Pullout strength variance among self-tapping screws inserted to different depths

The cortical self-tapping screw (STS) has replaced the non-STS as an aid in fracture fixation. In a recent biomechanical investigation, Berkowitz and colleagues found that STS pullout strength increased with insertion depth up to 1 mm past the far cortex only. In the present study, we wanted to apply a standardized protocol of assessing pullout strength to STSs of different compositions and manufacturers while eliminating the sample-size and block-variance issues that affected the previous investigation. Ninety STSs were randomly divided into 5 groups, each representing a different insertion depth. Peak force was determined with trials ending in screw pullout or failure. A statistically significant difference in pullout strength was identified with insertion depths up to 1 mm past the far cortex. No block variance was detected. These results support the recommendation that STSs be inserted only 1 mm past the far cortex in healthy cortical bone.     

Pedicle screw fixation strength: pullout versus insertional torque.

BACKGROUND CONTEXT: Researchers studying early pedicle screw designs have suggested that pullout strength and insertional torque are correlated. For surgeons using pedicle screws, insertional torque is widely believed to be a good predictor of pullout strength and initial stability of the screw and construct. How appropriate is this assumption when applied to new screw and thread designs? PURPOSE: This study investigated the correlation between insertional torque and pullout strength of three different pedicle screw designs, with different insertional torque characteristics. We hypothesized that a significant increase in insertional torque would indicate a commensurate increase in pullout strength. STUDY DESIGN: Biomechanical analysis of instrumented vertebral specimens. METHODS: Calf lumbar vertebra were prepared and instrumented with one of three pedicle screws. Pilot hole preparation was standardized and coaxial orientation was confirmed by direct inspection. Screws did not penetrate the pedicle cortex or abut or penetrate the anterior vertebral cortex. Any specimen with pedicle wall breach was discarded. The pedicles were instrumented with one of three screws: 1) 7.5 x 40 mm conical, asymmetric progressive thread (Xia; Stryker Spine, Allendale, NJ), 2) 7.5 x 40 mm conical with traditional V-shaped thread (Osteonics, Stryker Spine, Allendale, NJ)) or 3) 6.5 x 40 mm cylindrical with V thread (Osteonics, cylindrical). Paired testing allowed individual screws to be directly compared with a contralateral "control." Insertional torque and peak torque values were recorded for each rotation up to full insertion. Pullout testing was conducted at a rate of 1 mm/minute. Load-displacement data were recorded at 20 Hz. Stiffness was considered the slope of the most linear part of the curve before the yield point. RESULTS: Peak loads for 7.5 conical Xia screws measured 1,783+/-589.1 N compared with 1,943.0+/-625.8 N for 7.5 conical Osteonics screws and 1,641.0+/-356.7 N for 6.5 cylindrical Osteonics screws. The peak insertional torque values were 6.7+/-1.9 Nm (158% greater than control), 4.5+/-1.1 Nm (73% greater than control) and 2.6+/-0.7 Nm, respectively. Insertional torques for Xia screws were significantly greater than conical (p=.001) and cylindrical Osteonics screws (p<.0001), and insertional torques for Osteonics conical screws were significantly greater than those of cylindrical screws (p<.0001). Although pullout loads for the conical Osteonics screws were consistently higher than either the Xia or Osteonics cylindrical screws, the differences were not significant (p>.05). There was no significant correlation between pullout strength and insertional torque (p>.05). CONCLUSIONS: This unexpected result is best explained by the progressively narrowing flutes of the Xia screw, which compact the trabeculae into a smaller volume as the screw nears full insertion. The trapezoidal threads also increase contact with the cortical surface area and compress trabeculae toward the cortex, thus creating greater friction and higher torque values. This increase in torque did not translate into a commensurate increase in pullout strength, where trabeculae fail in shear.     

