BIOMECHANICAL STUDY OF 9 INTERLOCKING TIB IAL NAILS

Various different interlocking intramedullary nail designs are now ava ilable for the tibia the designs resulting from a different approaches to achieving stability for fracture fixation. During recent years sma ll-diameter unreamed interlocking nails have been developed. So far th ere are no data on stiffness of different nail designs that can justif y the use of certain nail types. We compared different nail types in t he bone-implant complex (BIC) with reference to stiffness under axial load, bending and torsion of the BIC. We simulated comminuted mid-shaf t fractures by a 2-cm defect osteotomy in paired human cadaver tibiae. We fixed each tibia with one of nine different interlocking nails (AO Unreamed Tibial Nail 9 mm [UTN9], AO Unreamed Tibial Nail 8 mm [UTN8] , Russell & Taylor Delta Tibial Nail 9 mm [RTD], Russell & Taylor Reco nstruction Tibial Nail 11 mm [RTR], Brooker & Wills Nail 11 mm [B-W], Grosse & Kempf Tibial Nail 11 mm [G-K], AO Universal Tibial Nail 11 mm [AOU], Klemm & Schellmann Tibial Nail 11 mm [K-S], Borner & Mattheck Tibial Nail 11 mm [B-M]) according to the manufacturers' recommendatio ns. Each BIC was tested mechanically using a Zwick universal testing m achine. Torque testing was performed with a maximum moment of 5 Nm in both directions. Maximum axial load was 1100 N. Four-point bending was done up to a moment of 66 Nm. For each implant bending and torque sti ffness of the nail were calculated and compared with the test results obtained in the cadaver study. A P-value less than 0.05 (least signifi cance difference test, ANOVA) was considered significant. In torque te sting the unslotted nails (UTN9, UTN8, RTD, RTR, B-W) showed significa ntly higher stiffness than the slotted nails (G-K, AOU, K-S, B-M). Bot h groups of nails were significantly less stiff than intact bone. In a xial load testing large-diameter nails interlocked with large-diameter interlocking bolts (G-K, K-S, B-M) showed significantly higher stiffn ess. For A-P bending no significant differences between implants were found, but isolated bone showed significantly higher A-P bending stiff ness. In varus-valgus bending large-diameter nails (RTR, G-K, K-S, B-M ) showed significantly higher bending stiffness than small-diameter (U TN8, B-W) implants. Calculations and mechanical tests showed much high er torque stiffness of unslotted implants. Slotted implants had compar ably higher stiffness of the BIC on testing than the stiffness calcula ted for the nail profile. Unslotted implants had higher calculated sti ffness than test stiffness. The calculation of bending stiffness showe d much less difference between different implants that between the val ues yielded by torque stiffness calculations. This study shows that st iffness of the bone-implant complex in interlocking tibial nails is mo re dependent on nail design than on the press-fit of the nails in the medullary canal. For torque stiffness the absence of a slot is of spec ial importance. Axial stiffness and bending stiffness are dependent ma inly on the nail diameter. According to our study unslotted nails prov ide significantly higher torque stiffness than slotted implants. There is experimental evidence for other groups that implants with higher t orque stiffness have advantages for fracture healing. With appropriate design small-diameter nails give adequate stability for fracture fixa tion.