The holding power of orthopedic screws in vivo

Biomechanical comparison of two different compression screws for the treatment of odontoid fractures in human dens axis specimen

BACKGROUND

Lag screw osteosynthesis for odontoid fractures has a high rate of pseudoarthrosis, especially in elderly patients. Besides biomechanical properties of the different screw types, insufficient fragment compression or unnoticed screw stripping may be the main causing factors for this adverse event. The aim of the study was to compare two screws in clinical use with different design principles in terms of compression force and stability against screw stripping.

METHODS

Twelve human cadaveric C2 vertebral bodies were considered. Bone density was determined. The specimens were matched according to bone density and randomly assigned to two experimental groups. An odontoid fracture was induced, which were fixed either with a 3.5 mm standard compression screw or with a 5 mm sleeve nut screw. Both screws are certified for the treatment of odontoid fractures. The bone samples were fixed in a measuring device. The screwdriver was driven mechanically. The tests were analyzed for peak interfragmentary compression and screw-in torque with a frequency of 20 Hz.

FINDINGS

The maximum fragment compression was significantly higher with screw with sleeve nut at 346.13(SD ±72.35) N compared with classic compression screw at 162.68(SD ±114.13) N (p = 0.025). Screw stripping occurred significantly earlier in classic compression screw at 255.5(SD ±192.0)° rotation after reaching maximum compression than in screw with sleeve nut at 1005.2(SD ±341.1)° (p = 0.0039).

INTERPRETATION

Screw with sleeve nut achieves greater fragment compression and is more robust to screw stripping compared to classic compression screw. Whether the better biomechanical properties lead to a reduction of pseudoarthrosis has to be proven in clinical studies.

Biomechanical comparison of compact versus standard flute drill bits, and interlocking versus buttress thread self-tapping cortical bone screws in cadaveric equine third metacarpal condyle

OBJECTIVES

To compare (1) performance of compact versus standard flute drill bits, (2) screw insertion properties and (3) pullout variables between interlocking thread (ITS) and buttress thread (BTS) self-tapping screws in third metacarpi.

STUDY DESIGN

In vitro experimental study.

SAMPLE POPULATION

Paired third metacarpi from 11 Thoroughbreds aged 2-4 years.

METHODS

Screws were inserted into the lateral condylar fossae following bone preparation using the respective drill bit for each screw type. Screw pullout was achieved using a mechanical testing system. Density and porosity of bone surrounding screw holes was measured with microcomputed tomography following each pullout test. Drilling, screw insertion and pullout variables were compared between drill bit and screw types using repeated measures ANOVA. Linear regression analyses were used to characterize relationships between bone tissue properties and drill bit and screw outcomes.

RESULTS

Maximum torque power spectral density (PSD) was lower for compact flute drill bits. Insertion torque was 50% higher for ITS. BTS had 33% greater preyield stiffness and 7% greater mean yield force. Bone tissue properties affected measured variables similarly for both screw and drill bit types.

CONCLUSIONS

Lower torque PSD may increase durability of the compact flute drill bit. ITS had greater insertional torque, which may reflect greater bone engagement. BTS had greater resistance to axial pullout forces.

CLINICAL SIGNIFICANCE

Metacarpal bone provides a simple model for comparison of drill bit and screw designs. Use of ITS to repair equine fractures subject to predominantly tensile forces is not justified based on the results of this study.

DIRECT PEDICLE SCREW INSERTION PULLOUT STRENGTH

Objective:

Study the in vitro pullout strength of SpineGuard/Zavation Dynamic Surgical Guidance Z-Direct Screw (DSG Screw), a screw pedicle designed to be inserted using a direct insertion technique.

Methods:

DSG Screws of 5.5 mm and 6.5 mm were introduced into polyurethane blocks with a density of 10 PCF (0,16 g/cm³). According to the experimental group, screws were inserted without pilot hole, with pilot without tapping, undertapping and line-to-line tapping. Screw pullout tests were performed using a universal test machine after screw insertion into polyurethane blocks.

Results:

Screws inserted directly into the polyurethane blocks without pilot hole and tapping showed a statistically higher pullout strength. Insertion of the screw without tapping or with undertapping increases the pullout screw strength compared to line-to-line tapping.

Conclusion:

DSG Screw showed the highest pullout strength after its insertion without pilot hole and tapping. Level of Evidence V, Expert Opinion.

Miniscrew-assisted rapid palatal expansion (MARPE): how to achieve greater stability. In vitro study

Objective:

Assess the influence of mono- and bicortical anchorage and diameter of mini-implants (MIs) on the primary stability of these devices.

Methods:

60 self-drilling MIs were distributed in six groups according to diameter (1.5mm, 1.8mm or 2.0mm) and type of anchorage (monocortical and bicortical) in bovine rib. The primary stability was evaluated by insertion torque, micromobility and pull-out strength tests. ANOVA and/or Tukey analysis were used to conduct intergroup comparisons (p< 0.05). Non-parametric statistics (Kruskal-Wallis and Mann-Whitney) were performed when normality was not found (p< 0.05).

Results:

MIs with larger diameters and bicortical anchorage showed greater primary stability regarding insertion torque (p< 0.05) and micromobility (p< 0.05). Only MI diameter had an effect on the pull-out strength test. Larger diameter MIs presented better retention in pull-out strength tests (p< 0.001), regardless of mono- or bicortical anchorage.

Conclusions:

MI primary stability is dependent on its diameter and type of anchorage. Bicortical anchorage showed greater stability when compared with monocortical anchorage, independently of other variables.

Biomechanical comparison of three different compression screws for treatment of odontoid fractures evaluation of a new screw design

BACKGROUND

Lag screw osteosynthesis in odontoid fractures shows a high rate of pseudarthrosis. Biomechanical properties may play a role with insufficient fragment compression or unnoticed screw stripping. A biomechanical comparison of different constructed lag-screws was carried out and the biomechanical properties determined.

METHODS

Two identical compression screws with different pilot holes (1.25 and 2.5 mm), a double-threaded screw and one sleeve-nut-screw were tested on artificial bone (Sawbone, densities 10-30pcf). Fragment compression and torque were continuously measured using thin-film force sensors (Flexiforce A201, Tekscan) and torque sensors (PCE-TM 80, PCE GmbH).

