Risk of serious adverse events after primary shoulder replacement: development and external validation of a prediction model using linked national data from England and Denmark.

BACKGROUND: Despite a rising rate of serious medical complications after shoulder replacement surgery, there are no prediction models in widespread use to guide surgeons in identifying patients at high risk and to provide patients with personalised risk estimates to support shared decision making. Our aim was to develop and externally validate a prediction model for serious adverse events within 90 days of primary shoulder replacement surgery. METHODS: Linked data from the National Joint Registry, National Health Service Hospital Episode Statistics Admitted Patient Care of England, and Civil Registration Mortality databases and Danish Shoulder Arthroplasty Registry and National Patient Register were used for our modelling study. Patients aged 18-100 years who had a primary shoulder replacement between April 1, 2012, and Oct 2, 2020, in England, and April 1, 2012, and Oct 2, 2018, in Denmark, were included. We developed a multivariable logistic regression model using the English dataset to predict the risk of 90-day serious adverse events, which were defined as medical complications requiring admission to hospital and all-cause death. We undertook internal validation using bootstrapping, and internal-external cross-validation across different geographical regions of England. The English model was externally validated on the Danish dataset. FINDINGS: Data for 40 631 patients undergoing primary shoulder replacement (mean age 72·5 years [SD 9·9]; 28 709 [70·7%] women and 11 922 [29·3%] men) were used for model development, of whom 2270 (5·6%) had a 90-day serious adverse event. On internal validation, the model had a C-statistic of 0·717 (95% CI 0·707-0·728) and was well calibrated. Internal-external cross-validation showed consistent model performance across all regions in England. Upon external validation on the Danish dataset (n=6653; mean age 70·5 years [SD 10·3]; 4503 [67·7%] women and 2150 [32·3%] men), the model had a C-statistic of 0·750 (95% CI 0·723-0·776). Decision curve analysis showed clinical utility, with net benefit across all risk thresholds. INTERPRETATION: This externally validated prediction model uses commonly available clinical variables to accurately predict the risk of serious medical complications after primary shoulder replacement surgery. The model is generalisable and applicable to most patients in need of a shoulder replacement. Its use offers support to clinicians and could inform and empower patients in the shared decision-making process. FUNDING: National Institute for Health and Care Research and the Department of Orthopaedic Surgery, Herlev and Gentofte Hospital, Denmark.

Reverse total shoulder replacement versus anatomical total shoulder replacement for osteoarthritis: population based cohort study using data from the National Joint Registry and Hospital Episode Statistics for England.

OBJECTIVES: To answer a national research priority by comparing the risk-benefit and costs associated with reverse total shoulder replacement (RTSR) and anatomical total shoulder replacement (TSR) in patients having elective primary shoulder replacement for osteoarthritis. DESIGN: Population based cohort study using data from the National Joint Registry and Hospital Episode Statistics for England. SETTING: Public hospitals and publicly funded procedures at private hospitals in England, 2012-20. PARTICIPANTS: Adults aged 60 years or older who underwent RTSR or TSR for osteoarthritis with intact rotator cuff tendons. Patients were identified from the National Joint Registry and linked to NHS Hospital Episode Statistics and civil registration mortality data. Propensity score matching and inverse probability of treatment weighting were used to balance the study groups. MAIN OUTCOME MEASURES: The main outcome measure was revision surgery. Secondary outcome measures included serious adverse events within 90 days, reoperations within 12 months, prolonged hospital stay (more than three nights), change in Oxford Shoulder Score (preoperative to six month postoperative), and lifetime costs to the healthcare service. RESULTS: The propensity score matched population comprised 7124 RTSR or TSR procedures (126 were revised), and the inverse probability of treatment weighted population comprised 12 968 procedures (294 were revised) with a maximum follow-up of 8.75 years. RTSR had a reduced hazard ratio of revision in the first three years (hazard ratio local minimum 0.33, 95% confidence interval 0.18 to 0.59) with no clinically important difference in revision-free restricted mean survival time, and a reduced relative risk of reoperations at 12 months (odds ratio 0.45, 95% confidence interval 0.25 to 0.83) with an absolute risk difference of -0.51% (95% confidence interval -0.89 to -0.13). Serious adverse events and prolonged hospital stay risks, change in Oxford Shoulder Score, and modelled mean lifetime costs were similar. Outcomes remained consistent after weighting. CONCLUSIONS: This study's findings provide reassurance that RTSR is an acceptable alternative to TSR for patients aged 60 years or older with osteoarthritis and intact rotator cuff tendons. Despite a significant difference in the risk profiles of revision surgery over time, no statistically significant and clinically important differences between RTSR and TSR were found in terms of long term revision surgery, serious adverse events, reoperations, prolonged hospital stay, or lifetime healthcare costs.

