Mechanical properties of the Pinless external fixator on human tibiae

In the treatment of either acute severe open tibial fractures or their sequelae, a convenient external fixator is desirable. The conventional transosseous fixation with pins entering the medullary cavity is associated with problems such as pin loosening and pin track infection. Due to the bacterial contamination of the medullary space via the pin track the change of treatment from primary external fixation to secondary medullary nailing is an infection risk. In order to minimize these problems an external clamp fixator, the Pinless, was created. Medullary penetration is avoided by substitution of the conventional pins with clamps. The latter are inserted by hand (removable handles) and anchored only in the bone cortex. The medullary cavity stays intact. But is this clamp fixation stable enough for clinical use?

On paired human cadaver tibiae, we compared the mechanical properties of the experimental Pinless, the conventional AO-tubular fixator and the Ultra-X fixator. Clamps differing in size (small/large) and material (steel/titanium) were used and compared to Schanz screws (steel, 5.0 mm diameter). We measured the stiffness of comparable configurations (1 or 2 bars) under axial compression, four-point-bending in two planes, and torsion. The pull-out force of the different clamps in relation to the bone diameter and number of rocking movements during insertion was also determined.

The Pinless configurations with small clamps and 1 bar showed stiffness values as follows (as a percentage of the corresponding AO-tubular fixator): 42/36% (steel/titanium clamp) axial stiffness, 61/43% bending stiffness perpendicular to the reference plane, 78/79% bending stiffness parallel to the reference plane, and 90/95% torsional stiffness. The corresponding Ultra-X device was not as stiff as the Pinless. The use of two longitudinal rods increased the relative stiffness only under axial compression. The mean pull-out force on the proximal tibia was 1011 N for the small steel clamp, 717 N for the large steel clamp, 681 N for the small and 777 N for the large titanium clamp. At the lowest tibial diameter the values were reduced by 10 to 43%. The rocking movements doubled the pull-out force, e.g. there was a pull-out force for the large clamp of 600 N with five rocking movements compared to 310 N without.

The Pinless was not as stiff as the conventional AO-tubular device but stiffer than the clinically used Ultra-X, especially in sagittal bending, the main load on a tibial fracture in the first weeks after trauma.(ABSTRACT TRUNCATED AT 400 WORDS)