Musculoskeletal strain: The patient's symptoms of back pain between the shoulder blades radiating into the chest, worsened by range of motion (ROM) and deep breathing, and a recent history of working in the garden, suggest a musculoskeletal origin. The absence of cough, shortness of breath (SOB), and the negative troponin level further support this diagnosis.

Costochondritis: Inflammation of the cartilage that connects the ribs to the breastbone can cause chest pain that worsens with deep breathing and movement, fitting the patient's symptoms.
Gastroesophageal reflux disease (GERD): Although the patient does not report nausea or vomiting, GERD can cause chest pain, especially after eating or with certain positions, and the mild intermittent diarrhea could be related to dietary factors that also exacerbate GERD.
Thoracic spine dysfunction: Issues such as a thoracic spine sprain or strain could explain the back pain radiating into the chest, especially with the mechanism of injury from gardening.

Pulmonary embolism (PE): Although the chest X-ray is clear and there's no reported shortness of breath, PE can present atypically, especially in older adults, and is a potentially life-threatening condition that must be considered.
Aortic dissection: This is a medical emergency that can present with sudden, severe chest or back pain, which can radiate. The absence of typical risk factors (e.g., hypertension) does not rule out this diagnosis.
Myocardial infarction (MI) with negative troponin: Early in the course of an MI, troponin levels may not be elevated. The absence of typical symptoms (e.g., arm pain, jaw pain) and a negative troponin does not completely rule out MI, especially in women, who can have atypical presentations.

Pneumothorax: A spontaneous pneumothorax could cause sudden chest pain and worsen with deep breathing, though typically it would also cause shortness of breath and might be visible on a chest X-ray.
Esophageal rupture or perforation: This would be an emergency situation, usually presenting with severe chest pain after vomiting or retching, which is not reported here.
Osteoporotic vertebral fracture: Although less common without a history of significant trauma, osteoporotic fractures can occur spontaneously or with minimal trauma in individuals with osteoporosis, causing back pain.