Biomechanical evaluation of the pullout strength of cervical screws

OBJECTIVE: In the process of anterior cervical fusion, little is known about the biomechanics of anterior cervical screw pullout. In this study, three different aspects of cervical screw fixation were evaluated: self-tapping (ST) versus self-drilling (SD) screws, the effect of screw geometry (length, diameter, thread pitch), and the use of rescue screws. METHODS: Nine screws consisting of different diameters, lengths, and thread pitch (cancellous and cortical) were tested in peak pullout force in an artificial bone model using an MTS 858 Mini Bionix test system. Rescue screws (4.5 mm) were then inserted in the failed holes of 4.0-mm screws and extracted to determine their holding strength. RESULTS: Length of screws and thread pitch both had a significant effect on the pullout force. Each 1 mm of increased screw length translates to 16 N of increased force to pullout in the foam bone model. Pullout strength did not vary significantly according to screw diameter or between SD and ST screws. However, the SD screw has an advantage because it can decrease the length of surgery. A decrease in pullout force of between 43% and 70% was found when using rescue screws. CONCLUSIONS: In situations in which the use of rescue/salvage screws is required, the surgeon should anticipate a significant decrease in the holding force compared with the original screw. Future directions for research include an evaluation of pullout force for screw and plate constructs.     

Biomechanical study on pullout strength of thoracic extrapedicular screw fixation

Objective:To identify the biomechanical feasibility of the thoracic extrapedicular approach to the placement of screws. Methods:Five fresh adult cadaveric thoracic spine from T1 to T8 were harvested. The screw was inserted either by pedicular approach or extrapedicular approach. The result was observed and the pullout strength by pedicular screw approach and extrapedicular screw approach via sagittal axis of the vertebrale was measured and compared statistically. Results:In thoracic pedicular approach, the pullout strength of pedicle screw was 1001.23 N±220 N (288.2-1561.7 N) and that of thoracic extrapedicular screw approach was 827.01 N±260 N when screw was inserted into the vertebrae through transverse process,and 954.25 N±254 N when screw was inserted into the vertebrae through the lateral cortex of the pedicle. Compared with pedicular group, the pullout strength in extrapedicular group was decreased by 4.7% inserted through transverse process (P>0.05) and by 17.3% inserted through the lateral cortex (P<0.05). The mean pullout strength by extrapedicular approach was decreased by 11.04% as compared with pedicular approach (P<0.05). Conclusions:It is feasible biomechanically to use extrapedicular screw technique to insert pedicular screws in the thoracic spine when it is hard to insert by pedicular approach.     

Evaluation of a new method of small fragment fixation in a medial malleolus fracture model

OBJECTIVE: To evaluate a new method of small fragment fixation in a medial malleolus fracture model. DESIGN/METHODS: The authors measured the pullout strength, resistance to shear stress, and speed of insertion of 4.0-millimeter partially threaded cancellous screws, 2.4-millimeter smooth K-wires, and a small fragment fixation system with 2.2-millimeter threaded K-wires. Pullout strength was tested in eighty-one synthetic foam blocks and resistance to shear stress in thirty synthetic tibias by use of a servohydraulic testing machine. Six randomized time trials with the threaded K-wires and cancellous screws were also conducted. RESULTS: Pullout strength increased with increasing foam density, increasing insertion depth, and varied with fixation method (p < 0.05). Maximum pullout strengths were as follows: partially threaded cancellous screws, 730+/-4 Newtons; threaded K-wires, 316+/-12 Newtons; and smooth K-wires, 172 +/-5 Newtons. Percent difference in pullout strength between the partially threaded cancellous screw and threaded K-wire diminished with increased depth of insertion and increased foam density. Offset axial load to initiate fracture displacement in a synthetic tibia model averaged 1540+/-138 Newtons for the partially threaded cancellous screws, 1,318+/-117 Newtons for the threaded K-wires, and 1,287+/-121 Newtons for the smooth K-wires (p > 0.05). Average time of fixation of a medial malleolar fragment by orthopedic surgeons with a variety of experience levels in a synthetic tibia with two threaded K-wires (114+/-8 seconds) was significantly faster (p < 0.05) than with two partially threaded cancellous screws (207+/-20 seconds). CONCLUSIONS: Threaded K-wires show substantial pullout strength and similar resistance to offset axial load when compared with partially threaded cancellous screws. These threaded K-wires offer an alternative for the internal fixation of medial malleolus fractures.     

