Safety Window for the Volar Needle Approach for Examination of the Pronator Quadratus Using Ultrasonography

Objectives To investigate pronator quadratus (PQ) anatomy and determine the proper volar needle insertion point based on landmarks, the tip of the ulnar styloid (an imaginary vertical line passing the tip of the ulnar styloid process [U line]), and the ulnar margin of the palmaris longus tendon (uPL) using ultrasonography. Design Descriptive study. Setting Department of physical medicine and rehabilitation. Participants Participants between 20 and 60 years without any diseases. (N=25; 13 men, 12 women; 50 forearms). Interventions Ultrasonography. Main Outcome Measures The proximal and distal volar surface points of origin, the proximal and distal insertion sites, and the midpoint of the PQ (PQ_M) were determined. The distance of each of the PQ surface indices from the U line was measured, and the probe was positioned at the level of PQ_M parallel to the U line. The relative distances from the vertical surface points of the median nerve and ulnar artery to the uPL were measured. Results The mean age and body mass index were 32.7±10.4 years and 21.98±2.83kg/m². The PQ_M was located at a mean distance of 2.63±0.35cm proximal from the U line (men 2.79±0.37cm and women 2.45±0.21cm; P<.05). The mean safety window for the volar approach was 0.72±1.8cm toward the radial side and 1.51±0.30cm toward the ulnar side from the uPL. The PQ was at a mean depth of 1.30±0.19cm from the skin and had a mean thickness of 1.19±0.24cm at the level of PQ_M. The distance between the U line and the proximal edge of the PQ, as well as the PQ thickness, was greater in men than in women. Conclusions The volar approach for needle electromyographic examination of the PQ can be performed precisely and safely.