The holding power of orthopedic screws in vivo

Mechanical characteristics of the new BONE-LOK bi-cortical internal fixation device

The purpose of this study was to evaluate the mechanical characteristics of a new and unique titanium compression anchor with BONE-LOK (Triage Medical, Inc, Irvine, CA) technology for compressive, bi-cortical internal fixation of bone. This device provides fixation through the use of a distal grasping anchor and an adjustable proximal collar that are joined by an axially movable pin and guide wire. The titanium compression anchor, in 2.0-, 2.7-, and 3.5-mm diameters, were compared with cortex screws (Synthes USA, Paoli, PA) of the same diameter and material for pullout strength in 20 lb/cu ft and 30 lb/cu ft solid rigid polyurethane foam; and for compression strength in 20 lb/cu ft foam. Retention strength of the collar was tested independently. The results showed significantly greater pullout strength of the 2.7-mm and 3.5-mm titanium compression anchor as compared with the 2.7-mm and 3.5-mm cortex screws in these test models. Pullout strength of the 2.0-mm titanium compression anchor was not statistically different in comparison with the 2.0-mm cortical screws. Compression strength of the titanium compression anchor was significantly greater than the cortical screws for all diameters tested. These differences represent a distinct advantage with the new device, which warrants further in vivo testing. Collar retention strength testing values were obtained for reference only and have no comparative significance.

Primäre Implantatstabilität bei unterschiedlichen Knochenaufbereitungstechniken

The treatment concept of osseointegration is based on a stable embodiment of implants in bone and the maintenance of stability during functional load. A goal of the surgical preparation technique is therefore to obtain a stable and firm implant anchorage. The aim of this study was to evaluate implant stability after different surgical treatment of the bony implantation bed. Thirty cylindrical solid-screw-shaped implants with standard SLA ITI configuration were implanted into the explanted mandibles of five minipigs. The implant sites were prepared either by a conventional burr technique (group A), by burr technique with additional thread cutting (group B), or by the osteotome technique (group C). Primary implant stability was evaluated by resonance frequency analysis and removal torque test. The average value of the resonance frequency analysis (RFA) was 6000+/-469 cycles/s in group A, 5700+/-557 cycles/s in group B, and 5540+/-527 cycles/s in group C. Removal torque values of group A (507+/-57 Nmm) were significantly higher than those of group B (466+/-45 Nmm) and group C (240+/-31 Nmm) (between group A and C p<0.05, group A to B p=0.39, and B to C p<0.05). It can be concluded from this study that the conventional burr technique achieves a statistically significantly better primary bone anchorage than the osteotome technique.

A biomechanical study of the cortex-anchorage vertebral screw

OBJECTIVE

To obtain a comprehensive understanding on the effect of the improvement of fixation strength and on the optimal design in various geometrical parameters of a new screw system through biomechanical analyses.

DESIGN

A new screw with the cortex-anchorage was designed and manufactured to improve the fixation of the instrumentation for osteoporotic spine. There were four expandable wings distributed around the screw after insertion.

BACKGROUND

Screw loosening or loss of correction caused by insufficient mechanical stability on the bone-screw interface is frequently found in osteoporotic subjects. Similarly, the removal and replacement of a screw in a revision procedure substantially decreases its mechanical fixation. Since cortex is the most rigid part in the vertebral body, emphasis on the cortex-anchorage may offer an optimal fixation of screws.

METHODS

The biomechanical evaluation that consists of the pullout test and the finite element analysis was applied to identify the stabilizing effect and the optimal design for the new screw system. In the pullout experiment, the porcine vertebral body with a hollow block of cancellous bone was proposed to simulate an osteoporotic spine. This osteoporotic model was specially simulated the degeneration and destruction of the cancellous bone in vertebrae. In the finite element analysis, the reduction of elastic modulus was used in various levels of vertebral degeneration.

RESULTS

Pulling screws out of vertebral bodies with a hollow block of cancellous bone, the mean pullout force was 729 (SD 159) N for the conventional screws, and 1072 (SD 179) N for the new screw system. The finite element analysis showed that the longer screw with bi-cortex fixation was the better option in reducing the bony stress and increased the stability. As the height of wings changed, the stress distributed on vertebral body indicated the lowest in fixation by a screw with the largest wings. Nevertheless, there existed a least displacement of vertebral body and moderately low stress on wings' lateral end when assembled with the middle size wings.

CONCLUSION

The stabilization function of expansive wings of the new screw system was enhanced in the osteoporotic vertebra and better than that of a conventional screw. The finite element analysis showed a middle size wing could help the screw to reduce the risk of failure and to improve the vertebral stability.

RELEVANCE

Screw loosening or loss of correction caused by insufficient mechanical stability on the bone-screw interface is frequently found in osteoporotic subjects. From the biomechanical point of view, this study had shown that a new design of screw could improve the fixation of the instrumentation for osteoporotic spine. With further investigations that includes the clinical proof and the development of a cortex-anchorage vertebral screw may provide a valuable alternative to the spinal instrumentation for the patients with osteoporosis.

Mechanical and histomorphometric evaluations of titanium implants with different surface treatments inserted in sheep cortical bone

Improvement of the implant-bone interface is still an open problem and the interest in chemical modification of implant surfaces for cementless fixation has grown steadily over the past decade. Mechanical and histomorphometric investigations were performed at different times on implants inserted into sheep femoral cortical bone to compare the in vivo osseointegration of titanium screws ( X 3.5 x 7 mm length) with different surface treatments. After 8 weeks of implantation, the push-out force of anodized and hydrothermally treated implants (ANODIC) was significantly higher than that of machined implants (MACH) (36%, p<0.0005), whereas a decrease of 39% was observed for acid-etched implants (HF) when compared to other surface treatments. After 12 weeks of implantation, the push-out force values of HF implants were still significantly lower than those observed for MACH (-19%, p<0.01) and hydroxyapatite vacuum plasma-sprayed implants (HAVPS, -25%, p<0.0005), and the highest push-out force was found in HAVPS (p<0.001) implants. After 8 and 12 weeks of implantation, the AI of HF implants was significantly (p<0.05) lower ( approximately -25%) than that of MACH, HAVPS and ANODIC implants. In conclusion, results appear to confirm that there are no specific differences between ANODIC and HAVPS implants in terms of behavior. Moreover, although MACH implants show some surface contaminating agents, they appear to ensure good osseointegration within 12 weeks both mechanically and histomorphometrically, as do ANODIC and HAVPS implants. However, further studies are required to investigate bone hardness and mineralization around implants.

Stability of the offset V osteotomy: effects of fixation, orientation, and surgical translocation in polyurethane foam models and preserved cadaveric specimens

Polyurethane foam models and cadaver specimens were used to examine the stability of the offset V first metatarsal osteotomy. Uniform osteotomies were performed in all specimens by using a specially designed jig. Specimens in the polyurethane foam model series (n = 10) varied with respect to fixation type, fixation orientation, and degree of lateral translocation of the osteotomy. All specimens were loaded to failure in an Instron testing machine (Instron, Canton, MA). The plantar wing-pin (Kirschner wire) osteotomy group showed statistically significantly greater stiffness (P =.0119) and load at failure (P =.0027) than the dorsal wing-pin group. Cadaveric offset V specimens received the same amount of capital fragment lateral translocation but had different fixation types and orientations. Using the identical protocol as the models, the cadaveric dorsal wing-screw group showed statistically significantly less displacement at failure than the plantar wing-screw, plantar wing-pin, and dorsal wing-pin groups (P =.0262). The dorsal wing-pin group with a synthetic tension band showed a statistically significant greater stiffness (P =.0054) and peak load at failure (P =.0004) compared with the dorsal wing-pin group without the tension band. The most stable offset V construct in the polyurethane foam model was the plantar wing-pin group. The preserved cadaveric specimens yielded different results. The cadaveric dorsal wing-pin group with the synthetic tension band showed superior stability compared with all other non-tension-band groups. These results indicate the importance of tension band effects provided by capsular and ligamentous structures, which are typically ignored in surgical optimization research.

