Plate-Rod Fixation

Introduction

Bone healing occurs by direct or indirect union. Direct bone union is characterized by remodeling of existing haversian systems through simultaneous bone resorption and bone deposition. Indirect bone union occurs through the sequential deposition of tissues with increasing mechanical strength: immature connective tissue - fibrocartilage - woven bone. Another method of indirect bone union, and possibly the mechanism associated with indirect fracture reduction techniques, is by omitting the stage of fibrocartilage deposition proceeding directly to the formation of woven bone. For either direct or indirect bone union to occur, certain biologic and mechanical events must be satisfied. Biologically, adequate vascular supply and appropriate histochemicals (BMP, growth factors) are needed to support existing bone cells and provide stimulus to differentiate pluripotential cells. Mechanically, the fixation must be strong and stiff enough to prevent excessive micromotion (strain) at the fracture interface but allow sufficient micromotion to stimulate bone formation. To satisfy mechanical conditions, surgeons often choose to apply a neutralization plate or a compression plate to provide interfragmentary compression. Coupled with atraumatic technique conditions are met to achieve direct bone union and an early return to clinical function. However, if in the application of the bone plate small fracture gaps are present on the transcortical surface, high local strain will occur (small fracture gaps concentrate strain). Additionally, with highly comminuted fractures, the vascular envelope is often damaged during reduction of fracture fragments delaying the appearance of the biological elements needed for bone union. High local strain and vascular compromise act synergistically in delaying the healing response. The delay in healing is coupled with the fact that the without the integrity of the bone column stress is carried by the plate and may lead to cyclic failure of the plate. The concept of indirect reduction is one which strives to preserve the biologic envelope of the fracture area. This is chiefly applied to comminuted fractures where reduction of fracture fragments is likely to destroy vascular attachments. Experience suggests that it is preferable not to disturb bone fragments or the fracture hematoma thereby preserving the biologic milieu. The surgeon should regain spatial alignment of the limb and then bridge the fractured zone with a buttress plate being careful to limit manipulation of the soft tissues to a minimum.

From a mechanical perspective, the plate must limit the strain at the fracture site to a level that is compatible for direct or indirect bone union. Comminuted fractures distribute strain over a large surface area which lowers interfragmentary strain to a level compatible with direct or indirect bone union. However, if the bony column is not reconstructed, the bone plate is placed under considerable stress since it must carry all the imposed physiologic load until callus (bio-buttress) is formed (Figure 52-1). If a standard plate is used, empty plate holes will be present overlying the area of comminution. In that an empty plate hole serves as a stress concentrator, plate failure can occur in this area. One method to reduce plate strain is to combine the use of an IM pin with that of the bone plate – i.e. a plate-rod construct (Figure 52-2).
 

Clinical Indications

Indications for application of plate-rod constructs are fractures where biological assessment indicates prolonged healing, mechanical assessment indicates the implants must sustain maximum stress, and clinical assessment indicates a comfortable, low maintenance system is needed. An example of a patient which fulfills these requirements would be a middle aged or older large or giant breed dog, having sustained multiple limb injuries with one injury being a multifragmented fracture with severe disruption of the soft tissue envelope. The plate/rod technique is mostly applied in the femur and humerus but may be applied to the tibia and radius as well.

Technique

Fractures of the Femur

Use a minimally invasive exposure or one employing the concept of OBDNT (Open But Do Not Touch). Insert (retrograde or normograde) an IM pin which occupies 40% the diameter of the marrow cavity. An IM pin of this approximate size reduces the stress on the plate by 50% or more. More importantly, the fatigue life of the plate is extended at least 10 fold. However, an IM pin which only occupies 25% of the marrow cavity reduces the stress in the plate by a factor of 10%. Therefore, the appropriate pin size is critical. Inserting the intra-medullary pin establishes varus-valgus alignment and assists in regaining appropriate length. Apply a buttress plate to the tension surface of the bone and contour it to the anatomic shape of the bone. Use a radiograph of the intact bone of the opposite leg as a template to help contour the plate if the bone of the affected leg is severely comminuted. Apply a plate of appropriate length to the tension surface of the bone. When applying minimally invasive technique, the plate must span the length of the bone from proximal metaphysis to distal metaphysis. Insert the most proximal and distal plate screws so that they avoid the IM pin and engage both near and far cortices. At this point examine for proper rotational alignment: As a general guide, the internal and external rotation of the hip should be equal when starting at a neutral position. Once rotational alignment is established, insert additional screws; place an additional screw proximally and an additional screw distally for a total of two screws in the end plate holes proximally and two screws in the most distal plate holes (Figure 52-3).

