Correction to: Prosthesis design influences segmental contribution to total cervical motion after cervical disc arthroplasty.

Unfortunately, figures 1 and 2 have been incorrectly published in the original publication. The complete correct figures are given below with the captions.

Prosthesis design influences segmental contribution to total cervical motion after cervical disc arthroplasty.

INTRODUCTION: We investigated a new metric for assessing the quality of motion of the cervical segments over the arc of extension-to-flexion motion after cervical disc arthroplasty (CDA). We quantified: (1) the amount of motion contributed by individual spinal segments to the total cervical spine motion, termed segmental motion fraction, and its variation throughout the arc of extension-to-flexion motion and (2) how cervical disc arthroplasty using two distinct prosthesis designs may influence the segmental motion contributions. MATERIALS AND METHODS: We tested 16 human C3-T1 spine specimens under physiologic loads; first intact, after CDA at C5-C6, and then at C5-C6 and C6-C7. The M6-C (Orthofix, USA) and Mobi-C (Zimmer, USA) disc prostheses were used in eight specimens each. RESULTS AND CONCLUSIONS: The designs of the cervical disc prostheses tested significantly influenced the variation in segmental motion fraction as the spine underwent motion between the endpoints of extension and flexion. While the mean segmental motion contribution to the total cervical motion was not influenced by prosthesis design, the way the motion took place between the extension and flexion endpoints was significantly influenced. The M6-C artificial disc restored physiologic motion quality such that implanted segments continued to function in harmony with other segments of the cervical spine as measured before arthroplasty. Conversely, the Mobi-C prosthesis, while maintaining average motion contributions similar to the pre-implantation values, demonstrated large deviations in motion contribution over the extension-to-flexion arc motion in ten of 16 implanted segments. Such non-physiologic implant kinematics could cause excessive prosthesis wear and motion and stress shielding at adjacent segments. These slides can be retrieved under Electronic Supplementary Material.

Cervical sagittal balance: a biomechanical perspective can help clinical practice.

PURPOSE: In this article, we summarize our work on understanding the influence of cervical sagittal malalignment on the mechanics of the cervical spine. METHODS: Biomechanical studies were performed using an ex vivo laboratory model to study the kinematic and kinetic response of human cervical spine specimens in the setting of cervical sagittal imbalance. The model allowed controlled variations of C2-C7 Sagittal Vertical Alignment (C2-C7 SVA) and T1-Slope so that clinically relevant sagittally malaligned profiles could be prescribed, while maintaining horizontal gaze, and their biomechanical consequences studied. RESULTS: Our results demonstrated that increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. An increase in C2-C7 SVA increased the lower cervical neural foraminal areas. Conversely, increasing T1-slope predominantly influenced subaxial cervical lordosis and, as a result, decreased cervical neural foraminal areas. Therefore, we believe patients with increased upper thoracic kyphosis and radicular symptoms may respond with increased forward head posture (FHP) as a compensatory mechanism to increase their lower cervical neural foraminal area and alleviate nerve root compression as well as reduce the burden on posterior muscles and soft and bony structures of the cervical spine. Increasing FHP (i.e., increased C2-C7 SVA) was associated with shortening of the cervical flexors and occipital extensors and lengthening of the cervical extensors and occipital flexors, which corresponds to C2-C7 flexion and C0-C2 extension. The greatest shortening occurred in the suboccipital muscles, suggesting considerable load bearing of these muscles during chronic FHP. Regardless, there was no evidence of nerve compression within the suboccipital triangle. Finally, cervical sagittal imbalance may play a role in exacerbating adjacent segment pathomechanics after multilevel cervical fusion and should be considered during surgical planning. CONCLUSIONS: The results of our biomechanical studies have improved our understanding of the impact of cervical sagittal malalignment on pathomechanics of the cervical spine. We believe this improved understanding will assist in clinical decision-making.

Dimensions of the cervical neural foramen in conditions of spinal deformity: an ex vivo biomechanical investigation using specimen-specific CT imaging.