Anterior Vertebral Body Screw Pullout Testing With The Hollow Modular Anchorage System - A Comparative in vitro Study. Hohltonnenschrauben als neues Verankerungskonzept an der Wirbelsäule - Lastauszugsversuche als biomechanische Vergleichsstudie

Pullout of implants at the proximal and distal ends of multilevel constructs represents a common spinal surgery problem. One goal concerning the development of new spinal implants is to achieve stable fixation together with the least invasive approach to the spinal column. This biomechanical study measures the influence of different modes of implantation and different screw designs, including a new monocortical system, on the maximum pullout strength of screws inserted ventrolaterally into calf vertebrae. The force pullout of eight different groups were tested and compared. Included were three bicortical used single screws (USS, Zielke-VDS, single KASS). To further increase pullout strength either a second screw (KASS) or a pullout-resistant nut can be added (USS with pullout nut). A completely new concept of anchorage represents the Hollow Modular Anchorage System (MACS-HMA). This hollow titanium implant has an increased outside diameter and is designed for monocortical use. Additionally two screw systems suitable for bicortical use were tested in monocortical mode of anchorage (USS, single KASS). We selected seven vertebrae equal in mean size and bone mineral density for each of the eight groups. The vertebral body and implant were connected to both ends of a servohydraulic testing machine. Displacement controlled distraction was applied until failure at the metal-bone-interface occurred. The maximum axial pullout force was recorded. Mean BMD was 312 +/- 55 mg CaHA/ml in cancellous bone and 498 +/- 98 mg CaHA/ml in cortical bone. The highest resistance to pullout found, measured 4.2 kN (KASS) and 4.0 kN (USS with pullout nut). The mean pullout strength of Zielke-VDS was 2.1 kN, of single KASS 2.5 kN, of MACS-HMA 2.6 kN and of USS 3.2 kN. There was no statistically significant difference (t-test, p > 0.05) between bicortical screws and the new monocortical implant. For the strongest fixation at the proximal or distal end of long spinal constructs the addition of a second screw or a pullout-resistant nut behind the opposite cortex offers even stronger fixation.

Screw pullout strength: a biomechanical comparison of large-fragment and small-fragment fixation in the tibial plateau

OBJECTIVES

To compare the pullout strengths of 6.5-millimeter diameter partially threaded cancellous screws and 4.5-millimeter diameter fully-threaded cortical screws versus 3.5-millimeter diameter cortical screws in the proximal tibia.

DESIGN

Three screws were inserted in the lateral tibial plateau of each leg of fifteen paired cadaveric tibias. In one tibia, large-fragment fixation was used, consisting of a unicortical 6.5-millimeter screw in the subchondral bone, and bicortical 4.5-millimeter screws in the metadiaphyseal and diaphyseal bone. In the contralateral tibia, small-fragment fixation consisting of three 3.5-millimeter screws was used, placing the screws in the same positions as described above.

MAIN OUTCOME MEASUREMENTS

A materials-testing machine was used to determine axial pullout strengths of each screw. The mean pullout strengths of large-fragment and small-fragment screws in each position were compared.

RESULTS

No significant difference in pullout strengths was found between the large-fragment and small-fragment screws in subchondral and metadiaphyseal bone. A statistically significant difference was found between pullout strengths of large-fragment and small-fragment screws in diaphyseal bone.

CONCLUSIONS

In human proximal tibial bone, the data from this study do not suggest that the pullout strength of 3.5-millimeter screws differs from that of 6.5-millimeter screws in subchondral bone, or that the pullout strength of 3.5-millimeter screws differs from that of 4.5-millimeter screws in metadiaphyseal bone. However, the pullout strength of 3.5-millimeter screws is significantly less than that of 4.5-millimeter screws in diaphyseal bone. The authors of the present study believe this supports the use of small-fragment fixation in the treatment of tibial plateau fractures.

Central and juxta-endplate vertebral body screw placement: a biomechanical analysis in a human cadaveric model

STUDY DESIGN

In vitro biomechanical testing of transvertebral body screws in different positions in both axial pull-out and toggle.

OBJECTIVES

To determine the relative strength of unicortical versus bicortical screw fixation within the vertebral body and to determine comparative strength of juxta-endplate and central screw positions with and without staples in both axial pull-out and toggle modes.

SUMMARY OF BACKGROUND DATA

Loss of fixation is common in centrally placed screws at the rostral end of a construct. To preserve segmental vessels, juxta-endplate screw positions are often used. The biomechanical strength of such screw placement methods has not been measured.

METHODS

Eighty-three human cadaveric vertebral bodies were tested for axial pull-out and toggle with and without staples. Screw positions included central, juxta superior, and inferior endplate. Juxta-endplate screws were toggled in both the rostral and caudal directions perpendicular to the screw axes.

RESULTS

Unicortical fixation resulted in a 93% decrease in axial pull-out strength compared with bicortical fixation. Centrally placed screws and juxta-endplate screws were equivalent in axial pull-out if no staples were used. The juxta-endplate screw with a staple that was toggled away from the endplate had the highest yield strength, followed by the central screw with a staple, and then the juxta-endplate screw without a staple toggled away from the endplate.

CONCLUSIONS

Bicortical fixation is much stronger than unicortical fixation. Centrally placed screws are significantly stronger when used with a staple. When preservation of segmental vessels is desirable, juxta-endplate screws should be placed in such a manner that compressive forces are directed away from the endplate.

Biomechanical evaluation of a double-threaded pedicle screw in elderly vertebrae

We sought to test the hypothesis that a pedicle screw that has two parallel threads of different heights throughout the full length of the screw could increase both bone purchase and pull-out strength compared with a standard single-threaded screw of similar dimensions. A single-threaded pedicle screw and a double-threaded pedicle screw were respectively placed into the paired pedicles of 21 vertebral bodies. The screws were then pulled out of the pedicles, and output parameters were measured. Although insertional torque was, on average, 14.5% higher (p = 0.039) for the single-threaded screw, maximum pull-out strength (p = 0.12), energy-to-failure (p = 0.39), and stiffness (p = 0.54) were not statistically different for the two screw types. It is concluded that a second, smaller inner thread on a double-threaded pedicle screw does not translate into either increased bone purchase or higher pull-out strengths.

Strength comparison of allogenic bone screws, bioabsorbable screws, and stainless steel screw fixation

Allogenic bone screws are new to the fixation market and have yet to be tested against current fixation materials. An in vitro comparison of the same sizes of stainless steel, bioabsorbable, and allogenic bone screws was undertaken to assess screw resistance to the forces of bending, pullout, and shear. Using aluminum plates to support the screws, forces up to 1000 Newtons were applied to six to eight samples of each type of screw. During each test, stainless steel screws withstood the maximum force that could be exerted by the testing apparatus without failing (bending, 113.9 +/- 11.8 N mean +/- SE; pullout 999.1 +/- 33.7 N; and shear, 997.5 +/- 108.8 N). In each test, compared to bioabsorbable screws, allogenic bone screws failed faster (pullout, allogenic: 12.4 +/- 1.1 seconds vs. bioabsorbable, 120.6 +/- 13.8 seconds; p = .001; bending, allogenic: 53.4 +/- 4.8 seconds vs. bioabsorbable, 201.9 +/- 11.1 seconds; p = .001; shear, allogenic 13.5 +/- 1.4 seconds vs. bioabsorbable, 43.8 +/- 0.9 seconds; p = .001) under equivalent (pullout: bioabsorbable, 385.0 +/- 18.4 N vs. allogenic, 401.0 +/- 35.9 N; p = .001) or lower (bending, allogenic: 4.7 +/- 0.2 N vs. bioabsorbable, 11.0 +/- 0.9 N; p = .675; shear, allogenic: 312.1 +/- 15.5 N vs. bioabsorbable 680.9 +/- 8.5 N; p = .001) loads, and in a highly variable fashion. Overall, the bioabsorbable screws withstood the forces of bending, pullout, and shear better than the allogenic screws, and stainless steel screws outperformed both bioabsorbable and allogenic screws. Despite these results, allogenic screws could still be useful in compliant patients who would benefit from their osteoconductive properties.

Arthrodesis of the equine proximal interphalangeal joint: a biomechanical comparison of two 7-hole 3.5-mm broad and two 5-hole 4.5-mm narrow dynamic compression plates

OBJECTIVE

To compare the biomechanical characteristics and mode of failure of two different dynamic compression plate (DCP) techniques for proximal interphalangeal joint (PIPJ) arthrodesis in horses.

STUDY DESIGN

Randomized block-design blocking on horse (1-5), method of fixation (two 7-hole, 3.5-mm broad DCP vs two 5-hole, 4.5-mm narrow DCP), side (left, right), and end (front, hind). Constructs were loaded to failure in 3-point bending in a dorsal-to-palmar (plantar) direction.

SAMPLE POPULATION

Ten paired limbs from 5 equine cadavers.

