MRI Vertebral Bone Quality Correlates With Interbody Cage Subsidence After Anterior Cervical Discectomy and Fusion.

STUDY DESIGN: Retrospective observational study of consecutive patients. OBJECTIVE: The purpose of the study was to evaluate VBQ as a predictor of interbody subsidence and to determine threshold values that portend increased risk of subsidence. SUMMARY OF BACKGROUND DATA: Many risk factors have been reported for the subsidence of interbody cages in anterior cervical discectomy and fusion (ACDF). MRI Vertebral Bone Quality (VQB) is a relatively new radiographic parameter that can be easily obtained from preoperative MRI and has been shown to correlate with measurements of bone density such as DXA and CT Hounsfield Units. METHODS: All patients who underwent 1- to 3-level ACDF using titanium interbodies with anterior plating between the years 2018 and 2020 at our tertiary referral center were included. Subsidence measurements were performed by 2 independent reviewers on CT scans obtained 6 months postoperatively. VBQ was measured on pre-operative sagittal T1 MRI by 2 independent reviewers, and values were averaged. RESULTS: Eight-five fusion levels in 44 patients were included in the study. There were 32 levels (38%) with moderate subsidence and 12 levels with severe subsidence (14%). The average VBQ score in those patients with severe subsidence was significantly higher than those without subsidence (3.80 vs. 2.40, P<0.01). A threshold value of 3.2 was determined to be optimal for predicting subsidence (AUC=0.99) and had a sensitivity of 100% and a specificity of 94.1% in predicting subsidence. CONCLUSIONS: VBQ strongly correlates with the subsidence of interbody grafts after ACDF. A threshold VBQ score value of 3.2 has excellent sensitivity and specificity for predicting subsidence. Spine surgeons can use VBQ as a readily available screening tool to identify patients at higher risk for subsidence. LEVEL OF EVIDENCE: Level-IV.

Fractures in the ankylosed spine are associated with poor bone quality and lower hounsfield units.

STUDY DESIGN: Retrospective study INTRODUCTION: Patients with ankylosing spinal disorders have a higher risk of fractures, highlighting the need for bone health surveillance. Bone assessment by dual energy x-ray absorptiometry (DXA) is challenging due to abnormal bone formation but measurements by quantitative computed tomography (qCT) have demonstrated higher sensitivity and specificity. However, no studies have analyzed bone quality using qCT in the ankylosed spine population to assess three-column fracture characteristics and subsequent outcomes. METHODS: 106 patients with 115 three-column fractures were identified from 1999 to 2020. Patient demographics, Charlson comorbidity index, and injury severity score were extracted. Bone quality measured in Hounsfield units (HU), fracture characteristics, neurologic injury, and mortality were obtained. RESULTS: Most injuries occurred in the thoracic spine (70.4%) following a ground level fall (60.5%). HU adjacent to the fracture (127 HU) was significantly lower than the mobile segments (173 HU) (p < 0.001). Fracture adjacent HU was significantly lower in AS patients compared to DISH (109 vs 150 HU, p = 0.02, respectively) and were lower in fractures that resulted in a non-union or revision surgery (88 vs 137 HU, p = 0.04). Patients with longer fused segments were associated with multilevel and displaced fractures. CONCLUSIONS: Fracture adjacent HUs within the autofused segments were significantly lower than in the mobile segments, and longer fusion segments were associated with displaced, multilevel fractures. This study reinforces the importance of assessing patients for decreased HUs as well as better understand how the length of fused segments is associated with displaced, multilevel fractures. LEVEL OF EVIDENCE: Level III.

Intraoperative Surgeon Assessment of Bone: Correlation to Bone Mineral Density, CT Hounsfield units and Vertebral Bone Quality.

