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1.  How pilot-hole size affects bone-screw pullout strength in human cadaveric cancellous bone 
Canadian Journal of Surgery  2005;48(3):207-212.
Screw failure of cancellous bone screws is not uncommon. To compare the effect of varying pilot-hole size on pullout strength of cancellous bone screws in human cadaveric bone, we designed and performed a biomechanical study to allow quantitative analysis.
Three pairs of distal femurs and 4 pairs of proximal tibias from embalmed human cadavers were stabilized in a mould, and the bone cortex was overdrilled. Four sites in a linear transverse plane were randomly assigned, anatomically matched with the paired bone and drilled with either pilot-hole size 3.2 mm or 2.5 mm. The cancellous screw (Synthes noncannulated 4.5-mm shaft, 6.5-mm external diameter) was guided into the pilot hole and pulled on by a test frame (Instron 8874 biaxial servo-hydraulic test frame) with increasing force to the point of failure, and the forces at which failure resulted were compared.
A comparison of 25 anatomically paired sites with a 2-tailed paired t test and Wilcoxon matched-pairs signed rank test indicated significantly stronger pullout strength (p = 0.047 and p = 0.047) of the 2.5-mm compared with the 3.2-mm pilot hole. Subanalysis of the 4 studied locations indicated that 3 supported the above findings and 1 supported a reverse trend.
Generally, cancellous screws demonstrated a significantly (p < 0.05) stronger hold using a smaller size pilot hole than the recommended standard diameter. All locations except the inner lateral site supported this finding.
PMCID: PMC3211539  PMID: 16013624
2.  Biomechanical comparison of pedicle screws versus spinous process screws in C2 vertebra 
Indian Journal of Orthopaedics  2014;48(6):550-554.
Biomechanical studies have shown C2 pedicle screw to be the most robust in insertional torque and pullout strength. However, C2 pedicle screw placement is still technically challenging. Smaller C2 pedicles or medial localization of the vertebral artery may preclude safe C2 pedicle screw placement in some patients. The purpose of this study was to compare the pullout strength of spinous process screws with pedicle screws in the C2.
Materials and Methods:
Eight fresh human cadaveric cervical spine specimens (C2) were harvested and subsequently frozen to −20°C. After being thawed to room temperature, each specimen was debrided of remaining soft tissue and labeled. A customs jig as used to clamp each specimen for screw insertion firmly. Screws were inserted into the vertebral body pairs on each side using one of two methods. The pedicle screws were inserted in usual manner as in previous biomechanical studies. The starting point for spinous process screw insertion was located at the junction of the lamina and the spinous process and the direction of the screw was about 0° caudally in the sagittal plane and about 0° medially in the axial plane. Each vertebrae was held in a customs jig, which was attached to material testing machine (Material Testing System Inc., Changchun, China). A coupling device that fit around the head of the screw was used to pull out each screw at a loading rate of 2 mm/min. The uniaxial load to failure was recorded in Newton'st dependent test (for paired samples) was used to test for significance.
The mean load to failure was 387 N for the special protection scheme and 465 N for the protection scheme without significant difference (t = −0.862, P = 0.403). In all but three instances (38%), the spinous process pullout values exceeded the values for the pedicle screws. The working distances for the spinous process screws was little shorter than pedicle screws in each C2 specimen.
Spinous process screws provide comparable pullout strength to pedicle screws of the C2. Spinous process screws may provide an alternative to pedicle screws fixation, especially with unusual anatomy or stripped screws.
PMCID: PMC4232822  PMID: 25404765
C2 vertebrae; screw fixation; spinous process screws; pedicle screws; Spine; cervical vertebrae; cadaver; bone screws
3.  Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study 
Indian Journal of Orthopaedics  2013;47(3):238-243.
The objective of this cadaveric study was to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of lower thoracic and lumbar pedicle screws. We also investigated the effect of bone mineral density (BMD), diameter of pedicle screws, and the region of spine on the pullout strength of pedicle screws.
Materials and Methods:
Sixty fresh human cadaveric vertebrae (D10–L2) were harvested. Dual-energy X-ray absorptiometry (DEXA) scan of vertebrae was done for BMD. Titanium pedicle screws of different diameters (5.2 and 6.2 mm) were inserted in the thoracic and lumbar segments after dividing the specimens into three groups: a) standard pedicle screw (no cortical perforation); b) screw with medial cortical perforation; and c) screw with lateral cortical perforation. Finally, pullout load of pedicle screws was recorded using INSTRON Universal Testing Machine.
Compared with standard placement, medially misplaced screws had 9.4% greater mean pullout strength and laterally misplaced screws had 47.3% lesser mean pullout strength. The pullout strength of the 6.2 mm pedicle screws was 33% greater than that of the 5.2 mm pedicle screws. The pullout load of pedicle screws in lumbar vertebra was 13.9% greater than that in the thoracic vertebra (P = 0.105), but it was not statistically significant. There was no significant difference between pullout loads of vertebra with different BMD (P = 0.901).
The mean pullout strength was less with lateral misplaced pedicle screws while medial misplaced pedicle screw had more pullout strength. The pullout load of 6.2 mm screws was greater than that of 5.2 mm pedicle screws. No significant correlation was found between bone mineral densities and the pullout strength of vertebra. Similarly, the pullout load of screw placed in thoracic and lumbar vertebrae was not significantly different.
