A Lateral Fracture Line Affects Femoral Trochanteric Fracture Instability and Swing Motion of the Intramedullary Nail: A Biomechanical Study

Abstract

Background: An unstable trochanteric femoral fracture is a serious injury, with a 1-year mortality rate of 5.4% to 24.9%, for which there is currently no standard treatment method. The lag screw insertion site is one of the primary contact areas between the cortical bone and an intramedullary nail. We hypothesized that a posterolateral fracture causes intramedullary nail instability when the posterolateral fracture line interferes with lag screw insertion. The purpose of the present study was to investigate the effect of posterolateral fracture line morphology on intramedullary nail stability by simulating unstable trochanteric femoral fractures with a posterolateral fracture fragment.

Methods: Eighteen custom-made synthetic osteoporotic bone samples were used in the present study. Nine samples had a posterolateral fracture line interfering with the lag screw insertion hole (Fracture A), and the other 9 had a fracture line 10 mm away from the hole (Fracture B). Cyclic loading (750 N) was applied to the femoral head 1,500 times. Movement of the end cap attached to the intramedullary nail was recorded. The amplitudes of motion in the coronal plane (coronal swing motion), sagittal plane (sagittal swing motion), and axial plane (total swing motion) were evaluated. The change in the neck-shaft angle was evaluated on photographs that were made before and after the test. Medial cortical displacement was measured before and after the test.

Results: Two Fracture-A samples were excluded because the amplitude of sagittal swing motion was too large. The mean values for coronal, sagittal, and total swing motion were 1.13 ± 0.28 mm and 0.51 ± 0.09 mm (p < 0.001), 0.50 ± 0.12 mm and 0.46 ± 0.09 mm (p = 0.46), and 1.24 ± 0.24 mm and 0.69 ± 0.11 mm (p < 0.001) for Fractures A and B, respectively. The mean neck-shaft angle change was -8.29° ± 2.69° and -3.56° ± 2.35° for Fractures A and B, respectively (p = 0.002). The mean displacement of the medial cortex was 0.38 ± 1.12 mm and 0.12 ± 0.37 mm for Fractures A and B, respectively (p = 0.57).

Conclusions: This study showed that an unstable trochanteric femoral fracture with a posterolateral fracture line that interferes with the lag screw insertion holes is a risk factor for increased intramedullary nail instability.

Fig. 1

Figs. 1-A through 1-D Overview of the specimens. Both fractures were 4-part comminuted trochanteric femoral fractures. Fracture A involved a lateral wall fracture line that interfered with the lag screw insertion hole (Figs. 1-A and 1-B). Fracture B involved a lateral wall fracture line 10 mm from the lag screw insertion hole (Figs. 1-C and 1-D). The structures indicated by the black arrows in Figures 1-A and 1-C were used to fix the depth gauges.

Fig. 2

Figs. 2-A and 2-B Photographs showing a specimen in the biomechanical testing machine. The specimen was placed on the machine 10° laterally from the gravitational line in the frontal plane and 12° posteriorly from the gravitational line in the sagittal plane with use of a clamp. We calculated the movement of the block attached to the top of the intramurally nail. The movement of the block was captured with digital depth gauges that were fixed perpendicularly to each other.

Fig. 3

Illustration showing the axis and the swing motion; the axis was set perpendicularly in relation to the femoral neck. A = anterior, M = medial, P = posterior, L = lateral.

Fig. 4

Bar graphs summarizing sagittal swing motion, coronal swing motion, and total swing motion in Fractures A and B. Fracture A had significantly greater coronal swing motion and total swing motion. The error bars represent the standard deviation .

Fig. 5

Figs. 5-A, 5-B, and 5-C Varus deformities were calculated with use of ImageJ2 software. We calculated the varus angles indicated by the bold dashed lines. The varus angle after loading (Fig. 5-B) was smaller than that before loading (Fig. 5-A), as shown for a representative fracture (A5). The measured displacement between before and after loading was calculated as the displacement of the medial cortex (Fig. 5-C), as shown for a representative fracture (A6). The arrow shows the displacement of the medial cortex.