- dial side between the ulnar carpal bone and the fourth carpal bone. In one horse they were located at the proximal palmar aspect of ulnar carpal bone (Fig. 4) and in the other horses at the palmar aspect of the fourth carpal bone (Figs. 1-3, 5).

ulnar carpal bone, RF = right front limb

Two of the standardbred half-siblings had ossicles. The other three horses were standardbred racehorses examined because of carpal lameness. All horses with clinical prob- lems related to the carpus had major defects within the joint to account for the lameness.

Fourteen of the horses had a first carpal bone; ten were bilateral and 4 were unilateral. Nine of the horses with a first carpal bone belonged to the group of 16 half-siblings. The other five were from the group of eleven patients.

Discussion

Separate ossification centers are described at numerous locations in humans. They tend to be bilateral and are de- scribed at approximately 25 different locations in the wrist of normal persons.” The most frequently affected bone in the human wrist is the pisiform bone. At the palmar aspect of the pisiform bone a secondary pisiform bone is found which is similar to the secondary ossification center at the palmar aspect of the accessory carpal bone described in

FIG. 1 . CT image of right carpus of horse 5 , a 10-year-old standardbred stallion. A 12 x 10 mm ossicle was identified palmar to the fourth carpal bone (arrow).

FIG. 2. CT image of right carpus of horse 2, a 3-year-old standard bred mare. A 3 x 4 mm ossicle was identified near the palmar aspect of the fourth carpal bone (arrow).

344 MARTENS 1999

FIG. 3. CT image of right carpus of horse 3, a 3-year-old standard bred mare. An 11 x 6 mm ossicle was identified palmar to the fourth carpal bone (arrow ) .

horse^.^,^^'^ In humans an accessory ossification center called hypotriquetrum is described at the palmar medial aspect between the ulnar carpal bone (triquetrum) and the fourth carpal bone (hamatum). This is approximately the same location as the ossicles seen in horses in this report.

In humans 3 types of tarsal accessory ossification centers are described. Type 1 are small ossicles within the substance of the posterior tibia1 tendon. Type 2 are ossicles united with the parent navicular by cartilaginous synchondrosis and type 3 is an accessory navicular that has fused to the parent navicular by a bony bridge.’’ Three of the accessory ossi- fication centers found in this study had a bony bridge to the ulnar or fourth carpal bone. All ossicles were situated in the area where a small ligament connects the ulnar and fourth carpal bone.14

A sesamoid bone may be ruled out as an explanation of the ossicle found in our study because the location is not in connection with any tendon. There is a small interosseous

FIG. 4. CT image of left carpus of horse 1, a 6-year-old standard bred stallion. A 5 x 5 mm ossicle was identified palmar to the ulnar carpal bone (arrow).

FIG. 5. CT image of left carpus of horse 4, a 4-year-old standard bred stallion. There is a 6 x 7 mm ossicle palmar to and connected with the fourth carpal bone by a bony bridge (solid arrow). Notice the lytic area representing an incomplete chip fracture of the dorsal aspect of the radial facet of the third carpal bone (open arrow). The subchondral bone in the ulnar facet of the third carpal bone is not within the image plane.

ligament between the palmaromedial parts of the ulnar and fourth carpal bones in the area where the ossicles were seen. It seems unlikely that a small ligament connecting two car- pal bones with a minimal movement would develop a sesa- moid bone. It is also unlikely that a sesamoid bone would develop a bony bridge to an adjacent bone.

In all horses the ossicles were incidental findings. In the lame horses there were osteoarthrotic changes, such as slab fractures of the third carpal bone, to account for the lameness. None of the other horses had lameness. In hu- mans, accessory ossification centers have been described as asymptotic, although they have been discovered as inciden- tal findings in patients.”” Instances of bilateral accessory iiavicular in the tarsus and bilateral accessory ossification centers of the malleoli in children have been associated with clinical symptoms, and during bone scintigraphy increased technetium uptake was noted in one affected limb whereas the same location in the contralateral limb was photo- peni~.’~’’ It was thought that an injury to the accessory ossification center caused the clinical symptoms.’”’ There- fore, it may be possible for these ossicles in horses to be associated with lameness.

In only one horse (no. 5 ) the ossicle was visible on ra- diographs (Fig. 6). In this horse the ossicle was situated further palmarly than in the other horses. This made it pos- sible to visualize the ossicle despite overlapping with other structures. In all other horses the overlapping with adjacent structures and the size of the ossicles obscure their detection in conventional radiographs. It is doubtful if additional ra- diographic projections would have improved the likelihood of visualizing the ossicles, because the overlapping of ad- jacent structures in would reduce the possibility of detecting them.

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FIG. 6. Dorsopalmar (A) and lateromedial (B) radiographs of the right carpus of horse 5, a 10-year-old standard bred stallion. A 12 x 10 mm ossicle was identified at the palmar aspect, between the ulnar and fourth carpal bones (arrow).

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