Resident Synovial Macrophages in Synovial Fluid: Implications for Immunoregulation in Infectious and Inflammatory Arthritis

Objectives

Resident synovial macrophages (RSM) provide immune sequestration of the joint space and are likely involved in initiation and perpetuation of the joint-specific immune response. We sought to identify RSM in synovial fluid (SF) and demonstrate migratory ability, in additional to functional changes that may perpetuate a chronic inflammatory response within joint spaces.

Methods

We recruited human patients presenting with undifferentiated arthritis in multiple clinical settings. We used flow cytometry to identify mononuclear cells in peripheral blood and SF. We used a novel transwell migration assay with human ex-vivo synovium obtained intra-operatively to validate flow cytometry findings. We used single cell RNA-sequencing (scRNA-seq) to further identify macrophage/monocyte subsets. ELISA was used to evaluate the bone-resorption potential of SF.

Results

We were able to identify a rare population of CD14dim, OPG+, ZO-1+ cells consistent with RSM in SF via flow cytometry. These cells were relatively enriched in the SF during infectious processes, but absolutely decreased compared to healthy controls. Similar putative RSM were identified using ex vivo migration assays when MCP-1 and LPS were used as migratory stimulus. scRNA-seq revealed a population consistent with RSM transcriptionally related to CD56+ cytotoxic dendritic cells and IDO+ M2 macrophages.

Conclusion

We identified a rare cell population consistent with RSM, indicating these cells are likely migratory and able to initiate or coordinate both acute (septic) or chronic (autoimmune or inflammatory) arthritis. RSM analysis via scRNA-seq indicated these cells are M2 skewed, capable of antigen presentation, and have consistent functions in both septic and inflammatory arthritis.

Did Rapid Expansion of Same Day Discharge Hip and Knee Arthroplasty During the COVID-19 Pandemic Increase Early Complications?

Background

The COVID-19 pandemic has had a lasting impact on patients seeking total hip and knee arthroplasty (THA, TKA) including more patients undergoing same day discharge (SDD) following total joint arthroplasty (TJA). The purpose of this study was to assess whether expansion of SDD TJA during the COVID-19 pandemic resulted in more early complications following TJA. We anticipated that as many institutions quickly launched SDD TJA programs there may be an increase in 30-day complications.

Methods

We retrospectively queried the ACS-NSQIP database for all patients undergoing primary elective TJA from January 1, 2018, to December 31, 2020. Participants who underwent THA or TKA between January 1, 2018 and March 1, 2020 were grouped into pre-COVID and between March 1, 2020 and December 31, 2020 were grouped into post-COVID categories. Patients with length of stay greater than 0 were excluded. Primary outcome was any complication at 30 days. Secondary outcomes included readmission and re-operation 30 days.

Results

A total of 14,438 patients underwent TKA, with 9,580 occurring pre-COVID and 4,858 post-COVID. There was no difference in rates of total complication between the pre-COVID (3.55%) and post-COVID (3.99%) groups (p=0.197). Rates of readmissions for were similar for the pre-COVID (1.75%) and post-COVID (1.98%) groups (p=0.381). There was no statistically significant difference in respiratory complications between the pre-COVID (0.41%) and post-COVID group (0.23%, p=0.03). A total of 12,265 patients underwent THA, with 7,680 occurring pre-COVID and 4,585 post-COVID. There was no difference in rates of total complication between the pre-COVID (3.25%) and post-COVID (3.49%) groups (p=0.52). Rates of readmissions for were similar for the pre-COVID (1.77%) and post-COVID (1.68%) groups (p=0.381). There was no statistically significant difference in respiratory complications between the pre-COVID (0.16%) and post-COVID group (0.07%, p=0.26). Combined data to include THA and TKA patients did not find a statistical difference in the rate of complications or readmission but did note a decrease in the rate of combined respiratory complications in the post-COVID group (0.15% vs. 0.30%, p=0.028).

Conclusion

Rapid expansion of SDD TJA during the COVID-19 pandemic did not increase overall complication, readmission, or re-operation rates. Level of Evidence: IV.

Bridge Plate Fixation of Distal Femur Fractures: Defining Deficient Radiographic Callus Formation and Its Associations

OBJECTIVE

To determine whether deficient early callus formation can be defined objectively based on the association with an eventual nonunion and specific patient, injury, and treatment factors.

METHODS

Final healing outcomes were documented for 160 distal femur fractures treated with locked bridge plate fixation. Radiographic callus was measured on postoperative radiographs until union or nonunion had been declared by the treating surgeon. Deficient callus was defined at 6 and 12 weeks based on associations with eventual nonunion through receiver-operator characteristic analysis. A previously described computational model estimated fracture site motion based on the construct used. Univariable and multivariable analyses then examined the association of patient, injury, and treatment factors with deficient callus formation.

RESULTS

There were 26 nonunions. The medial callus area at 6 weeks <24.8 mm 2 was associated with nonunion (12 of 39, 30.8%) versus (12 of 109, 11.0%), P = 0.010. This association strengthened at 12 weeks with medial callus area <44.2 mm 2 more closely associated with nonunion (13 of 28, 46.4%) versus (11 of 120, 9.2%), P <0.001. Multivariable logistic regression analysis found limited initial longitudinal motion (OR 2.713 (1.12-6.60), P = 0.028)) and Charlson Comorbidity Index (1.362 (1.11-1.67), P = 0.003) were independently associated with deficient callus at 12 weeks. Open fracture, mechanism of injury, smoking, diabetes, plate material, bridge span, and shear were not significantly associated with deficient callus.

CONCLUSION

Deficient callus at 6 and 12 weeks is associated with eventual nonunion, and such assessments may aid future research into distal femur fracture healing. Deficient callus formation was independently associated with limited initial longitudinal fracture site motion derived through computational modeling of the surgical construct but not more routinely discussed parameters such as plate material and bridge span. Given this, improved methods of in vivo assessment of fracture site motion are necessary to further our ability to optimize the mechanical environment for healing.

LEVEL OF EVIDENCE

Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

What Is the Fate of Total Joint Arthroplasty Patients Who Are Asked to Quit Smoking Prior to Surgery?

Background

Smoking is a known risk factor for complications following primary total joint arthroplasty (TJA). Little is known regarding the fate of patients who are asked to quit smoking before surgery. The purpose of this study was to evaluate the success of smoking cessation prior to primary TJA and the impact of smoking cessation on perioperative outcomes.

