Mesenchymal Stem Cell physiology can be affected by antibiotics: An in vitro study

The aim of this study was to investigate the effect of antibiotics used in clinical practice on Mesenchymal Stem Cells (MSCs) potential to proliferate and differentiate towards an osteogenic lineage. Trabecular bone was obtained from 10 patients (mean age of 36 years, range 18-72) suffering from long bone fractures. Mesenchymal Stem Cells (MSCs) were isolated and functional assays on their proliferation (CFU-F and XTT) and osteogenic differentiation (alkaline phosphatase activity and total calcium production) were performed. The effect of medium supplementation with gentamicin, vancomycin, benzyl-penicillin, flucloxacillin, cefuroxime and metronidazole was analysed. In concentrations found in peripheral circulation, none of the studies antibiotics had an effect on MSCs ability to proliferate and differentiate towards osteogenic lineage. Vancomycin and gentamicin in concentrations of 200 μg/ml and 75 μg/ml respectively, down-regulated the proliferation and osteogenic activity of MSCs. Some combination of the studied antibiotics found to inhibit both proliferation and osteogenesis. High antibiotic concentrations and the combination of different formulations can have detrimental effects on osteoprogenitor cells physiology and potentially bone healing.

Are intra-articular injections of Hylan G-F 20 efficacious in painful osteoarthritis of the knee? A systematic review & meta-analysis

AIM

To systematically review evidence and perform a meta-analysis of the efficacy of intra-articular (IA) injections of Hylan G-F 20 for the treatment of painful osteoarthritis (OA) of the knee.

METHODS

Systematic review of the Embase and PubMed databases up to July 2013 of randomised placebo-controlled trials studying the effect of Hylan G-F 20 in patients with painful knee OA, with a meta-analysis of trials reporting visual analogue scores (VAS) for weight-bearing pain in the knees of patients followed up for a minimum of 6 months.

RESULTS

Six placebo-controlled randomised trials were identified on systematic review of which two studies met criteria for inclusion in the meta-analysis. Meta-analysis demonstrated that at 6-month follow up, there was no significant difference between Hylan G-F 20 and control in terms of reduction in VAS for weight bearing pain. (Mean Difference - 12.96 (95% CI: -35.48, 9.56). Z tests used to test for overall effect showed that the difference between the two groups was not significant (p = 0.26).

DISCUSSION

A significant placebo effect exists for patients receiving IA injections for the treatment of painful knee OA. The withdrawal of fluid from the affected knee prior to any injectable therapy may itself have additional benefits which in isolation have not been studied. This may form the basis of future research. The authors' acknowledge that although limited conclusions can be drawn from the results of this study, the meta-analysis presented has not been performed previously and will further contribute to the knowledge on this subject.

CONCLUSION

Although Hylan G-F 20 may produce improvement in VAS scores for weight-bearing pain at 6-month follow up in OA knees treated with it, patients should be informed that this may be equivalent to that seen with control treatments.

The treatment of intramedullary osteomyelitis of the femur and tibia using the Reamer–Irrigator–Aspirator system and antibiotic cement rods

Intramedullary infection in long bones represents a complex clinical challenge, with an increasing incidence due to the increasing use of intramedullary fixation. We report a prospective case series using an intramedullary reaming device, the Reamer–Irrigator–Aspirator (RIA) system, in association with antibiotic cement rods for the treatment of lower limb long bone infections. A total of 24 such patients, 16 men and eight women, with a mean age of 44.5 years (17 to 75), 14 with femoral and 10 with tibial infection, were treated in a staged manner over a period of 2.5 years in a single referral centre. Of these, 21 patients had had previous surgery, usually for fixation of a fracture (seven had sustained an open fracture originally and one had undergone fasciotomies). According to the Cierny–Mader classification system, 18 patients were classified as type 1A, four as 3A (discharging sinus tract), one as type 4A and one as type 1B. Staphylococcus species were isolated in 20 patients (83.3%). Local antibiotic delivery was used in the form of impregnated cement rods in 23 patients. These were removed at a mean of 2.6 months (1 to 5). Pathogen-specific antibiotics were administered systemically for a mean of six weeks (3 to 18). At a mean follow-up of 21 months (8 to 36), 23 patients (96%) had no evidence of recurrent infection. One underwent a planned trans-tibial amputation two weeks post-operatively due to peripheral vascular disease and chronic recalcitrant osteomyelitis of the tibia and foot. The combination of RIA reaming, the administration of systemic pathogen-specific antibiotics and local delivery using impregnated cement rods proved to be a safe and efficient form of treatment in these patients.

