Dominic Gehweiler

Evaluation of a robot-assisted testing system for multisegmental spine specimens.

Mono- and multi-segmental testing methods are required to identify segmental motion patterns and evaluate the biomechanical behaviour of the spine. This study aimed to evaluate a new testing system for multisegmental specimens using a robot combined with an optical motion analysis system. After validation of the robotic system for accuracy, two groups of calf specimens (six monosegmental vs. six multisegmental) were mounted and the functional unit L3-4 was observed. Using rigid body markers, range of motion (ROM), elastic zone (EZ) and neutral zone (NZ), as well as stiffness properties of each functional spine unit (FSU) was acquired by an optical motion capture system. Finite helical axes (FHA) were calculated to analyse segmental movements. Both groups were tested in flexion and extension. A pure torque of 7.5 Nm was applied. Statistical analyses were performed using the Mann-Whitney U-test. Repeatability of robot positioning was -0.001±0.018 mm and -0.025±0.023° for translations and rotations, respectively. The accuracy of the optical system for the proposed set-up was 0.001±0.034 mm for translations and 0.075±0.12° for rotations. No significant differences in mean values and standard deviations of ROM for L3-4 compared to literature data were found. A robot-based facility for testing multisegmental spine units combined with a motion analysis system was proposed and the reliability and reproducibility of all system components were evaluated and validated. The proposed set-up delivered ROM results for mono- and multi-segmental testing that agreed with those reported in the literature. Representing the FHA via piercing points determined from ROM was the first attempt showing a relationship between ROM and FHA, which could facilitate the interpretation of spine motion patterns in the future.

Biomechanical evaluation of the Facet Wedge: a refined technique for facet fixation.

PurposePurpose of this paper is to evaluate the primary stability of a new approach for facet fixation the so-called Facet Wedge (FW) in comparison with established posterior fixation techniques like pedicle screws (PS) and translaminar facet screws (TLS) with and without anterior cage interposition.MethodsTwenty-four monosegmental fresh frozen non-osteoporotic human motion segments (L2–L3 and L4–L5) were tested in a two-arm biomechanical study using a robot-based spine tester. Facet Wedge was compared with pedicle screws and translaminar screws as a stand-alone device and in combination with an anterior fusion cage.ResultsPedicle screws, FW and translaminar screws could stabilize an intact motion segment effectively. Facet Wedge was comparable to PS for lateral bending, extension and flexion and slightly superior for axial rotation. Facet Wedge showed a superior kinematic capacity compared to translaminar screws.ConclusionsFacet Wedge offers a novel posterior approach in achieving primary stability in lumbar spinal fixation. The results of the present study showed that the Facet Wedge has a comparable primary stability to pedicle screws and potential advantages over translaminar screws.

Experimentally induced incomplete burst fractures - a novel technique for calf and human specimens

BackgroundFracture morphology is crucial for the clinical decision-making process preceding spinal fracture treatment. The presented experimental approach was designed in order to ensure reproducibility of induced fracture morphology.ResultsThe presented method resulted in fracture morphology, found in clinical classification systems like the Magerl classification. In the calf spine samples, 70% displayed incomplete burst fractures corresponding to type A3.1 and A3.2 fractures. In all human samples, superior incomplete burst fractures (Magerl A3.1) were identified by an independent radiologist and spine surgeon.ConclusionsThe presented set up enables the first experimental means to reliably model and study distinct incomplete burst fracture patterns in an in vitro setting. Thus, we envisage this protocol to facilitate further studies on spine fracture treatment of incomplete burst fractures.

