Sprain or Ligament Injury: The mechanism of injury (falling and catching with the hand) and the presence of significant hand swelling with limited range of motion (ROM) in the thumb, despite no fracture on X-ray, suggest a soft tissue injury such as a sprain or ligament injury. This is a common outcome from such falls, especially when the hand absorbs the impact.

Contusion or Bruising: Direct trauma to the hand can cause significant bruising and swelling, which might limit movement due to pain and inflammation rather than a structural injury.
Tendon Injury: The limited ROM in the thumb could indicate a tendon injury, which might not be visible on a standard X-ray but could significantly impact hand function.
Wrist or Hand Strain: Overstretching or pulling of the muscles in the hand or wrist during the fall could lead to strain, causing pain and limited mobility.

Scapholunate Ligament Injury: Although X-rays may not show a fracture, a scapholunate ligament injury is a serious condition that could be missed. It requires specific diagnostic tests and can lead to significant long-term disability if not properly treated.
Compartment Syndrome: This is a medical emergency that can occur after a crush injury or significant trauma to the hand, leading to increased pressure within the compartments of the hand. It requires immediate intervention to prevent permanent damage.
Vascular Injury: Damage to the blood vessels in the hand could lead to ischemia or other complications. While less common, it's crucial to assess vascular integrity, especially if there are signs of decreased circulation.

Nerve Injury or Neuropathy: Direct trauma to the nerves in the hand could result in neuropathy, leading to numbness, tingling, or weakness in the hand or fingers. This might not be immediately apparent and could be a rare but significant complication.
Osteochondral Injury: Although less likely without a visible fracture, an osteochondral injury (where a piece of cartilage and bone is damaged) could occur and might require an MRI for diagnosis.
Infection or Septic Arthritis: If the skin was broken during the fall, there's a small risk of infection, including septic arthritis, which is a medical emergency requiring prompt antibiotic treatment.