Probe versus drill: A biomechanical evaluation of two different pedicle preparation techniques for pedicle screw fixation in human cadaveric osteoporotic spine

BackgroundAim of this biomechanical study was to investigate the anchorage of pedicle screws in osteoporotic vertebrae using two different preparation techniques (probe versus drill-assisted).MethodsTwelve thoracic vertebrae were used for the study. The right and left pedicles of the vertebra were prepared with a thoracic probe or a 3.2 mm drill bit and divided into two groups. A standard titanium (diameter: 5.5 mm, length: 45 mm) pedicle screw was then inserted. All pedicle screws were initially loaded with −25 N to +25 N in the cranio-caudal direction. The load was increased by 5 N every 500 cycles up to a maximum load of 10,000 cycles. Loosening was defined as a displacement of the pedicle screw head of >5 mm. The two groups were compared in terms of maximum number of cycles and maximum force until loosening.FindingsThe pedicle screws prepared with the thoracic probe failed on average after 3819 cycles (SD 3281) and the pedicle screws prepared with the 3.2 mm drill after 3335 cycles (SD 3477). There was no significant difference between the two preparation techniques (P = .797). With regard to the maximum force until loosening, there was also no significant difference between the two techniques (thoracic probe: 61 N (SD 33), 3.2 mm drill bit: 56 N (SD 34), P = .791).InterpretationPreparation of the pedicle screw hole either with a probe or drill bit doesn't seem to have an influence on pedicle screw loosening rates in the osteoporotic spine.     

Impact of lumbar pedicle screw positioning on screw stability - A biomechanical investigation

BackgroundScrew loosening is a major complication following spondylodesis. While several modifications increase screw stability, some, such as screw augmentation, are associated with potential complications; new techniques are needed to minimize the risk of screw loosening without increasing complication rates.Methods13 fresh-frozen human lumbar vertebral bodies (L1 to L5) were dissected. In group 1 (n = 7), pedicle screws were implanted conventionally, while in group 2 (n = 6), the screws were positioned divergent in the sagittal pathway. Screw stability was tested under cyclic axial load; one testing-cycle included 1000 repetitions. The first cycle started with a load of 100 N while the load was increased by +20 N in each following cycle until failure. Failure was defined by either a >5 mm movement of the screw heads or triggering of the switch-off threshold.FindingsAverage number of cycles until failure was increased in group 2 compared with group 1 (12,046 vs 9761 cycles), as was the average load to failure (Fmax 313 N vs 260 N). Overall, in group 2, the number of cycles until screw loosening or failure increased by 23% (p = 0.28), while the required force increased by 20% (p = 0.3). Statistically significant correlation between BMD and increased number of cycles completed as well as with increased load (p < 0.01) could be observed.InterpretationThe results demonstrate, that divergent screw-drift of pairs of screws in the sagittal plane tends to increase stability, especially in vertebral bodies with lower bone density. Moreover, we could demonstrate a correlation between BMD and stability of screw-fixation.     

Cement augmentation as revision strategy for loosened thick-diameter non-fenestrated iliac screws – A biomechanical analysis

BackgroundLoosened/pulled-out iliac screws can be a challenging complication, as revision opportunities are limited because more distal anchorage is impossible. Insertion of thick-diameter screws is an option in cases with loosened thin-diameter screws. However, this is occasionally infeasible in patients with loosened thick-diameter screws. Therefore, this study aimed to biomechanically analyze whether loosened/pulled-out thick iliac screws regain strong anchorage following cement augmentation.MethodsSix human pelvis specimens were dissected to isolate 12 hemipelves, and 9 × 80 mm iliac screws were implanted. The pullout maneuver was performed followed by cement augmentation of the same screw within the same screw hole. After cement hardening, pullout was performed again. Forces needed for pullout and the insertion torque were measured for the 12 iliac screws.FindingsCement augmentation significantly increased stability, with the pullout forces required being as high as 953.23 N (standard deviation [SD] 1070.46 N; median 306.30 N) for the uncemented screws and 2897.53 N (SD 585.83 N; median 2907.81 N) after cement augmentation (p < 0.001). No correlation was found between insertion torques and pullout forces.InterpretationWhile revision of loosened iliac screws can often be achieved using higher diameters, this method is limited by the thickness of the screws already in place. We demonstrated that cement augmentation significantly increases stability even in thick-diameter screws, achieving more pullout stability than before the initial pullout. This method may be a cost-effective and rapid option for revision, even with significant defects and with no thicker-diameter iliac screw being available.     

