Subchondral bone sequestrum formation in the proximal intra-articular and osteochondral region of the third metatarsal bone of an Appaloosa mare treated for septic arthritis

Charlotte K. Barton Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO

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Monika A. Samol Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO

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Brad B. Nelson Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO

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Gabriella Piquini Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO

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Lauren E. Smanik Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO

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Laurie R. Goodrich Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO

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Abstract

OBJECTIVE

To raise awareness of the potential for intra-articular subchondral bone sequestrum formation secondary to a traumatic or septic process to enable more rapid identification of this uncommon but possible outcome in future cases.

ANIMAL

A client-owned 12-year-old Appaloosa mare.

CLINICAL PRESENTATION, PROGRESSION, AND PROCEDURES

The mare had a wound to the lateral aspect of the fourth metatarsal bone (MT4) that communicated with the distal tarsal joints. Radiographs revealed a displaced, comminuted fracture of MT4.

TREATMENT AND OUTCOME

The horse underwent aggressive debridement of the wound and MT4 as well as, on 2 occasions, needle joint lavage. Systemic, regional, and IA antibiotic therapy was also performed together with a bone graft from the tuber coxae. The horse’s comfort improved, and the wound appeared to be healing. Five weeks following discharge, the horse re-presented with a non–weight-bearing lameness and radiographs revealed marked osteomyelitis of the tarsometatarsal and distal intertarsal joints. Postmortem examination of the limb identified a sequestrum within the proximal articular surface of the third metatarsal bone.

CLINICAL RELEVANCE

The present report highlights the importance of arthroscopic lavage to visualize the cartilage surface and the benefits of advanced imaging to detect associated changes within the bone earlier than conventional radiographs. To our knowledge, no reports exist of intra-articular subchondral bone sequestra in the tarsometatarsal joint in horses.

Abstract

OBJECTIVE

To raise awareness of the potential for intra-articular subchondral bone sequestrum formation secondary to a traumatic or septic process to enable more rapid identification of this uncommon but possible outcome in future cases.

ANIMAL

A client-owned 12-year-old Appaloosa mare.

CLINICAL PRESENTATION, PROGRESSION, AND PROCEDURES

The mare had a wound to the lateral aspect of the fourth metatarsal bone (MT4) that communicated with the distal tarsal joints. Radiographs revealed a displaced, comminuted fracture of MT4.

TREATMENT AND OUTCOME

The horse underwent aggressive debridement of the wound and MT4 as well as, on 2 occasions, needle joint lavage. Systemic, regional, and IA antibiotic therapy was also performed together with a bone graft from the tuber coxae. The horse’s comfort improved, and the wound appeared to be healing. Five weeks following discharge, the horse re-presented with a non–weight-bearing lameness and radiographs revealed marked osteomyelitis of the tarsometatarsal and distal intertarsal joints. Postmortem examination of the limb identified a sequestrum within the proximal articular surface of the third metatarsal bone.

CLINICAL RELEVANCE

The present report highlights the importance of arthroscopic lavage to visualize the cartilage surface and the benefits of advanced imaging to detect associated changes within the bone earlier than conventional radiographs. To our knowledge, no reports exist of intra-articular subchondral bone sequestra in the tarsometatarsal joint in horses.

History

A 12-year-old Appaloosa mare was initially presented to the referring veterinarian with a 6 X 3-cm laceration to the lateral aspect of the right fourth metatarsal bone (MT4) that had occurred 3 days previously following a kick from another horse. Radiographs of the tarsus were performed and identified a fracture to MT4, and the horse was referred to Colorado State University Johnson Family Equine Hospital for further evaluation.

Diagnostic Findings and Interpretation

On presentation, the horse was consistently lame on the right hind limb at the walk and was reluctant to fully bear weight when standing. The laceration appeared grossly contaminated with the presence of necrotic tissue, bone fragments, and cloudy yellow drainage (Figure 1). Upon exploration of the wound, extensive communication with the fractured MT4 was appreciated. Following aseptic preparation, the tarsometatarsal (TMT) joint and distal intertarsal (DIT) joint were accessed from the medial aspect of the limb with a 20-gauge needle. Joint fluid from the TMT joint was obtained, although aspiration of fluid from the DIT joint was unsuccessful. Distention of both joints with sterile saline confirmed communication with the wound.

Figure 1
Figure 1

A 12-year-old Appaloosa mare with a 6 X 3-cm laceration to the lateral aspect of the right fourth metatarsal bone (MT4).

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0476

Fluid analysis and cytology of the TMT joint confirmed septic neutrophilic inflammation, with the presence of intracellular and extracellular cocci. The fluid had a nucleated cell count of 26,400 cells/µL (< 1,000 cells/µL), 97% degenerate neutrophils (< 10%), and a total protein of 6.8 g/dL (0.8 to 2.5 g/dL).

Radiographs of the right tarsus confirmed a 3.5-cm parasagittal comminuted fracture of MT4 that extended from the TMT joint proximally to the lateral cortex distally, with multiple fracture fragments (Figure 2). Ultrasound of the area confirmed communication between the wound and TMT with regional cellulitis and multifocal foreign debris. Mild diffuse desmitis of the long plantar ligament was noted; otherwise, no damage was noted to the surrounding soft tissue structures.

