Modified Lemaire Tenodesis Forces in Cadaveric Specimens Are Not Affected by Random Small-Scale Variations in the Femoral Insertion Point During Active Knee Joint Flexion-Extension.

Purpose: To directly measure lateral extra-articular tenodesis (LET) forces supporting anterior cruciate ligament reconstruction (ACLR) during dynamic flexion-extension cycles induced by simulated active muscle forces, to investigate the influence of random surgical variation in the femoral LET insertion point around the target insertion position, and to determine potential changes to the extension behavior of the knee joint in a cadaveric model. Methods: After iatrogenic anterior cruciate ligament deficiency and simulated anterolateral rotatory instability, 7 fresh-frozen cadaveric knee joints were treated with isolated ACLR followed by combined ACLR-LET. The specimens were tested on a knee joint test bench during active dynamic flexion-extension with simulated muscle forces. LET forces and the degree of knee joint extension were measured. Random variation in the LET insertion point around the target insertion position was postoperatively quantified by computed tomography. Results: In extension, the median LET force increased to 39 ± 2 N (95% confidence interval [CI], 36 to 40 N). In flexion over 70°, the LET was offloaded (2 ± 1 N; 95% CI, 0 to 2 N). In this study, small-scale surgical variation in the femoral LET insertion point around the target position had a negligible effect on the graft forces measured. We detected no difference in the degree of knee joint extension after combined ACLR-LET (median, 1.0° ± 3.0°; 95% CI, -6.2° to 5.2°) in comparison with isolated ACLR (median, 1.1° ± 3.3°; 95% CI, -6.7° to 6.1°; P = .62). Conclusions: LET forces in combined ACLR-LET increased to a limited extent during active knee joint flexion-extension independent of small-scale variation around 1 specific target insertion point. Combined ACLR-LET did not change knee joint extension in comparison with isolated ACLR under the testing conditions used in this biomechanical study. Clinical Relevance: Low LET forces can be expected during flexion-extension of the knee joint. Small-scale deviations in the femoral LET insertion point around the target insertion position in the modified Lemaire technique might have a minor effect on graft forces during active flexion-extension.

Pedicle screw augmentation in posterior constructs of the thoracolumbar spine: How many pedicle screws should be augmented?

BACKGROUNDS: To evaluate the effects of different pedicle screw augmentation strategies on screw loosening and adjacent segment collapse at the proximal end of long-segment instrumentation. METHODS: Eighteen osteoporotic (9 male, 9 female donors; mean age: 74.7 ± 10.9 [SD] years) thoracolumbar multi-segmental motion segments (Th11 - L1) were assigned as follows: control, one-level augmented screws (marginally), and two-level augmented screws (fully augmented) groups (3 × 6). Pedicle screw placement was performed in Th12 and L1. Cyclic loading in flexion started with 100-500 N (4 Hz) and was increased by 5 N every 500 cycles. Standardized lateral fluoroscopy images with 7.5 Nm loading were obtained periodically during loading. The global alignment angle was measured to evaluate the overall alignment and proximal junctional kyphosis. The intra-instrumental angle was used to evaluate screw fixation. FINDINGS: Considering screw fixation as a failure criterion, the failure loads of the control (683 N), and marginally (858 N) and fully augmented (1050 N) constructs were significantly different (ANOVA p = 0.032).Taking the overall specimen alignment as failure criteria, failure loads of the three groups (control 933 ± 271.4 N, marginally 858 N ± 196 N, and full 933 ± 246.3 N were in the same range and did not show any significance (p = 0.825). INTERPRETATION: Global failure loads were comparable among the three groups and unchanged with augmentation because the adjacent segment and not the instrumentation failed first. Augmentation of all screws showed significant improved in screw anchorage.

Actuator and Contact Force Modeling of an Active Soft Brace for Scoliosis.

