Contiguous-Level Unilateral Cervical Spine Facet Dislocation—A Report of a Less Discussed Subtype

Unilateral facet dislocation of subaxial cervical spine trauma is characterized by dislocation of inferior facet of superior vertebra over the superior facet of inferior vertebra. The injury is due to high-velocity trauma and associated with instability of spinal column. Such unilateral facet dislocations occurring at multiple adjacent levels for some reason are not reported or studied frequently. We have reported two cases of multiple-level dislocation of unilateral facets managed in our hospital with a review of available literature. The injury occurs as one side of the motion segment translates and rotates around an intact facet on the contralateral side. The major mechanism of injury is distractive flexion injury with axial rotation component. The injury is associated with instability secondary to loss of the discoligamentous complex. In cases with multiple-level dislocations of unilateral cervical facets, there are multiple mechanisms associated with significant neurological injury and most of them succumb at the site of injury. Only three other cases are available in English language literature. The neurological outcome is invariably poor. Multiple-level facet dislocations of subaxial cervical spine are reported sparsely in literature. We suspect that due to high-velocity nature of these injuries, most of them succumb soon after injury and not often reported. This article reports two cases of contiguous-level unilateral facet dislocation of subaxial cervical spine with associated injuries and the outcomes with review of literature.

Anteromedial Tibial Attachment in Single-Bundle Anterior Cruciate Ligament Reconstruction Can Represent Normal Kinematics in Computer Simulation

Tunnel position during anterior cruciate ligament (ACL) reconstruction is considered as an important factor to restore normal knee kinematics and to gain better clinical outcomes. It is still unknown where the optimal femoral and tibial tunnel position is located in single-bundle (SB) ACL reconstruction. The purposes of this study were to analyze the knee kinematics with various graft positions and to propose the optimal graft position during SB ACL reconstruction. A musculoskeletal computer simulation was used to analyze knee kinematics. Four attachments on the femoral side (anteromedial [AM], mid, posterolateral [PL], and over-the-top positions) and three attachments on the tibial side (AM, middle, and PL positions) were determined. The middle-bundle attachment was placed at the midpoint of the AM and PL bundle attachments for the femoral and tibial attachments. SB ACL reconstruction models were constructed to combine each of the four femoral attachments with each of three tibial attachments. Kinematic comparison was made among a double-bundle (DB) model and 12 SB reconstruction models during deep knee bend and stair descent activity. The tunnel position of the tibia had greater effect of knee kinematics than that of the femur. AM tibial attachment models showed similar medial and lateral anteroposterior positions to the DB model for both activities. Axial rotation in the AM tibial attachment models was similar to the DB model regardless of the femoral attachment, whereas greater maximum axial rotation was exhibited in the PL tibial attachment models, especially during stair descent activity. AM tibial attachment can represent normal knee kinematics, whereas the PL tibial attachment can induce residual rotational instability during high-demand activities. The AM tibial tunnel is recommended for SB ACL reconstruction.

Effects of Revision Rod Position on Spinal Construct Stability in Lumbar Revision Surgery: A Finite Element Study

Revision surgery (RS) is a necessary surgical intervention in clinical practice to treat spinal instrumentation–related symptomatic complications. Three constructs with different configurations have been applied in RS. One distinguishing characteristic of these configurations is that the revision rods connecting previous segments and revision segments are placed alongside, outside, or inside the previous rods at the level of facetectomy. Whether the position of the revision rod could generate mechanical disparities in revision constructs is unknown. The objective of this study was to assess the influence of the revision rod position on the construct after RS. A validated spinal finite element (FE) model was developed to simulate RS after previous instrumented fusion using a modified dual-rod construct (DRCm), satellite-rod construct (SRC), and cortical bone trajectory construct (CBTC). Thereafter, maximum von Mises stress (VMS) on the annulus fibrosus and cages and the ligament force of the interspinous ligament, supraspinous ligament, and ligamentum flavum under a pure moment load and a follower load in six directions were applied to assess the influence of the revision rod position on the revision construct. An approximately identical overall reducing tendency of VMS was observed among the three constructs. The changing tendency of the maximum VMS on the cages placed at L4-L5 was nearly equal among the three constructs. However, the changing tendency of the maximum VMS on the cage placed at L2-L3 was notable, especially in the CBTC under right bending and left axial rotation. The overall changing tendency of the ligament force in the DRCm, SRC, and CBTC was also approximately equal, while the ligament force of the CBTC was found to be significantly greater than that of the DRCm and SRC at L1-L2. The results indicated that the stiffness associated with the CBTC might be lower than that associated with the DRCm and SRC in RS. The results of the present study indicated that the DRCm, SRC, and CBTC could provide sufficient stabilization in RS. The CBTC was a less rigid construct. Rather than the revision rod position, the method of constructing spinal instrumentation played a role in influencing the biomechanics of revision.

