Effect of intramedullary pin size on reducing bone plate strain. ID - 20013170488
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The influence of pin size on the reduction of plate strain is investigated and compared with a plate alone construct. More specifically, the degree of plate strain with intramedullary Steinmann pins occupying 30, 40 and 50% the diameter of the medullary cavity of harvested canine femora (n=6, fracture gap of 20 mm in length) was determined; the fatigue life of the bone-implant constructs was estimated; and the stiffness of the bone-implant constructs was assessed in relation to plate alone constructs. Strain recorded from plate alone constructs was significantly different from that recorded in constructs having intramedullary pins occupying 40 and 50% the diameter of the marrow cavity. A significant difference in recorded strain was also observed between constructs having an intramedullary pin occupying 30% of the marrow cavity and those with pins occupying 50% the marrow cavity. In contrast to plate alone constructs, all plate/rod specimens were able to withstand an infinite number of cycles at the lowest load of 1200 N. At greater loads, plate/rod constructs had superior fatigue properties compared with plate alone constructs. The stiffness of plate/rod constructs increased as the size of the intramedullary pin occupying the marrow cavity increased. In comparison to plate alone constructs, the percentage increase in stiffness of plate/rod constructs increased from 6% with a pin occupying 30% the diameter of the marrow cavity to 78% with a pin occupying 50% of the marrow cavity. With a pin occupying 40% the diameter of the marrow cavity, stiffness increased an average of 40%. Based on these findings, the use of a rod (Steinmann pin) that occupies between 35-40% the diameter of the marrow cavity is recommended. For fracture gaps <20 mm long, the use of a pin size more closely occupying the 35% diameter mark is suggested. As the fracture gap length increases, it would be better to use a pin that more closely occupies the 40% mark. A pin occupying 50% the diameter of the marrow cavity is not necessary unless a plate that is structurally weaker than the standard 3.5 mm dynamic compression plates is used or in cases of possibly limited postoperative care of the patient. These results present the advantages of the plate/rod system as a buttressing device for indirect fracture reduction techniques. The intramedullary pin can help reestablish spatial alignment of the limb, maintains reduction of the main bone fragments as the plate is placed on the bone, reduces plate strain and consequently increases fatigue life of the construct, and allows microstrain of the fracture plane if it occupies 35-40% the diameter of the marrow cavity.
