Experimental determination of bone cortex holding power of orthopedic screw

Effect of coronal fracture angle on the stability of screw fixation in medial malleolar fractures: A finite element analysis

Malleolar screw fixation is the most widely used treatment method for medial malleolar (MM) fractures. Here, although buttress plate fixation is advocated for vertical MM fractures, the angular discrimination between oblique and vertical MM fractures is still not fully understood. The purpose of this study is to test the adequacy of screw fixation in MM fractures with different angles and determination of a ‘critical fracture angle’ to guide surgeons in the decision-making for screw fixation for MM fractures by utilizing an advanced engineering simulation approach. In addition to loading of the healthy tibia structure, various cases of the MM fracture double screw fixation (14 simulation scenarios in total with fracture angles between 30° and 90°, in 5° increments) were considered in this research and their static loading conditions just after fixation operation were simulated through nonlinear (geometric and contact nonlinearity) finite element analysis (FEA). Patient-specific computed tomography scan data, parametric three-dimensional solid modelling and finite element method (FEM) based engineering codes were employed in order to simulate the fixation scenarios. Visual and numerical outputs for the deformation and stress distributions, separation and sliding behaviours of the MM fracture fragments of various screw fixations were clearly exhibited through FEA results. Minimum and maximum separation distances (gap) of 3.75 and 150.34 µm between fracture fragments at fracture angles of 30° and 90° were calculated respectively against minimum and maximum sliding distances of 25.87 and 41.37 µm between fracture fragments at fracture angles of 90° and 35°, respectively. The FEA results revealed that while the separation distance was increasing, the sliding distance was decreasing and there were no distinct differences in sliding distances in the scenarios from fracture angles of 30°–90°. The limitations and errors in a FEA study are inevitable, however, it was interpreted that the FEA scenarios were setup in this study by utilizing acceptable assumptions providing logical outputs under pre-defined boundary conditions. Finally, the fracture healing threshold for separation and/or sliding distance between fracture fragments was assigned as 100 µm by referring to previous literature and it was concluded that the screws fixed perpendicular to the fracture in a MM fracture with more than 70° angle with the tibial plafond results in a significant articular separation (>100 µm) during single-leg stand. Below this critical angle of 70°, two screws provide sufficient fixation.

Bone: An Outstanding Composite Material

Bone is an outstanding, well-designed composite. It is constituted by a multi-level structure wherein its properties and behavior are dependent on its composition and structural organization at different length scales. The combination of unique mechanical properties with adaptive and self-healing abilities makes bone an innovative model for the future design of synthetic biomimetic composites with improved performance in bone repair and regeneration. However, the relation between structure and properties in bone is very complex. In this review article, we intend to describe the hierarchical organization of bone on progressively greater scales and present the basic concepts that are fundamental to understanding the arrangement-based mechanical properties at each length scale and their influence on bone’s overall structural behavior. The need for a better understanding of bone’s intricate composite structure is also highlighted.

Age-Related Effects on Right Femoral Bone of Male Wistar Rats: A Morphometric and Biomechanical Study

Introduction Study of the variations of bone characteristics with age in different animal models is important to design musculoskeletal studies. Thus, this study aimed to evaluate the bone mass, dimensions, and biomechanical parameters of the femur in young, middle-aged, and aged Wistar rats.

Materials and Methods Thirty male rats (Rattus norvegicus) were divided in three groups (n = 10 per group)—3-month-old young rats, 12-month-old middle-aged rats, and 18-months-old aged rats. The right femurs were subjected sequentially to morphometric study (bone weight, cortical thickness) and biomechanical tests (maximum resistance strength and bone stiffness).

Results We observed a significant increase in femur histological (cortical thickness) and biomechanical (maximum strength and bone stiffness) parameters with aging when compared with young animals.

Conclusions With the advancing age, the right femoral bone of middle-aged and old animals had greater variations when compared with young animals. However, further studies with the aid of a comparison between right and left femur and other long bones in both male and female rats are needed to corroborate with our findings.