Methods

This is a retrospective review of patients who presented between 2008 and 2020 to a single academic medical center with a documented smoking history and were asked to quit smoking prior to receiving a date for primary TJA. The cohort was surveyed about smoking cessation, smoking history, use of quit aids, seeking surgery elsewhere due to the cessation policy, and postoperative complications. Descriptive statistics evaluated the relationship between demographics, smoking cessation, and postoperative complications.

Results

A total of 101 patients completed the survey with an overall response rate of 48%. Sixty-two percent of patients quit smoking before surgery, and 51% of these patients reported remaining smoke-free at 6 months postoperatively. The average time to quit before TJA was 45 days (range: 1-365 days), and 62% quit without quit aids. The wound complication/infection rate was significantly higher for patients who did not stop smoking prior to TJA (4 of 16; 27%) than for those who did quit prior to surgery (3 of 63; 5%; P = .02).

Conclusions

This study demonstrates that most patients (62%) will stop smoking, if required, prior to primary TJA. Furthermore, 51% of patients reported abstinence from smoking at 6 months following TJA. TJA appears to be an effective motivator for smoking cessation.

Level of Evidence

III (retrospective cohort study).

'Tis Time 'Tis Time: The Importance of Operative Time, Thoroughness, and Shakespeare in Dair Procedures in Total Joint Arthroplasty

Background

Prosthetic joint infections (PJIs), while rare, are a devasting complication of both total joint arthroplasty (TJA). With most patients undergoing surgical treatment for PJI, options vary between one-stage or two-stage (the gold standard) procedures. Debridement, antibiotics, and implant retention (DAIR) procedures are a common, less morbid alternative to two-stage revisions, but patients undergoing DAIR procedures more often experience reinfections. This is likely in part due to non-standardized irrigation and debridement (I&D) methods within these procedures. Furthermore, DAIR procedures are often desired due to their cost effectiveness and lesser operative times, but no investigations have occurred regarding operative-time-based outcomes. This study aimed to compare reinfection incidence with procedure time in DAIR procedures. In addition, this study aimed to introduce the novel Macbeth Protocol for the I&D portion of DAIR procedures and assess its efficacy.

Methods

Records of unilateral DAIR procedures for primary TJA PJI performed by arthroplasty surgeons from 2015-2022 were retrospectively reviewed for patient demographics, select medical history, body mass index (BMI), joint, microbiology, and follow-up data. In addition, a single surgeon's DAIR procedures (for primary and revision TJA) were reviewed and use of The Macbeth Protocol was noted.

Results

A total of 71 patients (mean age 64.00 ± 12.81 years) who underwent unilateral DAIR were included. Patients with reinfections following their DAIR procedure had significantly (p = 0.034) lower procedure times (93.72 ± 15.01 min) compared to those without reinfections (105.87 ± 21.91 min). Twenty-two patients underwent 28 DAIR procedures by the senior author, where 11 (39.3%) DAIR procedures utilized The Macbeth Protocol. The use of this protocol did not significantly affect reinfection rate (p = 0.364).

Conclusion

This study concluded that increased operative time led to less reinfections for DAIR procedures treating unilateral primary TJA PJIs. Additionally, this study introduced The Macbeth Protocol, which demonstrated promising potential as an I&D technique despite not showing statistical significance. Arthroplasty surgeons should not sacrifice patient outcomes determined by reinfection rate for decreased operative time. Level of Evidence: III.

Projected Prevalence of Obesity in Aseptic Revision Total Hip and Knee Arthroplasty

Background

The purpose of this study was to develop projections of the prevalence of obesity in aseptic revision THA and TKA patients through the year 2029.

Methods

The National Surgical Quality Improvement Project (NSQIP) was queried for years 2011-2019. Current procedural terminology (CPT) codes 27134, 27137, and 27138 were used to identify revision THA and CPT codes 27486 and 27487 were used to identify revision TKA. Revision THA/TKA for infectious, traumatic, or oncologic indications were excluded. Participant data were grouped according to body mass index (BMI) categories: underweight/normal weight, <25 kg/m2; overweight, 25-29.9 kg/m2; class I obesity, 30.034.9 kg/m2; class II obesity, 35.0-39.9 kg/m2; morbid obesity ≥ 40 kg/m2. Prevalence of each BMI category was estimated from year 2020 to year 2029 through multinomial regression analyses.

Results

38,325 cases were included (16,153 revision THA and 22,172 revision TKA). From 2011 to 2029, prevalence of class I obesity (24% to 25%), class II obesity (11% to 15%), and morbid obesity (7% to 9%) increased amongst aseptic revision THA patients. Similarly, prevalence of class I obesity (28% to 30%), class II obesity (17% to 29%), and morbid obesity (16% to 18%) increased in aseptic revision TKA patients.

Conclusion

Prevalence of class II obesity and morbid obesity demonstrated the largest increases in revision TKA and THA patients. By 2029, we estimate that approximately 49% of aseptic revision THA and 77% of aseptic revision TKA will have obesity and/or morbid obesity. Resources aimed at mitigating complications in this patient population are needed. Level of Evidence: III.

Whole-body phase angle correlates with pre-operative markers in total joint arthroplasty

Background

Bioimpedance derived whole body phase angle (ϕ), a measure of cellular integrity, has been identified as an independent marker of morbidity and mortality in many medical and surgical specialties. While similar measures of water homeostasis like extracellular edema (EE) have been associated with pre-operative risk, ϕ has not been studied in orthopaedics, despite potential to serve as a pre-operative marker. This study aims to identify relationships between ϕ, EE, and body composition metrics, laboratory values, patient reported outcomes, and comorbidities.

Methods

Multi-frequency bioimpedance analysis (BIA) records, laboratory values, and patient reported outcomes of adult patients presenting to an academic arthroplasty clinic were retrospectively reviewed. Correlation coefficients between ϕ, EE, and reviewed information were conducted.

Results

ϕ was significantly correlated (p<0.001) most positively with measures of lean tissue such as skeletal muscle mass (r=0.48), appendicular skeletal muscle index (r=0.39), lean body mass (r=0.43), and dry lean mass (r=0.47), while it held negative correlations (p<0.001) with age (r= -0.55), and body fat mass (r= -0.11). ϕ was not correlated with body mass index (BMI, p = 0.204). In contrast, EE demonstrated its strongest positive correlations (p<0.001) with body fat mass (r=0.32), age (r=0.50), and BMI (r=0.26), and its strongest negative correlations (p<0.001) with serum albumin (r= -0.37) and total protein (r= -0.23).

