In patients with osteoporosis there is always a strong possibility that pedicle screws will loosen. This makes it difficult to select the appropriate osteoporotic patient for a spinal fusion. The purpose of this study was to determine the correlation between bone mineral density (BMD) and the magnitude of torque required to insert a pedicle screw. To accomplish this, 181 patients with degenerative disease of the lumbar spine were studied prospectively. Each underwent dual-energy x-ray absorptiometry (DEXA) and intra-operative measurement of the torque required to insert each pedicle screw. The levels of torque generated in patients with osteoporosis and osteopenia were significantly lower than those achieved in normal patients. Positive correlations were observed between BMD and T-value at the instrumented lumbar vertebrae, mean BMD and mean T-value of the lumbar vertebrae, and mean BMD and mean T-value of the proximal femur. The predictive torque (Nm) generated during pedicle screw insertion was [-0.127 + 1.62 × (BMD at the corresponding lumbar vertebrae)], as measured by linear regression analysis. The positive correlation between BMD and the maximum torque required to insert a pedicle screw suggests that pre-operative assessment of BMD may be useful in determining the ultimate strength of fixation of a device, as well as the number of levels that need to be fixed with pedicle screws in patients who are suspected of having osteoporosis.

Good lag screw holding power in trabecular bone of the femoral head is a requisite to achieve stability in the management of proximal femoral fractures. It has been demonstrated that insertion torque and pullout strength of lag screw are linearly correlated. Therefore, insertion torque measurement could be a method to estimate the achieved screw purchase. Manual perception is not reliable [1], but the use of an instrumented screwdriver would make the procedure feasible. The aim of this study was to assess the accuracy achievable using the insertion torque as predictor of lag screw purchase. Four different screw designs (two cannulated and two solid-core screws) were investigated in this study. Each screw was inserted into a block of trabecular bone tissue following a standardised procedure designed to maximise the experimental repeatability. The blocks of trabecular tissue were extracted from human as well as bovine femora to increase the range of bone mineral density. The prediction accuracy was evaluated by plotting pullout strength versus insertion torque, performing a linear regression analysis and calculating the difference (as percentage) between predicted and measured values. Insertion torque showed a strong linear correlation (coefficient of determination R. 2. : 0.95–0.99) with the pullout strength of lag screw. However the prediction error in pullout strength estimation was greater than 40% for small values of insertion torque, decreasing down to 15% when the lag screw was driven into good quality bone tissue. Measuring insertion torque can supply quantitative information about the achieved lag screw purchase. Since screw design and insertion procedure have been shown to affect both the insertion torque and the pullout strength [2], the prediction model must be screw-specific and determined, closely simulating the clinical procedure defined by the screw manufacturer. However, the surgeon must be aware that, even under highly repeatable experimental conditions, the prediction error was found to be high when small insertion torque was measured, i.e. when the screw was driven in low quality bone tissue. Therefore, insertion torque is not reliable in evaluating lag screw purchase in the management of proximal femur fracture of osteoporotic patients

Aims. To determine whether half-threaded screw holes in a new titanium locking plate design can substantially decrease the notch effects of the threads and increase the plate fatigue life. Methods. Three types (I to III) of titanium locking plates were fabricated to simulate plates used in the femur, tibia, and forearm. Two copies of each were fabricated using full- and half-threaded screw holes (called A and B, respectively). The mechanical strengths of the plates were evaluated according to the American Society for Testing and Materials (ASTM) F382-14, and the screw stability was assessed by measuring the screw removal torque and bending strength. Results. The B plates had fatigue lives 11- to 16-times higher than those of the A plates. Before cyclic loading, the screw removal torques were all higher than the insertion torques. However, after cyclic loading, the removal torques were similar to or slightly lower than the insertion torques (0% to 17.3%), although those of the B plates were higher than those of the A plates for all except the type III plates (101%, 109.8%, and 93.8% for types I, II, and III, respectively). The bending strengths of the screws were not significantly different between the A and B plates for any of the types. Conclusion. Removing half of the threads from the screw holes markedly increased the fatigue life of the locking plates while preserving the tightness of the screw heads and the bending strength of the locking screws. However, future work is necessary to determine the relationship between the notch sensitivity properties and titanium plate design. Cite this article: Bone Joint Res 2020;9(10):645–652