METHODS

Two 7-hole, 3.5-mm broad dynamic compression plates (bDCP) were used in 1 limb of a pair, and two 5-hole 4.5-mm narrow dynamic compression plates (nDCP) were used on the contralateral limb. Plates were positioned abaxially across the dorsomedial and dorsolateral aspect of the PIPJ. Arthrodesis constructs were loaded (19 mm/s) in 3-point bending in a dorsal-to-palmar (plantar) direction using a materials-testing machine. Composite stiffness, yield point, and maximal bending moment at failure were obtained from bending moment-angular deformation curves. Data were analyzed using ANOVA, X(2) analysis, and Fisher's exact tests; the power of the test was calculated when differences were not significant.

RESULTS

There were no significant differences in composite stiffness (P >.05; power = 0.8 @ delta = 21.9%), yield point (P >.05; power = 0.8 @ delta = 34.4%), or maximal bending moment (P >.05; power = 0.8 @ delta = 17.8%) between the two fixation techniques. For bDCP constructs, 11% (15 of 140) of the 3.5-mm screws were damaged; 7 of the screw heads pulled through plates where the plates bent, 1 screw head broke off, and 7 screws were bent or pulled out of the phalanx. For nDCP constructs, 8% (8 of 100) of the 4.5-mm screws were damaged; 1 screw head pulled through a plate, 1 screw head broke off, and 6 screws were bent or pulled out of the phalanx.

CONCLUSIONS

There were no biomechanical or failure differences between bDCP and nDCP fixation of the PIPJ in horses when evaluated in single-cycle 3-point bending to failure.

CLINICAL RELEVANCE

There is no biomechanical advantage to the use of two 7-hole, 3.5-mm bDCP in equine proximal interphalangeal arthrodesis compared with two 5-hole, 4.5-mm nDCP. Two 5-hole, 4.5-mm nDCP may be easier to place, whereas two 7-hole, 3.5-mm bDCP may provide more versatility in fracture repair.

A fatigue life analysis of small fragment screws

OBJECTIVES

To conduct a comparative fatigue analysis of several commonly used small fragment screws.

DESIGN

Biomechanical laboratory study.

SETTING

Research laboratory.

MAIN OUTCOME MEASUREMENTS

A fatigue life analysis of seven different types of small fragment screws was conducted using a Wohler fatigue-testing machine. Four different types of 3.5-millimeter cortical screws were subjected to fatigue analysis. These included solid stainless steel screws from Synthes Ltd. (core diameter 2.4 millimeters), Zimmer Inc. (core diameter 2.4 millimeter), and Smith and Nephew Richards Inc. (core diameter 2.4 millimeters) and cannulated stainless steel screws from Synthes Ltd. (core diameter 2.5 millimeters). In addition, three types of 4.0-millimeter cancellous screws were tested. These included stainless steel screws from Synthes Ltd. (core diameter 1.9 millimeters), titanium screws from Synthes Ltd. (core diameter 2.0 millimeters), and titanium alloy screws from DePuy-Ace (core diameter 2.8 millimeters). Fatigue lives, as reflected by mean cycles to failure, were compared.

RESULTS

The four types of cortical screws had longer fatigue lives than the Synthes cancellous screws did ( p < 0.001) but shorter fatigue lives than the DePuy-Ace cancellous screws did ( p < 0.0001). Among the cortical screws, the cannulated and solid Synthes screws and the solid Zimmer screws did not differ statistically. The Smith and Nephew Richards cortical screws failed at statistically fewer cycles than the Synthes solid and cannulated cortical screws did ( p < 0.003) but did not statistically differ from the Zimmer screws. The DePuy-Ace titanium alloy cancellous screw had the longest fatigue life of the tested implants by a large margin ( p < 0.0001). The Synthes pure titanium and stainless steel cancellous screws did not significantly differ.

CONCLUSIONS

This analysis supports core diameter as the principal factor determining fatigue life as the results paralleled implant geometry. This design modification to improve bending and fatigue strength may come at a price to pullout strength, however, because of a decreased major-to-minor diameter and increased pitch. Cortical screws differed in fatigue performance despite identical dimensions, presumably highlighting the importance of implant processing and machining. Cannulated cortical screws performed well relative to solid screws, thereby supporting their clinical use. Pure titanium and stainless steel cancellous screws performed similarly in fatigue despite differing material properties, presumably because of geometric design differences. This report highlights some of the differences in the in vitro fatigue performance among several commonly used small fragment screws.

Pullout properties of 3.5-mm AO/ASIF self-tapping and cortex screws in a uniform synthetic material and in canine bone

OBJECTIVES--To compare the pullout properties of 3.5-mm AO/ASIF self-tapping screws (STS) to corresponding standard cortex screws (CS) in a uniform synthetic test material and in canine femoral bone. The influence of screw-insertion technique, test material, and test-material thickness were also assessed. STUDY DESIGN--In vitro experimental study. SAMPLE POPULATION--Two independent studies: a uniform synthetic test material and paired femurs from mature dogs. METHODS-Mechanical testing was performed in accordance with standards established by the American Society for Testing and Materials for determination of axial pullout strength of medical bone screws. Completely inserted STS, completely inserted CS, and incompletely inserted STS were tested in 3 groups of 10 test specimens each in 4.96-mm and 6.8-mm thick sheets of synthetic material. In the bone study, group 1 consisted of 24 completely inserted STS compared with 24 completely inserted CS, and group 2 consisted of 24 incompletely inserted STS versus 24 completely inserted CS. Comparisons were made between paired femurs at corresponding insertion sites. Pullout data were normalized, thereby eliminating the effect of test-material thickness on pullout properties. Mean values were compared using 2-way ANOVA. Statistical significance was set at P <.05. RESULTS--In both the 4.96-mm and 6.8-mm synthetic material, pullout testing of the completely inserted STS demonstrated significantly greater yield strength and ultimate strength than completely inserted CS. There was no significant difference between incompletely inserted STS and completely inserted STS. The 6.8-mm test material significantly increased yield strength and ultimate strength for all test groups compared with the 4.96-mm test material. In canine bone, there was no significant difference in yield strength of completely inserted STS and completely inserted CS. Yield strength of completely inserted STS and completely inserted CS were significantly greater than incompletely inserted STS. CONCLUSIONS--Pullout properties of completely inserted STS were significantly greater than corresponding CS in a uniform test material. In canine bone, the pullout strength of STS and CS were not different. Incomplete STS insertion resulted in an 18% reduction in holding power as compared with completely inserted CS and STS in canine bone. CLINICAL RELEVANCE--The length of STS used in canine bone should be such that the cutting flutes extend beyond the trans cortex to maximize pullout strength.

Analysis of the osseous/metal interface of drill free screws and self-tapping screws

Aim: A comparison of metal/osseous interface and bone remodelling after insertion of different types of titanium bone screws in vivo. Material: Samples of five of each of the following bone screw types were inserted into the anterior wall of the frontal sinus of five Göttingen minipigs: self-tapping micro- (1.5 mm) and miniscrews (2.0 mm) or drill free micro- (1.5 mm) and miniscrews (2.0 mm) (Martin Medizintechnik, Tuttlingen, Germany). Screw length was 7 mm. Methods: Sequential intraperitoneal injections of fluorochromes were performed between the second and ninth postoperative week. After 6 months the pigs were sacrificed, the screw-bone-blocks resected, and microradiographic, histological and fluorescence microscopical examinations were carried out. Results: Using drill free screws, mean screw/bone contact was 88.4% (miniscrews), or 93.8% (microscrews). With self-tapping miniscrews it was 54.9%, but in microscrews 81%; the differences were statistically significant (t -test: p<0.05). By fluorescence microscopy, the amount of bone remodelling (ratio of residual vs. newly formed bone) was measured. Significantly more of the residual bone was found in the region of the screw threads using drill free screws (miniscrews: mean 71.8%, microscrews: mean 67.9%) than in the region of screw threads with self-tapping screws (miniscrews: mean 33.1%, microscrews: mean 42.4%). Conclusion: The present data support the view that screw/bone contact with drill free screws was superior to that of self-tapping screws; the greater amount of original bone in the threads of drill free screws demonstrated that the insertion of drill free screws did not cause harm to the surrounding bone. Both results are important for osteosynthesis in regions where thin cortical bone is present, such as the central midface. Copyright 2001 European Association for Cranio-Maxillofacial Surgery.

