Cyclic and Linear Tetrablock Copolymers Synthesized at Speed and Scale by Lewis Pair Polymerization of a One-Pot (Meth)acrylic Mixture and Characterized at Multiple Levels.

Cyclic block copolymers (cBCP) are fundamentally intriguing materials, but their synthetic challenges that demand precision in controlling both the monomer sequence and polymer topology limit access to AB and ABC block architectures. Here, we show that cyclic ABAB tetra-BCPs (cABAB) and their linear counterpart (lABAB) can be readily obtained at a speed and scale from one-pot (meth)acrylic monomer mixtures, through coupling the Lewis pair polymerization's unique compounded-sequence control with its precision in topology control. This approach achieves fast (<15 min) and quantitative (>99%) conversion to tetra-BCPs of predesignated linear or cyclic topology at scale (40 g) in a one-pot procedure, precluding the needs for repeated chain extensions, stoichiometric addition steps, dilute conditions, and postsynthetic modifications, and/or postsynthetic ring-closure steps. The resulting lABAB and cABAB have essentially identical molecular weights (Mn = 165-168 kg mol-1) and block degrees/symmetry, allowing for direct behavioral comparisons in solution (hydrodynamic volume, intrinsic viscosity, elution time, and refractive indices), bulk (thermal transitions), and film (thermomechanical and rheometric properties and X-ray scattering patterns) states. To further the morphological characterizations, allylic side-chain functionality is exploited via the thiol-ene click chemistry to install crystalline octadecane side chains and promote phase separation between the A and B blocks, allowing visualization of microdomain formation.

Low-Dose Irradiation and Rotating-Hinge Revision for the Treatment of Severe Idiopathic Arthrofibrosis Following Total Knee Arthroplasty: A Review of 60 Patients With a Mean 6-Year Follow-Up.

BACKGROUND: Severe idiopathic arthrofibrosis after total knee arthroplasty (TKA) is a challenging problem to treat. Low-dose irradiation may decrease fibro-osseous proliferation, while rotating-hinge (RH) revision allows for distal femur shortening and collateral ligament sacrifice. This study reports the clinical outcomes and implant survivorship in patients treated with low-dose irradiation and RH revision for severe idiopathic arthrofibrosis following TKA. METHODS: A retrospective review was performed on 60 consecutive patients. Patients who had greater than 80° arc of knee motion or less than 15° flexion contracture were excluded. Mean follow-up was 6 years (range, 2 to 14). Kaplan-Meier survivorship analyses were performed, and logistic regressions were used to determine associations between preoperative patient characteristics and clinical outcomes. RESULTS: Median flexion contracture and median terminal flexion at presentation were 20 and 70°, respectively; at final follow-up, 59 of 60 patients (98%) had ≤10° flexion contracture and 49 of 60 patients (82%) had ≥90° of flexion. The 10-year survivorship free from reoperation for any reason, revision for any reason, and revision for aseptic loosening were 63, 87, and 97%, respectively. There were 27% percent of patients who underwent a manipulation under anesthesia postoperatively, which was the most common reason for return to the operating room. A greater number of prior surgeries was significantly associated with worse range of motion at the final follow-up (P = .004). There were no known radiation-associated complications. CONCLUSIONS: Patients with severe idiopathic arthrofibrosis following TKA treated with low-dose irradiation and RH revision maintained a gain in knee range of motion of 60° with reliable flexion contracture correction at a mean 6-year follow-up. A manipulation under anesthesia was common in the postoperative period. Survivorship free from revision for aseptic loosening was excellent at 10 years.

Compounded Interplay of Kinetic and Thermodynamic Control over Comonomer Sequences by Lewis Pair Polymerization.

The design of facile synthetic routes to well-defined block copolymers (BCPs) from direct polymerization of one-pot comonomer mixtures, rather than traditional sequential additions, is both fundamentally and technologically important. Such synthetic methodologies often leverage relative monomer reactivity toward propagating species exclusively and therefore are rather limited in monomer scope and control over copolymer structure. The recently developed compounded sequence control (CSC) by Lewis pair polymerization (LPP) utilizes synergistically both thermodynamic (Keq) and kinetic (kp) differentiation to precisely control BCP sequences and suppress tapering and misincorporation errors. Here, we present an in-depth study of CSC by LPP, focusing on the complex interplay of the fundamental Keq and kp parameters, which enable the unique ability of CSC-LPP to precisely control comonomer sequences across a variety of polar vinyl monomer classes. Individual Lewis acid equilibrium and polymerization rate parameters of a range of commercially relevant monomers were experimentally quantified, computationally validated, and rationalized. These values allowed for the judicious design of copolymerizations which probed multiple hypotheses regarding the constructive vs conflicting nature of the relationship between Keq and kp biases, which arise during CSC-LPP of comonomer mixtures. These relationships were thoroughly explored and directly correlated with resultant copolymer microstructures. Several examples of higher-order BCPs are presented, further demonstrating the potential for materials innovation offered by this methodology.