PURPOSE: Patients with cervical spondylosis commonly present with neck pain, radiculopathy or myelopathy. As degenerative changes progress, multiple factors including disc height loss, thoracic kyphosis, and facetogenic changes can increase the risk of neural structure compression. This study investigated the impact of cervical deformity including forward head posture (FHP) and upper thoracic kyphosis, on the anatomy of the cervical neural foramen. METHODS: Postural changes of 13 human cervical spine specimens (Occiput-T1, age 50.6 years; range 21-67) were assessed in response to prescribed cervical sagittal malalignments using a previously reported experimental model. Two characteristics of cervical sagittal deformities, C2-C7 sagittal vertical alignment (SVA) and sagittal angle of the T1 vertebra (T1 tilt), were varied to create various cervical malalignments. The postural changes were documented by measuring vertebral positions and orientations. The vertebral motion data were combined with specimen-specific CT-based anatomical models, which allowed assessments of foraminal areas of subaxial cervical segments as a function of increasing C2-C7 SVA and changing T1 tilt. RESULTS: Increasing C2-C7 SVA from neutral posture resulted in increased neural foraminal area in the lower cervical spine (largest increase at C4-C5: 13.8 ± 15.7 %, P < 0.01). Increasing SVA from a hyperkyphotic posture (greater T1 tilt) also increased the neural foraminal area in the lower cervical segments (C5-C6 demonstrated the largest increase: 13.4 ± 9.6 %, P < 0.01). The area of the cervical neural foramen decreased with increasing T1 tilt, with greater reduction occurring in the lower cervical spine, specifically at C5-C6 (-8.6 ± 7.0 %, P < 0.01) and C6-C7 (-9.6 ± 5.6 %, P < 0.01). CONCLUSION: An increase in thoracic kyphosis (T1 tilt) decreased cervical neural foraminal areas. In contrast, an increase in cervical SVA increased the lower cervical neural foraminal areas. Patients with increased upper thoracic kyphosis may respond with increased cervical SVA as a compensatory mechanism to increase their lower cervical neural foraminal area.

Coupled rotation patterns in cervical spine axial rotation can change when the head is kept level.

The biomechanical literature describes axial rotation occurring coupled with lateral bending and flexion in the cervical spine. Since the head is kept level during some activities of daily living, we set out to investigate the changes in total and segmental motion that occur when a level gaze constraint is applied to cadaveric cervical spine specimens during axial rotation. 1.5Nm of left and right axial rotation moment was applied to sixteen C2-T1 cadaveric specimens with C2 unconstrained and C2 constrained to simulate level gaze. Overall and segmental motions were determined using optoelectronic motion measurement and specimen-specific kinematic modeling. Without a kinematic constraint on C2, motions were as described in the literature; namely, flexion and lateral bending to the same side as axial rotation. Keeping C2 level reduced that total axial rotation range of motion of the specimens. Changes were also produced in segmental coupled rotation in all specimens. The observed changes included completely opposite coupled motion than in the uncoupled specimens, and traditional coupled behavior at one load extreme and the opposite at the other extreme. Constraining C2 during axial rotation to simulate level gaze can produce coupled motion that differs from the classically described flexion and lateral bending to the same side as axial rotation. Statement of Clinical Significance: Activities of daily living that require the head to be kept level during axial rotation of the cervical spine may produce segmental motions that are quite different from the classically described motions with implications for biomechanical experiments and implant designers.

Prosthesis design and likelihood of achieving physiological range of motion after cervical disc arthroplasty: analysis of range of motion data from 1,173 patients from 7 IDE clinical trials.