Effect of cutting flute design on cortical bone screw insertion torque and pullout strength

OBJECTIVE

To determine the effect of the number and length of cutting flutes on the insertion torque and pullout strength for self-tapping 4.5-millimeter cortical bone screws.

DESIGN

Screws were self-tapped in the diaphysis of human cadaver femurs. Each of the six screw types studied had different designs with varying cutting flute lengths and numbers. Bone mineral density, insertion torque, and pullout strength were measured.

SETTING

The study was conducted at an experimental biomechanics laboratory associated with a university medical center.

OUTCOME MEASUREMENTS

Insertion torque and pullout strength were normalized by the local bone mineral density.

RESULTS

The mean normalized insertion torque of the design with four full-length cutting flutes was less than the design with three full-length flutes and the two designs with one-third length flutes (p < 0.05). The mean normalized pullout strength of the screw with four full-length flutes was significantly greater than that of all screws with fewer than three flutes (p < 0.05).

CONCLUSIONS

Priorities for a cutting flute design should ideally include ease of screw insertion, minimal soft tissue irritation, and maximal screw holding power. Screws with more than two flutes were easier to insert and did not cause cortical damage during insertion. The screw with four full-length flutes showed a trend toward being the easiest to insert and having the greatest holding strength.

Effects of hole preparation on screw pullout resistance and insertional torque: a biomechanical study

OBJECT

The authors conducted a study to assess the effect of a pilot hole preparation on screw pullout resistance and screw insertional torque.

METHODS

Three different screws were tested: cancellous lateral mass screws, cortical lateral mass screws, and pedicle screws. Synthetic bone blocks were used as the host material. Each screw group was separated into two subgroups. The first subgroup of screws was inserted into the test material following pilot hole preparation. Pilot holes were prepared; a drill bit diameter size smaller than the core diameter of the screws was used. The second group of screws was inserted into the test material without pilot hole preparation (a 3- or 4-mm hole drilled for entrance site preparation only). The insertional torque was measured as the screw was advanced into the material. The screws were axially extracted from the host material at a constant speed of 2.5 mm/minute. The pullout resistances and insertional torques for the pilot hole and the nonpilot hole groups were then statistically compared. The authors found that preparation of a pilot hole caused a significant decrease in the insertional torque. The screws inserted without a pilot hole showed greater pullout resistances compared with those inserted following a pilot hole preparation; however, there was no statistically significant difference.

CONCLUSIONS

The optimum screw insertion technique may involve drilling a short pilot hole and using a drill bit with a smaller diameter than the screw core diameter to increase bone-screw purchase. This applies to cancellous and cortical lateral mass screws as well as pedicle screws.

In vitro comparison of the use of two large-animal, centrally threaded, positive-profile transfixation pin designs in the equine third metacarpal bone

OBJECTIVE

To compare the in vitro holding power and associated microstructural damage of 2 large-animal centrally threaded positive-profile transfixation pins in the diaphysis of the equine third metacarpal bone.

SAMPLE POPULATION

25 pairs of adult equine cadaver metacarpal bones.

PROCEDURE

Centrally threaded positive-profile transfixation pins of 2 different designs (ie, self-drilling, self-tapping [SDST] vs nonself-drilling, nonself-tapping [NDNT] transfixation pins) were inserted into the middiaphysis of adult equine metacarpal bones. Temperature of the hardware was measured during each step of insertion with a surface thermocouple. Bone and cortical width, transfixation pin placement, and cortical damage were assessed radiographically. Resistance to axial extraction before and after cyclic loading was measured using a material testing system. Microstructural damage caused by transfixation pin insertion was evaluated by scanning electron microscopy.

RESULTS

The temperature following pin insertion was significantly higher for SDST transfixation pins. Periosteal surface cortical fractures were found in 50% of the bones with SDST transfixation pins and in none with NDNT transfixation pins. The NDNT transfixation pins were significantly more resistant to axial extraction than SDST transfixation pins. Grossly and microscopically, NDNT transfixation pins created less damage to the bone and a more consistent thread pattern.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro analysis revealed that insertion of NDNT transfixation pins cause less macroscopic and microscopic damage to the bone than SDST transfixation pins. The NDNT transfixation pins have a greater pull out strength, reflecting better initial bone transfixation pin stability.

A biomechanical comparison of Schuhli nuts or cement augmented screws for plating of humeral fractures

Schuhli locking nuts can be used in poor quality cortical bone to enhance fixation stability as an alternative to cement augmented screws. This study compared the fixation strength and stability of plate constructs using Schuhli locking nuts with standard screws and cement augmented screws for fixation of simulated humeral shaft fractures in a test model with osteoporosis. The constructs were tested in axial compression, 4-point bending, and torsion to determine fixation stability. The humeri were cycled in torsion (4.5 Nm) for 1000 cycles to simulate upper extremity use during the early postoperative period and retested for stability. The Schuhli locking nuts and cement augmented screws had significantly greater fixation stability than the standard screws before (range, 6-14 times greater) and after cycling in torsional loading (range, 3-3.6 times greater). Although cement augmented screws and Schuhli augmentation showed increased fixation stability compared with the standard screws in axial and 4-point bending before cycling (range, 1.3-1.4 times greater), this was not significant. Compared with Schuhli fixation, cement augmented screws showed no significant difference in fixation stability in all loading modes before and after cycling. Schuhli locking nuts offer the stability of cement augmentation while avoiding its potential adverse effects on fracture healing with extravasation and thermal necrosis.

Comparison of the in vitro holding strengths of conical and cylindrical pedicle screws in a fully inserted setting and backed out 180 degrees

Previous investigations have suggested that conical and cylindrical pedicle screws have comparable holding strengths. So far, the remaining performance in screws turned back or loose as a result of other reasons has not been determined. Twenty-four cadaveric spines from 6- to 8-week-old calves were examined. After bone mineral density was determined, four pedicle screws (two conical and two cylindrical screws) were inserted. The screws were fully inserted and half of them turned back 180 degrees. Twenty-four axial pullout and 24 cyclic loading tests with subsequent pullout tests were conducted. The pullout strengths of conical screws turned back 180 degrees are significantly smaller (1.8 kN) than those of cylindrical screws (4.3 kN). After cyclic loading, the displacement of conical screws is significantly greater (6.9 mm) than that of cylindrical screws (4.7 mm). Pedicle screws, especially conical ones, need to be placed to a correct depth, and they should not have to be backed out.

Mechanical properties of small fragment screws

For many years, stainless steel small fragment screws have been produced by one manufacturer. Recently, other implant makers have begun offering similar stainless steel screws. In addition, screw geometry and material composition have been modified in an attempt to produce screws for a wide range of clinical situations. This study compared the mechanical properties of several commonly used small fragment screws. Seven sets of screws were tested mechanically, including three brands of geometrically identical standard stainless steel cortical screws and one brand each of cannulated stainless steel cortical screws, titanium cortical screws, stainless steel cancellous screws, and bioabsorbable polylactic acid screws. Screws from each group were tested for pullout strength, torque to failure, and three-point bending to failure. There were no differences in the mechanical properties of the identical 3.5-mm standard stainless steel cortical screws. No difference in pullout strength was found between the five sets of cortical screws. However, the cancellous screws had 4% to 24% less pullout strength. Torsion tests showed that cannulated stainless steel cortical, titanium cortical screws, stainless steel cancellous screws, and polylactic acid screws failed at significantly less torque than did standard stainless steel cortical screws. Standard stainless steel cortical screws had the highest mean yield point and maximal load at failure of all screws in three-point bending. Other metal screws had lower yield strength and maximal load at failure than did the standard stainless steel cortical screws, and polylactic acid screws had the least bending strength.

Mechanical performance of standard and cannulated 4.0-mm cancellous bone screws

The mechanical performance of bone screws is determined by their pull-out strength (holding power), compressive force, stripping torque, yield bending moment, ultimate bending moment, and fatigue strength. These parameters are related to the parameters of the screw design, including major thread diameter, minor thread diameter, thread length, pitch, shaft diameter, cannulation diameter, and material properties. The goal of the study was to theoretically predict the static performance of five 4.0-mm, 45-46-mm-long, cancellous, partially threaded standard and cannulated bone screws and compare the predictions with experimental measurements. A secondary goal was to determine if cannulation of the bone screw diminished its mechanical performance. The predicted values for pull-out force, compressive force, and stripping torque were determined by the thread length, major thread diameter, and thread shape factor. The screws with the largest major thread diameter and longest thread length had the greatest pull-out force, compressive strength, and stripping torque. However, when correcting for the thread length, a higher thread shape factor compensated for a smaller major diameter. The coefficient of determination (r2) for the correlation between the predicted and measured pull-out force improved from 0.75 to 0.90 when the theoretical model included the thread shape factor. The yield and ultimate bending moments are a function of the section modulus and material properties of the screw. The Ace solid screw had the greatest section modulus and yield and ultimate bending moments. The experimental data support the theoretical models for predicting the mechanical performance of bone screws. The design of the bone screws can be optimized on the basis of theoretical modeling. The strong correlation between the predicted and measured parameters allows comparison between bone screws without repeated experimental tests. Theoretical and experimental results show that cannulation of the bone screw did not inherently diminish its mechanical performance.