Scoliosis is an abnormality of the spinal curvature that severely affects the musculoskeletal, respiratory, and nervous systems. Conventionally, it is treated using rigid spinal braces. These braces are static, rigid, and passive in nature, and they (largely) limit the mobility of the spine, resulting in other spinal complexities. Moreover, these braces do not have precise control over how much force is being applied by them. Over-exertion of force may deteriorate the spinal condition. This article presents a novel active soft brace that allows mobility to the spine while applying controlled corrective forces that are regulated by varying the tensions in elastic bands using low-power light weight twisted string actuators (TSAs). This article focuses on the actuator and contact force modeling of the active soft brace (ASB). The actuator modeling is required to translate the twisting of string in terms of contraction of the string's length, whereas the contact force modeling helps in estimating the net resultant force exerted by the band on the body using single point pressure/force sensors. The actuators (TSAs) are modeled as helix geometry and validated using a laser position sensor. The results showed that the model effectively tracked the position (contraction in length) with root mean square error (RMSE) of 1.7386 mm. The contact force is modeled using the belt and pulley contact model and validated by building a custom testbed. The actuator module is able to regulate the pressure in the range 0-6 Kpa, which is comparable to 0-8 Kpa pressure regulated in rigid braces. This makes it possible to verify and demonstrate the working principle of the proposed active soft brace.

Modified Lemaire tenodesis reduces anterior cruciate ligament graft forces during internal tibial torque loading.

PURPOSE: The aim of the study was to directly measure graft forces of an anterior cruciate ligament reconstruction (ACLR) and a lateral extra-articular tenodesis (LET) using the modified Lemaire technique in combined anterior cruciate ligament (ACL) deficient and anterolateral rotatory instable knees and to analyse the changes in knee joint motion resulting from combined ACLR + LET. METHODS: On a knee joint test bench, six fresh-frozen cadaveric specimens were tested at 0°, 30°, 60°, and 90° of knee flexion in the following states: 1) intact; 2) with resected ACL; 3) with resected ACL combined with anterolateral rotatory instability; 4) with an isolated ACLR; and 5) with combined ACLR + LET. The specimens were examined under various external loads: 1) unloaded; 2) with an anterior tibial translation force (ATF) of 98 N; 3) with an internal tibial torque (IT) of 5 Nm; and 4) with a combined internal tibial torque of 5 Nm and an anterior tibial translation force of 98 N (IT + ATF). The graft forces of the ACLR and LET were recorded by load cells incorporated into custom devices, which were screwed into the femoral tunnels. Motion of the knee joint was analysed using a 3D camera system. RESULTS: During IT and IT + ATF, the addition of a LET reduced the ACLR graft forces up to 61% between 0° and 60° of flexion (P = 0.028). During IT + ATF, the LET graft forces reached 112 N. ACLR alone did not restore native internal tibial rotation after combined ACL deficiency and anterolateral rotatory instability. Combined ACLR + LET was able to restore native internal tibial rotation values for 0°, 60° and 90° of knee flexion with decreased internal tibial rotation at 30° of flexion. CONCLUSION: The study demonstrates that the addition of a LET decreases the forces seen by the ACLR graft and reduces residual rotational laxity after isolated ACLR during internal tibial torque loading. Due to load sharing, a LET could support the ACLR graft and perhaps be the reason for reduced repeat rupture rates seen in clinical studies. Care must be taken not to limit the internal tibial rotation when performing a LET.

Standardized fracture creation in the distal humerus and the olecranon for surgical training and biomechanical testing.