BACKGROUND CONTEXT: The functional goals of cervical disc arthroplasty (CDA) are to restore enough range of motion (ROM) to reduce the risk of accelerated adjacent segment degeneration but limit excessive motion to maintain a biomechanically stable index segment. This motion-range is termed the "Physiological mobility range." Clinical studies report postoperative ROM averaged over all study subjects but they do not report what proportion of reconstructed segments yield ROM in the Physiological mobility range following CDA surgery. PURPOSE: To calculate the proportion of reconstructed segments that yield flexion-extension ROM (FE-ROM) in the Physiological mobility range (defined as 5°-16°) by analyzing the 24-month postoperative data reported by clinical trials of various cervical disc prostheses. STUDY DESIGN/SETTING: Analysis of 24-month postoperative FE-ROM data from clinical trials. PATIENT SAMPLE: Data from 1,173 patients from single-level disc replacement clinical trials of 7 cervical disc prostheses. OUTCOME MEASURES: 24-month postoperative index-level FE-ROM. METHODS: The FE-ROM histograms reported in Food and Drug Administration-Investigational Device Exemption (FDA-IDE) submissions and available for this analysis were used to calculate the frequencies of implanted levels with postoperative FE-ROM in the following motion-ranges: Hypomobile (0°-4°), Physiological (5°-16°), and Hypermobile (≥17°). The ROM histograms also allowed calculation of the average ROM of implanted segments in each of the 3 motion-ranges. RESULTS: Only 762 of 1,173 patients (implanted levels) yielded 24-month postCDA FE-ROM in the physiological mobility range (5°-16°). The proportions ranged from 60% to 79% across the 7 disc-prostheses, with an average of 65.0%±6.2%. Three-hundred and two (302) of 1,173 implanted levels yielded ROM in the 0°-4° range. The proportions ranged from 15% to 38% with an average of 25.7%±8.9%. One-hundred and nine (109) of 1,173 implanted levels yielded ROM of ≥17° with a range of 2%-21% and an average proportion of 9.3%±7.9%. The prosthesis with built-in stiffness due to its nucleus-annulus design yielded the highest proportion (103/131, 79%) of implanted segments in the physiological mobility range, compared to the cohort average of 65% (p<.01). Sixty-five of the 350 (18.6%) discs implanted with the 2 mobile-core designs in this cohort yielded ROM≥17° as compared to the cohort average of 9.3% (109/1,173) (p<.05). At 2-year postCDA, the "hypomobile" segments moved on average 2.4±1.2°, those in the "physiological-mobility" group moved 9.4±3.2°, and the hypermobile segments moved 19.6±2.6°. CONCLUSIONS: Prosthesis design significantly influenced the likelihood of achieving FE-ROM in the physiological mobility range, while avoiding hypomobility or hypermobility (p<.01). Postoperative ROM averaged over all study subjects provides incomplete information about the prosthesis performance - it does not tell us how many implanted segments achieve physiological mobility and how many end up with hypomobility or hypermobility. We conclude that the proportion of index levels achieving postCDA motions in the physiological mobility range (5°-16°) is a more useful outcome measure for future clinical trials.

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.

Biomechanical evaluation of a low profile, anchored cervical interbody spacer device in the setting of progressive flexion-distraction injury of the cervical spine.

INTRODUCTION: Anterior cervical decompression and fusion is a well-established procedure for treatment of degenerative disc disease and cervical trauma including flexion-distraction injuries. Low-profile interbody devices incorporating fixation have been introduced to avoid potential issues associated with dissection and traditional instrumentation. While these devices have been assessed in traditional models, they have not been evaluated in the setting of traumatic spine injury. This study investigated the ability of these devices to stabilize the subaxial cervical spine in the presence of flexion-distraction injuries of increasing severity. METHODS: Thirteen human cadaveric subaxial cervical spines (C3-C7) were tested at C5-C6 in flexion-extension, lateral bending and axial rotation in the load-control mode under ±1.5 Nm moments. Six spines were tested with locked screw configuration and seven with variable angle screw configuration. After testing the range of motion (ROM) with implanted device, progressive posterior destabilization was performed in 3 stages at C5-C6. RESULTS: The anchored spacer device with locked screw configuration significantly reduced C5-C6 flexion-extension (FE) motion from 14.8 ± 4.2 to 3.9 ± 1.8°, lateral bending (LB) from 10.3 ± 2.0 to 1.6 ± 0.8, and axial rotation (AR) from 11.0 ± 2.4 to 2.5 ± 0.8 compared with intact under (p < 0.01). The anchored spacer device with variable angle screw configuration also significantly reduced C5-C6 FE motion from 10.7 ± 1.7 to 5.5 ± 2.5°, LB from 8.3 ± 1.4 to 2.7 ± 1.0, and AR from 8.8 ± 2.7 to 4.6 ± 1.3 compared with intact (p < 0.01). The ROM of the C5-C6 segment with locked screw configuration and grade-3 F-D injury was significantly reduced from intact, with residual motions of 5.1 ± 2.1 in FE, 2.0 ± 1.1 in LB, and 3.3 ± 1.4 in AR. Conversely, the ROM of the C5-C6 segment with variable-angle screw configuration and grade-3 F-D injury was not significantly reduced from intact, with residual motions of 8.7 ± 4.5 in FE, 5.0 ± 1.6 in LB, and 9.5 ± 4.6 in AR. CONCLUSIONS: The locked screw spacer showed significantly reduced motion compared with the intact spine even in the setting of progressive flexion-distraction injury. The variable angle screw spacer did not sufficiently stabilize flexion-distraction injuries. The resulting motion for both constructs was higher than that reported in previous studies using traditional plating. Locked screw spacers may be utilized with additional external immobilization while variable angle screw spacers should not be used in patients with flexion-distraction injuries.