Mechanical and biomechanical measurements of five currently available osteosynthesis systems of self-tapping screws

Pressure force, uniaxial 'pullout', minimum torque, and peak torque tests were done to evaluate the effectiveness of three popular monocortical and non-compressing self-tapping screw systems 2 mm in diameter (Champy, Centre-Drive, and Wuerzburger) and two systems 1.5 mm in diameter (Champy and Wuerzburger). The screw systems were all tested on polyvinylchloride plate, skull, and molar mandibular bone from fresh human cadavers. The Champy screw 2 mm in diameter and 7 mm long produced the highest pressure force (mean (SD) 153.4 (58.5) N, n=40) of the systems tested. The Centre-Drive screw of the same size showed the highest retentive force in pullout tests (619.5 (169.9) N, n=40) and also the largest difference between minimum and peak torque in skull and molar mandibular bone (1.86 (0.65) kpcm, n=40). This was clinically relevant compared with reported human bite-force (range 216-740 N). The results showed that, the screw diameter and number of threads were the most important and significant of the mechanical variables tested. The skull bone also exerted more retentive force than the molar mandibular bone. The overall data indicate that there is no need to use screws more than 7 mm long or wider than 2 mm in diameter for monocortical non-compressive osteosynthesis in the craniofacial and the mandibular region.

Finite element method evaluation: articulations and diagonals in an 8-pin type 1B external skeletal fixator

OBJECTIVE

To determine the mechanical properties of articulations and diagonals in an 8-pin type 1b external skeletal fixator (ESF).

STUDY DESIGN

Finite element method-computer simulation.

METHODS

The control type 1b ESF was supplemented with different articulations and diagonals. The parameters of frame angle, articulation diameter, and pin- or connector-based fastening were altered. The configurations were loaded for axial compression, torsion, and craniocaudal and mediolateral bending as single loads and as a combination. Three-dimensional linear and rotational gap strain and Von Mises stress maxima were determined.

RESULTS

For 90 degrees , 60 degrees , and 30 degrees frame angles and 0.48-cm-diameter articulations and diagonals, the best configurations, based on lower gap strain combined with decreased or minimally increased stress maxima for the combined load, were single or double diagonals or four horizontal articulations. Combining double diagonals with double wide horizontal articulations further lowered gap strain. For a 90 degrees frame angle and 0.32-cm-diameter articulations and diagonals, the superior configurations, showing the lowest gap strain combined with decreased or minimally increased stress maxima for the combined load, were double diagonals, four horizontal, or wide double horizontal articulations. The 0.48-cm articulations and diagonals provided lower or similar gap strain than 0.32-cm articulations and diagonals. The connector-to-connector version of these superior configurations provided slightly lower or similar gap strain and stress maxima than the pin-to-pin version. Only pin-to-pin-fastened double diagonals provided better stress maxima than the corresponding connector-to-connector version.

CONCLUSIONS

Supplementing a type 1b ESF with double diagonals and wide double articulations results in the greatest increase in stability at the fracture gap for complex fractures or in the initial phase of bone healing.

Experimental determination of bone cortex holding power of orthopedic screw

Cylindrical specimens of bone measuring 15 mm in diameter were obtained from the lateral cortical layer of 10 pairs of femurs and tibias. A central hole 3.2 mm in diameter was drilled in each specimen. The hole was tapped, and a 4.5 mm cortical bone screw was inserted from the outer surface. The montage was submitted to push-out testing up to a complete strip of the bone threads. The cortical thickness and rupture load were measured, and the shear stress was calculated. The results were grouped according to the bone segment from which the specimen was obtained. The results showed that bone cortex screw holding power is dependent on the bone site. Additionally, the diaphyseal cortical bone tissue is both quantitatively and qualitatively more resistant to screw extraction than the metaphyseal tissue.

Clinical applications of drill free screws in maxillofacial surgery

Drill free screws are newly designed osteosynthesis screws with specially formed tips and cutting flutes, which act like a cork-screw and can be inserted into bone without predrilling. A prospective study on 82 patients was performed in order to investigate the efficiency of mini- and micro-drill free screws (DFS) in clinical use and to find out areas in maxillofacial traumatology and orthognathic surgery, where the application of this new type of screw may be recommended. Thirty-eight Le-Fort-osteotomies, 23 central and lateral midfacial fractures and 21 fractures of the mandible were fixed using the Champy titanium micro/miniplate system and in total 518 center-drive titanium micro-DFS (1.5 mm) and 392 center-drive titanium mini-DFS (2 mm) with lengths between 4 and 7 mm. The results showed that the grip of micro- and mini-DFS was sufficient for the fixation of bone fragments in the central and lateral midface and in the mandibular area. The insertion of DFS was simple and is recommended in the area of the central midface; the insertion of the screws was difficult, but possible in the anterior mandible and in the lateral midface. The application of DFS in the mandibular angle region is not recommended.

An in vitro comparison of hollow ground and trocar points on threaded positive-profile external skeletal fixation pins in canine cadaveric bone

OBJECTIVE

To compare the microstructural damage created in bone by pins with lathe-cut and rolled-on threads, and to determine the peak tip temperature and damage created by positive-profile external fixator pins with either hollow ground (HG) or trocar (T) tips during insertion.

STUDY DESIGN

An acute, in vitro biomechanical evaluation.

SAMPLE POPULATION

Twenty-seven canine tibiae.

METHODS

Lathe-cut thread design with T point (LT-T), rolled-on thread design with T point (RT-T), and rolled-on thread design with HG point (RT-HG) pins were evaluated. Twenty pins of each type were inserted under constant drilling pressure into 12 canine tibiae (12 diaphyseal and 8 metaphyseal sites per pin type). Peak pin tip temperature, drilling energy, end-insertional pin torque, and pullout force were measured for each pin. For the histologic study, five pins of each type were inserted into cortical and cancellous sites in 15 additional tibiae. Entry and exit damage, and thread quality were assessed from 100 micron histologic sections by using computer-interfaced videomicroscopy.

RESULTS

T-tipped pins reached higher tip temperature in both diaphyseal and metaphyseal bone compared with HG-tipped pins. RT-T pins had higher pullout strength (diaphyseal) and end-insertional torque compared with other combinations. No differences in drilling energy or insertional bone damage was found between the three pin types (P < .05).

CONCLUSIONS

T-tipped pins mechanically outperformed HG-tipped pins. Pin tip and thread design did not significantly influence the degree of insertional bone damage.

CLINICAL RELEVANCE

T-tipped pins may provide the best compromise between thermal damage and interface friction for maximizing performance of threaded external fixator pins.

Determination of the role of the cancellous bone in generation of screw holding power at metaphysis

OBJECTIVE

To determine whether cancellous bone at metaphysis plays a significant role in generation of holding power of the cancellous screw.

DESIGN

Maximal holding power of the cancellous screw at distal metaphysis of the femur with and without cancellous bone inside was determined by applying a load to push out the screw.

BACKGROUND

Generation of screw holding power from the cancellous bone can arise by a mechanism of compression of cancellous bone between screw threads when a screw is inserted by non-tapping technique. Metaphysis has intermediate amount of cancellous and cortical bone when compared with diaphysis and intercondyle of a long bone.

METHODS

Eight pairs of fresh cadaveric femurs were used. One femur of a pair was removed of cancellous bone at the distal metaphysis; cancellous bone of the other was preserved. A full thread cancellous screw was inserted into the distal femoral metaphysis. An axial load was applied at the screw tip to push out the screw by using a universal testing machine.

RESULT

Mean push-out force of the screw at distal femoral metaphysis without cancellous bone inside was 1824.76 N and stiffness was 746.76 N/mm. Mean push-out force of the screw at distal femoral metaphysis with preservation of cancellous bone was 2015.86 N and stiffiness was 853. 09 N/mm. The statistical analysis of both groups showed no significant differences.

