[Possibilities for improving fatigue properties of interlocking screws of solid tibial nails. A mathematical model with practical conclusions]. / Möglichkeiten zur Verbesserung der Ermüdungseigenschaften von Verriegelungsschrauben solider Tibianägel. Ein mathematisches Modell mit praktischen Schlussfolgerungen.

One major problem using small diameter nails in the treatment of tibial fractures is the high rate of fatigue fractures of the locking screws. The objective of this study was to develop a mathematical model for analysis of the stress concentration factor as well as edge fibre stress. This would allow an analysis of the stress peaks caused by the thread of the screw as well as showing ways to increase the fatigue limits of the screws. The main consideration was the fact that a thread can be calculated like a relief notch used in theory of strength to relieve the strain on building materials. The transformation from one notch to multiple notches obviously reduces the stress concentration factor. To improve the fatigue limit of locking screws one has to modify the stress concentration factor or the edge fibre stress on the implant. Absence of a thread at the location where the screw contacts the nail's aperture (where the main load is transmitted to the screw and where the screw therefore usually tends to break) may double the fatigue strength and fatigue limit by avoiding the negative notch effect of the screw's thread. If this option is not possible one has to consider that the optimal threaded screw should have a minimal edge distance, a high notch radius and a minimal edge depth.

Evoked potentials in acute head injured patients with MRI-detected intracerebral lesions.

BACKGROUND: Magnetic resonance imaging (MRI) allows precise detection of intracranial lesions in head injured patients. We compared intracranial lesions detected in MRI to somatosensory evoked potentials (SEP) and brainstem auditory evoked potentials (BAEP) concerning their prognostic value. METHODS: Thirty patients with traumatic brain injury and prolonged recovery were studied. Size, side and number of 474 intra- and extraparenchymal lesions as well as lesion localisation based on a specific anatomical classification were entered into a database (a total of 7080 data). In addition, we recorded median-nerve SEP (M-SEP), tibial nerve SEP (T-SEP) and BAEP in all of the patients. FINDINGS: M-SEP and Glasgow-Outcome-Score (GOS) one year after injury correlated significantly to patients with lesions in the brainstem (p<0.0001) and corpus callosum (p<0.001). Similar results were found for T-SEP (p<0.0001). All patients with bicortical loss of M-SEP had an unfavourable outcome (GOS 2). Among the analysis of lesion volume, only the volume of brainstem lesions correlated to GOS (p<0.001), but this was not found for callosal lesions. However, comparing the vegetative (GOS 2) to the non-vegetative group (GOS 3-5), for both callosal (p<0.02) and brainstem (p<0.005) lesions a significant correlation was found. INTERPRETATION: MRI does not improve the prognostic reliability of SEP in head injury but offers possibilities for clarifying electrophysiological and clinical pathologies. This explains that the volume of brainstem lesions, essentially influencing the clinical outcome, is strongly correlated to T-SEP and M-SEP. In contrast, callosal lesions did not show a clear relationship to outcome despite large callosal lesions (>4 ml) which tended to poor outcome. In conclusion, we suggest that MRI and SEP are supplementary to each other concerning prognostic evaluation.

Fatigue strength of locking screws and prototypes used in small-diameter tibial nails: a biomechanical study.

BACKGROUND: One major problem with the use of small-diameter nails in the treatment of tibial fractures is the high rate of fatigue fractures of the locking screws. The objective of this study was to correlate such parameters as diameter of locking screws, influence of notch effect, and material with fatigue strength and fatigue limit. METHODS: In a biomechanical study, the mechanical properties of 11 different locking screws and screw designs to be used in solid tibial nails were tested under typical fatigue conditions. RESULTS: The results showed that an increase in the screw's diameter significantly increased the fatigue strength of the tested locking screws and prototypes (p < 0.0005). A 20% increase of diameter improved fatigue strength by 25 to 70%. Absence of thread at the location where the screw contacts the nail's aperture (where the main load is transmitted to the screw and where the screw, therefore, usually tends to break) may double the fatigue strength and fatigue limit by avoiding the negative notch effect of the screw's thread. It was further significant that titanium alloys had a higher fatigue strength compared with stainless steel alloys (p < 0.001). However, the fatigue strength of titanium implants is massively reduced by minimal damage to the surface. CONCLUSION: Optimizing the core diameter of the locking screws at the aperture of the solid tibial nail by using screws without a continuous thread generates a fatigue strength two to three times higher compared with screws with a continuous thread. Increasing fatigue strength of locking screws might decrease malunion in fractures stabilized by small-diameter nails.