Effect of K-wire Reuse and Drill Mode on Heat Generation in Bone.

BACKGROUND: We examined the effect of Kirschner wire (K-wire) reuse and use of oscillating mode on heat generation within cortical bone. METHODS: Two trocar-tipped K-wires were drilled through the diaphysis of each of 30 human metacarpals and phalanges: one K-wire was inserted in rotary mode and another in oscillating mode. Each wire was reused once. Thermocouples placed within the dorsal and volar bone adjacent to the K-wire drill path measured temperatures throughout each test. RESULTS: Peak cortex temperatures were 25°C to 164°C. Rotary drilling achieves peak temperatures quicker (31 ± 78 seconds vs 44 ± 78 seconds, P = .19) than oscillating drilling, but insertion time is also less, resulting in lower overall heat exposure. This effect is also seen when the K-wire is reused (34 ± 70 seconds vs 41 ± 85 seconds, P = .4). The length of time that cortical bone was exposed to critical temperatures (47°C or more) was significantly higher when a wire was reused (36 ± 72 seconds vs 43 ± 82 seconds, P = .008). Peak temperatures greater than 70°C (a temperature associated with instantaneous cell death) were observed on many occasions. CONCLUSIONS: Overall heat exposure may be higher if a K-wire is reused or inserted in oscillating mode. In the absence of external cooling, K-wire insertion into cortical bone can easily expose bone to temperatures that exceed 70°C and may increase the risk of osteonecrosis.

Bone cutting efficiency and heat generation using a traditional fluted Burr and a novel fluteless resurfacing tool.

BACKGROUND: Powered instrumentation is often used for bone preparation and/or removal in many orthopaedic procedures but does risk thermogenesis. This study compares biomechanical properties of a fluted burr and a novel fluteless resurfacing tool. METHODS: Twenty cadaveric metatarsals were tested with four predetermined cutting forces to evaluate heat generation and cutting rate for the fluted burr and fluteless resurfacing tool over 40 s or until a depth of 4 mm was reached. Cutting rate was calculated from displacement transducer data. Heat generation was measured by thermocouples placed in the bone adjacent to the burring site. Assuming a body temperature of 37 °C, a 10 °C increase in heat was used as the threshold of inducing osteonecrosis. FINDINGS: At 1.0 N and 1.7 N, the thermal osteonecrosis threshold was reached at comparable times between burrs, while the bone removed by the resurfacing tool was on average five times greater than fluted burr at 1.0 N and over twice as great at 1.7 N. Statistical analysis of these common cutting forces showed the resurfacing tool had significantly higher cutting rates (P < 0.01). As a result, the fluted burr produced higher temperatures for the same amount of bone removal (P < 0.01). INTERPRETATION: In a cadaveric study, the fluteless resurfacing tool demonstrated higher bone cutting rates and lower heat generation for the same amount of bone removed than a traditional fluted burr.

Republication of "A Biomechanical Comparison of Limited Open Versus Krackow Repair for Achilles Tendon Rupture".