INTRODUCTION: Surgical training and biomechanical testing require models that realistically represent the in vivo injury condition. The aim of this work was to develop and test a method for the generation of distal humerus fractures and olecranon fractures in human specimens, while preserving the soft tissue envelope. METHODS: Twenty-one cadaveric upper extremity specimens (7 female, 14 male) were used. Two different experimental setups were developed, one to generate distal humerus fractures and one to generate olecranon fractures. Specimens were placed in a material testing machine and fractured with a predefined displacement. The force required for fracturing and the corresponding displacement were recorded and the induced energy was derived of the force-displacement graphs. After fracturing, CT imaging was performed and fractures were classified according to the AO classification. RESULTS: Eleven distal humerus fractures and 10 olecranon fractures with intact soft tissue envelope could be created. Distal humerus fractures were classified as AO type C (n = 9) and as type B (n = 2), all olecranon fractures were classified as AO type B (n = 10). Distal humerus fractures required significantly more load than olecranon fractures (6077 N ± 1583 vs 4136 N ± 2368, p = 0.038) and absorbed more energy until fracture than olecranon fractures (17.8 J ± 9.1 vs 11.7 J ± 7.6, p = 0.11), while the displacement at fracture was similar (5.8 mm ± 1.6 vs 5.9 mm ± 3.1, p = 0.89). CONCLUSION: The experimental setups are suitable for generating olecranon fractures and distal humerus fractures with intact soft tissue mantle for surgical training and biomechanical testing.

Evaluation of mushroom-shaped allograft for unstable proximal humerus fractures.

INTRODUCTION: Proximal humerus fractures are common injuries of the elderly. Different treatment options, depending on fracture complexity and stability, have been recommended in the literature. Particularly for varus displaced fractures with a lack of medial support, and patients suffering from osteoporosis, structural allografts can be used to enhance the stability of the construct. An individually shaped allograft has been suggested in the literature and investigated in a clinical setting. However, biomechanical properties have yet to be evaluated. MATERIALS AND METHODS: Twenty-four fresh-frozen humeri and 12 femoral heads were obtained, and an unstable three-part fracture of the humeral head was simulated. Fracture fixation was achieved by using a locking plate in both groups. In the test group, a mushroom-shaped allograft was tailored out of a femoral head to individually fit the void inside the humeral head. Specimens were fitted with a 3D motion analysis system and cyclically loaded with a stepwise increasing load magnitude in a varus-valgus bending test until failure or up to a maximum of 10,000 load cycles. RESULTS: The mushroom group reached a significantly higher number of load cycles (8342; SD 1,902; CI 7133-9550) compared to the control group (3475; SD 1488; CI 2530-4420; p < 0.001). Additionally, the test group showed significantly higher stiffness values concerning all observational points (p < 0.001). CONCLUSION: This mushroom-shaped allograft in combination with a locking plate significantly increased load to failure as well as stiffness of the construct when exposed to varus-valgus bending forces. Therefore, it might be a viable option for surgical treatment of unstable and varus displaced proximal humerus fractures to superiorly prevent loss of reduction and varus collapse.

Systematic Review of Back-Support Exoskeletons and Soft Robotic Suits.

Lower back pain and musculoskeletal injuries are serious concerns for workers subjected to physical workload and manual material handling tasks. Spine assistive exoskeletons are being developed to support the spine and distribute the spine load. This article presents a detailed up-to-date review on the back support exoskeletons by discussing their type (Active/Passive), structure (Rigid/Soft), power transmission methods, weight, maximum assistive force, battery technologies, tasks (lifting, bending, stooping work), kinematic compatibility and other important features. This article also assesses the back support exoskeletons in terms of their ability to reduce the physical load on the spine. By reviewing functional and structural characteristics, the goal is to increase communication and realization among ergonomics practitioners, developers, customers, and factory workers. The search resulted in reviewing 34 exoskeletons of which 16 were passive and 18 were active. In conclusion, back support exoskeletons have immense potential to significantly reduce the factors regarding work-related musculoskeletal injuries. However, various technical challenges and a lack of established safety standards limit the wide adaptation of exoskeletons in industry.

Single column plate plus other column lag screw fixation vs. both column plate fixation for anterior column with posterior hemitransverse acetabular fractures - a biomechanical analysis using different loading protocols.

