Figure 10.1. Foal. Neonate. (a) Distal forelimbs. Unilateral hemimelia with limb shortening and varus deformity. (b) Longitudinal sections reveal aplasia of the distal phalanx and navicular bone of the left forelimb.

Figure 10.2. Foal. Neonate. Axial skeleton (ventral view of spine). Congenital scoliosis (right lateral curvature of the spine) involving the caudal thoracic segment. (Courtesy Dr. P. Habecker, New Bolton Center, University of Pennsylvania.)

Figure 10.3. Foal. Neonates. Craniofacial dysplasias (i.e., malformations). (a) Lateral deviation of the rostral maxilla, (b) “Wry nose”; (b) palatoschisis, “cleft palate”; (c) maxillary brachygnathia. (Courtesy Dr. P. Habecker, New Bolton Center, University of Pennsylvania.)

Figure 10.4. Horse. Regional hypodontia and odontodysplasia of the maxillary arcade.(a) Computed tomography (CT) scan reveals abnormal development of the right second maxillary molar (right arrow) as compared with the left second maxillary molar (left arrow). (b) Sagittal sections of both second maxillary molars (M2) show lack of enamel and dentin production of the right M2 with retention of the deciduous tooth, or “cap” (asterisk) when compared with the normal left M2. (c, d) Malformation and abnormal orientation of the maxillary incisors with aplasia of the left canine tooth.

Figure 10.5. Horse. Juvenile. Osteochondritis dissecans (OCD), hock. (a) Digital radiograph reveals an osteochondral fragment (arrow) associated with the distal intermediate ridge of the tibia (“DIRT” lesion). (b, c) The osteochondral fragment is loosely associated with the subchondral bone (arrow) and manually dissects away from the subchondral bone, which has a soft, crumbly texture. (b) The distal fibula (F) is labeled for orientation. (Gross photos in [b] and [c] courtesy of Dr. P. Habecker, New Bolton Center, University of Pennsylvania)

Figure 10.6. Horse. Juvenile. OCD, stifle. Oblique projection digital radiographs with enlarged insets of the (a) left and (b) right femoropatellar joints reveal an OCD lesion of the lateral trochlear ridge (LTR) of the right stifle (arrow) as compared with the normal LTR of the left stifle (arrow). (c) Gross photos show a large dissecting osteochondral defect in the left LTR (right arrows); however, a smaller nondissecting cartilage defect is within the LTR of the right stifle (left arrows). The animal was euthanized due to poor prognosis for future athletic performance. (Gross photo [c] courtesy of Dr. P. Habecker, New Bolton Center, University of Pennsylvania.)

Figure 10.7. Horse. Juvenile. OCD, lateral trochlear ridge of distal femur. (a) Photomicrograph of the lesion shows a dissecting fissure through the articular cartilage (arrows; H&E) and (b) a retained core of hyaline cartilage (asterisk; H&E) surrounded by sclerotic subchondral bone. (c) A dissected fragment of cartilage, enmeshed in fibrin, is adjacent to the synovial membrane of a joint recess (H&E).

Figure 10.8. Horse. OCD, humeral head of glenohumeral (shoulder) joint. Digital radiographs of the (a) left and (b) right shoulder joints from an 8-month-old warmblood filly with chronic right forelimb lameness and shoulder joint effusion. (b) As compared with the left, the right shoulder shows an abnormal contour of the right humeral head with joint space incongruence, subchondral lysis (asterisk), subchondral sclerosis, and numerous free osteochondral fragments along the caudal distal aspect of the glenoid (arrow). (c) Gross evaluation reveals a small articular cartilage depression (mild OCD) within the left glenoid (arrow), but (d) the right glenoid has numerous depressions and dissecting cartilage flaps (arrow), with severe secondary degenerative osteoarthritis characterized by subchondral sclerosis and eburnation of the humeral head (asterisk), hyperplasia and hyperemia of the synovial membrane (arrowhead), and fibrous thickening of the joint capsule.

Figure 10.9. Horse. Dynamic cervical vertebral stenosis due to OCD and malarticulation of the cervical vertebral facets. (a) Digital radiograph with epidural contrast (myelogram) of the cervical spine shows vertebral canal stenosis with thinning of the dorsal and ventral dye columns (arrow) and enlarged facets (arrowhead). (b) Gross dissection of C4-C5 facet joint reveals articular cartilage fissures (arrows) and an isolated osteochondral island (asterisk) within the C4 caudal facet. Subchondral bone lipping along the articular margins of the cranial C5 facet (arrowheads) with synovial hyperplasia and hyperemia (arrows) indicates secondary degenerative osteoarthritis.

Figure 10.10. (a) Boiled specimen of a cervical spine from an 18-year-old Thoroughbred gelding that had dynamic cervical vertebral stenosis and prior arthrodesis with metallic basket surgical implants (arrows) placed as a juvenile. The surgically fused joints show smooth bony contours of the facet joints (a, open arrowhead; (b), whereas the adjacent, nonsurgically fused facet joint shows severe periarticular osteophytosis (a, arrowhead; (c) consistent with chronic vertebral instability and secondary severe degenerative osteoarthritis.

