Forces required to dynamize sliding screws in gamma nail and selfdynamizable internal fixator

BACKGROUND

Single limb support phase of the gait-cycle in patients who are treated for a pertrochanteric fracture is characterized by transversal loads acting on the lag screw, tending to block its dynamization. If the simultaneous axial force overcomes transversal loads of the sliding screw, the dynamization can still occur.

METHODS

Biomechanical investigation was performed for three types of dynamic implants: Gamma Nail, and two types of Selfdynamizable Internal Fixators (SIF) - SIF-7 (containing two 7 mm non-cannulated sliding screws), and SIF-10 (containing one 10 mm cannulated sliding screw). Contact surface between the stem and the sliding screws is larger in SIF implants than in Gamma Nail, as the stem of Gamma Nail is hollow. A special testing device was designed for this study to provide simultaneous application of a controlled sliding screws bending moment and a controlled transversal load on sliding screws (Qt) without using of weights. Using each of the implants, axial forces required to initiate sliding screws dynamization (Qa) were applied and measured using a tensile testing machine, for several values of sliding screws bending moment. Standard least-squares method was used to present the results through the linear regression model.

RESULTS

Positive correlation between Qt and Qa was confirmed (p < 0.05). While performing higher bending moments in all the tested implants, Qa was higher than it could be provided by the body weight. It was the highest in Gamma Nail, and the lowest in SIF-10.

CONCLUSIONS

A larger contact surface between a sliding screw and stem results in lower forces required to initiate dynamization of a sliding screw. Patients treated for a pertrochanteric fracture by a sliding screw internal fixation who have longer femoral neck or higher body weight could have different programme of early postoperative rehabilitation than lighter patients or patients with shorter femoral neck.

Treatment of subtrochanteric femoral fractures using Selfdynamisable internal fixator

Background

Surgical treatment is the preferred method for treating subtrochanteric femoral fractures and the variety of extramedullary and intramedullary implants continues to evolve. The purpose of our study was to retrospectively evaluate the clinical and radiological results of subtrochanteric fractures that are treated with the Selfdynamisable internal fixator.

Methods

From January 2000 to January 2004, we treated 49 consecutive patients who had subtrochanteric fractures. According to the AO classification, 8 (16.3%) fractures were type 32-A, 16 (32.7%) were type 32-B and 25 (51%) fractures were type 32-C. The mean follow-up time was 22.3 months.

Results

The average operating time was 45 minutes (range, 32 to 90 minutes). The average blood loss was 250 mL (range, 125 to 350 mL). The average hospital stay was 10 days (range, 7 to 59 days). Implant failure was not observed and union was achieved in all the patients. Deep infection occurred in one (2%) patient in the early postoperative period. Fracture union was achieved at a mean of 14 weeks. Varus malalignment less then 10 degree was noted in three (6.1%) patients at the end of follow-up. Thirty-five patients were pain-free and 14 had mild pain.

Conclusions

The selfdynamisable internal fixator was successfully used for subtrochanteric fracture. It provides a short operative time, low blood loss, spontaneous biaxial dynamisation and healing in an optimal period of time without the need for secondary intervention.