Augmented screws in angular stable plating of the proximal humerus: What to do when revision is needed?

BackgroundBone cement augmentation of modified cannulated locking screws shows biomechanically and clinically good results for osteoporotic fracture management. Nevertheless, complications need to be considered. Therefore implant removal should be tested for feasibility.MethodsImplant removal was simulated in 7 pairs of osteoporotic cadaveric humeri: During screw removal from an angular stable proximal humerus plate, we measured the maximum torque of 14 augmented screws and the corresponding 14 non-augmented screws on the contralateral humeri. After screw removal, specimens were cut along the screw axes to macroscopically investigate the impact of screw removal on the surrounding bone. In addition, we established a technique for cement removal in cases in which the screw head is obstructed with cement and therefore disables the insertion of the screwdriver.FindingsThe screw extraction torque measurements showed no significant differences between the two groups regarding one screw (screw 4 augmented: 1.52 Nm, SD 0.25 Nm vs. screw 4 non-augmented: 1.80 Nm, SD 0.40 Nm; P = 0.20), whereas torque values for the second screw in the augmented group were lower than in the control group (screw 5 augmented: 0.72 Nm, 0.31 Nm vs. screw 5 non-augmented: 1.42 Nm, 0.52 Nm; P = 0.009). Macroscopy of the bone showed no damage to the trabeculae within the humeral head due to the removal.InterpretationThe removal of cannulated, polymethylmethacrylate-augmented, 2.8 mm titanium screws from an angular stable plate was uncomplicated, without the need for special instruments or increased torque for screw removal. No additional damage was visible at the bone-cement interface.     

The quantity of bone cement influences the anchorage of augmented pedicle screws in the osteoporotic spine: A biomechanical human cadaveric study

Background: The aim of this comparative biomechanical human cadaveric study was to investigate the anchorage of augmented screws with two different volumes of bone cement. For this purpose the effect of cranio-caudal loadings on pedicle screws was evaluated and axial pullout tests were performed.Methods: A total of 50 pedicle screws (25 augmented/25 non-augmented) were instrumented into osteoporotic vertebra of fresh human cadavers. The augmented screws were grounded by two different volumes of bone cement (1.5cm³ vs 4cm³). Biomechanical performance was assessed by performing a cyclic loading protocol (frequency: 3 Hz, load range: 20–200 N, number of cycles: 100,000), followed by axial pullout (13 augmented/11 non-augmented) or by either directly measuring axial pullout strength (12 augmented/12 non-augmented).Findings: The median T-score of the specimens was − 4.25 (range: − 6.38 to − 2.4). Pullout tests with and without cyclic preloading showed significantly increased pullout strength in augmented screws (Fmax: augmented: 1159 N (SD 395 N); non-augmented: 532 N (SD 297 N); p < 0.05). No significant difference in the pullout strength was found concerning the quantity of cement (Fmax (direct pullout): 4.0 cm³: 1463 N (SD 307 N); 1.5 cm³: 1214 N (SD 236 N); p > 0.05). The pullout strength significantly decreased in high-volume augmented screws after cyclic loading (Fmax (4.0 cm³): direct pullout 1463 N (SD 307 N); cyclic preload: 902 N (SD 435 N); p < 0.05).Interpretation: Biomechanical advantages of augmented pedicle screws can also be found after cyclic preload. However, our results indicate that the anchoring stability of high-volume augmented pedicle screws after cyclic loading is disadvantageous compared to moderate augmented screws; thus high-volume augmentation should be avoided.     

