Introduction

Sternal luxation is a rarely reported but potentially more common than realized condition of the small animal patient. Although little literature exists documenting feline and canine sternal luxation and its cause, trauma is anecdotally believed to be the most prevalent etiology. Of the four existing case reports, one dog and one cat required rigid surgical fixation whereas one dog and one cat recovered with conservative medical management.^1–3 Because of the extremely limited available literature, definitive case management of sternal luxation is poorly defined but likely guided by the number of sternebrae luxated, the stability of the luxated sternebrae, and the concurrent sequelae. This report discusses successful medical management of a sternal luxation and proposes possible case management criteria for surgical versus conservative medical management.

Case Report

A 5 yr old spayed female indoor–outdoor domestic longhair red tabby cat presented for evaluation of a 1-day history of hiding and vocalizing when picked up. The owner reported that, for the previous day, the cat had deviated from its normal routine by not wanting to go outside at night, hiding under the bed, not eating, and urinating and defecating outside the litter box with a single episode of diarrhea.

On physical examination, the cat was quiet but responsive, weighed 3.8 kg, had a body condition score of 5/9, and had multifocal areas of lentigo on the lips and gums. Thoracic auscultation revealed no cardiac or respiratory abnormalities. The vital parameters were within normal limits for a stressed cat in the veterinary hospital (rectal temperature 37.7°C, pulse 222 beats/min, respiration 42 breaths/min). Palpation of the thorax revealed significant pain along the caudal sternum where an approximately 8 × 6 cm contusion was noted. Abdominal palpation was within normal limits with the exception of mild to moderate pain in the cranioventral area.

A urinalysis revealed severely concentrated urine (urine specific gravity 1.064, reference range 1.035–1.050), a slight haziness, and trace protein. A serum biochemical analysis revealed mild hyperglycemia (198 mg/dL, reference range 70–160 mg/dL), mild hypoproteinemia (6.1 g/dL, reference range 6.5–8.4 g/dL), mild hypoglobunlinemia (3.4 g/dL, reference range 4.1–6.0 g/dL), and markedly increased creatine kinase (6190 U/L, reference range 50–225 U/L). A complete blood count revealed mild hemoglobinemia (8.6 g/dL, reference range 9.0–12.0 g/dL), mild microcytic anemia (mean corpuscular volume 39.9 fL, reference range 40.0–55.0 fL; hematocrit 25.7%, reference range 9.0–13.0%; packed cell volume 25%, reference range, 30.0–46.0%), and mild thrombocytopenia (176 × 10³/μL, reference range 200–700 × 10³/μL). The patient was sedated with 0.02 mg/kg of buprenorphine IV and 0.01 mg/kg of dexmedetomidine IV, and orthogonal radiographs of the thorax and abdomen were obtained.

Thoracic radiographs revealed a luxated fifth intersternebral joint with the sixth sternebra being cranioventrally displaced (along with the seventh and eighth sternebrae) to the level of the mid fourth sternebra (Figure 1). There were sharply marginated, short oblique fractures of the distal sixth costal cartilages bilaterally with mild dorsal displacement of the distal segment. Moderate soft-tissue swelling was present ventral to the sternum, most severe at the site of luxation. The cardiac silhouette was mildly enlarged, occupying greater than 50% of the thoracic cavity width on the ventrodorsal projection. The abdominal radiographs were unremarkable.

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After obtaining orthogonal thoracic and abdominal radio-graphs, the sternal luxation of the sedated patient was palpated more aggressively and deemed to be stable. Because of the stability of the luxation and the absence of sternebral fractures, conservative medical management in the form of analgesics and rest was recommended. The owner was instructed to prevent the patient from going outside until the recheck in 14 days and to administer buprenorphine 0.4 mL intramuscularly q 12 hr for 3 days (0.3 mg/mL), gabapentin 50 mg per os q 12 hr for 14 days, and robenacoxib 6 mg per os q 24 hr for 3 days. At the recheck 14 days after presentation, the patient’s contusion had resolved, pain on palpation was absent, and the luxation was still palpably stable. Repeat thoracic radiographs 2 wk after presentation revealed an unchanged sternal luxation and resolution of the soft-tissue swelling. Twelve months after presentation, the patient presented for an unrelated lameness. In that timeframe, the patient had no consequences to the sternal luxation, which still palpated stable and was radiographically unchanged. This case was exempt from Institutional Animal Care and Use Committee approval, and owner consent to treat was obtained at each hospital visit.

