In multilevel posterior cervical instrumented fusions, extending the fusion across the cervico-thoracic junction at T1 or T2 (CTJ) has been associated with decreased rate of re-operation and pseudarthrosis but with longer surgical time and increased blood loss. The impact on patient reported outcomes (PROs) remains unclear. The primary objective was to determine whether extending the fusion through the CTJ influenced PROs at 3 and 12 months after surgery. Secondary objectives were to compare the number of patients reaching the minimally clinically important difference (MCID) for the PROs and mJOA, operative time duration, intra-operative blood loss (IOBL), length of stay (LOS), discharge disposition, adverse events (AEs), re-operation within 12 months of the surgery, and patient satisfaction.

This is a retrospective analysis of prospectively collected data from a multicenter observational cohort study of patients with degenerative cervical myelopathy. Patients who underwent a posterior instrumented fusion of 4 levels of greater (between C2-T2) between January 2015 and October 2020 with 12 months follow-up were included. PROS (NDI, EQ5D, SF-12 PCS and MCS, NRS arm and neck pain) and mJOA were compared using ANCOVA, adjusted for baseline differences. Patient demographics, comorbidities and surgical details were abstracted. Percentafe of patient reaching MCID for these outcomes was compared using chi-square test. Operative duration, IOBL, AEs, re-operation, discharge disposittion, LOS and satisfaction were compared using chi-square test for categorical variables and independent samples t-tests for continuous variables.

A total of 206 patients were included in this study (105 patients not crossing the CTJ and 101 crossing the CTJ). Patients who underwent a construct extending through the CTJ were more likely to be female and had worse baseline EQ5D and NDI scores (p> 0.05). When adjusted for baseline difference, there was no statistically significant difference between the two groups for the PROs and mJOA at 3 and 12 months. Surgical duration was longer (p 0.05). Satisfaction with the surgery was high in both groups but significantly different at 12 months (80% versus 72%, p= 0.042 for the group not crossing the CTJ and the group crossing the CTJ, respectively). The percentage of patients reaching MCID for the NDI score was 55% in the non-crossing group versus 69% in the group extending through the CTJ (p= 0.06).

Up to 12 months after the surgery, there was no statistically significant differences in PROs between posterior construct extended to or not extended to the upper thoracic spine. The adverse event profile did not differ significantly, but longer surgical time and blood loss were associated with construct extending across the CTJ.

Prolonged length of stay (LOS) is a significant contributor to the variation in surgical health care costs and resource utilization after elective spine surgery. The primary goal of this study was to identify patient, surgical and institutional variables that influence LOS. The secondary objective is to examine variability in institutional practices among participating centers.

This is a retrospective study of a prospectively multicentric followed cohort of patients enrolled in the CSORN between January 2015 and October 2020. A logistic regression model and bootstrapping method was used. A survey was sent to participating centers to assessed institutional level interventions in place to decrease LOS. Centers with LOS shorter than the median were compared to centers with LOS longer than the median.

A total of 3734 patients were included (979 discectomies, 1102 laminectomies, 1653 fusions). The median LOS for discectomy, laminectomy and fusion were respectively 0.0 day (IQR 1.0), 1.0 day (IQR 2.0) and 4.0 days (IQR 2.0). Laminectomy group had the largest variability (SD=4.4, Range 0-133 days). For discectomy, predictors of LOS longer than 0 days were having less leg pain, higher ODI, symptoms duration over 2 years, open procedure, and AE (p< 0.05). Predictors of longer LOS than median of 1 day for laminectomy were increasing age, living alone, higher ODI, open procedures, longer operative time, and AEs (p< 0.05). For posterior instrumented fusion, predictors of longer LOS than median of 4 days were older age, living alone, more comorbidities, less back pain, higher ODI, using narcotics, longer operative time, open procedures, and AEs (p< 0.05). Ten centers (53%) had either ERAS or a standardized protocol aimed at reducing LOS.