Custom-made polyurethane-based synthetic bones mimic screw cut-through of intramedullary nails in human long bones

Intramedullary nails are considered the gold standard for the treatment of tibial shaft fractures. Thereby, the screw-bone interface is considered the weakest link. For biomechanical evaluation of osteosyntheses synthetic bones are often used to overcome the disadvantages of human specimens. However, commercially available synthetic bones cannot adequately mimic the local mechanical properties of human bone. Thus, the aim of this study was to develop and evaluate novel cortical bone surrogate materials that mimic human tibial shafts in the screw-loosening mechanisms of intramedullary nails. Bone surrogates, based on two different polyurethanes, were developed and shaped as simple tubes with varying cortical thicknesses to simulate the diaphyseal cortex of human tibiae. Fresh frozen human tibiae and commercially available synthetic bones with similar cortical thickness were used as references. All specimens were treated with a nail dummy and bicortical locking screws to simulate treatment of a distal tibia shaft fracture. The nail-bone construct was loaded in a combined axial-torsional-sinusoidal loading protocol to simulate the physiological load during human gait. The loads to failure as well as the number of load cycles were evaluated. Furthermore, the cut-through length of the screws was analysed by additional micro computed -tomography images of the tested specimens. The failure load of custom made synthetic bone tubes with 6 mm cortical thickness (3242 ± 136 N) was in accordance with the failure load of human samples (3300 ± 307 N, p = 0.418) with a similar cortical thickness of 4.9 ± 1.4 mm. Commercially available synthetic bones with similar cortical thickness of 4.5 ± 0.7 mm were significantly stronger (4575 ± 795 N, p = 0.008). Oval-shaped migration patterns were "cut" into the cortices by the screws due to the cyclical loading. The cut-through length of the self-developed synthetic bones with 6 mm cortices (0.8 ± 0.6 mm, p = 0.516) matched the cut-through of the human tibiae (0.7 ± 0.6 mm). The cut-through of commercially available epoxy-based synthetic bones deviated from the human reference (0.2 ± 0.1 mm, p < 0.001). The results of this study indicate that the novel bone surrogates realistically mimic the failure and screw migration behaviour in human tibiae. Thus, they offer a new possibility to serve as substrate for biomechanical testing. The use of commercially available surrogates is discouraged for biomechanical testing as there is a risk of drawing incorrect conclusions.

Pullout Strength After Multiple Reinsertions in Radial Bone Fixation

Background: Due to bone cutting loss from self-tapping screws (STS), progressive destruction of bone can occur with each reinsertion during surgery. When considering the use of jigs that utilize multiple insertions such as those seen in ulnar and radial shortening osteotomy systems, or scenarios where a screw needs to be removed and reinserted due to some technical issue, this can be concerning, as multiple studies examining the effects of multiple reinsertions and the relationship between insertional torque and pullout strength have had mixed results. Methods: Insertional torque and pullout strength were experimentally measured following multiple reinsertions of STS for up to 5 total insertions for various densities and locations along radial sawbone shafts. Results: Torque and pullout strength were significantly greater in middle segments of the radial shaft. Our trials corroborate previous literature regarding a significant reduction in fixation between 1 and 2 insertions; beyond this, there was no significant difference between pullout strength across all segment locations as well as bone densities for 3 to 5 insertions. There was a moderate to high correlation of insertional torque to pullout strength noted across all bone densities and segments (Pearson r = 0.663, P < .001). Conclusion: While reinsertion of STS between 1 and 2 insertions has been shown to significantly differ in pullout strength, beyond this, there does not appear to be a significant difference in up to 5 insertions at any specific region of radial bone across a range of sawbone densities. Further insertions may be considered with caution.

