Screw insertion

Figure 15.5 Rotationaliy molded part with screw Insert...

Where thin sheets are to be joined and access is availabie from only one side, rivet bushes or rivet nuts are used. These provide an adequate length and strength of thread which is fixed and therefore allows ease of assembly, Fig. 12.5(c). Available in thread sizes up to 12 mm for lighter applications, blind nuts of the type shown in Fig. 12.5(d) can also be used. The nut is enclosed in a plastics body which is pressed into a predrilled hole. A screw inserted into the nut pulls it up and, in so doing, expands and traps the plastics body. 

Molded plastic parts acquire internal stresses during the molding process, and these increase when fasteners are installed. As a resulf bulk material is generally added to area of the joint in the form of a boss. The added material of a boss is intended to prevent cracking of the plastic when the fastener is installed and tightened. The boss outside diameter should also provide enough strength to withstand possible hoop stresses developed by screw insertion. 

Nurmi JT, Kannus P, Sievanen H, Jarvela T, Jarvinen M, Jarvinen TL (2004) Interference screw fixation of soft tissue grafts in anterior cruciate ligament reconstruction part 2 effect of preconditioning on graft tension during and after screw insertion. Am J Sports Med 32 (2) 418-424... 

Fig. 5.4. Marked artefact and signal distortion from metallic screws inserted for bilateral slipped capital femoral epiphysis...

Although absorbable plates have been relatively successful for these applications, there are limitations. Deforming the polymer plates prematurely can cause damage and weaken plates, and absorbable screws often require tapping of the bone resulting in over-tapping of drilled holes for screw insertion (9). Another concern is the application of excessive torsional forces that result in the failure between the head of the screw and the shaft. 

Bone absorption around screw or screw loosening for total joint replacement has been studied as one of failure factor. Stress concentration around the fixed screw seems to be an index to evaluate a possibility of failure after the TMJ prosthesis replacement. Therefore, the distribution of the fixed screws in the model C was evidenced more suitable for decreasing the stress concentration around screw holes. In addition, the distribution of the fixed screws at the model A could be better to prevent screw fracture due to less magnitude in peak stress concentration. Analyzing the different types of positions in the screw insertion showed that the magnitudes of peak stress in three condylar prostheses were not significant difference, but the regions of the peak stress were trivial dissimilar in the three FE models. 

FIGURE 15.1 Axial air swirlers (a)-dual-lead spiral (b)-the multiple-start screw insert and (b-a)multibladeaxial air swirler. 

Keywords— Ultrasound, imaging, screw insertion, scoliosis, spinal surgery. 

This study is part of our goal to investigate the potential application of ultrasonnd to guide screw insertion during surgery. The objective of this preliminary study is to investigate... 

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