In this study stratifying individual patient and institutional level factors across Canada, several independent predictors were identified to enhance the understanding of LOS variability in common elective lumbar spine surgery. The current study provides an updated detailed analysis of the ongoing Canadian efforts in the implementation of multimodal ERAS care pathways. Future studies should explore multivariate analysis in institutional factors and the influence of preoperative patient education on LOS.

Instrumented fusion for lumbar degenerative spondylolisthesis (LDS) has been challenged recently with high impact trials demonstrating similar changes in health-related quality of life (HRQOL) and less morbidity/cost with laminectomy alone. Randomized trials often fail, however, to evaluate a heterogeneous population of patients. A standardized clinical assessment and management plan (SCAMP) was created as a decision aid for surgeons based on the radiographic stability and clinical presentation of patients. The purpose of this study was to compare outcomes of those patients who followed the decision aid with respect to fusion/no fusion to those who did not.

Patients were prospectively enrolled from eleven different Canadian institutions and followed from 2015–2019. A degenerative spondylolisthesis instability classification system (DSIC) was created using best available evidence stratifying patients into three different subtypes (1. stable degenerative spondylolisthesis, 2. potentially unstable spondylolisthesis and 3. unstable spondylolisthesis). The decision aid recommends laminectomy alone for group 1 patients, posterolateral fusion with pedicle screws in type 2 patients and pedicle screw and interbody fusion for type 3 patients. One year changes in HRQOL, length of hospital stay (LOS), medication use and surgical time were compared between each group and in context of whether the treatment fell within the decision aid recommendation. Statistics were performed with STATA software.

There were 394 patients initially enrolled and 334 (84.8%) with full one year data available for comparison. There were 95 type 1 (stable), 224 type 2 (potentially unstable) and 75 type 3 (unstable) patients initially classified. Baseline Ostwestry disability index (ODI), EQ-5D, and SF-12 MCS scores were significantly worse for type 3 patients versus type 1 patients. One hundred and eight patients were treated within the recommendations of the DSIC system (108/334, 32.3%). Surgeons performed interbody fusions in 141 patients (42%) rather than follow DSIC recommending a less invasive approach. There were no significant differences EQ-5D, SF-12 PCS/MCS, PHQ-9 or ODI at one year between patient groups. There was a trend towards shorter operating times for those patients following the DSIC system (195 minutes non-followers versus 180 followers, p=0.078) and reduced hospital stay (4.46 days non-followers versus 3.98 followers, p=0.065).

There were no significant clinical differences in outcome at 1 year whether patients underwent decompression alone, decompression/posterolateral fusion or interbody fusion regardless of the stability classification. Surgeons were more likely to perform potentially unnecessary interbody fusions even in those patients with stable or potentially unstable spondylolisthesis. Although not statistically significant, there is some suggestion that following the DSIC system based on best evidence recommendations leads to more judicious/responsible use of hospital resources. Further study is required to determine why surgeons are more likely to choose more invasive, higher rigidity constructs in patients with LDS.

Equilibrative nucleoside transporter 1 (ENT1) transfers nucleosides, such as adenosine, across plasma membranes. We reported previously that mice lacking ENT1 (ENT1-KO) exhibit progressive ectopic calcification of spinal tissues, including the annulus fibrosus (AF) of intervertebral discs (J Bone Miner Res 28:1135–49, 2013, Bone 90:37–49, 2016). Our purpose was twofold: (1) to compare ectopic calcifications in ENT1-KO mice with those in human DISH, and (2) to investigate the molecular pathways underlying pathological calcification in ENT1-KO mice.

Studies were performed with age-matched wild-type (WT) and ENT1-KO mice, as well as human cadaveric vertebral columns meeting radiographic criteria for DISH. Mouse and human specimens were scanned using high-resolution, micro-computed tomography (micro-CT). As well, some samples were decalcified and processed for histological assessment. Calcified lesions in selected specimens were examined using energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). To investigate molecular changes associated with ectopic calcification, we isolated AF tissue from thoracic intervertebral discs of WT and ENT1-KO mice. Tissues were then subjected to transcriptomic and proteomic analyses.