FINDINGS

The lowest compression reached the double-threaded screw. Compression and sleeve-nut-screw achieved 214-298% and 325-546%, respectively, of the compression force of double-threaded-screw, depending on the test material. The pilot hole optimization led to a significant improvement in compression only in the densest test material. Screw stripping took place significantly later with increasing density of the test material on all screws. In compression screws this was done at a screw rotation of 180-270°, in sleeve nut screw at 270-720° and in double-threaded screws at 300-600° after reaching the maximum compression.

INTERPRETATION

Double-threaded screw is robust against screw stripping, but achieves only low fragment compression. The classic compression screws achieve better compression, but are sensitive to screw stripping. Sleeve-nut screw is superior in compression and as robust as double-threaded screw against screw stripping. Whether the better biomechanical properties lead to a reduction in pseudarthrosis must be proven in clinical trials.

Biomechanical evaluation of peak reverse torque (PRT) in a dynamic compression plate-screw construct used in a goat tibia segmental defect model

Background

Peak reverse torque (PRT) is a valid method to evaluate implants’ secondary stability in the healing bone. The secondary stability is achieved by the implant over time and it has been positively correlated with the implants’ osseointegration level. In other words, peak reverse torque is the force required to break the bone-implant interface. The purpose of this study was to compare the peak reverse torque for the self-tapping and non-self-tapping screws used in a dynamic compression plate–screw–bone construct after 60 days of loading when used to stabilize 2.5-cm defects in the tibia of goats. The second objective was to compare the peak removal torque of the screws placed in the different positions to evaluate the impact of construct biomechanics on implants osseointegration.

Results

In total, 176 non-self-tapping screws and 66 self-tapping screws were used to fix the 8-holes dynamic compression plates to the bones. The screws were placed in the tibiae from proximal (position sites 1,2, 3) to distal (position sites 4,5,6) and were removed 60 days post-implantation. The animals remained weight-bearing throughout the study period. The screws placed in the proximal diaphysis had significantly less peak reverse torque than screws placed in the distal diaphysis in both groups (p < 0.05). The peak reverse torque resistance was also significantly less for the non-self-tapping screws as compared with the self-tapping screws (p < 0.05). The intracortical fractures in the trans-cortex occurred significantly more frequently during the placement of non-self-tapping screws (p < 0.05) as compared with self-tapping screws (p < 0.05).

Conclusions

Based on these results, we concluded that self-tapping screws may be expected to maintain a more stable bone-implant interface during the first 60 days of loading as compared with non-self-tapping screws. This should be a consideration for orthopedic surgeons and scientists using bone plates to stabilize non-load sharing fractures when a stable plate-screw-bone interface is needed to ensure prolonged stability.

Risk factors for cage migration and cage retropulsion following transforaminal lumbar interbody fusion

BACKGROUND CONTEXT

Transforaminal lumbar interbody fusion (TLIF) is a widely accepted surgical procedure, but cage migration (CM) and cage retropulsion (CR) are associated with poor outcomes.

PURPOSE

This study seeks to identify risk factors associated with these serious events.

STUDY DESIGN

A prospective observational longitudinal study.

PATIENT SAMPLE

Over a 5-year period, 881 lumbar levels in 784 patients were treated using TLIF at three spinal surgery centers.

OUTCOME MEASURES

We evaluated the odds ratio of the risk factors for CM with and without subsidence and CR in multivariate analysis.

METHODS

Our study classified CM into two subgroups: CM without subsidence and CM with subsidence. Cases of spinal canal and/or foramen intrusion of the cage was defined separately as CR. Patient records, operative notes, and radiographs were analyzed for factors potentially related to CM with subsidence, CM without subsidence, and CR.

RESULTS

Of 881 lumbar levels treated with TLIFs, CM without subsidence was observed in 20 (2.3%) and CM with subsidence was observed in 36 (4.1%) patients. Among the CM cases, CR was observed in 17 (17/56, 30.4%). The risk factors of CM without subsidence were osteoporosis (OR 8.73, p < .001) and use of a unilateral single cage (OR 3.57, p < .001). Osteoporosis (OR 5.77, p < .001) and endplate injury (OR 26.87, p < .001) were found to be significant risk factors for CM with subsidence. Risk factors of CR were osteoporosis (OR 7.86, p < .001), pear-shaped disc (OR 8.28, p = .001), endplate injury (OR 18.70, p < .001), unilateral single cage use (OR 4.40, p = .03), and posterior cage position (OR 6.45, p = .04). A difference in overall fusion rates was identified, with a rate of 97.1% (801 of 825) for no CM, 55.0% (11 of 20) for CM without subsidence, 41.7% (15 of 36) for CM with subsidence, and 17.6% (3 of 17) for CR at 1.5 years postoperatively.

CONCLUSIONS

Our results suggest that osteoporosis is a significant risk factor for both CM and CR. In addition, a pear-shaped disc, posterior positioning of the cage, the presence of endplate injury and the use of a single cage were correlated with the CM with and without subsidence and CR.

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.

Role of autologous non-vascularised intramedullary fibular strut graft in humeral shaft nonunions following failed plating

BACKGROUND

Non-union humeral shaft fractures are seen frequently in clinical practice at about 2-10% in conservative management and 30% in surgically operated patients. Osteosynthesis using dynamic compression plate (DCP), intramedullary nailing, locking compression plate (LCP), Ilizarov technique along with bone grafting have been reported previously. In cases of prior failed plate-screw osteosynthesis the resultant osteopenia, cortical defect, bone loss, scalloping around screws and metallosis, make the management of non-union more complicated. Fibular graft as an intramedullary strut is useful in these conditions by increasing screw purchase, union and mechanical stability. This study is a retrospective and prospective follow up of revision plating along with autologous non-vascularised intramedullary fibular strut graft (ANVFG) for humeral non-unions following failed plate osteosynthesis.