Shoulder replacement surgery's rising demand, inequality of provision, and variation in outcomes: cohort study using Hospital Episode Statistics for England.

BACKGROUND: The aim of this study was to forecast future patient demand for shoulder replacement surgery in England and investigate any geographic and socioeconomic inequalities in service provision and patient outcomes. METHODS: For this cohort study, all elective shoulder replacements carried out by NHS hospitals and NHS-funded care in England from 1999 to 2020 were identified using Hospital Episode Statistics data. Eligible patients were aged 18 years and older. Shoulder replacements for malignancy or acute trauma were excluded. Population estimates and projections were obtained from the Office for National Statistics. Standardised incidence rates and the risks of serious adverse events (SAEs) and revision surgery were calculated and stratified by geographical region, socioeconomic deprivation, sex, and age band. Hospital costs for each admission were calculated using Healthcare Resource Group codes and NHS Reference Costs based on the National Reimbursement System. Projected rates and hospital costs were predicted until the year 2050 for two scenarios of future growth. RESULTS: A total of 77,613 elective primary and 5847 revision shoulder replacements were available for analysis. Between 1999 and 2020, the standardised incidence of primary shoulder replacements in England quadrupled from 2.6 to 10.4 per 100,000 population, increasing predominantly in patients aged over 65 years. As many as 1 in 6 patients needed to travel to a different region for their surgery indicating inequality of service provision. A temporal increase in SAEs was observed: the 30-day risk increased from 1.3 to 4.8% and the 90-day risk increased from 2.4 to 6.0%. Patients from the more deprived socioeconomic groups appeared to have a higher risk of SAEs and revision surgery. Shoulder replacements are forecast to increase by up to 234% by 2050 in England, reaching 20,912 procedures per year with an associated annual cost to hospitals of £235 million. CONCLUSIONS: This study reports a rising incidence of shoulder replacements, regional disparities in service provision, and an overall increasing risk of SAEs, especially in more deprived socioeconomic groups. These findings highlight the need for better healthcare planning to match local population demand, while more research is needed to understand and prevent the increase observed in SAEs.

Association between surgeon volume and patient outcomes after elective shoulder replacement surgery using data from the National Joint Registry and Hospital Episode Statistics for England: population based cohort study.

OBJECTIVE: To investigate the association between surgeon volume and patient outcomes after elective shoulder replacement surgery to improve patient outcomes and inform future resource planning for joint replacement surgery. DESIGN: Prospective cohort study. SETTING: Public and private hospitals in the United Kingdom, 2012-20. PARTICIPANTS: Adults aged 18 years or older who had shoulder replacement surgery, identified in the National Joint Registry, with linkage of participants in England to Hospital Episode Statistics data. MAIN OUTCOME MEASURES: The main outcome measure was revision surgery. Secondary outcome measures were reoperation within 12 months, serious adverse events, and prolonged hospital stay (>3 nights) after shoulder replacement surgery. RESULTS: 39 281 shoulder replacement procedures undertaken by 638 consultant surgeons at 416 surgical units met the inclusion criteria and were available for analysis. Multilevel mixed effects models and restricted cubic splines were fit to examine the association between a surgeon's mean annual volume and risk of adverse patient outcomes, with a minimum volume threshold of 10.4 procedures yearly identified. Below this threshold the risk of revision surgery was significantly increased, as much as twice that of surgeons with the lowest risk (hazard ratio 1.94, 95% confidence interval 1.27 to 2.97). A greater mean annual surgical volume was also associated with a significantly lower risk of reoperations, fewer serious adverse events, and shorter hospital stay, with no thresholds identified. Annual variation in surgeon volume was not associated with any of the outcomes assessed. CONCLUSIONS: In the healthcare system represented by these registry data, an association was found between surgeons who averaged more than 10.4 shoulder replacements yearly and lower rates of revision surgery and reoperation, lower risk of serious adverse events, and shorter hospital stays. These findings should inform resource planning for surgical services and joint replacement surgery waiting lists and improve patient outcomes after shoulder replacement surgery.

Modelling the effect of a dedicated hip fracture unit on patient outcomes using segmented robust linear regression techniques.

