The Evolution of Lateral Lumbar Interbody Fusion: A Journey from Past to Present.

Lumbar interbody fusion procedures have seen a significant evolution over the years, with various approaches being developed to address spinal pathologies and instability, including posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), and lateral lumbar interbody fusion (LLIF). LLIF, a pivotal technique in the field, initially emerged as extreme/direct lateral interbody fusion (XLIF/DLIF) before the development of oblique lumbar interbody fusion (OLIF). To ensure comprehensive circumferential stability, LLIF procedures are often combined with posterior stabilization (PS) using pedicle screws. However, achieving this required repositioning of the patient during the surgical procedure. The advent of single-position surgery (SPS) has revolutionized the procedure by eliminating the need for patient repositioning. With SPS, LLIF along with PS can be performed either in the lateral or prone position, resulting in significantly reduced operative time. Ongoing research endeavors are dedicated to further enhancing LLIF procedures making them even safer and easier. Notably, the integration of robotic technology into SPS has emerged as a game-changer, simplifying surgical processes and positioning itself as a vital asset for the future of spinal fusion surgery. This literature review aims to provide a succinct summary of the evolutionary trajectory of lumbar interbody fusion techniques, with a specific emphasis on its recent advancements.

Post-surgical thoracic spinal cord herniation following an unrepaired dural defect: a rare complication.

Surgery for ossification of the ligamentum flavum (OLF) comes with a relatively high risk of dural tear. We report a 50-year-old woman, who presented with symptomatic spinal stenosis from OLF at T11-T12 and lower lumbar spondylosis for which a single stage posterior decompression and instrumented fusion of both sites was done. Removal of the OLF resulted in a small dural tear with intact arachanoid which was covered using a fibrin sealant. In the first post-operative day, the patient's neurology started deteriorating. An MR scan was done to look for hematoma. It showed the spinal cord herniating out of the thecal sac at the operated level. Emergency re-operation was done to reduce the herniation and the dural defect was repaired. The patient gradually recovered to her best functional status. Based on this experience, we advise primary repair of inadvertent durotomies.

Frequency and predictors of implant-related complications after posterior cervical fusion.

STUDY DESIGN: Retrospective, observational study. PURPOSE: To determine the frequency and predictors of implant-related complications in adults after posterior cervical fusion. OVERVIEW OF LITERATURE: Published literature on lumbosacral fusion suggest that implant-related complications are not uncommon. Although posterior cervical fusion is a common operation, data on frequency and predictors of implant-related complications after posterior cervical fusion is still scarce. METHODS: 86 patients (with 740 screws) who underwent posterior cervical fusion were included. Implant-related complications were identified by the presence of: (1) halo sign, (2) screw pull-out/breakage (3) post-operative kyphosis and (4) implant-related complications requiring revision surgery. These were stratified into two groups: (a) minor - isolated halo sign or screw pull-out/breakage (b) major - post-operative kyphosis > 10 degrees, and revision surgery. Demographic, operative and radiological data was collected. Rates of implant-related complications were determined and associated risk factors identified. RESULTS: 33 (38.4%) patients had signs of implant-related complications. Of these, 29 (87.9%) had minor complications and 4 (12.1%) had major complications. Charlson Comorbidity Index (CCI) (p = 0.03179) and pre-op C2-C7 sagittal vertical alignment (SVA) (p = 0.02449) were the only significant risk factors for all-cause implant-related complications during multivariate logistic regression. Other intraoperative parameters (type of screw, length of fusion, levels decompressed, and extension of fusion beyond the levels decompressed) were not significantly associated with implant-related complications. CONCLUSIONS: Implant-related complications are not uncommon but rarely require revision surgery. Higher pre-operative SVA and CCI were significant risk factors; length of construct and extent of decompression were not. These findings may assist clinicians when deciding the extent of fusion and in selecting patients for closer follow-up.

We assessed the frequency and predictors of implant-related complications in adults after posterior cervical fusion. Implant-related complications (halo sign, screw pull-out/breakage, post-operative kyphosis) are not uncommon but rarely require revision surgery. Higher pre-operative SVA and CCI were significant risk factors; length of construct and extent of decompression were not.

Correction to: Staged surgery for tandem cervical and lumbar spinal stenosis: Which should be treated first?

