Analysis of implantation-induced nail deformation and roentgen morphometric studies as the principle for an aiming device for distal interlocking nailing without roentgen image intensification

While working to develop a distal locking device, we analyzed distal nail position with reference to nail deformation and a radiographic-morphometric investigation. The amount and the direction of implant deformation in unslotted stainless steel unreamed tibial nails (Synthes) were analyzed. Measurement of implant deformation (3 translations, 3 angles) in the center of the distal transverse locking hole was performed with a 3D magnetic motion tracker system before and after nail insertion. Unreamed tibial nails (diameter 8 mm, n = 10; diameter 9 mm, n = 10) were inserted in paired human cadaver tibiae. The results showed lateral translations of -4.5 +/- 3.5 mm (mean and standard deviation, range 14.3 mm) and dorsal translations of -7.8 +/- 5.8 mm (mean and standard deviation, range 19.2 mm). Rotational deformations around the longitudinal axis of the nail were 0.3 +/- 0.7 degree (mean and standard deviation, range 2.4 degrees). The results showed, that a simple aiming arm, mounted on the proximal nail end, alone or even in combination with a large working channel (e.g. 10 mm in diameter), was not adequate to the aiming process. A radiographic-morphometric analysis was subsequently performed in 40 tibiae after experimental (n = 20) or clinical (n = 20) implantation of nails 8 mm and 9 mm in diameter. These measurements showed a relatively constant distance between the upper distal transverse hole and the anterior aspect of the tibia (average 12.3 mm and only a narrow range (7.6 mm). These data were the basis for the development of an aiming technique that exploits the relatively constant distance between the distal nail hole and the anterior aspect of the tibia. This aiming device is set at a distance of 12.3 mm from the anterior cortex, and the fine tuning is finally resolved by the use of a "working channel' 10 mm in diameter.