Introduction: With non-fusion stabilization surgery technique, the demands on the pedicle screw system have increased. The screw implants require a high press fit for optimum bone integration and a high pullout strength to guarantee primary stability. We compared the cylindrical screw design from a pedicle screw system with the conical design in regard to the insertional torque and pullout strength. Methods: Three human cadaver specimens L1 – L5 (15 vertebrae, 30 pedicles) were fitted with pedicle screws on both sides. The pedicles were randomized to one of two screw types: 1) cylindrical pedicle screw, 2) conical. A computer tomographic bone density measurement was performed beforehand. The insertional torque was measured while inserting the pedicle screws. The correct position of the screw was verified using computer tomography. In order to test the pullout strength, the preparations were divided up into individual vertebrae and fixed. Results: The mean peak insertional torque for the conical screws was significantly higher than that for the cylindrical screws. The pullout strength showed no significant difference between the two types of pedicle screws. With both the conical and the cylindrical pedicle screws, the pullout strength and insertional torque significantly decreased with decreasing bone density in the preparations. Conclusions: Statements in the literature about the effect on insertional torque and pullout strength of using a conical or cylindrical pedicle screw design vary. The two screw designs in our study only differed in regard to their internal diameter. The significantly higher insertional torque values found for the conical screws could describe the improved screw press-fit behavior. The pullout strength was not significantly affected by the conical design. No correlation was found between the insertional torque and pullout strength. In order to optimize the adjustment of insertional torque and pullout strength, we recommend the conical pedicle screw design for non-fusion surgical techniques for the surgical treatment of degenerative diseases of the lumbar spine

Introduction. Femoral neck fractures are a large clinical and economical problem. One of the most common fixation options for femoral neck fractures are multiple cancellous screws. A previous clinical study has shown the lack of washers to be the single largest predictor of fixation failure in the treatment of femoral neck fractures with cancellous screws. This finding was somewhat surprising as washers do not prevent the screws from backing out and do not provide any increase resistance to varus collapse. Therefore a follow-up biomechanical study was designed to test this observation. The purpose was to evaluate the maximal insertional torque of screws in osteoporotic bone with and without washers. We hypothesized that the lateral cortex of an osteoporotic proximal femur does not provide sufficient counter resistance for the screw heads to obtain maximum torque upon screw insertion in the femoral head and that the use of washers would increase screw purchase by providing a larger rigid surface area and subsequent higher counter resistance thereby allowing a higher maximal screw insertion torque. Methods. We used eight matched pairs of osteoporotic fresh-frozen human cadaveric femurs (age >70 years, all female). Two screws each were inserted in each femur either with or without a washer and maximal insertional torque was measured using a 50 Nm torque transducer. The testing was performed using a customized device which allowed the torque transducer to apply a constant axial force and torque speed to the screws. A paired Student's t-test was used to compare the maximal screw insertional torque of screws with washers versus screws without washers in matched pairs. Results. Fifteen out of 16 times the maximal screw insertional torque was higher when a washer was used. The average maximal torque with a washer was 5.1 Newtonmeter (Nm) compared to 3.1 Nm without a washer (p<0.001). Conclusion. We conclude that the addition of washers increases the maximal insertion torque of cancellous screws in the treatment of osteoporotic femoral neck fractures by providing counter resistance to the screw heads at an otherwise weak lateral cortex. We have demonstrated that the washer prevents the screw heads from penetrating the lateral cortex and provides for an improved purchase of the screws in the femoral head. As a clinical reference value for interpretation of this data, the limit of torque limiting screw drivers used with locking plates is set between 4 and 6 Nm. Therefore the difference in insertion torques likely represents clinically relevant values. Since there is no apparent disadvantage in the use of washers and they are inexpensive and readily available even in less developed countries, we advocate for their routine use until larger clinical studies disprove their efficacy