Micro-CT of ENT1-KO mice revealed ectopic calcification of spinal tissues, first appearing in the cervical-thoracic region and extending caudally with advancing age. Histological examination of calcified lesions in mice revealed accumulations of amorphous, eosinophilic, acellular material in paraspinal ligaments and entheses, intervertebral discs, mandibular symphysis, and sternocostal articulations. There was no evidence of inflammation associated with these lesions. EDX of calcified lesions revealed a high content of calcium and phosphorus in a molar ratio of ∼1.6, with hydroxyapatite detected by micro-XRD. Ten human cadaveric spines (three females and seven males, mean age 81 years) that met radiographic criteria for DISH were analysed in detail by micro-CT. Remarkable heterogeneity in the density and morphology of ectopic calcifications was observed. Analyses of calcifications by EDX and XRD again yielded a calcium/phosphorus ratio of ∼1.6 and a crystalline diffraction pattern matching hydroxyapatite. Histological examination of human lesions revealed regions of mature ossification and other areas of irregular amorphous calcification that resembled lesions in ENT1-KO mice. Microarray analysis of AF tissue from WT and ENT1-KO mice showed extensive dysregulation of transcription in affected tissues. Cell cycle-associated transcripts were the most affected, including the E2f family of transcription factors and proliferating cell nuclear antigen. In addition, expression of genes involved in the regulation of mineralization and bone development were dysregulated. Proteomic analyses confirmed transcriptomic changes and revealed alterations in known modulators of biomineralization such as matrix Gla-protein.

Many of the characteristics of ectopic calcification in ENT1-KO mice resemble those of DISH in humans. Human lesions were found to be heterogeneous with regions of pathological ossification and amorphous calcification, the latter resembling lesions in the mouse model. Our studies of the molecular events associated with ectopic calcification in ENT1-KO mice may provide insights into the pathogenesis of DISH in humans. ENT1-KO mice may also be useful for evaluating therapeutics for the prevention of ectopic calcification in DISH and related disorders.

In order to reduce the risk of dislocation larger femoral heads in total hip arthroplasty (THA) are being used by surgeons in recent years. The standard head size of 28 mm used in 73% of all hip procedures in 2003 was used in only 29% in 2016; whereas head sizes of 32 mm and 36 mm combined, were used in 70%. The increase of head size effectively reduces the thickness of the acetabular cup, altering the load transfer. Herein, this research work investigates the effect of increasing the femoral head size on the stresses of the periacetabular bone at two selected regions: A1 (superior) and A2 (anterior). Three Finite Element models were developed from CT scan data of a hemipelvis implanted with a cemented all-polyethylene acetabular cup with a 50 mm outer diameter and inner diameter to accommodate three head sizes: 28 mm, 32 mm and 36 mm. The peak reaction force at the hip joint during one leg stand for an overweight patient with a body weight of 100 Kg was simulated for head sizes investigated. We found that highest average von Mises stress was 5.7 MPa and occurred in the cortical bone of region A1 which is located within Zone 1 boundaries (Charnley &DeLee); whereas a lower stress of 4.0 MPa occurred at region A2. In the two regions the stresses were the same for the three head sizes. Periacetabular bone was found to be insensitive to the increase of femoral head diameter in cemented THA.

The treatment of scapholunate (SL) ligament injuries is addressed by surgical procedures to stabilize the carpal joint. Open techniques include bone-ligament-bone transfers, tenodesis, partial fusions and carpectomies. Innovative procedures using wrist arthroscopy, offer minimally invasive fixation without full exposure of carpal bones; however, the success of the technique and its impact on the reduction on the range of carpal movement is as yet not well known. In this work, the performance of Corella tenodesis technique to repair the SL ligament is evaluated for a wrist type II by numerical methods. Human wrist can be classified based on the lunate morphology: type I for lunate that articulates with radius, scaphoid, capitate and triquetrum, and type II which has an extra surface to articulate with the hamate. A finite element model was constructed from CT-scan images, the model includes cortical and trabecular bones, articular cartilage and ligaments. Three scenarios were simulated representing healthy wrist, SL ligament sectioning and the Corella technique. The performance of the technique was assessed by measure the SL gap in dorsal and volar side as well as the SL angle to be compared to cadaveric studies. In intact position, the SL gap and the SL angle predicted by the numerical model is 2.8 mm and 44.8º, these values are consistent to the standard values reported in cadaveric experiments (2.0 ± 0.8 mm for SL gap and 45.8 ± 9.7 for SL angle). Virtual surgeries may help to understand and evaluate the performance of the techniques at clinical application.

