Bending strength and holding power of tibial locking screws

Comparison of insertion time, pull-out strength, and screw-media interface area of customized pedicle screw with different core and thread design against commercial pedicle screw: a pilot study on Indonesian Population

Objective

This research aimed to developing customized pedicle screw based on Indonesian vertebral anatomy and compare the insertion time, pull-out strength, and screw-media interface area of different screw design. We have developed 3 different types of pedicle screws (v-thread cylinder-core, square-thread cylinder-core and square-thread conical-core). The thread diameter was calculated from pedicle width of Indonesian population (6 mm). We used commercially available pedicle screw as control group (6.2 mm).

Result

The insertion time were significantly difference between v-thread cylinder-core pedicle screw (22.94 s) with commercially available pedicle screw (15.86 s) (p < 0.05). The pull-out strength was significantly difference between commercially available pedicle screw (408.60 N) with square-thread conical pedicle screw (836.60 N) (p < 0.05). The square-thread conical-core group have the highest interface area (1486.21 mm²). The data comparison showed that the square-thread conical-core customized pedicle screw group has comparable insertion time and has better pull-out strength than commercially available pedicle screw.

Biomechanical assessment of screw safety between far cortical locking and locked plating constructs

With the emerging concerns for more flexible and less stiff bridge constructs in the interest of stimulating bone healing, the technique of far cortical locking has been designed to reduce the stiffness of locked plating (LP) constructs while retaining construct strength. This study utilized simulation with diaphyseal bridge plating biomechanical models to investigate whether far cortical locking causes larger screw fracture risk than LP during rehabilitation. The fracture risk of the screws in the far cortical locking constructs increases in the non-osteoporotic and osteoporotic diaphysis compared with the screws in the LP constructs.

Osseointegration of Sandblasted or Anodised Hydrothermally-Treated Titanium Implants: Mechanical, Histomorphometric and Bone Hardness Measurements

The improvement of the implant-bone interface is still an open problem in the long-term mechanical stability of cementless fixed implants. Mechanical, histomorphometric and bone hardness measurements were performed in sheep femoral cortical bone implants at 8 and 12 weeks from surgery to compare in vivo the osseointegration of titanium screws (Ø 3.5 mm × 7 mm length) with two different surface treatments: sandblasting with 70–100 μm HA followed by acid etching with HNO3 (Group A) and Ca-P anodization followed by a hydrothermal treatment (Group B). No significant differences were found for maximum push-out force and interfacial strength between groups at both experimental times. No significant difference was observed for Bone Ingrowth between groups at both experimental times, while the Affinity Index of Group B was significantly higher (7.5%, p<0.05) and lower (10.2%, p<0.05) than that of Group A at 8 and 12 weeks, respectively. Finally, a significant increase in bone microhardness measured within 200 μm from the interface and inside the thread depth of Group A was observed between the two experimental times (p<0.05). In conclusion, present findings show that osseointegration may be accelerated by adequate surface roughness and bioactive ceramic coating such as current tested treatments which enhance bone interlocking and mineralization.

The Safety and Strength of a Novel Medial, Partial Nonthreaded Pedicle Screw: A Cadaveric and Biomechanical Investigation

STUDY DESIGN

Cadaveric and biomechanical study.

OBJECTIVE

The aim of this study was to assess the safety and pullout strength of medial, partial nonthreaded thoracic pedicle screws compared with conventional screws.

SUMMARY OF BACKGROUND DATA

The perforation rate of the pedicle screws has been reported as high as 41%. Nerve injury and irritation can result from the compression of malpositioned screw on neural structures.

METHODS

Ten fresh cadavers were studied. Screws, 5.0 and 6.0 mm, were inserted from T1 to T6 and T7 to T12, respectively. Pedicle perforations and fractures were recorded upon screw insertion and final positioning (nonthreaded portion facing medially) after a wide laminectomy. Pullout strength of novel and conventional screws were then tested using an Instron machine in an artificial bone substitute.