Figure 10.11. Horse. (a) Parasagittal section shows a fracture (arrows) with a hematoma (H) centered on the synarthrosis between the basisphenoid and basioccipital bone with dorsal subluxation and compression of the brainstem caudal to the hypothalamus (arrowhead). The cerebellum (C) is labeled for orientation. (b) Digital radiograph shows a linear lucency that extends ventrally from the nuchal crest and courses through the suture lines between the occipital and parietal bones, and basisphenoid and basioccipital bones (arrows), representing traumatic separation at the suture line. Early callus at the base of the basisphenoid bone is consistent with the duration of time since the injury was sustained.

Figure 10.12. Parasagittal section of a fractured cervical spine from a 4-year-old Thoroughbred colt that occurred immediately following a breakdown injury of the forelimb. Displaced, comminuted fractures of the third cervical vertebral body and dorsal lamina resulted in subluxation of the C2-C3 vertebral endplates with compression and complete transection of the spinal cord (arrow). Surprisingly, the dura mater remained intact.

Figure 10.13. Horse. Physeal fracture of the left tibia from a foal with surgical fixation, implant infection and removal, and progressive fracture remodeling. (a) A Salter-Harris fracture type II (Day 0) is surgically repaired (b) using metallic plates and screws (Day 1). Due to complications, the surgical implants were removed. (c) Progressive modeling and ossification of the fracture is demonstrated (Day 60), and shows (d) gradual reduction and smoothing of the callus with eventual incorporation of the metaphyseal spike (Day 180).

Figure 10.14. Foal. Chronic epiphyseal fracture of the femoral head with luxation and pseudoarthrosis. A portion of the epiphysis of the femoral head is fractured (arrowheads). The displaced fracture fragment is adhered within a fibrotic articular surface of the acetabular cup (black arrow) and surrounded by villous projections of a hyperplastic synovial membrane (V). The femoral head is luxated and sits within a soft tissue pocket, or “pseudoarthrosis,” located dorsocranial to the acetabulum (white arrows).

Figure 10.15. Racehorse. Juvenile. (a) Radiograph, antemortem. A displaced oblique fracture extends through the proximal to mid humeral diaphysis (black arrows). A slight periosteal proliferation is evident along the proximal caudal medial surface of the metaphysis (white arrows). (b) Nuclear scintigraphic scan: antemortem. Markedly increased radiopharmaceutical uptake within the proximal diaphyseal-metaphyseal junction, consistent with active bone remodeling, and indicative of a proximal humeral stress fracture. (c) Radiograph: postmortem. A diaphyseal fracture line extends into the caudal aspect of the metaphysis. Patchy medullary radiopacities (black arrows) surround this portion of the fracture, indicative of endosteal callus.

Figure 10.16. Racehorse. Juvenile. Gross photos of the humeral fracture imaged in Figure 10.15. (a) The normal left humerus is compared with (b) the right humerus that shows a displaced dorsal distal diaphyseal fracture bridged by hemorrhage that extends proximally through the medullary cavity and out through the caudal proximal diaphyseal-metaphyseal junction (white arrows). (c) Inset of the metaphyseal region shows intramedullary foci of new bone (e.g., endosteal callus) that surround the fracture line (black arrows), as well as slight periosteal callus along the caudal metaphysis (white arrow).

Figure 10.17. Racehorse. Juvenile. H&E photomicrographs of acute fracture and early phases of fracture healing. (a) Fracture fragments of necrotic bone (NB) are embedded within a hematoma (asterisk). (b) With time, the hematoma will organize into granulation tissue with proliferation of mesenchymal stem cells (arrows). (c) Osteoclasts (arrows) begin to resorb necrotic bone spicules (NB), as osteoblasts begin to produce woven “repair” bone (arrowhead). This process is also known as “creeping substitution.”

Figure 10.18. Racehorse. Juvenile. Photomicrographs (H&E) of later phases of fracture healing. (a) Woven bone (WB) produced by osteoblasts from differentiated osteoprogenitor cells (asterisk) forms (b) an endosteal callus (EC) adjacent to (c) partially resorbed fragments of compact lamellar bone (LB).

Figure 10.19. Horse. Lateral condylar fracture of the left third metacarpus with internal fixation surgical repair. (a) Preoperative and (b) postoperative digital radiographs of a Thoroughbred racehorse that sustained an incomplete simple, nondisplaced lateral condylar fracture. (b–d) Excellent anatomic reduction of the fracture was achieved by internal fixation using cortical bone screws placed in lag fashion, and articular cartilage damage is minimal. An excellent prognosis to return to racing was given; however, the horse was euthanized due to development of postoperative pneumonia.

