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At least 91 records · Page 5Linked to original sources
PubMed1987

Scaphoid osteosynthesis: early experience with the Herbert bone screw.

The results of our first fourteen patients with fractured scaphoids treated with the Herbert bone screw have been disappointing. Six have failed to unite. The operation is technically demanding and there is a high incidence of malpositioning of the screw which correlates with failure to achieve union. This is predictable from initial radiographs and peroperative radiology is advised. The osteosynthesis should be protected until union has occurred radiologically, especially in manual workers.

Bone Screws↗
PubMed1994

A six-year retrospective analysis of the T-Screw bone fixation device.

The authors present a retrospective study of the T-Screw bone fixation device and its use for rigid fixation of osteotomies, arthrodesis, and fractures in foot surgery. The following is a discussion of 395 procedures in which the T-Screw was used for fixation. The results of the data confirm that there are specific advantages when using the device.

Adolescent↗
PubMed1983

[Experimental study of improved threading of bone screws].

In order to improve the holding force between the bone and the screw, we have been carrying out experimental investigations of the stability of bone and screw connection. Especially Schanz screws with a diameter of 6 mm, used in connection with the Wagner leg lengthening device, have been taken into consideration. By use of a loading model we determined the sizes of the essential kinds of loadability of the bone-screw connection and therefrom derive directions for dimensioning. The influence of the thread parameters (e.g. flank angle, pitch, pitch angle) on the loadability of the bone-screw connection has been investigated experimentally for 17 different thread profiles. Summarizing up the results, a pitch angle of 7,5 degrees and a flank angle between 12 degrees and 15 degrees proved to enable the maximum loadability. Corresponding to the results of our trials with the loading model, the improvement of the thread was carried out on the premises of simultaneous failure of both the cortical thread and the Schanz screws by pressure or shear. The loosening behaviour of the improved thread has been tested together with the conventional thread form. The results showed a reduction of about 20% in loosening of the screws and an increase of about 30% in pressure and traction loadability.

Biomechanical Phenomena↗
PubMed1995

Predicting the integrity of vertebral bone screw fixation in anterior spinal instrumentation.

STUDY DESIGN: Using 2-week-old calf lumbar vertebrae as a model for the human adolescent spine, the strength and rigidity of different methods of anterior spinal screw fixation and the use of noninvasive techniques for predicting bone screw stability before surgery and screw insertional torque intraoperatively were investigated. OBJECTIVES: The objectives were to determine what factors most affect the strength and rigidity of screw fixation, to determine the strongest and most rigid type of screw fixation for anterior spinal instrumentation, and to determine if noninvasive measurements of bone density before surgery by dual energy x-ray absorptiometry or quantitative computed tomography and if intraoperative measurement of screw insertional torque can be used to predict the in vivo strength and rigidity of vertebral screw fixation. SUMMARY OF BACKGROUND DATA: Anterior spinal instrumentation is an efficient method to correct spinal deformities in the thoracolumbar and lumbar spine. Fewer vertebrae are instrumented and arthrodesed, allowing for greater spine mobility. The forces transmitted to each vertebra are higher, perhaps accounting for the clinical failure rate of 13-30% at the metal bone interface from screw cut out. METHODS: Quantitative computed tomography and dual energy x-ray absorptiometry were used to assess the bone density of 24, 2-week-old calf lumbar vertebrae. Four different methods of vertebral screw fixation were evaluated: unicortical screw, bicortical screw, bicortical screw and washer, and bicortical screws and staple. The maximal screw insertional torque was measured for each specimen. Each vertebral body-screw construct failed in a mode simulating in vivo screw cut out. The applied moment and rotatory displacement were recorded. Ash density was measured for each vertebral body after removing all hardware. RESULTS: Noninvasive measures of bone density varied linearly with ash density (P < 0.01). Screw insertional torque varied linearly with bone mineral content and bone mineral density (r2 = 0.50) and was correlated with the yield moment for all types of fixation except the staple. Density measured by quantitative computed tomography did not affect rigidity or yield moment. Post hoc analysis showed that the screw-staple construct was the strongest and most rigid form of fixation. CONCLUSIONS: Measurement of bone density before surgery using dual energy x-ray absorptiometry and intraoperative measurement of screw insertional torque can be used to assess the stability of anterior spinal instrumentation. A bicortical screw inserted through a Dwyer-type staple provided the strongest and most rigid form of fixation.