The authors would like to acknowledge the following funding information that was missing in.

Staged surgery for tandem cervical and lumbar spinal stenosis: Which should be treated first?

PURPOSE: Tandem spinal stenosis (TSS) refers to lumbar and cervical spinal canal stenosis. Staged surgery is often chosen, but sometimes, mere decompression of one stenosis is adequate to relieve symptoms. Therefore, we intend to analyze whether starting with the cervical or the lumbar region is the most logical option. METHODS: We retrospectively reviewed the data of 47 patients with TSS, having first-stage decompression for the most symptomatic stenosis, and classified into two groups (Group A: lumbar decompression first, Group B: cervical decompression first). Postoperative outcomes were analyzed for at least 2 years, and they were cautiously watched for symptoms of the non-operated stenosis; if such symptoms were debilitating, second-stage surgery for the non-operated stenosis was done. RESULTS: The demographic characteristics of Group A (n = 11) and Group B (n = 36) were comparable. One patient (9%) in Group A and 25 patients (67%) in Group B had resolution of symptoms and good functional recovery. The need for a second-stage surgery for the non-operated stenosis was significantly high (p = 0.001) among patients in Group A. They suffered a significant worsening of both the mJOA score and the Nurick's grade; whereas, patients in Group B experienced staged improvement of both scales. CONCLUSION: First-stage surgery for the cervical stenosis significantly lowers the need of the second-stage surgery. In contrast, if lumbar stenosis was treated first, a dramatic exacerbation of the symptoms related to the cervical stenosis can occur soon. Therefore, treatment of cervical stenosis first seems to be more appropriate. These slides can be retrieved under Electronic Supplementary Material.

Intraoperative electromyographic monitoring to optimize safe lumbar pedicle screw placement - a retrospective analysis.

BACKGROUND: The foremost concern of a surgeon during pedicle screw fixation is safety. Assistive modalities, especially intraoperative electromyographic monitoring (EMG) can function as an essential tool to recognize screw malposition that compromise neural integrity, so that the screws can be repositioned immediately rather than later. We intend to study the efficacy of intraoperative EMG monitoring to detect potential pedicle breach and evaluate whether reoperation rates were significantly reduced. METHODS: Retrospectively, patients who underwent posterior stabilization with pedicle screws for various pathologies were analysed and those with screws among L1-S1 levels were shortlisted. They were divided into two groups. Group 1 included patients in whom trigger EMG (t-EMG) was used to confirm appropriate screw placement and Group 2 included those in whom it was not used. Responses to t-EMG and corresponding stimulation thresholds were recorded for Group 1 patients. The sensitivity and specificity of the test was calculated. Reoperation rates due to postoperative neurologic compromise caused by malpositioned screws were compared between both the groups. RESULTS: A total of 518 patients had 3112 pedicle screws between L1-S1 levels. Among Group 1 [n = 296; Screws = 1856], 145 screws (7.8%) showed a positive response for t-EMG at stimulation thresholds ranging between 2.6 to 19.8 mA. The sensitivity and specificity of t-EMG to diagnose potential pedicle breach was found to be 93.33% and 92.88% respectively. Only one patient among Group 1 required reoperation. However, among Group 2 [n = 222; screws = 1256], six patients required reoperation. This indicated a significant decrease in the number of malpositioned screws that caused neurological compromise [p = 0.02], leading to subsequent decrease in reoperation rates [p = 0.04] among Group 1 patients. CONCLUSIONS: Trigger EMG is well efficient in detecting potential pedicle screw breaches that might endanger neural integrity. In combination with palpatory and radiographic assessment, it will certainly aid safe and secure pedicle screw placement. It can also efficiently reduce reoperation rates due to neurologic compromise provoked by a malpositioned screw.

Genetic regulation of injury-induced heterotopic ossification in adult zebrafish.