Biomechanical evaluation of fixation strength of conventional and expansive pedicle screws with or without calcium based cement augmentation

Background

The expansive pedicle screw was originally developed to be installed in the bone of compromised quality, but there are some concerns whether it can provide enough fixation strength in the spine with osteoporosis or severe osteoporosis.

Methods

Twelve fresh human cadaver spines were stratified into four levels: normal, osteopenia, osteoporosis and severe osteoporosis. The vertebra was bilaterally instrumented with pedicle screws according to four protocols, including conventional pedicle screw without augmentation, expansive pedicle screw without augmentation, conventional screw with augmentation and expansive screw with augmentation. Screw pullout tests were conducted.

Findings

Given the same specimen, the fixation strength of expansive screw was significantly higher than that of the conventional screw. When the same type of screw was used, the fixation strength of the calcium based cement augmented group was stronger than that of the non-augmented group. The pullout strength and stiffness of the expansive screw, augmented conventional screw and augmented expansive screw groups at the osteoporotic level were comparable to those of the conventional pedicle screw group at the osteopenic level. However, under the severely osteoporotic bone environment, the pullout strength of pedicle screw with whatever placement protocol was significantly lower than that of the conventional screw group at the osteopenic level.

Interpretation

Our results demonstrate that (i) the expansive pedicle screw appears feasible and safe in either osteopenic or osteoporotic spine; (ii) calcium based cement augmentation can offer improved initial fixation strength of pedicle screws.; and (iii) no screw placement protocol we examined is efficacious in the bone at the severely osteoporotic level.     

Biomechanical effects of posterior pedicle screw-based instrumentation using titanium versus carbon fiber reinforced PEEK in an osteoporotic spine human cadaver model

BackgroundAim of this biomechanical investigation was to compare the biomechanical effects of a carbon fiber reinforced PEEK and titanium pedicle screw/rod device in osteoporotic human cadaveric spine.MethodsTen human fresh-frozen cadaveric lumbar spines (L1-L5) have been used and were randomized into two groups according to the bone mineral density. A monosegmental posterior instrumentation (L3-L4) using titanium pedicle screws and rods was carried out in group A and using carbon fiber reinforced PEEK in group B. A cyclic loading test was performed at a frequency of 3 Hz, starting with a peak of 500 N for the first 2000 cycles, up to 950 N for 100,000 cycles under a general preload with 100 N. All specimens were evaluated with regard to a potential collapse of the implanted pedicle screws. A CT supported digital measurement of cavities around the pedicle at 3 defined measuring points was performed. Finally, the maximum zero-time failure load of all specimens was determined using a universal testing machine (80% F_max).FindingsRegarding maximum axial force (group A: 2835 N, group B: 3006 N, p = 0.595) and maximum compression (group A: 11.67 mm, group B: 15.15 mm, p = 0.174) no statistical difference could be shown between the two groups. However, significant smaller cavity formation around the pedicle screws could be observed in group B (p = 0.007), especially around the screw tip (p < 0.001).InterpretationCarbon fiber reinforced PEEK devices seem to be advantageous in terms of microscopic screw loosening compared to titanium devices.     

Biomechanical effects of insertion location and bone cement augmentation on the anchoring strength of iliac screw

Background

Iliac screw loosening has been a clinical problem in the lumbo-pelvic reconstruction. Although iliac screws are commonly inserted into either upper or lower iliac column, the biomechanical effects of the two fixations and their revision techniques with bone cement remain undetermined. The purpose of this study was to compare the anchoring strengths of the upper and lower iliac screws with and without cement augmentation.

Methods

5 pairs of formalin fixed cadaveric ilia with the bone mineral density values ranged from 0.82 to 0.97 g/cm² were adopted in this study. Using screws with 70-mm length and 7.5-mm diameter, 2 conventional iliac screw fixations and their revision techniques with cement augmentation were sequentially established and tested on the same ilium as follows: upper screw, upper cement screw, lower screw, and lower cement screw. Following 2000 cyclic compressive loading of − 300 N to −100 N to the screw on a material testing machine, the maximum pull-out strengths were measured and analyzed.