Stability of pedicle screws after kyphoplasty augmentation: an experimental study to compare transpedicular screw fixation in soft and cured kyphoplasty cement

OBJECTIVE: The goal of this cadaver study was to compare the stability of pedicle screws after implantation in soft or cured kyphoplasty cement. METHODS: Pedicle screws were inserted in a total of 30 thoracolumbar vertebrae of 10 different human specimens: 10 screws were implanted in nonaugmented vertebrae (group 1), each 10 screws were placed in soft (group 2) and cured (group 3) cement. Pedicle screws were than evaluated for biomechanical axial pullout resistance. RESULTS: Mean axial pullout strength was 232 N (range 60-600 N) in group 1, 452 N (range 60-1125 N) in group 2 and 367 N (range 112-840 N) in group 3. The paired Student t-test demonstrated a significant difference between pullout strength of groups 1 and 2 (P = 0.0300). Between pullout strength of groups 1 and 3 and between groups 2 and 3 no significant difference was seen. CONCLUSION: We achieved a 1.9 times higher pullout strength with kyphoplasty augmentation of osteoporotic vertebrae compared with the pullout strength of nonaugmented vertebrae. Implantation of pedicle screws in cured cement is a sufficient method. With this method we found a 1.6 times higher pullout strength then in nonaugmented vertebrae.     

Mechanical and structural characteristics of the new BONE-LOK cortical-cancellous internal fixation device

The purpose of this study was to evaluate the structural and mechanical characteristics of a new and unique titanium cortical-cancellous helical compression anchor with BONE-LOK (Triage Medical, Inc., Irvine, CA) technology for compressive internal fixation of fractures and osteotomies. This device provides fixation through the use of a distal helical anchor and a proximal retentive collar that are united by an axially movable pin (U.S. and international patents issued and pending). The helical compression anchor (2.7-mm diameter) was compared with 3.0-mm diameter titanium cancellous screws (Synthes, Paoli, PA) for pullout strength and compression in 7# and 12# synthetic rigid polyurethane foam (simulated bone matrix), and for 3-point bending stiffness. The following results (mean +/- standard deviation) were obtained: foam block pullout strength in 12# foam: 2.7-mm helical compression anchor 70 +/- 2.0 N and 3.0-mm titanium cancellous screws 37 +/- 11 N; in 7# foam: 2.7-mm helical compression anchor 33 +/- 3 N and 3.0-mm titanium cancellous screws 31 +/- 12 N. Three-point bending stiffness, 2.7-mm helical compression anchor 988 +/- 68 N/mm and 3.0-mm titanium cancellous screws 845 +/- 88 N/mm. Compression strength testing in 12# foam: 2.7-mm helical compression anchor 70.8 +/- 4.8 N and 3.0-mm titanium cancellous screws 23.0 +/- 3.1 N, in 7# foam: 2.7-mm helical compression anchor 42.6 +/- 3.2 N and 3.0-mm titanium cancellous screws 10.4 +/- 0.9 N. Results showed greater pullout strength, 3-point bending stiffness, and compression strength for the 2.7-mm helical compression anchor as compared with the 3.0-mm titanium cancellous screws in these testing models. This difference represents a distinct advantage in the new device that warrants further in vivo testing.     

Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out

BACKGROUND CONTEXT: Biomechanical studies show that bone-mineral density, pedicle morphology, and screw thread area affect pedicle screw pullout failure. The current literature is based on studies of cylindrical pedicle screw designs. Conical screws have been introduced that may provide better "fit and fill" of the dorsal pedicle as well as improved resistance to screw bending failure. However, there is concern about loss of fixation if conical screws must be backed out after insertion. PURPOSE: To determine that conical screws have comparable initial stiffness and fixation strength compared with standard, cylindrical screws, and to assess whether conical screw fixation deteriorates when screws are backed out from full insertion. STUDY DESIGN/SETTING: This biomechanical analysis compared pullout strength of cylindrical and conical pedicle screw designs, using porcine lumbar vertebrae in a paired testing format. METHODS: Porcine lumbar vertebrae were instrumented with conical and cylindrical pedicle screws with the same thread pitch, area and contour, and an equivalent diameter at the pedicle isthmus, 1.2 cm distal to the hub. Axial pullout was performed at 1.0 mm/minute displacement. Pullout loads, work and stiffness were recorded at 0.02-second intervals. Conical versus cylindrical screws were tested using three paired control configurations: fully inserted, backed out 180 degrees and backed out 360 degrees. Fully inserted values were compared with each set of back-out values to determine relative loss of fixation strength. Screw pullout data were analyzed using a Student's t test. RESULTS: Pullout loads in these porcine specimens were comparable to data from healthy human vertebrae. Conical screws provided a 17% increase in the pullout strength compared with cylindrical screws (P<.10) and a 50% increase in initial stiffness (P<.05) at full insertion. There was no loss in pullout strength, stiffness or work to failure when conical or cylindrical screws were backed out 180 or 360 degrees from full insertion. CONCLUSIONS: Conical screws offer improved initial fixation strength compared with cylindrical screws of the same size and thread design. Our results suggest that appropriately designed conical screws can be backed out 180 to 360 degrees for intraoperative adjustment without loss of pullout strength, stiffness or work to failure. Intraoperative adjustments of these specific conical screws less than 360 degrees should not affect initial fixation strength. These results may not hold true for screws with a smaller thread area or larger minor diameter.     

Improving the pullout strength of pedicle screws by screw coupling

The objective of this study was to determine the effect of pedicle screw coupling on the pullout strength of pedicle screws in the osteoporotic spine. The vertebral bone mineral density (BMD) of 33 cadaveric lumbar vertebrae were measured by quantitative computed tomography. Pedicle screws were inserted into each pedicle. The pullout strength and displacement of the screws, without coupling and with single or double couplers, were studied, and the relationship between pullout strength and BMD was analyzed. The average pullout strength of the pedicle screws without screw coupling was 909.3 +/- 188.6 N (n = 9), that coupled with a single coupler was 1,409.0 +/- 469.1 N (n = 9), and that with double couplers was 1,494.0 +/- 691.6 N (n = 9). The pullout strength of the screws coupled with single or double couplers was significantly greater than that of screws without couplers (p < 0.01); however, there was no significant difference between the groups of single and double couplers. The improvement of pullout strength by screw coupling was significant in a test group with BMD of more than 90 mg/ml (p < 0.01), but was not in the group with BMD less than 90 mg/ml (p = 0.55). These results suggest that the coupling of pedicle screws improves pullout strength; however, the effect tends to be less significant in severely osteoporotic spines.     

Screw pull-out force is dependent on screw orientation in an anterior cervical plate construct

Two common justifications for orienting cervical screws in an angled direction is to increase pull-out strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed versus variable angle systems may offer strength advantages. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pull-out load. Variable and fixed angle 4.0 x 15 mm and 4.0 x 13 mm self-tapping screws were used to affix a Medtronic Atlantis cervical plate to polyurethane foam bone samples (density 0.160/cm). This synthetic product is a model of osteoporotic cancellous bone. The fixed angle screws can only be placed at 12 degrees convergent to the midline and 12 degrees in the cephalad/caudal ("12 degrees up and in") direction. Three groups were tested: (1) all fixed angle screws, (2) variable angle, all screws 12 degrees up and in, (3) variable angle, all screws 90 degrees to the plate. Plate constructs were pulled off with an Instron DynaMight 8841 servohydrolic machine measuring for maximum screw pull-out force. There was no difference between group 1, fixed angle (288.4 +/- 37.7 N) (mean +/- SD) and 2, variable angle group (297.7 +/- 41.31 N P< or =0.73). There was a significant increase in maximum pull-out force to failure for the construct with all screws at 90 degrees (415.2+/-17.4 N) compared with all screws 12 degrees "up and in" (297.4 +/- 41.3 N, P< or =0.0016). Group 3 done with 13 mm screws, showed a trend toward better pull-out strength, compared to group 2 w/15 mm screws (345.2 +/- 20.5 vs. 297.4 +/- 41.3, P< or =0.06). In this plate pull-out model, screw orientation influences maximum force to failure. When all 4 screws are 90 degrees to the plate the construct has the greatest ability to resist pullout. Fixed angle designs show no advantage over variable angle. These findings are contrary to current teaching.     