PMCID: PMC3687899  PMID: 23798753
Misplaced pedicle screw; pullout strength; vertebra
4.  Screw fixation to the posterior cortical shell does not influence peak torque and pullout in anterior cervical plating 
European Spine Journal  2002;11(5):494-499.
There is no consensus over whether screw fixation for anterior cervical plating should include the posterior cortical shell of the vertebral bodies or not. Thus, the purpose of this study was to investigate the function of the posterior cortical shell with respect to maximal screw torque and pullout force. Twenty-four fresh frozen human cervical vertebrae coming from six spinal segments C4–C7 were used. They were scanned for bone mineral density (BMD) and then assigned to two groups with comparable bone density and segmental distribution. The posterior longitudinal ligament was resected carefully and two parallel burr holes were drilled into each vertebral body. The posterior cortical shell was removed in one burr hole, using a 6-mm steel burr, producing a shallow excavation with a depth of approximately 2 mm. An ABC screw was inserted into each burr hole. The screw to be inserted into the hole with the posterior excavation was called "monocortical". In contrast, the contralateral screw was called "bicortical". Peak torque was measured in one group, while pullout force was analyzed using the specimens of the second group. Mean value and standard deviation were calculated for peak torque and pullout force with respect to the type of fixation. A paired t-test was used to determine the effect of fixation type on peak torque and pullout force. Pearson moment correlation coefficients were calculated to determine the effect of BMD on peak torque and pullout force with respect to whether the screw was "mono- or bicortical". A 95% level of significance was used for all tests. No significant differences for peak torque and pullout force could be found comparing monocortical and bicortical screw fixation. However, for both monocortical and bicortical screw fixation, a positive correlation was seen for peak torque versus BMD and for pullout force versus bone mineral density, respectively. The importance of the posterior cortical shell for screw pullout force and screw peak torque seems to be negligible. In constrast, BMD greatly influences both peak torque and pullout force for both types of fixation.
PMCID: PMC3611321  PMID: 12384759
Cervical spine Bone Biomechanics In vitro Implant stabilisation Bone mineral density
5.  Tissue Quality Assessment Using a Novel Direct Elasticity Assessment Device (The E-Finger): A Cadaveric Study of Prostatectomy Dissection 
PLoS ONE  2014;9(11):e112872.
Minimally invasive radical prostatectomy (RP) (robotic and laparoscopic), have brought improvements in the outcomes of RP due to improved views and increased degrees of freedom of surgical devices. Robotic and laparoscopic surgeries do not incorporate haptic feedback, which may result in complications secondary to inadequate tissue dissection (causing positive surgical margins, rhabdosphincter damage, etc). We developed a micro-engineered device (6 mm2 sized) [E-finger]) capable of quantitative elasticity assessment, with amplitude ratio, mean ratio and phase lag representing this. The aim was to assess the utility of the device in differentiating peri-prostatic tissue types in order to guide prostate dissection.
Material and Methods
Two embalmed and 2 fresh frozen cadavers were used in the study. Baseline elasticity values were assessed in bladder, prostate and rhabdosphincter of pre-dissected embalmed cadavers using the micro-engineered device. A measurement grid was created to span from the bladder, across the prostate and onto the rhabdosphincter of fresh frozen cadavers to enable a systematic quantitative elasticity assessment of the entire area by 2 independent assessors. Tissue was sectioned along each row of elasticity measurement points, and stained with haematoxylin and eosin (H&E). Image analysis was performed with Image Pro Premier to determine the histology at each measurement point.
Statistically significant differences in elasticity were identified between bladder, prostate and sphincter in both embalmed and fresh frozen cadavers (p = <0.001). Intra-class correlation (ICC) reliability tests showed good reliability (average ICC = 0.851). Sensitivity and specificity for tissue identification was 77% and 70% respectively to a resolution of 6 mm2.
This cadaveric study has evaluated the ability of our elasticity assessment device to differentiate bladder, prostate and rhabdosphincter to a resolution of 6 mm2. The results provide useful data for which to continue to examine the use of elasticity assessment devices for tissue quality assessment with the aim of giving haptic feedback to surgeons performing complex surgery.
PMCID: PMC4226612  PMID: 25384014
6.  In vitro comparison of resistance to implant failure in unstable trochanteric fractures fixed with intramedullary single screw versus double screw device 
Indian Journal of Orthopaedics  2014;48(3):306-312.
The purpose of this study was to compare the resistance of intramedullary single screw device (Gamma nail) and double screw device proximal femoral nail (PFN) in unstable trochanteric fractures in terms of the number of cycles sustained, subsidence and implant failure in an axial loading test in cadaveric femora.
Materials and Methods:
The study was conducted on 18 dry cadaveric femoral specimens, 9 of these were implanted with a Gamma nail and 9 with PFN. There was no significant difference found in average dual energy X-ray absorptiometry value between both groups. The construct was made unstable (AO type 31A3.3) by removing a standard sized posteromedial wedge. These were tested on a cyclic physiological loading machine at 1 cycle/s with a load of 200 kg. The test was observed for 50,000 loading cycles or until implant failure, whichever occurred earlier. Peak displacements were measured and analysis was done to determine construct stiffness and gap micromotion in axial loading.
It was observed that there was statistically significant difference in terms of displacement at the fracture gap and overall construct stiffness of specimens of both groups. PFN construct group showed a mean subsidence of 1.02 mm and Gamma nail construct group showed mean subsidence of 2.36 mm after cycling. The average stiffness of Gamma nail group was 62.8 ± 8.4 N/mm which was significantly lower than average stiffness of the PFN group (80.4 ± 5.9 N/mm) (P = 0.03). In fatigue testing, 1 out of 9 PFN bone construct failed, while 5 of 9 Gamma nail bone construct failed.