STUDY DESIGN: Retrospective observational study of consecutive patients. OBJECTIVE: The purpose of the study is to determine if a surgeon's qualitative assessment of bone intraoperatively correlates with radiologic parameters of bone strength. SUMMARY OF BACKGROUND DATA: Preoperative radiologic assessment of bone can include modalities such as CT Hounsfield Units (HUs), dual-energy x-ray absorptiometry bone mineral density (DXA BMD) with trabecular bone score (TBS) and MRI vertebral bone quality (VBQ). Quantitative analysis of bone with screw insertional torque and pull-out strength measurement has been performed in cadaveric models and has been correlated to these radiologic parameters. However, these quantitative measurements are not routinely available for use in surgery. Surgeons anecdotally judge bone strength, but the fidelity of the intraoperative judgement has not been investigated. METHODS: All adult patients undergoing instrumented posterior thoracolumbar spine fusion by one of seven surgeons at a single center over a 3-month period were included. Surgeons evaluated the strength of bone based on intraoperative feedback and graded each patient's bone on a 5-point Likert scale. Two independent reviewers measured preoperative CT HUs and MRI VBQ. BMD, lowest T-score and TBS were extracted from DXA within 2 years of surgery. RESULTS: Eighty-nine patients were enrolled and 16, 28, 31, 13 and 1 patients had Likert grade 1 (strongest bone), 2, 3, 4, and 5 (weakest bone), respectively. The surgeon assessment of bone correlated with VBQ (τ=0.15, P=0.07), CT HU (τ=-0.31, P<0.01), lowest DXA T-score (τ=-0.47, P<0.01), and TBS (τ=-0.23, P=0.06). CONCLUSION: Spine surgeons' qualitative intraoperative assessment of bone correlates with preoperative radiologic parameters, particularly in posterior thoracolumbar surgeries. This information is valuable to surgeons as this supports the idea that decisions based on feel in surgery have statistical foundation.

Spinal artery syndrome following kyphoplasty in the setting of a non-compressive extradural cement extravasation: a case report.

INTRODUCTION: Cement extravasation (CE) during vertebroplasty or kyphoplasty for vertebral compression fracture (VCF) is not uncommon, though neurological deficits occur rarely and when paraparesis occurs severe cord compression has been described. We report a case of progressive paraparesis in the setting of non-compressive extradural CE during kyphoplasty with evidence for spinal artery syndrome and neurological recovery after treatment. CASE PRESENTATION: A 77-year-old female with T12 VCF failed conservative treatment and underwent kyphoplasty. In the recovery room, the patient was noted to have bilateral leg weakness, left worse than right, and had urgent CT scan that showed right paracentral CE without cord compression or arterial cement embolization. The patient was transferred to a tertiary hospital and had MRI of the spine that confirmed extradural CE and no cord compression. Because the patient had progression of lower extremity deficits despite medical management, she underwent surgical decompression, cement excision, and spinal fusion with instrumentation. Post op MRI showed T2 hyperintensities in the spinal cord consistent with spinal artery syndrome. One month post op, she had almost complete recovery of her neurological function. DISCUSSION: Spinal artery syndrome may be considered in patients with neurological deficit s/p kyphoplasty even if the extravasated cement does not compress the spinal cord and even if the deficits are worse contralateral to the cement extravasation. If spinal artery syndrome is present and medical management does not improve the deficits, surgery may be indicated even if there is no cord compression.

The Impact of Resident Involvement on Outcomes and Costs in Elective Hand and Upper Extremity Surgery.