CONCLUSIONS

This study confirmed that cancellous bone at metaphysis plays no significant role in generation of holding power of the cancellous screw.

RELEVANCE

Because metaphyseal cancellous bone plays no role in generation of screw holding power, only a well-inserted cancellous screw into bone cortices can achieve good screw holding power at metaphysis of a long bone.

Initial fixation strength of modified patellar tendon grafts for anatomic fixation in anterior cruciate ligament reconstruction

Recently it has been shown that anatomic tibial graft fixation in anterior cruciate ligament (ACL) reconstruction is preferable in order to increase isometry and knee stability. To facilitate anatomic patellar tendon graft fixation, customized graft length shortening is necessary. The purpose of this study was to compare the initial fixation strength of four different shortened patellar tendon grafts including three bone plug flip techniques and direct patellar tendon-to-bone interference fit fixation in a model with standardized bone density. Ninety calf tibial plateaus (22 to 24 weeks old) with adjacent patella and extensor ligaments were used. Tendon grafts were shortened by flipping the bone plug over the tendon leaving a tendon-tendon-bone (TTB) construct and, as the first modification in the opposite direction resulting in a tendon-bone-tendon (TBT) construct. The second modification consisted of the TBT construct with interference screw position at the lateral aspect of the bone plug (TBTlat). As the fourth modification the tendon graft was directly fixed (Tdirect) with an interference screw. In addition, a round-threaded titanium (RCI; Smith & Nephew DonJoy, Carlsbad, CA), a round-threaded biodegradable screw (Sysorb; Sulzer Orthopedics, Münsingen, Switzerland), and a conventional titanium interference screw (Arthrex Inc, Naples, FL) were compared. We found that TTB (mean 441 N for biodegradable screw, 357 N for RCI screw, 384 N for conventional screw) and TBT (mean 407 N for biodegradable screw, 204 N for RCI screw, 392 N for conventional screw) construct fixation achieves comparable fixation strength, although failure in the TTB was due to tendon strip off at its ligamentous insertion. The highest failure load was found in TBTlat fixation (mean 610 N for biodegradable screw, 479 N for RCI screw). Therefore, this technique should be recommended when using a tendon flip technique. The failure load for Tdirect fixation (mean 437 N for biodegradable screw, 364 N for RCI screw) was similar to that of TTB and TBT fixation, which may indicate that a patellar-tendon graft harvested without its patellar bone plug and directly fixed with an interference screw is equivalent to a flipped graft. This may additionally reduce harvest site morbidity and eliminates the risk of patellar fracture. The fixation strength of round-threaded biodegradable and conventional titanium interference screws was similar, whereas that of round-threaded titanium screws was significantly lower in the patellar tendon flip-techniques. However, it should be taken into consideration that round-threaded titanium screws are proposed for direct tendon-to-bone fixation.

Schuhli augmentation of plate and screw fixation for humeral shaft fractures: a laboratory study

OBJECTIVES

Schuhli locking nuts provide a mechanism to lock 4.5-millimeter bone screws to a standard dynamic compression plate (DCP plate). It has been proposed that Schuhlis can provide increased fixation stability in areas of a proximal cortical defect or osteopenic bone and may keep screws from loosening and backing out from the plate. A biomechanical study was performed to investigate the effect of Schuhli augmentation of a ten-hole broad DCP plate for fixation of a simulated humeral shaft fracture versus standard DCP plate fixation.

DESIGN

Biomechanical cadaver study.

INTERVENTION

Six pairs of cadaveric humeri from elderly individuals were tested in offset axial loading, torsion, and four-point bending to obtain load versus deformation curves and baseline mechanical properties. Each pair of humeri was then instrumented with a ten-hole broad DCP plate on one side and a DCP plate augmented with Schuhlis at each screw hole on the contralateral side. All screws were placed in cortical bone. The constructs were retested in all three modalities. The humeri were then cycled in torsion for 1,000 cycles and retested in all three modalities. Each humerus was then loaded to failure in torsion to determine the ultimate load and rotational displacement.

MAIN OUTCOME MEASUREMENTS

Resistance to displacement was determined from the load versus deformation curves in each testing modality before and after cycling; these data were normalized to the intact values determined prior to instrumentation. Paired Student's t tests were performed to determine statistically significant differences between the two modes of fixation.

RESULTS

There were no significant differences in stability between the two fixation techniques in all three testing modalities both before and after cycling. However, the Schuhli augmented constructs sustained significantly greater loads and rotational deformations prior to failure.

CONCLUSIONS

In this model of humeral shaft fractures in the elderly, the addition of Schuhlis did not significantly change the mechanical stability of plate and screw fixation. However, load and angular deformation at failure were significantly greater in the Schuhli augmented specimens.

Drill Free Screws: a new form of osteosynthesis screw

Although the application of self-tapping and non self-tapping screws is virtually universal in cranio-maxillofacial surgery, the inevitable, time consuming procedure of drilling a pilot hole has some potential disadvantages, such as damage to nerves, tooth roots or tooth germs, thermal necrosis of the bone and drill bit breakage. Drill Free Screws (DFS) are a recently developed type of osteosynthesis screws, having a tip like a cork screw and specially formed cutting flutes which enable insertion of the screws without drilling. DFS 1.5 and 2 mm were inserted into discs of wood, polyvinylchloride (PVC) and porcine mandibular bone of varying thicknesses between 2 and 4 mm. The values of insertion torque and maximum torque were recorded using an electric torque tester. Thereafter, the screws were inserted with a fixed torque and uniaxial pull out tests were performed. In comparison with this, the same procedure was performed using 1.5 and 2 mm self-tapping titanium screws. Ten trials for each screw-material-combination were conducted to determine insertion torque, maximum torque and pull out analysis. The results showed that the holding power of DFS lay between 70 and 104% of the holding power of self-tapping titanium screws; only in PVC was the difference more than 15%.

Self-tapping versus standard tapped titanium screw fixation in the upper extremity

Most screws used in fracture fixation necessitate a separate step for tapping of the screw hole. Titanium screw systems have been developed in which the screws can be inserted directly after a drill hole is made. These self-tapping screws thereby eliminate an operative step. A retrospective study was conducted that evaluated all wrist and hand procedures performed between January 1992 and December 1994 by 1 surgeon using screw fixation. The results of 39 cases treated with standard tapped titanium screws were compared with 28 cases treated with self-tapping titanium screws. Nearly identical union and complication rates were obtained in each group. Comparable results can be obtained with self-tapping screw fixation, which limits the number of instruments needed, eliminates an operative step, and thereby may diminish operative risk and shorten operative time.

Biodegradable interference screw fixation exhibits pull-out force and stiffness similar to titanium screws

Recently, increased interest in biodegradable interference screws for bone-tendon-bone graft fixation has led to numerous screws becoming available. The implants are made from different polymers and have different designs, which might influence their mechanical properties. Several studies have reported a wide range of mechanical results for these screws using different biomechanical models. The aim of the present study is to compare reliable biomechanical data for six different biodegradable interference screws, consisting of five different polymers, with a conventional titanium screw in a standardized model. Seventy proximal calf tibias were used to determine maximal pull-out force, stiffness of fixation, and insertion torque for interference screw fixation of bone-tendon-bone grafts. Additionally, maximal torque at failure was determined. Data were analyzed with respect to aspects of screw design, such as drive and thread shape. Five of the six biodegradable screws provided initial pull-out force and stiffness of fixation comparable with that of a conventional titanium screw. Torque at failure can be greatly increased by adapting the drive design to the mechanical properties of the polymeric raw material. A correlation between pull-out force and thread height indicates that fixation rigidity depends on screw design, even in a biodegradable implant.

The effect of screw holes on bone strength

Internal fixation devices continue to be used in the fixation of foot and ankle fractures and osteotomies. Because of the increased use of fixation techniques, postoperative care following removal of internal fixation devices assumes a vital role in the success of these procedures. Using 10 fibulas from 5 cadavers, the authors tested the resistance of 5 intact fibulas and 5 contralateral fibulas with one 3.5-mm. drill hole to bending forces. The mean load at failure of the intact fibulas was 360.86 N (range 181.69-800.14 N), whereas the mean load at failure of the drilled fibulas was 215.20 N (range 66.49-600.61 N). The drilled fibulas displayed failure with the application of 59.63% of the mean load applied to the intact fibulas at failure (p = 0.007).

