Reasons why dynamic compression plates are inferior to locking plates in osteoporotic bone: a finite element explanation

Abstract

While locking plate fixation is becoming increasingly popular for complex and osteoporotic fractures, for many indications compression plating remains the standard choice. This study compares the mechanical behaviour of the more recent locking compression plate (LCP) device, with the traditional dynamic compression plates (DCPs) in bone of varying quality using finite element modelling. The bone properties considered include orthotropy, inhomogeneity, cortical thinning and periosteal apposition associated with osteoporosis. The effect of preloads induced by compression plating was included in the models. Two different fracture scenarios were modelled: one with complete reduction and one with a fracture gap. The results show that the preload arising in DCPs results in large principal strains in the bone all around the perimeter of the screw hole, whereas for LCPs large principal strains occur primarily on the side of the screw proximal to the load. The strains within the bone produced by the two screw types are similar in healthy bone with a reduced fracture gap; however, the DCP produces much larger strains in osteoporotic bone. In the presence of a fracture gap, the DCP results in a considerably larger region with high tensile strains and a slightly smaller region with high compressive strains. These findings provide a biomechanical basis for the reported improved performance of locking plates in poorer bone quality.

Bone resorption triggered by high radial stress: The mechanism of screw loosening in plate fixation of long bone fractures

“…This indicates that LCP is superior at resisting axial loading and bending moments and DCP is superior at resisting torsional loading of unstable fractures. LCP has been reported to be more secure in fixing unstable and osteoporotic fractures due to its angle stability . However, no conclusive discovery could be made on the fixation stability of LCP and DCP in terms of resisting different forces.…”

“…Two idealized identical diaphyseal tibias with a length of 49.0 mm were created to represent the two long bone fracture fragments . The geometric characteristics for the bone were selected to match reported values of cortical thickness and cross‐sectional areas for healthy young adults, which are 5.1 mm and 319.2 mm 2 , respectively .…”

“…The material qualities to represent healthy bone included material orientations for orthotropy and heterogeneous variation in the radial direction. These qualities best represents the mechanical behavior of cortical bone and have been used in several studies to examine the stability of plate fixation configurations . These properties are summarized in Table .…”

“…Screws were modeled with an outer diameter of 5.0 mm and a thread depth of 0.6 mm. The screw threads were explicitly modeled with a triangular profile in idealized rings as described in several modeling studies . The plate and screws were stainless steel and were modeled as a homogeneous isotropic material with Young's modulus and Poisson's ratio equal to 205 GPa and 0.3, respectively …”

Pre-operative planning for fracture fixation using locking plates: device configuration and other considerations

“…On the other hand, studies have also shown that the pull-out strength of conventional screws increases with bone density [16,17] which can result in equivalent or even better results than locked plating in healthy bone [14]. These differences arise due to two main factors: (1) The preloads involved in compression screw tightening increase strain levels at the screw-bone interface even before physiological loads are applied, whereas locking screws have negligible screw-tightening preload and resulting strains [13,18]; and (2) During physiological loading, compression plating allows for frictional load transfer at the plate-bone interface; locked plating, on the other hand, transfers all physiological loads via the screw-bone interface [18]. In particular, the localised high tensile strains produced by conventional screws have been shown to be responsible for their poorer performance in osteoporotic bone [18].…”