Heterotopic ossification is the inappropriate formation of bone in soft tissues of the body. It can manifest spontaneously in rare genetic conditions or as a response to injury, known as acquired heterotopic ossification. There are several experimental models for studying acquired heterotopic ossification from different sources of damage. However, their tenuous mechanistic relevance to the human condition, invasive and laborious nature and/or lack of amenability to chemical and genetic screens, limit their utility. To address these limitations, we developed a simple zebrafish injury model that manifests heterotopic ossification with high penetrance in response to clinically emulating injuries, as observed in human myositis ossificans traumatica. Using this model, we defined the transcriptional response to trauma, identifying differentially regulated genes. Mutant analyses revealed that an increase in the activity of the potassium channel Kcnk5b potentiates injury response, whereas loss of function of the interleukin 11 receptor paralogue (Il11ra) resulted in a drastically reduced ossification response. Based on these findings, we postulate that enhanced ionic signalling, specifically through Kcnk5b, regulates the intensity of the skeletogenic injury response, which, in part, requires immune response regulated by Il11ra.

Living with Fibrodysplasia Ossificans Progressiva: Radiological Images of a Patient with Extensive Heterotopic Ossification.

Fibrodysplasia ossificans progressiva (FOP) is an exceptionally rare genetic disorder characterized by the progressive formation of heterotopic bone in soft tissues. Here, we present the radiological findings of an 18-year-old female diagnosed with FOP who had severe spinal and right-upper-limb abnormalities. Her SF-36 scores suggested significant impairment in physical function, affecting work and other regular daily activities. Radiographic evaluation with X-rays and CT scans revealed scoliosis and total fusion of almost all levels of the spine, with only a few disc spaces spared. A large mass of heterotopic bone was observed, corresponding to the location of the paraspinal muscles in the lumbar region, branching upwards and fusing with the scapulae on both sides. On the right side, this exuberant heterotopic bone mass fused with the humerus, resulting in a fixed right shoulder, while the rest of the upper and lower limbs are spared and have a range of motion. Our report highlights the extensive ossification that can manifest in patients with FOP, resulting in restricted mobility and a poor quality of life. While there is no definite treatment that can reverse the effects of the disease, preventing injuries and minimizing iatrogenic harm is of critical importance in this patient as inflammation is known to play a key role in triggering heterotopic bone. Meanwhile, ongoing research into therapeutic strategies holds the key to unlocking a potential cure for FOP in the future.

Musculoskeletal regeneration: A zebrafish perspective.

Musculoskeletal injuries are common in humans. The cascade of cellular and molecular events following such injuries results either in healing with functional recovery or scar formation. While fibrotic scar tissue serves to bridge between injured planes, it undermines functional integrity. Hence, faithful regeneration is the most desired outcome; however, the potential to regenerate is limited in humans. In contrast, various non-mammalian vertebrates have fascinating capabilities of regenerating even an entire appendage following amputation. Among them, zebrafish is an important and accessible laboratory model organism, sharing striking similarities with mammalian embryonic musculoskeletal development. Moreover, clinically relevant muscle and skeletal injury zebrafish models recapitulate mammalian regeneration. Upon muscle injury, quiescent stem cells - known as satellite cells - become activated, proliferate, differentiate and fuse to form new myofibres, while bone fracture results in a phased response involving hematoma formation, inflammation, fibrocartilaginous callus formation, bony callus formation and remodelling. These models are well suited to testing gene- or pharmaco-therapy for the benefit of conditions like muscle tears and fractures. Insights from further studies on whole body part regeneration, a hallmark of the zebrafish model, have the potential to complement regenerative strategies to achieve faster and desired healing following injuries without any scar formation and, in the longer run, drive progress towards the realisation of large-scale regeneration in mammals. Here, we provide an overview of the basic mechanisms of musculoskeletal regeneration, highlight the key features of zebrafish as a regenerative model and outline the relevant studies that have contributed to the advancement of this field.

Multiple Clay Shoveler's Fractures of the Thoracic Spine.

Typically, a clay shoveler's fracture is a stress-type avulsion fracture involving the spinous processes of the lower cervical or upper thoracic vertebrae. Even though C7 and T1 are the most commonly involved spinal levels, these avulsion fractures can occur at any lower cervical or upper thoracic level, either as solitary or multiple fractures. This fracture used to be common in workers who shovel heavy loads of clay for long periods, hence its name. It does not cause any structural, functional, or neurological impairments and is therefore considered a stable fracture. Management is mostly conservative, involving rest, analgesics, and activity modification for a period of 4-6 weeks. Here, we present a 35-year-old male who sustained a motor vehicle accident. Except for midline tenderness in the back, there were no other positive findings. Plain radiographs showed a T11 vertebral compression fracture and absent or deviated spinous process shadows for most of the upper thoracic vertebrae. Computed tomography (CT) imaging clearly revealed multiple spinous process fractures extending from T2 to T8 levels. Considering the stability of these fractures, the patient was managed conservatively with rest, bracing, and analgesics. The recovery was quick, and he was back to his full functional status by six weeks.