Computational anatomy of the dens axis evaluated by quantitative computed tomography: Implications for anterior screw fixation

The surgical fracture fixation of the odontoid process (dens) of the second cervical vertebra (C2/axis) is a challenging procedure, particularly in elderly patients affected by bone loss, and includes screw positioning close to vital structures. The aim of this study was to provide an extended anatomical knowledge of C2, the bone mass distribution and bone loss, and to understand the implications for anterior screw fixation. One hundred and twenty standard clinical quantitative computed tomography (QCT) scans of the intact cervical spine from 60 female and 60 male European patients, aged 18–90 years, were used to compute a three‐dimensional statistical model and an averaged bone mass model of C2. Shape and size variability was assessed via principal component analysis (PCA), bone mass distribution by thresholding and via virtual core drilling, and the screw placement via virtual positioning of screw templates. Principal component analysis (PCA) revealed a highly variable anatomy of the dens with size as the predominant variation according to the first principal component (PC) whereas shape changes were primarily described by the remaining PCs. The bone mass distribution demonstrated a characteristic 3D pattern, and remained unchanged in the presence of bone loss. Virtual screw positioning of two 3.5 mm dens screws with a 1 mm safety zone was possible in 81.7% in a standard, parallel position and in additional 15.8% in a twisted position. The approach permitted a more detailed anatomical assessment of the dens axis. Combined with a preoperative QCT it may further improve the diagnostic procedure of odontoid fractures.

Biomechanical evaluation of combined short segment fixation and augmentation of incomplete osteoporotic burst fractures

BackgroundTreating traumatic fractures in osteoporosis is challenging. Multiple clinical treatment options are found in literature. Augmentation techniques are promising to reduce treatment-related morbidity. In recent years, there have been an increasing number of reports about extended indication for augmentation techniques. However, biomechanical evaluations of these techniques are limited.MethodsNine thoracolumbar osteoporotic spinal samples (4 FSU) were harvested from postmortem donors and immediately frozen. Biomechanical testing was performed by a robotic-based spine tester. Standardized incomplete burst fractures were created by a combination of osteotomy-like weakening and high velocity compression using a hydraulic material testing apparatus. Biomechanical measurements were performed on specimens in the following conditions: 1) intact, 2) fractured, 3) bisegmental instrumented, 4) bisegmental instrumented with vertebroplasty (hybrid augmentation, HA) and 5) stand-alone vertebroplasty (VP). The range of motion (RoM), neutral zone (NZ), elastic zone (EZ) and stiffness parameters were determined. Statistical evaluation was performed using Wilcoxon signed-rank test for paired samples (p = 0.05).ResultsSignificant increases in RoM and in the NZ and EZ (p < 0.005) were observed after fracture production. The RoM was decreased significantly by applying the dorsal bisegmental instrumentation to the fractured specimens (p < 0.005). VP reduced fractured RoM in flexion but was still increased significantly (p < 0.05) above intact kinematic values. NZ stiffness (p < 0.05) and EZ stiffness (p < 0.01) was increased by VP but remained lower than prefracture values. The combination of short segment instrumentation and vertebroplasty (HA) showed no significant changes in RoM and stiffness in NZ in comparison to the instrumented group, except for significant increase of EZ stiffness in flexion (p < 0.05).ConclusionsStand-alone vertebroplasty (VP) showed some degree of support of the anterior column but was accompanied by persistent traumatic instability. Therefore, we would advocate against using VP as a stand-alone procedure in traumatic fractures.HA did not increase primary stability of short segment instrumentation. Some additional support of anterior column and changes of kinematic values of the EZ may lead one to suppose that additive augmentation may reduce the load of dorsal implants and possibly reduce the risk of implant failure.

Biomechanical characteristics of pedicle screws in osteoporotic vertebrae—comparing a new cadaver corpectomy model and pure pull-out testing

