Management of delayed posttraumatic cervical kyphosis

We describe three patients with misdiagnosed unstable fractures of the cervical spine, who were treated conservatively and developed kyphotic deformity, myelopathy, and radiculopathy. All three patients were then managed with closed reductions by crown halo traction, followed by instrumented fusions. Their neurologic function was regained without permanent disability in any patient. Unstable fractures of the cervical spine will progress to catastrophic neurologic injuries without surgical fixation. Posttraumatic kyphosis and the delayed reduction of partially healed fracture dislocations by preoperative traction are not well characterized in the subaxial cervical spine. The complete evaluation of any subaxial cervical spine fracture requires CT scanning to assess for bony fractures, and MRI to assess for ligamentous injury. This allows for assessment of the degree of instability and appropriate management. In patients with delayed posttraumatic cervical kyphosis, preoperative closed reduction provided adequate realignment, facilitating subsequent operative stabilization.     

Segmental kinematic analysis of planovalgus feet during walking in children with cerebral palsy

Pes planovalgus (flatfoot) is a common deformity among children with cerebral palsy. The Milwaukee Foot Model (MFM), a multi-segmental kinematic foot model, which uses radiography to align the underlying bony anatomy with reflective surface markers, was used to evaluate 20 pediatric participants (30 feet) with planovalgus secondary to cerebral palsy prior to surgery. Three-dimensional kinematics of the tibia, hindfoot, forefoot, and hallux segments are reported and compared to an age-matched control set of typically-developing children. Most results were consistent with known characteristics of the deformity and showed decreased plantar flexion of the forefoot relative to hindfoot, increased forefoot abduction, and decreased ranges of motion during push-off in the planovalgus group. Interestingly, while forefoot characteristics were uniformly distributed in a common direction in the transverse plane, there was marked variability of forefoot and hindfoot coronal plane and hindfoot transverse plane positioning. The key finding of these data was the radiographic indexing of the MFM was able to show flat feet in cerebral palsy do not always demonstrate more hindfoot eversion than the typically-developing hindfoot. The coronal plane kinematics of the hindfoot show cases planovalgus feet with the hindfoot in inversion, eversion, and neutral. Along with other metrics, the MFM can be a valuable tool for monitoring kinematic deformity, facilitating clinical decision making, and providing a quantitative analysis of surgical effects on the planovalgus foot.     

Comparison of the anatomic risk for vertebral artery injury associated with percutaneous atlantoaxial anterior and posterior transarticular screws

Background contextAs a minimally invasive spine surgery, percutaneous atlantoaxial fixation techniques using anterior transarticular screw (ATS) and posterior transarticular screw (PTS) have promising clinical results. However, transarticular screw fixation is technically demanding and carries a potential risk of iatrogenic vertebral artery (VA) injury. There were no available data comparing the anatomic risk of VA injury associated with these screws.PurposeTo evaluate the trajectories of percutaneous atlantoaxial ATS and PTS through three-dimensional (3D) computerized tomography.Study designTo compare the anatomic risk of VA injury between percutaneous ATS and PTS.Patient sampleSixty patients ranged in age from 19 to 75 years (mean, 45.08 years) and included 35 men and 25 women.Outcome measuresImage measurement of C2 isthmus height and C2 isthmus width and the distance between the medial-most superior articular facet to the medial-most edge of the VA groove of the C2 (D).MethodsSixty consecutive patients (in total) with lower cervical lesions were evaluated through 3D images reconstructed by a rapid 3D system. The maximum possible diameters of the percutaneous atlantoaxial ATS and PTS trajectories were compared and examined. Mean, range, and standard deviations for each type of screw, for left and right trajectories, and for men and women were calculated from 120 percutaneous atlantoaxial ATS and PTS measurements through SPSS.ResultsThe maximum mean diameter differed significantly between the trajectories of 120 percutaneous atlantoaxial ATS and PTS. For screw trajectories ≤3.5 mm in diameter, 19.2% of the PTS trajectories were judged as risky, whereas all the anterior ones were judged as safe.ConclusionsFrom an anatomic perspective, percutaneous ATS fixation poses less anatomic risk of VA injury than percutaneous PTS fixation. As an alternative surgical therapy for atlantoaxial subluxation, percutaneous ATS fixation may play a more important role in the future.     