Early Postoperative Loss of Disc Height Following Transforaminal and Lateral Lumbar Interbody Fusion: A Radiographic Analysis.

STUDY DESIGN: Retrospective comparative radiological study. PURPOSE: To analyze the difference in early disc height loss following transforaminal and lateral lumbar interbody fusion (TLIF and LLIF). OVERVIEW OF LITERATURE: Minimal disc height loss facilitated by the polyaxial screw heads can occur naturally due to mechanical loading following lumbar fusion procedures. This loss does not usually cause any significant foraminal narrowing. However, when there is concomitant cage subsidence, symptomatic foraminal compromise could occur, especially when posterior decompression is not performed. It is not known whether the type of procedure, TLIF or LLIF, could influence this phenomenon. METHODS: Retrospectively, patients who underwent TLIF and LLIF for various degenerative conditions were shortlisted. Each of their fused levels with the cage in situ was analyzed independently, and the preoperative, postoperative, and follow-up disc height measurements were compared between the groups. In addition, the total disc height loss since surgery was calculated at final follow-up and was compared between the groups. RESULTS: Forty-six patients (age, 64.1±8.9 years) with 70 cage levels, 35 in each group, were selected. Age, sex, construct length, preoperative disc height, cage height, and immediate postoperative disc height were similar between the groups. By 3 months, disc height of the TLIF group was significantly less and continued to decrease over time, unlike in the LLIF group. By 1 year, the TLIF group demonstrated greater disc height loss (2.30±1.3 mm) than the LLIF group (0.89±1.1 mm). However, none of the patients in either group had any symptomatic complications throughout follow-up. CONCLUSIONS: Although our study highlights the biomechanical advantage of LLIF over TLIF in maintaining disc height, none of the patients in our cohort had symptomatic complications or implant-related failures. Hence, TLIF, as it incorporates posterior decompression, remains a safe and reliable technique despite the potential for greater disc height loss.

A Novel Scale for Assessing the Burden of Caregiving for Functionally Compromised Patients: Proposal and Validation.

BACKGROUND: Disability not only burdens the patient, but also the caregiver. To quantify this caregiving burden, we propose a simple four-part questionnaire tool. Our objective is to validate this questionnaire by administering it to caregivers who oversee patients with low back pain and are functionally compromised. METHODS: Twenty-five spouse caregivers who were taking care of in-patients awaiting surgery for various lumbar spine pathologies were shortlisted. The content-validated questionnaire was administered on different occasions during the care recipient's treatment. Cronbach's α was calculated to assess internal consistency. Interrelationships between the care recipient's pain score, extent of functional compromise, and caregiver burden were calculated. The questionnaire's ability to track changes in the caregivers' attitudes over time was assessed. RESULTS: The percentage of caregiver burden before the surgery of the care recipient was 52.5. This increased significantly to 61.1% (P=0.001) 3 days after surgery, but was found to decrease to 32.5% (P<0.001) a month after the surgery; demonstrating the questionnaire's efficacy to track changes. Cronbach's alpha of 0.948 signifies the questionnaire's excellent internal consistency. Pearson's correlation coefficient (r) between the care recipient's pain score and caregiver's burden score was 0.41 (P=0.04), and between the care recipient's disability score and caregiver's burden score was 0.9 (P<0.001). CONCLUSION: The proposed questionnaire is consistent and can track changes in a caregiver's attitude over time. It can be adopted for clinical use to assess the burden of caregiving for functionally compromised patients.

Morphological Parameters of the Thoracic Pedicle in an Asian Population: A Magnetic Resonance Imaging-Based Study of 3324 Pedicles.