MULTI-OBJECTIVE OPTIMIZATION FOR SURGICAL DRILLING OF HUMAN FEMURS: A METHODOLOGY FOR BETTER PULL-OUT STRENGTH OF FIXATION USING TAGUCHI METHOD BASED ON MEMBERSHIP FUNCTION

Drilling through bone is one of the common cutting processes involved in many of the orthopedic surgeries. In bone drilling, spindle speed, feed rate, diameter of the drill bit, drill bit geometry and method of cooling are the important parameters to influence the in-situ temperature, drill thrust force and quality characteristics of the drilled hole. Because of the selection of inappropriate drilling parameters, uncontrolled large drilling forces, continuous increase in temperature and mechanical damage to the local host bone were observed. As these adverse effects lead to poor bone–implant contact and often a revision surgery, performing a surgical drilling with optimal parameters is essential to succeed in the surgical procedure. It was observed that in addition to the variations in apparent bone density, the orientation of osteons influences the drilling thrust force and temperature in bone drilling. Ten adult cadaveric human femurs from the age group of 32–65 years were considered and drilling experiments were conducted on proximal-diaphysis, mid-diaphysis and distal-diaphysis regions in the longitudinal, radial and circumferential directions. Bone drilling with different spindle speeds (500, 1000 and 1500[Formula: see text]rpm), feed rates (40, 60 and 80[Formula: see text]mm/min), and apparent density in the range of 0.98[Formula: see text]g/cm3 to 1.98[Formula: see text]g/cm3 was investigated in this work using a 3.20[Formula: see text]mm diameter surgical drill-bit. The generation of in-situ temperature as well as thrust force at each target location was measured using [Formula: see text]-type thermocouple and Kistler[Formula: see text] dynamometer, respectively. Taguchi method based on membership function was used to optimize the drilling process. Then the efficacy of the method in reducing the in-situ temperature and thrust force, and quality of the drilled hole in respect of anatomical region and drilling direction was investigated using pull-out strength of the bone screws. Results revealed that the optimal parameters obtained from the Taguchi method based on membership function could simultaneously minimize the temperature as well as thrust force in bone drilling. The proposed method can be adopted to minimize the temperature and thrust force, and choose the best location nearest to the defect site for strong implant fixation by using CT datasets of the patient as the only input.

Characteristics of anterior inferior calcaneal cortex

BACKGROUND

Minimal invasive surgery of calcaneal fracture provided satisfactory outcomes. In tongue type calcaneal fracture, percutaneous screw usually purchases in anterior inferior calcaneal cortex. However, there was no detail about the cortex of anterior inferior calcaneus so the surface anatomy and cortical thickness of this area were studied.

METHODS

88 calcaneus from embalmed cadavers were enrolled. Anterior part of the inferior cortex was identified. Surface anatomy was examined. Length, anterior and posterior widths were measured. Anterior inferior calcaneal cortex was divided into anterior, middle and posterior segments. The cortical thickness at middle, medial most and lateral most of 3 segments were measured.

RESULTS

Anterior inferior calcaneal cortex was a long trapezoidal shape with well-defined borders as a dense and thick cortical bone, convex relief from medial and lateral walls. Mean(SD) length was 33.40(3.46) millimeters (mm). Median(min,max) of anterior and posterior width were 10.50(8.21,19.26) mm and 14.00(10.05,20.42) mm, respectively. Mean(SD) of middle cortical thickness of anterior and middle segment were 3.12(0.76) and 3.72(0.74). Median(min,max) middle cortical thickness of posterior segment was 3.13(1.62,6.51) mm. Whereas, of the medial most were 1.31(0.78,3.11), 1.31(0.90,2.57) and 1.26(0.85,2.61) mm and of the lateral most were 1.17(0.67,2.64), 1.38(0.80,2.55) and 1.31(0.84,2.61) mm, respectively. Inter-intraobserver reliabilities of the measurements were >0.79. The statistical analysis showed the middle cortex is significantly the thickest (P<0.001) and posterior width is significant wider than the anterior (P<0.001).

CONCLUSIONS

Anterior inferior calcaneal cortex has special characteristics in term of surface anatomy, width and thickness. For the percutaneous screw insertion from posterosuperior to anterior inferior calcaneus in tongue type calcaneal fracture, we recommend that screw should purchase in middle cortex due to maximal cortical thickness as well as its cortical width could accept 6.5 or 7.0mm screw without screw extrusion.