INTRODUCTION: Typically, a healthcare intervention is evaluated by comparing data before and after its implementation using statistical tests. Comparing group means can miss underlying trends and lead to erroneous conclusions. Segmented linear regression can be used to reveal secular trends but is susceptible to outliers. We described a novel method using segmented robust regression techniques to evaluate the effect of introducing a dedicated hip fracture unit (HFU). METHODS: We retrospectively analysed patient outcomes from a total of 2777 patients sustaining proximal femoral fragility fractures over a 6-year period at a Level 1 Major Trauma Centre. We compared time to surgical intervention and length of hospital stay before and after the implementation of the HFU using group comparison tests, segmented ordinary regression and robust regression techniques to evaluate the effect of the intervention. RESULTS: Group comparison tests did not identify a significant difference in time to surgery pre and post- HFU. Segmented regression revealed that there was a significant reduction in time to surgery but that this predated the introduction of the HFU. Group comparison tests did not identify a significant difference in length of stay pre and post-HFU. Ordinary segmented regression demonstrated that there was a constant reduction in length of stay, which accelerated after the introduction of the HFU. Robust regression identified that this change occurred prior to the HFU. DISCUSSION: There was a significant decrease in time to surgical intervention during the study period that occurred long before the introduction of the HFU, and that cannot be attributed to the HFU itself. Length of stay started dropping early in the study period and was unrelated to the HFU. However, with robust regression we concluded that the HFU was effective in reducing relatively long hospital stays (outliers). Several explanatory factors that may have affected the observed trends in time to surgery and length of stay were identified. CONCLUSION: Robust regression is a useful adjunct to ordinary segmented linear regression techniques in modelling retrospective time-series and dealing with outliers. The changes observed in hip fracture patient outcomes over a 6-year period was likely multifactorial.

Dual-incision minimally invasive fasciotomy of the anterior and peroneal compartments for chronic exertional compartment syndrome of the lower leg.

To evaluate the risk of iatrogenic injury when using a dual-incision minimally invasive technique to decompress the anterior and peroneal compartments of the lower leg. Forty lower extremities from 20 adult cadavers, embalmed with Thiel's method, were subject to fasciotomy of the anterior and peroneal compartment using a dual-incision minimally invasive fasciotomy. The first incision was made 12 cm proximal to the lateral malleolus to identify and protect the superficial peroneal nerve (SPN). The second incision was made at the mid-point of the Fibula (half-way between the fibular head and the lateral malleolus). Release of the anterior and peroneal compartments was successful in all specimens. Two nerve injuries of the superficial peroneal nerve were reported. More precisely, in these cases the medial dorsal cutaneous nerve got injured during the fascial opening of the extensor compartment. Two incision minimally invasive fasciotomy to decompress the anterior and peroneal compartments of the lower leg appears to be safe with regard to the results of this study.

Minimally Invasive Lower Leg Fasciotomy for Chronic Exertional Compartment Syndrome-How Safe Is It? A Cadaveric Study.

BACKGROUND: Chronic exertional compartment syndrome (CECS) is a recognized clinical diagnosis in running athletes and military recruits. Minimally invasive fasciotomy techniques have become increasingly popular, but with varied results and small case numbers. Although decompression of the anterior and peroneal compartments has demonstrated a low rate of iatrogenic injury, little is known about the safety of decompressing the deep posterior compartment. PURPOSE: To evaluate the risk of iatrogenic injury when using minimally invasive techniques to decompress the anterior, peroneal, and deep posterior compartments of the lower leg. STUDY DESIGN: Descriptive laboratory study. METHODS: A total of 60 lower extremities from 30 adult cadavers were subject to fasciotomy of the anterior, peroneal, and deep posterior compartments using a minimally invasive technique. Two common variations in surgical technique were employed to decompress each compartment. Anatomical dissection was subsequently carried out to identify incomplete division of the fascia, muscle injury, neurovascular injury, and the anatomical relationship of key neurovascular structures to the incisions. RESULTS: Release of the anterior and peroneal compartments was successful in all but 2 specimens. There was no injury to the superficial peroneal nerve or any vessel in any specimen. A transverse incision crossing the anterior intermuscular septum resulted in muscle injury in 20% of the cases. Release of the deep posterior compartment was successful in all but 1 specimen when a longitudinal skin incision was used, without injury to neurovascular structures. Compared with a longitudinal incision, a transverse skin incision resulted in fewer complete releases of the deep posterior compartment and a significantly higher rate of injury to the saphenous nerve (16.7%; P = .052) and long saphenous vein (23.3%; P = .011). CONCLUSION: Minimally invasive fasciotomy of the anterior, peroneal, and deep posterior compartments using longitudinal incisions had a low rate of iatrogenic injury in a cadaveric model. Complete compartment release was achieved in 97% to 100% of specimens when employing this technique. CLINICAL RELEVANCE: Minimally invasive fasciotomy techniques for CECS have become increasingly popular with purported low recurrence rates, improved cosmesis, and faster return to sport. It is important to determine whether this technique is safe, particularly given the variable rates of neurovascular injury reported in the literature.