RESULTS

A total of 240 thoracic pedicle screws were inserted. Of them, 88.8% (213 screws) were fully contained during screw insertion. There were 5.0% (12 screws) grade 1 medial perforations and 6.2% (15 screws) grade 1 lateral perforations during screw insertion. Upon final positioning, 93.8% (225 screws) were fully contained. All grade 1 medial perforations, which occurred during insertion, were converted to grade 0. No dural or nerve root injuries occurred. Pedicle split fractures were noted in 6.7% (16 screws). The use of medial, partial nonthreaded screws reduced the overall perforation rate from 11.2% to 6.2%. The mean pullout load for the 5 mm fully threaded screw versus medial, partial nonthreaded was 1419.3±106.1 N (1275.8-1538.8 N) and 1336.6±44.2 N (1293.0-1405.1 N) respectively, whereas 6 mm pullout load averaged 2126.0±134.8 N (1986.3-2338.3 N) and 2036.5±210.0 N (1818.4-2355.9 N). The difference was not statistically significant.

CONCLUSIONS

The use of medial, partial nonthreaded pedicle screws reduced the medial perforation rate from 5.0% to 0%; however, the pullout strength was not significantly reduced. The use of this novel screw can potentially reduce the incidence of nerve injury or irritation after medial pedicle perforations.

A novel intramedullary nail for use in the treatment of supramalleolar malunion and nonunion: A preliminary report of three cases

Introduction

The fractures around the supramalleolar region are difficult to manage and deformities may occur following insufficient or inadequate treatment attempts. The treatment of those deformities is even more challenging. The available options for surgical fixation have inherent problems considering the soft tissue problems, infection and compliance of the patients. In this study, we report the preliminary outcome of new IMN system in use of supramalleolar deformities.

Patients and Methods

Three patients with supramalleolar deformities were recruited for this study. Two patients had periarticular distal tibia malunions and one patient had nonunion in same region. All patients were operated with using newly designed intramedullary nail system with a unique distal interlocking bolt screw. The outcome for each patient was followed-up and evaluated with using AOFAS score.

Results

The patients were followed up for 3 years, 3 months, and 6 months, respectively. The AOFAS scores were 36, 33, and 21 preoperatively; and 75, 68, and 75 postoperatively in Patients 1, 2 and 3, respectively.

Conclusion

In our patient series, adequate fixation following correction of the deformity was seen. Although the number of the cases are limited this study provides encouraging results regarding the outcome of new IMN system in use of supramalleolar deformities.

Biomechanical measurements of stopping and stripping torques during screw insertion in five types of human and artificial humeri

During orthopedic surgery, screws are inserted by “subjective feel” in humeri for fracture fixation, that is, stopping torque, while trying to prevent accidental over-tightening that causes screw–bone interface failure, that is, stripping torque. However, no studies exist on stopping torque, stripping torque, or stopping/stripping torque ratio in human or artificial humeri. This study evaluated five types of humeri, namely, human fresh-frozen (n = 19), human embalmed (n = 18), human dried (n = 15), artificial “normal” (n = 13), and artificial “osteoporotic” (n = 13). An orthopedic surgeon used a torque screwdriver to insert 3.5-mm-diameter cortical screws into humeral shafts and 6.5-mm-diameter cancellous screws into humeral heads by “subjective feel” to obtain stopping and stripping torques. The five outcome measures were raw and normalized stopping torque, raw and normalized stripping torque, and stopping/stripping torque ratio. Normalization was done as raw torque/screw–bone interface area. For “gold standard” fresh-frozen humeri, cortical screw tests yielded averages of 1312 N mm (raw stopping torque), 30.4 N/mm (normalized stopping torque), 1721 N mm (raw stripping torque), 39.0 N/mm (normalized stripping torque), and 82% (stopping/stripping torque ratio). Similarly, fresh-frozen humeri gave cancellous screw average results of 307 N mm (raw stopping torque), 0.9 N/mm (normalized stopping torque), 392 N mm (raw stripping torque), 1.2 N/mm (normalized stripping torque), and 79% (stopping/stripping torque ratio). Of the five cortical screw parameters for fresh-frozen humeri versus other groups, statistical equivalence (p ≥ 0.05) occurred in four cases (embalmed), three cases (dried), four cases (artificial “normal”), and four cases (artificial “osteoporotic”). Of the five cancellous screw parameters for fresh-frozen humeri versus other groups, statistical equivalence (p ≥ 0.05) occurred in five cases (embalmed), one case (dried), one case (artificial “normal”), and zero cases (artificial “osteoporotic”). Stopping/stripping torque ratios were relatively constant for all groups at 77%–88% (cortical screws) and 79%–92% (cancellous screws).

