[Very rare cases of periprosthetic malignant neoplasms : Data from 4000 cases of endoprosthetic joint replacements from the histopathologic implant register]. / Nachweis hoch seltener periprothetischer maligner Neoplasien : Ergebnisse von 4000 Gelenkendoprothesen aus dem histopathologischen Implantatregister.

BACKGROUND: In 2016, the AG 11 (work group for implant-material-intolerance) of the German society for Orthopaedics and Orthopaedic Surgery (DGOOC) created a histopathologic implant register (HIR). The goal was to conduct a retrospective data analysis based on the revised SLIM-consensus-classification, which defines eight different failure mechanisms. QUESTIONS: The analysis of 4000 cases of endoprosthetic joint replacements addressed the following questions: 1. What is the frequency distribution of different SLIM-types? 2. How does durability of endoprosthetic joint replacements differ among SLIM-types? 3. What kind of periprosthetic malignant neoplasia can be detected and how often? RESULTS: SLIM-type I was diagnosed in 1577 cases (n = 1577, 39.4%), SLIM-type II in 577 cases (n = 577; 14.4%), SLIM-type III in 146 cases (n = 146; 3,7%), SLIM-type IV in 1151 cases (n = 1151; 28.8%), SLIM-type V in 361 cases (n = 361; 9.0%), SLIM-type VI in 143 cases (n = 143; 3.6%), SLIM-type VII in 42 cases (n = 42; 1.0%), and SLIM-type VIII in 3 cases (n = 3; 0.075%). There was statistical significance in implant durability between the different SLIM types. Among the different reasons for endoprosthetic joint replacement failure, non-infectious causes have the biggest share at 81%, with SLIM-type I (39.5%), and SLIM-type IV (29.4%) being the predominant SLIM types. Three cases of periprosthetic malignant neoplasia (SLIM-type VIII) were detected: one case of small B lymphocytic lymphoma/BCLL (C85.9; ICD-O: 9670/3), one case of diffuse large B­cell lymphoma/DLBCL (C83.3; ICD­O 9680/3), and one case of anaplastic large cell lymphoma (C84.7; ICD-O: 9714/3), with the latter ones being the causes for joint replacement , which indicates that malignant neoplasia is a very rare cause of endoprosthetic joint replacement (n = 2; 0.05%). DISCUSSION: These data are complete new, especially as concerns arthrofibrosis (SLIM-type V), adverse inflammatory reactions (SLIM-type VI), and the very rare cases of periprosthetic malignant neoplasia, SLIM-type VIII, as a reason for revision. Since neither the annual review (2017) of the EPRD, nor the national evaluation report (2017) of the IQTIG provide sufficient data, this indicates the relevance of the HIR of the AG 11 of the DGOOC.

CD3+ lymphocytosis in the peri-implant membrane of 222 loosened joint endoprostheses depends on the tribological pairing.

Background and purpose - The most frequent cause of arthroplasty failure is aseptic loosening-often induced by particles. Abrasion material triggers inflammatory reactions with lymphocytic infiltration and the formation of synovial-like interface membranes (SLIM) in the bone-implant interface. We analyzed CD3 quantities in SLIM depending on articulating materials and possible influences of proven material allergies on CD3 quantities. Patients and methods - 222 SLIM probes were obtained from revision surgeries of loosened hip and knee arthroplasties. SLIM cases were categorized according to the SLIM-consensus classification and to the particle algorithm. The CD3 quantities were analyzed immunohistochemically, quantified, and correlated to the particle types. Results - Metal-metal pairings showed the highest CD3 quantities (mean 1,367 counted cells). CD3 quantities of metal-polyethylene (mean 243), ceramic-polyethylene (mean 182), and ceramic-ceramic pairings (mean 124) were significantly smaller. Patients with contact allergy to implant materials had high but not statistically significantly higher CD3 quantities than patients without allergies. For objective assessment of the CD3 response as result of a pronounced inflammatory reaction with high lymphocytosis (adverse reaction), a defined CD3 quantity per high power field was established, the "CD3 focus score" (447 cells/0.3 mm2, sensitivity 0.92; specificity 0.90; positive predictive value 0.71; negative predictive value 0.98). Interpretation - The high CD3 quantities for metal-metal pairings may be interpreted as substrate for previously described adverse reactions that cause severe peri-implant tissue destruction and SLIM formation. It remains unclear whether the low CD3 quantities with only slight differences in the various non-metal-metal pairings and documented contact allergies to implant materials have a direct pathogenetic relevance.