The Management of Symptomatic Lumbar Disc Herniation in Pregnancy: A Systematic Review.

STUDY DESIGN: Systematic review. OBJECTIVES: Lumbar disc herniation (LDH) has been reported to affect 1 in 10 000 pregnant women. There is limited evidence available regarding the optimal management of LDH in pregnant patients. We aimed to review the current evidence for the management of symptomatic LDH in pregnancy through critical appraisal and analysis of the available literature. METHODS: Searches were conducted in Medline, Embase, PubMed, Science Direct, and The Cochrane Library from inception using predetermined search terms. All peer-reviewed studies of pregnant women with symptomatic LDH were included. The quality of eligible articles was assessed and extracted data and characteristics were pooled for analysis. References cited by studies were screened to identify other relevant publications. RESULTS: Thirty studies involving 52 patients were identified. Compared to surgically managed patients, conservatively managed patients had a higher full recovery rate (61.54% vs 56.41%) and reported a lower rate of persistent symptoms (30.77% vs 38.54%). Compared to patients who were treated surgically for cauda equina syndrome, patients treated surgically for sciatica had a higher full recovery rate (80.95% vs 27.78%) and reported a lower rate of persistent symptoms (14.29% vs 66.67%). CONCLUSION: There is limited evidence to guide the management of pregnant patients with LDH. Despite a suggestion toward improved outcomes with conservative management, the presence of selection bias and the overall poor quality of current research precludes reliable conclusions from being drawn. Decision making for this patient group should be undertaken within a multidisciplinary setting.

Suprapatellar tibial nailing: a learning curve analysis.

PURPOSE: The suprapatellar approach for intramedullary nailing of tibial fractures is gaining popularity with reported improved patient outcomes when compared to infrapatellar techniques. The aim of this study was to investigate the learning curve of the suprapatellar technique using radiation exposure as an outcome measure. METHODS: Data were analysed from a prospectively collected database over a 3-year period at a Major Trauma Centre in the United Kingdom. 83 study patients with an acute isolated extra-articular fracture of the tibia treated with intramedullary tibial nailing were included. Cases requiring additional intra-operative procedures were excluded. Four consultant trauma surgeons with no previous experience of the suprapatellar technique used this approach for 40 consecutive operations. Six consultant trauma surgeons used the infrapatellar approach for 43 patients and acted as a control group. Patient demographics, fluoroscopy time and radiation dose area product (DAP) were collected for each operation. A segmented linear regression modelling method was employed to analyse learning. RESULTS: Fluoroscopy time and DAP per surgeon showed no evidence of a learning curve when using a suprapatellar tibial nailing technique in group or individual analysis. Fluoroscopy time and DAP were stationary in the infrapatellar group analysis, confirming the absence of time-dependent trends over the study period. CONCLUSIONS: Consultant trauma surgeons experienced no significant learning-related increase in radiation exposure when introducing a suprapatellar technique for intramedullary nailing of uncomplicated tibial fractures. Future work is required to investigate the effects of learning on other outcome measures.

Evaluating learning and change in orthopaedics: What is the evidence-base?

Learning and change are key elements of clinical governance and are responsible for the progression of our specialty. Although orthopaedics has been slow to embrace quality improvement, recent years have seen global developments in surgical education, quality improvement, and patient outcome research. This review covers recent advances in the evaluation of learning and change and identifies the most important research questions that remain unanswered. Research into proxies of learning is improving but more work is required to identify the best proxy for a given procedure. Learning curves are becoming commonplace but are poorly integrated into postgraduate training curricula and there is little agreement over the most appropriate method to analyse learning curve data. With various organisations promoting centralisation of care, learning curve analysis is more important than ever before. The use of simulation in orthopaedics is developing but is yet to be formally mapped to resident training worldwide. Patient outcome research is rapidly changing, with an increased focus on quality of life measures. These are key to patients and their care. Cost-utility analysis is increasingly seen in orthopaedic manuscripts and this needs to continue to improve evidence-based care. Large-scale international, multi-centre randomised trials are gaining popularity and updated guidance on sample size estimation needs to become widespread. A global lack of surgeon equipoise will need to be addressed. Quality improvement projects frequently employ interrupted time-series analysis to evaluate change. This technique's limitations must be acknowledged, and more work is required to improve the evaluation of change in a dynamic healthcare environment where multiple interventions frequently occur. Advances in the evaluation of learning and change are needed to drive improved international surgical education and increase the reliability, validity, and importance of the conclusions drawn from orthopaedic research.