Multiobjective optimization design of spinal pedicle screws using neural networks and genetic algorithm: mathematical models and mechanical validation

Short-segment instrumentation for spine fractures is threatened by relatively high failure rates. Failure of the spinal pedicle screws including breakage and loosening may jeopardize the fixation integrity and lead to treatment failure. Two important design objectives, bending strength and pullout strength, may conflict with each other and warrant a multiobjective optimization study. In the present study using the three-dimensional finite element (FE) analytical results based on an L₂₅ orthogonal array, bending and pullout objective functions were developed by an artificial neural network (ANN) algorithm, and the trade-off solutions known as Pareto optima were explored by a genetic algorithm (GA). The results showed that the knee solutions of the Pareto fronts with both high bending and pullout strength ranged from 92% to 94% of their maxima, respectively. In mechanical validation, the results of mathematical analyses were closely related to those of experimental tests with a correlation coefficient of −0.91 for bending and 0.93 for pullout (P < 0.01 for both). The optimal design had significantly higher fatigue life (P < 0.01) and comparable pullout strength as compared with commercial screws. Multiobjective optimization study of spinal pedicle screws using the hybrid of ANN and GA could achieve an ideal with high bending and pullout performances simultaneously.

Biomechanical measurements of cortical screw stripping torque in human versus artificial femurs

Femur fracture plates are applied using cortical bone screws. Surgeons do this manually by subjective ‘feel’ without monitoring torque. Few studies have quantified stripping torque in human bone. No studies have measured stripping torque in the artificial bones from Sawbones (Vashon, WA, USA) that are frequently used in biomechanical studies. The present aim was to measure stripping torque of cortical screws in human versus artificial femurs. Sixteen fresh-frozen human femurs and eight artificial femurs were used. Using a digital torque screwdriver, each femur had a 3.5-mm diameter unicortical screw manually inserted into the anterior midshaft until failure of the screw–bone interface. Results were normalized by cortical thickness and the screw–bone interfacial area. There were no statistical differences in human versus artificial data, respectively, for stripping torque (1741 ± 442 N.mm, 2012 ± 176 N.mm, p = 0.11), stripping torque/thickness (313 ± 59 N, 305 ± 30 N, p = 0.74), and stripping torque/area (28.5 ± 5.3 N/mm, 27.8 ± 2.8 N/mm, p = 0.74). Artificial unicortical thickness (6.6 ± 0.3 mm) was greater than human thickness (5.6 ± 1.1 mm) ( p = 0.02). For human specimens, there was a moderate linear correlation of absolute and normalized stripping torque versus standardized bone mineral density (R ≥ 0.32) and clinical T-score (R = 0.29), but not with age (R ≤ 0.29). Surgeons should be aware of the stripping torque limits for human femurs and potentially take steps to monitor these values during surgery. The artificial femurs being increasingly used in research accurately replicate human cortical properties during screw insertion. To date, this is the first series of human femurs evaluated for cortical screw stripping.