Inter- and intra-operator variability associated with extracapsular suture tensioning

Objectives: To determine inter- and intra- operator variability associated with extracapsular suture tensioning as performed during lateral fabello-tibial suture placement.

Study design: Ex vivo study.

Methods: Fifteen Greyhound cadaveric pelvic limbs were prepared by cutting the cranial cruciate ligament and placing an extracapsular fabello-tibial suture. On two occasions, three surgeons tensioned the extracapsular suture of each stifle. Stifles were returned to 135 degrees of flexion and the suture tension was measured using a commercially available suture tensioner with inbuilt tensiometer.

Statistical analysis: Intra-operator and inter-operator agreement were assessed using the limits of agreement method. A linear mixed effects model was specified to assess the effect of operator, repeated estimates and stifle order on tension applied.

Results: The mean difference within the three operators ranged from 0 to 14.7N. With 95% limits of agreement, on most occasions for all three operators, the difference was between –31.7 and 41.0 N. The mean difference between the three operators ranged from 6.0 to 30.7 N. With 95% limits of agreement, on most occasions the difference between operators was between –25.6 and 62.5 N.

Clinical significance: Marked variation exists in the tension applied during fabello- tibial suture application, both within and between surgeons. This variation may lead to inconsistent clinical outcomes. Further studies are required to determine the clinical consequences of this marked variation in extracapsular suture tensioning.

Analysis on the mechanical resistance of fixation of femoral neck fractures in synthetic bone, using the dynamic hip system and an anti-rotation screw

OBJECTIVE

To statistically analyze the results obtained from biomechanical tests on fixation of femoral neck fractures of Pauwels III type, in synthetic bone, using the dynamic hip system with an anti-rotation screw, versus a control group.

METHODS

Ten synthetic bones from a Brazilian manufacturer (model C1010) were used and divided into two groups: test and control. In the test group, fixation of an osteotomy was performed with 70° of inclination at the level of the femoral neck, using DHS with an anti-rotation screw. The resistance of this fixation was evaluated, along with its rotational deviation at 5 mm of displacement (phase 1) and at 10 mm of displacement (phase 2), which was considered to be failure of synthesis. In the control group, the models were tested in their entirety until femoral neck fracturing occurred.

RESULTS

The test values in the test group (samples 1-5) in phase 1 were: 1512 N, 1439 N, 1205 N, 1251 N and 1273 N, respectively (mean = 1336 N; standard deviation [SD] = 132 N). The rotational deviations were: 4.90°, 3.27°, 2.62°, 0.66° and 0.66°, respectively (mean = 2.42°; SD = 1.81°). In phase 2, we obtained: 2064 N, 1895 N, 1682 N, 1713 N and 1354 N, respectively (mean = 1742 N; SD = 265 N). The failure loading values in the control group were: 1544 N, 1110 N, 1359 N, 1194 N and 1437 N, respectively (mean = 1329 N; SD = 177 N). The statistical analysis using the Mann-Whitney test showed that the test group presented maximum loading at a displacement of 10 mm, i.e. significantly greater than the failure loading of the control group (p = 0.047).

CONCLUSION

The mechanical resistance of the test group was significantly greater than that of the control group.

Validity of synthetic bone as a substitute for osteoporotic cadaveric femoral heads in mechanical testing: A biomechanical study

Introduction

The objective of this study was to determine if a synthetic bone substitute would provide results similar to bone from osteoporotic femoral heads during in vitro testing with orthopaedic implants. If the synthetic material could produce results similar to those of the osteoporotic bone, it could reduce or eliminate the need for testing of implants on bone.

Methods

Pushout studies were performed with the dynamic hip screw (DHS) and the DHS Blade in both cadaveric femoral heads and artificial bone substitutes in the form of polyurethane foam blocks of different density. The pushout studies were performed as a means of comparing the force displacement curves produced by each implant within each material.

Results

The results demonstrated that test material with a density of 0.16 g/cm³ (block A) produced qualitatively similar force displacement curves for the DHS and qualitatively and quantitatively similar force displacement curves for the DHS Blade, whereas the test material with a density of 0.08 g/cm³ (block B) did not produce results that were predictive of those recorded within the osteoporotic cadaveric femoral heads.