Wear patterns of taper connections in retrieved large diameter metal‐on‐metal bearings

Wear of the modular taper between head and shaft has been related to clinical failure resulting from adverse reactions to metallic debris. The problem has become pronounced in large metal‐on‐metal bearings, but the mechanism has not yet been fully understood. We analyzed retrieved components from five patients revised with various diagnoses. Two distinct wear patterns were observed for the head tapers. Three samples demonstrated “asymmetric” wear towards the inner end of the head taper. The other two showed “axisymmetric” radial wear (up to 65 µm) presenting the largest wear volumes (up to 20 mm³). Stem tapers demonstrated relatively little wear, and the fine thread on the stem taper surface was observed to be imprinted on the taper inside of the head. Our findings demonstrate that the cobalt‐chrome head wears preferentially to the titanium stem taper. “asymmetric” wear suggests toggling due to the offset of the joint force vector from the taper. In contrast, samples with “axisymmetric” radial wear and a threaded imprint suggested that corrosion led to head subsidence onto the stem taper with gradual rotation. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1116–1122, 2013     

Parametric and cadaveric models of lumbar flexion instability and flexion restricting dynamic stabilization system

Purpose

Development of a dynamic stabilization system often involves costly and time-consuming design iterations, testing and computational modeling. The aims of this study were (1) develop a simple parametric model of lumbar flexion instability and use this model to identify the appropriate stiffness of a flexion restricting stabilization system (FRSS), and (2) in a cadaveric experiment, validate the predictive value of the parametric model.

Methods

Literature was surveyed for typical parameters of intact and destabilized spines: stiffness in the high flexibility zone (HFZ) and high stiffness zone, and size of the HFZ. These values were used to construct a bilinear parametric model of flexion kinematics of intact and destabilized lumbar spines. FRSS implantation was modeled by iteratively superimposing constant flexion stiffnesses onto the parametric model. Five cadaveric lumbar spines were tested intact; after L4–L5 destabilization (nucleotomy, midline decompression); and after FRSS implantation. Specimens were loaded in flexion/extension (8 Nm/6 Nm) with 400 N follower load to characterize kinematics for comparison with the parametric model.

Results

To accomplish the goal of reducing ROM to intact levels and increasing stiffness to approximately 50 % greater than intact levels, flexion stiffness contributed by the FRSS was determined to be 0.5 Nm/deg using the parametric model. In biomechanical testing, the FRSS restored ROM of the destabilized segment from 146 ± 13 to 105 ± 21 % of intact, and stiffness in the HFZ from 41 ± 7 to 135 ± 38 % of intact.

Conclusions

Testing demonstrated excellent predictive value of the parametric model, and that the FRSS attained the desired biomechanical performance developed with the model. A simple parametric model may allow efficient optimization of kinematic design parameters.     

The biomechanical influence of the facet joint orientation and the facet tropism in the lumbar spine

Background contextFacet joint orientation and facet tropism (FT) are presented as the potential anatomical predisposing factors for lumbar degenerative changes that may lead in turn to early degeneration and herniation of the corresponding disc or degenerative spondylolisthesis. However, no biomechanical study of this concept has been reported.PurposeTo investigate the biomechanical influence of the facet orientation and FT on stress on the corresponding segment.Study designFinite element analysis.MethodsThree models, F50, F55, and F60 were simulated with different facet joint orientations (50°, 55°, and 60° relative to coronal plane) at both L2–L3 facet joints. A FT model was also simulated to represent a 50° facet joint angle at the right side and a 60° facet joint angle at the left side in the L2–L3 segment. In each model, the intradiscal pressures were investigated under four pure moments and anterior shear force. Facet contact forces at the L2–L3 segment were also analyzed under extension and torsion moments and anterior shear force. This study was supported by 5000 CHF grant of 2011 AO Spine Research Korea fund. The authors of this study have no topic-specific potential conflicts of interest related to this study.ResultsThe F50, F55, and F60 models did not differ in the intradiscal pressures generated under four pure moments: but under anterior shear force, the F60 and FT models showed increases of intradiscal pressure. The F50 model under extension and the F60 model under torsion each generated an increase in facet contact force. In all conditions tested, the FT model yielded the greatest increase of intradiscal pressure and facet contact force of all the models.ConclusionsThe facet orientation per se did not increase disc stress or facet joint stress prominently at the corresponding level under four pure moments, but FT could make the corresponding segment more vulnerable to external moments or anterior shear force.