OBJECTIVES: Open reduction and internal fixation of both columns is considered the treatment of choice for displaced anterior column with posterior hemitransverse (ACPHT) fractures in non-geriatric patients. Plate fixation of one column combined with lag screw fixation of the other column allows to decrease operative time and approach-related morbidity compared to conventional both column plating. The aim of this biomechanical study was to evaluate whether single column plate plus other column lag screw fixation confers similar stability to both column plate fixation. Physiological loads were simulated using both the single-leg stance (SLS) as well as the sit-to-stand (STS) loading protocols. METHODS: A clinically relevant ACPHT fracture model was created using fourth-generation composite hemipelves. Fractures were stabilized with three different fixation constructs: (1) anterior column plate plus posterior column screw fixation (AP+PCS), posterior column plate plus anterior column screw fixation (PP+ACS) and anterior column plate plus posterior column plate fixation (AP+PP). Specimens were loaded from 50 to 750 N with a ramp of 100 N/s. Fracture gap motion (FGM) and relative interfragmentary rotation (RIFR) between the three main fracture fragments were assessed under loads of 750 N using an optical 3D measurement system. RESULTS: STS loading generally resulted in higher mean FGM and RIFR than STS loading in the AP+PCS and AP+PP groups, while no significant differences were found in the PP+ACS group. Compared to conventional both column plate fixation (AP+PP), PP+ACS displayed significantly higher FGM and RIFR between the iliac wing and the posterior column during SLS loading. No significant differences in FGM and RIFR were identified between the AP+PCS and the AP+PP group. CONCLUSION: Overall, single column plate plus other column lag screw fixation conferred similar stability to conventional both column plate fixation. From a clinical point of view, AP+PCS appears to be the most attractive alternative to conventional AP+PP for internal fixation of ACPHT fractures.

Nail Versus Plate: A Biomechanical Comparison of a Locking Plate Versus an Intramedullary Nail With an Angular Stable Locking System in a Shoulder Simulator With Active Muscle Forces Using a Two-Part Fracture Model.

OBJECTIVES: To compare a locking compression plate versus an intramedullary nail with an angular stable locking system (ASLS) using a 2-part fracture model in a shoulder test bench. METHODS: Twelve fresh frozen humeri were used for biomechanical testing in a shoulder simulator. A 2-part fracture model, with and without medial cortical support, was used to compare the locking plate and a nail with an ASLS. The varus impaction, varus per cycle motion, tilt, and tilt per cycle were analyzed. RESULTS: No significant differences for the resulting forces in the glenoid fossa were evaluated. The stable fracture model showed no significant differences for the 2 groups. The median varus impaction was -0.96 degrees (range -0.55 to -4.26 degrees) in the plate group and 0.5 degrees (range -3.06 to 0.98 degrees) in the nail group, after 500 cycles of cyclic loading in the unstable fracture model. The plate group showed a significantly higher median varus impaction per cycle motion and median varus impaction at the 200th, 300th, and 400th cycle of physiological loading. CONCLUSIONS: The intramedullary nail with the ASLS could be an alternative for patients suffering from osteoporosis and comorbidities.

The Effect of Rod Pattern, Outrigger, and Multiple Screw-Rod Constructs for Surgical Stabilization of the 3-Column Destabilized Cervical Spine - A Biomechanical Analysis and Introduction of a Novel Technique.

OBJECTIVE: Anterior-only reconstructions for cervical multilevel corpectomies are prone to fail under continuous mechanical loading. This study sought to define the mechanical characteristics of different constructs in reducing a range of motion (ROM) of the 3-column destabilized cervical spine, including posterior cobalt-chromium (CoCr)-rods, outrigger-rods (OGR), and a novel triple rod construct using lamina screws (6S3R). The clinical implications of biomechanical findings are discussed in depth from the perspective of the challenges surgeons face cervical deformity correction. METHODS: Three-column deficient cervical spinal models were produced based on reconstructed computed tomography scans. The corpectomy defect between C3 and C7 end-level vertebrae was restored with anterior titanium (Ti) mesh-cage. The ROM was evaluated in a customized 6-degree of freedom spine tester. Tests were performed with different rod materials (Ti vs. CoCr), varying diameter rods (3.5 mm vs. 4.0 mm), with and without anterior plating, and using different construct patterns: bilateral rod fixation (standard-group), OGR-group, and 6S3R-Group. Construct stability was expressed in changes and differences of ROM (°). RESULTS: The largest reduction of ROM was noticed in the 6S3R-group compared to the standard- and the OGR-group. All differences observed were emphasized with an increasing number of corpectomy levels and if anterior plating was not added. For all simulated 1-, 2-, and 3-level corpectomy constructs, the OGR-group revealed decreased ROM for all motion directions compared to the standard-group. An increase of construct stiffness was also recorded for increased rod diameter (4.0 mm) and stiffer rod material (CoCr), though these effects lacked behind the more advanced construct pattern. CONCLUSION: A novel reconstructive technique, the 6S3R-construct, was shown to outperform all other constructs and might resemble a new standard of reference for advanced posterior fixation.