Introduction

The use of larger femoral heads in Total Hip Arthroplasty has increased in order to reduce the risk of dislocation and to improve the range of motion of the joint. In 2003, within the UK, the “standard” head size of 28mm was used in 73% of all hip procedures, whereas by 2012, this figure dropped to 36%. Concerns regarding the impact of this increment in head size on the cement and bone stresses have arisen; however, this has yet to be clearly determined.

Purpose: To investigate subsidence of the Charite total disc arthroplasty (TDA) and to identify if a discrepancy between vertebral endplate and the Charite footprint predispose to subsidence.

Method: Between July 2001 and May 2008, 69 patients underwent a Charite TDA (DePuy Spine, Raynham, MA). They were prospectively followed at 3, 6, 12 months, and once a year thereafter. The following measurements were performed on the replaced motion segment using a lateral radiograph:

The anterior-posterior (AP) dimension of the end plates.

The ratio between the actual and radiographic AP length of the metal endplate was calculated and utilized as the correction factor for the error of magnification on all other radiographic measurements.

Results: At L5-S1 the mean subsidence was 1.87 mm and occurred exclusively at the posterior part of the inferior end plate of L5. The mean posterior uncoverage was 3.5 mm (L5) and 0.27mm (S1). At L4-L5 the mean subsidence was 1.48 mm (L4) and 0.56 mm (L5). Posterior uncoverage of L4 and L5 vertebrae were 4.81 and 2.22 mm, respectively. Subsidence of more than 1 mm was present in all cases where the posterior uncoverage of the end plate with the TDA was more than 2 mm (odds ratio: 5.7). Subsidence was non – progressive in all cases. An anatomic mismatch exists between L5 and S1 endplates in the AP dimension; in more than half the patients S1 is shorter than L5.

Conclusion: The radiographic measurements suggest an increased likelihood of subsidence with more than 2 mm of posterior uncoverage of the end plate by the TDA. The endplate AP length of S1 is frequently less than that of L5. Implant selection based on the smaller S1 endplate may produce worrisome uncoverage of the L5 inferior endplate leading to an increased risk of subsidence and possible catastrophic failure. TDA design should afford modularity to compensate.

Purpose: Bone mineral density (BMD) is an important factor in the performance of orthopaedic instrumentation both in and ex-vivo, and until now, there has not existed a reliable technique for determining BMD at the precise location of such hardware. This paper describes such a technique using cadaveric human sacra as a model.

Method: Nine fresh-frozen sacra had solid and hollow titanium screw placed into the S1 pedicles from a posterior approach. High-resolution micro-computed tomography (CT) was performed on each specimen before and after screw placement. All images were reconstructed with an isotropic spatial resolution of 0.308 mm, reoriented, and the pre-screw and post-screw scans were registered and transformed using a six-degree rigid-body transformation matrix. Once registered, two points, corresponding to the center of the screw at the cortex and at the screw tip, were determined in each scan. These points were used to generate cylindrical regions of interest (ROI) with the same trajectory and dimensions as the screw. BMD measurements were obtained within each of the ROI in the pre-screw scan. To examine the effect of artefact on BMD measurements around the titanium screws, annular ROI of 1 mm thickness were created expanding from the surface of the screws, and BMD was measured within each in both the pre-and post-screw scans.

Results: The registration process was accurate, with an error of 0.2 mm. Four specimens were scanned five times with repositioning, and error in BMD measurements was ± 2%. BMD values in the cylindrical ROI corresponding to screw trajectories were not statistically different from side to side of each specimen (p = 0.23). Artefact-related differences in BMD values followed an exponential decay curve as distance from the screws increased, approaching a low value of approximately 20 mg HA/cc, but not disappearing completely.