STUDY DESIGN: A cross-sectional magnetic resonance imaging (MRI)-based anatomical study. OBJECTIVES: Instrumentation of the thoracic spine may be challenging due to the unique pedicle morphology and the proximity of vital structures. As prior morphological studies have mostly been done in Caucasians, our study aims to determine the optimal pedicle screw size for transpedicular fixation in an Asian population. METHODS: A retrospective analysis of 400 patients who had undergone MRI of the thoracic spine was performed. A total of 3324 pedicles were included. Pedicle morphology was graded qualitatively based on the size of its cancellous channel, and quantitatively with the following parameters: pedicle transverse diameter, pedicle screw path length, and pedicle angle. Subgroup analysis based on gender was performed. RESULTS: Mean pedicle transverse diameter was the narrowest at the T4 (2.9 ± 1 mm) and T5 (3.1 ± 1.1 mm) level. The mean pedicle screw path length progressively increased from T1 (34 ± 4.6 mm) to T12 (47 ± 4.6 mm). The mean pedicle angle was the largest at T1 (34° ± 7.9°) and decreased caudally, to 9.4° ± 3.8° at the T12 level. Females had significantly lower mean pedicle diameter and screw path length than males at every vertebral level; however, they had a larger pedicle angle at T8 to T10. The most common size of the pedicle cancellous channel was more than 4 mm. CONCLUSION: Morphological differences in the Asian pedicle suggest that caution needs to be taken during thoracic spine instrumentation, particularly in Asian females who have significantly smaller pedicles. In such cases, the use of alternative techniques or intraoperative navigation may be useful.

Spinal Navigation for Cervical Pedicle Screws: Surgical Pearls and Pitfalls.

STUDY DESIGN: Retrospective cohort study. OBJECTIVE: We intend to evaluate the accuracy and safety of cervical pedicle screw (CPS) insertion under O-arm-based 3-dimensional (3D) navigation guidance. METHODS: This is a retrospective study of patients who underwent CPS insertion under intraoperative O-arm-based 3D navigation during the years 2009 to 2018. The radiological accuracy of CPS placement was evaluated using their intraoperative scans. RESULTS: A total of 297 CPSs were inserted under navigation. According to Gertzbein classification, 229 screws (77.1%) were placed without any pedicle breach (grade 0). Of the screws that did breach the pedicle, 51 screws (17.2%) had a minor breach of less than 2 mm (grade 1), 13 screws (4.4%) had a breach of between 2 and 4 mm (grade 2), and 4 screws (1.3%) had a complete breach of 4 mm or more (grade 3). Six screws were revised intraoperatively. There was no incidence of neurovascular injury in this series of patients. 59 of the 68 breaches (86.8%) were found to perforate laterally, and the remaining 9 (13.2%) medially. It was noted that the C5 cervical level had the highest breach rate of 33.3%. CONCLUSIONS: O-arm-based 3D navigation can improve the accuracy and safety of CPS insertion. The overall breach rate in this study was 22.9%. Despite these breaches, there was no incidence of neurovascular injury or need for revision surgery for screw malposition.

Postoperative Urinary Retention Following Thoracolumbosacral Spinal Fusion: Prevalence, Risk Factors, and Outcomes.

Objective Postoperative urinary retention (POUR) is an often-underestimated common complication following spine surgery, and it is essential to avoid its untoward long-term consequences. Besides, a dilemma exists regarding the appropriate timing for the postoperative removal of indwelling catheter (IDC). Hence, we aim to describe the prevalence, risk factors, and outcomes of POUR and also come up with recommendations for the removal of IDC. Methods Electronic records of patients who underwent elective thoracolumbosacral spinal fusion surgery from January 2017 to December 2019 were retrospectively reviewed. Excluded were those who underwent fusion for indications such as trauma, cauda equina syndrome, infection, and malignancy. Both surgery-related and patient-related risk factors were tabulated, and their association with the likely development of POUR was assessed by univariate and multivariate analysis. Results One hundred sixty-eight patients (median age=64.1 years; 58.9% female) were included, with the incidence of POUR being 7.8%. Our findings suggest surgery-related factors, both intra- and postoperative, including operating time (p=0.008), anesthetic time (p=0.005), number of fusion levels (p<0.001), mobilization status prior to trial off catheter (TOC; p=0.021), and TOC timing (p=0.029) may have an association with POUR. In addition, patient-related factors, including the use of beta-blockers (p=0.020) and pre-operative mobility status (p<0.001), may also be associated with the likely development of POUR. Conclusion POUR seems to be a frequent complication following thoracolumbosacral spinal fusion surgery, which was found to have an association with some surgery-related and patient-related factors. While most of these factors are non-modifiable, certain modifiable risk factors provide the surgeon an opportunity to prevent POUR. Considering these factors, we recommend appropriate and timely mobilization of the patient prior to removal of IDC, which is to be performed preferably in the daytime.