Activated human B cells induce inflammatory fibroblasts with cartilage-destructive properties and become functionally suppressed in return.

BACKGROUND: Cross-talk between synovial fibroblasts (SF) and immune cells is suggested to play a crucial role in inflammation and chronification of rheumatoid arthritis (RA). The contribution of B cells in this process is poorly defined. METHODS: Here, primary B cells from healthy donors were polyclonally activated and cocultured with SF of non-synovitic origin from patients with osteoarthritis. RESULTS: In B-SF cocultures the concentrations of interleukin 6 (IL-6) and IL-8 increased manifold compared with single cultures even under physical separation and remained stable for several days after B-cell removal. Intracellular staining confirmed SF as key producers of IL-6 and IL-8, and B cells as main producers of tumour necrosis factor alpha (TNFα) and IL-1ß. Blocking experiments with a combination of anti-TNFα-antibodies and rIL-1RA significantly reduced SF cytokine production by up to 90%, suggesting that B-cell-derived TNFα and IL-1ß were crucial mediators of SF activation. Interestingly, B-cell cytokine production, CD25 expression and proliferation decreased in cocultures by at least 50%, demonstrating a negative regulatory loop towards the activated B cells. Inhibition of activin receptor-like kinase 5, a crucial component of the tumour growth factor ß (TGFß) signalling pathway, partly restored B-cell proliferation, suggesting a contribution of SF-derived TGFß in B-cell suppression. Besides cytokines, B-cell-activated SF also upregulated secretion of matrix metalloproteases such as MMP-3, thereby acquiring potential tissue destructive properties. This was confirmed by their invasion into human cartilage in the severe combined immunodeficiency mouse fibroblast invasion model in vivo. CONCLUSIONS: Interaction with activated B cells leads to conversion of non-arthritic SF into SF with a proinflammatory and aggressive RA-like phenotype, thereby suggesting a new, so far unrecognised role for B cells in RA pathogenesis.

Fixation pattern of conical and cylindrical modular revision hip stems in different size bone defects.

PURPOSE: Femoral defects often make it difficult to achieve sufficient fixation of the stem during revision surgery. No clinical studies comparing modular stems are available and differentiated recommendations are rare. The aim of this study was to compare the fixation of different revision stems in an experimental and standardised manner. METHODS: Segmental AAOS type I and III defects were reproduced in four femur pairs and two modular stems of different shape (cylindrical and conical) were implanted. Interfacial stem-bone movements were measured under axial torque application to analyze the stem fixation depending on defect extension. RESULTS: Both stems showed adequate fixation in AAOS type I defect. The defect extension significantly reduced the fixation of both implants. The fixation pattern changed significantly for the cylindrical-shaped stem but was maintained for the conical-shaped stem. CONCLUSIONS: Shape as well as the extension of femoral defect have an impact on primary fixation. A type I defect seems to be bridgeable for both stems albeit in a different way. In contrast, stem-demanding activities during the healing phase have to be avoided for the conical-shaped stem in a type III defect, whereas the cylindrical-shaped stem has already ceased to be sufficiently stable in this case.

Strut grafts in revision hip arthroplasty faced with femoral bone defects: an experimental analysis.

PURPOSE: In total hip arthroplasty fixation of revision stems can be demanding due to femoral bone loss. Strut grafts are often used for bone augmentation and stabilization of the newly inserted prosthesis. The aim of this study was to assess the effect of strut grafts on primary stability under various stem fixation conditions. METHODS: Two different revision stems (cylindrical and conical shape) were implanted into synthetic femora. Following a semicircular transfemoral osteotomy, three deficient femoral bearings were simulated (bony lid reattached with cable wires; weakened lid reattached with cable wires; strut grafts placed to the weakened lid with cable wires). Relative micro-movements were measured between prostheses and bones due to an axial moment applied to the stems. RESULTS: Relative movements correlated to the stem shape. The cylindrical stem showed higher movements increasing significantly with a weakened bony lid and portrayed a slight decrease of movements with strut graft application. No unequivocal influence of the weakened lid could be detected for the conical implant. Strut graft application did not show an additional stabilizing effect. CONCLUSIONS: The primary stability of the cylindrical fixation concept decreases with impaired fixation conditions of the femur. A clear restabilizing effect with strut grafts could not be proven. A decrease of primary stability due to the impaired bone could not be observed for the conical stem shape. Additionally, strut grafts do not enhance fixation for this stem shape. We conclude that surgeons should not rely on a stabilizing effect of strut grafts in revision hip surgery.