A Neurogenetic Approach to a Multiobjective Design Optimization of Spinal Pedicle Screws

A pedicle screw fixation has been widely used to treat spinal diseases. Clinical reports have shown that the weakest part of the spinal fixator is the pedicle screw. However, previous studies have only focused on either screw breakage or screw loosening. There have been no studies that have addressed the multiobjective design optimization of the pedicle screws. The multiobjective optimization methodology was applied and it consisted of finite element method, Taguchi method, artificial neural networks, and genetic algorithms. Three-dimensional finite element models for both the bending strength and the pullout strength of the pedicle screw were first developed and arranged on an L25 orthogonal array. Then, artificial neural networks were used to create two objective functions. Finally, the optimum solutions of the pedicle screws were obtained by genetic algorithms. The results showed that the optimum designs had higher bending and pullout strengths compared with commercially available screws. The optimum designs of pedicle screw revealed excellent biomechanical performances. The neurogenetic approach has effectively decreased the time and effort required for searching for the optimal designs of pedicle screws and has directly provided the selection information to surgeons.

Influence of a zirconia sandblasting treated surface on peri-implant bone healing: An experimental study in sheep

A sandblasting process with round zirconia (ZrO(2)) particles might be an alternative surface treatment to enhance the osseointegration of titanium dental implants. Our previous study on sheep compared smooth surface titanium implants (control) with implant surfaces sandblasted with two different granulations of ZrO(2). As the sandblasted surfaces proved superior, the present study further compared the ZrO(2) surface implant with other surface treatments currently employed: machined titanium (control), titanium oxide plasma sprayed (TPS) and alumina sandblasted (Al-SL) at different times after insertion (2, 4 and 12weeks). Twelve sheep were divided into three groups of four animals each and underwent implant insertion in tibia cortical bone under general anaesthesia. The implants with surrounding tissues were subjected to histology, histomorphometry, scanning electron microscopy and microhardness tests. The experimentation indicated that at 2weeks Zr-SL implants had the highest significant bone ingrowth (p<0.05) compared to the other implant surfaces, and a microhardness of newly formed bone inside the threads significantly higher than that of Ti. The present work shows that the ZrO(2) treatment produces better results in peri-implant newly formed bone than Ti and TPS processing, whereas its performance is similar to the Al-SL surface treatment.

Increasing bending strength of tibial locking screws: mechanical tests and finite element analyses

BACKGROUND

Healing of tibial fractures treated by locked nailing is threatened by locking screw failure. However, the effects of the design factors of the screws on their mechanical strength have rarely been studied.

METHOD

Three-point bending tests and finite element analyses were used to investigate the bending strength of five types of commercially available tibial locking screws and two types of specially designed screws. Yielding strength and fatigue life measured in bending tests were correlated to total strain energy and maximal tensile stress computed in finite element analyses. Parametric analysis and design optimization were done according to the Taguchi method. Validation studies to assess the stress rising effect of the threads on the fatigue strength were conducted in two types of new screws made of either stainless steel or titanium alloy.

FINDINGS

The yielding strength of the screws was closely related to their total strain energy, and the logarithm of the fatigue life was closely related to the maximal tensile stress with correlation coefficients of -0.95 and -0.90, respectively. Parametric studies indicated that fatigue strength of the screws was affected mainly by inner diameter (contribution, 63.8%) and root radius (27.8%). The yielding strength was determined primarily by inner diameter (88.5%). Titanium screws had a longer fatigue life than stainless steel screws, especially in screws with larger root radii.

INTERPRETATION

A screw's strength is closely related to its design factors. Finite element models, which can reliably reflect the mechanical strength of screws can save time and effort during screw design. Larger root radius can effectively improve the fatigue strength, especially for titanium screws as compared with stainless steel screws.

A comparison of pullout strength for pedicle screws of different designs: a study using tapped and untapped pilot holes

STUDY DESIGN

The pullout strengths of various pedicle screw designs are compared using tapped and untapped pilot holes.