Introduction: Deterioration of bone-pin interface, causing pin loosening and infection, is a major cause of postoperative complications following external fixation. This paper presents the results obtained using hydroxyapatite (HA)-coated pins in various bone types, such as osteoporotic bone. Materials And Methods; In an animal study comparing HA-coated and standard pins, pin insertion and extraction torque were measured. Subsequently, a clinical study compared insertion, extraction torque and pin-tract infection rate of standard and HA-coated pins implanted in healthy and osteoporotic bone. Results: hi the animal study, mean final insertion torque was 4360±1050Nmm in the standard group and 3420 ± 676 Nmm in the HA-coated group. Mean extraction torque was 253 ± 175 Nmm in the standard group and 3360 ± 1260 Nmm in the HA-coated group (p = 0.002). In the clinical study on healthy bone, the mean insertion torque was 4130 ± 2160 Nmm in the standard group and 3440 ± 1930 Nmm in the HA-coated group (p = 0.03). The mean extraction torque was 1470 ± 1700 Nmm in the standard group and 5130 ± 2300 Nmm in the HA-coated group (p < 0.001). There were 33 pin-tract infections in the standard group and 20 pin-tract infections in the HA-coated group (p < 0.05, power 0.9). The standard pin group had a mean extraction torque of 730 (0–5500) Nmm in the infected pin-tracks and 2110 (0–7000) Nmm in the uninfected pin-tracts (p < 0.0001). The HA-coated group had a mean extraction torque of 5310 (1000–8000) Nmm in the infected pin-tracts and 5080 (0–8000) Nmm in the uninfected pin tracts (NS). In the clinical study on osteoporotic bone, mean final insertion torque was 461 ± 254 Nmm in the standard group and 331 ± 175Nmm in the HA-coated group (p = 0.01). Mean extraction torque was 191 ± 154 Nmm in the standard group and 600 ± 214 Nmm in the HA-coated group (p < 0.0005). Pin infection rate was greater in the standard group (p < 0.05). Conclusion: There was no deterioration of the bone-pin interface strength (measured by torque resistance at the time of pin extraction) with HA-coated pins. The improved strength of fixation of the HA-coated pins was associated with a lower incidence of pin-tract infection

Objectives. Cement augmentation of pedicle screws could be used to improve screw stability, especially in osteoporotic vertebrae. However, little is known concerning the influence of different screw types and amount of cement applied. Therefore, the aim of this biomechanical in vitro study was to evaluate the effect of cement augmentation on the screw pull-out force in osteoporotic vertebrae, comparing different pedicle screws (solid and fenestrated) and cement volumes (0 mL, 1 mL or 3 mL). Materials and Methods. A total of 54 osteoporotic human cadaver thoracic and lumbar vertebrae were instrumented with pedicle screws (uncemented, solid cemented or fenestrated cemented) and augmented with high-viscosity PMMA cement (0 mL, 1 mL or 3 mL). The insertion torque and bone mineral density were determined. Radiographs and CT scans were undertaken to evaluate cement distribution and cement leakage. Pull-out testing was performed with a material testing machine to measure failure load and stiffness. The paired t-test was used to compare the two screws within each vertebra. Results. Mean failure load was significantly greater for fenestrated cemented screws (+622 N; p ⩽ 0.001) and solid cemented screws (+460 N; p ⩽ 0.001) than for uncemented screws. There was no significant difference between the solid and fenestrated cemented screws (p = 0.5). In the lower thoracic vertebrae, 1 mL cement was enough to significantly increase failure load, while 3 mL led to further significant improvement in the upper thoracic, lower thoracic and lumbar regions. Conclusion. Conventional, solid pedicle screws augmented with high-viscosity cement provided comparable screw stability in pull-out testing to that of sophisticated and more expensive fenestrated screws. In terms of cement volume, we recommend the use of at least 1 mL in the thoracic and 3 mL in the lumbar spine. Cite this article: C. I. Leichtle, A. Lorenz, S. Rothstock, J. Happel, F. Walter, T. Shiozawa, U. G. Leichtle. Pull-out strength of cemented solid versus fenestrated pedicle screws in osteoporotic vertebrae. Bone Joint Res 2016;5:419–426

Introduction. Conventional screws achieve sufficient insertion torque in healthy bone. In poor bone screw stripping can occur prior to sufficient torque generation. It was hypothesized that a screw with a larger major/minor diameter ratio would provide improved purchase in poor bone as compared to conventional screws. We evaluated the mechanical characteristics of such a screw using multiple poor bone quality models. Methods. Testing groups included: conventional screws, osteopenia screws used in bail-out manner (ie, larger major/minor diameter screws inserted into a hole stripped by a conventional screw), and osteopenia screws used in a preemptive manner (ie, no screw stripping occurrence). Stripping Torque: Screws were inserted through standard straight plates into a low density block of foam with a predrilled hole. Stripping torque was defined as maximum insertion torque reached by the screw before the screw began to spin freely in the foam. Pullout. Pullout tests were conducted on screws inserted into the same test media. Axial pull-out testing was then conducted by applying a tensile load to the screws. Compression. Screws were inserted through standard straight plates by hand while the amount of compression achieved between plate and bone was measured using a pressure sensor. The same foam test media was utilized in addition to osteoporotic fresh-frozen femoral diaphyseal cadaver (bone mineral density<0.60 g/cm2). The screws were tightened across a range of possible insertion torques with pressure measurements taken at multiple intervals. Results. The osteopenia bone screws showed a 67% increase in torque before stripping occurred (p<0.01) when compared to the conventional screw. The osteopenia screw used in a bail-out manner showed a 57% increase in stripping torque (p<0.01) and a 76% increase in pullout strength (p<0.01) when compared to the conventional screw. Additionally, the bail-out screw showed a minimal decrease in both stripping torque (6%, p = 0.45) and pullout strength (11%, p<0.01) when compared to the osteopenia screw tested in preemptive manner. There was a linear relationship between applied torque and compressive force generation for both osteopenia and conventional screws. The osteopenia screws were able to gain greater compression against bone across a range of insertion values as compared to conventional bone screws. Discussion. The osteopenia screw achieved superior stripping torque, pullout strength, and compressive forces when compared to conventional screws in simulated poor quality bone and osteoporotic cadaver bone. When used as a bail-out screw, it also achieved superior stripping torque and pullout strength. The results of this study indicate that a screw of larger major/minor diameter ratio could be an effective bail-out option for screw stripping associated with osteopenic fracture fixation