Background: Soft tissue complications after Achilles tendon repair has led to increased interest in less invasive techniques. Various limited open techniques have gained popularity as an alternative to open operative repair. The purpose of this study was to biomechanically compare an open Krackow and limited open repair for Achilles tendon rupture. We hypothesized that there would be no statistical difference in load to failure, work to failure, and initial linear stiffness. Methods: A simulated Achilles tendon rupture was created 4 cm proximal to its insertion in 18 fresh-frozen cadaveric below-knee lower limbs. Specimens were randomized to open or limited open PARS Achilles Jig System repair. Repairs were loaded to failure at a rate of 25.4 mm/s to reflect loading during normal ankle range of motion. Load to failure, work to failure, and initial linear stiffness were compared between the 2 repair types. Results: The average load to failure (353.8 ± 88.8 N vs 313.3 ± 99.9 N; P = .38) and work to failure (6.4 ± 2.3 J vs 6.3 ± 3.5 J; P = .904) were not statistically different for Krackow and PARS repair, respectively. Mean initial linear stiffness of the Krackow repair (17.8 ± 5.4 N/mm) was significantly greater than PARS repair (11.8 ± 2.5 N/mm) (P = .011). Conclusion: No significant difference in repair strength was seen, but higher initial linear stiffness for Krackow repair suggests superior resistance to gap formation, which may occur during postoperative rehabilitation. With equal repair strength, but less soft tissue devitalization, the PARS may be a favorable option for patients with risk factors for soft tissue complications.

Effect of prosthesis endplate lordosis angles on L5-S1 kinematics after disc arthroplasty.

OBJECTIVE: We hypothesized that L5-S1 kinematics will not be affected by the lordosis distribution between the prosthesis endplates. MATERIALS AND METHODS: Twelve cadaveric lumbosacral spines (51.3 ± 9.8 years) were implanted with 6° or 11° prostheses (ProDisc-L) with four combinations of superior/inferior lordosis (6°/0°, 3°/3°, 11°/0°, 3°/8°). Specimens were tested intact and after prostheses implantation with different lordosis distributions. Center of rotation (COR) and range of motion (ROM) were quantified. RESULTS: Six-degree lordosis prostheses (n = 7) showed no difference in flexion-extension ROM, regardless of design (6°/0° or 3°/3°) (p > 0.05). In lateral bending (LB), both designs reduced ROM (p < 0.05). In axial rotation, only the 3°/3° design reduced ROM (p < 0.05). Eleven-degree lordosis prostheses (n = 5) showed no difference in flexion-extension ROM for either design (p > 0.05). LB ROM decreased with distributed lordosis prostheses (3°/8°) (p < 0.05). Overall, L5-S1 range of motion was not markedly influenced by lordosis distribution among the two prosthesis endplates. The ProDisc-L prosthesis design where all lordosis is concentrated in the superior endplate yielded COR locations that were anterior and caudal to intact controls. The prosthesis with lordosis distributed between the two endplates yielded a COR that tended to be closer to intact. CONCLUSIONS: Further clinical and biomechanical studies are needed to assess the long-term impact of lordosis angle distribution on the fate of the facet joints.

Would an anatomically shaped lumbar interbody cage provide better stability? An in vitro cadaveric biomechanical evaluation.

STUDY DESIGN: A biomechanical cadaveric study of lumbar spine segments. OBJECTIVE: To compare the immediate stability provided by parallel-shaped and anatomically shaped carbon fiber interbody fusion I/F cages in posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) constructs with posterior pedicle screw instrumentation. SUMMARY OF BACKGROUND DATA: Few biomechanical data are available on the anatomically shaped cages in PLIF and TLIF constructs. METHODS: Twenty human lumbar segments were tested in flexion-extension (FE) (8 N m flexion, 6 N m extension), lateral bending (LB) (± 6 N m), and torsional loading (± 5 N m). Each segment was tested in the intact state and after insertion of interbody cages in one of 3 constructs: PLIF with 2 parallel-shaped or anatomically shaped cages and TLIF with 1 anatomically shaped cage. All cages received supplementary pedicle screw fixation. The range-of-motion (ROM) values after cage insertion and posterior fixation were compared with the intact specimen values using analysis of variance and multiple comparisons with Bonferroni correction. RESULTS: All constructs significantly reduced segmental motion relative to intact (P < 0.001). The motion reductions in FE, LB, and axial rotation were 85 ± 15%, 83 ± 18%, and 67 ± 6.8% for the PLIF construct using parallel cages, 79 ± 5.5%, 87 ± 10%, and 66 ± 20% for PLIF using anatomically shaped cages, and 90 ± 6.8%, 87 ± 12%, and 77 ± 22% for TLIF with an anatomically shaped cage. In FE and LB, the reductions in the ROM caused between the 3 constructs were equivalent (P > 0.05). In axial rotation, the TLIF cage provided significantly greater limitation in the ROM compared with the parallel-shaped PLIF cage (P = 0.01). CONCLUSIONS: The parallel-shaped and anatomically shaped I/F cages provided similar stability in a PLIF construct. The greater stability of the TLIF construct was likely due to a more anterior placement of the TLIF cage and preservation of the contralateral facet joint.