MATERIALS AND METHODS

Seventy eight cases of nonunion humeral shaft fractures were managed in our institute between 2008 and 2015. Of these, 57 cases were failed plate osteosynthesis, in which 15 cases were infected and 42 cases were noninfected. Out of the 78 cases, bone grafting was done in 55 cases. Fibular strut graft was used in 22 patients, of which 4 cases were of primary nonunion with osteoporotic bone. Applying the exclusion criteria of infection and inclusion criteria of failed plate osteosynthesis managed with revision plating using either LCP or DCP and ANVFG, 17 cases were studied. The mean age of the patients was 40.11 yrs (range: 26-57 yrs). The mean duration of non-union was 4.43 yrs (range: 0.5-14 yrs). The mean follow-up period was 33.41 months (range: 12-94 months). The average length of fibula was 10.7 cm (range: 6-15 cm). Main outcome measurements included bony union by radiographic assessment and pre- and postoperative functional evaluation using the DASH (Disabilities of the Arm, Shoulder and Hand) score. Results: Sixteen out of 17 fractures united following revision plating and fibular strut grafting. Average time taken for union was 3.5 months (range: 3-5 months). Complications included one each of implant failure with bending, transient radial nerve palsy and transient ulnar nerve palsy. No case had infection, graft site morbidity or peroneal nerve palsy. Functional assessment by DASH score improved from 59.14 (range: 43.6-73.21) preoperatively to 23.39 (range: 8.03-34.2) postoperatively (p = 0.0003). Conclusion: The results of our study indicate that revision plating along with ANVFG is a reliable option in humeral diaphyseal non-unions with failed plate-screw osteosynthesis providing adequate screw purchase, mechanical stability and high chances of union with good functional outcome.

Pullout strength of 2.0 mm cancellous and cortical screws in synthetic bone

OBJECTIVE

To determine whether 2.0 mm cancellous screws are superior to 2.0 mm cortical screws when inserted into cancellous and bicortical bone.

STUDY DESIGN

Biomechanical study.

METHODS

The 2.0 mm cancellous screws and 2.0 mm cortical screws were inserted according to the recommended guidelines in synthetic cancellous and bicortical blocks. Fifteen screw-block constructs per group were tested to failure in axial pullout. Axial pullout strength and yield strength were calculated. Data were analyzed using a one-way ANOVA.

RESULTS

The 2.0 mm cortical screws achieved lower axial pullout strength than 2.0 mm cancellous screws in cancellous blocks. The 2.0 mm cortical screws achieved greater pullout strength than 2.0 mm cancellous screws in bicortical blocks.

CONCLUSION

The 2.0 mm cancellous screws may offer a biomechanical advantage in bone with thin cortices (<1 mm thick), whereas 2.0 mm cortical screws may be preferred in cortical bone with cortices measuring at least 1 mm in thickness.

Screw Stripping After Repeated Cortical Screw Insertion-Can We Trust the Cancellous "Bailout" Screw?

BACKGROUND

During osteosynthesis standard nonlocking cortical screws often require reinsertion, raising concern over possible decrease in their effectiveness. This study aims to quantify that potential loss of fixation with reinsertions as well as examine the ability of a cancellous "bailout screw" to regain insertion torque in a previously stripped screw hole.

METHODS

Four different types of bone surrogates were chosen to represent normal cortical bone, osteoporotic cortical bone, high-density (normal) cancellous bone, and low-density (osteoporotic) cancellous bone; nonlocked 3.5-mm cortical screws were inserted into the predrilled holes 1, 2, 3, 4, or 5 times before being torqued maximally to the point of stripping. A 4.0-mm cancellous "bailout" screw was then placed into the same hole and torqued until stripping. Torque was measured continuously using a torque-measuring screwdriver and maximal insertion torque (MIT) of 3.5 and 4.0 screws before stripping was recorded.

RESULTS

MIT decreased with reinsertion of nonlocked cortical screws. By the third reinsertion in all but the normal bone surrogates, the screws lost approximately one third to one half of their original MIT (50%-71% of original torque). The bailout screw succeeded in restoring the original MIT in the osteoporotic cancellous bone surrogate and the normal cortical bone surrogate. In the normal cancellous and osteoporotic cortical bone surrogates, the bailout screw was only able to restore an average of 50% (range 31%-63%) of the original MIT.

CONCLUSIONS

Screw reinsertion may significantly reduce the MIT of 3.5-mm nonlocked cortical screws. Use of the bailout cancellous screw for a stripped cortical screw should be expected to restore MIT only in normal cortical bone and osteoporotic cancellous bone. In other scenarios, the bailout screw should not be expected to uniformly restore full insertion torque.

Management of humeral fracture nonunion in severe osteoporosis by a combination of locking plating and intramedullary fibular grafting

Nonunion of the humerus in a severely osteoporotic bone is a likely event especially if the fracture is transverse. The management of such a combination is a challenge. Most of the conventional fixation methods are unlikely to succeed as the bone failure precedes implant failure in osteoporosis. The challenge is further compounded in severe osteoporosis when the cortical thickness is affected more severely. We used a combination of an intramedullary fibula with a locking plate in 5 cases. The results show that it may be a good combination in such situations as the bone strength is augmented and the plate pullout is less likely.

Optimizing compression: Comparing eccentric plate holes and external tensioning devices

OBJECTIVE

Elimination of interfragmentary motion in fracture fixation using plates to impart compression and promote primary bone healing through absolute stability has been well described as a reliable and successful method to treat simple transverse and short oblique fracture morphologies. Our hypothesis is that dynamic compression plating augmented by external compression techniques would produce and maintain a significantly greater amount of compression than using the plate alone.

METHODS

Simple transverse diayphyseal fractures were simulated in nine 4th generation composite bone models. A load cell was placed within the transverse fracture osteotomy and stabilized and compressed using either eccentric screw placement in a dynamic compression plate alone or augmented with an opposite segment Verbrugge clamp or articulated tensioning device (ATD) compressing using a screw outside of the plate. Dynamic plate compression was evaluated independently and in conjunction with the external compression techniques. Statistical analyses were carried out using a linear mixed effects model and pairwise comparisons between conditions with a significance set at a P-value <0.05.