Comparison of the tissue response to absorbable self-reinforced polylactide screws and metallic screws in the fixation of cancellous bone osteotomies: an experimental study on the rabbit distal femur

The availability of absorbable fracture-fixation devices for clinical use calls for better knowledge of the reaction of bone tissue to absorbable polyester implants as compared with similar metallic devices. To examine and compare the tissue response to biodegradable and metallic screws within cancellous bone, a transverse transcondylar osteotomy of the distal femur was fixed with absorbable self-reinforced polylevolactide screws in 35 rabbits and with stainless-steel screws in 35 rabbits. New bone formation and consolidation of the osteotomy were examined histologically, histomorphometrically, and microradiographically within standardized sample fields 1, 3, 6, 12, 24, 36, and 48 weeks postoperatively. The intact contralateral femur served as the control. A vigorous osteoconductive response to the polylevolactide screws was observed at 3 weeks postoperatively, and the osteoid surface fraction was significantly higher in all follow-ups than in the contralateral femora. In the femora with metallic screws, new bone formation was seen 3, 6, and 12 weeks postoperatively, but at 24, 36, and 48 weeks the osteoid surface fraction did not differ significantly from that of the intact control femora. The total bone area was significantly larger in the femora with self-reinforced polylevolactide screws than in the control bone 6-48 weeks postoperatively; in the femora with metallic screws, this was found only at 6 and 12 weeks. After 48 weeks, the femora fixed with metallic screws had statistically smaller total bone area than the intact control femora. Solid bone union was seen in 84% of the osteotomies in the self-reinforced polylevolactide group and in 76% of those in the metallic group after 3 weeks or more. No signs of degradation of the self-reinforced polyleuolactide implant and only a mild foreign-body reaction with no accumulations of inflammatory cells to either self-reinforced polylevolactide or metallic screws were observed during the follow-up period. Both types of screws seemed to induce an osteostimulatory response around their threads. This phenomenon was transient for metallic screws but lasted for at least 48 weeks for self-reinforced polylevolactide screws. The polylevolactide screw does not seem to cause osteopenia at the implantation site. The fixation properties of both self-reinforced polylevolactide screws and metallic screws appear to be sufficient for the fixation of small fragments of cancellous bone.

Comparative pull-out strength of tapped and untapped pilot holes for bicortical anterior cervical screws

STUDY DESIGN

This biomechanical study analyzed the axial pull-out strength of tapped versus untapped pilot holes for bicortical screws in the anterior cervical spine.

OBJECTIVE

To determine which pilot hole preparation method was mechanically better.

SUMMARY OF BACKGROUND DATA

Tapping pilot holes in the lumbar spine was previously shown significantly to reduce pull-out strength of pedicle screws. No study was found investigating the effect of tapping on pilot holes for anterior cervical bicortical screws.

METHODS

Twenty-five unembalmed human cadaveric cervical vertebrae (C3-C7) were tested. Two identical pilot holes were drilled into each vertebra: one pilot hole was tapped, and the control pilot hole was not tapped. A fully threaded cortical bone screw was inserted into each pilot hole. Screw pull-out strength was determined using a servocontrolled hydraulic materials testing system and an axial load cell. Force-deformation and failure curves were obtained.

RESULTS

There were no statistically significant differences between the axial pull-out strength of tapped and untapped pilot holes at any vertebral level. Mean force to-failure was 386 +/- 42 N in the untapped pilot holes and 397 +/- 48 N in the tapped pilot holes.

CONCLUSIONS

Tapping a pilot hole for bicortical screws of the anterior cervical spine neither weakens nor strengthens the axial pull-out strength of fully threaded cortical bone screws. Tapping may be unnecessary; however, it may be desirable in patients with dense bone to cut the thread profile into the bone or if the screws have dull tips and threads.

In vitro biomechanical and histological assessment of pilot hole diameter for positive-profile external skeletal fixation pins in canine tibiae

This study was conducted to evaluate the effect of pilot hole (PH) diameter (0, 1.5, 2.0, 2.7, 3.1, 3.3, 3.5, and 3.7 mm) on the biomechanical and microstructural performance of positive-profile threaded external skeletal fixation pins (3.18 mm inner diameter, 3.97 mm outer diameter) using cadaveric canine tibiae. Eight pins per pilot hole diameter (four pins per bone) were used to assess differences in end-insertional torque and pin pull-out strength. Histological evaluation of eight bicortical pin tracts per pilot hole diameter was accomplished using computer-interfaced videomicroscopy on specimens processed using a bulk-staining technique. Compared with no predrill, use of 2.7 mm PH increased end-insertional torque and pull-out strength by 25% and 13.5%, respectively. No significant differences were observed in biomechanical variables for the PH diameter range of 2.0 to 3.1 mm. Compared with no predrill, use of a 3.1 mm PH increased thread area by 18%. Microfracturing around the threads decreased as PH diameter increased. Damage to the interface at the entry and exit sites of both near and far cortices also decreased as PH diameter increased. It was concluded that predrilling a PH whose diameter approximates, but does not exceed the inner diameter of the positive profile pin will not only improve initial pin stability compared with no predrilling, but it will also reduce microstructural damage that may lead to excessive bone resorption and premature pin loosening.

Cancellous bone screw thread design and holding power

This study was designed to isolate and evaluate the parameters of host density, outer diameter (OD), root diameter (RD), and pitch in cancellous bone screw design and their effect on holding power. Special emphasis was placed on screw pitch, which has been evaluated infrequently in the literature. Three groups of stainless steel V thread screws (group I, OD 4.5 mm, RD 3.0 mm; group II, OD 6.4 mm, RD 3.5 mm; group III, OD 6.4 mm, RD 4.2 mm) were machined with progressive increases in pitch from 12 to 32 threads per inch (TPI). Two densities of synthetic cancellous bone material (Pedilen, Ottobock, Minneapolis, MN, U.S.A.), 0.15 g/ml and 0.22 g/ml, were then prepared and molded into sheets 1.9 cm thick and the screw threads completely engaged. Push-out tests were performed using a servohydraulic testing machine (MTS, Minneapolis, MN, U.S.A.). Fifteen trials of each screw were tested in each material. The effect on holding power of the different parameters of the custom screws in order of importance was (a) host material density, (b) OD (c) pitch, and (d) RD. The groups with a 6.4-mm OD had a much greater holding power than did the group with a 4.5-mm OD (p < 0.001). A decrease in screw pitch (increased threads per inch) did itself have a significant improved effect on fixation for all groups in both pedilen densities (p < 0.001). In the two 6.4-mm screw groups studied, the difference in the two root diameters (4.2 mm vs. 3.5 mm) showed the smaller root diameter to give a greater holding power in the less dense 0.15 g/ml pedilen (p < 0.001). In the more dense 0.22 g/ml pedilen there was no difference (p = 0.26) between the root diameters. To optimize holding power, cancellous screws may be designed with a decreased pitch (increased TPI) over those commercially available today. Cannulated screws must have a larger cancellous thread root diameter to leave room for the central cannulation; this may decrease their holding power in less dense cancellous bone but not in denser bone.

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.

External skeletal fixation in ruminants

External skeletal fixation (ESF) techniques are increasingly being adapted for treatment of fractures in large animals. This article includes detailed information regarding decision making for ESF, biomechanics of ESF, interaction of the bone and implants, general surgical techniques for application of ESF (including traditional ESF, use of acrylic sidebars for ESF, and transfixation, pinning, and casting), discussions of application of ESF to specific bones, and complications associated with the use of ESF. Also, techniques for the management of open, infected fractures are presented. Future directions of ESF research and application also are presented.

Strength of fixation of anterior vertebral body screws

STUDY DESIGN

The technique of hole preparation and the placement or orientation of anterior transvertebral screws in relation to the endplates were the subject of two experiments.

OBJECTIVES

Experiment 1--to determine if anterior transvertebral body screws have higher pull-out strengths after unicortical predrilling compared with a bicortical predrilling; Experiment 2--to determine the effect of the screw's orientation in the vertebral body on its resistance to loosening.

SUMMARY AND BACKGROUND DATA

There are a variety of surgical techniques for using anterior transvertebral screws in the vertebral body without any clear guidelines as to which techniques optimize the strength of fixation.

METHODS

In experiment 1, 31 cadaveric vertebral bodies were tested for pull-out strength of transvertebral body screws placed after unicortical predrilling compared with bicortical predrilling. In experiment 2, 48 cadaveric vertebral bodies were tested with transvertebral screws inserted using four different screw orientations.

