Traditional compression plates were originally designed to provide absolute stability, targeting primary bone healing without callus formation ()(9
). The axial rigidity of modern locked plating constructs is comparable to that of non-locked plating constructs (10
). In contrast, external fixators were designed to provide sufficient interfragmentary motion to stimulate secondary bone healing by callus formation. External fixators can provide over 10 times more interfragmentary motion in response to a given load than rigid fixation with locked or non-locked plates (11
). Since locked plating relies on secondary rather than on primary bone healing (5
), reducing the stiffness of a locked plating construct becomes essential to target secondary bone healing.
Figure 1 The conundrum: Locked plating constructs are comparably stiff than non-locked constructs that were designed to induce primary bone healing by rigid fixation (10, 21, 25, 26). However, locked bridge plating constructs rely on secondary bone healing with (more ...)
FCL reduces the stiffness of a locked plating construct by means of FCL screws that are fixed in the plate and in the far cortex, while retaining a controlled motion envelope in the near cortex of a diaphysis (). FCL screws have a flexible shaft with a reduced diameter that can elastically deflect within the near cortex motion envelope. The motion envelope is controlled by the diameter of a collar segment adjacent to the FCL screw head. FCL constructs therefore resemble a monolateral external fixator, the bar of which has been applied close to the bone surface, and the pins of which are secured in the far cortex rather than in the near cortex (). Similar to the external fixator, FCL constructs provide fixed-angle yet flexible connections between a bridging member and the bone segments, whereby FCL screws approach the working length of external fixator pins. In contrast, screws of standard locked constructs are rigidly confined between the near and far cortices and therefore have an insufficient working length to enable flexible fixation.
Figure 2 FCL fixation: A) FCL screws are locked in the plate and in the far cortex, while retaining a controlled motion envelope Δd in the near cortex. B) Similar to the pins of an external fixator, flexible shafts of FCL screws provide a sufficient working (more ...)
Biomechanical studies demonstrated that FCL screws reduce the axial stiffness of a standard locked plating construct by 88% for bridge plating of the femoral diaphysis (2
), by 84% for bridge plating of the tibial diaphysis (3
), and by 80% for stabilization of metaphyseal femur fractures (7
). For stiffness correlation, three diaphyseal fixation constructs were evaluated under axial compression (): a standard locked plating (LP) construct, using the proximal 9-hole segment of a femoral locking plate applied with standard locking screws (NCB, Zimmer, Warsaw, IN); an FCL construct, using the same NCB plate applied with FCL screws (MotionLoc, Zimmer); and a monolateral external fixator (EBI, Parsippany, NJ). The stiffness of the FCL construct (682 N/mm) was 84% lower than that of the standard locked construct (4,286 N/mm), and approached that of the external fixator (488 N/mm) (). The stiffness of the FCL construct is suitable to generate interfragmentary strain (IFS) in the 30% range known to promote fracture healing by callus formation (15
). The actual amount of IFS depends on the applied load and the fracture gap size. For example, under 400 N partial post-operative weight-bearing (16
) of a 1–3 mm wide fracture gap, a construct stiffness of 444–1,333 N/mm will be required to induce 30% IFM. Moreover, at 400 N partial post-operative loading the FLC construct generated interfragmentary motion of approximately 0.6 mm, which is within the 0.2 – 1 mm envelope of interfragmentary motion known to stimulate secondary bone healing (17
). Conversely, the standard locked construct induced less than 0.1 mm motion, and may therefore not reliable promote callus formation.
FCL may therefore be essential to reduce the stiffness of locked plating constructs in order to actively promote callus proliferation in the early healing phase, and to enable load sharing for callus maturation in the late remodeling stages of fracture healing.