Purpose: Four external fixation pin types differing in coating, design and implantation technique were tested in an animal study. Methods: Forty tapered pins were divided into 4 Groups according to pin design type: Group A consisted of 10 standard self-tapping pins (ø5–6mm, pitch 1.75mm), Group B 10 hydroxyapatite (HA)-coated self-tapping (ø5–6mm, pitch 1.75mm), Group C 10 standard, self-drilling, self-tapping (ø5–6mm, pitch 1.25mm) and Group D 10 HA-coated, self-drilling, self-tapping (ø5–6mm, pitch 1.25mm). Four pins were randomly implanted into the femoral diaphysis of 10 sheep. The pins were implanted at 2-cm intervals apart. Pre-drilling was used for Groups A and B but not for Groups C and D. Sheep were euthanized 6 weeks after surgery. Results: There were no major complications. Mean pin insertion torque was 3100 ± 915 Nmm in Group A, 2808 ± 852 Nmm in Group B, 2589 ± 852 Nmm in Group C and 2180 ± 652 Nmm in Group D. Mean pin extraction torque was 1570 ± 504 Nmm in Group A, 2128 ± 1159 Nmm in Group B, 1599 ± 809 Nmm in Group C and 2200 ± 914 Nmm in Group D. Insertion torque of the coated groups was lower than insertion torque of the standard groups (p < 0.05). However, extraction torque of Groups B and D was higher than Groups A and C (p < 0.05). No differences in pin fixation were found between the two coated pin groups (Group B and D). Morphologic analysis showed extensive bone to pin contact without fibrous tissue interposition in the coated pin groups and fibrous tissue interposition in the uncoated pin groups. Conclusion/Significance: This study demonstrated that coating pins with hydroxyapatite is effective regardless of the pin design and the implantation technique

Purpose: Healing may be problematic after lag screw osteosynthesis of pertrochanteric fractures in osteoporotic bone due to the greater risk of defective fixation. Acrylic cement has been proposed to reinforce the fixation of the lag screw in these patients, principally to avoid the risk of cutout, but the acrylic cement’s thermal toxicity, its poor biocompatibility, and the difficult manipulation are important drawbacks. Cortoss® is a new composite biomaterial composed of bisphenol-aglycidly (bis-GMA) which could be an attractive alternative to classical cements. Cortoss is an injectable material with mechanical properties similar to human cortical bone. The purpose of this clinical study was to describe the new method for injection the material and to assess the anchoring force and safety of Cortoss in osteoporotic patients undergoing surgical fixation of pertrochanteric fractures. Material and methods: This prospective study was approved by the local ethics committee. Twenty consecutive patients aged 70 years or more with pertro-chanteric fracture were included. The lag screw was inserted under fluoroscopic guidance, and the maximum insertion torque was measured. The screw was then unscrewed seven turns (length of the threaded head), and 2.5 cm Cortoss was injected via a polyimide catheter measuring 2.5 mm in diameter. The screw was then rescrewed in place to a troque 30% above that measured without Cortoss. Results: Eighteen women and two men, age 70 – 96 years, gave their informed consent to participate in the study. Mean maximal insertion torque without Cortoss was 1.23 Nm (min 0, max 4.8 Nm), which was increased 30% with Cortoss. Posto-operative x-rays showed that the screw head was embedded in an envelope of Cortoss and that the Cortoss had diffused into the adjacent bone. There were no adverse effects. Discussion and conclusion: Cortoss provided effective reinforcement of the fixation as demonstrated by the higher insertion torque. Cortoss can also increase the mechanical resistance of the screw-cancellous bone interface, limit fixation default, and reduce secondary displacement of the lag screw in patients with osteoporotic bone. Improved stability can also reduce immobilisation time and facilitate bone healing without increasing the risk of secondary displacement and subsequent morbidity. Due to its safety, easy use, and efficacy, Cortoss provides a better alternative than acrylic cement for reinforcement of lag screw fixation of porotic bone