When considering micromotion (subsidence) and incidence of implant/screw failure, double screw device (PFN) had statistically significant lower micromotion across the fracture gap with axial compression and lower incidence of implant failure. Hence, double screw device (PFN) construct had higher stability compared to single screw device (GN) in an unstable trochanteric fracture femur model.
PMCID: PMC4052032  PMID: 24932039
Cyclic loading; Gamma nail; proximal femoral nail; subsidence
7.  Pullout strength of pedicle screws with cement augmentation in severe osteoporosis: A comparative study between cannulated screws with cement injection and solid screws with cement pre-filling 
Pedicle screws with PMMA cement augmentation have been shown to significantly improve the fixation strength in a severely osteoporotic spine. However, the efficacy of screw fixation for different cement augmentation techniques, namely solid screws with retrograde cement pre-filling versus cannulated screws with cement injection through perforation, remains unknown. This study aimed to determine the difference in pullout strength between conical and cylindrical screws based on the aforementioned cement augmentation techniques. The potential loss of fixation upon partial screw removal after screw insertion was also examined.
The Taguchi method with an L8 array was employed to determine the significance of design factors. Conical and cylindrical pedicle screws with solid or cannulated designs were installed using two different screw augmentation techniques: solid screws with retrograde cement pre-filling and cannulated screws with cement injection through perforation. Uniform synthetic bones (test block) simulating severe osteoporosis were used to provide a platform for each screw design and cement augmentation technique. Pedicle screws at full insertion and after a 360-degree back-out from full insertion were then tested for axial pullout failure using a mechanical testing machine.
The results revealed the following 1) Regardless of the screw outer geometry (conical or cylindrical), solid screws with retrograde cement pre-filling exhibited significantly higher pullout strength than did cannulated screws with cement injection through perforation (p = 0.0129 for conical screws; p = 0.005 for cylindrical screws). 2) For a given cement augmentation technique (screws without cement augmentation, cannulated screws with cement injection or solid screws with cement pre-filling), no significant difference in pullout strength was found between conical and cylindrical screws (p >0.05). 3) Cement infiltration into the open cell of the test block led to the formation of a cement/bone composite structure. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, whereas the composite remained well bonded to the screws. This result implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. 4) The back-out of the screw by 360 degrees from full insertion did not decrease the pullout strength in any of the studied cases. 5) Generally, larger standard deviations were found for the screw back-out cases, implying that the results of full insertion cases are more repeatable than those of the back-out cases.
Solid screws with retrograde cement pre-filling offer improved initial fixation strength when compared to that of cannulated screws with cement injection through perforation for both the conically and cylindrically shaped screw. Our results also suggest that the fixation screws can be backed out by 360 degrees for intra-operative adjustment without the loss of fixation strength.
PMCID: PMC3224375  PMID: 21284883
8.  Comparison of Bioabsorbable Suture Anchor Fixation on the Tibial Side for Anterior Cruciate Ligament Reconstruction Using Free Soft Tissue Graft: Experimental Laboratory Study on Porcine Bone 
Yonsei Medical Journal  2014;55(3):760-765.
The use of graft tissue fixation using bioabsorbable interference screws (BISs) in anterior cruciate ligament (ACL) reconstruction offers various advantages, but limited pullout strength. Therefore, additional tibial fixation is essential for aggressive rehabilitation. We hypothesized that additional graft tissue fixation using bioabsorbable suture anchors (BSA) would provide sufficient pull-out strength.
Materials and Methods
Twenty four fresh frozen porcine distal femur and patellar tendon preparations were used. All specimens were divided into three groups based on additional fixation methods: A, isolated BIS; B, BIS and BSA; and C, BIS and post cortical screw. Tensile testing was carried out under an axial load. Ultimate failure load and ultimate failure load after cyclic loading were recorded.
The ultimate failure loads after load to failure testing were 166.8 N in group A, 536.4 N in group B, and 438 N in group C; meanwhile, the ultimate failure loads after load to failure testing with cyclic loading were 140 N in group A, 466.5 N in group B, and 400 N in group C. Stiffness after load to failure testing was 16.5 N/mm in group A, 33.5 N/mm in group B, and 40 N/mm in group C. An additional BSA fixation resulted in a significantly higher ultimate failure load and stiffness than isolated BIS fixation, similar to post screw fixation.
Additional fixation using a BSA provided sufficient pullout strength for ACL reconstruction. The ultimate failure load of the BSA technique was similar to that of post cortical screws.
PMCID: PMC3990092  PMID: 24719145
Anterior cruciate ligament reconstruction; graft; tissue fixation; suture anchor
9.  Pullout strength of anterior spinal instrumentation: a product comparison of seven screws in calf vertebral bodies 
European Spine Journal  2007;16(7):1047-1054.
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.
PMCID: PMC2219666  PMID: 17273838
Product comparison; Vertebral body screws; Pullout strength; Bone mineral density; Screw design
10.  The effect of starting point placement technique on thoracic transverse process strength: an ex vivo biomechanical study 
Scoliosis  2010;5:14.