Kinematics of a cervical disc prosthesis implanted above or below one- and two-level fusions.

Background: The theoretical advantages of hybrid constructs over multi-level fusion have been illustrated in clinical and biomechanical studies. However, there is no biomechanical data on hybrid constructs using load control analyses. There is also no clear data on whether there is a biomechanical difference if the arthroplasty is below or above a 1- or 2-level fusion. This work investigated the effect on segmental motion of having a cervical total disc arthroplasty implanted above or below a 1- or 2-level fusion. Methods: Segmental motions of 16 C2-T1 cervical spine specimens were measured as the specimens were tested to 1.5Nm in axial rotation and in flexion-extension under compressive preload. Tests were conducted on intact specimens, and then after arthroplasty with a 1-level and 2-level fusion. 8 specimens were in test Group 1, where the hybrid configuration had a total disc arthroplasty above a 1- or 2-level fusion. The arthroplasty was below the 1- and 2-level fusion in Group 2. Load control and displacement control analyses were conducted to determine the effect of the hybrid configurations on segmental motion. Results: In load control, compensatory motion increases were found at all non-instrumented cervical spine segments in flexion-extension and axial rotation. Flexion-extension and axial rotation ranges of motion at the total disc arthroplasty level were less than 1° different than intact.In displacement control, there was no consistent pattern of compensatory motion. Range of motion at the arthroplasty level was within 3.5° of intact. Conclusions: The total disc arthroplasty segmental level in a hybrid construct has similar amounts of motion as intact. This may shield the arthroplasty level and adjacent levels from supra-physiological motion and loading. These results suggest that a hybrid construct may be protective of adjacent segments, whether the total disc arthroplasty is above or below the fusion.

Comparison of the biomechanical stability of transverse and oblique screw trajectories in retrograde intramedullary nailing of supracondylar femur fractures.

BACKGROUND: The goal was to determine the effect of addition of oblique trajectory distal interlock screws to a retrograde intramedullary femoral nail on implant stability (stiffness), cycles to failure and mode of failure. The hypothesis was that addition of oblique screws would increase implant stability and number of loading cycles to failure. METHODS: Eight matched pairs were tested; one femur implanted with a femoral nail with only transverse distal interlock screws and the other with transverse and oblique interlock screws. Axial compressive load was applied to the femoral head and the gluteal tendon was tensioned vertically to simulate standing or at 45° to the sagittal plane to simulate stair climbing. Loads were cycled to increasing amplitude until failure of fixation (10 mm displacement or 10° rotation). FINDINGS: In simulated standing, oblique screw specimen had greater sagittal bending (bowing) than transverse only specimen. Transverse (axial) plane motion was higher in simulated stair climbing in oblique screw specimen. Oblique screw specimen had higher sagittal plane translation at 600 N of load. At 300 N, oblique screw specimen had lower internal-external rotation than transverse only specimen. A larger number of cycles to failure were observed in four oblique screw of seven paired specimen. Failure (10 mm or 10 degrees of motion) was only achieved during simulated stair climbing. INTERPRETATION: Our hypothesis that adding oblique screws improves fixation was rejected. Activities of daily living other than standing may constitute a challenge to fracture fixation; fixation failure occurred at lower loads in simulated stair climbing than standing.

Sex-based Difference in Response to Recombinant Human Bone Morphogenetic Protein-2 in a Rat Posterolateral Fusion Model.