Screw orientation and plate type (variable- vs. fixed-angle) effect strength of fixation for in vitro biomechanical testing of the Synthes CSLP

BACKGROUND CONTEXT: Two common justifications for orienting cervical screws in an angled direction are to increase pullout strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed- versus variable-angle systems may offer strength advantages. Despite these teachings, there is a paucity of supporting biomechanical evidence. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pullout force. PURPOSE: This study evaluates the effect of screw orientation and plate type (fixed- vs. variable-angle) on screw pullout strength. STUDY DESIGN: Anterior cervical plates of both a fixed- and variable-angle CSLP, were tested for peak pullout strength in a direct plate pullout model using polyurethane foam bone, which models osteoporotic bone. METHODS: Self-tapping, locking screws (4.0x14mm and 4.0x16mm) were used. Screws were oriented in the fixed-angle plate in the standard fashion. In the variable plate, screws were instrumented in three different orientations. Biomechanical testing was performed on an Instron DynaMight 8841 servohydraulic testing machine, measuring maximum pullout force under a displacement-controlled pullout rate of 1mm/min. Five samples were tested per group. RESULTS: When all screws were placed 90 degrees to the plate, there was a significantly increased peak pullout strength (412.8+/-22.2N) compared with when all screws were placed 12 degrees "up and in" (376.2+/-24.3N, p less than or equal to .03). When the 90 degrees construct was reproduced using 14-mm screws and compared with 16-mm screws oriented 12 degrees "all up and in," there was still significantly higher pullout strength with the all 90 degrees construct (434.2+/-28.7N vs. 376.2+/-24.3N, p less than or equal to .009). The fixed-angle plate had a significantly decreased peak pullout strength (288.2+/-15.7N) compared with the variable-angle plate (416.6+/-12.6N) (p less than .00001) when the screws were placed in the same orientation. Overall, the variable-angle plate, regardless of the orientation of screws, had a significantly greater pullout strength than the fixed-angle plate (p less than .001). CONCLUSIONS: In this system, a variable-angle plate has greater pullout strength than a fixed-angle plate, regardless of the orientation of screws. With the variable-angle plate, a construct of all screws 12 degrees "up and in" is the weakest configuration. We found that with the 90 degrees construct, both 16- and 14-mm screws performed significantly better than 16-mm convergent screws. These findings are remarkable because they contradict the current doctrine. This may be a function of plate-dependent factors and should not be applied universally to all plate systems. Further study of screw orientation in additional plating systems is warranted.     

Mechanical performance of cylindrical and dual-core pedicle screws after repeated insertion

Study Design: Ex vivo study of the mechanical performance of cylindrical and dual-core pedicle screws after insertion, removal, and reinsertion in the same hole. Objective: To evaluate the effect of repeated use of same screw hole on the insertion torque and the retentive strength of the cylindrical and dual-core screws. Summary of Background Data: Insertion and removal of pedicle screws is sometimes necessary during surgical procedure to assess the integrity of the pilot-hole wall. However, this maneuver may compromise the implant-holding capacity. Methods: Sixty thoracolombar vertebrae (T13–L5), harvested from 10 healthy calves, were used to insert 2 different designs of pedicle screws: cylindrical (5.0-mm outer diameter) and dual-core screws (5.2-mm outer diameter). Three experimental groups were created on the basis of the number of insertions of the screws and 2 subgroups were established according to the core pedicle screw design (dual-core and cylindrical). The insertion torque was measured during initial insertion, second insertion, and third insertion. Pullout screw tests were performed using a universal testing machine to evaluate the pullout strength after initial insertion, second insertion, and third insertion. Results: Significant reductions of 38% in mean insertion torque and 30% in mean pullout strength of dual-core screw were observed between the initial insertion and the third insertion. The cylindrical screw observed significant reductions of 52.5% in mean insertion torque and 42.3% in mean pullout strength between the initial insertion and the third insertion. A reduction of mean insertion torque and pullout strength between the first insertion and the second insertion but without significance was also observed for both types of screws. Conclusion: Insertions and reinsertion of either cylindrical or dual-core pedicle screws have compromised insertion torque and pullout strength of the implants as measured by mechanical tests.     