RESULTS

Both of the external compression techniques (Verbrugge and ATD) achieved significantly higher compression than the plate compression technique alone with 78% (P<0.001) and 134% (P<0.001) more compression respectively. The measured compression across the osteotomy after screw application and removal of external compression decreases by 17% for the Verbrugge device (P=0.215) and by 22%, after removal of the ATD device (P=0.038). For both techniques, adding additional screws in eccentric (load) position further increases compression.

CONCLUSION

Plate compression is a reliable method for inducing compression across transverse and short oblique fractures. Augmenting plate compression technique with external compression techniques (Verbrugge clamp or ATD) allows for a significantly greater compressive load to be achieved. Compression lost after removal of the external compression device indicates that the maximal compression attainable across a fracture may not be reliably maintained with standard dynamic compression plating techniques.

Residual Hole Orientation After Plate Removal: Effect on the Clavicle

Clavicle fractures account for 2.6% to 4% of all fractures. Surgical stabilization of this type of injury is becoming more common. Anterior inferior plating and superior plating are 2 popular approaches to open reduction and internal fixation. Reports of plate removal have raised concerns about reinjury. The goal of the current study was to determine whether the orientation of screw holes in clavicles after removal of an anterior inferior plate vs a superior plate have different biomechanical effects on stiffness and load to failure. The medial and lateral ends of 28 matched pairs of fresh clavicles were potted. Pilot holes, 2.5 mm in diameter, were drilled and oriented anterior inferiorly or superiorly, simulating those left after removal of a plate for a middle-third fracture. The clavicles underwent dynamic axial compression and 3-point load to failure, replicating forces associated with reinjury. Clavicles with anterior inferior holes had a statistically significant higher median maximal load difference of 139 N compared with those with superior holes (P=.013). Anterior inferior holes showed a statistically significant median increase in stiffness of 16.3 N/mm compared with superior holes (P=.036). Clavicles with anterior inferior holes had a statistically significant increase in median maximal load to failure and an increase in median stiffness compared with those with superior holes. This finding is relevant for patients who undergo hardware removal and return to activities that put them at risk for repeat high-impact injuries to the clavicle.

Effect of pilot hole on biomechanical and in vivo pedicle screw-bone interface

PURPOSE

To experimentally study the influence of pilot hole diameter (smaller than or equal to the internal (core) diameter of the screw) on biomechanical (insertion torque and pullout strength) and histomorphometric parameters of screw-bone interface in the acute phase and 8 weeks after pedicle screw insertion.

METHODS

Fifteen sheep were operated upon and pedicle screws inserted in the L1-L3 pedicles bilaterally. The pilot hole was smaller (2.0 mm) than the internal diameter (core) of the screw on the left side pedicle and equal (2.8 mm) to the internal diameter (core) of the screw on the right side pedicle. Ten animals were sacrificed immediately (five animals were assigned to pullout strength tests and five animals were used for histomorphometric bone-screw interface evaluation). Five animals were sacrificed 8 weeks after pedicle screw insertion for histomorphometric bone-screw interface evaluation.

RESULTS

The insertion torque and pullout strength were significantly greater in pedicle screws inserted into pilot holes smaller than internal (core) diameter of the screw. Histomorphometric evaluation of bone-screw interface showed that the percentage of bone-implant contact, the area of bone inside the screw thread and the area of bone outside the screw thread were significantly higher for pilot holes smaller than the internal (core) diameter of the screw immediately after insertion and after 8 weeks.

CONCLUSION

A pilot diameter smaller than the internal (core) diameter of the screw improved the insertion torque and pullout strength immediately after screw insertion as well the pedicle screw-bone interface contact immediately and 8 weeks after screw placement in sheep with good bone mineral density.

Reduction of pullout strength caused by reinsertion of 3.5-mm cortical screws

INTRODUCTION

Osteosynthesis of the tibia, tibial plafond, and calcaneus is commonly performed with plates and 3.5-mm self-tapping cortical screws. Screw insertion and reinsertion within the same hole in the bone may occur during surgery. Therefore, the purpose of this study was to evaluate the pullout strength of 3.5-mm self-tapping screws with up to 5 re-insertions in the diaphysis of the tibia, metaphysis of the distal tibia, calcaneus, and a polyurethane synthetic bone model.

METHODS

Screws were inserted into a synthetic bone model and 5 pairs of human cadaveric diaphyseal tibiae, distal tibiae, and calcanei. The bone was predrilled, and then 3.5-mm cortical self-tapping 316 L stainless steel screws with a washer were inserted bicortically. Screws were inserted from 1 to 5 times at each location. The screws were grasped and subjected to 5-mm/min tensional force via the biaxial material testing systems machine. Statistical significance was determined using a paired 2-tailed t test.

RESULTS

There was a significant difference in the pullout strength of the tibial diaphysis (1710 ± 550 N), tibial metaphysis (471 ± 266 N), and calcaneus (238 ± 90 N; P < 0.01). The tibial diaphysis pullout strength was 1710 ± 550 N for one insertion differing significantly relative to the groups with 4 (average 1030 ± 543 N, P = 0.004) or 5 (average 364 ± 209 N, P < 0.001) insertions. The tibial metaphyseal pullout strength for the single insertion group was 471 ± 266 N and differed significantly relative to the 3 (P = 0.026), 4 (P = 0.044), and 5 (P = 0.042) insertion groups. The calcaneal pullout strength for the single insertion group was 238 ± 90 N with a significant difference of the 1, 3, and 4, versus the 5 insertion group (P = 0.027, 0.040, and 0.033, respectively). The synthetic bone model pullout strength decreased significantly from the one insertion group relative to all other insertion groups (group 1, 1167 ± 263 N; group 2, 768 ± 199 N; group 3, 694 ± 295 N; group 4, 662 ± 356 N; and group 5, 154 ± 183 N; P < 0.02).

CONCLUSIONS

There is a significant decrease in relative pullout strength of 3.5-mm self-tapping cortical screws when comparing the tibial diaphysis, tibial metaphysis, and calcaneus. There is also a significant decrease in 3.5-mm self-tapping screw pullout strength after repeated reinsertions in the synthetic bone model, mid-shaft tibia, metaphyseal tibia, and calcaneus. We recommend that during osteosynthesis, careful screw insertion, and minimal reinsertion be performed.