Nickel (Ni) allergic patients with complications to Ni containing joint replacement show preferential IL-17 type reactivity to Ni.

BACKGROUND: Some nickel (Ni) allergic patients develop complications following Ni-containing arthroplasty. In the peri-implant tissue of such patients, we had observed lymphocyte dominated inflammation together with IFN-gamma and IL-17 expression. OBJECTIVES: To determine whether Ni stimulation of peripheral blood mononuclear cells (PBMCs) of such patients would lead to a different cytokine pattern as compared to Ni-allergic patients with symptom-free arthroplasty. PATIENTS AND METHODS: Based on history and patch testing in 15 Ni-allergic patients (five without implant, five with symptom-free arthroplasty, five with complicated arthroplasty) and five non-allergic individuals, lymphocyte transformation test (LTT) was performed using PBMC. In parallel in vitro cytokine response to Ni was assessed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). RESULTS: All 15 Ni-allergic individuals showed enhanced LTT reactivity to Ni (mean SI = 8.42 +/- 1.8) compared to the non-allergic control group. Predominant IFN-gamma expression to Ni was found both in the five allergic patients without arthroplasty and also in the five allergic, symptom-free arthroplasty patients. In contrast, in the five Ni-allergic patients with arthroplasty-linked complications a predominant, significant IL-17 expression to Ni was seen but not in patients with symptom-free arthroplasty. CONCLUSIONS: The predominant IL-17 type response to Ni may characterize a subgroup of Ni-allergic patients prone to develop lymphocytic peri-implant hyper-reactivity.

The influence of age, bone quality and body mass index on periprosthetic femoral fractures: a biomechanical laboratory study.

BACKGROUND: A periprosthetic femoral fracture after total hip arthroplasty is a serious but uncommon complication, and therefore difficult to analyze clinically in scientifically and statistically valid study samples. With the aim of preventing these fractures the purpose of this study was to investigate potential risk factors associated with cementless hip arthroplasty in a standardized laboratory setup. Additionally, we aimed to clarify if the treatment with a cementless hip stem is fraught with higher risk of periprosthetic fracture for older patients compared to younger patients. MATERIAL/METHODS: A biomechanical setup was developed to provide analysis on sixteen femoral specimens of different age. A cementless hip stem was implanted into the specimens and loads - representing hip contact forces - were applied under standardized conditions until fracture occurred. The femurs were divided into two age groups (<70 and >or=77 years of age). RESULTS: The elderly specimens fractured at significantly lower maximum forces (<70: Fmax=5,308N; >or=77: Fmax=2,519N; p<0.01). Maximum fracture loads were found to correlate strongly with age (p=0.01), BMD (e.g. for the Ward's triangle: p<0.01) and BMI (p=0.04). CONCLUSIONS: In patients with advanced age treated with cementless hip stems the risk of suffering a periprosthetic fracture is significantly higher. It increases in patients with an age of 80 years or older, a Ward's triangle BMD below 0.500 g/cm2 and a BMI >33 kg/m2. Whereas one single factor must not be viewed as an exclusion criterion for a cementless hip stem treatment, cumulation of these factors should alert the orthopaedic surgeon.

Myoelectric hand prostheses in very young children.

Myoelectric prostheses have generally been provided for adolescent or adult patients. The availability of smaller-sized electric hands has enabled the introduction of myoelectric prostheses to preschool children, mainly in the Scandinavian countries. This study evaluates the acceptance of myoelectric prostheses in 41 children with unilateral upper limb deficiency between the ages of two and five years. The prosthesis was used for an average time of 5.8 hours per day. The level of amputation was found to influence the acceptance rate. Furthermore, prosthetic use training by an occupational therapist is related to successful use of the prosthesis. The general drop-out rate in preschool children is very low compared to adults. Therefore, infants can profit from myoelectric hand prostheses. Since a correct indication and an intense training program significantly influence the acceptance rate, introduction of myoelectric prostheses to preschool children should take place at specialised centres with an interdisciplinary team.