Biomechanical comparison of fixation techniques for transverse acetabular fractures - Single-leg stance vs. sit-to-stand loading.

OBJECTIVES: To biomechanically compare five different fixation techniques for transverse acetabular fractures using both the single-leg stance (SLS) and the sit-to-stand (STS) loading protocols and to directly compare fracture gap motion (FGM) and relative interfragmentary rotation (RIFR). METHODS: Transtectal transverse acetabular fractures were created on fourth-generation composite hemipelves in a reproducible manner. Five different fixation techniques were biomechanically assessed using both an SLS and STS loading protocol: anterior plate (AP) only, posterior plate (PP) only, anterior plate plus posterior column screw (AP+PCS), posterior plate plus anterior column screw (PP+ACS) and anterior plus posterior plate (AP+PP). After preconditioning, the specimens were loaded from 50 to 750 N with a ramp of 100 N/s. FGM and RIFR under loads of 750 N were measured using an optical 3D measurement system. RESULTS: In the three groups of fixation techniques addressing both columns, STS loading resulted in higher mean FGM and in RIFR than SLS loading. No construct failure was observed. In the single plate groups (AP only and PP only), STS loading resulted in failure of all specimens before reaching loads of 750 N, while no failure occurred after SLS loading. No significant differences in FGM and RIFR were found between the double plate (AP+PP) and the single plate plus column screw (AP+PCS and PP+ACS) techniques. CONCLUSION: SLS loading appeared to overestimate the strength of acetabular fracture fixation constructs and STS loading may be more appropriate to provide clinically relevant biomechanical data. Internal fixation of a single column might not provide adequate stability for transverse fractures, while strength of single plate plus column screw fixation and double plate fixation was comparable.

Local osteo-enhancement of osteoporotic vertebra with a triphasic bone implant material increases strength-a biomechanical study.

PURPOSE: The aim of this study was to assess the biomechanical properties of intact vertebra augmented using a local osteo-enhancement procedure to inject a triphasic calcium sulfate/calcium phosphate implant material. METHODS: Twenty-one fresh frozen human cadaver vertebra (Th11-L2) were randomized into three groups: treatment, sham, and control (n = 7 each). Treatment included vertebral body access, saline lavage to displace soft tissue and marrow elements, and injection of the implant material to fill approximately 20% of the vertebral body by volume. The sham group included all treatment steps, but without injection of the implant material. The control group consisted of untreated intact osteoporotic vertebra. Load at failure and displacement at failure for each of the three groups were measured in axial compression loading. RESULTS: The mean failure load of treated vertebra (4118 N) was significantly higher than either control (2841 N) or sham (2186 N) vertebra (p < 0.05 for: treatment vs. control, treatment vs. sham). Treated vertebra (1.11 mm) showed a significantly higher mean displacement at failure than sham vertebra (0.80 mm) (p < 0.05 for: treatment vs. sham). In the control group, the mean displacement at failure was 0.99 mm. CONCLUSIONS: This biomechanical study shows that a local osteo-enhancement procedure using a triphasic implant material significantly increases the load at failure and displacement at failure in cadaveric osteoporotic vertebra.

Functional and radiographic evaluation of an interspinous device as an adjunct for lumbar interbody fusion procedures.