The use of thoracic pedicle screws in spinal deformity, trauma, and tumor reconstruction is becoming more common. Unsuccessful screw placement may require salvage techniques utilizing transverse process hooks. The effect of different starting point placement techniques on the strength of the transverse process has not previously been reported. The purpose of this paper is to determine the biomechanical properties of the thoracic transverse process following various pedicle screw starting point placement techniques.
Forty-seven fresh-frozen human cadaveric thoracic vertebrae from T2 to T9 were disarticulated and matched by bone mineral density (BMD) and transverse process (TP) cross-sectional area. Specimens were randomized to one of four groups: A, control, and three others based on thoracic pedicle screw placement technique; B, straightforward; C, funnel; and D, in-out-in. Initial cortical bone removal for pedicle screw placement was made using a burr at the location on the transverse process or transverse process-laminar junction as published in the original description of each technique. The transverse process was tested measuring load-to-failure simulating a hook in compression mode. Analysis of covariance and Pearson correlation coefficients were used to examine the data.
Technique was a significant predictor of load-to-failure (P = 0.0007). The least squares mean (LS mean) load-to-failure of group A (control) was 377 N, group B (straightforward) 355 N, group C (funnel) 229 N, and group D (in-out-in) 301 N. Significant differences were noted between groups A and C, A and D, B and C, and C and D. BMD (0.925 g/cm2 [range, 0.624-1.301 g/cm2]) was also a significant predictor of load-to-failure, for all specimens grouped together (P < 0.0001) and for each technique (P <0.05). Level and side tested were not found to significantly correlate with load-to-failure.
The residual coronal plane compressive strength of the thoracic transverse process is dependent upon the screw starting point placement technique. The funnel technique significantly weakens transverse processes as compared to the straightforward technique, which does not significantly weaken the transverse process. It is also dependent upon bone mineral density, and low failure loads even in some control specimens suggest limited usefulness of the transverse process for axial compression loading in the osteoporotic thoracic spine.
PMCID: PMC2909925  PMID: 20624320
11.  Effect of bone material properties on effective region in screw-bone model: an experimental and finite element study 
There have been numerous studies conducted to investigate the pullout force of pedicle screws in bone with different material properties. However, fewer studies have investigated the region of effect (RoE), stress distribution and contour pattern of the cancellous bone surrounding the pedicle screw.
Screw pullout experiments were performed from two different foams and the corresponding reaction force was documented for the validation of a computational pedicle screw-foam model based on finite element (FE) methods. After validation, pullout simulations were performed on screw-bone models, with different bone material properties to model three different age groups (<50, 50–75 and >75 years old). At maximum pullout force, the stress distribution and average magnitude of Von Mises stress were documented in the cancellous bone along the distance beyond the outer perimeter pedicle screw. The radius and volume of the RoE were predicted based on the stress distribution.
The screw pullout strengths and the load–displacement curves were comparable between the numerical simulation and experimental tests. The stress distribution of the simulated screw-bone vertebral unit showed that the radius and volume of the RoE varied with the bone material properties. The radii were 4.73 mm, 5.06 mm and 5.4 mm for bone properties of ages >75, 75 > ages >50 and ages <50 years old, respectively, and the corresponding volumes of the RoE were 6.67 mm3, 7.35 mm3 and 8.07 mm3, respectively.
This study demonstrated that there existed a circular effective region surrounding the pedicle screw for stabilization and that this region was sensitive to the bone material characteristics of cancellous bone. The proper amount of injection cement for augmentation could be estimated based on the RoE in the treatment of osteoporosis patients to avoid leakage in spine surgery.
PMCID: PMC4071020  PMID: 24952724
Biomechanics; Pedicle screw; Cancellous bone; Finite element analysis
12.  Non-invasive quantification of lower limb mechanical alignment in flexion 
Computer Aided Surgery  2014;19(4-6):64-70.
Non-invasive navigation techniques have recently been developed to determine mechanical femorotibial alignment (MFTA) in extension. The primary aim of this study was to evaluate the precision and accuracy of an image-free navigation system with new software designed to provide multiple kinematic measurements of the knee. The secondary aim was to test two types of strap material used to attach optical trackers to the lower limb.
Seventy-two registrations were carried out on 6 intact embalmed cadaveric specimens (mean age: 77.8 ± 12 years). A validated fabric strap, bone screws and novel rubber strap were used to secure the passive tracker baseplate for four full experiments with each knee. The MFTA angle was measured under the conditions of no applied stress, valgus stress, and varus stress. These measurements were carried out at full extension and at 30°, 40°, 50° and 60° of flexion. Intraclass correlation coefficients, repeatability coefficients, and limits of agreement (LOA) were used to convey precision and agreement in measuring MFTA with respect to each of the independent variables, i.e., degree of flexion, applied coronal stress, and method of tracker fixation. Based on the current literature, a repeatability coefficient and LOA of ≤3° were deemed acceptable.