PURPOSE: The purpose of this study was to assess the impact of resident involvement on periprocedural outcomes and costs after common procedures performed at an academic hand surgical practice. METHODS: A retrospective review was performed in all patients undergoing 7 common elective upper extremity procedures between January 2008 and December 2018: carpal tunnel release, distal radius open reduction and internal fixation (ORIF), trigger finger release, thumb carpometacarpal arthroplasty, phalanx closed reduction and percutaneous pinning, cubital tunnel release, and olecranon ORIF. The medical record was reviewed to determine the impact of surgical assistants (resident, fellow, or physician assistant) on periprocedural outcomes, periprocedural costs, and 1-year postoperative outcomes. The involvement of surgical trainees operating under direct supervision was compared with the entire operation performed by the attending surgeon with a physician assistant present. RESULTS: A total of 396 procedures met the inclusion criteria. Analysis of the whole study sample revealed low rates of intraoperative complications, wound complications, medical complications, readmissions, and mortality. Subgroup analysis of carpal tunnel releases revealed significantly greater tourniquet times for residents compared with physician assistants (7 ± 2 min, 6 ± 1 min), as well as longer overall operating room times for residents compared to fellows or physician assistants (17 ± 5 min, 13 ± 3 min, 12 ± 3 min). Operating room times for distal radius ORIF were significantly greater among residents compared to fellows or physician assistants (68 ± 19 min, 57 ± 17 min, 56 ± 14 min). There were no differences in any other perioperative metrics or periprocedural costs for the trigger finger release or cubital tunnel release cohorts. CONCLUSIONS: Resident involvement in select upper extremity procedures can lengthen operative times but does not have an impact on blood loss or operating room costs. CLINICAL RELEVANCE: Surgeons should be aware that having a resident assistant slightly increases operative times in elective hand surgery.

Magnetic Resonance Imaging Is Inadequate to Assess Cervical Sagittal Alignment Parameters.

STUDY DESIGN: Retrospective radiographic study. OBJECTIVE: To evaluate cervical sagittal alignment measurement reliability and correlation between upright radiographs and magnetic resonance imaging (MRI). SUMMARY OF BACKGROUND DATA: Cervical sagittal alignment (CSA) helps determine the surgical technique employed to treat cervical spondylotic myelopathy. Traditionally, upright lateral radiographs are used to measure CSA, but obtaining adequate imaging can be challenging. Utilizing MRI to evaluate sagittal parameters has been explored; however, the impact of positional change on these parameters has not been determined. METHODS: One hundred seventeen adult patients were identified who underwent laminoplasty or laminectomy and fusion for cervical spondylotic myelopathy from 2017 to 2019. Two clinicians independently measured the C2-C7 sagittal angle, C2-C7 sagittal vertical axis (SVA), and the T1 tilt. Interobserver and intraobserver reliability were assessed by intraclass correlation coefficient. RESULTS: Intraobserver and interobserver reliabilities were highly correlated, with correlations greater than 0.85 across all permutations; intraclass correlation coefficients were highest with MRI measurements. The C2-C7 sagittal angle was highly correlated between x-ray and MRI at 0.76 with no significant difference ( P =0.46). There was a weaker correlation with regard to C2-C7 SVA (0.48) and T1 tilt (0.62) with significant differences observed in the mean values between the 2 modalities ( P <0.01). CONCLUSIONS: The C2-C7 sagittal angle is highly correlated and not significantly different between upright x-ray and supine MRIs. However, cervical SVA and T1 tilt change with patient position. Since MRI does not accurately reflect the CSA in the upright position, upright lateral radiographs should be obtained to assess global sagittal alignment when planning a posterior-based cervical procedure.

Iatrogenic vertebral artery injury with concomitant contralateral idiopathic vertebral artery dissection managed with intravascular intervention: a case report.

Background: Vertebral artery injury (VAI) is a known potential complication of posterior cervical fusion surgery. Pre-operative imaging is used to determine the patency of bilateral vertebral arteries during the planning and execution of surgery. This case illustrates an example of a staged anterior/posterior cervical reconstruction in which an iatrogenic VAI combined with a contralateral idiopathic vertebral artery dissection not identified on pre-operative imaging resulted in absent basilar artery anterograde flow. Case Description: A 61-year-old female underwent planned staged anterior cervical decompression C4-T1 with posterior cervical fusion C2-T4 for the treatment of degenerative cervical myeloradiculopathy. During the second stage posterior fusion, iatrogenic VAI occurred during drilling for placement of the right C2 pars screw. Upon post-operative angiography, in addition to the known right VAI, there was a new left vertebral artery dissection that occurred during/after the anterior stage. The basilar artery was only filled in retrograde fashion from the right internal carotid artery across the right posterior communicating artery. The left vertebral artery dissection was treated with telescoping flow diverting stents to restore flow to the basilar artery and the right VAI was treated with coiling. Conclusions: Surgeons should be aware of the possibility, while rare, that an occult injury to the non-injured artery is always a possibility if significant deformity correction or alignment change has occurred during cervical spine surgery. Working closely with neurointerventional colleagues can be invaluable to quickly assess and if necessary, restore blood flow to the brain through these life saving techniques.