We compared thirty-eight patients from three orthopaedic centres who had external fixation of the femur or the tibia. Patients were randomised to receive standard (tapered 5/6 millimeter) pins (Group A) or OsteoTite (hydroxyapatite coated tapered 5/6 mm) pins (Group B). In Group A there were 18 patients who received 71 pins. Average age of the patients was 49±12 years. Three patients had external fixation in the femur and 15 in the tibia. External fixation was used in three types of treatment: fracture fixation (4), knee osteotomy (12), and bone-transport (2). Two different fixator frames were used: a unilateral fixator was mounted in 15 patients and a circular fixator in three patients. In Group B, 20 patients (average age: ± 48 years) received 86 pins. Four patients had external fixation in the femur and 16 in the tibia. External fixation was used in three types of treatments: fracture fixation in three patients, knee osteotomy in 13 patients, and bone-transport in four patients. Two different fixator frames were used: a unilateral fixator was mounted in 16 patients and a circular fixator in four patients. No significant difference regarding sex, age, external fixation treatment type, external fixation frame, and length of treatment were observed between the two groups. Mean final pin insertion torque was 477 ± 214 Newton cm in Group A and 339 ± 184 in Group B (p< 0.01). Mean pin extraction torque was 205 ± 169 Newton cm in Group A and 532 ± 211 in Group B (p< 0.01). Pin extraction torque was significantly lower compared to the corresponding insertion torque in Group A (p< 0.01). Pin extraction torque was significantly higher compared to the corresponding insertion torque in Group B (p< 0.01). Pin tract infection rate was lower in Group B compared to Group A (p< 0.01). This study shows that hydroxyapatite coating was clinically effective in the improvement of the bone pin interface strength of tapered pins. By using these pins, deterioration of the bone pin interface strength can be avoided and external fixation complications minimised

Introduction: External fixation is used widely in the management of fractures, despite a relatively high incidence of complication, arising from pin loosening and infection. Diamond like carbon (DLC) is a low surface energy coating that can be applied to external fixator pins and may reduce biofilm formation and infection resulting in a lower incidence of pin loosening. Hydroxyapatite (HA) is well established as a coating to enhance fixation of external fixator pins. This study tests the hypothesis that HA and DLC coatings on stainless steel (SS) external fixator pin shafts modify integration of the implant with soft/hard tissues. Materials and Methods: An Orthofix external fixator was used to stabilise a tibial osteotomy with 6 self-drilling/tapping 6mm pins in 32 skeletally mature Friesland ewes. Animals were divided into four groups; SS, DLC, HA partially coated (threads only) and HA fully coated (threads and pin shaft). Pin insertion torque was measured using a torque wrench and extraction torque similarly obtained at 10 weeks when animals underwent euthanasia. Pin performance indices (PPI) were calculated as a ratio of extraction to insertion torque x100%. Pin site 2 was preserved for hard grade resin histology and subsequent pin tissue integration analysis. Pin site 3 was used for analysis of the soft tissue pin shaft interface using transmission electron microscopy. Pin site 5 was examined for the presence of biofilm formation using scanning electron microscopy. Pin site 6 was swabbed for microbiological analysis. Results: SS and DLC pins achieved significantly higher insertion torques compared to HA partially coated pins (p=0.001, 0.002). Both groups of HA coated pins demonstrated a significantly higher, extraction torque and therefore PPI for all pin site positions compared to SS and DLC (p< 0.001– 0.025). The epithelium was found not to be in contact with the pin shaft in all cases. No significant differences were found between the different pin groups for epidermal down growth and dermal contact. Both groups of HA coated pins showed a significantly higher percentage of new bone in direct contact with the embedded threads compared to SS and DLC pins (p< 0.001, p=0.004). The proportion of soft tissue in contact and within the thread, of fully coated HA pins was significantly lower compared to stainless steel (p=0.003, p=0.017), DLC (p=0.004, p=0.002) and HA partially coated pins (p=0.006, p=0.02). Biofilms were evident on all pins except those coated with DLC. More bacteria were observed on the fully HA coated pins. DLC had significantly lower number of bacterial colonies in culture compared to SS (p=0.028) and fully coated HA pins (p=0.005). Discussion: Coatings of DLC and HA do have a significant affect on hard/soft tissue reactions. However coatings do not have a significant effect on epidermal down growth or dermal attachment to the pin shaft surface. DLC coated pins had the cleanest surface with no bio-film present and significantly lower numbers of bacteria present. Fully HA coated pins despite evidence of bio-film formation, bacteria and high microbiological counts had significantly higher PPI. In addition fully coated HA pins demonstrated significantly reduced amounts of soft tissue at the pin bone interface. Therefore soft tissue reactions may affect bone integration

