BACKGROUNDSubtalar joint (STJ) neutral position is the position typically used by clinicians to obtain a cast representation of a patient's foot before fabrication of biomechanical functional orthosis. But no method for measuring STJ neutral position has been proven accurate and reproducible by different testers. This study was conducted to investigate the STJ neutral position in normal feet in cadavers.

METHODSTwelve fresh-frozen specimens of amputated lower legs were used. Pressure-sensitive films were inserted into the anterior and posterior articulation of STJ. The contact areas for various foot positions and under axial loads of 600 N were determined based on the gray level of the digitized film. The STJ neutral positions were determined as the ankle-foot position where the maximum contact area was achieved, because the neutral position of a joint was defined as the position where the concave and convex surfaces were completely congruous.

RESULTSIn ankle-foot neutral position, the contact area of STJ was (2.79 +/- 0.24) cm(2). In the range of motion of adduction-abduction (ADD-ABD), the maximum contact area was (3.00 +/- 0.26) cm(2) when the foot was positioned 10 degrees of ABD (F = 221.361, P < 0.05). In the range of motion of dorsiflexion-plantarflexion (DF-PF), the maximum contact area was (3.61 +/- 0.25) cm(2) when the foot was positioned 20 degrees of DF (F = 121.067, P < 0.05). In the range of motion of inversion-eversion (INV-EV), the maximum contact area was (3.14 +/- 0.26) cm(2) when the foot was positioned 10 degrees of EV (F = 256.252, P < 0.05).

CONCLUSIONSJoints, such as STJ, therefore, are not necessarily in neutral position when the ankle-foot is placed in the traditional concept of neutral position. The results demonstrate that the most approximate STJ neutral position was in the foot position of 10 degrees of abduction, 20 degrees of dorsiflexion and 10 degrees of eversion.