Correction of degenerative lumbar coronal deformity using asymmetrical interbody cages: Surgical technique and case report.

In adult degenerative spondylosis, much emphasis has been placed upon recognizing the sagittal plane deformity and techniques to restore this alignment. However, the coronal plane deformity has not received much attention and, if left uncorrected, may lead to poorer outcomes. Here, we present a case of degenerative lumbar scoliosis with a rigid coronal malalignment secondary to a dysplastic sacrum. We performed staged T11-pelvis lateral and posterior approach to address this deformity. For the first stage, a lateral lumbar interbody fusion was performed at the concavity of the curve from L3 to L5. For the second stage, through posterior approach, a long-segment instrumentation from T11 to pelvis was done along with bilateral asymmetrical posterior lumbar interbody fusion of L5-S1 to level the L5 vertebra at the hemi-curve, thereby leveling the coronal deformity. We propose, for cases with a rigid coronal deformity due to bony dysplasia, correction through the disc space using asymmetrical interbody cages as in this case offers the surgeon an option to achieve a desired correction, without the need for vertebral osteotomy.

Neuromonitoring in Cervical Spine Surgery: When Is a Signal Drop Clinically Significant?

STUDY DESIGN: Retrospective cohort study. PURPOSE: To identify the clinical significance of different patterns of intraoperative neuromonitoring (IONM) signal alerts. OVERVIEW OF LITERATURE: IONM is a long-established valuable adjunct to complex spine surgeries. IONM for cervical spine surgery is in the form of somatosensory evoked potential (SSEP) and motor evoked potential (MEP). The efficacy of both modalities (individually or in combination) to detect clinically significant neurological compromise is constantly being debated and requires conclusive suggestions. METHODS: Clinical and neuromonitoring data of 207 consecutive adult patients who underwent cervical spine surgeries at multiple surgical centers using bimodal IONM were analyzed. Signal changes were divided into three groups. Group 0 had transient signal changes in either MEPs or SSEPs, group 1 had sustained unimodal changes, and group 2 had sustained changes in both MEPs and SSEPs. The incidences of true neurological deficits in each group were recorded. RESULTS: A total of 25% (52/207) had IONM signal alerts. Out of these signal drops, 96% (50/52) were considered to be false positives. Groups 0 and 1 had no incidence of neurological deficits, while group 2 had a 29% (2/7) rate of true neurological deficits. The sensitivities of both MEP and SSEP were 100%. SSEP had a specificity of 96.6%, while MEP had a lower specificity at 76.6%. C5 palsy rate was 6%, and there was no correlation with IONM signal alerts (p=0.73). CONCLUSIONS: This study shows that we can better predict its clinical significance by dividing IONM signal drops into three groups. A sustained, bimodal (MEP and SSEP) signal drop had the highest risk of true neurological deficits and warrants a high level of caution. There were no clear risk factors for false-positive alerts but there was a trend toward patients with cervical myelopathy.

Fibrodysplasia ossificans progressiva: current concepts from bench to bedside.

Heterotopic ossification (HO) is a disorder characterised by the formation of ectopic bone in soft tissue. Acquired HO typically occurs in response to trauma and is relatively common, yet its aetiology remains poorly understood. Genetic forms, by contrast, are very rare, but provide insights into the mechanisms of HO pathobiology. Fibrodysplasia ossificans progressiva (FOP) is the most debilitating form of HO. All patients reported to date carry heterozygous gain-of-function mutations in the gene encoding activin A receptor type I (ACVR1). These mutations cause dysregulated bone morphogenetic protein (BMP) signalling, leading to HO at extraskeletal sites including, but not limited to, muscles, ligaments, tendons and fascia. Ever since the identification of the causative gene, developing a cure for FOP has been a focus of investigation, and studies have decoded the pathophysiology at the molecular and cellular levels, and explored novel management strategies. Based on the established role of BMP signalling throughout HO in FOP, therapeutic modalities that target multiple levels of the signalling cascade have been designed, and some drugs have entered clinical trials, holding out hope of a cure. A potential role of other signalling pathways that could influence the dysregulated BMP signalling and present alternative therapeutic targets remains a matter of debate. Here, we review the recent FOP literature, including pathophysiology, clinical aspects, animal models and current management strategies. We also consider how this research can inform our understanding of other types of HO and highlight some of the remaining knowledge gaps.

