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AAOS Now

Published 6/1/2008
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William W. Cross III, MD; Gregory A. Brown, MD, PhD

Achieving stable fixation: Biomechanical designs for fracture healing

Orthopaedic surgeons have a choice of conventional, locked, and hybrid designs

Locked plating has dramatically changed the clinical practice of most orthopaedic surgeons. This new technology has been rapidly adopted because of the perceived improved fixation of fixed-angle devices. As with most technological advances, however, significantly increased costs are associated with locking plate technology. If locked plating technology is being misapplied, the result is overuse and unnecessary added costs, as well as increased construct stiffness and unknown effects on fracture healing biology.

To develop indications for the use of conventional and locked screw internal fixation, an understanding of screw fixation biomechanics is needed. Screw pullout is a function of the bone strength, outer thread diameter, engaged thread length in bone, and thread shape factor. Because the thread diameter, thread shape factor, and engaged thread length (assuming bicortical screws, not unicortical screws) are not fundamentally different for conventional and locking screws, the different failure mechanisms must be explained by bone properties and other fixation mechanics.

Conventional internal fixation functions by compressing the plate to the bone and generating friction between the plate/bone interface to resist fracture fragment motion. Locking internal fixation resists fragment motion because the locking screw head engages the plate, creating a fixed-angle construct, but the locking screw does not compress the plate to the bone.

The in vivo screw force has two components: the compression force compressing the plate to the bone, and the physiologic force from activities such as weight bearing or muscle forces.

Locking screws and plates
Because the compression force for locking screws is zero, the screw force is the physiologic force. In osteoporotic bone, the pullout force for conventional and locking screws is the same given the same thread design, but conventional screws use most of the pullout force for plate compression; little pullout force remains to resist physiologic forces.

Locking screws, on the other hand, use the entire pullout force (pullout strength) to resist physiologic forces. The screw pullout strength for conventional and locked screws is equal, but locked screws use it more effectively because they do not have a compression force component.

Locking plates have a different failure mode than conventional plates. Conventional screw constructs frequently fail because individual screws toggle, loosen, and pull out. Because locking screws are fixed to the plate, the screws must all fail or pull out simultaneously. This can be both an advantage and a disadvantage. Because locking plate constructs tend to have higher failure loads, if a locking plate fails, the greater forces and energy imparted can lead to catastrophic consequences, including additional fracture lines and comminution.

Choosing the “right” fixation
Engineers analyze mechanics and failures to improve designs. By analogy, orthopaedic surgeons can use an understanding of conventional and locked screw/plate biomechanics and failures to design internal fixation constructs that promote fracture healing.

The orthopaedic surgeon must first make a conscious choice between primary and secondary bone healing as part of the preoperative planning process. This decision guides the selection of conventional, locked, or hybrid fixation. Primary bone healing requires anatomic reduction and rigid fixation. Secondary bone healing does not require anatomic reduction or rigid fixation—only stable fixation.

Locked plates do not have embedded microchips with fuzzy logic to determine whether the surgeon plans to achieve primary or secondary bone healing. Although locked plate constructs are usually combined with indirect reduction techniques for secondary bone healing, the increased stiffness and rigidity of locked screws are ideal for primary bone healing, particularly in osteoporotic bone.

Conversely, conventional screws are usually combined with direct reduction techniques for primary bone healing. Mast, Jakob, and Ganz (Planning and Reduction Technique in Fracture Surgery, Berlin, Springer-Verlag, 1989), however, developed their indirect reduction techniques with conventional screw/plate systems.

Follow basic principles
The second major consideration in designing a fracture healing construct is to recall standard fracture reduction principles. Intra-articular fractures require anatomic reduction; extra-articular fractures require achievement of correct length, alignment, and rotation. Percutaneous or indirect reduction techniques that violate these fracture reduction principles will fail or result in poorer clinical outcomes regardless of the fixation used (conventional, locking, or hybrid).

The acronym ORIF (open reduction and internal fixation) emphasizes the primary importance of the reduction and not the fixation method. Percutaneous and indirect reduction techniques preserve the biologic fracture healing potential by preserving the bone fragment blood supply. Reduction techniques can be used with conventional, locked, and hybrid fixation, but the percutaneous and indirect techniques must not compromise the reduction principles.