A comparative study on screw loosening in osteoporotic lumbar spine fusion between expandable and conventional pedicle screws

INTRODUCTION

The aim of this study is to compare the rate of screw loosening and clinical outcomes of expandable pedicle screws (EPS) with those of conventional pedicle screws (CPS) in patients treated for spinal stenosis (SS) combined with osteoporosis.

METHODS

One hundred and fifty-seven consecutive patients with SS received either EPS fixation (n = 80) or CPS fixation (n = 77) to obtain lumbosacral stabilization. Patients were observed for a minimum of 24 months. Outcome measures included screw loosening, fusion rate, Japanese Orthopaedic Association (JOA) scores and Oswestry disability index (ODI) scoring system, and complications.

RESULTS

In the EPS group, 20 screws became loose (4.1%) in 6 patients (7.5%), and two screws (0.4%) had broken. In the CPS group, 48 screws became loose (12.9%) in 15 patients (19.5%), but no screws were broken. The fusion rate in the EPS group (92.5%) was significantly higher than that of the CPS group (80.5%). The rate of screw loosening in the EPS group (4.1%) was significantly lower than that of the CPS group (12.9%). Six EPS (1.8%) screws were removed. In the EPS group, two screws had broken but without neural complications. Twelve months after surgeries, JOA and ODI scores in the EPS group were significantly improved. There were four cases of dural tears, which healed after corresponding treatment.

CONCLUSIONS

EPS can decrease the risk of screw loosening and achieve better fixation strength and clinical results in osteoporotic lumbar spine fusion.

Experimental in vivo acute and chronic biomechanical and histomorphometrical comparison of self-drilling and self-tapping anterior cervical screws

PURPOSE

Self-drilling screws (SDS) and self-tapping screws (STS) allow for quicker bone insertion and are associated with increased anchorage. This is an experimental in vivo comparison of anterior cervical SDS and STS in the post-insertion acute and chronic phases.

METHODS

Thirty C2-C6 vertebrae from six Santa Inês hair sheep were used. Each screw design was randomly assigned to five of each spinal level. Insertion torque was measured using a torque device. Three animals were killed in each phase. Vertebrae were randomly assigned to pullout tests and histomorphometrical bone-screw interface evaluation (percent screw-bone contact and bone density inside and outside the threaded area). Statistical significance was set at P < 0.05.

RESULTS

SDS insertion torque was greater than STS (P = 0.0001). SDS pullout strength was significantly greater than STS in the acute and chronic phases (P = 0.0001, 0.0003, respectively). SDS percent screw-bone contact and inside area bone density were significantly greater in both phases. No outside area bone density differences were observed in either phase.

CONCLUSIONS

SDS had higher insertion torque and better anchorage than STS in both phases. SDS percent bone-screw contact and inside area bone density were higher in both phases.

Non-vascularised fibular graft as an intramedullary strut for infected non-union of the humerus

PURPOSE. To review outcomes of 7 patients who underwent revision surgery for infected non-union of the humerus using a fibular graft as an intramedullary strut. METHODS. Records of 7 men aged 29 to 59 (mean, 40) years with humeral diaphyseal infected non-union who underwent fixation using a compression plate and a non-vascularised fibular graft as an intramedullary strut were reviewed. The mean number of previous surgeries was 2.7 (range, 2-4). Three of the patients had active draining sinuses previously. Their C-reactive protein levels were normal and tissue cultures negative. The remaining 4 patients had active draining sinuses. They first underwent implant removal and debridement. Tissue cultures confirmed infection in 3 of them. The mean duration between debridement and the index surgery was 5 (range, 3-10) months. RESULTS. The mean length of the fibulae harvested was 13 (range, 12-15) cm. All 7 non-unions healed. The mean time to healing was 5.4 (range, 4-8) months. The mean follow-up period was 15 (range, 13-24) months. All patients had weakness of the extensor hallucis longus, which improved to near normal at month 3. There was no donor-site morbidity. Three patients with active infection at presentation underwent repeat surgery. Two of them had wound washouts, and their non-unions went on to heal successfully; one underwent implant removal after union due to an active sinus. Six of the patients returned to their pre-injury activity level, and one endured a brachial plexus injury. CONCLUSION. Fixation using a compression plate and a non-vascularised fibular graft as an intramedullary strut achieved good outcome for infected non-union of the humerus despite prior multiple failed surgeries.

Advances in bone repair with nanobiomaterials: mini-review

Nanotechnology has emerged to be one of the most powerful engineering approaches in the past half a century. Nanotechnology brought nanomaterials for biomedical use with diverse applications. In the present manuscript we summarize the recent progress in adopting nanobiomaterials for bone healing and repair approaches. We first discuss the use of nanophase surface modification in manipulating metals and ceramics for bone implantation, and then the use of polymers as nanofiber scaffolds in bone repair. Finally we briefly present the potential use of the nanoparticle delivery system as adjunct system in promoting bone regeneration following fracture.

The pin-bone interface in external fixator: a standardized analysis in a sheep osteotomy model

OBJECTIVES

The hypothesis of the study was that the incidence of pin loosening and pin infection would increase, whereas the general stability of the pin-bone interface would decrease with ongoing implantation time. The aim of this study was to analyze the biologic reactions of the bone tissue adjacent to the pin to determine the relationship among the osseous anchorage of pins, the incidence of infections, and the histologic appearance.

METHODS

Three groups of sheep received a tibial osteotomy stabilized by external fixators. The pin-bone interface was analyzed biomechanically, radiologically, microbiologically, and histologically after 3, 6, and 9 weeks.

RESULTS/CONCLUSIONS

Contrary to common opinion, pin anchorage was not altered biomechanically throughout the 9 weeks of the study. This effect might be attributed to an increasing remodeling found in the callus and cortex around the pins and was likely assisted by a strict pin care routine and a low infection rate.