Effect of screw torque level on cortical bone pullout strength

OBJECTIVES: The objectives of this study were 2-fold: (1) to perform detailed analysis of cortical screw tightening stiffness during automated insertion, and (2) to determine the effect of 3 torque levels on the holding strength of the bone surrounding the screw threads as assessed by screw pullout. METHODS: Ten pairs of ovine tibiae were used with 3 test sites spaced 20 mm apart centered along the shaft. One side of each pair was used for measuring ultimate failure torque (Tmax). These Tmax and bone-density values were used to predict Tmax at contralateral tibia sites. Screws were inserted and tightened to 50%, 70%, and 90% of predicted Tmax at the contralateral sites to encompass the average clinical level of torque (86% Tmax). Pullout tests were performed and maximum force values were normalized by cortical thickness. RESULTS: Torque to failure tests indicated tightening to 86% Tmax occurs after yield and leads to an average 51% loss in stiffness. Normalized pullout strength for screws tightened to 50% Tmax, 70% Tmax, and 90% Tmax were 2525 +/- 244, 2707 +/- 280, and 2344 +/- 346 N, respectively, with a significant difference between 70% Tmax and 90% Tmax groups (P < 0.05). CONCLUSIONS: Within the limitations of our study involving the testing of 1 type of screw purchase in ovine tibiae, results demonstrate that clinical levels of lag screw tightening (86% Tmax) are past the yield point of bone. Tightening to these high torque levels can cause damage leading to compromised holding strength. Further research is still required to establish the appropriate level of torque required for achieving optimal fracture fixation and healing.     

Augmentation of anterior vertebral body screw fixation by an injectable, biodegradable calcium phosphate bone substitute.

STUDY DESIGN: A biomechanical study to evaluate the effects of a biodegradable calcium phosphate (Ca-P) bone substitute on the fixation strength and bending rigidity of vertebral body screws. OBJECTIVES: To determine if an injectable, biodegradable Ca-P bone substitute provides significant augmentation of anterior vertebral screw fixation in the osteoporotic spine. SUMMARY OF BACKGROUND DATA: Polymethylmethacrylate (PMMA) augmented screws have been used clinically; however, there is concern about thermal damage to the neural elements during polymerization of the PMMA as well as its negative effects on bone remodeling. Injectable, biodegradable Ca-P bone substitutes have shown enhanced fixation of pedicle screws. METHODS: Sixteen fresh cadaveric thoracolumbar vertebrae were randomly divided into two groups: control (no augmentation) (n = 8) and Ca-P bone substitute augmentation (n = 8) groups. Bone-screw fixation rigidity in bending was determined initially and after 10(5) cycles, followed by pullout testing of the screw to failure to determine pullout strength and stiffness. RESULTS: The bone-screw bending rigidity for the Ca-P bone substitute group was significantly greater than the control group, initially (58%) and after cyclic loading (125%). The pullout strength for Ca-P bone substitute group (1848 +/- 166 N) was significantly greater than the control group (665 +/- 92 N) (P < 0.01). Stiffness in pullout for the Ca-P bone substitute groups (399 +/- 69 N/mm) was significantly higher than the control group (210 +/- 51 N/mm) (P < 0.01). CONCLUSION: This study demonstrated that augmentation of anterior vertebral body screw fixation with a biodegradable Ca-P bone substitute is a potential alternative to the use of PMMA cement.     