Segmented Linear Regression Modelling of Time-Series of Binary Variables in Healthcare.

INTRODUCTION: In healthcare, change is usually detected by statistical techniques comparing outcomes before and after an intervention. A common problem faced by researchers is distinguishing change due to secular trends from change due to an intervention. Interrupted time-series analysis has been shown to be effective in describing trends in retrospective time-series and in detecting change, but methods are often biased towards the point of the intervention. Binary outcomes are typically modelled by logistic regression where the log-odds of the binary event is expressed as a function of covariates such as time, making model parameters difficult to interpret. The aim of this study was to present a technique that directly models the probability of binary events to describe change patterns using linear sections. METHODS: We describe a modelling method that fits progressively more complex linear sections to the time-series of binary variables. Model fitting uses maximum likelihood optimisation and models are compared for goodness of fit using Akaike's Information Criterion. The best model describes the most likely change scenario. We applied this modelling technique to evaluate hip fracture patient mortality rate for a total of 2777 patients over a 6-year period, before and after the introduction of a dedicated hip fracture unit (HFU) at a Level 1, Major Trauma Centre. RESULTS: The proposed modelling technique revealed time-dependent trends that explained how the implementation of the HFU influenced mortality rate in patients sustaining proximal femoral fragility fractures. The technique allowed modelling of the entire time-series without bias to the point of intervention. Modelling the binary variable of interest directly, as opposed to a transformed variable, improved the interpretability of the results. CONCLUSION: The proposed segmented linear regression modelling technique using maximum likelihood estimation can be employed to effectively detect trends in time-series of binary variables in retrospective studies.

Segmented Linear Regression Models for Assessing Change in Retrospective Studies in Healthcare.

INTRODUCTION: In retrospective studies, the effect of a given intervention is usually evaluated by using statistical tests to compare data from before and after the intervention. A problem with this approach is that the presence of underlying trends can lead to incorrect conclusions. This study aimed to develop a rigorous mathematical method to analyse temporal variation and overcome these limitations. METHODS: We evaluated hip fracture outcomes (time to surgery, length of stay, and mortality) from a total of 2777 patients between April 2011 and September 2016, before and after the introduction of a dedicated hip fracture unit (HFU). We developed a novel modelling method that fits progressively more complex linear sections to the time series using least squares regression. The method was used to model the periods before implementation, after implementation, and of the whole study period, comparing goodness of fit using F-tests. RESULTS: The proposed method offered reliable descriptions of the temporal evolution of the time series and augmented conclusions that were reached by mere group comparisons. Reductions in time to surgery, length of stay, and mortality rates that group comparisons would have credited to the hip fracture unit appeared to be due to unrelated underlying trends. CONCLUSION: Temporal analysis using segmented linear regression models can reveal secular trends and is a valuable tool to evaluate interventions in retrospective studies.

Learning curves in surgery: variables, analysis and applications.

Learning curves graphically represent the relationship between learning effort and learning outcome. Learning curves are increasingly used in research, the design of randomised controlled trials, the assessment of competency, healthcare education and training programme design. In this review we have outlined the principles behind plotting learning curves, described the common methods used to analyse learning curves, how to interpret learning curves, the multitude of learning models, their applications and potential pitfalls, and the importance of a mathematically rigorous approach to learning curve analytics.

Imageless navigation total hip arthroplasty - an evaluation of operative time.

INTRODUCTION: Imageless navigation has been successfully integrated in knee arthroplasty but its effectiveness in total hip arthroplasty (THA) has been debated. It has consistently been shown that navigation adds significant time and cost to the operation. Further, the relative success of traditional hip replacements has impeded the adoption of new techniques. METHODS: We compared the operative time between fifty total hip replacements with and without the use of imageless navigation by a single senior surgeon in a retrospective study. We employed standard statistical tools to compare the two methods. A correlation-based analysis was used to delimit the "learned" phase of imageless navigation to make comparisons meaningful. RESULTS: Contrary to what has previously been reported, there was no significant difference between operative time in navigated, when compared to traditional operations (p = 0.498). Only fourteen operations were required to delimit the learning phase of this operation. DISCUSSION: This is the first study that demonstrates no added operative time when using imageless navigation in THA, achieved with an improved workflow. The results also demonstrate a very reasonable learning curve.