Conclusions

Based on their associations with markers of health and fitness, BIA determined ϕ and EE demonstrate relationships to markers currently implemented in orthopaedic practice. This likely indicates that ϕ has potential as a comprehensive surrogate for several commonly used markers to quantify pre-operative risk. In the future, ϕ may aid in developing risk-stratifications for intervention and prevention of complications.

Not all Total Hip and Knee Arthroplasties Are the Same: What Are the Implications in Large Database Studies?

BACKGROUND

The use of claims databases for research after total hip and knee arthroplasty (THA, TKA) has increased exponentially. These studies rely on accurate coding, and inadvertent inclusion of patients with nonroutine indications may influence results. The purpose of this study was to evaluate the complexity of THA and TKA captured by CPT code and determine if complication rates vary based on the indication.

METHODS

The NSQIP database was queried using CPT codes 21730 and 27447 to identify patients undergoing THA and TKA from 2018 to 2019. The surgical indication was classified based on the ICD-10 diagnosis code as routine primary, complex primary, inflammatory, fracture, oncologic, revision, infection, or indeterminant. Patient factors and 30-day complications, readmission, reoperation, and wound complications were compared.

RESULTS

A total of 86,009 THA patients had 703 ICD-10 diagnosis codes and 91.4% were routine primary indications. Complication rates were: routine primary 7.4%, complex primary 11.3%, inflammatory 12.5%, fracture 23.9%, oncologic 32.4%, revision 26.9%, infection 38.7%, and indeterminant 10.3% (P < .0001). 137,500 TKA patients had 552 ICD-10 diagnosis codes and 96.1% were routine primary cases. Complication rates were: routine primary 5.9%, complex primary 8.0%, inflammatory 7.2%, fracture 38.9%, oncologic 32.7%, revision 13.3%, infection 37.7%, and indeterminant 9.6% (P < .0001). Routine primary arthroplasty had significantly lower rates of reoperation, readmission, and wound complications.

CONCLUSION

Using CPT code alone captures 10% of THA and 4% of TKA patients with procedures for nonroutine primary indications. It is essential to recognize identification of patients simply by CPT code has the potential to inadvertently introduce bias, and surgeons should critically assess methods used to define the study populations.

Are Body Mass Index Cutoffs Creating Racial, Ethnic, and Gender Disparities in Eligibility for Primary Total Hip and Knee Arthroplasty?

BACKGROUND

Unabated increases in the prevalence of obesity among American adults have disproportionately affected women, Black persons, and Hispanic persons. The purpose of this study was to evaluate for disparity in rates of patient eligibility for primary total hip arthroplasty (THA) and total knee arthroplasty (TKA) based on race and ethnicity and gender by applying commonly used body mass index (BMI) eligibility criteria to two large national databases.

METHODS

We retrospectively reviewed data from the American College of Surgeons National Surgical Quality Improvement Program (ACS-NSQIP) database for the years 2015-2019 for primary THA and TKA and the National Health and Nutrition Examination Survey (NHANES) from 2011-2018. Designations of race and ethnicity were standardized between cohorts. BMI cutoffs of <50 kg/m², <45 kg/m², <40 kg/m², and <35 kg/m² were then applied. Rates of eligibility for surgery were examined for each respective BMI cutoff and stratified by age, race and ethnicity, and gender.

RESULTS

143,973 NSQIP THA patients, 242,518 NSQIP TKA patients, and 13,255 NHANES participants were analyzed. Female patients were more likely to be ineligible for surgery across all cohorts for all modeled BMI cutoffs (P < .001 for all). Black patients had relatively lower rates of eligibility across all cohorts for all modeled BMI cutoffs (P < .0001 for all). Hispanic patients had disproportionately lower rates of eligibility only at a BMI cutoff of <35 kg/m².

CONCLUSION

Using BMI cutoffs alone to determine the eligibility for primary THA and TKA may disproportionally exclude women, Black persons, and Hispanic persons. These data raise concerns regarding further disparity and restriction of arthroplasty care to vulnerable populations that are already marginalized.

LEVEL OF EVIDENCE

Retrospective Cohort Study, Level III.

Does Isolated Unilateral Hip or Knee Osteoarthritis Lead to Adverse Changes in Extremity Composition?

BACKGROUND

While muscle atrophy is a function of normal aging, loss of muscle in the setting of hip and knee osteoarthritis (OA) has been observed using radiographic studies. There is limited data available regarding changes in extremity composition using bioimpedance (BIA). The purpose of this study was to assess the changes in extremity composition in patients with isolated, unilateral hip or knee OA using BIA.

METHODS

Patients presenting to our institution's adult reconstruction clinic from February 2020 to April 2021 were retrospectively reviewed to identify those with isolated, unilateral hip and knee OA. The InBody 770 Body Composition Analyzer (InBody USA, Cerritos, California) was used to perform a complete body composition assessment, per protocol. Lean extremity mass (LEM), fat mass (FM), intracellular water (ICW), extremity body water (EBW = ICW + extracellular water (ECW)) and phase angle (PA) were determined. Differences between the affected (OA) and unaffected (no OA) extremities were compared using t-tests.

RESULTS

38 patients had isolated hip OA. The mean age was 60.8 (±11.7) years, mean BMI was 31.7 (±6.8) kg/m2, and 39.5% were female. LEM, FM, EBW, ICW, and PA were significantly decreased in the hip OA extremity (LEM: 20.0 vs. 20.4 kg, p=0.0008, FM: 8.8 vs. 8.9 kg, p=0.0049, EBW: 15.7 vs 16.0, p=0.0011, ICW: 9.5 vs. 9.7 L, p=0.0004, PA: 4.5 vs 4.9º, p<0.0001). There were 25 patients with isolated knee OA. Mean age was 62.8 (±11.3) years, mean BMI was 33.6 (±6.9) kg/m2, and 52.0% were female. FM and PA were significantly lower in the knee OA extremity (11.3 vs 11.4 kg, p=0.0291, 4.5 vs 4.9º, p<0.0001). There were no significant differences in LEM, EBW, and ICW between the knee OA extremity and the unaffected extremity.

CONCLUSION

Patients with isolated, unilateral hip OA had decreased LEM, FM, EBW, and ICW in the affected extremity. Both unilateral hip and knee OA was associated with decreased PA, suggestive of greater underlying dysfunction in muscle or cellular performance. Further study is needed to better define when these abnormalities develop, how they progress over time, and the impact of targeted interventions in reversing these changes. Level of Evidence: III.