Absorptiometry, Photon↗
PubMed1996

The reinforcement of cancellous bone screws with calcium phosphate cement.

The ability of calcium phosphate cement (CPC) to reinforce cancellous screws placed in previously stripped holes was studied in vitro. The distal end of canine femurs were harvested. A total of 15 screws were placed in six femurs. The pullout strength (failure force), failure displacement, stiffness, and energy absorbed were determined for the screws in the intact cancellous bone. Next, these stripped screw holes were packed with CPC. The pullout test was repeated, and the results were compared using a paired, Student's t test. We found that the CPC was able to reinforce the previously stripped holes and significantly increase the pullout strength (1,159 +/- 278 N versus 678 +/- 297 N) and the stiffness (1,990 +/- 569 N/mm versus 1,519 +/- 609 N/mm) of the constructs, as well as the energy absorbed by the constructs until failure (467 +/- 180 N.mm versus 278 +/- 140 N.mm). There was no difference in the failure displacement (0.94 +/- 0.23 versus 0.85 +/- 0.51 mm). This study documents the ability of CPC to acutely reinforce cancellous bone screws in a region with no or poor-quality cancellous bone.

Animals↗
PubMedDate not supplied

A biomechanical comparison of headless tapered variable pitch and AO cortical bone screws for fixation of a simulated slab fracture in equine third carpal bones.

OBJECTIVE: To compare the mechanical shear strengths and stiffnesses obtained from in vitro testing of a simulated complete third carpal bone (C3) frontal plane radial facet slab fracture (osteotomy) stabilized with either a 4/5 Acutrak (AT) compression screw or a 4.5-mm AO cortical bone (AO) screw inserted in lag fashion. Drilling, tapping, and screw insertion torques, forces, and times also were compared between AT and AO implants. STUDY DESIGN: In vitro biomechanical assessment of site preparation, screw insertion, and shear failure test variables of bone screw stabilized simulated C3 slab fracture in paired cadaveric equine carpi. SAMPLE POPULATION: Eight pairs of cadaveric equine C3 without orthopedic abnormalities. METHODS: Standardized simulated C3 slab fractures were repaired with either AO or AT screws (AO/C3 and AT/C3 groups, respectively). Drilling, tapping, and screw insertion torques, forces, and times were measured with a materials testing machine for each screw type. Repaired specimens were tested in axially oriented shear until failure. Paired Students t-tests were used to assess differences between site preparation, screw insertion, and shear testing variables. Significance was set at P <.05. RESULTS: There were no significant differences in bone fragment measurements of the standardized simulated C3 slab fractures created for AO or AT screws. There were no significant differences for mean and maximum drilling torques; however, the tapered AT drill had greater maximum drilling force compared with the 3.2-mm and 4.5-mm AO drill bits. Mean insertion torque and force measured from the self-tapping AT screw were not significantly different compared with the 4.5-mm AO tap. There were no significant differences in maximum screw torque among constructs. Total procedure time was significantly longer for the AT group (5.8 +/- 1.6 minutes) compared with the AO group (2.9 +/- 1.1 minutes; P =.001). AT stabilized specimens had significantly greater mean +/- SD initial shear stiffness (3.64 +/- 1.08 kN/mm) than AO specimens (1.64 +/- 0.73 kN/mm; P =.005). All other shear mechanical testing variables were not statistically different among screw types. CONCLUSION: The 4/5 Acutrak insertion technique was accurate and safe, and the AT screw effectively stabilized simulated equine C3 frontal plane slab fractures. When tested in shear, this screw type was mechanically comparable to the 4.5-mm AO screw; however, AT constructs had greater initial shear stiffness. Initial shear stiffness was likely an indirect measure of interfragmentary compression, and thus may indicate that the AT screw provides a more rigid fixation for frontal plane C3 slab fractures in horses. CLINICAL RELEVANCE: Considering the comparable mechanical behavior, greater initial shear stiffness for AT screw stabilized C3 slab fracture fragments, the ability to accurately insert the screw with the aid of a guide pin, and the potential for less persistent soft tissue irritation with the headless screw design, the 4/5 tapered AT screw is an attractive alternative for repair of C3 slab fractures in horses.