Conclusion

This study demonstrates that synthetic material with a density of 0.16 g/cm³ can provide a good substitute for cadaveric osteoporotic femoral heads in the testing of implants. However we do recognise that no synthetic material can be considered as a definitive substitute for bone, therefore studies performed with artificial bone substrates may need to be validated by further testing with a small bone sample in order to produce conclusive results.

The effect of the screw pull-out rate on cortical screw purchase in unreamed and reamed synthetic long bones

Orthopaedic fracture fixation constructs are typically mounted on to human long bones using cortical screws. Biomechanical studies are increasingly employing commercially available synthetic bones. The aim of this investigation was to examine the effect of the screw pull-out rate and canal reaming on the cortical bone screw purchase strength in synthetic bone. Cylinders made of synthetic material were used to simulate unreamed (foam-filled) and reamed (hollow) human long bone with an outer diameter of 35 mm and a cortex wall thickness of 4 mm. The unreamed and reamed cylinders each had 56 sites along their lengths into which orthopaedic cortical bone screws (major diameter, 3.5 mm) were inserted to engage both cortices. The 16 test groups ( n = 7 screw sites per group) had screws extracted at rates of 1 mm/min, 5 mm/min, 10 mm/min, 20 mm/min, 30 mm/min, 40 mm/min, 50 mm/min, and 60 mm/min. The failure force and failure stress increased and were highly linearly correlated with pull-out rate for reamed ( R2 = 0.60 and 0.60), but not for unreamed ( R2 = 0.00 and 0.00) specimens. The failure displacement and failure energy were relatively unchanged with pull-out rate, yielding low coefficients for unreamed ( R2 = 0.25 and 0.00) and reamed ( R2 = 0.27 and 0.00) groups. Unreamed versus reamed specimens were statistically different for failure force ( p = 0.000) and stress ( p = 0.000), but not for failure displacement ( p = 0.297) and energy (0.054< p<1.000). This is the first study to perform an extensive investigation of the screw pull-out rate in unreamed and reamed synthetic long bone.

The effect of repetitive pilot-hole use on the insertion torque and pullout strength of vertebral system screws

STUDY DESIGN

In vitro biomechanical investigation of the screw-holding capacity.

OBJECTIVE

To evaluate the effect of repetitive screw-hole use on the insertional torque and retentive strength of vertebral system screws.

SUMMARY AND BACKGROUND DATA

Placement and removal of vertebral system screws is sometimes necessary during the surgical procedures in order to assess the walls of the pilot hole. This procedure may compromise the holding capacity of the implant.

METHODS

Screws with outer diameter measuring 5, 6, and 7 mm were inserted into wood, polyurethane, polyethylene, and cancellous bone cylindrical blocks. The pilot holes were made with drills of a smaller, equal, or wider diameter than the inner screw diameter. Three experimental groups were established based on the number of insertions and reinsertions of the screws and subgroups were created according to the outer diameter of the screw and the diameter of the pilot hole used.

RESULTS

A reduction of screw-holding capacity was observed between the first and the following insertions regardless the anchorage material. The pattern of reduction of retentive strength was not similar to the pattern of torque reduction. The pullout strength was more pronounced between the first and the last insertions, while the torque decreased more proportionally from the first to the last insertions.

CONCLUSION

Insertion and reinsertion of the screws of the vertebral fixation system used in the present study reduced the insertion torque and screw purchase.

Cortical screw pullout strength and effective shear stress in synthetic third generation composite femurs

BACKGROUND

The use of artificial bone analogs in biomechanical testing of orthopaedic fracture fixation devices has increased, particularly due to the recent development of commercially available femurs such as the third generation composite femur that closely reproduce the bulk mechanical behavior of human cadaveric and/or fresh whole bone. The purpose of this investigation was to measure bone screw pullout forces in composite femurs and determine whether results are comparable to cadaver data from previous literature.