Avaliação da trajetória dos parafusos bicorticais pela técnica de harms e melcher em relação à artéria carótida interna: estudo experimental em cadáveres

OBJETIVO: O objetivo deste trabalho é estudar, em peças anatômicas; a relação entre os parafusos bicorticais pela técnica de Harms e Melcher e a artéria carótida interna. MÉTODOS: Nossa amostra consiste em cinco cadáveres. RESULTADOS: Os resultados encontrados foram: a média da menor distância entre o orifício de saída do parafuso e a borda medial da artéria carótida interna direita foi de 11,55 mm (com variação de 10,05 a 14,23 mm), enquanto do lado esquerdo a média foi de 7,50 mm (variando de 2,75 a 12,42 mm). A média da menor distância entre a borda posterior da artéria carótida interna e a cortical anterior da massa lateral de C1 à direita foi de 4,24 mm (variando de 2,08 a 7,48 mm), enquanto do lado esquerdo a média obtida foi de 2,98 mm (com variação de 1,83 a 3,83 mm). CONCLUSÃO: Os resultados encontrados estão de acordo com os estudos similares existentes na literatura que enfatizam a necessidade de uma avaliação imaginológica criteriosa da posição anatômica da artéria carótida interna antes da utilização de parafusos bicorticais na massa lateral de C1 por via posterior.

EFFECT OF PILOT HOLE TAPPING ON PULLOUT STRENGTH AND INSERTION TORQUE OF DUAL CORE PEDICLE SCREWS

Objective: To evaluate the influence of pilot hole tapping on pullout resistance and insertion torque of pedicle screws with a conical core. Methods: Mechanical tests using a universal testing machine were performed on pedicle screws with a conical core that were inserted into pedicles in the fifth lumbar vertebra of calves. The insertion torque was measured using a torque meter with a capacity of 10 Nm, which was considered to be the highest torque value. The pilot holes were prepared using a probe of external diameter 3.8 mm and tapping of the same dimensions and thread characteristics as the screw. Results: Decreased insertion torque and pullout resistance were observed in the group with prior tapping of the pilot hole. Conclusions: Pilot hole tapping reduced the insertion torque and pullout resistance of pedicle screws with a conical core that had been inserted into the pedicle of the fifth lumbar vertebra of calves.

Self-drilling and self-tapping screws: an ultrastructural study

OBJECTIVE

The aim of the current study was to compare self-drilling and self-tapping screws with regard to bone contact and the production of bone debris using scanning electron microscopy.

MATERIALS AND METHODS

Three New Zealand rabbit calvariae were used. Self-tapping and self-drilling screws were inserted into the outer surface of the skull with and without saline solution irrigation. All screws were 5 mm in length and were inserted until their tips projected through the endosteal side. Sixteen screws were used--8 with a head diameter of 1.5 mm and 8 with 2.0 mm. All self-tapping screws were inserted through a drill bit hole (1.6 mm for 2.0-mm screws and 1.3 mm for 1.5-mm screws).

RESULTS

There was no damage to the screws after insertion. Bone damage occurred when irrigation was not used during the installment of the self-tapping screws. Bone debris formed during the installment of the self-drilling screws, which is considered beneficial.

CONCLUSIONS

Because the insertion of self-drilling screws is performed with manual pressure, irrigation is not essential. Unlike the drilling that occurs with self-tapping screw, the bone debris formed with self-drilling screws is not the result of the heat generated, but rather the result of biologically active bone tissue capable of reacting with the screw and improving its performance. The animal model used proved highly appropriate for comparisons with human beings because the bone structures of the head have the same density and thickness.

Pitch and longitudinal fluting effects on the primary stability of miniscrew implants

OBJECTIVE

To test the hypotheses that pitch and fluting have no effect on the primary stability of miniscrew implants (MSIs).

MATERIALS AND METHODS

Maximum placement torque and pullout strength of experimental MSIs were compared with those of control MSIs with the use of synthetic and cadaver bone. MSIs with 1.00 mm pitch were compared with those with 1.25 mm and 0.75 mm pitch; MSIs with three longitudinal flutes were compared with the same MSIs without flutes. A total of 60 MSIs (15 of each design) were evaluated with synthetic bone; a split-mouth cadaver model was used to compare the three experimental designs against the 1 mm control MSIs (total of 90 MSIs).

RESULTS

The synthetic bone model showed higher placement torque and pullout strength for the 0.75 pitch than for the 1.0 mm and 1.25 mm pitch MSIs, but differences were significant (P < .05) only for pullout strength. The cadaver model showed no significant differences in placement torque or pullout strength associated with pitch. Both synthetic and cadaver bone models showed that MSIs with flutes had significantly (P < .05) higher placement torque and pullout strength. Spearman correlations between placement torque and pullout strength were statistically significant for both synthetic (r = .504) and cadaver (r = .502) bone.

CONCLUSION

Within limits, decreasing MSI pitch increases pullout strength, and fluting increases both placement torque and pullout strength.

Does placing screws off-centre in tubular bone alter their pullout strength?

Screws are used to fix broken bones either directly or through plates. Surgeons sometimes find that a screw they have used is not quite in the centre of the bone but to one side or maybe even the edge. It has been postulated that screws catching the edge of the bone do not give good fixation and may even predispose to fractures. We conducted the present experiment using porcine femora to see if a screw's transverse plane position in the bone made any difference to its pullout strength. 20 cortical screws were inserted into the cortical segments of 5 pig femora (4 screws per femur) using the standard AO technique. The screws were inserted in one of 5 randomly chosen positions-centre, medial off-centre, lateral off-centre, medial edge and lateral edge. The screws were tested to failure in axial pullout using a Losenhausen universal testing machine. We found that 4 of the 8 'edge' screws failed with fractures developing around the screw track during pullout testing. These 4 screws were noted on cross-section to have 100% bone contact with their threads completely embedded in the cortical bone. They also had significantly lower pullout resistance than the 4 'edge' screws without fractures (p=0.05) and the 12 'non-edge' screws (in the central 75% of the bone) (p=0.03). This was most likely due to the associated fractures. There was a statistically significant association between the 'edge' screw position (i.e. within 12.5% of the medial or lateral edge of the bone) and the likelihood of fracture (p=0.000). We conclude that in the transverse plane, cortical screws either on their own or through plates should be inserted in the central three-fourths of the bone. Screws placed outside this zone carry a higher risk of fixation failure due to fractures around the screw track with axial loading.

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.