The Impact of Orthopaedic Surgical Training on Body Composition

BACKGROUND

Residency is known to consist of rigorous training that has contributed to increased rates of resident burn out, depression, and suicide. There have been recent efforts to attempt to combat and solve the rising levels of mental health concerns amongst physicians and physicians in training. While studies have examined the adverse effects of medical training on mental health, few have examined the associated changes in physical health. This study aimed to identify and compare baseline body composition and hand grip strength of orthopaedic surgery residents. The second aim was to identify and compare changes that may occur over the course of the training program.

METHODS

First year orthopaedic surgery residents ("interns") were recruited to undergo body composition measurements via bioimpedance analysis (BIA) during their first, third, and twelfth month of post-graduate training. At each interval, three hand-grip-strength measurements per hand were captured. Additionally, orthopaedic surgery residents who had already completed their first year ("non-interns") were recruited to undergo baseline and 12-month BIA for comparison.

RESULTS

Six interns and six non-interns were recruited. The interns lost 2.88 ± 4.26 kg (-3.31% ± 4.75%) of their initial body mass with most of the loss being body fat mass (1.97 ± 2.62 kg) by three months. Interns recovered a fair amount of mass loss by 12 months with a net change of -0.78 ± 3.14 kg (-1.09% ± 3.90%). Non-interns experienced an overall net weight gain (1.20 ± 3.64 kg; 1.68% ± 5.55%) over the same period. Intern HGS changed by -1.92 ± 2.49 kg and 3.39 ± 2.34 kg at 3- (n=6) and 12-months (n=3), respectively.

CONCLUSION

This study demonstrates that there is an appreciable decrease in overall body mass, lean tissue mass, and body fat throughout the orthopaedic resident's intern year. The results demonstrate an initial fall in each body metric and strength by 3-months followed by partial recovery by 12-months. This pattern contrasts the average gain of body mass in each measured metric by non-interns. This study is limited by population sizes and by incompleteness of HGS data. Level of Evidence: II.

Projected Prevalence of Obesity in Primary Total Hip Arthroplasty: How Big Will the Problem Get?

BACKGROUND

Obesity is associated with higher rates of adverse outcomes following primary total hip arthroplasty (THA). The purpose of this study is to utilize 3 national databases to develop projections of obesity within the general population and primary THA patients in the United States through 2029.

METHODS

Data from the National Surgical Quality Improvement Program (NSQIP), the Behavior Risk Factor Surveillance System (BRFSS), and the National Health and Nutrition Examination Survey were queried for years 1999-2019. Current Procedural Terminology code 27130 was used to identify primary THA patients in NSQIP. Individuals were categorized according to body mass index (kg/m²) by year: normal weight (≤24.9); overweight (25.0-29.9); obese (30.0-39.9); and morbidly obese (≥40). Multinomial logistic regression was used to project categorical body mass index data for years 2020-2029.

RESULTS

A total of 8,222,013 individuals were included (7,986,414 BRFSS, 235,599 NSQIP THA). From 2011 to 2019, the prevalence of normal weight and overweight individuals declined in the general population (BRFSS) and in primary THA. Prevalence of obese/morbidly obese individuals increased in the general population from 31% to 36% and in primary THA from 42% to 49%. Projection models estimate that by 2029, 46% of the general population will be obese/morbidly obese and 55% of primary THA will be obese/morbidly obese.

CONCLUSION

By 2029, we estimate ≥55% of primary THA to be obese/morbidly obese. Increased resources dedicated to care pathways and research focused on improving outcomes in obese arthroplasty patients will be necessary as this population continues to grow.

LEVEL OF EVIDENCE

Level III, Retrospective Cohort Study.

Body Composition Changes in the Immediate Peri-operative Period Following Total Joint Arthroplasty

BACKGROUND

Bioimpedance analysis (BIA) is a readily available tool to assess body composition in a clinical setting. BIA has received little attention in orthopaedics and namely joint arthroplasty. This study aims to quantify changes in body composition in the immediate peri-operative period following total joint arthroplasty.

METHODS

Adults scheduled for elective total joint arthroplasty were recruited to participate. Patients underwent BIA scans in the immediate peri-operative period: pre-operative on their day of surgery, post-operative day 0, and post-operative day 1.

RESULTS

67 patients were enrolled to undergo BIA scans. Mean age was 62.64 ± 10.28 years old, and 49.2% were females. The all-supine cohort exhibited a 0.36 ± 0.61 kg increase in dry lean mass (p < 0.001) and 1.30 ± 2.14 kg increase in lean body mass on postoperative day 0 (p < 0.001). Patients received to 1.16 ± 0.58 kg of fluid mass, on average.

CONCLUSION

BIA is a rapid, portable tool that allows for body composition analysis of an inpatient surgical population. This study demonstrated that BIA can detect net fluid changes and may approximate implant mass following total joint arthroplasty. This may aid surgeons in interpreting post-operative body composition changes.

Reliability of Spinopelvic Measurements That May Influence the Cup Position in Total Hip Arthroplasty

BACKGROUND

Spinopelvic pathology has been identified as a potential risk factor for instability after total hip arthroplasty. Spinopelvic radiographic parameters used to diagnose spinopelvic disease may also impact optimal cup placement. The purpose of this study was to assess the arthroplasty surgeon accuracy in making spinopelvic measurements.

METHODS

Five fellowship-trained or in-training arthroplasty surgeons reviewed 44 lateral lumbar radiographs in two sessions. All evaluators were instructed how to perform measurements but had little experience in doing so. Traditional measurements included the pelvic tilt (PT), pelvic incidence (PI), and sacral slope (SS), and novel measurements included the acetabular ante-inclination (AI) and pelvic femoral angle. Surgeon measurements were compared with those made by experienced engineers. intraclass coefficients (ICCs) were calculated and interpreted.

RESULTS

The interobserver reliability for 4 of 5 of the spinopelvic parameters was good to excellent (ICC >0.75) with the highest reliability seen for PI measurement (ICC = 0.939). Only moderate interobserver reliability was observed for AI measurement (ICC = 0.559). Intraobserver reliability was the highest for the PI and SS, ranging from moderate to good (ICC, 0.718 to 0.896). The lowest intraobserver reliability was seen for the AI (ICC range, 0.026 to 0.545) and pelvic femoral angle (ICC range, 0.035 to 0.828). Surgeon measurements of PT and SS were compared with engineer measurements with extremely poor correlation observed (ICC <0 for all).

