Development and mechanical testing of a short intramedullary nail for fixation of femoral rotational osteotomy in cerebral palsy patients

Abstract

Background: Rotational osteotomy is frequently indicated to correct excessive femoral anteversion in cerebral palsy patients. Angled blade plate is the standard fixation device used when performed in the proximal femur, but extensile exposure is required for plate accommodation. The authors developed a short locked intramedullary nail to be applied percutaneously in the fixation of femoral rotational osteotomies in children with cerebral palsy and evaluated its mechanical properties.

Methods: The study was divided into three stages. In the first part, a prototype was designed and made based on radiographic measurements of the femoral medullary canal of ten-year-old patients. In the second, synthetic femoral models based on rapid-prototyping of 3D reconstructed images of patients with cerebral palsy were obtained and were employed to adjust the nail prototype to the morphological changes observed in this disease. In the third, rotational osteotomies were simulated using synthetic femoral models stabilized by the nail and by the AO-ASIF fixed-angle blade plate. Mechanical testing was done comparing both devices in bending-compression and torsion.

Results: The authors observed proper adaptation of the nail to normal and morphologically altered femoral models, and during the simulated osteotomies. Stiffness in bending-compression was significantly higher in the group fixed by the plate (388.97 ± 57.25 N/mm) than in that fixed by the nail (268.26 ± 38.51 N/mm) as torsional relative stiffness was significantly higher in the group fixed by the plate (1.07 ± 0.36 Nm/°) than by the nail (0.35 ± 0.13 Nm/°).

Conclusions: Although the device presented adequate design and dimension to fit into the pediatric femur, mechanical tests indicated that the nail was less stable than the blade plate in bending-compression and torsion. This may be a beneficial property, and it can be attributed to the more flexible fixation found in intramedullary devices.

Figure 1

Nail prototype. Nail prototype with the 6.5-mm cannulated proximal locking screw and the 4.5-mm cortical distal locking screw.

Figure 2

Simulated rotational osteotomy performed in a morphologically altered femoral model. Simulated rotational osteotomy fixed by the nail prototype, performed in a morphologically altered synthetic femoral model based on reconstructed tomographic image of a 10-year-old patient with cerebral palsy. Frontal view (a) and lateral view (b).

Figure 3

Simulated rotational osteotomy performed in a morphologically normal femoral model and fixed by the nail prototype. Simulated rotational osteotomy fixed by the nail prototype, performed in a synthetic adolescent-sized femoral model. Frontal view (a) and lateral view (b).

Figure 4

Simulated rotational osteotomy performed in a morphologically normal femoral model and fixed by a 90° angled-blade plate. Simulated rotational osteotomy fixed by a 90° angled-blade children's plate, performed in a synthetic adolescent-sized femoral model. Frontal view (a) and lateral view (b).

Figure 5

Femoral model within the testing machine for bending-compression test. Photograph demonstrating a femoral model within the testing machine for bending-compression test at 11° in adduction.

Figure 6

Femoral model within the torsion testing machine. Photograph demonstrating a femoral model placed horizontally in the testing machine for torsion test.

Figure 7

Relative stiffness found in the bending-compression tests. Graphic representation of relative stiffness values found in the bending-compression tests according to the groups fixed with plate and nail.

Figure 8

Linear displacement found in the bending-compression tests. Graphic representation of linear displacement values found in the bending-compression tests according to the groups fixed with plate and nail.

Figure 9

Relative torsional stiffness found in torsion tests. Graphic representation of relative torsional stiffness values found in torsion tests according to the groups fixed with plate and nail.

Figure 10

Angular displacement found in the torsion tests. Graphic representation of angular displacement values found in the torsion tests according to the groups fixed with plate and nail.