Surgical Intervention is Associated With Improvements in the ASIA Impairment Scale in Gunshot-induced Spinal Injuries of the Thoracic and Lumbar Spine.

STUDY DESIGN: Retrospective cohort study of patients from the National Spinal Cord Injury Statistical Center (NSCISC). OBJECTIVE: The aim was to compare the outcomes of patients with gunshot-induced spinal injuries (GSIs) treated operatively and nonoperatively. SUMMARY OF BACKGROUND DATA: The treatment of neurological deficits associated with gunshot wounds to the spine has been controversial. Treatment has varied widely, ranging from nonoperative to aggressive surgery. METHODS: Patient demographics, clinical information, and outcomes were extracted. Surgical intervention was defined as a "laminectomy, neural canal restoration, open reduction, spinal fusion, or internal fixation of the spine." The primary outcome was the American Spinal Injury Association (ASIA) Impairment Scale. Statistical comparisons of baseline demographics and neurological outcomes between operative and nonoperative cohorts were performed. RESULTS: In total, 961 patients with GSI and at least 1-year follow-up were identified from 1975 to 2015. The majority of patients were Black/African American (55.6%), male (89.7%), and 15-29 years old (73.8%). Of those treated surgically (19.7% of all patients), 34.2% had improvement in their ASIA Impairment Scale score at 1 year, compared with 20.6% treated nonoperatively. Overall, surgery was associated with a 2.0 [95% confidence interval (CI): 1.4-2.8] times greater likelihood of ASIA Impairment Scale improvement at 1 year. Specifically, benefit was seen in thoracic (odds ratio: 2.5; 95% CI: 1.4-4.6) and lumbar injuries (odds ratio: 1.7; 95% CI: 1.1-3.1), but not cervical injuries. CONCLUSIONS: While surgical indications are always determined on an individualized basis, in our review of GSIs, surgical intervention was associated with a greater likelihood of neurological recovery. Specifically, patients with thoracic and lumbar GSIs had a 2.5 and 1.7-times greater likelihood of improvement in their ASIA Impairment Scale score 1 year after injury, respectively, if they underwent surgical intervention.

Does Operative Management of Epidural Abscesses Increase Healthcare Expenditures up to 1 Year After Treatment?

