Femoral Iatrogenic Subtrochanteric Fatigue Fracture Risk is not Increased by Placing Drill Holes Below the Level of the Lesser Trochanter.

BACKGROUND: Iatrogenic subtrochanteric fractures of the femur can occur postoperatively following placement of screws in the lateral femoral cortex. Drilling holes below the lesser trochanter is generally avoided to prevent fatigue failure; however, there is little biomechanical evidence to support this recommendation. We hypothesized that hole placement below the level of the lesser trochanter will not accelerate fatigue failure compared to holes at the level of the lesser trochanter. METHODS: Twelve matched-pairs of male fresh-frozen cadaveric femurs were used for biomechanical testing. A single screw hole was drilled through the lateral femoral cortex either at the level of the lesser trochanter (proximal-hole group) or below the lesser trochanter (distal-hole group). Each femur was cycled to failure using a physiologically-relevant loading model. Paired t-test was used to evaluate for a difference in cycles to failure between groups. RESULTS: There was no statistical difference in cycles to failure between the groups with the hole drilled at or below the lesser trochanter. CONCLUSIONS: The traditional recommendation to avoid drilling holes below the level of the lesser trochanter is based mainly on experience and case reports in the literature. The results of this study indicate that placing holes below the level of the lesser trochanter, in and of itself, may not pose any additional risk of fracture. Other important factors need to be considered, such as tapering of the lateral femoral cortex. CLINICAL RELEVANCE: There are often situations where the patient's anatomy and facture pattern is more conducive to placing a screw distal to the lesser trochanter. This study may allow surgeons greater flexibility in placing screws more distally in the lateral femoral cortex by demonstrating the safety of doing so, at least in the population studied.

A fatigue loading model for investigation of iatrogenic subtrochanteric fractures of the femur.

BACKGROUND: Biomechanics of iatrogenic subtrochanteric femur fractures have been examined. Previously-described loading models employed monotonic loading on the femoral head, which is limited in emulating physiological features. We hypothesize that cyclic loading combined with the engagement of abductor forces will reliably cause iatrogenic subtrochanteric fractures. METHODS: Finite element analysis determined the effects of adding the abductor muscle forces to the hip contact force around holes located in the lateral femoral cortex. Finite element analysis predictions were validated by strain gage measurements using Sawbones™ femurs (Pacific Research Laboratories, Inc., Vashon, Washington, USA) with or without abductor muscle forces. The newly developed physiologically-relevant loading model was tested on cadaveric femurs (N=8) under cyclic loading until failure. FINDINGS: Finite element analysis showed the addition of the abductor muscle forces increased the maximum surface cortical strain by 107% and the strain energy density by 332% at the lateral femoral cortex. Strain gages detected a 72.9% increase in lateral cortical strain using the combined loading model. The cyclic, combined loading led to subtrochanteric fractures through the drill hole in all cadaveric femurs. INTERPRETATION: Finite element analysis simulations, strain gage measurements, and cyclic loading of fresh-frozen femurs indicate the inclusion of abductor forces increases the stress and strain at the proximal-lateral femoral cortex. Furthermore, a cyclic loading model that incorporates a hip contact force and abductor muscles force creates the clinically encountered subtrochanteric fractures in vitro. This physiologically-relevant loading model may be used to further study iatrogenic subtrochanteric femur fractures.

Impact of defects on the surface chemistry of ZnO(0001 macro)-O.

Scanning tunneling microscopy and core level photoelectron spectroscopy measurements have been used to investigate the morphology of ZnO(0001 macro)-O, and its reactivity with carbon monoxide and carbon dioxide, as a function of surface preparation. Real space images of the surface indicate that increasing the substrate anneal temperature during preparation significantly reduces the surface step density. Surface defect concentration is also monitored by employing formic acid as a chemical probe, which is shown to adsorb dissociatively (HCOOH --> [HCOO](-) + H(+)) only on zinc cations at step edges. Carbon 1s X-ray photoelectron spectra show that carbon monoxide and carbon dioxide both react to form surface carbonate species. Spectra, recorded both as a function of surface preparation and following coadsorption, demonstrate that the carbonate formed from either reactant molecule is located at oxygen vacancies at step edges, evidencing the significant role that defects can play in the surface chemistry of ZnO(0001 macro)-O.