Pullout resistance of conventional pedicle screw implantation in comparison to fluoroscopic computer-assisted technique

AIM: Aim of the study was to compare pullout resistance of pedicle screws after conventional and fluoroscopic computer-assisted implantation in the cadaveric thoracic and lumbar spine. METHODS: Pedicle screws were inserted in a total of 10 vertebrae of different human specimens: 10 screws were placed using conventional technique (group 1) and 10 screws were inserted with fluoroscopic computer-assisted system contralaterally (group 2). Then pedicle screws were evaluated for biomechanical axial pullout resistance. RESULTS: Mean pullout force was 232 N (range 60-600 N) in group 1 and 353 N (range 112-625 N) in group 2. The difference was significant (p=0,0425). CONCLUSION: Fluoroscopic navigated implantation of pedicle screws increases the pullout strength in thoracic and lumbar cadaveric spines as compared with conventional methods.     

Biomechanical evaluation of patellar- and hamstring tendon graft fixation for anterior cruciat ligament reconstruction using a poly-(D, L-lactide) interference screw

Anterior cruciate ligament (ACL) reconstruction using autologous hamstring tendons are being performed more frequently and satisfactory results have been reported. Advantages such as low donor site morbidity and ease of harvest as well as disadvantages like low initial construct stiffness have been described. Recently, it has been demonstrated that graft fixation close to the original ACL insertion sites increases anterior knee stability and graft isometry. Hamstring tendon fixation techniques using interference screws offer this possibility. To reduce the risk of graft laceration, a round threaded titanium interference screw (RCI) was developed. To improve initial fixation strength, fixation techniques for hamstring tendons with separate or attached tibial bone plugs were introduced. However, data on fixation strength do not yet exist. With respect to the proposed advantages of biodegradable implants, like undistorted magnetic resonance imaging, uncompromised revision surgery and a decreased potential of graft laceration during screw insertion, we performed pullout tests of round threaded biodegradable and round threaded titanium interference screw fixation of semitendinosus (ST) grafts with and without distally attached tibial bone plugs. Data were compared with bone-tendon-bone (BTB) graft fixation using biodegradable and conventional titanium interference screws. We used 56 proximal calf tibiae to compare maximum pullout force, screw insertion torque, and stiffness of fixation for biodegradable direct ST tendon and bone plug fixation (group I: without bone plug, group II: with bone plug) versus titanium interference screw fixation (group III: without bone plug, group IV: with bone plug). A round threaded biodegradable poly-(D, L-lactide) (Sysorb) and a round threaded titanium interference screw (RCI) were used. As a control calf bone-tendon-bone (BTB) grafts fixed with either poly-(D, L-lactide) (group V) or conventional titanium (group VI) interference screws were used. ST tendons were harvested either with or without their distally attached tibial bone plugs from human cadavers and were folded to a three-stranded graft. Specimen were loaded in a material testing machine with the applied load parallel to the long axis of the bone tunnel. Maximum pullout force of ST bone plug (group III: 717 N +/- 90, group IV: 602 N +/- 117) fixation was significantly higher than that of direct tendon (group I: 507 N +/- 93, group III: 419 N +/- 77) fixation. Maximum pullout force of biodegradable screw ST fixation was higher than that of titanium screw fixation in both settings. There was no significant difference in pullout force between biodegradable (713 N +/- 210) and titanium (822 N +/- 130) BTB graft fixation or between ST fixation with bone plug and biodegradable screw with BTB fixation. Pullout force of hamstring tendon interference screw fixation can be improved by using a biodegradable implant. In addition, initial pullout force can be greatly improved by harvesting the hamstring tendon graft with its distally attached tibial bone plug. This may be important, especially in improving tibial graft fixation. This study encourages further research in tendon-bone healing with direct interference screw fixation to confirm the potential of this advanced method.     