Aims. To devise a method to quantify and optimize tightness when inserting cortical screws, based on bone characterization and screw geometry. Methods. Cortical human cadaveric diaphyseal tibiae screw holes (n = 20) underwent destructive testing to firstly establish the relationship between cortical thickness and experimental stripping torque (T. str. ), and secondly to calibrate an equation to predict T. str. Using the equation’s predictions, 3.5 mm screws were inserted (n = 66) to targeted torques representing 40% to 100% of T. str. , with recording of compression generated during tightening. Once the target torque had been achieved, immediate pullout testing was performed. Results. Cortical thickness predicted T. str. (R. 2. = 0.862; p < 0.001) as did an equation based on tensile yield stress, bone-screw friction coefficient, and screw geometries (R. 2. = 0.894; p < 0.001). Compression increased with screw tightness up to 80% of the maximum (R. 2. = 0.495; p < 0.001). Beyond 80%, further tightening generated no increase in compression. Pullout force did not change with variations in submaximal tightness beyond 40% of T. str. (R. 2. = 0.014; p = 0.175). Conclusion. Screw tightening between 70% and 80% of the predicted maximum generated optimum compression and pullout forces. Further tightening did not considerably increase compression, made no difference to pullout, and increased the risk of the screw holes being stripped. While further work is needed for development of intraoperative methods for accurate and reliable prediction of the maximum tightness for a screw, this work justifies insertion torque being considerably below the maximum. Cite this article: Bone Joint Res 2020;9(8):493–500

Introduction The AO/ASIF 3.5 mm STS is increasingly used for internal fixation of large bones with the recent introduction of the 3.5 mm periarticular plating system. Our study aims to compare the insertion torque and mechanical properties of the screw after insertion into bovine femora using non tapped and pretapped methods. Methods Three groups of ten 3.5 mm AO/ASIF STSs of variable lengths were used. One group was put aside as the control. One group was inserted into fresh bovine femora using pre-tapped drill holes and the final group using non-tapped drill holes. The insetion torques were measured and compared using an analogue torque screw driver. All screws were removed. The three groups were then tested for mechanical strength. The results of the groups were compared we found the insertion torque to be six times greater in the non tapped group compared to the pre tapped group. We noted the non tapped group failed later than the pre-tapped group, this was statistically not significant however. The mechanical strength of the screw was not statistically altered. Conclusions We conclude that the 3.5 mm STS is easier to insert when pre tapped. However pre tapping is not necessary to preserve the mechanical strength of the screw

The effect of screw geometry on the pullout strength of Anterior Cruciate Ligament [ACL] reconstruction is well documented. Most research has looked at the effect of screw length and diameter, however other factors such as the degree of taper may also be important. Tapered screws should in theory be associated with increased pullout strength. This has not been demonstrated either clinically or in vitro before. The aim of this study was to compare the pullout strength of ACL reconstruction with a parallel against a tapered screw. A parallel and tapered screw were manufactured which were identical in all other respects. Sixty superficial digital flexors from the hind legs of sheep were harvested. The tendons were paired and combined to form a quadruple tendon reconstruction of approximately 7mm diameter as measured with graft sizer. An ACL reconstruction was performed on the proximal tibia of 30 bovine knees, which had been harvested in right and left knee pairs, using the quadruple tendon. Fifteen reconstructions were fixed using tapered screws and fifteen with non-tapered screws. The insertion torque of both tapered and non tapered screws were recorded using an instrumented torque screwdriver. The reconstructions were mounted in an Instron materials testing machine with an x-ray bearing system to eliminate horizontal forces, to ensure that the forces were all directed along the line of the tibial tunnel. The maximum pullout strengths were recorded in each case. Five knee pairs were subjected to bone densitometry scanning to ensure that any difference in pull out strength was not due to changes in bone density between right and left knee pairs. Results indicated that there was no difference between right and left knee pairs [p = 0.58] and that tapered screws were associated with significantly higher pull-out strengths [p=0.007] and insertion torques [p = 0.001]