OBJECTIVE

The objective of this study is to compare the pullout strength of various pedicle screw designs. The designs are compared using tapped and untapped pilot holes. By using several different screw designs, it is possible to gain an understanding of whether there is a correlation between tapping a pilot hole and the ultimate pullout strength.

SUMMARY OF BACKGROUND DATA

Most bone screws originally developed were intended to be installed in a pretapped pilot hole. This same technology has been carried over to the development of more modern bone screws for use in spinal fixation applications. Many pedicle screws in use today are still intended to be installed in a tapped hole. Preparing the vertebrae and tapping of a pilot hole involve additional trauma to the patient as well as increased operating time.

METHODS

Pedicle screws from various manufacturers are installed in tapped and untapped pilot holes and then loaded to failure. A uniform synthetic material was used to provide a consistent test of each screw design by eliminating variability seen in bone.

RESULTS

Tapping pilot holes did not increase the pullout strength of the screws tested in this study. It was observed during testing that tapping some of the holes degraded the material. This degradation led to pullout strengths that were lower than in the untapped case, and generally larger standard deviations.

CONCLUSIONS

The pullout strength was not increased by tapping for the screws in this study. Screws placed in untapped holes generally had higher pullout strengths and lower standard deviations. The results of this study suggest that tapping does not increase pullout strength in bone with densities near 20 lb/ft3, which correlates with low density cancellous or osteoporotic bone.

Multiobjective optimization of tibial locking screw design using a genetic algorithm: Evaluation of mechanical performance

Breakage or loosening of locking screws may impair fracture fixation or bone healing in locked nailing of tibial fractures. Bending strength and bone holding power, two important design objectives of locking screws, may conflict with each other. The present study used multiobjective optimization with a genetic algorithm to investigate the optimal designs with respect to these two objectives. Three-dimensional finite element models for analyzing bending strength and bone holding power of locking screws were created first. Through use of a Taguchi L25 orthogonal array, two objective functions were developed by least-squares regression analyses. Then, the trade-off solutions between the two objectives known as Pareto optima were explored by a weighted-sum aggregating approach under geometric constraints. The objective functions, reliably reflecting the finite element results, were valid for multiobjective studies. The Pareto fronts of the screws with 4.5-mm and 5.0-mm outer diameters were similar. The "knee" region of the Pareto front, characterized by the fact that a small improvement in either objective will cause a large deterioration in the other objective, might be the favored choice of optimal designs. The commercially available locking screws compared with the Pareto optima were found to be dominated designs and could be improved. In conclusion, the multiobjective optimization with a genetic algorithm was useful for optimization of locking screw design with many variables and conflicting objectives. Choosing an optimal design requires a thorough knowledge of the inherent problems. This method could reduce the time, cost, and labor associated with the screw development process.

Mechanical tests and finite element models for bone holding power of tibial locking screws

OBJECTIVE

To investigate the bone holding power of tibial locking screws.

DESIGN

The bone holding power was assessed by mechanical testing and finite element analysis.

BACKGROUND

Screw loosening might threaten fracture fixation and bone healing.

METHODS

In mechanical tests, six types of different tibial locking screws were inserted into low-density polyurethane foam tubes, which simulated osteoporotic bone. The screws were pushed out of the foam bone by an axial load, and the maximal pushout load was recorded. In finite element analysis, three-dimensional finite element models with a nonlinear contact interface between the screws and the bones were created to simulate the mechanical testing. The total strain energy of the bone and total reaction force of the screws were recorded. The contribution of the design factors was analyzed by the Taguchi method.

RESULTS

In the mechanical tests, foam bone was stripped by the screw threads without screw deformation. The testing results were closely related to those of finite element analysis. The Taguchi analysis showed that the descending order of contribution of the design factors was outer diameter, pitch, half angle, and inner diameter. Root radius and thread width had minimal effects.

CONCLUSIONS

The bone holding power of the screws could be reliably assessed by finite element models, which could analyze the effects of all the design factors independently and were potentially applicable to screws with irregular thread patterns.