Findings

The average pull-out strengths of upper, upper cement, lower, and lower cement screws were 964 N, 1462 N, 1537 N, and 1964 N, respectively. The lower screw showed significantly higher pull-out strength than the upper one (P = 0.008). The cement augmentation notably increased the pull-out strengths of both upper and lower screws. The positive correlation between pull-out strength and bone mineral density value was obtained for the 4 fixations.

Interpretation

The lower iliac screw technique should be the preferred choice in lumbo-pelvic stabilization surgery; cement augmentation may serve as a useful salvage technique for iliac screw loosening; preoperative evaluation of bone quality is crucial for predicting fixation strength of iliac screw.     

Biomechanical comparison of cervical discectomy/fusion model using allograft spacers between anterior and posterior fixation methods (lateral mass and pedicle screw)

BackgroundThe purpose of this study is to investigate effects of different fixation methods on the physical stress on allospacers, endplate-vertebral body, and implants using finite element model analyses.MethodsStress distribution and subsidence risk according to the fixation methods under the condition of hybrid motion control were analyzed. The detailed finite element model of a previously validated, three-dimensional, intact cervical spinal segment model, with C5–C6 segmental fusion using allospacer, was used to evaluate the biomechanical characteristics of different fixation combinations, such as anterior plate/screws, lateral mass screw, and posterior pedicle screw.FindingsThe load sharing on allospacers increased in extension in order of posterior pedicle screws (21.4%), lateral mass screws (31.5%), and anterior plate/screws (56.6%). lateral mass screw demonstrated the highest load sharing (68.1%) on the allospacer in flexion. The Peak von Mises stress of the allospacer was the lowest in flexion and axial rotation but the highest in extension with anterior plate/screws. Allospacer subsidence risk was the lowest in extension, lateral bending, and axial rotation with posterior pedicle screws but the lowest in flexion with anterior plate/screws. The bone-screw loosening risk was the lowest in all modes with posterior pedicle screws but the highest with anterior plate/screws.InterpretationPosterior pedicle screws demonstrated the best mechanical stability of allospacer failure-subsidence and the lowest risk of screw loosening. Different motion restrictions depending on the fixation method should be considered for implant and allospacer safety.     

Perfusion pressure of a new cannulating fenestrated pedicle screw during cement augmentation

BackgroundCannulating fenestrated pedicle screws are effective for fixating osteoporotic vertebrae. However, a major limitation is the excessive pressure required to inject a sufficient amount of cement into the vertebral body through the narrow hole of a pedicle screw. We have recently proposed a new cannulating fenestrated pedicle screw with a large hole diameter and a matched inner pin for screw-strength maintenance. Our purpose was to determine whether the new screw can significantly reduce bone-cement perfusion pressure during cement augmentation,MethodsTwo different methods were used to examine perfusion pressure. Hagen–Poisseuille's flow model in a tube was used to calculate pressure drop in the bone-cement channel. Experimentally, both Newtonian silicone oil and bone-cement (polymethyl methacrylate) were tested using a cement pusher through the cannulating screw at a constant rate of 2 ml/min.FindingsThe internal hollow portion of the screw was the bottleneck of the perfusion, and the new design significantly reduced the perfusion pressure. Specifically, perfusion pressure dropped by 59% (P < 0.05) when diameter size was doubled.InterpretationThe new design effectively improved the application of bone-cement augmentation with the ease of bone-cement perfusion, thereby enhancing operational safety.     

