Complications of trochanteric fracture fixation
Goffin, Jerome Moon Ho
SHS vs. Gamma Nail Using FE analysis, the behaviour of the Gamma nail and the SHS was compared in an osteoporotic bone model for the fixation of three- and fourpart 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. Results show 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. The volume of bone susceptible to yielding increases with an increase in size of the medial fragment. On the other hand, Evans’ type IV with a Gamma nail is not predicted to cut out. These findings suggest that future clinical trials investigating fixation of unstable proximal fractures should include the size of the medial fragment and the integrity of the greater trochanter as covariables and be powered to evaluate whether intramedullary devices are superior to SHSs for Evans’ type IV fractures and inferior/equivalent for type V. PFNA Because of the contradictory body of evidence related to the potential benefits of helical blades in trochanteric fracture fixation, the e↵ect of bone compaction resulting from the insertion of a PFNA was evaluated. A subject-specific computational model of a trochanteric fracture (31-A2 in the AO classification) was developed with lack of medial support and bone density was varied to account for variability in bone properties amongst hip fracture patients. Results show that for a bone density corresponding to 100% of the bone density of the cadaveric femur, there does not seem to be any advantage in using a PFNA with respect to the risk of blade cut-out. On the other hand, in a more osteoporotic femoral head characterized by a density corresponding to 75% of the initial bone density, local bone compaction around the helical blade provides additional bone purchase, thereby decreasing the risk of cut-out, as quantified by the volume of bone susceptible to yielding. These findings indicate benefits of using a PFNA over an intramedullary nail with a conventional lag screw and suggest that any clinical trial reporting surgical outcomes regarding the use of helical blades should include a measure of the femoral head bone density as a covariable. TAD and Screw Position 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 TAD variations as the location of the lag screw tip was varied were described. Findings show that TAD should be adjusted for the size of the femoral head (a variable which varies a lot according to the sex of the patient) and that no correlation exists between TAD and bone morphometry indices obtained from μCT data (BV/TV and Tb.Th). Therefore, these results seem to suggest that TAD lacks mechanical justification and that predictors which are based on mechanical properties, such as bone density, should be investigated further. Cut-out and Screw Position The biomechanical performance of a CT scan-based three-part trochanteric fracture model (31-A2 in the AO classification) stabilized with a SHS was compared for nine di↵erent positions of the lag screw (3 x 3 arrangement, from anterior to posterior and from inferior to superior). Results show 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. In this study, TAD was anti-correlated with the risk of cut-out, as quantified by the volume of bone susceptible to yielding, which suggests that a TAD > 25 mm cannot be considered to be an accurate predictor of lag screw cut-out. Further clinical studies investigating lag screw cut-out should attempt to find more reliable predictors of cut-out that should better reflect the biomechanics and subject-specificity of the femoral head. Plasticity Formulations Using an FE model of unstable trochanteric fracture stabilized with a SHS, 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 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 di↵use. Linear elasticity associated with a minimum principal strain criterion provides the smallest volume of elements susceptible to yielding for all loading modes. The region likely to undergo plastic deformation, as predicted by the linear elastic model, is similar to that obtained from plasticity-based formulations, which indicates that this simple criterion provides an adequate estimate of the risk of cut-out.