The mean fixed flexion for the 6 specimens was 12.8° (range: 6–20°). The mean repeatability coefficient measuring MFTA in extension with screws or fabric strapping of the baseplate was ≤2°, compared to 2.3° using rubber strapping. When flexing the knee, MFTA measurements taken using screws or fabric straps remained precise (repeatability coefficient ≤3°) throughout the tested range of flexion (12.8–60°); however, using rubber straps, the repeatability coefficient was >3° beyond 50° flexion. In general, applying a varus/valgus stress while measuring MFTA decreased precision beyond 40° flexion. Using fabric strapping, excellent repeatability (coefficient ≤2°) was observed until 40° flexion; however, beyond 50° flexion, the repeatability coefficient was >3°. As was the case with precision, agreement between the invasive and non-invasive systems was satisfactory in extension and worsened with flexion. Mean limits of agreement between the invasive and non-invasive system using fabric strapping to assess MFTA were 3° (range: 2.3–3.8°) with no stress applied and 3.9° (range: 2.8–5.2°) with varus and valgus stress. Using rubber strapping, the corresponding values were 4.4° (range: 2.8–8.5°) with no stress applied, 5.5° (range: 3.3–9.0°) with varus stress, and 5.6° (range: 3.3–11.9°) with valgus stress.
Acceptable precision and accuracy may be possible when measuring knee kinematics in early flexion using a non-invasive system; however, we do not believe passive trackers should be mounted with rubber strapping such as was used in this study. Flexing the knee appears to decrease the precision and accuracy of the system. The functions of this new software using image-free navigation technology have many potential clinical applications, including assessment of bony and soft tissue deformity, pre-operative planning, and post-operative evaluation, as well as in further pure research comparing kinematics of the normal and pathological knee.
PMCID: PMC4266097  PMID: 24856249
Image-free navigation; mechanical femorotibial alignment; total knee arthroplasty
13.  Biomechanical Comparison of Different Volar Fracture Fixation Plates for Distal Radius Fractures 
Hand (New York, N.Y.)  2007;3(2):96-101.
The purpose of this study was to compare the biomechanical properties of four volar fixed-angle fracture fixation plate designs in a novel sawbones model as well as in cadavers. Four volar fixed angle plating systems (Hand Innovations DVR-A, Avanta SCS/V, Wright Medical Lo-Con VLS, and Synthes stainless volar locking) were tested on sawbones models using an osteotomy gap model to simulate a distal radius fracture. Based on a power analysis, six plates from each system were tested to failure in axial compression. To simulate loads with physiologic wrist motion, six plates of each type were then tested to failure following 10,000 cycles applying 100N of compression. To compare plate failure behavior, two plates of each type were implanted in cadaver wrists and similar testing applied. All plate constructs were loaded to failure. All failed with in apex volar angulation.The Hand Innovations DVR-A plate demonstrated significantly more strength in peak load to failure and failure after fatigue cycling (p value < 0.001 for single load and fatigue failure). However, there was no significant difference in stiffness among the four plates in synthetic bone. The cadaveric model demonstrated the same mode of failure as the sawbones. None of the volar plates demonstrated screw breakage or pullout, except the tine plate (Avanta SCS/V) with 1 mm of pullout in 2 of 12 plates. This study demonstrates the utility of sawbones in biomechanical testing and indicates that volar fixation of unstable distal radius fractures with a fixed angle device is a reliable means of stabilization.
PMCID: PMC2529136  PMID: 18780084
Distal radius fracture; Biomechanics; Volar plate
14.  Pullout of a lumbar plate with varying screw lengths 
Screw length pertains to stability in various orthopedic fixation devices. There is little or no information on the relationship between plate pullout strength and screw length in anterior lumbar interbody fusion (ALIF) plate constructs in the literature. Such a description may prove useful, especially in the treatment of osteoporotic patients where maximizing construct stability is of utmost importance. Our purpose is to describe the influence of screw length on ALIF plate stability in severely and mildly osteoporotic bone foam models.
Testing was performed on polyurethane foam blocks with densities of 0.08 g/cm3 and 0.16 g/cm3. Four-screw, single-level ALIF plate constructs were secured to the polyurethane foam blocks by use of sets of self-tapping cancellous bone screws that were 20, 24, 28, 32, and 36 mm in length and 6.0 mm in diameter. Plates were pulled out at 1 mm/min to failure, as defined by consistently decreasing load despite increasing displacement.
Pullout loads in 0.08-g/cm3 foam for 20-, 24-, 28-, 32-, and 36-mm screws averaged 303, 388, 479, 586, and 708 N, respectively, increasing at a mean of 25.2 N/mm. In 0.16-g/cm3 foam, pullout loads for 20-, 24-, 28-, 32-, and 36-mm screws averaged 1004, 1335, 1569, 1907, and 2162 N, respectively, increasing at a mean of 72.2 N/mm.
The use of longer screws in ALIF plate installation is expected to increase construct stability. Stabilization from screw length in osteoporotic patients, however, is limited.
PMCID: PMC4300880
ALIF; Lumbar spine; Anterior plate; Pullout; Polyurethane foam; Screw length
15.  Biomechanical effects of hardware configuration after union of proximal femoral and shaft fractures 
Bone & Joint Research  2012;1(6):104-110.
The use of two implants to manage concomitant ipsilateral femoral shaft and proximal femoral fractures has been indicated, but no studies address the relationship of dynamic hip screw (DHS) side plate screws and the intramedullary nail where failure might occur after union. This study compares different implant configurations in order to investigate bridging the gap between the distal DHS and tip of the intramedullary nail.
A total of 29 left synthetic femora were tested in three groups: 1) gapped short nail (GSN); 2) unicortical short nail (USN), differing from GSN by the use of two unicortical bridging screws; and 3) bicortical long nail (BLN), with two angled bicortical and one unicortical bridging screws. With these findings, five matched-pairs of cadaveric femora were tested in two groups: 1) unicortical long nail (ULN), with a longer nail than USN and three bridging unicortical screws; and 2) BLN. Specimens were axially loaded to 22.7 kg (50 lb), and internally rotated 90°/sec until failure.