后凸成形和传统钉道骨水泥强化对骶骨钉松动的生物力学作用（英文）

背景：后凸成形骨水泥强化可应用于骨质疏松患者的腰椎椎弓根钉固定。目的：评价松动的骶骨钉经后凸成形和传统钉道骨水泥强化后的固定强度。方法：纳入9具骨质疏松症患者的新鲜尸体标本。在同一骶骨标本上,分别测试单皮质和双皮质骶骨椎弓根钉最大拔出力后,分别建立传统钉道骨水泥强化与后凸成形骨水泥强化椎弓根钉固定模型。在MTS材料试验机上,对螺钉尾部施加2 000次周期性压力载荷后,进行螺钉最大拔出力测试。结果与结论：9个标本的骨密度均值为0.71 g/cm2（0.61～0.77 g/cm2）。4种骶骨钉固定技术单皮质、双皮质、传统钉道骨水泥强化和后凸成形骨水泥强化骶骨钉的平均最大拔出力分别为203,325,437及565 N。双皮质骶骨钉的拔出力显著高于单皮质钉（P〈0.05）;但此2固定均显著低于骨水泥强化组（P〈0.05）。后凸成形骨水泥强化组的拔出力显著高于传统钉道骨水泥强化组（P〈0.05）。此外,4种骶骨钉固定技术的最大拔出力与骨密度值均呈现显著的正相关（P〈0.05）。结果证实,传统钉道骨水泥强化技术和后凸成形骨水泥强化技术均可做为骶骨椎弓根钉松动的补救手段,但后凸成形骨水泥强化可获得更为坚强的锚定。     

Guidelines for cortical screw versus pedicle screw selection from a fatigued decompressive lumbar laminectomy model show similar stability and less bone mineral density dependency

BackgroundTraditional pedicle screws are the gold standard for lumbar spine fixation; however, cortical screws along the midline cortical bone trajectory may be advantageous when lumbar decompression is required. While biomechanic investigation of both techniques exists, cortical screw performance in a multi-level lumbar laminectomy and fusion model is unknown. Furthermore, longer-term viability of cortical screws following cyclic fatigue has not been investigated.MethodsFourteen human specimens (L1–S1) were divided into cortical and pedicle screw treatment groups. Motion was captured for the following conditions: intact, bilateral posterior fixation (L3–L5), fixation with laminectomy at L3–L5, fixation with laminectomy and transforaminal lumbar interbody fusion at L3–L5 both prior to, and following, simulated in vivo fatigue. Following fatigue, screw pullout force was collected and “effective shear stress” [pullout force/screw surface area] (N/mm²) was calculated; comparisons and correlations were performed.FindingsIn flexion-extension and lateral bending, all operative constructs significantly reduced motion compared to intact (P < 0.05), regardless of pedicle or cortical screws; only posterior fixation with and without laminectomy significantly reduced motion in axial rotation (P < 0.05). Pedicle screws significantly increased average pullout strength (944.2 N vs. 690.2 N, P < 0.05), but not the “effective shear stress” (1.01 N/mm² vs. 1.1 N/mm², P > 0.05).InterpretationIn a posterior laminectomy and fusion model, cortical screws provided equivalent stability to pedicle screw fixation, yet had significantly lower screw pullout force. No differences in “effective shear stress” warrant further investigation of the effect of screw length/diameter in the aforementioned screw trajectories.     

Comparison of trans-cortical and cancellous screws to press fit for acetabular shell fixation in total hip arthroplasty: A cadaveric study

BackgroundTotal hip arthroplasty complications are associated with mechanical loosening of the acetabular component, which may be attributed to the type of fixation used (press fit, trans-cortical screws, cancellous screws). Therefore, the purpose of this study was to compare trans-cortical and cancellous screws to press fit for fixation of the acetabular shell.MethodsFive cadaveric pelvis specimens were hemisected (N = 10) at the sacroiliac joint. Each hemi-pelvis was initially tested with a press fit cup followed by the left and right pairs being randomized to either a cancellous or trans-cortical screw condition. Each fixation was tested by applying a load to a rod inserted into the centre of the acetabular cup at 0.5 mm/s, until failure occurred. The failure force, failure moment, and the rotation angle of the cup at failure were calculated.FindingsThe cups fixated with a trans-cortical screw failed at a significantly greater mean [SD] force (1046.20 [386.52] N). The trans-cortical screws also significantly increased the angle of failure 46.29 (16.90) ° compared to the press-fit cups (6.73 [4.59] °). Finally, there was a significant increase in the failure moment, such that, the trans-cortical condition failed at a mean (SD) moment of 53.75 (16.24) Nm compared to 9.59 (1.85) Nm and 32.15 (18.16) Nm for the press fit and cancellous (p = 0.044) conditions, respectively.InterpretationThe acetabular shells that were fixated with trans-cortical screws provide greater stability compared to the press-fit cups or cancellous screws.     