Mechanism of Spatial and Temporal Control in Precision Cyclic Vinyl Polymer Synthesis by Lewis Pair Polymerization.

In typical cyclic polymer synthesis via ring-closure, chain growth and cyclization events are competing with each other, thus affording cyclic polymers with uncontrolled molecular weight or ring size and high dispersity. Here we uncover a mechanism by which Lewis pair polymerization (LPP) operates on polar vinyl monomers that allows the control of where and when cyclization takes place, thereby achieving spatial and temporal control to afford precision cyclic vinyl polymers or block copolymers with predictable molecular weight and low dispersity (≈1.03). A combined experimental and theoretical study demonstrates that cyclization occurs only after all monomers have been consumed (when) via conjugate addition of the propagating chain end to the specific site of the initiating chain end (where), allowing the cyclic polymer formation steps to be regulated and executed with precision in space and time.

Closing the "One Monomer-Two Polymers-One Monomer" Loop via Orthogonal (De)polymerization of a Lactone/Olefin Hybrid.

Two well-known low-ceiling-temperature (LCT) monomers, γ-butyrolactone (γ-BL) toward ring-opening polymerization (ROP) to polyester and cyclohexene toward ring-opening metathesis polymerization (ROMP) to poly(cyclic olefin), are notoriously "nonpolymerizable". Here we present a strategy to render not only polymerizability of both the γ-BL and cyclohexene sites, orthogonally, but also complete and orthogonal depolymerization, through creating an LCT/LCT hybrid, bicyclic lactone/olefin (BiL=). This hybrid monomer undergoes orthogonal polymerization between ROP and ROMP, depending on the catalyst employed, affording two totally different classes of polymeric materials from this single monomer: polyester P(BiL=)ROP via ROP and functionalized poly(cyclic olefin) P(BiL=)ROMP via ROMP. Intriguingly, both P(BiL=)ROP and P(BiL=)ROMP are thermally robust but chemically recyclable under mild conditions (25-40 °C), in the presence of a catalyst, to recover cleanly the same monomer via chain unzipping and scission, respectively. In the ROP, topological and stereochemical controls have been achieved and the structures characterized. Furthermore, the intact functional group during the orthogonal polymerization (i.e., the double bond in ROP and the lactone in ROMP) is utilized for postfunctionalization for tuning materials' thermal and mechanical performances. The impressive depolymerization orthogonality further endows selective depolymerization of both the ROP/ROMP copolymer and the physical blend composites into the same starting monomer.

Synchronous Control of Chain Length/Sequence/Topology for Precision Synthesis of Cyclic Block Copolymers from Monomer Mixtures.

Precision synthesis of cyclic polymers with predictable molecular weight and low dispersity is a challenging task, particularly concerning cyclic polar vinyl polymers through a rapid chain-growth mechanism and without high dilution. Harder yet is the precision synthesis of cyclic block copolymers (cBCPs), ideally from comonomer mixtures. Here we report that Lewis pair polymerization (LPP) capable of thermodynamically and kinetically compounded sequence control successfully addressed this longstanding challenge. Thus, LPP of acrylate/methacrylate mixtures under ambient temperature and normal concentration conditions rapidly and selectively affords well-defined cBCPs with high molecular weight (Mn = 247 kg/mol) and low dispersity (D = 1.04) in one step. Such cBCPs have been characterized by multiple techniques, including direct structural observation by imaging.

High-performance pan-tactic polythioesters with intrinsic crystallinity and chemical recyclability.