Figure 2
Figure 2

Non–weight-bearing lateromedial radiograph of the horse described in Figure 1. The radiograph shows a comminuted, mildly displaced proximal MT4 traumatic fracture with extension into the tarsometatarsal joint.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0476

Treatment and Outcome

An IV catheter was placed in the right jugular vein, and the horse was administered IV 22,000 IU of potassium penicillin/kg, 6.6 mg of gentamicin/kg, and 4.4 mg of phenylbutazone/kg prior to the induction of general anesthesia. The horse was placed in dorsal recumbency and the skin surrounding the wound thoroughly cleaned with 4% chlorhexidine and saline. The wound was sharply debrided, removing bone fragments and necrotic tissue. The medial aspect of the tarsus was then aseptically prepared, and, under radiographic guidance, the TMT and DIT joints were accessed with 20-gauge needles and a single 14-gauge needle was placed within the lateral aspect of the TMT to aid egress. The joints were lavaged utilizing a fluid pump and 10 L of sterile saline. Upon completion, 125 mg of amikacin was injected into both the TMT and DIT joints. A regional limb perfusion was performed with an Esmarch tourniquet placed just proximal to the tarsus, and 2 g of amikacin was injected into the saphenous vein along with 60 mL of sterile saline. The tourniquet was left in place for 20 minutes. Following the procedure, saline-soaked gauze (AMD roll; Kerlix) was placed over the wound and a full limb compressive bandage was applied. The horse recovered from anesthesia uneventfully.

The horse was continued to be managed on IV antibiotics (potassium penicillin, 22,000 IU/kg, and gentamicin, 6.6 g/kg) and a decreased dose of phenylbutazone (2.2 mg/kg, IV, q 12 h). Daily bandage changes and regional limb perfusions were performed with 2 g of amikacin for a further 5 days. The horse’s comfort level remained unchanged; therefore, 2 days later, a second joint lavage under standing sedation was performed.

Six days following presentation, repeat radiographic imaging was obtained due to continued discomfort. Radiographs identified mildly increased lucency of the fracture margins at the plantar aspect with widening of the fracture gap but no further abnormalities. Under standing sedation, a cancellous bone marrow graft was delivered to provide osteogenic stimulation and bone formation as well as antimicrobial effects. Cancellous bone was harvested from the ileum and immediately packed into the wound bed after aseptic preparation. A collagen sponge was placed over the graft, and a compressive stacked bandage was applied.

The horse’s comfort improved gradually, and 10 days following presentation the horse was switched to oral antibiotic medication (sulfadiazine/trimethoprim, 23 mg/kg, PO, q 12 h for 10 days) and discharged to a rehabilitation center. The horse was initially doing well, with lameness improving and the wound healing appropriately. However, 5 weeks later the horse was readmitted to the hospital as she became increasingly lame at the walk. The wound bed had healthy granulation tissue and good epithelization; thus, the source of persistent lameness was suspected to be the fractured MT4. A CT was recommended to more completely characterize the diseased tissues, but it was declined. The owners opted for removal of the fractured MT4 under general anesthesia, which was performed without complication.

The horse’s comfort improved initially; however, 3 days later she became non–weight-bearing. Repeat radiographic examination of the tarsus was performed, revealing marked progression of the TMT osteomyelitis, and the horse was humanely euthanized (pentobarbital, 85 mg/kg, IV) due to the poor prognosis for resolution of the infection.

Postmortem Examination

A postmortem CT scan of the right hind limb was performed in bone window (window width, 15,000 HU; window level, 450 HU) and soft tissue window (window width, 450 HU; window level, 60 HU) with 1-mm slice thickness. CT was followed by detailed autopsy of the limb from the mid-diaphysis of the tibia to the hoof. CT identified a large osseous fragment (4.8 mm height X 3.5 mm width X 5 mm depth; 1,390 HU) within the subchondral bone at the dorsomedial aspect of the proximal third metatarsal bone (MT3), surrounded by a hypoattenuating rim (158 HU) in relation to the surrounding cortical bone and focally mildly sclerotic trabecular bone (Figure 3). The aforementioned hypoattenuating halo corresponded with a bordering rim of subchondral lysis partially filled with pink/red-tinged fluid that was identified on gross examination. In the opposing articular surface of the third tarsal bone in relation to the sequestrum, there was a focal, ill-defined region of subchondral demineralization that was identified on CT and radiographs. Further changes associated with persistent, septic osteoarthritis of the TMT joint were noted, including regionally extensive cartilage erosion extending from the lateral aspect of the interosseous fossa recognized on gross examination, as well as irregular bone loss along the dorsolateral and lateral articular margin of the MT3 seen on the necropsy and CT (Figures 3 and 4). The TMT joint capsule and synovium were diffusely thickened and discolored (dark red).