Influence of additional cement augmentation on endplate stability in circumferential stabilisation of osteoporotic spine fractures

BackgroundAnterior stabilisation of osteoporotic spine fractures is uncommon but necessary in the case of complex vertebral body comminution. The purpose of this study was to investigate the effect of additional cement-augmentation on the endplate stability.MethodsTwelve human cadaveric lumbar spines were divided in two groups: (A) posterior cement-augmented pedicle screw/rod-based instrumentation of L3 to L5, posterior decompression of L4/5 and partial corpectomy of L4 and (B) same experimental setup with additional cement-augmentation of the adjacent endplates. A cyclic loading test was performed at a frequency of 3 Hz, starting with a peak of 500 N for the first 2.000 cycles, up to 950 N for 100.000 cycles under a general preload with 50 N. All specimens were evaluated with regard to a potential collapse of the adjacent endplates. Subsequently, the maximum zero-time failure load of all specimens was determined using a universal testing machine.FindingsThe median T-score of bone density was −4.32 (range −2.97 to −5.59), distributed equally in the two groups (average age 83 years). The specimen of the endplate-augmented group showed a significant higher failure load compared to non-endplate-augmented cadavers (group A: 2038 N, group B: 2990 N, p = 0.03). All specimens passed the full cyclic loading protocol with 100.000 cycles. No significant difference was observable regarding the adjacent endplate subsidence.InterpretationAdditional cement augmentation in circumferential stabilisation resulted in a significant enhancement of the endplate stability regarding the maximum axial load, while the cyclic loading did not significantly enhance the fatigue endurance of the vertebral endplates over the 100,000 cycles tested.     

Does pedicle screw fixation of the subaxial cervical spine provide adequate stabilization in a multilevel vertebral body fracture model? An in vitro biomechanical study

BackgroundCervical vertebral body fractures generally are treated through an anterior-posterior approach. Cervical pedicle screws offer an alternative to circumferential fixation. This biomechanical study quantifies whether cervical pedicle screws alone can restore the stability of a three-column vertebral body fracture, making standard 360° reconstruction unnecessary.MethodsRange of motion (2.0 Nm) in flexion-extension, lateral bending, and axial rotation was tested on 10 cadaveric specimens (five/group) at C2–T1 with a spine kinematics simulator. Specimens were tested for flexibility of intact when a fatigue protocol with instrumentation was used to evaluate construct longevity. For a C4–6 fracture, spines were instrumented with 360° reconstruction (corpectomy spacer + plate + lateral mass screws) (Group 1) or cervical pedicle screw reconstruction (C3 and C7 only) (Group 2).FindingsResults are expressed as percentage of intact (100%). In Group 1, 360° reconstruction resulted in decreased motion during flexion-extension, lateral bending, and axial rotation, to 21.5%, 14.1%, and 48.6%, respectively, following 18,000 cycles of flexion-extension testing. In Group 2, cervical pedicle screw reconstruction led to reduced motion after cyclic flexion-extension testing, to 38.4%, 12.3%, and 51.1% during flexion-extension, lateral bending, and axial rotation, respectively.InterpretationThe 360° stabilization procedure provided the greatest initial stability. Cervical pedicle screw reconstruction resulted in less change in motion following cyclic loading with less variation from specimen to specimen, possibly caused by loosening of the shorter lateral mass screws. Cervical pedicle screw stabilization may be a viable alternative to 360° reconstruction for restoring multilevel vertebral body fracture.     