Cite this article: Bone Joint J 2014; 96-B:783–8.

Distraction osteogenesis in the treatment of long bone defects of the lower limbs: effectiveness, complications and clinical results; a systematic review and meta-analysis

We carried out a systematic review of the literature to evaluate the evidence regarding the clinical results of the Ilizarov method in the treatment of long bone defects of the lower limbs. Only 37 reports (three non-randomised comparative studies, one prospective study and 33 case-series) met our inclusion criteria. Although several studies were unsatisfactory in terms of statistical heterogeneity, our analysis appears to show that the Ilizarov method of distraction osteogenesis significantly reduced the risk of deep infection in infected osseous lesions (risk ratio 0.14 (95% confidence interval (CI) 0.10 to 0.20), p < 0.001). However, there was a rate of re-fracture of 5% (95% CI 3 to 7), with a rate of neurovascular complications of 2.2% (95% CI 0.3 to 4) and an amputation rate of 2.9% (95% CI 1.4 to 4.4).The data was generally not statistically heterogeneous. Where tibial defects were > 8 cm, the risk of re-fracture increased (odds ratio 3.7 (95% CI 1.1 to 12.5), p = 0.036). The technique is demanding for patients, illustrated by the voluntary amputation rate of 1.6% (95% CI 0 to 3.1), which underlines the need for careful patient selection.

Treatment of AVN using the induction chamber technique and a biological-based approach: indications and clinical results

OBJECTIVE

To determine the efficacy of core decompression (CD) technique combined with recombinant morphogenetic proteins, autologous mesenchymal stem cells (MSCs) and xenograft bone substitute into the necrotic lesion of the femoral head on clinical symptoms and on the progression of osteonecrosis of the femoral head.

PATIENTS AND METHODS

A total of 38 patients (40 hips) with early stage osteonecrosis of the femoral head were studied over a 4-year period.

RESULTS

CD technique combined with recombinant morphogenetic proteins, autologous MSCs and xenograft bone substitute was associated with a significant reduction in both pain and joint symptoms and reduced the incidence of fractural stages. At 36 months, 33 patients achieved clinical and radiographic healing.

CONCLUSION

This long-term follow-up study confirmed that CD technique combined with recombinant morphogenetic proteins, autologous MSCs and xenograft bone substitute may be an effective treatment for patients with early stage osteonecrosis of the femoral head.

The impact of traumatic pelvic fractures on sporting activity and quality of life

BACKGROUND

Pelvic fractures (PFX) reflect high-energy trauma with high mortality and morbidity.

AIM

We attempted to determine: whether there is a decrease in levels of sporting and physical activity in patients with operatively-treated PFX; risk factors for decreased sporting activity; any correlation between sporting activity and quality of life in this group.

METHODS

Retrospective demographics on mechanism of injury, fracture type, associated injury and injury severity score, as well as prospective documentation of the level and frequency of sporting activity, were collected from adult patients treated operatively for a PFX between 2007 and 2010, using a specifically designed questionnaire. Quality of life before and after injury was also recorded using the EuroQol-5D health-outcome tool.

RESULTS

80 patients without pre-existing musculoskeletal disability were enrolled. The mean age was 44.9 years (18-65). The mean follow-up was 30.5 months (12-39). A decrease in level and frequency of sporting activity was observed. It was associated with lower-extremity associated injuries, but not with injury severity score, PFX severity, PFX type, age, or timing of follow-up. Sporting activity before injury predicted higher levels of sporting participation after injury. Decreased sporting activity after injury was associated with decreased EuroQol-SD score.

