Are plasticity models required to predict relative risk of lag screw cut-out in finite element models of trochanteric fracture fixation?

Using a finite element model of unstable trochanteric fracture stabilized with a sliding hip screw, the benefits of two plasticity-based formulations, Drucker-Prager and crushable foam, were evaluated and compared to the commonly used linear elastic model of trabecular bone in order to predict the relative risk of lag screw cut-out for five distinct load cases. The crushable foam plasticity formulation leads to a much greater strain localization, in comparison to the other two models, with large plastic strains in a localized region. The plastic zone predicted with Drucker-Prager is relatively more diffuse.

A computational study on the effect of fracture intrusion distance in three- and four-part trochanteric fractures treated with Gamma nail and sliding hip screw.

Using finite element analysis, the behaviors of the Gamma nail and the sliding hip screw (SHS) were compared in an osteoporotic bone model for the fixation of three- and four-part trochanteric fractures (31-A2 in the AO classification, types IV and V in Evans' classification). The size of the medial fragment was varied based on clinical data, and the case of a fractured greater trochanter was also considered. Our results showed that for Evans' type V stabilized with a Gamma nail and for Evans' types IV and V with the SHS, cancellous bone around the lag screw is susceptible to yielding, thus indicating a risk of cut-out.

A micro-architectural evaluation of osteoporotic human femoral heads to guide implant placement in proximal femoral fractures.

Background And Purpose: The micro-architecture of bone has been increasingly recognized as an important determinant of bone strength. Successful operative stabilization of fractures depends on bone strength. We evaluated the osseous micro-architecture and strength of the osteoporotic human femoral head.

What is the relevance of the tip-apex distance as a predictor of lag screw cut-out?

Using a simple mathematical formulation, the relationship between the position of the lag screw tip (relevant to both intramedullary and extramedullary devices) and the concept of tip-apex distance (TAD) was derived. TAD is widely used in operating theaters as a surgical guideline in relation to the fixation of trochanteric fractures, and in clinical studies as a predictor of lag screw cut-out. In order to visualize better this concept, the locus of points having the same TAD was plotted and the dependence of TAD on the location of the lag screw tip was also reported.

The importance of lag screw position for the stabilization of trochanteric fractures with a sliding hip screw: a subject-specific finite element study.

Using finite element analysis, we compared the biomechanical performance of a CT scan-based three-part trochanteric fracture model (31-A2 in the AO classification) stabilized with a sliding hip screw for nine different positions of the lag screw (3 × 3 arrangement, from anterior to posterior and from inferior to superior). Our results showed that the volume of bone susceptible to yielding in the head and neck region is the lowest for inferior positions and increases as the lag screw is moved superiorly. Overall, for this specific subject, the models less likely to lead to cut-out are the ones corresponding to inferior middle and inferior posterior positions of the lag screw.

Focal adhesion size controls tension-dependent recruitment of alpha-smooth muscle actin to stress fibers.

Expression of alpha-smooth muscle actin (alpha-SMA) renders fibroblasts highly contractile and hallmarks myofibroblast differentiation. We identify alpha-SMA as a mechanosensitive protein that is recruited to stress fibers under high tension. Generation of this threshold tension requires the anchoring of stress fibers at sites of 8-30-microm-long "supermature" focal adhesions (suFAs), which exert a stress approximately fourfold higher (approximately 12 nN/microm2) on micropatterned deformable substrates than 2-6-microm-long classical FAs.