A Mechanical Comparison of the Compressive Force Generated by Various Headless Compression Screws and the Impact of Fracture Gap Size

Background: There is evidence that interfragmentary fracture gap size may affect the compression achievable with a modern headless compression screw (HCS). This mechanical study compared the compression achieved by 3 commercial HCS systems through various fracture gaps: CAPTIVATE Headless (Globus Medical, Inc, Audubon, Pennsylvania), Synthes (DePuy Synthes, Westchester, Pennsylvania), and Acumed Acutrak 2 (Acumed LLC, Hillsboro, Oregon). Methods: Screws were inserted into a custom test fixture composed of polyurethane synthetic bone foam fragments, separated by a layer of easily compressible polyurethane foam simulating a fracture gap. Compression was measured after final insertion and countersinking. The effect of the interfragmentary fracture gap size on the compression generated was also investigated. Results: The CAPTIVATE Headless 3.0 mm screw (70.1 ± 5.7 N) and the Synthes 3.0 mm screw (64.9 ± 7.3 N) achieved similar compressive forces after final countersink. Similar comparisons were found for the CAPTIVATE Headless 2.5 mm and Synthes 2.4 mm screws, and the CAPTIVATE Headless 4.0 mm and Acutrak 2 Standard screws. The final compression of the CAPTIVATE Headless 2.5 mm and Synthes 2.4 mm screws was not significantly affected when the fracture gap was doubled from 2 to 4 mm, but was reduced significantly by 95.9% with the Acutrak 2 Micro screw. Conclusion: When comparing like-sized screws, the CAPTIVATE, Synthes, and Acutrak 2 HCS systems demonstrated similar potential compressive forces. However, compared with the CAPTIVATE Headless and Synthes HCS systems, which are inserted with a compression sleeve that is not gap distance–dependent, the Acutrak 2 HCS system demonstrated less compression when the simulated fracture gap size was increased to 4 mm.     

Pullout strength comparison of two methods of orienting screw insertion in the lateral masses of the bovine cervical spine.

We undertook a biomechanical study to compare the pullout strength of 3.5-mm AO screws placed in two different orientations within the bovine cervical spine. The first set of screws were oriented obliquely and passed through the lateral mass, as recommended by the AO group. The orientation of the second set was anterior to posterior through the lateral mass, as recommended by Roy-Camille. All screw holes were drilled and tapped by a spinal surgeon experienced with both techniques. Pullout force was measured on an Instron materials testing machine using a self-centering screw-holding chuck and loading rate of 0.833 mm/sec. Although the bone strength in the Roy-Camille orientation was greater (46.7 N/mm versus 36.1 N/mm, p < 0.05), the overall mean pullout force for the AO orientation was greater (607 N versus 471 N, p < 0.025) due to the longer length of bone available for screw purchase (17.0 mm versus 10.3 mm).     

Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone

It has recently been shown that graft fixation close to the ACL insertion site is optimal in order to increase anterior knee stability. Hamstring tendon fixation using interference screws offers this possibility and a round threaded titanium interference screw has been previously developed. The use of a round threaded biodegradable interference screw may be equivalent. In addition, to increase initial fixation strength, graft harvest with a distally attached bone plug may be advantageous, but biomechanical data do not exist. This study compares the initial pullout force, stiffness of fixation, and failure modes of three strand semitendinosus grafts in 36 proximal calf tibiae using either biodegradable poly-(D,L-lactide) (Sysorb; Sulzer Orthopaedics Ltd, Munsingen, Switzerland) or round threaded titanium (RCI; Smith & Nephew DonJoy, Carlsbad, CA) interference screws, harvested either without (biodegradable: group I, titanium III) or with (biodegradable: group II, titanium: group IV) attached tibial bone plugs. Maximum pullout force in group I (507 ± 93 N) was significantly higher than in group III (419 ± 77 N). Pullout force of bone plug fixation was significantly higher than that of direct tendon fixation (717 ± 90 N in group II and 602 ± 117 N in group IV). Pullout force of biodegradable fixation was significantly higher in both settings. These results indicate that initial pullout force of hamstring-tendon graft interference screw fixation can be increased by using a biodegradable interference screw. In addition, initial pullout force of hamstring-tendon graft fixation with an interference screw can be greatly increased by harvesting the graft with its distally attached tibial bone plug.Arthroscopy 1998 Jan-Feb;14(1):29-37