CONCLUSIONS

Patients should be counselled on the likelihood of a reduction in sporting activities after surgically treated PFX. A larger multi-centre study is needed to further expand on the evidence of the true impact of PFX and its associated injuries on sporting activity.

Fracture non-union: Can biomarkers predict outcome?

Delayed bone healing and non-union occurs in approximately 10-15% of long bone fractures. Both pathologies may result in prolonged period of pain, disability and repetitive operative interventions. Despite intense investigations and progress done in understanding the pathophysiologic processes governing bone healing, the diagnostic tools have not been altered. The clinical findings and radiographic features remain the two important landmarks of diagnosing non-union and even when the diagnosis is established there is debate on the ideal timing and mode of intervention. Emerging evidence suggest that there are certain molecules and genes that can serve as predictors of potentially unsuccessful fracture union. This article summarises the current evidence on the available 'bio-markers'to predict fracture non-union.

Partial anterior sacrectomy with nerve preservation to treat locally advanced rectal cancer

AIM

Most studies that have reported outcomes after composite abdomino-sacral resection for locally advanced/recurrent rectal cancer have involved resections below the S2/3 disc space. Involvement of the sacrum above this level is uncommon and, until recently, was considered a contraindication to resection.

METHOD

We report here a surgical technique to deal with high sacral involvement with an anterior approach and maintenance of sacropelvic stability.

RESULTS

The operative findings confirmed a locally perforated rectal cancer with an associated abscess cavity and direct invasion into S2. Given the likelihood that a complete dislocation of the sacrum would cause significant neurological damage and pelvic instability without oncological benefit, we opted for a partial high anterior sacrectomy with nerve preservation. The patient made an uncomplicated recovery without neurological deficit and was able to walk with the aid of crutches from postoperative day 3.

CONCLUSION

While a high sacral transection is appropriate for some patients with locally advanced/recurrent rectal cancer, operative decisions and options should be tailored to each individual.

Acetabular fractures with marginal impaction

Over a five-year period, adult patients with marginal impaction of acetabular fractures were identified from a registry of patients who underwent acetabular reconstruction in two tertiary referral centres. Fractures were classified according to the system of Judet and Letournel. A topographic classification to describe the extent of articular impaction was used, dividing the joint surface into superior, middle and inferior thirds. Demographic information, hospitalisation and surgery-related complications, functional (EuroQol 5-D) and radiological outcome according to Matta’s criteria were recorded and analysed. In all, 60 patients (57 men, three women) with a mean age of 41 years (18 to 72) were available at a mean follow-up of 48 months (24 to 206). The quality of the reduction was ‘anatomical’ in 44 hips (73.3%) and ‘imperfect’ in 16 (26.7%). The originally achieved anatomical reduction was lost in12 patients (25.8%). Radiologically, 33 hips (55%) were graded as ‘excellent’, 11 (18.3%) as ‘good’, one (1.7%) as ‘fair’ and 15 (25%) as ‘poor’. A total of 11 further operations were required in 11 cases, of which six were total hip replacements.

Univariate linear regression analysis of the functional outcome showed that factors associated with worse pain were increasing age and an inferior location of the impaction. Elevation of the articular impaction leads to joint preservation with satisfactory overall medium-term functional results, but secondary collapse is likely to occur in some patients.

Cite this article: Bone Joint J 2013;95-B:230–8.

Genetic predisposition to non-union: evidence today

Atrophic non-union represents a complex clinical condition and research is ongoing in an effort to elucidate its pathophysiology and to offer new and more efficient treatment modalities. Differences seen in fracture healing responses and final outcome may be attributed among other factors to biological variations between patients resulting in a "disturbed" signalling pathway and an "inert or deficient local biology with reduced potentials for bone regeneration". The genetic contribution with or without the interaction of other exogenous factors in cases of impaired fracture healing, is yet to be elucidated. However, preliminary animal and human studies demonstrate the molecular basis of fracture non-unions and correlate genetic variants of the molecules regulating fracture healing and their expression patterns with impaired bone healing and fracture non-union. Further research is needed to clarify the genetic component and its role and interaction with other risk factors that may result in increased susceptibility of a patient to develop this complication.