CONCLUSION

Surgeon intraobserver and interobserver reliability in making novel radiographic measurements was low compared with traditional radiographic measurements. Surgeon reliability in making traditional measurements of the PT and SS was very poor compared with experienced assessors using software-based measurements.

Effect of COVID-19 on Hip and Knee Arthroplasty Surgical Volume in the United States

BACKGROUND

In an effort to help combat the COVID-19 pandemic and preserve essential health care resources, starting in mid-March 2020, surgeons have been instructed to only perform essential surgical procedures. The vast majority of hip and knee arthroplasty surgery does not meet the definition of essential surgery. This study estimated the number of arthroplasty procedures that would be canceled because of these important restrictions.

METHODS

The US hip and knee arthroplasty procedure volume projections for 2020 were estimated from four recently published studies. Data from the American Joint Replacement Registry were utilized to determine what percentage of these cases would be considered nonessential surgery. Monthly and weekly estimates of nonessential hip and knee arthroplasty procedures that would have occurred had there not been any restrictions due to COVID-19 were calculated.

RESULTS

After excluding essential procedures, it was estimated that approximately 30,000 primary and 3000 revision hip and knee arthroplasty procedures will be canceled each week while COVID-19 restrictions regarding nonessential surgery are in place. If only 50% of nonessential cases were actually canceled across the United States, that would still result in the cancellation of 15,001 primary and 1435 revision hip and knee arthroplasty procedures per week while restrictions are in place.

CONCLUSION

This study highlights the profound impact COVID-19 is having on our current hip and knee arthroplasty volume. The large number of cases canceled because of COVID-19 translates into major financial losses for health care institutions and may have a profound impact on our patients.

Cutibacterium colonization of the anterior and lateral thigh

AIMS

Of growing concern in arthroplasty is the emergence of atypical infections, particularly Cutibacterium (formerly Propionibacterium) sp. infections. Currently, the dermal colonization rate of Cutibacterium about the hip is unknown. Therefore, the aim of this study was to investigate colonization rates of Cutibacterium sp. at locations approximating anterior and posterolateral approaches to the hip joint.

METHODS

For this non-randomized non-blinded study, 101 adult patients scheduled for hip or knee surgery were recruited. For each, four 3 mm dermal punch biopsies were collected after administration of anaesthesia, but prior to antibiotics. Prebiopsy skin preparation consisted of a standardized preoperative 2% chlorhexidine skin cleansing protocol and an additional 70% isopropyl alcohol mechanical skin scrub immediately prior to biopsy collection. Two skin samples 10 cm apart were collected from a location approximating a standard direct anterior skin incision, and two samples 10 cm apart were collected from a lateral skin incision (suitable for posterior, direct-lateral, or anterolateral approaches). Samples were cultured for two weeks using a protocol optimized for Cutibacterium.

RESULTS

A total of 23 out of 404 cultures (collected from 101 patients) were positive for a microorganism, with a total of 22 patients having a positive culture (22%). Overall, 15 of the cultures in 14 patients were positive for Cutibacterium sp. (65%), of which Cutibacterium acnes comprised the majority (n = 13; 87%). Other isolated microorganisms include coagulase-negative Staphylococcus (n = 6), Clostridium (n = 1), and Corynebacterium (n = 1). Of all positive cultures, 15 were obtained from the anterior location (65%), of which seven (60%) were from the most proximal biopsy location. However, these findings were not statistically significant (anterior vs lateral, p = 0.076; proximal vs distal, p =0.238).

CONCLUSION

Approximately 14% (14/101) of the patients demonstrated a positive Cutibacterium colonization about the hip, the majority anteriorly. Given the high colonization rate of Cutibacterium, alternative skin preparations for total hip arthroplasty should be considered. Cite this article: Bone Joint J 2020;102-B(7 Supple B):52-56.

Lower Extremity Geometry in Morbid Obesity-Considerations for Total Knee Arthroplasty

BACKGROUND

Obesity has been associated with varying adverse outcomes related to total knee arthroplasty (TKA) and has long been associated with various mechanical complications. Computational modeling holds promise for investigating biomechanical aberrations related to TKA in obese patients. However, basic anthropometric information regarding the shape of the lower extremity in obese patients is lacking. We, therefore, developed a method to determine lower extremity shape both generally and subject specific as a function of body mass index (BMI) and sex.

METHODS

A unique set of 4 radiographs (knee anteroposterior/lateral; EOS long-leg alignment anteroposterior/lateral) were reviewed for patients with BMI exceeding 30 kg/m² before undergoing TKA. Soft tissue thickness in both the coronal and sagittal planes, including anterior adipose tissue thickness overlying the tibial tubercle and patella, was measured. Elliptical parameters were then determined for any distance along the lower extremity. Additional measurements were obtained, including prepatellar thickness and pretubercular thickness and anteroposterior and mediolateral dimensions of the proximal tibia. A total of 232 obese subjects were analyzed.

RESULTS

Girth increased as a function of BMI. Anterior prepatellar and pretubercular subcutaneous fat thickness in females exceeded that in males for all values of BMI. Wide variation was seen with regard to overall patterns of adiposity among same-sex subjects with similar BMI. Proximal tibial osseous dimensions did not demonstrate an association with BMI; however, males were found to have deeper and wider tibias compared with females.

CONCLUSIONS

BMI and sex influence lower extremity shape. The prediction algorithms developed here holds implications for future biomechanical studies of TKA in obese patients.

The Scarcity of Orthopaedic Physician Scientists

Breakthrough advances in medicine almost uniformly result from the translation of new basic scientific knowledge into clinical practice, rather than from assessment, modification or refinement of current methods of diagnosis and treatment. However, as is intuitively understood, those most responsible for scientific conception and creation-scientists - are generally not the ones applying these advances at the patient's bedside or the operating room, and vice versa. Recognition of the scarcity of clinicians with a background that prepares them to develop new basic knowledge, and to critically evaluate the underlying scientific basis of methods of diagnosis and treatment, has led to initiatives including federally funded Physician-Scientist programs, whereby young, motivated scholars begin a rigorous training, which encompasses education and mentorship within both medical and scientific fields, culminating in the conferment of both MD and PhD degrees. Graduates have demonstrated success in integrating science into their academic medical careers. However, for unknown reasons, orthopaedic surgery, more than other specialties, has struggled to recruit and retain physician-scientists, who possess a skill set evermore rare in today's increasingly complicated medical and scientific landscape. While the reasons for this shortfall have yet to be completely elucidated, one thing is clear: If orthopaedics is to make significant advances in the diagnosis and treatment of musculoskeletal diseases and injuries, recruitment of the very best and brightest physician-scientists to orthopaedics must become a priority. This commentary explores potential explanations for current low-recruitment success regarding future orthopaedic surgeon-scientists, and discusses avenues for resolution.