Factors Influencing Early Disc Height Loss Following Lateral Lumbar Interbody Fusion.

STUDY DESIGN: Retrospective radiological analysis. PURPOSE: To analyze the factors influencing early disc height loss following lateral lumbar interbody fusion (LLIF). OVERVIEW OF LITERATURE: Postoperative disc height loss can occur naturally as a result of mechanical loading. This phenomenon is enabled by the yielding of the polyaxial screw heads and settling of the cage to the endplates. When coupled with cage subsidence, there can be significant reduction in the foraminal space which ultimately compromises the indirect decompression achieved by LLIF. METHODS: Seventy-two cage levels in 37 patients aged 62±10.2 years who underwent single or multilevel LLIF for degenerative spinal conditions were selected. Their preoperative and postoperative follow-up radiographs were used to measure the anterior disc height (ADH), posterior disc height (PDH), mean disc height (MDH), disc space angle (DSA), and segmental angle. Correlations between the loss of disc height and several factors, including age, construct length, preoperative lordosis, postoperative lordosis, disc height, cage dimensions, and cage position, were analyzed. RESULTS: We found that the lateral interbody cages significantly increased ADH, PDH, MDH, and DSA after surgery (p<0.0001). However, there was a loss of disc height over time. All postoperative disc height parameters, especially the amount of increase in MDH (r=0.413, p<0.0001) after surgery, showed a significant positive association with early disc height loss. The levels demonstrating a significant (≥25%) height loss were those that exhibited a substantial height increase (128.3%, 4.6±3.0 to 10.5±5.6 mm) postoperatively. However, the levels that showed less than 25% height loss were those that exhibited, on average, only a 57.4% height increase post-operatively. CONCLUSIONS: The greater the postoperative increase in disc height, the greater the disc height loss throughout early follow-up. Therefore, achieving an optimal disc height rather than overcorrection is an important surgical strategy to adopt when performing LLIF.

Does Lumbar Interbody Cage Size Influence Subsidence? A Biomechanical Study.

STUDY DESIGN: An experimental laboratory-based biomechanical study. OBJECTIVE: To investigate the correlation between cage size and subsidence and to quantify the resistance to subsidence that a larger cage can provide. SUMMARY OF BACKGROUND DATA: The assumption that a bigger interbody cage confers less subsidence has not been proven. There was no previous study that has shown the superiority of lateral cages to bullet cages in terms of subsidence and none that has quantified the correlation between cage size and subsidence. METHODS: A cage was compressed between two standardized polyurethane foam blocks at a constant speed. Four sizes of bullet cages used for transforaminal lumbar interbody fusion (TLIF) and six sizes of lateral cages used for lateral lumbar interbody fusion (LLIF) were tested. The force required for a 5 mm subsidence, axial area of cages, and stiffness were analyzed. RESULTS: A larger cage required a significantly higher force for a 5 mm subsidence. Longer bullet cages required marginal force increments of only 6.2% to 14.6% compared to the smallest bullet cage. Lateral cages, however, required substantially higher increments of force, ranging from 136.4% to 235.7%. The average force of lateral cages was three times that of bullet cages (6426.5 vs. 2115.9 N), and the average stiffness of the LLIF constructs was 3.6 times that of the TLIF constructs (635.5 vs. 2284.2 N/mm). There was a strong correlation between the axial area of cages and the force for a 5 mm subsidence. Every 1 mm increment of axial area corresponded to approximately 8 N increment of force. CONCLUSION: Cage size correlated strongly with the force required for a 5 mm subsidence. The LLIF constructs required higher force and were stiffer than the TLIF constructs. Among bullet cages, longer cages only required marginal increments of force. Lateral cages, however, required substantially higher force. LEVEL OF EVIDENCE: N/A.