Gustilo IIIC fractures in the lower limb: our 15-year experience

Controversy continues to surround the management of patients with an open fracture of the lower limb and an associated vascular injury (Gustilo type IIIC). This study reports our 15-year experience with these fractures and their outcome in 18 patients (15 male and three female). Their mean age was 30.7 years (8 to 54) and mean Mangled Extremity Severity Score (MESS) at presentation was 6.9 (3 to 10). A total of 15 lower limbs were salvaged and three underwent amputation (two immediate and one delayed). Four patients underwent stabilisation of the fracture by external fixation and 12 with an internal device. A total of 11 patients had damage to multiple arteries and eight had a vein graft. Wound cover was achieved with a pedicled flap in three and a free flap in six. Seven patients developed a wound infection and four developed nonunion requiring further surgery. At a mean follow-up of five years (4.1 to 6.6) the mean visual analogue scale for pain was 64 (10 to 90). Depression and anxiety were common. Activities were limited mainly because of pain, and the MESS was a valid predictor of the functional outcome. Distal tibial fractures had an increased rate of nonunion when associated with posterior tibial artery damage, and seven patients (39%) were not able to return to their previous occupation.

A 2-year experience, management and outcome of 200 clavicle fractures

INTRODUCTION

Clavicle fractures can cause pain and functional impairment if not managed appropriately. This article evaluates the prevalence of clavicular fractures, estimates the number of cases requiring operative treatment, evaluates whether removal of implant is a frequent necessity and compares the final functional outcome of the operative and non-operative groups.

PATIENTS AND METHODS

Between November 2005 and November 2007, patients with clavicular fractures were eligible for participation. Patients below 18 years of age and those with pathological fractures were excluded. Demographic details, mechanism of injury, operative versus non-operative treatment, radiographic classification (Allman system), complications, implant removal and functional outcome using the University of California, Los Angeles (UCLA) shoulder-rating score were documented and analysed.

RESULTS

Out of 16,280 fractures that presented to our University Teaching Hospitals, 200 (1.23%) met the inclusion criteria. As many as 20 patients were lost due to natural attrition. A total of 159 (88.3%) patients were treated non-operatively and 21 (11.7%) patients were operated upon, over half of them for symptomatic non-union. All clavicles united postoperatively. Eighty-one conservatively managed undisplaced medial, middle and lateral end fractures had excellent mean UCLA shoulder scores. A statistical significance in UCLA scores (p<0.05) was noted between the operative and non-operative patient groups in mid-shaft fractures. There was no statistical difference between the operative and non-operative groups in lateral-end fractures. A total of 42.9% required removal of metal implant due to soft tissue irritation with complete resolution of symptoms.

CONCLUSION

The incidence of clavicle fractures was 1.23%. A small number of patients (11.7%) required operative treatment. We recommend surgical management of symptomatic non-union and removal of metal implant for hardware-related irritation.

Management of bone infections in adults: the surgeon's and microbiologist's perspectives

Bone infection in adults is a potentially devastating complication following trauma or surgery. The clinician should diagnose osteomyelitis based on certain clinical manifestations and on laboratory and imaging findings. For pathogen identification, the treating surgeon should take appropriate tissue samples. Close collaboration with microbiologists is of paramount importance to dictate the appropriate duration and type of antibiotics to be administered. Treatment of acute osteomyelitis requires surgical debridement and prolonged course of antibiotics. Debate exists regarding the maintenance or the removal of any internal fixation device. Treatment of chronic osteomyelitis is more complicated. For its eradication the treatment course is often prolonged and frustrating. Based on the current literature an algorithm of treatment for both acute and chronic bone infections is recommended.

