[An in vitro model for producing femoral fractures and for the study of primary stability of cerclage]

PURPOSE

Intraoperative femoral fracture is a well recognized intraoperative complication of cementless total hip arthroplasty (THA). This study was designed to establish an in-vitro model for initiation of fractures of the femur in cementless THA and to assess the effect of fracture fixation by cerclage wiring using steel wire (Protasul).

METHODS

Ten human femur specimens were studied. Longitudinal cracks were created by pressing a well fixed conical stem deeper into the femur in a material testing machine.

RESULTS

The force necessary to propagate a longitudinal crack in the femur was 1915 N to 9288 N (median 6531 N). This forces are according to 3 to 15 times (median 11 times) body weight. There was no difference of force between left and right femurs from the same individual (U-test). The femoral cortex fractured in 5 cases ventrally, in 6 cases medially and in 1 case laterally. After monofile cerclage wire application the force required to press prosthesis 5 mm deeper into the medullary canal was 0.3 to 2.7 times body weight. After monofile cerclage wire application pressing in the prosthesis 30 mm needed 1.8 to 8.1 times body weight.

CONCLUSIONS

During insertion of a conical stem the orthopaedic surgeon should look for fractures of the medial cortex of the femur which is the most frequent location for fractures. This in-vitro model provides a reproductable method for the initiation and propagation of longitudinal femoral fractures in press-fit noncemented THA system and to assess the effect of fracture fixation in the form of cerclage wiring.

Electrical and magnetic stimulation of the intercostal nerves: a comparative study

The present study was performed in order to re-evaluate a previous report on motor nerve conduction measurement of the intercostal nerves (IN) by electrical stimulation (ES). In addition, it was aimed to compare ES with magnetic stimulation (MS). The 7th to 9th IN were investigated in 50 healthy volunteers, aged 20 to 80 years (mean age 35 +/- 16 years). The recording electrodes were placed over the cranial part of the rectus abdominal muscle. Stimulation was performed paravertebrally, and at a distance of 10 cm between stimulating and recording electrodes. The mean distal motor latency was slightly, but significantly longer after ES than MS (2.5 +/- 0.4 ms and 2.7 +/- 0.3 ms, respectively, p < 0.01). ES and MS resulted in similar mean motor nerve conduction velocities (NCV; 77 +/- 10 m/s and 79 +/- 10 m/s, respectively; p = 0.2). The amplitudes of the compound motor action potentials (CMAP) after distal stimulation versus proximal stimulation were significantly different in ES (2.5 vs. 1.9 mV; p < 0.01) and MS (3.6 vs. 1.6 mV; p < 0.001, resp.). In one subject, neither distal nor proximal ES evoked a CMAP. In eight subjects, proximal ES failed to elicit a CMAP. In all subjects, distal MS resulted in a visible CMAP, whereas in two subjects proximal MS was unsuccessful. Thus, MS proved to be significantly more effective in eliciting a CMAP (p < 0.05). Moreover, MS was agreed to be less unpleasant than ES. In summary, MS facilitates nerve conduction studies of the IN and is superior to ES regarding performance and acceptance.