Biomechanical feasibility of semi-rigid stabilization and semi-rigid lumbar interbody fusion: a finite element study

Background Semi-rigid lumbar fusion offers a compromise between pedicle screw-based rigid fixation and non-instrumented lumbar fusion. However, the use of semi-rigid interspinous stabilization (SIS) with interspinous spacer and ligamentoplasty and semi-rigid posterior instrumentation (SPI) to assist interbody cage as fusion constructs remained controversial. The purpose of this study is to investigate the biomechanical properties of semi-rigidly stabilized lumbar fusion using SIS or SPI and their effect on adjacent levels using finite element (FE) method. Method Eight FE models were constructed to simulate the lumbosacral spine. In the non-fusion constructs, semi-rigid stabilization with (i) semi-rigid interspinous spacer and artificial ligaments (PD-SIS), and (ii) PI with semi-rigid rods were simulated (PD + SPI). For fusion constructs, the spinal models were implanted with (iii) PEEK cage only (Cage), (iv) PEEK cage and SIS (Cage+SIS), (v) PEEK cage and SPI (Cage+SPI), (vi) PEEK cage and rigid PI (Cage+PI). Result The comparison of flexion-extension range of motion (ROM) in the operated level showed the difference between Cage+SIS, Cage+SPI, and Cage+PI was less than 0.05 degree. In axial rotation, ROM of Cage+SIS were greater than Cage+PI by 0.81 degree. In the infrajacent level, while Cage+PI increased the ROM by 24.1, 27,7, 25.9, and 10.3% and Cage+SPI increased the ROM by 26.1, 30.0, 27.1, and 10.8% in flexion, extension, lateral bending and axial rotation respectively, Cage+SIS only increased the ROM by 3.6, 2.8, and 11.2% in flexion, extension, and lateral bending and reduced the ROM by 1.5% in axial rotation. The comparison of the von Mises stress showed that SIS reduced the adjacent IVD stress by 9.0%. The simulation of the strain energy showed a difference between constructs less than 7.9%, but all constructs increased the strain energy in the infradjacent level. Conclusion FE simulation showed semi-rigid fusion constructs including Cage+SIS and Cage+SPI can provide sufficient stabilization and flexion-extension ROM reduction at the fusion level. In addition, SIS-assisted fusion resulted in less hypermobility and less von Mises stress in the adjacent levels. However, SIS-assisted fusion had a disadvantage of less ROM reduction in lateral bending and axial rotation. Further clinical studies are warranted to investigate the clinical efficacy and safety of semi-rigid fusions.

New anterior controllable antedisplacement and fusion surgery for cervical ossification of the posterior longitudinal ligament: a biomechanical study

OBJECTIVE The traditional anterior approach for multilevel severe cervical ossification of the posterior longitudinal ligament (OPLL) is demanding and risky. Recently, a novel surgical procedure—anterior controllable antedisplacement and fusion (ACAF)—was introduced by the authors to deal with these problems and achieve better clinical outcomes. However, to the authors’ knowledge, the immediate and long-term biomechanical stability obtained after this procedure has never been evaluated. Therefore, the authors compared the postoperative biomechanical stability of ACAF with those of more traditional approaches: anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF). METHODS To determine and assess pre- and postsurgical range of motion (ROM) (2 Nm torque) in flexion-extension, lateral bending, and axial rotation in the cervical spine, the authors collected cervical areas (C1–T1) from 18 cadaveric spines. The cyclic fatigue loading test was set up with a 3-Nm cycled load (2 Hz, 3000 cycles). All samples used in this study were randomly divided into three groups according to surgical procedures: ACDF, ACAF, and ACCF. The spines were tested under the following conditions: 1) intact state flexibility test; 2) postoperative model (ACDF, ACAF, ACCF) flexibility test; 3) cyclic loading (n = 3000); and 4) fatigue model flexibility test. RESULTS After operations were performed on the cadaveric spines, the segmental and total postoperative ROM values in all directions showed significant reductions for all groups. Then, the ROMs tended to increase during the fatigue test. No significant crossover effect was detected between evaluation time and operation method. Therefore, segmental and total ROM change trends were parallel among the three groups. However, the postoperative and fatigue ROMs in the ACCF group tended to be larger in all directions. No significant differences between these ROMs were detected in the ACDF and ACAF groups. CONCLUSIONS This in vitro biomechanical study demonstrated that the biomechanical stability levels for ACAF and ACDF were similar and were both significantly greater than that of ACCF. The clinical superiority of ACAF combined with our current results showed that this procedure is likely to be an acceptable alternative method for multilevel cervical OPLL treatment.