In the last decades, several interspinous process devices were designed as a minimally invasive treatment option for spinal stenosis. In order to minimise surgical trauma, interspinous process devices were recently discussed as an alternative posterior fixation in vertebral interbody fusions. Therefore, the purpose of this study was to evaluate the effect of a newly designed interspinous device with polyester bands (PBs) on range of motion (RoM) and centre of rotation (CoR) of a treated motion segment in comparison with an established interspinous device with spikes (SC) as well as with pedicle screw instrumentation in lumbar fusion procedures. Flexibility tests with an applied pure moment load of 7.5 Nm were performed in six monosegmental thoracolumbar functional spinal units (FSUs) in the following states: (a) native, (b) native with PB device, (c) intervertebral cage with PB device, (d) cage with SC and (e) cage with internal fixator. The resulting RoM was normalised to the native RoM. The CoR was determined of X-ray images taken in maximal flexion and extension during testing. In flexion and extension, the PB device without and with the cage reduced the RoM of the native state to 58% [standard deviation (SD) 17.8] and 53% (SD 15.7), respectively. The SC device further reduced the RoM to 27% (SD 16.8), while the pedicle screw instrumentation had the most reducing effect to 17% (SD 17.2) (p < 0.01). In lateral bending and axial rotation, the interspinous devices had the least effect on the RoM. Compared to the native state, for all instrumentations the CoR showed a small shift towards cranial. In the anterior-posterior direction, the SC device and the pedicle screw instrumentation shifted the CoR towards the posterior wall. The interspinous devices significantly reduced the RoM in flexion/extension, while in axial rotation and lateral bending only the internal fixator had a significant effect on the RoM.

Biomechanical Study of a Novel, Expandable, Non-Metallic and Radiolucent CF/PEEK Vertebral Body Replacement (VBR).

Vertebral body replacement is well-established to stabilize vertebral injuries due to trauma or cancer. Spinal implants are mainly manufactured by metallic alloys; which leads to artifacts in radiological diagnostics; as well as in radiotherapy. The purpose of this study was to evaluate the biomechanical data of a novel carbon fiber reinforced polyetheretherketone (CF/PEEK) vertebral body replacement (VBR). Six thoracolumbar specimens were tested in a six degrees of freedom spine tester. In all tested specimens CF/PEEK pedicle screws were used. Two different rods (CF/PEEK versus titanium) with/without cross connectors and two different VBRs (CF/PEEK prototype versus titanium) were tested. In lateral bending and flexion/extension; range of motion (ROM) was significantly reduced in all instrumented states. In axial rotation; the CF/PEEK combination (rods and VBR) resulted in the highest ROM; whereas titanium rods with titanium VBR resulted in the lowest ROM. Two cross connectors reduced ROM in axial rotation for all instrumentations independently of VBR or rod material. All instrumented states in all planes of motion showed a significantly reduced ROM. No significant differences were detected between the VBR materials in all planes of motion. Less rigid CF/PEEK rods in combination with the CF/PEEK VBR without cross connectors showed the smallest reduction in ROM. Independently of VBR and rod material; two cross connectors significantly reduced ROM in axial rotation. Compared to titanium rods; the use of CF/PEEK rods results in higher ROM. The stiffness of rod material has more influence on the ROM than the stiffness of VBR material.

How does a novel knitted titanium nucleus prosthesis change the kinematics of a cervical spine segment? A biomechanical cadaveric study.

BACKGROUND: Total disc replacement is a possible treatment alternative for patients with degenerative disc disease, especially in the cervical spine. The aim is to restore the physiological flexibility and biomechanical behavior. A new approach based on these requirements is the novel nucleus prosthesis made of knitted titanium wires. METHODS: The biomechanical functionalities of eight human cervical (C4-C7) spine segments were investigated. The range of motion was quantified using an ultra-sound based motion analysis system. Moreover, X-rays in full flexion and extension of the segment were taken to define the center of rotation before and after implantation of the nucleus prosthesis as well as during and after complex cyclic loading. FINDINGS: The mean range of motion of the index segment (C5/6) in flexion/extension showed a significant reduction of range of motion from 9.7° (SD 4.33) to 6.0° (SD 3.97) after implantation (P = 0.037). Lateral bending and axial rotation were not significantly reduced after implanting and during cyclic loading in our testing. During cyclic loading the mean range of motion for flexion/extension increased to 7.2° (SD 3.67). The center of rotation remained physiological in the ap-plane and moved cranially in the cc-plane (-27% to -5% in cc height) during the testing. INTERPRETATION: The biomechanical behavior of the nucleus implant might lower the risk for adjacent joint disorders and restore native function of the index segment. Further in vivo research is needed for other factors, like long-term effects and patient's satisfaction.

