Effects of Various Modalities of Internal Fixation on The Blood Supply of Bone

• Iatrogenic bone necrosis is the result of the surgical approach to bone, the manipulations required to reduce the fracture, and any procedures preparatory to implant insertion and fixation eg, medullary reaming, periosteal stripping, or endosteal perforation.

Effects of Intramedullary Nailing

· Medullary blood supply is important to the diaphysis for bone healing, an intramedullary nailing disrupts this source.

· Intramedullary nailing damages the endosteal blood supply, resulting in partial cortical necrosis.

· The amount of necrosis increases after endosteal reaming caused either

1. By Direct damage to the vascularity of endosteal surface or
2. By intravasation of marrow elements and fat into the iontra-cortical blood supply, which leads to vessel thrombosis.

· Periosteal blood flow alone cannot reach the endosteum and endosteal Callus may be inhibited.

· The endosteal blood supply regenerates around the nail, and the greater the space between the nail and the cortex, the more rapidly this revascularization progresses.

· The extraosseous periosteal blood supply also increases after nailing and can traverse the outer cortex to revascularize the necrotic endosteum.

· The regeneration of the nutrient artery itself may not occur for six months and is further delayed in the presence of a close-fitting nail.

· Heat and pressure are by-products of reaming.

· Hydraulic pressure builds up in the cavity which far exceeds that of the blood pressure and is independent of the size of the reamer.

· The reamer acts as a piston-in-a-sleeve which is filled with a mixture of medullary fat, blood, blood clots and bone debris.

·  High intramedullary pressure leads to injection of the medullary fat into the trans-cortical vessels and to pressure-related fat infiltration into the venous system.

·  As a result the trans-cortical vessels close, leading to reduced perfusion of the cortex.

·  Fat infiltration into the venous system may cause fat embolism.

Effect of Plating

· Plate fixation preserves the medullary and metaphyseal vessels as well as periosteal vessels on the opposite side of the “footprint” caused by the plate.

· Impairment of the blood supply to the periosteum at the plate-bone interface also causes porosity of the underlying bone.

· For this reason, the LCP and the LC-DCP have a trapezoidal cross-section and lateral undercuts that reduce the contact area and facilitate the removal.

· The internal fixator (ie, noncontact plates) aims to preserve blood flow under the plate by minimizing contact with the bone.

· Much of the vascular supply to the Callus area is derived from the surrounding soft tissue.

· Callus perfusion is of the utmost importance and may determine the outcome of healing.

· Bone can only form When supported by a vascular network, and cartilage will persist in the absence of sufficient perfusion.

· However, this angiogenic response is sensitive to both the method of treatment and the induced mechanical conditions.

· The vascular response appears to be greater after more flexible fixation, possibly due to larger amounts of osseous callus.

· Large tissue strains caused by instability reduce the blood supply, especially in the fracture gap.

Effect of Drilling & Screws

· The medullary blood vessels can be injured by drilling or screw insertion, or both.

· In stable conditions, the axial blood flow recovers rapidly.

· Multiple screws crossing the medullary canal can affect the regional vascular pattern and cause abnormal remodelling of the endosteum.

· In addition, the presence of a number of aligned screws in a single cortex impairs the normal perfusion of the cortical bone.

· Monocortical screws only affect one cortex and possibly also the central medullary blood flow.

· It has been shown in vitro that drilling generates heat that is locally incompatible with vital biological structures.

· In addition, repeated drilling can increase heat necrosis in bone.

· Drill bits have to be kept sharp and must be replaced if they become blunt, the same applies to self-drilling, self-tapping locking head screws.

Irrigation and cooling during drilling procedures are important for minimizing of heat.