Currently, evaluation of the stability of spinal instrumentations often focuses on simple pull‐out or cyclic loading. However, the loading characteristics and the specimen alignment rarely simulate physiological loading conditions, or the clinical situation itself. The purpose of this study was to develop an alternative setup and parameters to compare static and dynamic characteristics of pedicle screws at the bone‐implant interface in lumbar osteoporotic cadavers. A corpectomy model development was based on ASTM‐1717 standard, allowing a deflection of the cranial and caudal element under loading. Twelve human osteoporotic vertebrae (L1–L4) were analyzed for morphological CT‐data and T‐Score. For group A (n = 6) loads were simulated as in vivo measurements during walking, representing 2 months postoperatively. A subsequent pull‐out was performed. Group B (n = 6) was tested with pure pull‐out. Screw loosening at the tip/head was optically measured and analyzed with respect to clinical patterns. Correlations between CT‐data, T‐Score, and in vitro parameters were determined. For group A, the subsidence for the head/tip was measured towards the upper/lower endplate, resulting in visible deflections. The progress of the subsidence was greatest within the first and last cycles until failure. The predominant patterns were pure rotation and toggling. However, the pull‐out between groups was not significantly different. Pedicle‐angle and cyclic‐subsidence correlated with R = 0.806/0.794. T‐Score and pull‐out correlated only in group A. With the corpectomy setup, clinically observed wipe effects and a loss of correction could be simulated. The presented parameters facilitate analysis of the complex changing load distributions and interactions between the left and right bone‐implant interface.

Double plating in Vancouver type B1 periprosthetic proximal femur fractures: A biomechanical study.

Periprosthetic hip fractures are an increasing problem in modern orthopedic and trauma surgery. Many options for the operative treatment are available to the surgeon ranging from modern variable angular systems to standard plates, screws, and cerclages. However, there is no gold standard and therefore, the aim of this study, was to investigate the biomechanical characteristics of double plating versus a lateral standard plate in a Vancouver B1 fracture model. Ten 4th generation composite femora were used to implant cementless total hip prosthesis and create Vancouver B1 periprosthetic fractures. Afterwards, the osteotomies were fixed using the locking compression plate in combination with the locking attachment plate (LCP, LAP, DePuy Synthes, Solothurn, Switzerland)‐group I. Group II additionally achieved a 5‐hole 4.5/5.0 mm LCP anteriorly. Each construct was cyclically loaded to failure in axial compression. Axial construct stiffness was 50.87 N/mm (SD 1.61) for group I compared to 738.68 N/mm (SD 94.8) for group II, this difference was statistically significant (p = 0.016). The number of cycles to failure was also significant higher for group II (2,375 vs. 13,000 cycles; p = 0.016). Double plating can significantly increase construct stiffness and stability, and thus, is an option in the treatment of complex periprosthetic fractures, in revision surgery and for patients with the inability to partial weight bear.

Complications of intramedullary nailing—Evolution of treatment

Intramedullary nailing of diaphyseal long bone fractures is a standard procedure in todays trauma and orthopedic surgery due to the numerous advantages (e.g. minimal invasive, limited soft tissue damage, load stability). In the last decade indications have been extended to the metaphyseal region. This was associated with problems and complications due to the reduced bone-implant interface. The changed anatomical conditions lead to decreased implant anchorage. Newly developed locking solutions overcome most of these problems. First, the number and also the orientation of the locking screws were adapted to allow a multiplanar locking. This results in increased implant anchorage in the soft metaphyseal bone, thus construct stability significantly improved. Additional options like angular stable locking have been introduced and furthermore enhanced construct stability especially in poor bone stock. As a perspective locking screw augmentation shows promising results in first biomechanical testing.

How does free rod-sliding affect the posterior instrumentation for a dynamic stabilization using a bovine calf model?

Study Design. A biomechanical cadaveric study in lumbar calf spine. Objective. Evaluation of the effects of selected degrees of freedom (df) on the dynamic stabilization of the spine in terms of segmental range of motion (RoM), center of rotation (CoR), and implant loadings. Summary of Background Data. For dorsal stabilization, rigid implant systems are becoming increasingly complemented by numerous dynamic systems based on pedicle screws and varying df. However, it is still unclear which df is most suitable to accomplish a physiologically related dynamic stabilization, and which loadings are induced to the implants. Human and calf specimens are reported to show certain similarities in their biomechanics. Young healthy calf specimens are not degenerated and show less interindividual differences than elderly human specimens. However, the existing differences between species limit the conclusions in a preclinical setting. Methods. Six calf specimens from level L3–L4 were analyzed in flexion and extension with a 6-df robotic spine simulator. A clinical functional radiological examination tool was used and parameters such as RoM, CoR, and implant loadings were determined for 6 configurations: (1) intact, (2) defect, (3) rigid fixation, (4) free craniocaudal (CC) rod-sliding, (5) free polyaxiality, and (6) combined free rod-sliding and free polyaxiality. The location of the CoR was determined relative to vertebral body dimensions. A CoR repositioning was defined as sufficient when its median differed less than 5% of the vertebral body dimensions. Results. Free rod-sliding in the CC direction restored the CoR from the defect back to the intact condition. The RoM could be significantly reduced to approximately 1/2 of the intact condition. Compared with the rigid condition, the implant bending moments increased from 0.3/−0.8 Nm (flexion/extension) to 1.3/−1.2 Nm for the free CC rod-sliding condition. Conclusion. Free CC rod-sliding restores the intact conditions of the tested kinematic parameters most suitably and at the same time reduces the RoM. Stabilization toward the intact condition could decrease the risk of stress shielding and the progress of segment degeneration. Level of Evidence: N/A