For synthetic femora, a difference was detected between GSN and BLN in energy to failure (p = 0.04) and torque at failure (p = 0.02), but not between USN and other groups for energy to failure (vs GSN, p = 0.71; vs BLN, p = 0.19) and torque at failure (vs GSN, p = 0.55; vs BLN, p = 0.15). For cadaveric femora, ULN and BLN performed similarly because of the improvement provided by the bridging screws.
Our study shows that bicortical angled screws in the DHS side plate are superior to no screws at all in this model and loading scenario, and suggests that adding unicortical screws to a gapped construct is probably beneficial.
PMCID: PMC3626196  PMID: 23610679
Dynamic hip screw; Intramedullary nail; Femur fracture; Femoral neck; Intertrochanteric; Biomechanical
16.  Comparison and prediction of pullout strength of conical and cylindrical pedicle screws within synthetic bone 
This study was designed to derive the theoretical formulae to predict the pullout strength of pedicle screws with an inconstant outer and/or inner diameter distribution (conical screws). For the transpedicular fixation, one of the failure modes is the screw loosening from the vertebral bone. Hence, various kinds of pedicle screws have been evaluated to measure the pullout strength using synthetic and cadaveric bone as specimens. In the literature, the Chapman's formula has been widely proposed to predict the pullout strength of screws with constant outer and inner diameters (cylindrical screws).
This study formulated the pullout strength of the conical and cylindrical screws as the functions of material, screw, and surgery factors. The predicted pullout strength of each screw was compared to the experimentally measured data. Synthetic bones were used to standardize the material properties of the specimen and provide observation of the loosening mechanism of the bone/screw construct.
The predicted data from the new formulae were better correlated with the mean pullout strength of both the cylindrical and conical screws within an average error of 5.0% and R2 = 0.93. On the other hand, the average error and R2 value of the literature formula were as high as -32.3% and -0.26, respectively.
The pullout strength of the pedicle screws was the functions of bone strength, screw design, and pilot hole. The close correlation between the measured and predicted pullout strength validated the value of the new formulae, so as avoid repeating experimental tests.
PMCID: PMC2694760  PMID: 19402917
17.  Biomechanical comparison of different combinations of hook and screw in one spine motion unit - an experiment in porcine model 
The biomechanical performance of the hooks and screws in spinal posterior instrumentation is not well-characterized. Screw-bone interface failure at the uppermost and lowermost vertebrae is not uncommon. Some have advocated for the use of supplement hooks to prevent screw loosening. However, studies describing methods for combined hook and screw systems that fully address the benefits of these systems are lacking. Thus, the choice of which implant to use in a given case is often based solely on a surgeon’s experience instead of on the biomechanical features and advantages of each device.
We conducted a biomechanical comparison of devices instrumented with different combinations of hooks and screws. Thirty-six fresh low thoracic porcine spines were assigned to three groups (12 per group) according to the configuration used for of fixation: (1) pedicle screw; (2) lamina hook and (3) combination of pedicle screw and lamina hook. Axial pullout tests backward on transverse plane in the direction normal to the rods were performed using a material testing machine and a specially designed grip with self-aligned function.
The pullout force for the pedicle screws group was significantly greater than for the hooks and the combination (p < 0.05). However, no significant difference was found between the hooks and the combination (p > 0.05).
Pedicle screws achieve the maximal pullout strength for spinal posterior instrumentation.
PMCID: PMC4062888  PMID: 24913189
Pedicle screw; Lamina hook; Biomechanical study; Porcine model
18.  Augmentation of tibial plateau fractures with Trabecular Metal™: a biomechanical study 
Restoration and maintenance of the plateau surface are the key points in the treatment of tibial plateau fractures. Any deformity of the articular surface jeopardizes the future of the knee by causing osteoarthritis and axis deviation. The purpose of this study is to evaluate the effect of Trabecular Metal (porous tantalum metal) on stability and strength of fracture repair in the central depression tibial plateau fracture.
Six matched pairs of fresh frozen human cadaveric tibias were fractured and randomly assigned to be treated with either the standard of treatment (impacted cancellous bone graft stabilized by two 4.5 mm screws under the comminuted articular surface) or the experimental method (the same screws supporting a 2 cm diameter Trabecular Metal (TM) disc placed under the comminuted articular surface). Each tibia was tested on a MTS machine simulating immediate postoperative load transmission with 500 Newton for 10,000 cycles and then loaded to failure to determine the ultimate strength of the construct.
The trabecular metal construct showed 40% less caudad displacement of the articular surface (1, 32 ± 0.1 mm vs. 0, 80 ± 0.1 mm) in cyclic loading (p < 0.05). Its mechanical failure occurred at a mean of 3275 N compared to 2650 N for the standard of care construct (p < 0, 05).
The current study shows the biomechanical superiority of the trabecular metal construct compared to the current standard of treatment with regards to both its resistance to caudad displacement of the articular surface in cyclic loading and its strength at load to failure.
PMCID: PMC2758832  PMID: 19772649
19.  Assessment of different screw augmentation techniques and screw designs in osteoporotic spines 
European Spine Journal  2008;17(11):1462-1469.