The 2014 Frank Stinchfield Award: The 'landing zone' for wear and stability in total hip arthroplasty is smaller than we thought: a computational analysis

BACKGROUND

Positioning of total hip bearings involves tradeoffs, because cup orientations most favorable in terms of stability are not necessarily ideal in terms of reduction of contact stress and wear potential. Previous studies and models have not addressed these potentially competing considerations for optimal total hip arthroplasty (THA) function.

QUESTIONS/PURPOSES

We therefore asked if component positioning in total hips could be addressed in terms of balancing bearing surface wear and stability. Specifically, we sought to identify acetabular component inclination and anteversion orientation, which simultaneously resulted in minimal wear while maximizing construct stability, for several permutations of femoral head diameter and femoral stem anteversion.

METHODS

A validated metal-on-metal THA finite element (FE) model was used in this investigation. Five dislocation-prone motions as well as gait were considered as were permutations of femoral anteversion (0°-30°), femoral head diameter (32-48 mm), cup inclination (25°-75°), and cup anteversion (0°-50°), resulting in 4320 distinct FE simulations. A novel metric was developed to identify a range of favorable cup orientations (so-called "landing zone") by considering both surface wear and component stability.

RESULTS

When considering both wear and stability with equal weight, ideal cup position was more restrictive than the historically defined safe zone and was substantially more sensitive to cup anteversion than to inclination. Ideal acetabular positioning varied with both femoral head diameter and femoral version. In general, ideal cup inclination decreased with increased head diameter (approximately 0.5° per millimeter increase in head diameter). Additionally, ideal inclination increased with increased values of femoral anteversion (approximately 0.3° per degree increase in stem anteversion). Conversely, ideal cup anteversion increased with increased femoral head diameter (0.3° per millimeter increase) and decreased with increased femoral stem anteversion (approximately 0.3° per degree increase). Regressions demonstrated strong correlations between optimal cup inclination versus head diameter (Pearson's r=-0.88), between optimal cup inclination versus femoral anteversion (r=0.96), between optimal cup anteversion versus head diameter (r=0.99), and between optimal cup anteversion and femoral anteversion (r=-0.98). For a 36-mm cup with a 20° anteverted stem, the ideal cup orientation was 46°±12° inclination and 15°±4° anteversion.

CONCLUSIONS

The range of cup orientations that maximized stability and minimized wear (so-called "landing zone") was substantially smaller than historical guidelines and specifically did not increase with increased head size, challenging the presumption that larger heads are more forgiving. In particular, when the cup is oriented to improve not only stability, but also wear in the model, there was little or no added stability achieved by the use of larger femoral heads. Additionally, ideal cup positioning was more sensitive to cup anteversion than to inclination.

CLINICAL RELEVANCE

Positioning THA bearings involves tradeoffs regarding stability and long-term bearing wear. Cup positions most favorable to minimization of wear such as low inclination and elevated anteversion were detrimental in terms of construct stability. Orientations were identified that best balanced the competing considerations of wear and stability.

Computational techniques for the assessment of fracture repair

The combination of high-resolution three-dimensional medical imaging, increased computing power, and modern computational methods provide unprecedented capabilities for assessing the repair and healing of fractured bone. Fracture healing is a natural process that restores the mechanical integrity of bone and is greatly influenced by the prevailing mechanical environment. Mechanobiological theories have been proposed to provide greater insight into the relationships between mechanics (stress and strain) and biology. Computational approaches for modelling these relationships have evolved from simple tools to analyze fracture healing at a single point in time to current models that capture complex biological events such as angiogenesis, stochasticity in cellular activities, and cell-phenotype specific activities. The predictive capacity of these models has been established using corroborating physical experiments. For clinical application, mechanobiological models accounting for patient-to-patient variability hold the potential to predict fracture healing and thereby help clinicians to customize treatment. Advanced imaging tools permit patient-specific geometries to be used in such models. Refining the models to study the strain fields within a fracture gap and adapting the models for case-specific simulation may provide more accurate examination of the relationship between strain and fracture healing in actual patients. Medical imaging systems have significantly advanced the capability for less invasive visualization of injured musculoskeletal tissues, but all too often the consideration of these rich datasets has stopped at the level of subjective observation. Computational image analysis methods have not yet been applied to study fracture healing, but two comparable challenges which have been addressed in this general area are the evaluation of fracture severity and of fracture-associated soft tissue injury. CT-based methodologies developed to assess and quantify these factors are described and results presented to show the potential of these analysis methods.

Stability and trunnion wear potential in large-diameter metal-on-metal total hips: a finite element analysis

BACKGROUND

Large-diameter femoral heads for metal-on-metal THA hold theoretical advantages of joint stability and low bearing surface wear. However, recent reports have indicated an unacceptably high rate of wear-associated failure with large-diameter bearings, possibly due in part to increased wear at the trunnion interface. Thus, the deleterious consequences of using large heads may outweigh their theoretical advantages.

QUESTIONS/PURPOSES

We investigated (1) to what extent femoral head size influenced stability in THA for several dislocation-prone motions; and the biomechanics of wear at the trunnion interface by considering the relationship between (2) wear potential and head size and (3) wear potential and other factors, including cup orientation, type of hip motion, and assembly/impaction load.

METHODS

Computational simulations were executed using a previously validated nonlinear contact finite element model. Stability was determined at 36 cup orientations for five distinct dislocation challenges. Wear at the trunnion interface was calculated for three separate cup orientations subjected to gait, stooping, and sit-to-stand motions. Seven head diameters were investigated: 32 to 56 mm, in 4-mm increments.

RESULTS

Stability improved with increased diameter, although diminishing benefit was seen for sizes of greater than 40 mm. By contrast, contact stress and computed wear at the trunnion interface all increased unabatedly with increasing head size. Increased impaction forces resulted in only small decreases in trunnion wear generation.