Spontaneous changes of skull morphology in premature babies: A preliminary study using plagiocephalometric analysis

Aim — to develop a reliable protocol to study the spontaneous changes in te skull morphology, specifically plagiocephaly in premature infants.Materials and methods. Evaluation of the degree of asymmetry using a plagiocephalometric tool and passive motion assessment for axial rotation and clinical examination of the neck muscles in six preterm infants. All measures are taken four times over a two months period.Results. About the placement of thermoplastic bands, a maximal variability of 3 % and 5 % was found for intra-and inter- observations respectively for the indices of interest (ODDI, CPI, CVAI). The variability of measures taken on photocopies was less than 1 %. 67 % of children had a preferential position on the third measure (T3) and 83 % on the fourth measure (T4). The prevalence of plagiocephaly was 17, 67, 33 and 50 % at T1, T2, T3 and T4 respectively considering a threshold of oblique diameters difference (ODDI) of 104 % . No influence of gender, gestational age, primiparity or asymmetry in muscle tension and/or rotation has been highlighted. In comparison with previous data, a very highly significant difference (p=0,001) was found for the index head values.Conclusion. This study has demonstrated the feasibility of the method. More consistent data should be considered, with a broader sample in order to provide a relevant analysis of the morphometric changes of the skull base. According to the criteria of the literature, three premature infants out of six had a plagiocephaly at T4 and we observed a normalization of morphometric values was observed in two infants.

Axial rotation as a cause of loss of interference in a shrink fitted split disc assembly

На примере составного диска, созданного посредством операции горячей посадки, изучается прочность такого соединения на отрыв. Отрывные усилия создаются в качестве центробежных инерционных сил при вращении диска относительное его центральной оси. Показывается, что созданный натяг в сборке имеет выраженную тенденцию к уменьшению. Однако для его обнуления необходимы достаточно значительные угловые скорости вращения. Приведен пример расчета придельной угловой скорости в зависимости от первоначального нагрева охватывающей детали сборки, термомеханических свойств материала сборки и геометрических параметров итоговой конструкции. Подобные расчеты в снижающемся натяге совершенно необходимы, когда эксплуатационные условия требуют принять вращение составного диска в качестве части его функциональных обязанностей. On the example of a composite disk, created by means of a hot-fit operation, the pulloff strength of such a disk is studied. separation forces as centrifugal forces with relative rotation of its central axis. It is shown that the created interference fit in the assembly has a pronounced tendency to decrease. However, for vanishing, sufficiently significant angular velocities of rotation are required. An example of calculating the near-limit angular velocity, depending on the original covered part of the assembly, of the thermomechanical properties of the material and the geometric characteristics of the final structure is given. Such computations in decreasing interference are essential when operating conditions require the rotation of the composite disc to be accepted as part of its functionality.

Computer Simulation of Optimal Lipped Polyethylene Liner Orientation Against Prosthetic Impingement

Background: Lipped or elevated acetabular liners are to improve posterior stability and are widely used in hip arthroplasty. However, concerns of increasing impingement exist when using such liners and optimal orientation of the elevated rim remains unknown. We aimed to identify the impact of lipped liner on the range of motion (ROM) before impingement and propose its optimal orientation.Methods: An isochoric three-dimensional model of a general hip-replacement prosthesis was generated and flex-extension, add-abduction, axial rotation was simulated on a computer. The maximum ROM of the hip was measured before the neck impinged on the liner. Different combinations of acetabular anteversion angles ranging from 5 to 30 degrees and lipped liner orientations from posterior to anterior were tested. Results: When acetabular anteversion was 10 or 15 degrees, placing the lip of the liner in the posterosuperior of the acetabulum allowed satisfactory ROM in all directions. When acetabular anteversion was 20 degrees, extension and external rotation were restricted. Adjusting the lip to the superior restored satisfactory ROM. When acetabular anteversion was 25 degrees, only placing the lip into the antero-superior could increase extension and external rotation to maintain satisfactory ROM.Conclusions: This study showed that optimal lipped liner orientation should be depend on acetabular anteversion. When acetabular anteversion was smaller than 20 degrees, placing lip in the posterior allowed an optimally ROM. When acetabular anteversion was greater than 20 degrees, adjusting lip to the anterior allowed a comprehensive larger ROM to avoid early impingement.

