ANALYSIS OF NAIL DEFORMATION CAUSED BY IM PLANTATION AND RADIOGRAPHIC-MORPHOMETRIC INVESTIGATIONS AS THE DESIGN BASIS FOR AN AIMING DEVICE FOR DISTAL LOCKING WITHOUT AN X-RAY INTENSIFIER

While working to develop a distal locking device, we analyzed distal n ail position with reference to nail deformation and a radiographic-mor phometric investigation. The amount and the direction of implant defor mation in unslotted stainless steel unreamed tibial nails (Synthes) we re analyzed. Measurement of implant deformation (3 translations, 3 ang les) in the center of the distal transverse locking hole was performed with a 3D magnetic motion tracker system before and after nail insert ion. Unreamed tibial nailas (diameter 8 mm, n=10; diameter 9 mm, n=10) were inserted in paired human cadaver tibiae. The results showed late ral 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 d eviation, range 19.2 mm). Rotational deformations around the longitudi nal axis of the nail were 0.3+/-0.7 degrees (mean and standard deviati on, 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 nor adequate to th e aiming process. A radiographic-morphometric analysis was subsequentl y performed in 40 tibiae after experimental (n=20) or clinical (n=20) implantation of nails 8 mm and 9 mm in diameter. These measurements sh owed a relatively constant distance between the upper distal transvers e 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. Th is aiming device is set at a distance of 12.3 mm from the anterior cor tex, and the fine tuning is finally resolved by the use of a 'working channel' 10 mm in diameter.