Conclusion: CT in the presence of metal creates artefact, making measured BMD values near implants unreliable. This technique is accurate for determination of BMD, non-destructive, and eliminates the problem of this metal artefact through the use of co-registration of a pre- and post-screw scan. This technique has applications both in-vitro and in-vivo.

Purpose: To report the clinical and radiographic prospective results of a consecutive series of patient with a minimum two year follow-up with the Charite Total Disc Arthroplasty (TDA).

Method: Between 2001 and 2005, sixty patients underwent a Charite TDA (Depuy Spine, Raynham, MA) at either L4-5 or L5-S1. The primary indication for surgery was discogenic low back pain confirmed by provocative discography. Clinical assessment was carried out preoperatively and postoperatively at 3, 6, 12 months, and once a year thereafter using the Oswestry Disability Index (ODI), Visual Analogue Scale (VAS) for back and leg pain, and SF-36. Radiographic analysis included: angle of sagittal rotation, translation of the rostral vertebra onto the caudal vertebra, anterior vertical motion (AVM), middle vertical motion (MVM), posterior vertical motion (PVM), pre- and post-operative lumbar lordosis, disc height and subsidence of the TDA. The radiographic measurements were performed using the GE Medical Systems Centricity PACS Software Version 1.0.

Results: There were 36 female and 24 male patients with a mean age of 39 (range 21–59). The mean duration of low back pain was 70 months. Twenty-five percent claimed work compensation status. The mean postoperative hospital stay was 4.8 days. A statistical significant improvement was demonstrated between the mean pre-operative ODI (50) and all post-operative intervals (p< 0.0001) which had declined to 27.7 by one year. Similarly, pre-operative VAS back pain (8.0), leg pain (6.1), SF-36 physical component summary score (33.5) and mental component summary score (41.8) remained improved (p< 0.0001) by three months (4.1, 3.1, 51.7, 62.0 respectively). One patient with an L5-S1 TDA has since undergone a posterolateral instrumented fusion. The mean pre- and post-operative lumbar lordosis was 34.58 and 53.48 respectively. The mean saggital rotation was 6.5 degrees at 5 year follow-up, while the mean translation was 0.83 mm. The mean AVM, MVM and PVM were 0.59 mm, −3.96 mm and 3.69 mm respectively at 5 year follow-up.

Conclusion: This study demonstrates satisfactory clinical results in carefully selected patients. The radiographic assessment confirmed preservation of movement at the replaced disc during flexion and extension of the lumbar spine.

Purpose: To report the clinical and radiographic prospective results of a consecutive series of patient with a minimum two year follow-up with the Charite Total Disc Arthroplasty (TDA).

Methods: Between 2001 and 2005 sixty patients underwent a Charite TDA (Depuy Spine, Raynham, MA) at either L4–5 or L5-S1. The primary indication for surgery was discogenic low back pain confirmed by provocative discography. Clinical assessment was carried out preoperatively and postoperatively at 3, 6, 12 months, and once a year thereafter using the Oswestry Disability Index (ODI), Visual Analogue Scale (VAS) for back and leg pain, and SF-36. Radiographic analysis included: angle of sagittal rotation, translation of the rostral vertebra onto the caudal vertebra, anterior vertical motion (AVM), middle vertical motion (MVM), posterior vertical motion (PVM), pre- and post-operative lumbar lordosis, disc height and subsidence of the TDA. The radiographic measurements were performed using the GE Medical Systems Centricity PACS Software Version 1.0.

Results: There were 36 female and 24 male patients with a mean age of 39 (range 21–59). The mean duration of low back pain was 70 months. Twenty-five percent claimed work compensation status. The mean post-operative hospital stay was 4.8 days. A statistical significant improvement was demonstrated between the mean pre-operative ODI (50) and all post-operative intervals (p< 0.0001) which had declined to 27.7 by one year. Similarly, pre-operative VAS back pain (8.0), leg pain (6.1), SF-36 physical component summary score (33.5) and mental component summary score (41.8) remained improved (p< 0.0001) by three months (4.1, 3.1, 51.7, 62.0 respectively).