CONCLUSIONS

These data suggest that the theoretical advantages of large-diameter femoral heads have a limit. Diameters of greater than 40 mm demonstrated only modest improvement in terms of joint stability yet incurred substantial increase in wear potential at the trunnion.

CLINICAL RELEVANCE

Our model has potential to help investigators and designers of hip implants to better understand the optimization of trunnion design for long-term durability.

Impingement and dislocation in total hip arthroplasty: mechanisms and consequences

In contemporary total hip arthroplasty, instability has been a complication in approximately 2% to 5% of primary surgeries and 5% to 10% of revisions. Due to the reduction in the incidence of wear-induced osteolysis that has been achieved over the last decade, instability now stands as the single most common reason for revision surgery. Moreover, even without frank dislocation, impingement and subluxation are implicated in a set of new concerns arising with advanced bearings, associated with the relatively unforgiving nature of many of those designs. Against that backdrop, the biomechanical factors responsible for impingement, subluxation, and dislocation remain under-investigated relative to their burden of morbidity. This manuscript outlines a 15-year program of laboratory and clinical research undertaken to improve the scientific basis for understanding total hip impingement and dislocation. The broad theme has been to systematically evaluate the role of surgical factors, implant design factors, and patient factors in predisposing total hip constructs to impinge, sublux, and/or dislocate. Because this class of adverse biomechanical events had not lent itself well to study with existing approaches, it was necessary to develop (and validate) a series of new research methodologies, relying heavily on advanced finite element formulations. Specific areas of focus have included identifying the biomechanical challenges posed by dislocation-prone patient activities, quantifying design parameter effects and component surgical positioning effects for conventional metal-on-polyethylene implant constructs, and the impingement/dislocation behavior of non-conventional constructs, quantifying the stabilizing role of the hip capsule (and of surgical repairs of capsule defects), and systematically studying impingement and edge loading of hard-on-hard bearings, fracture of ceramic liners, confounding effects of patient obesity, and subluxation-mediated worsening of third body particle challenge.

Effect of ACL reconstruction graft size on simulated Lachman testing: a finite element analysis

INTRODUCTION

ACL reconstructions are frequently performed following ACL injury. The most common treatment is single bundle reconstruction. While ACL reconstructions have been studied clinically and experimentally, quantitative information regarding the local biomechanics the knee following ACL reconstruction is generally lacking. Specifically, the role of graft size on joint stability and soft tissue injury propensity is currently unknown.

METHODS

Therefore, a non-linear contact finite element model was developed to systematically evaluate the relationship between ACL graft size and knee joint biomechanics following ACL reconstruction. A simulated Lachman maneuver was utilized to assess knee joint laxity, meniscal stress, in situ graft loading, and peak articular cartilage contact pressure for ACL graft sizes between 5 and 9 mm, as well as an ACL-deficient knee. The model was validated by corroboration with previously published experimental (cadaveric) data on ACL reconstruction.

RESULTS

The 5 mm graft resulted in 30% greater relative AP translation compared to the 9 mm graft; the ACL deficient knee resulted in 2.56-times greater AP translation than the average graft reconstruction. Contact pressure and peak meniscal stresses decreased monotonically for increased values of ACL graft diameter. For all graft diameters, soft tissue stress and articular contact pressure was reduced versus the ACL-deficient knee.

CONCLUSIONS

ACL reconstruction dramatically affects the local biomechanics of the knee. Stresses occurring in the soft tissues, as well as contact pressure at the articular surfaces, were found to be highly sensitive to ACL graft size. Larger grafts were associated with lower meniscal stress, decreased joint laxity, and less articular cartilage contact stress. Therefore, the current data suggests that increased graft size confers a biomechanical advantage in the ACL reconstructed knee.

Fracture propagation propensity of ceramic liners during impingement-subluxation: a finite element exploration

Although improvements in materials engineering have greatly reduced fracture rates in ceramic femoral heads, concerns still exist for liners. Ceramics are vulnerable to fracture due to impact and from stress concentrations (point and line loading) such as those associated with impingement-subluxation. Thus, ceramic cup fracture propensity is presumably very sensitive to surgical cup positioning. A novel fracture mechanics finite element formulation was developed to identify cup orientations most susceptible to liner fracture propagation for several impingement-prone patient maneuvers. Other factors being equal, increased cup inclination and increased anteversion were found to elevate fracture risk. Squatting, stooping, and leaning shoe-tie maneuvers were associated with the highest fracture risk. These results suggest that fracture risk can be reduced by surgeons' decreasing cup abduction and by patients' avoiding of specific activities.

Edge-loading severity as a function of cup lip radius in metal-on-metal total hips--a finite element analysis

While favorable tribological properties and allowance for larger femoral head sizes have made metal-on-metal (MoM) bearings an increasingly popular choice for total hip arthroplasty, concerns have mounted regarding adverse reactions to metal wear debris and ions. MoM cups differ from conventional polyethylene cups in terms of edge profile design and reductions from full hemisphericity, suggesting differences in loading at or near the cup edge, especially during subluxation events. Finite element analysis was used to investigate the effects of cup orientation and lip edge curvature on damage propensity for edge or near-edge loading during subluxation. Increased cup lip radius (resulting in reduced articular arc) had a detrimental effect upon subluxation-free hip range of motion and upon dislocation resistance. Contact stresses near the cup edge demonstrated complex relationships between edge radius and cup orientation, with peak stresses being influenced by both variables. The tendency for scraping wear at the egress site demonstrated similarly complex dependencies. These data indicate that acetabular cup design is an important determinant of edge and near-edge loading damage propensity.

Bone-on-bone versus hardware impingement in total hips: a biomechanical study

Dislocation remains a serious concern for total hip arthroplasty (THA). Impingement, typically between the implant femoral neck and the acetabular cup, remains the most common dislocation impetus. Wear reductions from recent bearing technology advancements have encouraged introduction of substantially increased femoral head diameters. However, there is some evidence that range of motion with larger head sizes is limited by bone-on-bone, rather than hardware, impingement. While all impingement events are of course undesirable, currently little is known biomechanically if these two impingement modes differ in terms of generation of potentially deleterious stress concentrations or with regard to dislocation resistance. Finite element (FE) analysis was therefore used to parametrically investigate the role of head diameter on the local biomechanics of bone-on-bone versus component-on-component impingement events. Of several dislocation-prone patient motion challenges considered, only squatting consistently resulted in bone-on-bone (as opposed to hardware) impingement. Implant stress concentrations arising from hardware impingement during squatting were greater than those from bony impingement, for all head sizes considered. Additionally, dislocation resistance was substantially greater for instances of bony impingement versus hardware-only impingement. These findings suggest that hardware impingement may still be a/the the predominant mode of impingement even with the use of larger femoral heads, for sub-optimally positioned cups. Additionally, the data indicate that, should impingement occur, impingements between the implant neck and cup are (1) more likely to dislocate, and (2) have a greater propensity for causing damage to the implant compared to impingement events involving bony members.