METHOD OF APPROACH

The pullout strengths of 3.5 and 4.5 mm standard bicortical screws inserted into synthetic third generation composite femurs were measured and compared to existing adult human cadaveric and animal data from the literature.

RESULTS

For 3.5 mm screws, the measured extraction shear stress in synthetic femurs (23.70-33.99 MPa) was in the range of adult human femurs and tibias (24.4-38.8 MPa). For 4.5 mm screws, the measured values in synthetic femurs (26.04-34.76 MPa) were also similar to adult human specimens (15.9-38.9 MPa). Synthetic femur results for extraction stress showed no statistically significant site-to-site effect for 3.5 and 4.5 mm screws, with one exception. Overall, the 4.5 mm screws showed statistically higher stress required for extraction than 3.5 mm screws.

CONCLUSIONS

The third generation composite femurs provide a satisfactory biomechanical analog to human long-bones at the screw-bone interface. However, it is not known whether these femurs perform similarly to human bone during physiological screw "toggling."

A surrogate long-bone model with osteoporotic material properties for biomechanical testing of fracture implants

In vitro comparative testing of fracture fixation implants is limited by the highly variable material properties of cadaveric bone. Bone surrogate specimens are often employed to avoid this confounding variable. Although validated surrogate models of normal bone (NB) exist, no validated bone model simulating weak, osteoporotic bone (OPB) is available. This study presents an osteoporotic long-bone model designed to match the lower cumulative range of mechanical properties found in large series of cadaveric femora reported in the literature. Five key structural properties were identified from the literature: torsional rigidity and strength, bending rigidity and strength, and screw pull-out strength. An OPB surrogate was designed to meet the low range for each of these parameters, and was mechanically tested. For comparison, the same parameters were determined for surrogates of NB. The OPB surrogate had a torsional rigidity and torsional strength within the lower 2% and 16%, respectively, of the literature based cumulative range reported for cadaveric femurs. Its bending rigidity and bending strength was within the lower 11% and 8% of the literature-based range, respectively. Its pull-out strength was within the lower 2% to 16% of the literature based range. With all five structural properties being within the lower 16% of the cumulative range reported for native femurs, the OPB surrogate reflected the diminished structural properties seen in osteoporotic femora. In comparison, surrogates of NB demonstrated structural properties within 23-118% of the literature-based range. These results support the need and utility of the OPB surrogate for comparative testing of implants for fixation of femoral shaft fractures in OPB.

Evaluation of 2 Cement Techniques for Augmentation of Stripped 1.5 mm Screw Sites in the Distal Metaphysis of Feline Radii

OBJECTIVE

To evaluate the effect of 2 cement augmentation techniques on pullout strength of 1.5 mm screws placed in stripped 1.5 mm screw sites in the distal metaphysis of feline radii.

STUDY DESIGN

Experimental study.

SAMPLE POPULATION

Feline radii (21 pairs).

METHODS

Treatment groups (n=4) were allocated according to a Latin square design to 4 sites in each pair of radii. Positive and negative controls were a 1.5 mm screw and a screw of the same diameter in a previously stripped screw hole, respectively. Treatment groups were a 1.5 mm screw implanted in a previously stripped screw hole after injection of polymethylmethacrylate (PMMA) or a bioresorbable calcium phosphate cement (CPC, Norian skeletal repair system (SRS)). The ultimate pullout strength was compared between groups.

RESULTS

The mean (+/-SEM) pullout strength of screws augmented with either bone cement was less than that of the positive control group and greater than that of the negative control. Injection of CPC or PMMA before screw implantation increased the pullout strength of the negative control by 86.8+/-22.9% and 104.1+/-32.1%, respectively. Holding power of the positive control screws differed from these 2 groups, and was 274.8+/-39.17% higher than that of the negative control.

CONCLUSION

Injection of CPC or PMMA increases but does not restore the holding power of stripped 1.5 mm diameter screws.

CLINICAL RELEVANCE

The use of CPC (Norian SRS) augmentation of stripped 1.5 mm diameter screws warrants clinical investigation as it combines biomechanical results similar to PMMA with osteoconduction and resorbability.

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.