Effectiveness Of Plain Shoulder Radiograph In Detecting Degenerate Rotator Cuff Tears.

BACKGROUND: Studies have demonstrated radiographic findings of sclerosis and cortical irregularity at the greater tuberosity can suggest a rotator cuff tear. Plain radiographs are the most easily attainable first-line investigations in evaluating shoulder injuries. This study determines the effectiveness in predicting degenerate rotator cuff tears by detecting radiographic changes on shoulder x-rays. METHODS: Retrospective cross-sectional study with a consecutive series of patients conducted in Hinchingbrooke Hospital, Huntingdon, United Kingdom from January 2015 to June 2017. Anteroposterior shoulder radiographs of 150 symptomatic patients who underwent shoulder arthroscopy were independently analysed by surgeons who were blinded from the arthroscopic results. Patients aged fewer than 30 and over 70 years were excluded. Patients with advanced osteoarthritis and cuff tear arthropathy evident on x-rays were also excluded. Sixty-five patients included in the study had rotator cuff tears on arthroscopy. Radiographic changes were correlated with arthroscopic findings to determine this test's ability to predict degenerate rotator cuff tears. RESULTS: When both cortical irregularity and sclerosis were present on the plain radiograph, these signs had a sensitivity of 78.8% [95% CI 65.7, 87.8%] and specificity 77.4% [95% CI 67.2, 85.0%] with a positive predictive value of 68.3%, using contingency table analysis. The presence of cortical irregularity was found to be a better predictor of a tear as compared to sclerosis. CONCLUSIONS: This study concludes that plain radiograph are good modality for initial evaluation of rotator cuff tears and detecting when both cortical irregularity and sclerosis. Consideration of these radiographic findings serves as a useful adjunct in diagnostic workup and can guide subsequent investigations and treatment when evaluating rotator cuff tears of the shoulder.

Modeling Learning in Surgical Practice.

OBJECTIVE: Methods that model surgical learning curves are frequently descriptive and lack the mathematical rigor required to extract robust, meaningful, and quantitative information. We aimed to formulate a method to model learning that is tailored to dealing with the high variability seen in surgical data and can readily extract important quantitative information such as learning rate, length of learning, and learnt level of performance. METHODS: We developed a method where progressively more complex models are fitted to learning data. These include novel models that split the learning data into 2 linear phases and fit adjoining lines using least squares regression. The models were compared and the least complex model was selected unless a more complex one was significantly better. Significance was tested by Fischer tests. We applied this method to total hip and knee replacements using imageless navigation, analyzing the operative time for a surgeon's first 50 and 60 operations, respectively. This method was then tested against 4 sets of simulated learning data. RESULTS: The proposed method of progressive model complexity successfully modeled the learning curve among real operative data. It was also effective in deducing the underlying trends in simulated scenarios, created to represent typical situations that can practically arise in any learning process. CONCLUSIONS: The novel modeling method can be used to extract meaningful and quantitative information from learning data displaying high variability seen in surgical practice. By using simple and intuitive models, the method is accessible to researchers and educators without the need for specialist statistical knowledge.

An unusual cause of ventilatory failure in motor neurone disease.

A patient previously diagnosed with motor neurone disease (MND) and gastrostomy-fed was under surveillance for ventilatory decline via our respiratory centre. At a planned review she was found to be hypercapnic, which would usually prompt an offer of non-invasive ventilation for home use. However, she was alkalotic and not acidotic as we might expect. Her serum potassium was checked urgently and confirmed as low. It was established that the community team had prescribed a feeding regime with insufficient potassium. Correction of hypokalaemia resolved her ventilatory failure. This case demonstrates the importance of co-ordinated care in the management of patients with MND.

Granulocytic sarcoma: a rare cause of sciatica.

We describe a case report of a man aged 56 years with a 4-month history of right-sided sciatica-type pain with subclinical disc prolapse evident on MRI. Worsening pain together with the appearance of a tender mass in his right buttock prompted further imaging, which demonstrated an infiltrative mass engulfing the lumbosacral plexus. This was later shown to be a granulocytic sarcoma on biopsy. Intervertebral disc herniation can be an incidental finding and is not always the cause of sciatica.