BACKGROUND: The incidence of spinal epidural abscesses is increasing. What is more, they are associated with high rates of morbidity and mortality. Advances in diagnostic imaging and antibiotic therapies have made earlier diagnosis and nonoperative management feasible in appropriately selected patients. Nonoperative treatment also has the advantage of lower immediate healthcare charges; however, it is unknown whether initial nonoperative care leads to higher healthcare charges long term. QUESTIONS/PURPOSES: (1) Does operative intervention generate higher charges than nonoperative treatment over the course of 1 year after the initial treatment of spinal epidural abscesses? (2) Does the treatment of spinal epidural abscesses in people who actively use intravenous drugs generate higher charges than management in people who do not? METHODS: This retrospective comparative study at two tertiary academic centers compared adult patients with spinal epidural abscesses treated operatively and nonoperatively from January 2016 through December 2017. Ninety-five patients were identified, with four excluded for lack of billing data and one excluded for concomitant intracranial abscess. Indications for operative management included new or progressive motor deficit, lack of response to nonoperative treatment including persistent or progressive systemic illness, or initial sepsis requiring urgent source control. Of the included patients, 52% (47 of 90) received operative treatment with no differences in age, gender, BMI, and Charlson comorbidity index between groups, nor any difference in 30-day all-cause readmission rate, 1-year reoperation rate, or 2-year mortality. Furthermore, 29% (26 of 90) of patients actively used intravenous drugs and were younger, with a lower BMI and lower Charlson comorbidity index, with no differences in 30-day all-cause readmission rate, 1-year reoperation rate, or 2-year mortality. Cumulative charges at the index hospital discharge and 90 days and 1 year after discharge were compared based on operative or nonoperative management and secondarily by intravenous drug use status. Medical records, laboratory results, and hospital billing data were reviewed for data extraction. Demographic factors including age, gender, region of abscess, intravenous drug use, and comorbidities were extracted, along with clinical factors such as symptoms and ambulatory function at presentation, spinal instability, intensive care unit admission, and complications. The primary outcome was charges associated with care at the index hospital discharge and 90 days and 1 year after discharge. All covariates extracted were included in this analysis using negative binomial regression that accounted for confounders and the nonparametric nature of charge data. Results are presented as an incidence rate ratio with 95% confidence intervals. RESULTS: After adjusting for demographic and clinical variables such as age, gender, BMI, ambulatory status, presence of mechanical instability, and intensive care unit admission among others, we found higher charges for the group treated with surgery compared with those treated nonoperatively at the index admission (incidence rate ratio [IRR] 1.62 [95% CI 1.35 to 1.94]; p < 0.001) and at 1 year (IRR 1.36 [95% CI 1.10 to 1.68]; p = 0.004). Adjusted analysis also showed that active intravenous drug use was also associated with higher charges at the index admission (IRR 1.57 [95% CI 1.16 to 2.14]; p = 0.004) but no difference at 1 year (IRR 1.11 [95% CI 0.79 to 1.57]; p = 0.55). CONCLUSION: Multidisciplinary teams caring for patients with spinal epidural abscesses should understand that the decreased charges associated with selecting nonoperative management during the index admission persist at 1 year with no difference in 30-day readmission rates, 1-year reoperation rates, or 2-year mortality. On the other hand, patients with active intravenous drug use have higher index admission charges that do not persist at 1 year, with no difference in 30-day readmission rates, 1-year reoperation rates, or 2-year mortality. These results suggest possible economic benefit to nonoperative management of epidural abscesses without increases in readmission or mortality rates, further tipping the scale in an evolving framework of clinical decision-making. Future studies should investigate if these economic implications are mirrored on the patient-facing side to determine whether any financial burden is shifted onto patients and their families in nonoperative management. LEVEL OF EVIDENCE: Level III, therapeutic study.

Effects of Single-Dose Versus Hypofractionated Focused Radiation on Vertebral Body Structure and Biomechanical Integrity: Development of a Rabbit Radiation-Induced Vertebral Compression Fracture Model.

PURPOSE: Vertebral compression fracture is a common complication of spinal stereotactic body radiation therapy. Development of an in vivo model is crucial to fully understand how focal radiation treatment affects vertebral integrity and biology at various dose fractionation regimens. We present a clinically relevant animal model to analyze the effects of localized, high-dose radiation on vertebral microstructure and mechanical integrity. Using this model, we test the hypothesis that fractionation of radiation dosing can reduce focused radiation therapy's harmful effects on the spine. METHODS AND MATERIALS: The L5 vertebra of New Zealand white rabbits was treated with either a 24-Gy single dose of focused radiation or 3 fractionated 8-Gy doses over 3 consecutive days via the Small Animal Radiation Research Platform. Nonirradiated rabbits were used as controls. Rabbits were euthanized 6 months after irradiation, and their lumbar vertebrae were harvested for radiologic, histologic, and biomechanical testing. RESULTS: Localized single-dose radiation led to decreased vertebral bone volume and trabecular number and a subsequent increase in trabecular spacing and thickness at L5. Hypofractionation of the radiation dose similarly led to reduced trabecular number and increased trabecular spacing and thickness, yet it preserved normalized bone volume. Single-dose irradiated vertebrae displayed lower fracture loads and stiffness compared with those receiving hypofractionated irradiation and with controls. The hypofractionated and control groups exhibited similar fracture load and stiffness. For all vertebral samples, bone volume, trabecular number, and trabecular spacing were correlated with fracture loads and Young's modulus (P < .05). Hypocellularity was observed in the bone marrow of both irradiated groups, but osteogenic features were conserved in only the hypofractionated group. CONCLUSIONS: Single-dose focal irradiation showed greater detrimental effects than hypofractionation on the microarchitectural, cellular, and biomechanical characteristics of irradiated vertebral bodies. Correlation between radiologic measurements and biomechanical properties supported the reliability of this animal model of radiation-induced vertebral compression fracture, a finding that can be applied to future studies of preventative measures.