Metal release and corrosion effects of modular neck total hip arthroplasty.

Modular neck implants are an attractive treatment tool in total hip replacement. Concerns remain about the mechanical stability and metal ion release caused by the modular connection. Five different implant designs were investigated in an experimental set-up. In vivo conditions were simulated and the long-term titanium release was measured. Finally, the modular connections were inspected for corrosion processes and signs of fretting. No mechanical failure or excessive corrosion could be identified for the implants tested. The titanium releases measured were extremely low compared to in vivo and in vitro studies and were not in a critical range.

Do short-stemmed-prostheses induce periprosthetic fractures earlier than standard hip stems? A biomechanical ex-vivo study of two different stem designs.

INTRODUCTION: The causes of periprosthetic fractures of the femur due to the design of the prosthesis and the individual parameters of the patient are unexplored. By different anchorage techniques in cementless total hip arthroplasties, it is assumed that there are various load limits of the implant's bearing femur. MATERIALS AND METHODS: In the present study, we compared a standard hip stem (cementless Spotorno) and a short-stemmed design (Mayo) by an artificial reproduction of periprosthetic fractures in 20 femur specimens. RESULTS: The measured fracture loads showed an extensive range, with higher maximum loads in the standard stem group. The bone mineral density and the subsiding pattern of the standard stems showed a significant correlation to the incidence of the periprosthetic fractures. In the experimental setup, a slightly lower fracture resistance was shown for the short-stemmed prosthesis. Additionally, it was shown that donors with a higher body mass index had a significantly increased fracture risk. CONCLUSIONS: Short-stemmed prostheses, especially the Mayo hip, do not constitute a higher fracture risk. In general, an increased body mass index among patients with a cementless hip stem is associated with an increased fracture risk, particularly at high load values, i.e., resulting from a step during stumbling. Taking into account the ascertained results, the danger of provoking a femoral periprosthetic fracture can be reduced.

Primary rotational stability of cylindrical and conical revision hip stems as a function of femoral bone defects: an in vitro comparison.

Bone stock losses in cementless femoral stem revisions compromise a stable fixation. The surgeon has to rely on his wealth of experience in deciding which stem shape to use. The aim of our study was to compare the primary rotational stability of cylindrical and conical revision hip stems subjected to femoral defects. Four current prostheses (two cylindrical, two conical) were implanted into four synthetic femora. Micro-motion was measured under torque application and femoral neck osteotomy and segmental AAOS Type I and III defects were simulated. The relative movements of all prostheses were significantly influenced by the extent of bone loss (p<0.01). Major differences were seen in fixation behavior (p<0.01). The main fixation area of conical stems is within the distal femoral isthmus, whereas cylindrical implants are dependent on proximal bone stock. In our study, cylindrical stems are advantageous for minor defects because they provide a proximal fixation. In cases of extensive substance loss, the conical implants showed lesser relative movements. These findings should be taken into account for clinical decisions.

Good survival of uncemented tapered stems for failed intertrochanteric osteotomy: a mean 16 year follow-up study in 45 patients.

INTRODUCTION: Inferior survival of cemented total hip arthroplasty has been reported after previous femoral osteotomy. We previously presented 5-15 years results of uncemented femoral stems for this subgroup of patients. The purpose of the present study was to re-evaluate that same patient group at 10-20 years follow-up. MATERIALS AND METHODS: Forty-eight hips in 45 patients had undergone conversion THA for a failed intertrochanteric osteotomy of the hip after a mean of 12 years (2-33 years). Mean time of follow-up was 16 years (10-20 years). RESULTS: At the latest follow-up five patients had died (five hips), and one patient (one hip) remained lost to follow-up. Compared to the previous evaluation, one more patient required femoral revision for aseptic loosening giving a total of four patients (four hips) with femoral revision--one for infection and three for aseptic loosening of the stem. Survival of the stem was 91% at 15 and 20 years respectively; survival with femoral revision for aseptic loosening as an end point was 93%. The median Harris-Hip-Score at final follow-up was 76 points (previously 80 points). Radiolucent lines in Gruen zones 1 and 7 were present in 20 and 17% of hips, respectively. Radiolucencies in other zones were not detected. There was no radiographic evidence of femoral osteolysis, stress-shielding or loosening. CONCLUSION: The long-term results with this type of uncemented tapered titanium femoral component after proximal femoral osteotomy remain encouraging and compare favorably to those achieved in patients with regular femoral anatomy.