Ultrasound melted polymer sleeve for improved primary pedicle screw anchorage: A novel augmentation technique.

BACKGROUND: Cement augmentation of pedicle screws to prevent screw loosening is associated with significant complications, such as cement leakage or bone necrosis. Therefore, an alternative strategy to improve pedicle screw anchorage has been recently developed: Polymer reinforcement of pedicle screws uses an in situ melted polymer sleeve in order to enhance screw anchorage. This biomechanical study evaluated the effect of polymer-reinforcement by comparing polymer-reinforced pedicle screws to non-augmented as well as cement-augmented screws under cyclic loading. METHODS: For each of the two comparisons (polymer-reinforced vs. non-augmented screws and polymer-reinforced vs. cement-augmented screws), polymer-reinforced screws and control screws were placed into the left and right pedicle of seven vertebrae (mean age: 74.0 (SD 9.3) years) to allow for pairwise left-right comparisons. Each screw was subjected to cyclic cranio-caudal loading with an initial load ranging from -50 N to +50 N and with stepwise increasing compressive loads (5 N every 100 cycles) until screw loosening. FINDINGS: Polymer-reinforced pedicle screws resisted a higher number of load cycles until loosening than the contralateral non-augmented control screws (4300 SD 2018 vs. 2457 SD 1116 load cycles, p = 0.015). Screw anchorage of polymer-reinforced pedicle screws was comparable to that of cement augmented control screws (3857 (SD2085) vs. 4300 (SD1257) load cycles until failure, p = 0.64). INTERPRETATION: Our findings indicate that polymer-reinforcement significantly enhances pedicle screw anchorage in low quality bone and that its effect is similar in size than that of cement augmentation.

Effect of pedicle screw augmentation with a self-curing elastomeric material under cranio-caudal cyclic loading-a cadaveric biomechanical study.

BACKGROUND: Pedicle screws can be augmented with polymethylmethacrylate (PMMA) cement through cannulated and fenestrated pedicle screws to improve screw anchorage. To overcome the drawbacks of PMMA, a modified augmentation technique applying a self-curing elastomeric material into a balloon-created cavity prior to screw insertion was developed and evaluated. The aim of the study was to compare the effect of the established and novel augmentation technique on pedicle screw anchorage in a biomechanical in vitro experiment. METHODS: In ten lumbar vertebral bodies, the right pedicles were instrumented with monoaxial cannulated and fenestrated pedicle screws and augmented in situ with 2 ml PMMA. The left pedicles were instrumented with monoaxial cannulated pedicle screws. Prior to left screw insertion, a balloon cavity was created and filled with 3 ml of self-curing elastomer (silicone). Each screw was subjected to a cranio-caudal cyclic load starting from - 50 to 50 N while the upper load was increased by 5 N every 100 load cycles until loosening or 11,000 cycles (600 N). After cyclic loading, a pullout test of the screws was conducted. RESULTS: The mean cycles to screw loosening were 9824 ± 1982 and 7401 ± 1644 for the elastomer and PMMA group, respectively (P = 0.012). The post-cycling pullout test of the loosened screws showed differences in the failure mode and failure load, with predominantly pedicle/vertebrae fractures in the PMMA group (1188.6 N ± 288.1) and screw pullout through the pedicle (671.3 N ± 332.1) in the elastomer group. CONCLUSION: The modified pedicle screw augmentation technique involving a balloon cavity creation and a self-curing elastomeric silicone resulted in a significantly improved pedicle screw anchorage under cyclic cranio-caudal loading when compared to conventional in situ PMMA augmentation.