Post-operative hyperglycemia and its association with surgical site infection after instrumented spinal fusion.

OBJECTIVE: To evaluate the correlation between postoperative hyperglycemia and surgical site infection among patients who underwent primary instrumented spinal fusion surgery. PATIENTS AND METHODS: We collected data on all eligible patients treated at our institution over the course of 2005-2017. We defined serum hyperglycemia using a primary threshold of serum glucose ≥140 mg/dL and used ≥115 mg/dL as a secondary test. We used logistic regression techniques to evaluate unadjusted results for serum hyperglycemia on revision surgeries for infection, followed by sequential adjustment for sociodemographic and procedural characteristics. RESULTS: We included 3664 patients. Surgical site infections occurred in 4%. Post-operative hyperglycemia was significantly associated with a higher rate of revision surgery for infection (p = 0.02). Following adjusted analysis, hyperglycemia remained a statistically significant predictor for revision surgery due to infection (OR 2.19; 95 % CI 1.13, 4.25). Similar results were evident when using the lower threshold of ≥115 mg/dL (OR 2.36; 95 % CI 1.06, 5.23). CONCLUSIONS: This study highlights the importance of measuring serum glucose after spinal fusion and the need for heightened surveillance and/or treatment in those who exhibit postoperative hyperglycemia. In this context, it could be advantageous to use a lower threshold for hyperglycemia (115 mg/dL) in order to trigger interventions for glycemic control.

Laminoplasty versus laminectomy and fusion for cervical spondylotic myelopathy: a cost analysis.

BACKGROUND CONTEXT: Laminectomy with fusion (LF) and laminoplasty (LP) are commonly used to treat cervical spondylotic myelopathy (CSM). The decision regarding which procedure to perform is largely a matter of surgeon's preference, while financial implications are rarely considered. PURPOSE: We aimed to better understand the financial considerations of LF compared to LP in the treatment of CSM. STUDY DESIGN: Retrospective comparative study. PATIENT SAMPLE: Adult patients, 18 years of age or older, who had undergone LF or LP for CSM from 2017 to 2019 at 2 large academic centers were included. Patients who had undergone previous cervical spine surgery or procedures that extended above C2 or below T2 were excluded. OUTCOME MEASURES: The primary outcome was defined as the total cost of the procedure, which was calculated as the sum of the implant and non-implant supply costs. METHODS: Patient demographics, surgical parameters, including estimated blood loss and operative time, and length of stay were collected. Operating room material - both implant and non-implant - cost data was also obtained. Variables were analyzed individually as well as after adjustment based on the number of operative levels involved. Statistical analysis was performed using either Student t test with unequal variance or Wilcoxon rank sum test for continuous variables and chi-squared analysis for categorical variables. RESULTS: Two hundred fifty patients were identified who met inclusion criteria. There was no statistical difference in the mean age at time of surgery (p=.25), gender distribution (p=.33), or re-operation rate between the LF and LP groups (p=.39). Overall, operative time was similar between the LF (165.7 ± 61.9 min) and LP (173.8 ± 58.2 min) groups (p=.29), but the LP cohort had a shorter length of stay at 3.8 ± 2.7 days compared to the LF cohort at 4.8 ± 3.7 days. Implant costs in the LF group were significantly more at $6,204.94 ± $1426.41 compared to LP implant costs at $1994.39 ± $643.09. Mean total costs of LP were significantly less at $2,859.08 ± $784.19 compared to LF total costs of $6,983.16 ± $1,589.17. Furthermore, when adjusted for the number of operative levels, LP remained significantly less costly at $766.12 ± $213.64 per level while LF cost $1,789.05 ± $486.66 per operative level. Additional subgroup analysis limiting the cohorts to patients with either three or four involved vertebral levels demonstrated nearly identical cost savings with LP as compared to LF. CONCLUSIONS: This study demonstrates that LF is on average at least 2.4 times the total operative supply cost of LP and at least 2.3 times the operative supply cost of LP when adjusted for the number of operative levels. In patients deemed appropriate for either LP or LF, these data may be incorporated into decision-making for the treatment of CSM.