Nearly a quarter of screws cause damage during insertion by stripping the bone, reducing pullout strength by over 80%. Studies assessing surgically achieved tightness have predominately shown that variations between individual surgeons can lead to underpowered investigations. Further to the variables that have been previously explored, several basic aspects related to tightening screws have not been evaluated with regards to how they affect screw insertion. This study aims to identify the achieved tightness for several variables, firstly to better understand factors related to achieving optimal intraoperative screw purchase and secondly to establish improved methodologies for future studies. Two torque screwdrivers were used consecutively by two orthopaedic surgeons to insert 60 cortical, non-locking, stainless-steel screws of 3.5 mm diameter through a 3.5 mm plate, into custom-made 4 mm thick 20 PCF sheets of Sawbone, mounted on a custom-made jig. Screws were inserted to optimal tightness subjectively chosen by each surgeon. The jig was attached to a bench for vertical screw insertion, before a further 60 screws were inserted using the first torque screwdriver with the jig mounted vertically, enabling horizontal screw insertion. Following the decision to use the first screwdriver to insert the remaining screws in the vertical position for the other variables, the following test parameters were assessed with 60 screws inserted per surgeon: without gloves, double surgical gloves, single surgical gloves, non-sterile nitrile gloves and, with and then without augmented feedback (using digitally displayed real-time achieved torque). For all tests, except when augmented feedback was used, the surgeon was blinded to the insertion torque. Once the stopping torque was reached, screws were tightened until the stripping torque was found, this being used to calculate tightness (stopping/stripping torque ratio). Screws were recorded to have stripped the material if the stopping torque was greater than the stripping torque. Following tests of normality, Mann-Whitney-U comparisons were performed between and combining both surgeons for each variable, with Bonferroni corrections for multiple comparisons. There was no significant (p=0.29) difference in the achieved tightness between different torque screw drivers nor different jig positions (p=0.53). The use of any gloves led to significant (p < 0 .001) increases in achieved tightness compared to not using gloves for one surgeon but made no difference for the other (p=0.38–0.74). Using augmented feedback was found to virtually eliminate stripping. For one surgeon average tightness increased significantly (p < 0 .001) when torque values were displayed from 55 to 75%, whilst for the other, this was associated with significantly decreases (p < 0 .001), 72 to 57%, both surgeons returned to their pre-augmentation tightness when it was removed. Individual techniques make a considerable difference to the impact from some variables involved when inserting screws. However, the orientation of screws insertion and the type of screwdriver did not affect achieved screw tightness. Using visual feedback reduces rates of stripping and investigating ways to incorporate this into clinical use are recommended. Further work is underway into the effect of other variables such as bone density and cortical thickness

Introduction. Loosening of the baseplate is one of the most common causes of failure in Reverse Shoulder Arthroplasty. To allow osteo-integration to occur and thus provide long-term stability, initial screws fixation plays a pivotal role. In particular, tightening torque and force of nonlocking screws are two parameters that are considered to have a clear impact on implant stability, yet the relation is not fully understood. For this reason, this study aims to define an experimental set-up, to measure force and torque in artificial bone samples of different quality, in order to estimate ranges of optimal surgical values and give guidelines to maximize screw fixation and therefore initial implant stability. Methods. A custom-made torque sensor (Figure 1a) was built and calibrated using a lever deadweight system. To measure the compression force generated by the screw head, three thin FlexiForce sensors (Tekscan, South Boston, US) were enclosed between two 3D printed plates with a central hole to allow screw insertion (Figure 1b). The tightening force, represented by the sum of the three sensors, was calibrated using a uniaxial testing machine (Zwick/Roell, Ulm, Germany). Multiple screw lengths (26mm, 32mm and 47mm) were selected in the protocol. Synthetic bone blocks (Sawbones; Malmö, Sweden) of 20 and 30 PCF were used to account for bone quality variation. To evaluate the effect of a cortical bone layer, for each density three blocks were considered with 0 mm (no layer), 1.5 mm and 3 mm of laminate foam of 50 PCF. The holes for the screws were pre-drilled in the same way as in the operation room. For each combination of screw dimensions and bone quality, ten measurements were performed by acquiring the signal of the insertion torque and tightening force until bone breaking. Results. The typical output signal shows a maximum in the torque and force measurements, corresponding to bone breaking. After failure, a drop in the torque is visible, while a residual force remains present. For the base case (20 PCF), both torque and force show increasing mean values with longer screws, passing from 0.39 Nm (26mm) to 1.12 Nm (47mm) and from 180 N (26mm) to 419 N (47mm) respectively. Similar patterns were observed when the cortical layer was present or the bone quality was increased. Discussion. The findings of this study demonstrate that tightening force and torque are strongly impacted by bone quality and screw length. As main outcome, the maximum torque values could be used in clinical practice as a safety threshold for the surgeon. Compression force could also be used as input parameter in stability predictions of numerical models. Since only bone substitute was used, future research should include the extension to cadaveric bones. For any figures or tables, please contact authors directly