If it is necessary to place screws in more central plate holes, insert the plate screws so that they engage only the near cortex – i.e.monocortical screws (Figure 52-4). If large fragments are identified without disruption of the soft tissue envelope, they can be gently “lassoed” with absorbable suture and pulled into alignment. A cancellous bone graft should be harvested from the ipsilateral humerus or ilium and packed into the area of comminution.

Fractures of the Humerus

Use a minimally invasive exposure or one employing the concept of OBDNT (Open But Do Not Touch); the lateral approach is commonly used. The intra-meduallary pin should approximate 40% the diameter of the marrow cavity. An IM pin of this approximate size reduces the stress on the plate by 50% or more. More importantly, the fatigue life of the plate is extended at least 10 fold. The pin may be retrograded or normograded. Apply a plate of appropriate length to the tension surface of the bone. When applying minimally invasive technique, the plate must span the length of the bone from proximal metaphysis to distal metaphysis. Insert the most proximal and distal plate screws so that they avoid the IM pin and engage both near and far cortices. At this point examine for proper rotational alignment: As a general guide, the internal and external rotation of the shoulder should be equal when starting at a neutral position. Once rotational alignment is established, insert additional screws; place an additional screw proximally and an additional screw distally for a total of two screws in the end plate holes proximally and two screws in the most distal plate holes. If central screws are inserted, they should be monocortical screws (Figure 52-5).
 

Fractures of the Tibia

Use a minimally invasive exposure or one employing the concept of OBDNT (Open But Do Not Touch); the anteromedial approach is commonly used. The intra-meduallary pin should approximate 40% the diameter of the marrow cavity. An IM pin of this approximate size reduces the stress on the plate by 50% or more. More importantly, the fatigue life of the plate is extended at least 10 fold. The pin must be normograded; the pin will assist in re-establishing appropriate varus-valgus alignment and length. Rotation alignment is judged by aligning the fabella of the femur with the medial and lateral malleoli of the distal tibia. Apply a plate of appropriate length to the medial surface of the bone. When applying minimally invasive technique, the plate must span the length of the bone from proximal metaphysis to distal metaphysis. Insert the most proximal and distal plate screws so that they avoid the IM pin and engage both near and far cortices. Place an additional screw proximally and an additional screw distally for a total of two screws in the end plate holes proximally and two screws in the most distal plate holes. If central screws are inserted, they should be monocortical screws (Figure 52-6).

Fractures of the Radius/Ulna

Use a minimally invasive exposure or one employing the concept of OBDNT (Open But Do Not Touch); the anteromedial approach is commonly used for the radial exposure, whereas a posterolateral approach is used for the ulna. The intra-meduallary pin is placed in the ulna for this construct. The size of the pin approximates the diameter of the marrow cavity of the ulna. The pin can be normograded or retrograded and should be placed prior to applying the plate on the radius. The pin will assist in re-establishing appropriate varus-valgus alignment, rotational alignment and length. Next apply the bone plate to the cranial surface of the radius. When applying minimally invasive technique, the plate must span the length of the bone from proximal metaphysis to distal metaphysis. All the plate screws are inserted as bicortical screws (Figure 52-7).
 

Suggested Readings

Hulse D, Ferry K, Fawcett A, et. al. Effect of intramedullary pin size on reducing bone plate strain. Vet Comp Orthop Traumatol 2000;13:185-190.