Figure 10.20. Horse. Medial condylar fracture (Y configuration) of (a) the left hind limb with (b, c) surgical internal fixation and subsequent catastrophic failure. The postoperative fracture extends through two proximal screws placed in the mid‑diaphyseal region (arrows).

Figure 10.21. Horse. Chronic articular wing fracture of the distal phalanx with surgical implant infection and fibrous nonunion. (a) Preoperative digital radiographs of a mildly displaced articular wing fracture (arrow). (b) Digital radiographs obtained 3 months postoperatively reveal widening of the fracture line and mild lysis surrounding the surgical implants. (c) Dissection of the distal phalanx shows poor anatomic reduction with proximal and caudal displacement of the largest fracture fragment forming a prominent “step” in the articular cartilage surface.

Figure 10.22. Horse. Photomicrographs (H&E) show (a) persistence of the fracture (arrows) with incongruence of the articular cartilage surface (AC). (b) A section through the deeper portion of bone shows fracture fragments bridged by 5 mm of fibrous connective tissue (arrow). (c) Small necrotic bone fragments, sequestra (S) are surrounded by suppurative inflammation and multinucleate osteoclasts (arrows). (d) Persistent infection is evidenced by bacteria observed within necrotic bone fragments (arrow) surrounded by neutrophils (N).

Figure 10.23. Horse. Traumatically induced chronic septic arthritis of the metacarpophalangeal joint. (a) A cutaneous ulcer and bud of granulation tissue surrounds a chronic fistula that communicates with the metacarpophalangeal joint. (b) The synovium is severely hyperplastic and hyperemic, and the articular cartilage is thinned and yellow. (c) Pannus extends from the periarticular margins over portions of the articular cartilage, which has an irregular “scored” surface.

Figure 10.24. Septic arthritis of the tarsocrural (hock) joint in a young foal with recent history of enterocolitis. (a) Prolonged suppurative inflammation resulted in diffuse cartilage atrophy and synovial hyperplasia (arrow) with fibrinous exudate. (b) Close inspection reveals few remaining islands of articular cartilage (arrow). (c) Photomicrograph of the synovial membrane effaced by an exudate of fibrin and inflammatory cells (double-headed arrow) and subtended by granulation tissue proliferation (GT).

Figure 10.25. Septic epiphysitis of the femoral head in a young foal. (a) Articular cartilage atrophy exposes the vascular channels that feed the deeper layers of cartilage (arrow). (b) Sagittal section reveals a discrete yellow-tan focus of inflammation within the subchondral bone of the femoral capital epiphysis (arrow). Low (c) and high (d) magnification H&E photomicrographs of the inflammatory focus in (b) show suppurative inflammation filling the medullary spaces and (c) extending into an expanded vascular canal within the overlying cartilage. (d) Fractured primary trabeculae (F) are surrounded by neutrophils and multinucleate osteoclasts (arrows).

Figure 10.26. Septic arthritis and osteomyelitis of the distal tibia with physeal abscess and sequestrum in a young foal. (a) A digital radiograph reveals soft tissue swelling involving the tarsocrural joint (small outer arrows) and a well-defined radiolucency adjacent to the distal tibial physis (large arrows). (b) The joint is packed with gelatinous, fibrino-suppurative exudate (arrowhead), and there is articular cartilage atrophy of the talus (T). (c) The distal tibia contains a well-demarcated dull yellow focus of necrotic bone, that is, sequestrum (arrows). (d) A sagittal section reveal a well-demarcated abscess proximal to the metaphyseal physis surrounded by a sclerotic rim (arrows). The abscess crosses the physis, extending to the sequestrum (arrowheads).

Figure 10.27. (a) Septic osteomyelitis of the central tarsal bone with pathologic fracture (black arrows) and displaced comminuted fracture fragments (white arrows) in a foal. (b) This fracture was initially thought to be of traumatic origin; however, necropsy revealed suppurative inflammation within the joint that extended from the inner portions of the bone. (c) Septic epiphysitis of the lateral femoral condyle of a foal can resemble an OCD lesion on a radiograph (arrowhead); however, the predilection site was inappropriate for OCD, and (d) necropsy revealed a wedge-shaped yellow suppurative focus at the site of the osteochondral defect (arrowheads), indicative of infectious etiology.

Figure 10.28. Septic osteomyelitis of the right hind medial sesamoid bone from a Thoroughbred racehorse euthanized for chronic intermittent grade 1-3 lameness. (a) A wedge-shaped radiolucency within the medial sesamoid bone seen on the dorsomedial-plantarolateral oblique projection (arrow) corresponds to (b) severe increased radiopharmaceutical uptake within the same sesamoid bone (arrow). (c) Metatarsophalangeal (fetlock) joint dissection reveals a semi-lunar depression within the central articular surface of the medial sesamoid bone (arrowhead) with mild clear-yellow effusion and synovial hyperplasia. (d) Sagittal sections reveal a wedge-shaped sequestrum (arrow) surrounded by an involucrum.