STUDY DESIGN: This was a preclinical study. OBJECTIVE: Evaluate sex-dependent differences in the bone healing response to recombinant human bone morphogenetic protein-2 (rhBMP-2) in a rat posterolateral spinal fusion model. SUMMARY OF BACKGROUND DATA: Minimal and conflicting data exist concerning potential sex-dependent differences in rhBMP-2-mediated bone regeneration in the context of spinal fusion. MATERIALS AND METHODS: Forty-eight female and male Sprague-Dawley rats (N=24/group), underwent L4-L5 posterolateral fusion with bilateral placement of an absorbable collagen sponge, each loaded with 5 µg of bone morphogenetic protein-2 (10 µg/animal). At eight weeks postoperative, 10 specimens of each sex were tested in flexion-extension with quantification of range of motion and stiffness. The remaining specimens were evaluated for new bone growth and successful fusion via radiography, blinded manual palpation and microcomputed tomography (microCT). Laboratory microCT quantified bone microarchitecture, and synchrotron microCT examined bone microstructure at the 1 µm level. RESULTS: Manual palpation scores differed significantly between sexes, with mean fusion scores of 2.4±0.4 in females versus 3.1±0.6 in males ( P <0.001). Biomechanical stiffness did not differ between sexes, but range of motion was significantly greater and more variable for females versus males (3.7±5.6° vs. 0.27±0.15°, P <0.005, respectively). Laboratory microCT showed significantly smaller volumes of fusion masses in females versus males (262±87 vs. 732±238 mm 3 , respectively, P <0.001) but significantly higher bone volume fraction (0.27±0.08 vs. 0.12±0.05, respectively, P <0.001). Mean trabecular thickness was not different, but trabecular number was significantly greater in females (3.1±0.5 vs. 1.5±0.4 mm -1 , respectively, P <0.001). Synchrotron microCT showed fine bone structures developing in both sexes at the eight-week time point. CONCLUSIONS: This study demonstrates sex-dependent differences in bone regeneration induced by rhBMP-2. Further investigation is needed to uncover the extent of and mechanisms underlying these sex differences, particularly at different doses of rhBMP-2.

Quantitative analysis of the long- and short-arm crescentic shelf bunionectomy osteotomies in fresh cadaveric matched pair specimens.

Two variations of crescentic shelf osteotomies have been described for the treatment of moderate to severe hallux abductovalgus: a short arm and a long arm. This study tested the hypothesis that the short-arm osteotomy will have a greater moment to failure and angular stiffness than the long arm. Eighteen first metatarsal specimens were dissected from 9 matched pairs of fresh frozen cadaveric specimens. One metatarsal from each pair received a short-arm osteotomy, whereas the other received a long-arm osteotomy. Each osteotomy was fixed with 2 screws. The short arm was fixed with 1 oblique screw and 1 dorsal-to-plantar screw. The long arm was fixed with 2 dorsal-to-plantar screws: 1 at the proximal aspect and 1 at the distal aspect of the shelf. Each specimen was loaded in a materials testing machine to measure moment to failure and angular stiffness. The base of the first metatarsal was potted and load applied to the plantar aspect of the metatarsal head at a constant rate until failure of the osteotomy. The mean maximum moment to failure of the short arm was significantly greater than the long arm (2.04 ± 0.96 Newton meter [Nm] vs. 1.48 ± 0.67 Nm, P = .03). The mean angular stiffness was significantly greater for short arm versus long arm (23.8 ± 19.11 Nm/radian vs. 0.98 ± 9.08 Nm/radian, P = .01). We report statistically significant data supporting the short-arm crescentic shelf osteotomy to have a greater moment to failure and angular stiffness compared with the long-arm crescentic shelf osteotomy.

Biomechanics of Cervical Disc Arthroplasty Devices.

Prosthesis design has an influence on the quantity and quality of postoperative motion after cervical disc arthroplasty. Prostheses with built-in resistance to angular and translational motion may have an advantage in restoring physiologic motion. The ability of a prosthesis to work with remaining bony and soft tissues to restore motion and load-sharing is a function of the kinematic degrees of freedom DOF, axis of rotation for a given motion, and device stiffness. How these characteristics allow the prosthesis to work with the patient's anatomy will determine whether the prosthesis is successful at restoring motion and mitigating adjacent-level stresses.

Characterising acetabular component orientation with pelvic motion during total hip arthroplasty.