Three-Dimensional Upper Body Kinematics and Inter-articular Kinematic Sequence During a Canoe Polo Throw

Canoe polo is an increasingly popular discipline requiring both kayaking and ball-handling skills. While the kinematics of the upper body during throw has been investigated for several overhead sports, the canoe polo throw has still to be studied. Therefore, the aim of this study is to analyze the canoe polo throw kinematics in terms of angles and inter-articular sequencing to understand its specificity. A secondary aim was to investigate whether adding pelvis mobility has an impact. Nineteen male players of canoe polo were equipped with reflective body markers for the throw analysis. They performed 5 throws with the pelvis fixed and 5 throws with additional pelvic mobility in rotation around a vertical axis. Inverse kinematics was performed with OpenSim providing pelvis, trunk, and glenohumeral rotations. Angular velocities were calculated to build the inter-articular sequences relative to these throws. Statistical parametric mapping was used to assess the effect of pelvis mobility on the throwing kinematics. Similar kinematics patterns as in other overhead sports were observed, however, a different inter-articular sequence was found for the canoe polo throw with a maximal angular velocity occurring sooner for the thorax in axial rotation than for the pelvis in rotation. While the limitation of rotation of the pelvis around a vertical axis has an influence on the pelvis and trunk kinematics, it did not modify the kinematic sequence.

Biomechanical analysis of stand-alone lumbar interbody cages versus 360° constructs: an in vitro and finite element investigation

OBJECTIVE Low fusion rates and cage subsidence are limitations of lumbar fixation with stand-alone interbody cages. Various approaches to interbody cage placement exist, yet the need for supplemental posterior fixation is not clear from clinical studies. Therefore, as prospective clinical studies are lacking, a comparison of segmental kinematics, cage properties, and load sharing on vertebral endplates is needed. This laboratory investigation evaluates the mechanical stability and biomechanical properties of various interbody fixation techniques by performing cadaveric and finite element (FE) modeling studies. METHODS An in vitro experiment using 7 fresh-frozen human cadavers was designed to test intact spines with 1) stand-alone lateral interbody cage constructs (lateral interbody fusion, LIF) and 2) LIF supplemented with posterior pedicle screw-rod fixation (360° constructs). FE and kinematic data were used to validate a ligamentous FE model of the lumbopelvic spine. The validated model was then used to evaluate the stability of stand-alone LIF, transforaminal lumbar interbody fusion (TLIF), and anterior lumbar interbody fusion (ALIF) cages with and without supplemental posterior fixation at the L4–5 level. The FE models of intact and instrumented cases were subjected to a 400-N compressive preload followed by an 8-Nm bending moment to simulate physiological flexion, extension, bending, and axial rotation. Segmental kinematics and load sharing at the inferior endplate were compared. RESULTS The FE kinematic predictions were consistent with cadaveric data. The range of motion (ROM) in LIF was significantly lower than intact spines for both stand-alone and 360° constructs. The calculated reduction in motion with respect to intact spines for stand-alone constructs ranged from 43% to 66% for TLIF, 67%–82% for LIF, and 69%–86% for ALIF in flexion, extension, lateral bending, and axial rotation. In flexion and extension, the maximum reduction in motion was 70% for ALIF versus 81% in LIF for stand-alone cases. When supplemented with posterior fixation, the corresponding reduction in ROM was 76%–87% for TLIF, 86%–91% for LIF, and 90%–92% for ALIF. The addition of posterior instrumentation resulted in a significant reduction in peak stress at the superior endplate of the inferior segment in all scenarios. CONCLUSIONS Stand-alone ALIF and LIF cages are most effective in providing stability in lateral bending and axial rotation and less so in flexion and extension. Supplemental posterior instrumentation improves stability for all interbody techniques. Comparative clinical data are needed to further define the indications for stand-alone cages in lumbar fusion surgery.