Non-setting,injectable biomaterials containing particulate hydroxyapatite can increase primary stability of bone screws in cancellous bone

BackgroundFracture fixation in weak bone is still a clinical challenge. Screw augmentation was shown to successfully increase their primary stability. The currently used calcium phosphate or polymeric bone cements, however, present important drawbacks such as induced toxicity and/or impaired bone neo-formation. A new approach to enhance bone screw primary stability without affecting bone formation is the use of non-setting, calcium phosphate loaded soft materials as the augmentation material.MethodsTwo types of biomaterials (non-crosslinked hyaluronic acid as viscous fluid and agar as hydrogel) were loaded with 40 wt/vol% of hydroxyapatite particles and characterized. The screw augmentation effect of all materials was evaluated through pull-out tests in bovine cancellous bone and compared to the non-augmented situation (control). The bone mineral density of each test sample was measured with μCT scans and was used to normalize the pull-out strength.FindingsBoth materials loaded with hydroxyapatite increased the normalized pull-out strength of the screws compared to control samples and particle-free materials. This counter-intuitive augmentation effect increased with decreasing bone mineral density and was independent from the type of the soft materials used.InterpretationWe were able to demonstrate that non-setting, injectable biomaterials loaded with ceramic particles can significantly enhance the primary stability of bone screws. This material combination opens the unique possibility to achieve a screw augmentation effect without impairing or even potentially favoring the bone formation in proximity to the screw. This effect would be particularly advantageous for the treatment of osteoporotic bone fractures requiring a stabilization with bone screws.     

Pullout strength for cannulated pedicle screws with bone cement augmentation in severely osteoporotic bone: Influences of radial hole and pilot hole tapping

Background

Pedicle screw fixation in a severely osteoporotic spine remains a challenge for orthopedic surgeons. The previous literature does not adequately address the effects of radial holes for cannulated screws with cement injection and pilot hole tapping on the bone/screw interfacial strength.

Methods

Specially designed cannulated pedicle screws, with or without radial holes, were installed in tapped and untapped pilot holes and then injected with cement. A uniform synthetic bone (test block) was used to provide a platform for each screw design. Specimens with inserted screws were then tested for axial pullout failure.

Findings

(1) Cannulated screws with cement augmentation significantly increased the pullout strength in comparison to solid screws. Additionally, the amount of cement exuded from the cannulated screws increased with an increasing number of radial holes, leading to an increase in the average ultimate pullout strength for cannulated screws with a large number of radial holes. (2) Radiological examination indicated that the cement was exuded from the most proximal holes at the very beginning of its flow path, whereas no cement exudation was found at the remaining distal holes. (3) Cement exudation from the holes of cannulated screws into the open cell of the test block led to a composite (cement/bone) structure at the area of cement exudation. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, while the composite was well bonded to the screws. This implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. (4) Tapping pilot holes decreased the pullout strength of the screws. Generally, larger standard deviations were found for the tapped cases, implying that untapped cases results are more repeatable than tapped cases results.

Interpretation

Cannulated pedicle screws with radial holes combined with PMMA cement augmentation but without tapping may be a viable clinical option for achieving fixation in severely osteoporotic bone.     

