Introduction: Traditional knowledge suggests that independent extension of index finger was caused solely by extensor indicis proprius (EIP). The "horn" sign is hence used to diagnose the presence of EIP. Patients who had EIP transfers have been observed to be able to independently extend their index finger, this hence raised some doubts about the reliability of this "horn" sign.
Method: Patients who underwent EIP transfer in Middlemore Hospital between 2007-2013 were contacted, followed up in clinic and examined. Independent (without long finger extension) and dependent (with concurrent long finger extension) extension range of movement and strength of extension of the operated hand (absent EIP) were measured and compared against the non-operated (intact EIP) hand.
Results: 11 patients were followed up in clinic. The independent extension lag in the operated hand ranged from 3-45o compared to -16-30o. There was a 16o difference in independent extension lag between the operated and non-operated side. There were minimal differences in dependent extension. The operated finger was found to have a weaker independent and dependent extension strength. Independent extension lag was observed in some non-operated hands.
Discussion: There is a variation in independent extension of index finger regardless of presence of EIP. Patients with an absent EIP can still independently extend their index finger but have a greater independent extension lag and weaker extension. When assessing for presence of EIP using the "horn" sign, it may be more reliable that the clinician examines both hands and compare their extension lag and strength.