The capsule's contribution to total hip construct stability--a finite element analysis

Instability is a significant concern in total hip arthroplasty (THA), particularly when there is structural compromise of the capsule due to pre-existing pathology or due to necessities of surgical approach. An experimentally grounded fiber-direction-based finite element model of the hip capsule was developed, and was integrated with an established three-dimensional model of impingement/dislocation. Model validity was established by close similarity to results from a cadaveric experiment in a servohydraulic hip simulator. Parametric computational runs explored effects of graded levels of capsule thickness, of regional detachment from the capsule's femoral or acetabular insertions, of surgical incisions of capsule substance, and of capsule defect repairs. Depending strongly upon the specific site, localized capsule defects caused varying degrees of construct stability compromise, with several specific situations involving over 60% decrement in dislocation resistance. Construct stability was returned substantially toward intact-capsule levels following well-conceived repairs, although the suture sites involved were often at substantial risk of failure. These parametric model results underscore the importance of retaining or robustly repairing capsular structures in THA, in order to maximize overall construct stability. 

A new osteophyte segmentation algorithm using partial shape model and its applications to rabbit femur anterior cruciate ligament transection via micro-CT imaging

Osteophyte is an additional bony growth on a normal bone surface limiting or stopping motion at a deteriorating joint. Detection and quantification of osteophytes from CT images is helpful in assessing disease status as well as treatment and surgery planning. However, it is difficult to distinguish between osteophytes and healthy bones using simple thresholding or edge/texture features due to the similarity of their material composition. In this paper, we present a new method primarily based active shape model (ASM) to solve this problem and evaluate its application to anterior cruciate ligament transection (ACLT) rabbit femur model via CT imaging. The common idea behind most ASM based segmentation methods is to first build a parametric shape model from a training dataset and apply the model to find a shape instance in a target image. A common challenge with such approaches is that a diseased bone shape is significantly altered at regions with osteophyte deposition misguiding an ASM method and eventually leading to suboptimum segmentations. This difficulty is overcome using a new partial ASM method that uses bone shape over healthy regions and extrapolates it over the diseased region according to the underlying shape model. Finally, osteophytes are segmented by subtracting partial-ASM derived shape from the overall diseased shape. Also, a new semi-automatic method is presented in this paper for efficiently building a 3D shape model for an anatomic region using manual reference of a few anatomically defined fiducial landmarks that are highly reproducible on individuals. Accuracy of the method has been examined on simulated phantoms while reproducibility and sensitivity have been evaluated on CT images of 2-, 4- and 8-week post-ACLT and sham-treated rabbit femurs. Experimental results have shown that the method is highly accurate ( R2 = 0.99), reproducible (ICC = 0.97), and sensitive in detecting disease progression (p-values: 0.065,0.001 and < 0.001 for 2- vs. 4, 4- vs. 8- and 2- vs. 8-weeks, respectively).

The role of iron and 2-oxoglutarate oxygenases in signalling

Sensing of ambient dioxygen levels and appropriate feedback mechanisms are essential processes for all multicellular organisms. In animals, moderate hypoxia causes an increase in the transcription levels of specific genes, including those encoding vascular endothelial growth factor and erythropoietin. The hypoxic response is mediated by hypoxia-inducible factor (HIF), an alphabeta heterodimeric transcription factor in which both the HIF subunits are members of the basic helix-loop-helix PAS (PER-ARNT-SIM) domain family. Under hypoxic conditions, levels of HIFalpha rise, allowing dimerization with HIFbeta and initiating transcriptional activation. Two types of dioxygen-dependent modification to HIFalpha have been identified, both of which inhibit the transcriptional response. Firstly, HIFalpha undergoes trans -4-hydroxylation at two conserved proline residues that enable its recognition by the von Hippel-Lindau tumour-suppressor protein. Subsequent ubiquitinylation, mediated by an ubiquitin ligase complex, targets HIFalpha for degradation. Secondly, hydroxylation of an asparagine residue in the C-terminal transactivation domain of HIFalpha directly prevents its interaction with the co-activator p300. Hydroxylation of HIFalpha is catalysed by enzymes of the iron(II)- and 2-oxoglutarate-dependent dioxygenase family. In humans, three prolyl hydroxylase isoenzymes (PHD1-3) and an asparagine hydroxylase [factor inhibiting HIF (FIH)] have been identified. The role of 2-oxoglutarate oxygenases in the hypoxic and other signalling pathways is discussed.

Alternative oxidation by isopenicillin N synthase observed by X-ray diffraction

BACKGROUND

Isopenicillin N synthase (IPNS) catalyses formation of bicyclic isopenicillin N, precursor to all penicillin and cephalosporin antibiotics, from the linear tripeptide delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-valine. IPNS is a non-haem iron(II)-dependent enzyme which utilises the full oxidising potential of molecular oxygen in catalysing the bicyclisation reaction. The reaction mechanism is believed to involve initial formation of the beta-lactam ring (via a thioaldehyde intermediate) to give an iron(IV)-oxo species, which then mediates closure of the 5-membered thiazolidine ring.

RESULTS

Here we report experiments employing time-resolved crystallography to observe turnover of an isosteric substrate analogue designed to intercept the catalytic pathway at an early stage. Reaction in the crystalline enzyme-substrate complex was initiated by the application of high-pressure oxygen, and subsequent flash freezing allowed an oxygenated product to be trapped, bound at the iron centre. A mechanism for formation of the observed thiocarboxylate product is proposed.

CONCLUSIONS

In the absence of its natural reaction partner (the N-H proton of the L-cysteinyl-D-valine amide bond), the proposed hydroperoxide intermediate appears to attack the putative thioaldehyde species directly. These results shed light on the events preceding beta-lactam closure in the IPNS reaction cycle, and enhance our understanding of the mechanism for reaction of the enzyme with its natural substrate.