Headless compression screw for horizontal medial malleolus fractures

BackgroundHorizontal medial malleolus fractures are caused by the application of rotational force through the ankle joint in several orientations. Multiple techniques are available for the fixation of medial malleolar fractures.MethodsHorizontal medial malleolus osteotomies were performed in eighteen synthetic distal tibiae and randomized into two fixation groups: 1) two parallel unicortical cancellous screws or 2) two Acutrak 2 headless compression screws. Specimens were subjected to offset axial tension loading. Frontal plane interfragmentary motion was monitored.FindingsThe headless compression group (1699 (SD 947) N/mm) had significantly greater proximal-distal stiffness than the unicortical group (668 (SD 298) N/mm), (P = 0.012). Similarly, the headless compression group (604 (SD 148) N/mm) had significantly greater medial-lateral stiffness than the unicortical group (281 (SD 152) N/mm), (P < 0.001). The force at 2 mm of lateral displacement was significantly greater in the headless compression group (955 (SD 79) N) compared to the unicortical group (679 (SD 198) N), (P = 0.003). At 2 mm of distal displacement, the mean force was higher in the headless compression group (1037 (SD 122) N) compared to the unicortical group (729 (SD 229) N), but the difference was not significant (P = 0.131).InterpretationA headless compression screw construct was significantly stiffer in both the proximal-distal and medial-lateral directions, indicating greater resistance to both axial and shear loading. Additionally, they had significantly greater load at clinical failure based on lateral displacement. The low-profile design of the headless compression screw minimizes soft tissue irritation and reduces need for implant removal.     

后凸成形和传统钉道骨水泥强化对骶骨钉松动的生物力学作用(英文)

背景:后凸成形骨水泥强化可应用于骨质疏松患者的腰椎椎弓根钉固定。目的:评价松动的骶骨钉经后凸成形和传统钉道骨水泥强化后的固定强度。方法:纳入9具骨质疏松症患者的新鲜尸体标本。在同一骶骨标本上,分别测试单皮质和双皮质骶骨椎弓根钉最大拔出力后,分别建立传统钉道骨水泥强化与后凸成形骨水泥强化椎弓根钉固定模型。在MTS材料试验机上,对螺钉尾部施加2 000次周期性压力载荷后,进行螺钉最大拔出力测试。结果与结论:9个标本的骨密度均值为0.71 g/cm2(0.61～0.77 g/cm2)。4种骶骨钉固定技术单皮质、双皮质、传统钉道骨水泥强化和后凸成形骨水泥强化骶骨钉的平均最大拔出力分别为203,325,437及565 N。双皮质骶骨钉的拔出力显著高于单皮质钉(P<0.05);但此2固定均显著低于骨水泥强化组(P<0.05)。后凸成形骨水泥强化组的拔出力显著高于传统钉道骨水泥强化组(P<0.05)。此外,4种骶骨钉固定技术的最大拔出力与骨密度值均呈现显著的正相关(P<0.05)。结果证实,传统钉道骨水泥强化技术和后凸成形骨水泥强化技术均可做为骶骨椎弓根钉松动的补救手段,但后凸成形骨水泥强化可获得更为坚强的锚定。     

Superior cortical screw in osteoporotic lumbar vertebrae: A biomechanics and microstructure-based study

BackgroundOsteoporosis reduces the bone-screw purchase, potentially reducing pullout strength and other biomechanical properties. However, the existing pedicle screw approach may not compensate for the detrimental effects of decreased vertebral bone mineral density.MethodsTwo methods of screw insertion were performed in thirteen cadaveric osteoporotic lumbar vertebrae: Magerl's method in the left pedicle, and superior cortical screw method in the right (its entry point located vertically 3 mm above Magerl's point). Before screw fixations, the pedicle and its corresponding vertebral body were divided into six equal layers from cranial to caudal by performing micro-CT and tested for microstructure properties, such as bone mineral density, trabecular bone volume fraction, trabecular thickness, trabecular separation and trabecular number. Further, pedicle was horizontally divided into three regions and tested. After screw fixations, microstructure properties of the bone surrounding the screws were analyzed. Finally, the screw pullout strength was tested biomechanically.FindingsThe bone structure is denser in the upper third of the pedicle and its corresponding vertebral body. A similar microstructure is seen within the pedicle. This study reveals that the pullout strength is significantly correlated to the bone mineral density, trabecular bone volume fraction and trabecular thickness. Biomechanical test showed pullout strength in the superior cortical screw group with mean 613.3 N (SD 200.4) was 22.4% higher than that in the Magerl group with mean 501.2 N (SD 256.6).InterpretationThe superior cortical screw method can be a reliable alternative, to provide better pullout strength for posterior lumbar instrumentation, especially in osteoporotic patients.