Figure 3
Figure 3

Intraoperative dorsolateral-palmaromedial radiographic projection (A) and postmortem dorsal reconstruction of CT image (window width, 1,571 HU; window level, 385 HU; 1-mm slice thickness; B) of the right tarsus and proximal metatarsus of the horse described in Figure 1. A—Note the osseous fragment within the dorsomedial-proximal aspect of the third metatarsal bone (MT3) bordered by a radiolucent halo, representing subchondral sequestration (arrow) as seen on gross images from Figure 2. B—Subchondral sequestrum visualized on CT as an osseous fragment with a hypoattenuating rim (158 HU) in relation to the surrounding cortical bone and medially adjacent focally sclerotic trabecular bone (asterisk). The opposing articular surface of the MT3 has a focally decreased, ill-defined region of demineralization representing subchondral bone lysis (arrowhead), which can also be identified on the provided radiographic view (A; arrowhead). The MT4 has been surgically removed.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0476

Figure 4
Figure 4

Skeletal abnormalities in the tarsometatarsal joint including subchondral sequestrum at the dorsomedial aspect of the proximal articular surface of the MT3 identified on postmortem gross examination of the horse described in Figure 1. The necrotic osseous fragment marked with a yellow circle on upper images and yellow arrow on dorsal cross sections is bordered by lytic subchondral bone replaced by pink/red-tinged tissue admixed with fluid. The overlaying cartilage is focally thinned and fibrillated, with a bordering rim of gray discoloration. There is a subtle rim of compacted (sclerotic) trabecular bone medially adjacent to the described sequestrum (arrowhead). Additionally, the cartilage adjacent to the lateral margin of the tarsometatarsal interosseous fossa has regionally extensive erosion (asterisk). The dorsolateral- and lateral-proximal articular margin of the MT3 is mildly to moderately irregular/lytic (green arrows). The MT4 has been surgically removed. MT2 = Second metatarsal bone.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0476

Comments

This report describes a case of sequestrum formation within the proximal articular surface and associated subchondral bone of the proximal MT3 associated with traumatic synovial sepsis, a finding that, to the authors’ knowledge, has not been previously described in the published literature.

The development of a sequestrum at the unusual location described in this case was likely caused by ischemic damage to the subchondral bone at the time of wounding. A sequestrum is a fragment of necrotic bone that is formed when the blood supply to the parent bone is compromised. Often, cortical ischemia is caused by damage to the periosteum, which disrupts the venous drainage, resulting in local vascular stasis.1 The necrotic bone then incites an inflammatory response, which leads to exudative fluid accumulation. The fluid both provides a growth medium for bacteria and exerts pressure on the surrounding soft tissues, which can result in the formation of a draining tract and nonhealing wound.1

While the rapid recognition and treatment of osteomyelitis is imperative to optimize outcome, early diagnosis is hampered by the inability to identify lesions on plain radiographs. Studies in the human literature comparing CT and plain radiography in osteomyelitis diagnosis determined that CT had a sensitivity of 67% while plain radiography only had a sensitivity of 16%,2 thus highlighting the importance of advanced diagnostic imaging in such cases. With the advent and gaining popularity of standing CT units, these imaging modalities are more reasonable in terms of practicality and cost and could greatly inform the clinician as to whether surgical (or otherwise) intervention would be helpful.3

Only 1 previous case of intra-articular sequestrum formation has been described in the published literature, which occurred in a 5-month-old Connemara foal.4 The foal was diagnosed with septic physitis of the proximal humerus, and a large defect involving the central portion of the subchondral bone plate of the glenoid and a sequestered bony fragment were detected following CT imaging.4 In this foal, the sequestrum was associated with septic physitis rather than septic arthritis as described in this report.

The objective of this case report was to raise awareness of the potential for intra-articular subchondral bone sequestrum formation associated with traumatic septic arthritis to enable earlier recognition and treatment. Practitioners should be especially vigilant in cases involving the distal tarsal joints where arthroscopic lavage and visualization of the cartilage surface is not possible, meaning that these lesions may go undetected for many weeks before radiographic signs become visible.

Acknowledgments

We would like to thank Dr Keri Belsito, the referring veterinarian, for all her care and efforts in managing this case.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.

References

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    Gift LJ, DeBowes RM. Wounds associated with osseous sequestration and penetrating foreign bodies. Vet Clin North Am Equine Pract. 1989;5(3):695708. doi:10.1016/s0749-0739(17)30583-7

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    Malcius D, Jonkus M, Kuprionis G, et al. The accuracy of different imaging techniques in diagnosis of acute hematogenous osteomyelitis. Medicina (Kaunas). 2009;45(8):624631. doi:10.3390/medicina45080081

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    Mathee N, Robert M, Higgerty SM, et al. Computed tomographic evaluation of the distal limb in the standing sedated horse: technique, imaging diagnoses, feasibility, and artifacts. Vet Radiol Ultrasound. 2023;64(2):243252. doi:10.1111/vru.13182

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    Clements PE, Jones B, Coomer R. Septic epiphysitis and sequestrum formation in the glenoid of the scapula in a five-month-old foal diagnosed by computed tomography. Vet Rec Case Rep. 2019;7(3):e000837. doi:10.1136/vetreccr-2019-000837

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