Delayed, Reversible Cervical Paralysis After Scoliosis Corrective Surgery in a Child with Osteogenesis Imperfecta: A Case Report.

CASE: A 12-year-old girl with osteogenesis imperfecta (OI) underwent posterior spinal arthrodesis (from T2 to the sacrum) for double major-curve scoliosis. She developed complete paralysis of all of the extremities 24 hours after surgery, without evidence of ischemia or infarction. The rods were removed, and the neurologic status improved; there was full restoration of strength within 1 week. She then underwent in situ fixation. At the 2-year follow-up, there had been no lapse in neurologic function. CONCLUSION: Reversible, distraction-induced neurologic deficits can occur outside of the instrumented spinal segment after corrective scoliosis surgery, particularly in patients with ligamentous laxity, as seen with OI.

NGF-TrkA Signaling by Sensory Nerves Coordinates the Vascularization and Ossification of Developing Endochondral Bone.

Developing tissues dictate the amount and type of innervation they require by secreting neurotrophins, which promote neuronal survival by activating distinct tyrosine kinase receptors. Here, we show that nerve growth factor (NGF) signaling through neurotrophic tyrosine kinase receptor type 1 (TrkA) directs innervation of the developing mouse femur to promote vascularization and osteoprogenitor lineage progression. At the start of primary ossification, TrkA-positive axons were observed at perichondrial bone surfaces, coincident with NGF expression in cells adjacent to centers of incipient ossification. Inactivation of TrkA signaling during embryogenesis in TrkA(F592A) mice impaired innervation, delayed vascular invasion of the primary and secondary ossification centers, decreased numbers of Osx-expressing osteoprogenitors, and decreased femoral length and volume. These same phenotypic abnormalities were observed in mice following tamoxifen-induced disruption of NGF in Col2-expressing perichondrial osteochondral progenitors. We conclude that NGF serves as a skeletal neurotrophin to promote sensory innervation of developing long bones, a process critical for normal primary and secondary ossification.

Circadian mechanisms in murine and human bone marrow mesenchymal stem cells following dexamethasone exposure.

A core group of regulatory factors control circadian rhythms in mammalian cells. While the suprachiasmatic nucleus in the brain serves as the central core circadian oscillator, circadian clocks also exist within peripheral tissues and cells. A growing body of evidence has demonstrated that >20% of expressed mRNAs in bone and adipose tissues oscillate in a circadian manner. The current manuscript reports evidence of the core circadian transcriptional apparatus within primary cultures of murine and human bone marrow-derived mesenchymal stem cells (BMSCs). Exposure of confluent, quiescent BMSCs to dexamethasone synchronized the oscillating expression of the mRNAs encoding the albumin D binding protein (dbp), brain-muscle arnt-like 1 (bmal1), period 3 (per3), rev-erb alpha (Rev A), and rev-erb beta (Rev B). The genes displayed a mean oscillatory period of 22.2 to 24.3 h. The acrophase or peak expression of mRNAs encoding "positive" (bmal1) and "negative" (per3) components of the circadian regulatory apparatus were out of phase with each other by approximately 8-12 h, consistent with in vivo observations. In vivo, phosphyrylation by glycogen synthase kinase 3beta (GSK3beta) is known to regulate the turnover of per3 and components of the core circadian regulatory apparatus. In vitro addition of lithium chloride, a GSK3beta inhibitor, significantly shifted the acrophase of all genes by 4.2-4.7 h oscillation in BMSCs; however, only the male murine BMSCs displayed a significant increase in the length of the period of oscillation. We conclude that human and murine BMSCs represent a valid in vitro model for the analysis of circadian mechanisms in bone metabolism and stem cell biology.