Open exchange locked nailing in humeral nonunions after intramedullary nailing

Humeral nonunions after cannulated intramedullary nailing have been difficult to reconstruct. In the current study, 23 consecutive patients were treated by open exchange locked nailing with bone grafting. There were 16 men and seven women with a mean age of 46.2 years. The nonunions followed humeral locked nailing in eight patients, Seidel nailing in 13, and Küntscher nailing in two. The average nonunion time was 14.7 months. The nonunions, located at the proximal (1/3) in four humeri, middle (1/3) in 15, and distal (1/3) in four, were antegrade nailed in 10 and retrograde nailed in 13. Nineteen had 8-mm nails and four had 7-mm nails. Supplementary wiring was used in 19 patients. The average followup was 21.4 months. With one surgery, all but one patient (96%) achieved osseous union in, on average, 16.3 weeks. One patient with chronic renal dialysis had persistent nonunion and an osteolytic supracondylar fracture. Other complications included one postoperative radial nerve palsy, one brachial artery injury, and one wire infection. At followup, all patients with solid union had excellent or satisfactory recovery of shoulder function. The average postoperative Neer score (90.7) was significantly better than the average preoperative score (68.5). Two patients had losses of elbow motion of 10 degrees and 20 degrees, respectively. This study shows that humeral nonunion after cannulated intramedullary nailing can be treated effectively by open exchange locked nailing with bone grafting. Supplementary wiring can compress the nonunion and facilitate bone healing.

Effect of bone diameter and eccentric loading on fatigue life of cortical screws used with interlocking nails

OBJECTIVE

To test the effects of bone diameter and eccentric loading on fatigue life of 2.7-mm-diameter cortical bone screws used for locking a 6-mm-diameter interlocking nail.

SAMPLE POPULATION

Eighteen 2.7-mm-diameter cortical bone screws.

PROCEDURE

A simulated bone model with aluminum tubing and a 6-mm-diameter interlocking nail was used to load screws in cyclic 3-point bending. Group 1 included 6 screws that were centrally loaded within 19-mm-diameter aluminum tubing. Group 2 included 6 screws that were centrally loaded within 31.8-mm-diameter aluminum tubing. Group 3 included 6 screws that were eccentrically loaded (5.5 mm from center) within 31.8-mm-diameter aluminum tubing. The number of cycles until screw failure and the mode of failure were recorded.

RESULTS

An increase in the diameter of the aluminum tubing from 19 to 31.8 mm resulted in a significant decrease in the number of cycles to failure (mean +/- SD, 761,215 +/- 239,853 to 16,941 +/- 2,829 cycles, respectively). Within 31.8-mm tubing, the number of cycles of failure of eccentrically loaded screws (43,068 +/- 14,073 cycles) was significantly greater than that of centrally loaded screws (16,941 +/- 2,829 cycles).

CONCLUSIONS AND CLINICAL RELEVANCE

Within a bone, locking screws are subjected to different loading conditions depending on location (diaphyseal vs metaphyseal). The fatigue life of a locking screw centrally loaded in the metaphyseal region of bone may be shorter than in the diaphysis. Eccentric loading of the locking screw in the metaphysis may help to improve its fatigue life.

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.

Mechanical strength, fatigue life, and failure analysis of two prototypes and five conventional tibial locking screws

OBJECTIVE

To investigate the effects of the design and microstructure on the mechanical strength of tibial locking devices.

DESIGN AND METHODS

The mechanical strength of two prototypes of specially developed locking devices (a both-ends-threaded screw and an unthreaded bolt) was tested and compared with that of five types of commercially available tibial locking screws (Synthes, Howmedica, Richards, Osteo AG, and Zimmer) with similar dimensions. The devices were inserted into a polyethylene tube and loaded at their midpoint by a materials testing machine to simulate a three-point bending test. Single-loading yielding strength and cyclic-loading fatigue life were then measured. Failure analysis of the fractured screws was performed to investigate the microstructure and potential causes of the fatigue fracture.

