Lengthening over nails using the double plate system ONAS-DPS

Abstract

Stable insertion of large Schanz screws behind an intramedullary (IM) nail when lengthening over nails (LON) may be difficult due to the limited bone stock. Additionally, the highly probable contact between the screws and IM nail (which is difficult to avoid) increases the likelihood of infection spreading from the skin via Schanz screws directly to the IM nail. A new device for LON has been developed. Instead of inserting Schanz screws from the external fixator beside the IM nail (as in standard LON), a system of two overlaying plates was constructed. Schanz screws can be fixed to the plates without entering the bone. The plates are fixed to the bone using four angle stability screws. The holes in the plates offer stabile fixation for a chosen angle under which the screw is positioned through the cortical bone. Using the new system there is no need to place Schanz screws behind the IM nail. Instead, Schanz screws pass to the plate and not through the bone. The new system for elongation over IM nail is called "Over Nail Angle Stability-Double Plate System" (ONAS-DPS) [Antolič V (2013) Modular side device with an intramedullary nail for guiding a bone during its lengthening. World Intellectual Property Organization. International Publication number: WO 2013/176632 A1].

Keywords: Angle stability plate; External fixation; Intramedullary nail; Lengthening over nail.

Figure 1.

ONAS-DPS: a retrograde femoral IM nail and unilateral frame are shown on the bone model. Two Schanz screws are fixed proximally and distally (Figures 2 and 3) into the plate. Note that the screws are not entering the underlying cortical bone and in this way the contact with IM nail is avoided. A wedge subplate is used distally. In the case of anterograde IM femoral nail (not shown) the “wedge plate” is used proximally. The most distal screw goes through the IM nail in order to provide adequate stability of the whole system.

Figure 2.

ONAS-DPS. Two plates are needed distally (Figure 1) since the femoral metaphysis is curved: the lower plate (subplate) is a specially designed “wedge plate” which makes the unilateral external fixator parallel to the bone. If ONAS-DPS is used with anterograde IM nail the wedge plate must be used proximally (not shown). Wedge plates with four different angles (thicknesses) are available with ONAS-DPS, each having an equivalent probe. The upper plate provides fixation to the bone with four angle stability screws. Each screw hole in the plate offers the possibility of placing the screw at any chosen angle from 90° to 65° with respect to the sagittal axis of the bone. Please note that the IM nail is in the medullary canal and that the plates are fixed with screws going anteriorly and posteriorly with respect to the IM nail. Proximally, a separate screw passes through both plates and cortical bone and goes through the IM nail and finally anchors into the opposite cortical bone. The two-plate system of the ONAS-DPS enables stable fixation of Schanz screws and also stable fixation of both plates to the bone, in this way avoiding direct contact between Schanz screws and IM nail, which is responsible for the high incidence of infection in standard LON.

Figure 3.

Proximal fixation of Schanz screws in the case of retrograde technique (Figure 1). On the diaphysis one plate of the ONAS-DPS can be used only (distally a wedge subplate must be used in order to compensate for the femoral shape – Figures 1 and 2). Further, no screw should go through the IM nail proximally (in contact with the distal fixation – Figures 1 and 2) as this would prevent elongation. Four angle stability screws and asymmetrically drilled corresponding screw holes are the same as shown in Figure 2.

Figure 4.

Intraoperative view of ONAS-DPS implantation in a 28-year-old female patient with congenital femoral shortening and deformity (osteotomy, intramedullary nail, plate with two Schanz screws). Note that there is no direct contact between Schanz screws and the bone or the IM nail.

Figure 5.

Radiograph of the distal femur in the patient from Figure 4 after the acute phase of lengthening has been completed.

Figure 6.

Force-deflection diagram of the shear force load (in N) plotted as a function of end deflection (in mm) of the Schanz screws inserted in the ONAS-DPS. The distance between the point force load and the fixation of the specimen was 60 mm.