This is an experimental study on human cadaver spines. The objective of this study is to compare the pullout forces between three screw augmentation methods and two different screw designs. Surgical interventions of patients with osteoporosis increase following the epidemiological development. Biomechanically the pedicle provides the strongest screw fixation in healthy bone, whereas in osteoporosis all areas of the vertebra are affected by the disease. This explains the high screw failure rates in those patients. Therefore PMMA augmentation of screws is often mandatory. This study involved investigation of the pullout forces of augmented transpedicular screws in five human lumbar spines (L1–L4). Each spine was treated with four different methods: non-augmented unperforated (solid) screw, perforated screw with vertebroplasty augmentation, solid screw with vertebroplasty augmentation and solid screw with balloon kyphoplasty augmentation. Screws were augmented with Polymethylmethacrylate (PMMA). The pullout forces were measured for each treatment with an Instron testing device. The bone mineral density was measured for each vertebra with Micro-CT. The statistical analysis was performed with a two-sided independent student t test. Forty screws (10 per group and level) were inserted. The vertebroplasty-augmented screws showed a significant higher pullout force (mean 918.5 N, P = 0.001) than control (mean 51 N), the balloon kyphoplasty group did not improve the pullout force significantly (mean 781 N, P > 0.05). However, leakage occurred in some cases treated with perforated screws. All spines showed osteoporosis on Micro-CT. Vertebroplasty-augmented screws, augmentation of perforated screws and balloon kyphoplasty augmented screws show higher pullout resistance than non-augmented screws. Significant higher pullout forces were only reached in the vertebroplasty augmented vertebra. The perforated screw design led to epidural leakage due to the position of the perforation in the screw. The position of the most proximal perforation is critical, depending on screw design and proper insertion depth. Nevertheless, using a properly designed perforated screw will facilitate augmentation and instrumentation in osteoporotic spines.
PMCID: PMC2583182  PMID: 18781342
Osteoporosis; Screw augmentation; Perforated screws; Vertebroplasty; Balloon kyphoplasty; Transpedicular fixation/stabilization
20.  Balancing Rigidity and Safety of Pedicle Screw Fixation via a Novel Expansion Mechanism in a Severely Osteoporotic Model 
BioMed Research International  2015;2015:318405.
Many successful attempts to increase pullout strength of pedicle screws in osteoporotic bone have been accompanied with an increased risk of catastrophic damage to the patient. To avoid this, a single-armed expansive pedicle screw was designed to increase fixation strength while controlling postfailure damage away from the nerves surrounding the pedicle. The screw was then subsequently tested in two severely osteoporotic models: one representing trabecular bone (with and without the presence of polymethylmethacrylate) and the other representing a combination of trabecular and cortical bone. Maximum pullout strength, stiffness, energy to failure, energy to removal, and size of the resulting block damage were statistically compared among conditions. While expandable pedicle screws produced maximum pullout forces less than or comparable to standard screws, they required a higher amount of energy to be fully removed from both models. Furthermore, damage to the cortical layer in the composite test blocks was smaller in all measured directions for tests involving expandable pedicle screws than those involving standard pedicle screws. This indicates that while initial fixation may not differ in the presence of cortical bone, the expandable pedicle screw offers an increased level of postfailure stability and safety to patients awaiting revision surgery.
PMCID: PMC4331393
21.  Pullout Strength of Suture Anchors: Effect of Mechanical Properties of Trabecular Bone 
Journal of biomechanics  2010;43(6):1138-1145.
This study investigated the relationships between trabecular microstructure and elastic modulus, compressive strength, and suture anchor pullout strength. Twelve fresh-frozen humeri underwent mechanical testing followed by micro-computed tomography (μCT). Either compression testing of cylindrical bone samples or pullout testing using an Arthrex 5 mm Corkscrew was performed in synthetic sawbone or at specific locations in the humerus such as the greater tuberosity, lesser tuberosity, and humeral head. Synthetic sawbone underwent identical mechanical testing and μCT analysis. Bone volume fraction (BVF), structural model index (SMI), trabecular thickness (TbTh), trabecular spacing (TbSp), trabecular number (TbN), and connectivity density were compared against modulus, compressive strength, and pullout strength in both materials. In cadaveric bone, modulus showed correlations to all of the microstructural properties, while compressive and pullout strength were only correlated to BVF, SMI, and TbSp. The microstructure of synthetic bone differed from cadaveric bone as SMI and TbTh showed little variation across the densities tested. Therefore, SMI and TbTh were the only microstructural properties that did not show correlations to the mechanical properties tested in synthetic bone. This study helps identify key microstructure-property relationships in cadaveric and synthetic bone as well as illustrate the similarities and differences between cadaveric and synthetic bone as biomechanical test materials.
PMCID: PMC2849904  PMID: 20117785
Suture Anchors; Pullout Strength; Micro-CT; Microstructure; Synthetic Bone
22.  Defining the impaction frequency and threshold force required for femoral impaction grafting in revision hip arthroplasty 
Acta Orthopaedica  2011;82(4):433-437.
Background and purpose
The two most common complications of femoral impaction bone grafting are femoral fracture and massive implant subsidence. We investigated fracture forces and implant subsidence rates in embalmed human femurs undergoing impaction grafting. The study consisted of two arms, the first examining the force at which femoral fracture occurs in the embalmed human femur, and the second examining whether significant graft implant/subsidence occurs following impaction at a set force at two different impaction frequencies.