Influência do macheamento na interface do parafuso e do tecido ósseo na fase imediata pós-implante

OBJETIVO: analisar experimentalmente as alterações agudas da interface entre o osso e o implante nas vértebras cervicais após a realização do macheamento do orifício piloto. MÉTODOS: foram utilizados cinco carneiros da raça Santa Inês deslanados. Na terceira vértebra cervical, foram feitos orifícios de 2,5 mm, de ambos os lados, na parte proximal e distal. No orifício proximal direito, foi realizado o macheamento antes da inserção do parafuso cortical de 3,5 mm (Grupo A); no orifício proximal esquerdo, o parafuso foi inserido sem macheamento (Grupo B); os outros dois orifícios distais foram utilizados como controle (Grupo C). As alterações da interface entre o osso e o implante foram analisadas por meio de estudo histomorfométrico, considerando o número de trabéculas fraturadas entre os filetes de rosca; a maior distância da fratura trabecular transversal provocada pela inserção do parafuso; a maior extensão da fratura trabecular longitudinal provocada pela inserção do parafuso e a linha de contato na interface osso-parafuso. RESULTADOS: o macheamento do orifício piloto provocou alterações da microestrutura do tecido ósseo ao redor do implante quando comparado às alterações produzidas pela inserção do parafuso sem o macheamento ou orifício piloto. A avaliação do contato entre o osso e o implante foi o parâmetro que apresentou diferença estatística na comparação entre a colocação do implante com e sem machemanto. CONCLUSÃO: na comparação com o orifício piloto, todos os parâmetros analisados apresentaram diferença estatística.

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.

Use of intramedullary fibular strut graft: a novel adjunct to plating in the treatment of osteoporotic humeral shaft nonunion

Humeral shaft fractures respond well to conservative treatment and unite without much problem. Since it is uncommon, there is not much discussion regarding the management of nonunion in the literature, and hence this is a challenge to the treating orthopaedic surgeon. Osteoporosis of the fractured bone and stiffness of the surrounding joints compounds the situation further. The Ilizarov fixator, locking compression plate, and vascularised fibular graft are viable options in this scenario but are technically demanding. We used a fibular strut graft for bridging the fracture site in order to enhance the pull-out strength of the screws of the dynamic compression plate. Six patients in the study had successful uneventful union of the fracture at the last follow-up. The fibula is easy to harvest and produces less graft site morbidity. None of the study patients needed additional iliac crest bone grafting. This is the largest reported series of patients with osteoporotic atrophic nonunion of humerus successfully treated solely using the combination of an intramedullary fibular strut graft and dynamic compression plate.

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."

Pullout strength of anterior spinal instrumentation: a product comparison of seven screws in calf vertebral bodies

A lot of new implant devices for spine surgery are coming onto the market, in which vertebral screws play a fundamental role. The new screws developed for surgery of spine deformities have to be compared to established systems. A biomechanical in vitro study was designed to assess the bone-screw interface fixation strength of seven different screws used for correction of scoliosis in spine surgery. The objectives of the current study were twofold: (1) to evaluate the initial strength at the bone-screw interface of newly developed vertebral screws (Universal Spine System II) compared to established systems (product comparison) and (2) to evaluate the influence of screw design, screw diameter, screw length and bone mineral density on pullout strength. Fifty-six calf vertebral bodies were instrumented with seven different screws (USS II anterior 8.0 mm, USS II posterior 6.2 mm, KASS 6.25 mm, USS II anterior 6.2 mm, USS II posterior 5.2 mm, USS 6.0 mm, USS 5.0 mm). Bone mineral density (BMD) was determined by quantitative computed tomography (QCT). Failure in axial pullout was tested using a displacement-controlled universal test machine. USS II anterior 8.0 mm showed higher pullout strength than all other screws. The difference constituted a tendency (P = 0.108) when compared to USS II posterior 6.2 mm (+19%) and was significant in comparison to the other screws (+30 to +55%, P < 0.002). USS II posterior 6.2 mm showed significantly higher pullout strength than USS 5.0 mm (+30%, P = 0.014). The other screws did not differ significantly in pullout strength. Pullout strength correlated significantly with BMD (P = 0.0015) and vertebral body width/screw length (P < 0.001). The newly developed screws for spine surgery (USS II) show higher pullout strength when compared to established systems. Screw design had no significant influence on pullout force in vertebral body screws, but outer diameter of the screw, screw length and BMD are good predictors of pullout resistance.

Long-term tissue response to bioabsorbable poly-L-lactide and metallic screws: an experimental study

Late, clinically manifest, adverse inflammatory reactions have sometimes occurred after the use of slowly degrading bioabsorbable poly-l-lactide (PLLA) devices in clinical series of bone fixation. In this study, long-term tissue response to bioabsorbable fracture fixation screws made of poly-l-lactide and to similar metallic screws in cancellous bone was examined and compared with intact bone. The postoperative evaluation of the rabbit femora was performed by using plain radiography, microradiography, histology, histomorphometry, and oxytetracycline labeling studies. The follow-up times were 36 and 51 months in groups of 15 and 14 rabbits, respectively. A walling-off response by formation of trabecular bone which outlined the screw profile was observed in the PLLA and metallic groups both. Connective tissue between this bone front and the implant was seen only in the PLLA group. There was no difference in the thickness of the layer between the two follow-up groups. Between the surgically handled femora and the intact control bone, there was no statistically significant difference in the amount of trabecular bone. The osteoid formation activity in the tissue-implant interface showed no differences between the groups. However, active osteoblasts were visible only in the PLLA group. The amount of birefringent PLLA material diminished between the 36-month and the 51-month follow-up groups. Within the follow-up times of this study, both the PLLA screws and the metallic screws were rather inert. Also, long-term walling-off was a typical response to both PLLA and metallic screws in cancellous bone. PLLA screws did not evoke any osteostimulatory reaction over the long-term follow-up. The findings clearly demonstrated that the overall degradation process of PLLA was very slow and accompanied by fibrous tissue formation. Macrophage activity seemed to be related to the slow degradation process of PLLA and might be associated with the formation of connective tissue replacing the original implant. The results of this study showed no significant differences between the bioabsorbable PLLA screws and the metallic screws in biocompatibility, and no signs of inflammatory foreign-body reactions occurred during the follow-up.