Three types of seemingly unyielding trade-offs have continued to challenge the rational design for circular polymers with both high chemical recyclability and high-performance properties: depolymerizability/performance, crystallinity/ductility, and stereo-disorder/crystallinity. Here, we introduce a monomer design strategy based on a bridged bicyclic thiolactone that produces stereo-disordered to perfectly stereo-ordered polythiolactones, all exhibiting high crystallinity and full chemical recyclability. These polythioesters defy aforementioned trade-offs by having an unusual set of desired properties, including intrinsic tacticity-independent crystallinity and chemical recyclability, tunable tacticities from stereo-disorder to perfect stereoregularity, as well as combined high-performance properties such as high thermal stability and crystallinity, and high mechanical strength, ductility, and toughness.

Compounded Sequence Control in Polymerization of One-Pot Mixtures of Highly Reactive Acrylates by Differentiating Lewis Pairs.

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Diverse Catalytic Systems and Mechanistic Pathways for Hydrosilylative Reduction of CO2.

Catalytic hydrosilylation of carbon dioxide has emerged as a promising approach for carbon dioxide utilization. It allows the reductive transformation of carbon dioxide into value-added products at the levels of formate, formaldehyde, methanol, and methane. Tremendous progress has been made in the area of carbon dioxide hydrosilylation since the first reports in 1981. This focus review describes recent advances in the design and catalytic performance of leading catalyst systems, including transition-metal, main-group, and transition-metal/main-group and main-group/main-group tandem catalysts. Emphasis is placed on discussions of key mechanistic features of these systems and efforts towards the development of more selective, efficient, and sustainable carbon dioxide hydrosilylation processes.

Shoulder and Elbow Range of Motion Can Be Maintained in Major League Baseball Pitchers Over the Course of the Season, Regardless of Pitching Workload.

BACKGROUND: Studies examining the evolution of shoulder and elbow range of motion (ROM) in baseball pitchers over a single season have yielded inconsistent results. PURPOSE: To evaluate shoulder and elbow ROM in Major League Baseball (MLB) pitchers over a single season and to assess for correlations between these changes and measures of a pitcher's workload. STUDY DESIGN: Case-control study; Level of evidence, 3. METHODS: Bilateral shoulder ROM (internal rotation [IR], external rotation [ER], and total range of motion [TROM]) was assessed in 92 MLB pitchers pre- and postseason. Shoulder forward flexion (FF) was measured in 64 pitchers, horizontal adduction (HA) was measured in 87 pitchers, and elbow ROM was measured in 86 pitchers. Data collected included demographics (age, height, weight, and body mass index) and measures of workload for pitchers (pitches thrown, innings pitched, and mean fastball velocity). Pitchers were not specifically excluded if they had an injury during the season but had recovered and were actively pitching at the time of postseason measurements. The change in motion from pre- to postseason was measured and the associations with player demographics and workload were calculated by use of the Pearson correlation coefficient. RESULTS: On average, pitchers demonstrated an increase in dominant shoulder ER from 118.8° to 125.4° (P < .001) and TROM from 173.1° to 181.7° (P < .001). HA was increased by an average of 15.7° (P < .001). The average shoulder IR (P = .189), FF (P = .432), and elbow ROM (flexion, P = .549; extension, P = .185) remained similar over the span of the season. Average glenohumeral IR deficit did not increase. Pitcher demographics and measures of throwing workload (total pitches thrown, innings pitched, mean fastball velocity) were not significantly correlated with changes in shoulder or elbow ROM. CONCLUSION: Significant increases in dominant shoulder ER, TROM, and HA were identified in MLB pitchers over the course of a single baseball season. These ROM changes were not correlated with measures of pitching workload.

Trends in the Surgical Management of Acromioclavicular Joint Arthritis Among Board-Eligible US Orthopaedic Surgeons.

