Chronic Tendon Injury (e.g., Mallet Finger or Tendon Contracture): This is the most likely diagnosis given the chronic nature of the flexion deformity and the mention of "findings may represent chronic tendon injury" in the radiology report. The absence of an acute fracture and preserved joint spacing also supports this diagnosis.

Dupuytren's Contracture: A condition characterized by thickening of the palmar fascia leading to flexion deformity of the fingers, which could explain the flexion deformity at the PIP joint.
Osteoarthritis: Although the joint spacing is preserved, early stages of osteoarthritis could lead to flexion deformity, especially if there's a component of ligamentous laxity or previous trauma.
Previous Trauma with Malunion or Nonunion: Even though the current radiographs do not show an acute fracture, a previous fracture that healed in a malunited or nonunited position could result in a chronic deformity.

Infection (e.g., Osteomyelitis or Septic Arthritis): Although less likely given the chronic presentation and lack of acute findings, infection could lead to destruction of the joint and surrounding tissues, resulting in deformity. Missing this diagnosis could have severe consequences.
Tumor (e.g., Giant Cell Tumor of the Tendon Sheath): A tumor could cause erosion of the bone or pressure on the tendons, leading to deformity. This is a critical diagnosis not to miss due to its potential for malignancy and need for early intervention.

Scleroderma: A systemic disease that can cause skin and connective tissue tightening, potentially leading to flexion contractures of the fingers.
Diabetic Cheiroarthropathy: Diabetes can lead to limited joint mobility, including flexion deformities, due to changes in the connective tissue.
Ehlers-Danlos Syndrome: A genetic disorder affecting connective tissues, which could result in joint hypermobility or, in some cases, contractures due to repetitive strain or injury.