The mean pre and post-operative lumbar lordosis was 34.58 and 53.48 respectively. The mean sagittal rotation was 6.5 degrees at 5 year follow-up, while the mean translation was 0.83 mm. The mean AVM, MVM and PVM were 0.59 mm, −3.96 mm and 3.69 mm respectively at 5 year follow-up.

Conclusion: This study demonstrates satisfactory clinical results in carefully selected patients. The radiographic assessment confirmed preservation of movement at the replaced disc during flexion and extension of the lumbar spine.

Interest Statement: No financial benefits or funding has been received for the completion of this study.

Purpose: To investigate subsidence of the Charite total disc arthroplasty (TDA) and to identify if a discrepancy between vertebral endplate and the Charite footprint predispose to subsidence.

Methods: Between July 2001 and May 2008 64 patients underwent a Charite TDA (DePuy Spine, Raynham, MA). They were prospectively followed at 3, 6, 12 months, and once a year thereafter.

The following measurements were performed on the replaced motion segment using a lateral radiograph:

The anterior-posterior (AP) dimension of the end plates.

The ratio between the actual and radiographic AP length of the metal endplate was calculated and utilized as the correction factor for the error of magnification on all other radiographic measurements.

Results: At L5-S1 the mean subsidence was 1.87 mm and occurred exclusively at the posterior part of the inferior end plate of L5. The mean posterior uncoverage was 3.5 mm (L5) and 0.27 mm (S1).

At L4–L5 the mean subsidence was 1.48 mm (L4) and 0.56 mm (L5). Posterior uncoverage of L4 and L5 vertebrae were 4.81 and 2.22 mm, respectively.

Subsidence of more than 1 mm was present in all cases where the posterior uncoverage of the end plate with the TDA was more than 2 mm (odds ratio: 5.7). Subsidence was non – progressive in all cases.

An anatomic mismatch exists between L5 and S1 endplates in the AP dimension; in more than half the patients S1 is shorter than L5.

Conclusion: The radiographic measurements suggest an increased likelihood of subsidence with more than 2 mm of posterior uncoverage of the end plate by the TDA. The endplate AP length of S1 is frequently less than that of L5. Implant selection based on the smaller S1 endplate may produce worrisome uncoverage of the L5 inferior endplate leading to an increased risk of subsidence and possible catastrophic failure. TDA design should afford modularity to compensate.

No financial benefits or funding has been received for the completion of this study.

The purpose of this study was to investigate the strength profile of the thoracolumbar endplate. Indentation testing was performed on the T9, T12, and L2 endplates of six fresh-frozen human cadaver vertebrae. Indentations were performed in a standardized rectangular grid pattern of seven columns and five rows. There was an incremental increase in the strength of each row moving anterior and posterior from the central row. The relative strength of the anterior regions of the endplate increased with rostral ascent into the thoracic spine.

The purpose of this study was to map the strength profile of the thoracolumbar endplates using indentation testing.

Indentation testing was performed on the T9, T12, and L2 endplates of six fresh-frozen human cadaver spines using a materials testing machine (Dynamight, Instrom Corporation, Canton, MA). A minimum of twenty-five indentations was performed in a rectangular grid (seven columns by five rows). A 3mm hemispherical indendor was lowered at 0.2mm/s to a depth of 3mm producing endplate failure.

The failure load significantly varied with the AP and LAT positions (p< .0001). Each row was significantly stronger than the rows anterior to it (p < 0.04), except for the most row. The most lateral columns were stronger than the central (range: p = .04 – .0002). The mean strength of the L2 posterior row was greater than that for the thoracic endplates (p< .01), while no difference existed between levels within the two anterior rows. The ratio of the mean strength for the posterior row compared to that of the anterior row was significantly different across level (P< 0.036). The ratios for L2, T12, and T9 were 1.35, 0.97, and 0.91 respectively.