The February 2024 Spine Roundup³⁶⁰ looks at: Surgeon assessment of bone – any good?; Robotics reduces radiation exposure in some spinal surgery; Interbody fusion cage versus anterior lumbar interbody fusion with posterior instrumentation; Is robotic-assisted pedicle screw placement an answer to the learning curve?; Acute non-traumatic spinal subarachnoid haematomas: a report of five cases and a systematic review of the literature; Is L4-L5 lateral interbody fusion safe and effective?

We investigated the effects of hydroxyapatite (HA) coating on the purchase of pedicle screws. A total of 23 consecutive patients undergoing lumbar fusion was randomly assigned to one of three treatment groups. The first received uncoated stainless-steel screws, the second screws which were partly coated with HA, and the third screws which were fully coated. The insertion torque was recorded. After 11 to 16 months, 21 screws had been extracted. The extraction torque was recorded. Radiographs were taken to assess fusion and to detect loosening of the screws. At removal, the extraction torques exceeded the upper limit of the torque wrench (600 Ncm) for many HA-coated screws. The calculated mean extraction torque was 29 ± 36 Ncm for the uncoated group, 447 ± 114 Ncm for the partly-coated group and 574 ± 52 Ncm for the fully-coated group. There were significant differences between all three groups (p < 0.001). There were more radiolucent zones surrounding the uncoated screws than the HA-coated screws (p < 0.001). HA coating of pedicle screws resulted in improved fixation with reduced risk of loosening of the screws

Summary. A rotational limit for screw insertion may improve screw purchase and plate compression by reducing stripping, as compared to a torque based limit. Introduction. Over-tightening screws results in inadvertent stripping of 20% of cortical bone screws. The current method of “two-fingers tight” to insert screws relies on the surgeon receiving torque feedback. Torque, however, can be affected by screw pitch, bone density and bone-thread friction. An alternative method of tightening screws is the “turn-of-the-nut” model, commonly used in engineering applications. In the “turn-of-the-nut” method, nuts used to fasten a joint are rotated a specific amount in order to achieve a pre-specified bolt tension. When applied to orthopaedics, bone assumes the role of the nut and the screw is the bolt. The screw is turned a set angular rotation that is independent of torque feedback. Potentially the “turn-of-the-nut” method provides an easier way of screw insertion that might lessen inadvertent screw stripping. The purpose of the current study was to use the “turn-of-the-nut” method to determine the angular rotation that results in peak plate compression and peak screw pullout force. Methods. Three pairs of human humeri in each of three groups (osteopenic, osteoporotic, and normal) underwent plate compression and pullout protocols. For plate compression, 3.5-mm screws were tightened into strain gauge instrumented plate until screw stripping occurred. Insertion torque, plate compression, and screw rotation were measured. For pullout, 3.5-mm screws were inserted until the head contacted the plate, additionally rotated (90, 180, 270, or 360 degrees), and then pulled out. A generalised linear and latent mixed model was used to check for significant associations (P < 0.05). Results. Mean (95% CI) peak plate compression occurred at 286 degrees (range, 261 – 311 degrees) beyond screw seating. Plate compression significantly increased at 90 to 135 degrees but not after 180 degrees. At 270 degrees, 39% of the screws had already reached their peak ability to compress. Peak screw torque lagged behind peak plate compression by 31 ± 50 degrees, and in seeking peak screw torque, a loss of 104 ± 115 N in plate compression resulted. Screw pullout force was greatest at 90 degrees, but it was not significantly different from that of the other angle groups. Conclusions. Screw rotation at 180 degrees provides plate compression and pullout strength statistically similar to those at greater rotations but without the loss of purchase associated with greater rotations