PURPOSE: (1) Define the epidemiologic trend of distal clavicle excision (DCE) for acromioclavicular (AC) joint arthritis among board-eligible orthopaedic surgeons in the United States, (2) describe the rates and types of reported complications of open and arthroscopic DCE, and (3) evaluate the effect of fellowship training on preferred technique and reported complication rates. METHODS: The American Board of Orthopaedic Surgery (ABOS) database was used to identify DCE cases submitted by ABOS Part II Board Certification examination candidates. Inclusion criteria were predetermined using a combination of ICD-9 and CPT codes. Cases were dichotomized into 2 groups: open or arthroscopic DCE. The 2 groups were then analyzed to determine trends in annual incidence, complication rates, and surgeon fellowship training. RESULTS: From April 2004 to September 2013, there were 3,229 open and 12,782 arthroscopic DCE procedures performed and submitted by ABOS Part II Board Eligible candidates. Overall, the annual incidence of open DCE decreased (78-37 cases per 10,000 submitted cases, P = .023). Although the annual number of arthroscopic DCE remained steady (1160-1125, P = .622), the percentage of DCE cases performed arthroscopically increased (65%-79%, P = .033). Surgeons without fellowship training were most likely to perform a DCE via an open approach (31%) whereas surgeons with sports medicine training were more likely to perform DCE arthroscopically compared with other fellowship groups (88%, P < .001). Open DCE was associated with a higher surgical complication rate overall when compared with arthroscopic DCE (9.4% vs 7.6%, respectively; P < .001). When compared with other fellowship-trained surgeons, sports medicine surgeons maintained a lower reported surgical complication rate whether performing open or arthroscopic DCE (5.5%, P = .027). CONCLUSIONS: In recent years, open management of AC joint arthritis has declined among newly trained, board-eligible orthopaedic surgeons, possibly because of an increased complication rate associated with open treatment. Fellowship training was significantly associated with the type of treatment (open vs arthroscopic) rendered and complication rates. LEVEL OF EVIDENCE: Level IV, case series.

Trends in Surgical Practices for Lateral Epicondylitis Among Newly Trained Orthopaedic Surgeons.

BACKGROUND: Much controversy exists regarding the optimal surgical intervention for lateral epicondylitis because of a multitude of options available and the lack of comparative studies. Knowledge of the current practice trends would help guide the design of comparative studies needed to determine which surgical technique results in the best outcome. PURPOSE: To review the latest practice trends for the surgical treatment of lateral epicondylitis among newly trained surgeons in the United States utilizing the American Board of Orthopaedic Surgery (ABOS) database. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: The ABOS database was utilized to identify surgical cases for lateral epicondylitis submitted by Part II board certification examination candidates from 2004 through 2013. Inclusion criteria were predetermined using a combination of International Classification of Diseases, Ninth Revision (ICD-9) and Current Procedural Terminology (CPT) codes. Cases were organized by open and arthroscopic treatment groups and by fellowship training and were analyzed to determine differences in surgical techniques, complication rates, and concomitant procedures. RESULTS: In total, 1150 surgeons submitted 2106 surgical cases for the treatment of lateral epicondylitis. The number of surgical cases for lateral epicondylitis performed per 10,000 submitted cases significantly decreased from 26.7 in 2004 to 21.1 in 2013 (P = .002). Among all cases, 92.2% were open and 7.8% were arthroscopic, with no change in the incidence of arthroscopic surgeries over the study period. Shoulder and elbow (18.1%) and sports medicine (11.4%) surgeons were more likely to perform surgery arthroscopically compared with hand surgeons (6.1%) (P < .001). There was no difference in overall self-reported complication rates between open (4.4%) and arthroscopic (5.5%) procedures (P = .666). Percutaneous tenotomy, debridement only, and debridement with tendon repair comprised 6.4%, 46.3%, and 47.3% of open treatment, respectively. Sports medicine, hand, and shoulder and elbow surgeons were more likely to repair the tendon after debridement compared with other surgeons, who were more likely to perform debridement alone (P < .001). Hand surgeons were most likely to perform concomitant procedures, of which the majority were neuroplasties. CONCLUSION: Although comparative studies are ultimately necessary for determining the optimal surgical technique, researchers should be mindful of the differences in practices according to training and the extent to which concomitant procedures are being performed, as both these factors may confound any future results.

All-Epiphyseal ACL Reconstruction in Children: Review of Safety and Early Complications.