Discussion

Although little literature exists documenting feline and canine sternal luxation and its cause, trauma is anecdotally believed to be the most prevalent etiology. Because of the many possible sequelae of trauma and their varying severity, thoracic orthogonal radiographs are recommended to help guide proper case management.⁴ Disruption of thoracic integrity can lead to paradoxical thoracic wall movement (flail chest), sternebrae/rib fractures, pneumothorax, life-threatening hemorrhage, pyothorax, breathing abnormalities, pulmonary contusions, and significant pain.⁵ The stability of the patient and the presence of pain can make accurate palpation of the sternum and associated structure quite challenging. Therefore, orthogonal thoracic radiographs are vital for assessing the presence and severity of trauma associated pathology.

Definitive case management of sternal luxation is poorly defined but likely guided by the number of sternebrae luxated, the stability of the luxated sternebrae, and the concurrent pathology. The feline sternum is composed of eight total bones, consisting of a cranial manubrium, six articulated sternebrae (body), and a caudal xiphoid process with the intersternebral joints comprised of synchondroses.⁶ The literature provides that the manubrium is often referred to as the first sternebra and the xiphoid the eighth. The sternum itself serves multiple important purposes, including stabilizing the thoracic cavity and thereby contributing to protecting vital thoracic structures such as the heart, lungs, and major blood vessels of the thoracic wall.⁷ Thoracic radiography can identify disruption of the thoracic wall integrity, which can lead to a loss of negative pressure within the thorax and a subsequent pneumothorax. These situations would necessitate surgical intervention to restore integrity and re-establish negative pressure within the thorax. Thoracic radiographs will also aid in identification of sternebral fractures, which will potentially require rigid surgical fixation because of the disruption in sternum stability.

Stability of the entire sternum and the abnormal sternebrae become the basis for treatment in the absence of sternebral fractures or a disruption of the thoracic wall leading to the loss of negative pressure in the thoracic cavity. One case report documented a 1 yr old spayed female cat with a stable seventh sternebra and xiphoid luxation, no clinical respiratory abnormalities, and no thoracic abnormalities who responded well to rest and analgesics at the 3 wk follow-up.³ Another cat with a palpably unstable fourth and fifth sternebrae luxation and dyspnea required rigid surgical fixation of the instability with a return to normal cardiopulmonary parameters and ambulation at the 5 wk postsurgical follow-up.² Interestingly, these authors also reported that a cat with polytrauma, dyspnea, multiple long bone fractures, and an unstable seventh sternebra luxation had surgical repair of the long bone fractures on presentation and initial medical management for the sternal luxation.² Ultimately, the sternal luxation required rigid surgical fixation 14 mo later because of chronic instability, which suggests that instability alone may not lead to cardiopulmonary abnormalities.

Because of the risks associated with surgical intervention as well as the skill level and equipment required to perform rigid surgical fixation of luxated sternebrae, many veterinarians likely opt to conservatively manage these cases. More severe sequelae of trauma may also lead to sternal luxations being underdiagnosed because of euthanasia before a definitive diagnosis. All of these issues could contribute to the lack of peer-reviewed literature and case reports. This also highlights the importance of orthogonal thoracic radiographs in trauma patients or when sternal luxation is suspected, which the authors suspect would alter the perceived rare prevalence of sternebrae abnormalities. The existing limited literature suggests that patient cardiopulmonary status, thoracic wall integrity, and sternal stability all contribute to case management strategies, but that sternal stability may not be clinically significant as long as the cardiopulmonary status is stable.

Conclusion

To the authors’ knowledge, this is the fifth reported case of small animal sternebral luxation and only the second (both cats) to demonstrate successful outcome using conservative medical therapy. This case provides further insight and strengthens the limited literature regarding the application of conservative medical therapy for dogs and cats with sternebral luxation. Because trauma is the likely etiology of the majority of sternebral luxations, basic trauma stabilization and patient assessment should occur before determining sternebral luxation management strategies. Ultimately, trauma sequelae and sternum stability should determine whether surgical intervention is needed. This case report and the one other case in which conservative medical therapy were instituted provide growing evidence of the practicality of conservative medical therapy in the presence of stable luxated sternebrae, potentially regardless of the degree of displacement.