RESULTS

Test results showed that both yielding strength and fatigue life were closely related to the section modulus of the inner diameter of screws. Among the threaded screws, the both-ends-threaded screws had a higher yielding strength and longer fatigue life than the Osteo AG, Howmedica, Richards, and Zimmer screws. The unthreaded bolts had a lower yielding strength than Synthes screws, but they demonstrated the longest fatigue life among all. In failure analysis of broken screws, no metallurgical or manufacturing defects were found except for surface microimperfections.

CONCLUSIONS

The implants investigated in this study are manufactured with high-quality materials and manufacturing processes. The main cause of hardware failure was mechanical overloading. The five commercially used tibial locking screws had a relatively short fatigue life under high loading. Removing the screw threads might substantially increase the fatigue life of the locking devices. In unthreaded bolts, this increase might be tenfold to a hundredfold.

Bending strength and holding power of a prototype tibial locking screw

To improve mechanical performance, a prototype tibial locking screw with two components was developed for the current study: a both-ends-threaded screw with a smooth shank and a small set screw inserted obliquely through the cap of the both-ends-threaded screw. The bending strength and holding power of this prototype screw were compared with that of five commercially available tibial locking screws: Synthes, Howmedica, Richards, Osteo AG, and Zimmer. To test bending strength, the screws were inserted into a polyethylene tube and loaded at their midpoint to simulate a three-point bending test. Single-loading yielding strength and cyclic-loading fatigue life then were measured. To test holding power, the screws were inserted into polyurethane foam tubes, and stripping torque and pushout strength were measured. The results showed that the yielding strength and the fatigue life were related closely to the inner diameter of the fully threaded screws. The stripping torque reflected the pushout strength, which also was estimated by the formula D[0.5 + 0.57735 (D - d)/2p] (D, outer diameter; d, inner diameter; p, pitch). Even though, among all of the tested screws, the prototype had the smallest outer diameter at its middle, it had the highest fatigue strength, and simultaneously preserved its high bone-holding power. A clinical trial using this prototype screw is warranted, one in which the difficult surgical technique of inserting the set screws should be investigated.

Locked nailing with interfragmentary wiring for humeral nonunions

OBJECTIVE

Locked nailing for humeral nonunions is threatened by residual fracture gap and fracture motion. This article describes the clinical experience of using interfragmentary wiring to solve these problems.

METHODS

Interfragmentary wiring was used in 21 consecutive humeral nonunions treated with humeral locked nails. The average age of patients was 49.5 years, with an average nonunion duration of 14.4 months. Eighteen patients had previous operations. Nonunions were located at the proximal third in 4, the middle third in 10, and the distal third in 7. Antegrade nailing was used in 11 and retrograde nailing in 10. Sixteen nonunions were nailed with 8-mm nails and five with 7-mm nails. Interfragmentary wiring was applied to either the posterior or the lateral cortex of humeri in a figure-of-eight configuration. Bone grafting was performed in all and average follow-up time was 22 months.

RESULTS

With a single operation, all patients achieved osseous union in, on average, 18.3 weeks. One patient with segmental nonunion suffered acute renal failure 4 months after operation, but fracture healing was not affected. Wire infection occurred in one patient with preoperative infection at the nail entry site and was treated by implant removal. Other complications including two transient radial nerve palsies and one brachial artery injury did not affect the final outcome. At follow-up, all but four patients had complete recovery of shoulder flexion and abduction. The average postoperative Neer score (91.1 points) was significantly better than the average preoperative score (65.5 points). All but three patients had complete recovery of elbow motion.

CONCLUSION

Interfragmentary wiring, a safe procedure if properly performed, could effectively decrease the residual fracture gap and fracture motion in locked nailing of humeral nonunions. Further biomechanical studies and prospective, randomized, controlled studies are warranted.