BACKGROUND: All-epiphyseal anterior cruciate ligament (ACL) reconstruction is a well-described technique for skeletally immature patients. The purpose of this study was to elucidate the early complication rate and identify associated risk factors for rerupture after this procedure in children. METHODS: We retrospectively reviewed patients who underwent all-epiphyseal ACL reconstructions performed at a large, tertiary care children's hospital between January 2007 and April 2013. Relevant postoperative data including the development of leg-length discrepancy, angular deformity, rerupture, infection, knee range of motion, arthrofibrosis, and other complications were recorded. Independent variables analyzed for association with rerupture included age, body mass index, graft type, graft size, and associated injuries addressed at surgery. RESULTS: A total of 103 patients (average 12.1 y old; range, 6.3 to 15.7) were analyzed. The mean follow-up was 21 months. The overall complication rate was 16.5% (17/103), including 11 reruptures (10.7%), 1 case (<1.0%) of clinical leg-length discrepancy of <1 cm, and 2 cases (1.9%) of arthrofibrosis requiring manipulation under anesthesia. Two patients (1.9%) sustained contralateral ACL ruptures and 3 (2.9%) sustained subsequent ipsilateral meniscus tears during the study period. There were no associations found between age, sex, graft type, graft thickness, body mass index, or associated injuries addressed during surgery and rerupture rate. Knee flexion continued to improve by 20 degrees on an average between the 6 weeks and 6 months postoperative visits (P<0.001; paired samples Student's t test). CONCLUSIONS: When taken in the context of known risk of future injury in an ACL-deficient knee, all-epiphyseal ACL reconstruction in children is safe. The rate of growth disturbance in this study is similar to previous reports in this patient demographic. The rerupture rate in this cohort is slightly higher compared with ACL reconstruction in older patients. LEVEL OF EVIDENCE: Level IV-retrospective case series.

Outcomes and toxicities in patients treated with definitive focal therapy for primary prostate cancer: systematic review.

AIM: This systematic review summarizes the clinical data on focal therapy (FT) when used alone as definitive therapy for primary prostate cancer (PCa). METHODS: The protocol is detailed in the online PROSPERO database, registration No. CRD42014014765. Articles evaluating any form of FT alone as a definitive treatment for PCa in adult male patients were included. RESULTS: Of 10,419 identified articles, 10,401 were excluded, and thus leaving 18 for analysis. In total, 2288 patients were treated using seven modalities. The outcomes of FT in PCa seem to be similar to those observed with whole gland therapy and with fewer side effects. CONCLUSION: Further research, including prospective randomized trials, is warranted to elucidate the potential advantages of focal radiation techniques for treating PCa. Prospero Registration Number: CRD42014014765.

Surgical management of osteonecrosis of the femoral head in patients with sickle cell disease.

Sickle cell disease is a known risk factor for osteonecrosis of the hip. Necrosis within the femoral head may cause severe pain, functional limitations, and compromise quality of life in this patient population. Early stages of avascular necrosis of the hip may be managed surgically with core decompression with or without autologous bone grafting. Total hip arthroplasty is the mainstay of treatment of advanced stages of the disease in patients who have intractable pain and are medically fit to undergo the procedure. The management of hip pathology in sickle cell disease presents numerous medical and surgical challenges, and the careful perioperative management of patients is mandatory. Although there is an increased risk of medical and surgical complications in patients with sickle cell disease, total hip arthroplasty can provide substantial relief of pain and improvement of function in the appropriately selected patient.

Levosimendan Reduces Mortality in Adults with Left Ventricular Dysfunction Undergoing Cardiac Surgery: A Systematic Review and Meta-analysis.

INTRODUCTION: Levosimendan is implemented in patients with low cardiac output after cardiac surgery. However, the strength of evidence is limited by randomized controlled trials enrolling a small number of patients. Hence we have conducted a systematic review to determine the role of levosimendan in adult cardiac surgery. METHODS: PUBMED, WoS, Cochrane database, and SCOPUS were systematically queried to identify original English language peer-reviewed literature (inception-October 2014) comparing clinical results of adult cardiac surgery between levosimendan and control. Pooled odds ratio (OR) was calculated using the Peto method; p < 0.05 is significant; results are presented within 95% confidence intervals. Continuous data was compared using standardized mean difference/mean difference. RESULTS: Fourteen studies were included in the analysis. Levosimendan reduced early mortality in patients with reduced ejection fraction (5.5% vs. 9.1%) (OR 0.48 [0.23-0.76]; p = 0.004). This result was confirmed using sensitivity analysis. Postoperative acute renal failure was lower with levosimendan therapy (7.4% vs. 11.5%). Intensive care unit stay was shorter in the levosimendan cohort comparable in both groups (standardized mean difference -0.31 [-0.53, -0.09]; p = 0.006; I(2) = 33.6%). Levosimendan-treated patients stayed 1.01 (1.61-0.42) days shorter when compared to control (p = 0.001). CONCLUSION: Our meta-analysis demonstrates that Levosimendan improves clinical outcomes in patients with left ventricular dysfunction undergoing cardiac surgery. Results of the ongoing multicenter randomized controlled trial are awaited to provide more conclusive evidence regarding the benefit of this drug.

