Abnormal kinematics of the distal radioulnar joint due to flat bone dysplasia in nail-patella syndrome: a 3D evaluation with dynamic radiostereometry.

Nail-patella syndrome is a genetic disease with different phenotypes often affecting nails and knees. We present a woman with nail-patella syndrome with instability and dysfunction of the distal radioulnar joint. This is a trait which, to our knowledge, has not previously been described in nail-patella syndrome. In the clinical examination, the ulna head subluxed from the distal radioulnar joint when the wrist was fully supinated, creating a dorsal visible indentation and a volar protrusion of the ulnar head. The distal radioulnar joint instability of the case patient was quantified with a 3D visualisation and kinematic evaluation using dynamic radiostereometry analysis during a forearm rotation test and a press test. The investigation revealed limited pronation and hyper-supination during the forearm rotation compared with a healthy control. The dynamic radiostereometry analysis enabled a better understanding of the pathomechanics behind the instability. Patient-specific CT-derived bone models of ulna and radius revealed flat end dysplastic joint surfaces. The dynamic radiostereometry analysis proved to be a useful tool to describe, visualise and understand the pathomechanics of the case patient's distal radioulnar joint.