Screw oversizing for anterior cruciate ligament graft fixation in primary and enlarged tibial tunnels: A biomechanical study in a porcine model.

BACKGROUND: Ideal diameter for tibial interference screw fixation of the anterior cruciate ligament (ACL) graft remains controversial. Tibial graft fixation with screws matching the tunnel diameter vs. one-millimetre oversized screws were compared. METHODS: In 32 cadaveric porcine tibiae, bovine extensor tendons with a diameter of eight millimetres were fixed in (I) a primary ACL reconstruction scenario with eight-millimetre tibial tunnels (pACL), with eight-millimetre (pACL-8) vs. nine-millimetre (pACL-9) screws, and (II) a revision ACL reconstruction scenario with enlarged tunnels of 10 mm (rACL), with 10-mm (rACL-10) vs. 11-mm (rACL-11) screws. Specimens underwent cyclic loading with low and high load magnitudes followed by a load-to-failure test. Graft slippage and ultimate failure load were recorded. RESULTS: In comparison with matched-sized screws (pACL-8), fixation with oversized screws (pACL-9) showed with significantly increased graft slippage during cyclic loading at higher load magnitudes (1.19 ±â€¯0.23 vs. 1.98 ±â€¯0.67 mm; P = 0.007). There were no significant differences between the two screw sizes in the revision scenario (rACL-10 vs. rACL-11; P = 0.38). Graft fixation in the revision scenario resulted in significantly increased graft slippage in comparison with fixation in primary tunnels at higher loads (pACL vs. rACL; P = 0.004). Pull-out strengths were comparable for both scenarios and all screw sizes (P > 0.316). CONCLUSIONS: Matched-sized interference screws provided better ACL graft fixation in comparison with an oversized screw diameter. In revision cases, the fixation strength of interference screws in enlarged tunnels was inferior to the fixation strength in primary tunnels.

Biomechanical evaluation of cable and suture cerclages for tuberosity reattachment in a 4-part proximal humeral fracture model treated with reverse shoulder arthroplasty.

BACKGROUND: Sufficient tuberosity fixation in proximal humeral fractures treated with shoulder arthroplasty is essential to gain a good clinical outcome. This biomechanical study evaluated the strength of the reattached tuberosities in reverse total shoulder arthroplasty fixed with cables or with sutures in a cerclage-like technique. Considering the mechanical advantages of flexible titanium alloy cables compared with conventional sutures for cerclage-like fixations, we hypothesized that titanium alloy cables would achieve higher fixation strengths of the tuberosities compared with heavy nonabsorbable sutures. METHODS: A 4-part fracture was created on 8-paired proximal human humeri. The tuberosities were reduced anatomically and fixed by 2 heavy nonabsorbable sutures (suture group) or by two 1-mm titanium alloy cables (cable group) in a cerclage-like technique around the neck of the prosthesis. The humeri were placed in a custom-made test setup enabling internal and external rotation. Cyclic loading with a stepwise increasing load magnitude was applied with a material testing machine, starting with 1 Nm and increasing the load by 0.25 Nm after each 100th cycle until failure of the fixation occurred (>15° rotation of the tuberosities). Any motion of the tuberosities was measured with a 3-dimensional ultrasound motion analysis system. RESULTS: Overall, the cable group reached 1414 ± 372 cycles, and the suture group reached 1257 ± 230 cycles until the fixations failed (P = .313). The suture group showed a significantly higher rotation of the lesser tuberosity relative to the humerus shaft axis after 200, 400, and 600 cycles compared with the cable group (P = .018-.043). CONCLUSIONS: Tuberosities reattached with cable cerclages showed higher fixation strength and therefore less rotation compared with suture cerclages in a 4-part proximal humeral fracture model treated with reverse total shoulder arthroplasty. Whether this higher fixation strength results in higher bony ingrowth rates of the tuberosities and thus leads to a better clinical outcome needs to be investigated in further clinical studies.