Are two really always better than one? Results, concerns and controversies in the use of bilateral internal thoracic arteries for coronary artery bypass grafting in the elderly: a systematic review and meta-analysis.

INTRODUCTION: Bilateral internal thoracic artery grafting appears to be the preferred method to achieve durable long-term coronary artery revascularization. However, data reporting the benefit of this technique in the elderly is very conflicting. METHOD: We performed a systematic review of available literature (till November 2014) using multiple databases to identify studies comparing clinical events in patients undergoing coronary artery bypass grafting using either a single or double internal thoracic artery in the elderly. While early mortality was the primary end-point of inclusion, other adverse events compared were sternal wound infection (deep and superficial), stroke and peri-operative myocardial infarction. Individual and pooled odd's ratios were calculated using the Mantel-Haenzel method (random effect model); sensitivity analysis was performed. Results are presented using 95% confidence intervals. RESULT: Nine retrospective studies (4479 BITA, 7733 LITA patients) fulfilled search criteria. Deep sternal wound infection was significantly higher after BITA harvest [OR 1.86 (1.3-2.5); I(2) = 0%; p < 0.01]. Early mortality (BITA 3.6% vs SITA 3.1%; p = 0.86), stroke [OR 0.7(0.4-1.1); p = 0.1], and peri-operative myocardial infarction (BITA 4.3% vs SITA 2.3%; p = 0.1) were comparable in both cohorts. Long-term survival favored the BITA cohort in two propensity matched studies. CONCLUSION: The incidence of deep sternal wound infection may be significantly higher after the harvest of both internal thoracic arteries in the elderly. While other post-operative adverse events are comparable, data regarding the long-term survival advantage in this cohort is conflicting. Hence, the use of both internal thoracic arteries in this age group needs to be invidualized.

Revision THA in obese patients is associated with high re-operation rates at short-term follow-up.

We performed a retrospective review of 309 consecutive revision THAs from 2005 to 2009. We identified a subgroup of patients with BMI >35 and compared the operative time, rate of complications, ICU admissions, re-admissions, and re-operations to patients with BMI <35 undergoing revision THA. At a mean follow-up of 36.3 months, there was no significant difference in operative time, perioperative complications, or re-admission rate between the two groups. However, a significantly higher rate of re-operation was observed in the obese group (46% vs. 28%, P=0.015). Obese patients were more likely to undergo reoperation for infection (P=0.017). Patients with high BMI contemplating primary THA should be aware of the potential subsequent complications associated with revision surgery should it become necessary.

Bilateral agenesis of the anterior cruciate ligament: MRI evaluation.

Bilateral agenesis of the anterior cruciate ligament (ACL) is extremely rare. We describe a 13-year-old girl who presented with bilateral knee pain without history of trauma; she has two family members with knee instability. Magnetic resonance imaging showed bilateral absence of the ACL, and medial posterior horn meniscal tears. Bilateral arthroscopic partial meniscectomy and anterior cruciate ligament reconstruction was performed.

Severe persistent synovitis after cobalt-chromium total knee arthroplasty requiring revision.

Implant-related hypersensitivity is a well-established cause of failure after total hip arthroplasty but is a rare complication after total knee arthroplasty (TKA). It remains a relatively unpredictable and poorly understood cause of implant-related failure. This article describes 5 patients (6 knees) who presented with persistent pain and hypertrophic synovitis after TKA using a cobalt-chromium component. Extensive perioperative workup, including white blood cell count, erythrocyte sedimentation rate, C-reactive protein, joint aspiration, and intraoperative cultures, ruled out infection as a cause of the symptoms. All knees demonstrated appropriate ligamentous balance and were well aligned, with all components noted to be well fixed at revision. In all patients, the clinical condition improved dramatically after revision to zirconium femoral and titanium metal-backed tibial components. Pain and functional outcome scores improved in all patients. Intraoperative histopathology revealed a thickened synovium with either a predominantly lymphocytic or histiocytic monocellular response. Final pathology confirmed that no infection was present in any patient. The goal of TKA is to produce a well-performing, pain-free joint. When patients present with recurrent pain and synovitis after TKA, infection must be excluded. When infection and instability have been excluded, metal hypersensitivity should be considered as a cause of primary TKA failure. In these patients, revision to a zirconium femoral component can provide predictable and effective clinical improvement.