INTRODUCTION: Suboptimal acetabular component position can result in impingement, dislocation, and accelerated wear. Intraoperative pelvic motion has led to surgeon error and acetabular cup malposition. This study characterises the relationship between pelvic rotation and postoperative acetabular cup orientation. METHODS: A device was constructed to allow cadaveric pelvis rotation along three axes about an acetabular cup in fixed orientation. The acetabular cup was fixed in space at 40° of radiographic inclination and 15° of anteversion relative to the anterior pelvic plane to represent consistent surgeon intraoperative placement. Active marker clusters were fixed to surgical equipment while the cadaveric pelvis was cemented with passive reflective markers, both identified with the Optotrak Certus motion capture system. The reamed cadaveric pelvis was rotated along three axes from -45° to 45° of roll, -30° to 30° of tilt, and -35° to 35° of pitch. The change in component inclination and anteversion was recorded at each 5° interval. Using computed tomography 3D reconstruction, the experimental setup was duplicated computationally to assess against a greater range of pelvis and implant sizes. RESULTS: Radiographic anteversion and inclination showed a non-linear relationship dependent on pelvic roll, tilt, and pitch. Radiographic anteversion changed -0.59°, 0.76° and 0.01° while radiographic inclination changed 0.23°, 0.18° and 1.00° for every 1° of pelvic roll, tilt and pitch, respectively. Computationally, anteversion changed -0.61°, 0.75° and 0.00° while inclination changed 0.22°, 0.19° and 1.00° for every 1° of pelvic roll, tilt and pitch, respectively. These results were independent of cup and pelvis size. CONCLUSIONS: Intraoperative pelvic motion can significantly affect final cup position, and this should be accounted for when placing acetabular components during total hip arthroplasty. Based on this study, intraoperative adjustment of the acetabular component position based on pelvis motion may be implemented to improve postoperative component position.

Loading of the lumbar spine during transition from standing to sitting: effect of fusion versus motion preservation at L4-L5 and L5-S1.

BACKGROUND CONTEXT: Transition from standing to sitting significantly decreases lumbar lordosis with the greatest lordosis-loss occurring at L4-S1. Fusing L4-S1 eliminates motion and thus the proximal mobile segments maybe recruited during transition from standing to sitting to compensate for the loss of L4-S1 mobility. This may subject proximal segments to supra-physiologic flexion loading. PURPOSE: Assess effects of instrumented fusion versus motion preservation at L4-L5 and L5-S1 on lumbar spine loads and proximal segment motions during transition from standing to sitting. STUDY DESIGN: Biomechanical study using human thoracolumbar spine specimens. METHODS: A novel laboratory model was used to simulate lumbosacral alignment changes caused by a person's transition from standing to sitting in eight T10-sacrum spine specimens. The sacrum was tilted in the sagittal plane while constraining anterior-posterior translation of T10. Continuous loading-data and segmental motion-data were collected over a range of sacral slope values, which represented transition from standing to different sitting postures. We compared different constructs involving fusions and motion preserving prostheses across L4-S1. RESULTS: After L4-S1 fusion, the sacrum could not be tilted as far posteriorly compared to the intact spine for the same applied moment (p<.001). For the same reduction in sacral slope, L4-S1 fusion induced 2.9 times the flexion moment in the lumbar spine and required 2.4 times the flexion motion of the proximal segments as the intact condition (p<.001). Conversely, motion preservation at L4-S1 restored lumbar spine loads and proximal segment motions to intact specimen levels during transition from standing to sitting. CONCLUSIONS: In general, sitting requires lower lumbar segments to undergo flexion, thereby increasing load on the lumbar disks. L4-S1 fusion induced greater moments and increased flexion of proximal segments to attain a comparable seated posture. Motion preservation using a total joint replacement prosthesis at L4-S1 restored the lumbar spine loads and proximal segment motion to intact specimen levels during transition from standing to sitting. CLINICAL SIGNIFICANCE: After L4-S1 fusion, increased proximal segment loading during sitting may cause discomfort in some patients and may lead to junctional breakdown over time. Preserving motion at L4-S1 may improve patient comfort and function during activities of daily living, and potentially decrease the need for adjacent level surgery.