Computational Anatomy of C2 Based on Quantitative Computed Tomography and its Implications for Screw Positioning

Introduction Surgical fixation of the axis (C2) includes screw positioning through bony corridors close to vital structures. However, human bones exhibit large variations in size and shape across individuals and populations. A 3D statistical model of C2 using Quantitative Computed Tomography (QCT) was created to analyze the anatomical variations and to understand their implications for screw positioning. Material and Methods 106 standard clinical QCTs from 46 adult female and 60 male European patients, aged 52.0 years ± 19.9 years, with intact C2s have been included in this study. The mean image resolution was 0.5mm × 0.5mm in the axial plane and 0.6mm in craniocaudally direction. After anonymization 3D statistical modeling of C2 was performed. This included the computation of averaged 3D surface and volumetric bone mineral density (vBMD) models and principal component analysis (PCA). Transpedicular and odontoid screw templates were virtually implanted and their corridors were analyzed. Results PCA revealed a highly variable anatomy of C2 in which size was the predominant variation in the 1st principal component (PC), whereas shape changes were primarily described by the remaining PCs. The largest shape variation was observed at the spinous and transverse processes and the transverse foramina. Comparison of the averaged 3D surface models of C2 for men and women separately revealed mainly a difference in size: The model for males was ~7% larger in the axial plane (anterior-posterior and left-right directions) and ~9% in craniocaudal direction. 3.5mm odontoid screw templates could be virtually implanted in all C2. The average corridor length was 39.2mm ± 2.8mm and the median screw length 38mm. The average corridor length difference between the genders was ~8%. The corridor exhibited the lowest average vBMD value (199.0 mgCaHA/ml) underneath the basis of the odontoid process and a maximum value (911.2 mgCaHA/ml) at its upper part near the cortical shell. Virtual implantation of a 3.5mm C2 pedicle screw with 1mm safety zone was not possible in 31.1% due to interference with the vertebral artery. In 26.4% a 3.5mm screw and in 42.5% a 4.5mm screw could be positioned. The median screw length was 32mm. The average pedicle corridor length was 28.7mm ± 1.9mm with an average convergence of 21.0° ± 3.6° to the sagittal plane and an average ascent of 18.7° ± 3.5° in cranial direction. The lowest average vBMD value for the corridor was located in the trabecular bone of the vertebral body (314.8 mgCaHA/ml) whereas the maximum value (689.2 mgCaHA/ml) was measured near the anterior cortical shell of the vertebral body. The average entry point was ~5mm lateral from the medial border of the inferior articular process and ~10mm cranial from its caudal border. Conclusion We established a 3D statistical model of C2 using QCTs. It revealed largely variable surfaces, bone quality and corridor dimensions and allowed the efficient 3D assessment of parameters relevant for screw positioning. There are anatomical conditions that allowed/not allowed for screw positioning. Surgical decision making must rely on both, a thorough anatomical understanding and the given individual situation. Acknowledgments This work was supported by AOSpine TK of the TK System of the AO Foundation, Davos, Switzerland. D. Gehweiler received a research fellowship grant from the AO Research Institute Davos, Davos, Switzerland.