Effects of pilot hole preparation technique on pedicle screw fixation in different regions of the osteoporotic thoracic and lumbar spine

OBJECT

The authors evaluated the effects of pilot hole preparation technique on insertional torque and axial pullout resistance in osteoporotic thoracic and lumbar vertebrae.

METHODS

Using a probe technique and fluoroscopy, 102 pedicle screws were placed in 51 dual-energy x-ray absorptiometry-proven osteoporotic thoracic and lumbar levels. Screws were inserted using the same-size tapping, one-size-under tapping, or no-tapping technique. Insertional torque and axial pullout resistance were measured. Analysis of variance, Fisher exact test, and regression analysis were performed. Same-size tapping decreased pullout resistance in the lumbar spine. There was no effect on pullout resistance in the thoracic spine. Pullout resistance values were lower for all insertion techniques in the upper thoracic spine. Insertional torque and bone mineral density correlated with pullout resistance in the thoracic and lumbar spine.

CONCLUSIONS

Tapping decreased pedicle screw pullout resistance in the osteoporotic human lumbar spine, although it did not affect pullout strength in the thoracic spine. Tapping decreased insertional torque in upper thoracic levels. Surgeons should optimize overall construct rigidity when placing thoracic pedicle screws in patients with spinal segment osteoporosis.

Spontaneous ejection of a ring sequestrum in a 10-year-old: a case report

A case of a 10-year-old patient with a femur fracture treated with an external fixator who subsequently went on to develop a free-floating ring sequestrum is presented. The patient was largely asymptomatic throughout his course of treatment until seen emergently at 2 years postinjury with local and systemic signs of infection. He was successfully treated with intravenous and oral antibiotics as well as thorough irrigation and debridement. Ring sequestrum is the end point for pin tract infection that is left untreated. When it occurs, a course of antibiotics and thorough irrigation and debridement can lead to a successful outcome.

Pull-out strength of monocortical screws placed in the maxillae and mandibles of dogs

BACKGROUND

Mini-implants can facilitate orthodontic tooth movement by serving as anchors. The purpose of this investigation was to determine whether the pull-out strength of screws in bone varies depending on the site of insertion in the maxilla or the mandible.

MATERIALS

Fifty-six titanium screws (2 mm diameter, 6 mm length, Synthes USA, Monument, Colo) were placed in 4 beagle dogs (14 screws per dog) within 30 minutes after they were killed. The screws were inserted to obtain monocortical anchorage, at predetermined sites in the anterior, middle, and posterior regions of the jaws bilaterally. Two screws were placed in the posterior palate in each dog. The jaws were harvested, and bone blocks, each containing a screw, were prepared for mechanical testing. The bone/screw block was aligned on a custom-made fixture, and the maximum force (F max ) at pullout was recorded. Cortical bone thickness was measured after extraction of the screw. Statistical analyses to test for differences were conducted with ANOVA and Tukey-Kramer tests.

RESULTS

Screws placed in the anterior mandibular region had significantly ( P < .05) lower F max (134.5 +/- 24N, mean +/- SE) than those placed in the posterior mandibular region (388.3 +/- 23.1N). Regression analyses suggested a weak (r = 0.39, P = .02) but significant correlation between F max and cortical bone thickness.

CONCLUSIONS

The bone supporting monocortical screws would most likely withstand immediate loading and support tooth-moving forces.

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.

A Mechanical Comparison of 4.5 mm Narrow and 3.5 mm Broad Plating Systems for Stabilization of Gapped Fracture Models

OBJECTIVE

To evaluate and compare the mechanical properties of 4.5 narrow and 3.5 broad plating systems using their respective cortical and cancellous screws in unstable, central, and eccentric gap fracture models.

STUDY DESIGN

Mechanical evaluation and comparison of 2 dynamic compression plate (DCP) systems.

SAMPLE POPULATION

Eighteen cortical and 30 cancellous gapped fracture models.

METHODS

DCP (4.5 mm narrow, 3.5 mm broad) with their respective cortical screws were applied to cortical bone density polyurethane foam blocks to construct center gap cortical fracture models that were tested in gap closing monotonic 4-point bending. DCP (4.5 mm narrow, 3.5 mm broad) with their respective cancellous screws were applied to cancellous bone density polyurethane foam blocks to construct eccentric gap cancellous fracture models. The cancellous constructs were tested in monotonic gap opening and gap closing cantilever bending and in cyclic axial loading. Univariate and multivariate repeated measures ANOVA were used to compare the maximum loads at failure of the 4.5 mm constructs and 3.5 mm constructs.

RESULTS

The 4.5 mm narrow plating system withstood significantly higher loads at failure than the 3.5 mm broad plating system in 4-point bending (P<.0001) and gap opening cantilever bending (P<.0001). The 4.5 mm system failed in gap closing cantilever bending by plastic deformation of the plate, whereas the 3.5 mm system failed by screw pullout. There was no difference between the 2 systems in cyclic axial loading.

CONCLUSION

Results indicate that the 4.5 mm narrow plating system has a mechanical advantage over the 3.5 mm broad plating system for stabilization of gapped fracture models.

CLINICAL RELEVANCE

The 4.5 mm narrow plating system may be mechanically advantageous compared with the 3.5 mm broad plating system for stabilizing unreconstructed comminuted long bone fractures in large dogs.

The Effect of Divergent Screw Placement on the Initial Strength of Plate-to-Bone Fixation

BACKGROUND

Numerous implants exist that allow screws to be placed at varying angles for lag fixation or to fix additional fragments. This study determined how placing screws at different angles affects fixation strength.

METHODS

Using a bone model, we first investigated the pullout strength of screws inserted at varying angles; then, we studied the strength of plate-bone constructs with end screws placed at divergent angles.

RESULTS

Varying the screw angle from 0 to 10 to 20 degrees progressively weakened the screw pullout resistance. No additional decrease was found in varying the angle further. In contrast, the strength of fixation of plate to bone was higher for constructs with screws placed at 20 or 30 degrees off of perpendicular when tested in gap-open bending and axial compression (all p < 0.05). No such differences were found in torsion.

CONCLUSION

The pullout strength with angled screws is reduced, but this does not translate into reduced strength of the bone-to-plate interface.