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A screw and a bolt (see Differentiation between bolt and screw below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a male thread (external thread). Screws and bolts are used to fasten materials by the engagement of the screw thread with a similar female thread (internal thread) in a matching part. Screws are often self-threading (also known as self-tapping) where the thread cuts into the material when the screw is turned, creating an internal thread that helps pull fastened materials together and prevents pull-out. There are many screws for a variety of materials; materials commonly fastened by screws include wood, sheet metal, and plastic. Explanation A screw is a combination of simple machines: it is, in essence, an inclined plane wrapped around a central shaft, but the inclined plane (thread) also comes to a sharp edge around the outside, which acts as a wedge as it pushes into the fastened material, and the shaft a

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A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the

Fastener

A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints;

Drill bit

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Non-setting, injectable biomaterials containing particulate hydroxyapatite can increase primary stability of bone screws in cancellous bone

Ulrike Kettenberger, Dominique Pioletti, Jorge Solana Munoz

BACKGROUND: Fracture fixation in weak bone is still a clinical challenge. Screw augmentation was shown to successfully increase their primary stability. The currently used calcium phosphate or polymeric bone cements, however, present important drawbacks such as induced toxicity and/or impaired bone neo-formation. A new approach to enhance bone screw primary stability without affecting bone formation is the use of non-setting, calcium phosphate loaded soft materials as the augmentation material. METHODS: Two types of biomaterials (non-crosslinked hyaluronic acid as viscous fluid and agar as hydrogel) were loaded with 40 wt/vol% of hydroxyapatite particles and characterized. The screw augmentation effect of all materials was evaluated through pull-out tests in bovine cancellous bone and compared to the non-augmented situation (control). The bone mineral density of each test sample was measured with μCT scans and was used to normalize the pull-out strength. FINDINGS: Both materials loaded with hydroxyapatite increased the normalized pull-out strength of the screws compared to control samples and particle-free materials. This counter-intuitive augmentation effect increased with decreasing bone mineral density and was independent from the type of the soft materials used. INTERPRETATION: We were able to demonstrate that non-setting, injectable biomaterials loaded with ceramic particles can significantly enhance the primary stability of bone screws. This material combination opens the unique possibility to achieve a screw augmentation effect without impairing or even potentially favoring the bone formation in proximity to the screw. This effect would be particularly advantageous for the treatment of osteoporotic bone fractures requiring a stabilization with bone screws.

2018

Strength of the baseplate fixation of a reversed shoulder prosthesis

Stephanie Kochbeck

A reversed shoulder prosthesis is the general treatment of glenohumeral arthritis and concomitant rotator cuff arthropathy. However, failure due to loosening of the components remains a concern. The purpose of this study was to examine the forces that occur at the screws of the glenoid implant during the fixation process. An experimental setup was developed allowing for measuring both force and torque depending on turn angle for each screw separately. Experiments were performed on artificial bone of two different densities (0.16 g/cm3 and 0.24 g/cm3) and on six human glenoid bones of unknown age and gender. Characteristic graphs could be obtained for every material and each screw position. On average maximal force values were 180 N in lower density foam and 341 N in higher density foam. Corresponding mean peak values for torque were 0.65 Nm and 1.33 Nm, respectively. In the human bone experiments force peak values for the anterior compression screw ranged from 179 N to 317 N, torque values from 0.26 Nm to 2.36 Nm. For the posterior compression screw force peak values ranged from 320 N to 545 N, torque values from 1.35 Nm to 4.66 Nm. Peak values were consistently higher in higher density foam and at the posterior screw position. These results show a high dependency of the occurring screw forces on material density and position within the glenoid and contribute to further research in glenoid implant design optimization.

2008

Calcium phosphate cement augmentation of cancellous bone screws can compensate for the absence of cortical fixation

Dominique Pioletti, Vincent Stadelmann, Alexandre Terrier

An obvious means to improve the fixation of a cancellous bone screw is to augment the surrounding bone with cement. Previous studies have shown that bone augmentation with Calcium Phosphate (CaP) cement significantly improves screw fixation. Nevertheless, quantitative data about the optimal distribution of CaP cement is not available. The present study aims to show the effect of cement distribution on the screw fixation strength for various cortical thicknesses and to determine the conditions at which cement augmentation can compensate for the absence of cortical fixation in osteoporotic bone. In this study, artificial bone materials were used to mimic osteoporotic cancellous bone and cortical bone of varying thickness. These bone constructs were used to test the fixation strength of cancellous bone screws in different cortical thicknesses and different cement augmentation depths. The cement distribution was measured with microCT. The maximum pullout force was measured experimentally. The microCT analysis revealed a pseudo-conic shape distribution of the cement around the screws. While the maximum pullout strength of the screws in foam only was 30±7 N, it could increase up to approximately 1000 N under optimal conditions. Cement augmentation significantly increased pullout force in all cases. The effect of cortical thickness on pullout force was reduced with increased cement augmentation depth. Indeed, cement augmentation without cortical fixation increased pullout forces over that of screws without cement augmentation but with cortical fixation. Since cement augmentation significantly increased pullout force in all cases, we conclude that the loss of cortical fixation can be compensated by cement augmentation.

Elsevier2010