Fracture load for periprosthetic femoral fractures in cemented versus uncemented hip stems: an experimental in vitro study.

This cadaveric study examined fracture loads in cemented and uncemented hip stems. Additionally, individual data and bone quality were analyzed and correlated to fracture patterns and fracture load. Cemented or uncemented hip stems were implanted in a randomized fashion in 10 matched paired fresh-frozen femora (donor median age, 78 years, and donor median weight, 74.2 kg). Bone density was measured before the femurs were fractured under load (maximum load of 10,000 N), and fracture patterns were analyzed according to the Vancouver and Johansson classification systems. In the uncemented group, all of the femurs fractured with a median load of 2625 N (range, 1725-7647 N). In the cemented group, 5 femurs fractured with a median maximum load of 9127 N (range, 2845-10,000 N) and 5 femurs did not fracture with a maximum load of 10,000 N. Fracture load corresponded to 4 times and 8.8 times body weight in the uncemented and cemented groups, respectively. Fracture patterns corresponded to Vancouver type A fractures in uncemented stems and Vancouver type C fractures in cemented hip stems. Analysis showed a significant correlation between fracture load and bone density in the uncemented group, whereas there was no correlation in the cemented group. Patients with poor bone quality treated with an uncemented hip stem are at higher risk for periprosthetic fractures; therefore, we recommend cemented stems in this group of patients. Cementation appears to protect against periprosthetic fractures, probably from internal stiffening of the femoral cavity.

Patellar blood flow during knee arthroplasty surgical exposure: Intraoperative monitoring by laser doppler flowmetry.

Patellofemoral complications following knee arthroplasty are a well-known problem. Patellar ischemia has been suspected to be causative for fracture, anterior knee pain, and patella component failure. The purpose of this study was to assess the influence of knee arthroplasty surgical dissection on patellar blood flow. Patellar blood flow was measured by means of intraosseous laser Doppler flowmetry (LDF) in 10 patients undergoing total knee arthroplasty by a standard medial parapatellar approach. The initial blood flow was 121.6 +/- 114.7 AU. The signal significantly decreased by 71% (p = 0.0051) when the knee was flexed and lost the pulsatile signal pattern in 80%. After arthrotomy, the signal was 100.1 +/- 120.3 AU in extension. The lowest signal was found in flexion and eversion of the patella (mean, 18 +/- 10.7 AU) and all signals lost pulsatility. As compared to the initial values, completion of the soft tissue dissection did not lead to a significant change of the blood flow signal (121.3 +/- 104.8; p = 0.6835). Flexion of the knee joint markedly reduced patellar perfusion. Standard medial parapatellar approach did not significantly change patellar blood flow. This study does not support the theory of postoperative patellar ischemia as a cause of anterior knee pain or patellofemoral problems.

Cementing a polyethylene cup into a well fixed acetabular metal-on-metal resurfacing component? An experimental investigation.

INTRODUCTION: Adverse reactions to metal debris often indicate revision surgery in metal-on-metal (MoM) hip arthroplasty and an exchange of the MoM bearing into either a metal on polyethylene or a ceramic-on-polyethylene articulation. At the moment the removal of the entire implant system is the most reasonable method. In order to avoid bone loss caused by the removal of a well-fixed acetabular component, the purpose of this study was to measure the stability of a cemented polyethylene (PE) cup in an acetabular hip resurfacing component and to examine if such a method could be suitable for clinical use. METHODS: PE cups were cemented into 2 different hip resurfacing components and biomechanical tests were applied to measure failure torques under lever out and rotational load. RESULTS: In all cases failure of the interface between the resurfacing components and the cement layer occurred at a very low load (0.14 Nm-61.50 Nm). DISCUSSION: The early failure occurred due to lacking interdigitation of cement and the polished metal surface. Thus we warn against cementing a PE cup into acetabular hip resurfacing components for clinical use.

Supramacroparticulate PE in 6 different joint endoprostheses localisations: An indicator for PE damage?