Advantages and disadvantages
Locking screws are not better or worse than conventional screws; they are inherently different. Consider the following differences:

  • Locking screws use the screw pullout strength more effectively, but cannot compress fractures as conventional screws can.
  • Conventional screws can reduce fracture fragments to an anatomically precontoured plate, but locking screws cannot use precontoured plates as a reduction aid.
  • The fixed-angle locking screw increases the stiffness and stability of the fracture/implant construct, but is technically difficult and requires more accurate plate application to the fracture fragments to provide adequate screw purchase. The variable-angle conventional screw is technically more forgiving and allows better bicortical screw purchase.

This yin and yang of conventional and locked plating technology are combined in hybrid plate fixation to provide the advantages of both technologies.

Failure modes
The conventional plating and locked plating have different failure modes. Conventional screw/plate constructs typically fail because the screws break or loosen and pull or back out. Locked screw/plate constructs typically fail due to intra-articular screw penetration as bone collapses onto the fixed screws that are unable to back out (
Fig. 1) or catastrophic fracture extension/comminution because the locked screws and plate fail simultaneously resulting in a new fracture (Fig. 2).

Plate length can also affect construct failure. The screw forces are inversely proportional to the plate length for axial loading conditions. Doubling the plate length reduces the screw forces by one half. A three-hole plate with screws in all three holes will have twice the screw forces of a five-hole plate with screws in holes one, three, and five. The physiologic force on the middle screw is zero for axial weight-bearing loading, just as the neutral axis stress for a bending beam is zero. This principle is similar to maximizing the external fixator pin spread in a fracture fragment to enhance construct stability.

Bridge plating
Bridge plating can be performed with conventional, locked, or hybrid fixation. The longer the “bridged” or comminuted segment, the more empty holes in the plate. Unfilled holes can double or triple the plate stresses by concentrating the stress, which reduces the fatigue life of the plate. Filling the locking holes with plugs, which can be made by cutting off the threaded head of a locking screw (
Fig. 3), increases the fatigue life of the plate.

Alternatively, the effective “working length” of the bridge plate can be reduced by lagging a large comminuted or butterfly fragment through the plate and stabilizing the cortex opposite from the plate (Fig. 4). Preserving the soft-tissue envelope and fracture healing biology through indirect reduction is imperative in this technique.

Cerclage cables
Cables can provide supplemental fixation for plates when the intramedullary canal is filled with a stemmed arthroplasty component or intramedullary nail. Cables distant from the fracture site will not disrupt the blood supply. Except in the case of a periprosthetic proximal femur fracture, using cables at the fracture site is not advisable because the cable strips soft tissue during passage and disrupts periosteal blood supply.

Clinical applications
The following clinical situations demonstrate how these principles can be applied. An antiglide plate applied to an unstable lateral malleolus fracture (
Fig. 5) provides both fracture reduction and stable fixation. Applying the first screw through the one-third tubular plate proximal to the fracture reduces the distal malleolar fragment through a buttress effect by compressing the plate against the fibula. Additional screws are applied through the distal plate holes perpendicular to the fracture line with a lag screw technique to compress the fracture fragments for stability and primary bone healing. The antiglide plating technique can only be applied with conventional screw/plate constructs.

Hybrid fixation is used in the second situation (Fig. 6). A renal transplant patient with a subtrochanteric femoral stress fracture treated with a long intramedullary hip screw fell and fractured the distal femur through the distal interlocking screw holes. The proximal fracture had insufficient healing to remove the intramedullary hip screw, so the distal fracture was treated with a lateral locking plate. A long plate was chosen to reduce the physiologic forces in the proximal screws. A conventional screw was used to reduce the distal fragment to the anatomically contoured plate as a reduction aid. A cable was used to reduce the plate to the proximal femoral shaft distal to the fracture sites. Multiple locking screws were used for distal fixation. Four unicortical locking screws were used for femoral shaft fixation. A plug was used to fill the open hole in the plate to reduce the risk of plate fatigue failure through an open hole. The resulting hybrid fixation exploited the advantages of multiple technologies.

Summary
Locked plating has evolved to hybrid fixation, but the new technology is not the preferred method for all internal fixation. Conventional screws are used for initial fracture reduction and plate application using indirect reduction techniques. Locking screws can then be added for unstable fracture patterns and/or osteoporotic bone. Certainly, not all fracture patterns require locked plating technology.

William W. Cross III, MD, is a resident in the department of orthopaedic surgery at the University of Minnesota; Gregory A. Brown, MD, PhD, is assistant professor in the department of orthopaedic surgery at the University of Minnesota and a member of the AAOS Biomedical Engineering Committee.