The periphery of the thoracolumbar endplate is stronger than the centre. The interaction identified between position and level suggests a relative strength increase in the anterior aspect of the endplate with rostral ascent into the thoracic spine.

This knowledge may assist in preventing intervertebral inplant subsidence by influencing implant positioning and design.

Funding: Canadian Institutes of Health Research Please contact author for figures and/or graphs.

This study examined the occurrence of type II (Beta) error in the spine surgical literature. A literature search from 1966 until present identified twenty-nine randomized control trials, which had a two-group parallel design and reported a non-significant difference in the primary outcome measure. We determined whether these trials had sufficient sample size to detect a 25% and 50% relative change in the primary outcome. Nine studies specifically identified a primary outcome. Four studies reported a sample size calculation. Therefore, twenty-five trials were at risk of committing a type II error.

The purpose of this study was to determine the frequency of potential type II error in randomized controlled trails reported in the spine surgical literature.

A literature search was conducted of the Medline, Pubmed, and the Cochrane databases using the keywords of “spine” and “surgery”, between 1967 until present to identify randomized controlled trials involving spine surgery. Trials were included in this study if they were of a two-group design, with at least one of the groups being a surgical cohort, and that reported a non-significant difference in the primary outcome. We determined the frequency for which the primary outcome and sample size calculation was reported. The sample size was assessed to determine whether the trial had sufficient subjects to detect a 25% and 50% relative difference in the primary outcome for a power of 80%.

Twenty-nine studies satisfied the inclusion criteria. Nine studies specifically identified a primary outcome. All others reported multiple outcomes with no specified primary measure. Four studies reported a sample size calculation. Of the remaining twenty-five that did not, three had sufficient power and the rest were at significant risk of committing a type II error.

The spine surgical literature is plagued with a high potential for type II errors in the published trails with a non-significant outcome.

In the spine surgical literature, a randomized controlled trial that fails to reject its null hypothesis, requires careful scrutiny of its methodology to prevent misinterpretation of the results.

A biomechanical study assessing compressive failure load, strength and stiffness with three different interbody device shapes was performed in human cadaveric vertebrae. The custom-made interbody devices had similar cross-sectional areas and specimens were tested with 20% or 40% coverage of indentor to endplate area. Axial compressive load was applied at 0.2mm/s to a depth equivalent to 20% of the vertebral height. The clover-leaf shaped device resulted in significantly higher failure load, strength and stiffness over the elliptical and the kidney shaped devices for both areas of coverage. The clover-leaf shaped devices extended over stronger periphery regions of the endplates and resulted in stronger interface properties.

To determine if two novel interbody cage shapes, the kidney and the clover-leaf, are biomechanically superior to a standard elliptical shape of similar cross sectional area.

Uniaxial compression tests with unrestricted rotations were carried out on the superior endplates of forty-eight thoracolumbar (T9-L2) vertebrae with one of three shaped indentors covering 20% or 40% of the endplate area. Compressive load was applied using a servohydraulic testing machine at 0.2mm/s, to depth equivalent to 20% of the vertebral height. Failure load, strength and stiffness were compared.

The clover-leaf shaped indentors resulted in higher failure load (53% average increase), higher strength (67% average increase) and higher construct stiffness (43% average increase), and these results were significant (p< 0.05). Larger indentor coverage area of 40% also resulted in significantly higher failure loads over 20% coverage (75% average increase).

Current elliptical interbody devices are placed over the central region of the endplate, which is also the weakest. A clover-leaf shaped device extended over the stronger peripheral regions of the endplates and resulted in improved bone-implant interface properties. This implant if implemented in vivo could potentially reduce implant subsidence and lead to better long-term outcomes in osteoporotic patients.

The novel clover-leaf shaped indentor displayed superior bone-implant interface properties. Larger interbody devices should be used when possible to improve interface properties.

Implant subsidence in osteoporotic patients could be significantly reduced with a clover-leaf shaped device, leading to better long-term outcomes.

Funding: Funding from the Canadian Institutes for Health Research.