Using a standardized impaction grafting technique with modifications, an initial group of 17 femurs underwent complete destructive impaction testing, allowing sequentially increased, controlled impaction forces to be applied until femoral fracture occurred. A second group of 8 femurs underwent impaction bone grafting at constant force, at an impaction frequency of 1 Hz or 10 Hz. An Exeter stem was cemented into the neomedullary canals. These constructs underwent subsidence testing simulating the first 2 months of postoperative weight bearing.
No femurs fractured below an impaction force of 0.5 kN. 15/17 of the femurs fractured at or above 1.6 kN of applied force. In the second group of 8 femurs, all of which underwent femoral impaction grafting at 1.6 kN, there was no correlation between implant subsidence and frequency of impaction. Average subsidence was 3.2 (1–9) mm.
It is possible to calculate a force below which no fracture occurs in the embalmed human femur undergoing impaction grafting. Higher impaction frequency at constant force did not reduce rates of implant subsidence in this experiment.
PMCID: PMC3237033  PMID: 21689068
23.  Bioabsorbable Tricalcium Phosphate Bone Cement Strengthens Fixation of Suture Anchors 
Failure of suture anchor fixation in rotator cuff repair can occur at different interfaces. Prior studies show fixation at the bone-anchor interface can be augmented using polymethylmethacrylate (PMMA) cement, and screw fixation into bone can be strengthened using bioabsorbable tricalcium phosphate cement.
We wished to determine whether augmentation of suture anchor fixation using bioabsorbable tricalcium phosphate cement would increase pullout strength of suture anchors from bone and the number of cycles to failure, to determine the mode of failure after cement augmentation, and to compare strength and mode of failure with those after augmentation with PMMA.
We used 10 matched pairs of cadaveric proximal humeri and implanted a metal screw-type suture anchor in one side and on the other side injected tricalcium phosphate cement into the anchor holes before anchor placement. We tested all specimens to failure using a ramped cyclic loading protocol.
Tricalcium phosphate cement augmentation increased the final load to failure by 29% and the number of cycles to failure by 20%. Visual inspection confirmed that failure occurred at the cement-bone interface.
Tricalcium phosphate cement appears to augment suture anchor fixation into bone, reducing the risk of anchor pullout and failure.
Clinical Relevance
When relying on suture anchor fixation in bone of questionable quality, we suggest considering augmentation of suture anchor fixation with bioabsorbable cement. This method also provides potential for bioabsorbability and may be more amenable to arthroscopic application.
PMCID: PMC2974857  PMID: 20521128
24.  A biomechanical study of conventional acetabular internal fracture fixation versus locking plate fixation 
Canadian Journal of Surgery  2009;52(3):221-228.
Conventional internal fixation entails the use of an interfragmentary lag screw along with a plate. Not all acetabular fractures are amenable to the placement of an interfragmentary lag screw, and the fracture may be displaced during tightening of the interfragmentary lag screw. Locking plates are a possible solution. We sought to determine whether a locking plate construct can provide stability equivalent to that provided with a conventional construct for transverse acetabular fractures.
We used 5 paired fresh-frozen cadaveric acetabula. We fixed one side with the conventional technique and the other side with a locking plate. We subjected each fixation to a cyclic compressive force up to 500 cycles, followed by compressive force until failure. We monitored 3-dimensional motion of the fracture.
The average fracture gap at 50 N compressive force after 500 loading cycles was 0.41 (standard deviation [SD] 0.49) mm for the conventional plate and lag screw construct compared with 0.76 (SD 0.62) mm for the locked plate construct (p = 0.46). The force to failure, as defined by 2 mm of fracture gap, was 848 (SD 805) N for the conventional plate and lag screw construct compared with 506 (SD 277) N for the locked plate fixation (p = 0.34).
The locking plate construct is as strong as the conventional plate plus interfragmentary lag screw construct for fixing transverse acetabular fractures. Locking plates may improve management of acetabular fractures by eliminating the need for placement of an interfragmentary lag screw. Furthermore, they may be helpful in revision hip arthroplasty in patients with pelvic discontinuity.
PMCID: PMC2689727  PMID: 19503667
25.  Cortical Margining Capabilities of Fins Associated with Ventral Cervical Spine Instrumentation 
Yonsei Medical Journal  2005;46(3):372-378.
Fins incorporated into the design of a dynamic cervical spine implant have been employed to enhance axial load-bearing ability, yet their true biomechanical advantages, if any, have not been defined. Therefore, the goal of this study was to assess the biomechanical and axial load-bearing contributions of the fin components of the DOC ventral cervical stabilization system. Eighteen fresh cadaveric thoracic vertebrae (T1-T3) were obtained. Three test conditions were devised and studied: Condition A (DOC implants with fins were placed against the superior endplate and bone screws were not inserted); Condition B (DOC implant without fins was placed and bone screws were inserted); and Condition C (DOC implant with fins were placed against the superior endplate and bone screws were inserted). Specimens were tested by applying a pure axial compressive load to the superior platform of the DOC construct, and load-displacement data were collected. Condition C specimens had the greatest stiffness (459 ± 80 N/mm) and yield load (526±168 N). Condition A specimens were the least stiff (266±53 N/mm), and had the smallest yield loads (180±54 N). The yield load of condition A plus condition B was approximately equal to that of condition C, with condition A contributing about one-third and condition B contributing two-thirds of the overall load-bearing capacity. Although the screws alone contributed to a substantial portion of axial load-bearing ability, the addition of the fins further increased load-bearing capabilities.
PMCID: PMC2815814  PMID: 15988809
Biomechanics; cervical plate; fins

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