Animals↗
PubMed1996

Factors affecting the pullout strength of cancellous bone screws.

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

Animals↗
PubMed1993

[Radiologic determination of bone screw exit site on the femoral condyle. A method of exact imaging and length determination of internal fixation devices (screws, K-wires) in metaphyses of larger tubular bones exemplified by the distal femur].

Distal femur imaging (A-P and lateral X-rays) after internal fixation often give only poor visualization of the exact position. A model with two anteriorly positioned cortex screws was used to show that an additional oblique X-ray, or fluoroscopy of the tip of the screw, allows much more exact determination of its length, so that it can be changed if necessary. This method is considered by the authors as a simple check of screw length measurement in "foyer fermée" procedures.

Adolescent↗
PubMed1984

Management of the fractured scaphoid using a new bone screw.

A new and simple operative technique has been developed to provide rigid internal fixation for all types of fractures of the scaphoid. This involves the use of a double-threaded bone screw which provides such good fixation that, after operation, a plaster cast is rarely required and most patients are able to return to work within a few weeks. A classification of scaphoid fractures is proposed. The indications for operation included not only acute unstable fractures, but also fractures with delayed healing and those with established non-union; screw fixation was combined with bone grafting to treat non-union. In a prospective trial, 158 operations using this technique were carried out between 1977 and 1981. The rate of union was 100 per cent for acute fractures and 83 per cent overall. This method of treatment appears to offer significant advantages over conventional techniques in the management of the fractured scaphoid.

Adolescent↗
PubMed2005

Cortical bone screw fixation in ionically modified apatite cements.

Hydroxyapatite cements are used in reconstruction of the face; usually in well-defined cavities where the cement can be stabilized without the need for internal fixation. A hydroxyapatite cement that could enable screw fixation and some loading therefore has considerable potential in maxillofacial reconstruction. It has been demonstrated recently that water demand of calcium phosphate cements can be reduced by ionically modifying the liquid component. This study investigated the capacity of an ionically modified precompacted apatite cement to retain self-tapping cortical bone screws. Screw pullout forces were determined in the direction of the screw long axis and perpendicular to it, using cortical bone and polymethylmethacrylate cement as a control. In bending pullout tests, measured forces to remove screws from ionically modified precompacted cement were insignificantly different from cortical bone. However, pullout forces of bone screws from hydroxyapatite cement decreased with aging time in vitro.

Bone Cements↗
PubMedDate not supplied

Arthrodesis of the hallux interphalangeal joint using a diagonally placed 2-mm. cortical bone screw.

Arthrodesis of the interphalangeal joint of the hallux is quite often needed in conjunction with first metatarsophalangeal joint interpositional arthroplasty. With the exception of two cited techniques, the authors believe all previously described methods of achieving interphalangeal joint arthrodesis are inconsistent in their results and in their ability to be performed in conjunction with first metatarsophalangeal joint procedures. The authors, therefore, sought a more practical, efficacious method of achieving hallux interphalangeal joint arthrodesis in the presence of first metatarsophalangeal joint implants, while still using ASIF-AO compressive technique. Presented is their technique of hallux IPJ arthrodesis using a diagonally placed 2-mm. cortical bone screw.

Arthrodesis↗
PubMed2001

Effect of insertion torque on bone screw pullout strength.

The effect of insertion torque on the holding strength of 4.5-mm ASIF/AO cortical bone screws was studied in vitro. Screw holding strength was determined using an Instron materials testing machine (Bristol, United Kingdom) on 55 lamb femora and 30 human tibiocortical bone sections. Holding strength was defined as tensile stress at pullout with rapid loading to construct failure. Different insertion torques were tested, normalizing to the thickness of cortical bone specimen engaged. These represented low, intermediate, high, and thread-damaging insertion torque. All screws inserted with thread-damaging torque and single cortex engaging screws inserted to high torque tightening moments showed diminished holding strength. This loss of strength amounted to 40%-50% less than screws inserted with less torque.

Biomechanical Phenomena↗