In the histopathological particle algorithm polyethylene (PE) particles with maximum lengths of more than 100µm - called PE supramacroparticles - are identified exclusively for knee joint and hip prostheses. However, a definitive characterisation, detection in all joint localisations and a causal clarification of the pathogenesis are lacking. In this study a total of 175 SLIM (synovial-like interface membrane) cases with PE supramacroparticles of knee joint prostheses (n=89), hip joint prostheses (n=44), ankle joint prostheses (n=36) and prostheses in three localisations of the upper extremities (n=6) were systematically investigated. The arithmetic mean of the particle length varied greatly within the prosthesis types. This had a significant positive correlation with the prosthesis lifetime and negative correlation with the date of implantation. It can be concluded that both the lifetime and the time of implantation have an influence on the particle length. The prostheses with supramacroparticulate damage moreover showed a clearly reduced survival rate compared with other data published on the prosthesis lifetime. The material wear therefore could not be attributed solely to the usual fatigue factors. Since loosening of the prostheses, decentring of the PE components or damage to the PE inlay existed in all cases, mechanical dysloading seems to be the most probable cause of PE supramacroparticle genesis. Due to the striking length and for demarcation from PE macroparticles, the term supramacroparticulate PE is proposed for a length of more than 100µm. In the extended histopathological particle algorithm supramacroparticulate PE has been included in the macroparticles category and should be taken into account and interpreted causally in histopathological diagnostics of joint prosthesis failure.

Imaging in scoliosis from the orthopaedic surgeon's point of view.

For treating patients with scoliosis orthopaedic surgeons need diagnostic imaging procedures in order to provide answers about a possible underlying disease, choice of treatment, and prognosis. Once treatment is instituted, imaging is also critical for monitoring changes of the deformity so as to optimize therapy. The combined effort of orthopaedic surgeons and radiologists helps detect treatable causes of scoliosis at an early stage, define the need and timing for surgery, and ensure that every precaution is taken to minimize the risks of surgery. Neurosurgical causes, with particular reference to spinal cord tumours and syringomyelia, need to be addressed before scoliosis surgery can be contemplated.

Influence of Surface Coating on Metal Ion Release: Evaluation in Patients With Metal Allergy.

Nickel, chromium, and cobalt in stainless steel and Cobalt-chrome-molybdenum (CoCrMo) alloys may induce allergy. The objectives of this study were to evaluate surface coating regarding ion release, patch test reactivity, and arthroplasty performance. Materials and methods included patch test in 31 patients with metal allergy and 30 patients with no allergy to stainless steel and CoCrMo disks that are uncoated or coated by titanium nitride/zirconium nitride (TiN/ZrN). Assessment include atomic absorption spectrometry of released nickel, cobalt, and chromium from the disks after exposure to distilled water, artificial sweat and culture medium. Results showed that both coatings reduced the nickel and chromium release from stainless steel and CoCrMo disks and mostly the cobalt release from the disks (maximally 11.755 µg/cm(2)/5 d to 1.624 by Ti-N and to 0.442 by ZrN). Six of the 31 patients with metal allergy reacted to uncoated disks, but none reacted to the coated disks. The current authors report on exemplary patients with metal allergy who had symptom relief by revision with surface-coated arthroplasty. The authors concluded that the surface coating may prevent cutaneous and peri-implant allergic reactions. [Orthopedics. 2016; 39(3):S24-S30.].

Failure of Polyethylene Inlays in Cementless Total Hip Arthroplasty: A Retrieval Analysis.

A retrieval analysis has been performed on 50 polyethylene inlays of cementless screw ring implants (Mecring, Mecron, Berlin, Germany) to investigate the failure mechanism of this specific open cup hip arthroplasty design that has shown a high clinical failure rate. Design-specific damage modes like rim creep, collar fatigue, and backside wear were assessed. Furthermore, the inlays were measured using a CMM to determine deformation. In 90% backside wear was observed and collar fatigue occurred in 68% of the cases. Rim creep was present in 38% of the polyethylene inlays. In 90% of the cases the cup opening diameter was 32.1 mm or less and 46% had a diameter less than 32 mm. It seems that creep and deformation of the polyethylene leads to a reduced diameter at the cup opening and consequently decreased clearance. To avoid this type of failure, polyethylene inlays should be supported at the back by the cup to reduce the risk of ongoing creep deformation.