RESULTS

There is no statistically significant difference in pull-out strength of anterior transvertebral body screws inserted after unicortical compared with bicortical predrilling (Experiment 1). The resistance to loosening is greatest when transvertebral screws are oriented in the "superior oblique" position (Experiment 2).

CONCLUSIONS

Experiment 1--preparation of the far cortex by predrilling is not necessary. Experiment 2--transvertebral screws should be obliquely oriented so that the forces applied to the screws are resisted by both of the endplates.

Pull-out strength of pedicle hooks with fixation screws: influence of screw length and angulation

The pull-out force of thoracic spinal pedicle hooks secured by long fixation screws engaging the posterior portion of the vertebral endplate was measured. The performance of these hooks was compared with that of hooks using a shorter screw and different screw orientation such that the vertebral endplates were not perforated. The longer and differently angulated screws, engaging the endplate, significantly enhanced the fixation potential of the hooks.

The Osteomed M3-X Fixation System: a new bone screw with applications in foot surgery

The purpose of this article is to familiarize the reader with the Osteomed M3-X Fixation System. The authors present a description of the instrumentation and design characteristics of this system, as well as some comparisons to other similar fixation devices. The indications for the screw's use and the correct techniques of insertion are also discussed. In addition, a review of over 200 forefoot procedures utilizing this system, and performed by the podiatric surgeons at Hutzel Hospital, is presented.

Tibial interference screw removal following anterior cruciate ligament reconstruction

A small number of patients developed pain and tenderness at the tibial tunnel following anterior cruciate ligament reconstruction. Twenty-three knees in 22 patients underwent removal of the tibial interference screw. Ten knees had a preoperative flexion contracture and underwent a concomitant procedure to address the loss of motion at the time of hardware removal. In the 13 knees with full extension, the interval between ligament reconstruction and screw removal averaged 16 months. Eleven of these knees also underwent arthroscopy, but no intra-articular causes of pain were identified. Roentgenographic analysis showed protrusion of the interference screw above the tibial cortex in three cases. Follow-up after hardware removal averaged 2 years. Tibial tunnel tenderness resolved in 21 of 23 knees, including those of the two patients who underwent hardware removal alone. Although it cannot be stated with certainty that tibial interferences screws may cause pain, this review suggests an association. This is an uncommon problem and it is estimated to be a factor in less than 3% of the author's anterior cruciate ligament reconstructions. More common causes of knee pain should be sought before electing to remove the tibial interference screw.

Pullout strength of fixation screws from polymethylmethacrylate bone cement

Polymethylmethacrylate bone cement is often used to fill voids and increase the strength of osteoporotic and pathological bone. However, it is unclear as to which method of cement augmentation provides optimal screw fixation. This study was conducted to determine which of the current cement augmentation techniques provides the strongest construct when used in association with orthopaedic fixation screws. Pullout strength was determined for screws placed in sawbones with no cement, soft cement, doughy cement and hard cement after drilling and tapping. All cement-screw constructs were significantly stronger than the no cement group. Screws placed in doughy cement had a significantly higher pullout force than those placed in hard cement. Pullout strength of screws placed in soft cement was intermediate between the other cement techniques but not significantly different from either group.

Biomechanical properties of small bone screws

PURPOSE

To evaluate systematically the biomechanical properties of 13 popular screw designs, ranging from 0.8 to 2.0 mm in diameter.

METHODS

Screws were characterized in terms of external, core, and drill diameter; cutting flute and head design; material; pitch, thread depth; and height of shank (unthreaded portion) and shank with plate. They were tested in standardized bone specimens (2 x 2.5cm slabs of fresh bovine femur) 1, 2, 3, and 4-mm thick. For each screw-bone thickness combination, 10 trials were conducted to determine push-out force (POF) and another 10 trials to determine insertion (IT) and maximum torque (MT) yielding a total of 1,040 tests.

RESULTS

Among the 13 different screws, in 1-mm thick bone, both the lowest (108.5 N) and highest (294.9 N) POF were created by 2-mm screws (P < .001); that with the lowest POF had a long unthreaded shank and pitch, that with highest POF had a short unthreaded shank and pitch. Screws with 0.8- to 1.5-mm diameters showed no differences in POF. The 2-mm screw with the lowest POF also had the lowest MT in 1-mm thick bone compared with the other 2-mm screws (P < .001). In thicker bones (> 2 mm), two 2-mm screws showed 30% to 50% lower MT than the other same size screws (P < .001) because their head slots stripped easily. When all screws were considered together for a particular bone thickness, torque was strongly predicted by screw diameter (MT: r = .94, P < .001; IT: r = 0.92, P < .001). Screws with the same diameters varied significantly in IT because of differences in self-tapping cutting flute design.

CONCLUSION

External diameter, unthreaded shank height, head slot, and self-tapping cutting flute design had the greatest impact on screw strength and efficiency in thin cortical bone. Thread depth, core diameter, and metal type did not affect performance significantly. Under these highly standardized in vitro conditions, the ideal 2-mm screw has an unthreaded shank that is as short as possible, and the pitch is about 0.8 mm. Additional aspects of a clinical situation beside holding strength must, however, be considered when choosing a screw.

Holding power of threaded external skeletal fixation pins in the near and far cortices of cadaveric canine tibiae

We compared the pin-bone interfaces at the near and far cortical penetration sites of positive-profile end-threaded external fixation pins in cadaveric canine tibiae. The holding power of the pins in each cortical surface was independently measured in 21 pin-bone sections. Scanning electron microscopy (SEM) was used to compare subjectively the microstructural appearance of the pin-bone interfaces at the near and far cortical penetration sites in eight pin-bone sections. The far cortical penetration site provided greater holding power than did the near cortical site. SEM evaluation suggested more bony microfractures and debris with less pin-bone SEM evaluation suggested more bony microfractures and debris with less pin-bone interlock in the near cortical penetration sites than in the corresponding far cortical penetration sites. This study showed that after low-speed power insertion of positive-profile end-threaded pins in canine cadaveric tibiae, the near cortical penetration site contributes approximately 25% less to the overall holding power of the pin than does the far cortical penetration site.

Pattern of screw loosening in fractures fixed with conventional and functional plates

Plate fixation depends mainly on the holding power of the screws. In the present study the pattern of screw loosening was investigated. Thirty-two adult female sheep divided into four groups were used. A mid-diaphyseal transverse osteotomy was made on the right radius of each animal, and then plated on the anterior (tension) surface. Half of each group were plated using a standard narrow 7-hole AO Dynamic Compression Plate (DCP), whereas in the remaining animals a 6-hole newly designed sliding plate (SP) was applied. The required torque of tightening intraoperatively, as well as for releasing the screws after killing the animals was recorded with a tension-calibrated screwdriver. The pattern of loosening was similar for all the screws and the three phases could be recognized. An initial loosening was observed 1 month after the operation. A slow recovery of the torque was measured from the second month onwards, becoming highest by the fourth month after the operation, whereas a slow decrease in torque was observed from the fourth to the sixth month. In the SP group, the overall loosening was much lower than the DCP group (P < 0.05), whereas there was no difference in the loosening between the proximally placed screws and the distally placed ones (P > 0.05) for both plates.

Ingrowth reduces implant-to-bone relative displacements in canine acetabular prostheses

We examined bone-to-implant relative displacement of acetabular prostheses acutely and after ingrowth in a canine model. Uncemented hemispherical acetabular cups with titanium mesh pads comprising approximately 26% of the surface of the cup were inserted in eight adult canine hemipelves ex vivo. The acetabular prostheses were fixed with 13 mm titanium screws. Zero, one, and two-screw configurations were tested, with the order of testing randomly assigned. A load simulating 1,000 cycles of canine gait as applied to the acetabular component, and relative displacements were measured at three locations between implant and bone to determine acute fixation. A repeated measures analysis of variance showed that two screws produced only 42% of the average relative displacement of one screw and 14% that of zero screws. Eight adult mixed-breed dogs then underwent unilateral total hip arthroplasty. All acetabula were biologically fixed with two cancellous screws. The results at 4 months showed significantly less relative displacement between the implant and bone than was measured in ex vivo implantations (p = 0.014). Bone ingrowth filled 20 +/- 6% (mean +/- SD) of the available space. The relative displacements of these implants were small in all cases (12 +/- 13 microns) and did not correlate with the amount of bone ingrowth. These data suggest that acetabular fixation with two screws can lead to bone ingrowth and reduced relative motion of the prosthesis under functional loading.