Treatment of post-operative infections following proximal femoral fractures: our institutional experience

Proximal femoral fractures (PFFs) are a major health concern in the elderly population. Improvements made in implants and surgical techniques resulted in faster rehabilitation and shorter length of hospital stay. Despite this, the reduced physiological reserve, associated co-morbidities and polypharmacy intake of the elderly population put them at high risk of postoperative complications particularly of infectious origin. Out of 10061 patients with proximal femoral fractures 105 (1.05%) developed surgical site infection; 76 (72%) infections occurred in patients who had sustained intracapsular (IC) fractures with the remaining 29 (28%) infections occurring in patients with extracapsular (EC) neck of femur fractures. The median number of additional surgical debridements was 2 (range 1-7). MRSA was isolated in 49 (47%) of the cases; 38 patients (36%) ultimately underwent a Girdlestone's excisional arthroplasty. Mortality at 30 days and 3 months was 10% and 31%, respectively. It was noted that post-operative hip infection predisposed to a prolonged length of stay in the acute unit and subsequently to a more dependent destination after discharge.

Second hit phenomenon: existing evidence of clinical implications

The last two decades extensive research evidence has been accumulated regarding the pathophysiology of trauma and the sequelae of interventions that follow. Aim of this analysis has been to collect and categorise the existing data on the so-called "second hit" phenomenon that includes the biochemical and physiologic alterations occurring in patients having surgery after major trauma. Articles were extracted from the PubMed database and the retrieved reports were included in the study only if pre-specified eligibility criteria were fulfilled. Moreover, a constructed questionnaire was utilised for quality assessment of the outcomes. Twenty-six articles were eligible for the final analysis, referring to a total of 8262 patients that underwent surgery after major trauma. Sixteen retrospective clinical studies including 7322 patients and 10 prospective ones, including 940 patients were evaluated. Several variables able to reproduce a post-operative second hit were identified; mostly related to pulmonary dysfunction, coagulopathy, fat or pulmonary embolism, and the inflammatory immune system. Indicative conclusions were extracted, as well as the need for further prospective randomised trials. Suggestions on the content and the rationale of future studies are provided.

The surgical treatment of anterior column and anterior wall acetabular fractures

Isolated fractures of the anterior column and anterior wall are a relatively rare subgroup of acetabular fractures. We report our experience of 30 consecutive cases treated over ten years. Open reduction and internal fixation through an ilioinguinal approach was performed for most of these cases (76.7%) and percutaneous techniques were used for the remainder. At a mean follow-up of four years (2 to 6), 26 were available for review. The radiological and functional outcomes were good or excellent in 23 of 30 patients (76.7%) and 22 of 26 patients (84.6%) according to Matta’s radiological criteria and the modified Merlé d’Aubigné score, respectively.

Complications of minor to moderate severity were seen in six of the 30 cases (20%) and none of the patients underwent secondary surgery or replacement of the hip.

The use of the reamer-irrigator-aspirator to harvest mesenchymal stem cells

The scarcity of mesenchymal stem cells (MSCs) in iliac crest bone marrow aspirate (ICBMA), and the expense and time in culturing cells, has led to the search for alternative harvest sites. The reamer-irrigation-aspirator (RIA) provides continuous irrigation and suction during reaming of long bones. The aspirated contents pass via a filter, trapping bony fragments, before moving into a 'waste' bag from which MSCs have been previously isolated. We examined the liquid and solid phases, performed a novel digestion of the solid phase, and made a comparative assessment in terms of number, phenotype and differentiation capacity with matched ICBMA. The solid fraction from the filtrate was digested for 60 minutes at 37° C with collagenase. Enumeration was performed via the colony-forming unit fibroblast (CFU-F) assay. Passage (P2) cells were differentiated towards osteogenic, adipogenic and chondrogenic lineages, and their phenotypes assessed using flow cytometry (CD33, CD34, CD45, CD73, CD90, and CD105). MSCs from the RIA phases were able to differentiate at least as well as those from ICBMA, and all fractions had phenotypes consistent with other established sources. The median number of colonies for the three groups was: ICBMA = 8.5 (2 to 86), RIA-liquid = 19.5 (4 to 90), RIA-solid = 109 (67 to 200) per 200 μl. The mean total yield of cells for the three groups was: ICBMA = 920 (0 to 4275), RIA-liquid = 114,983 (16,500 to 477,750), RIA-solid = 12,785 (7210 to 28 475). The RIA filtrate contains large numbers of MSCs that could potentially be extracted without enzymatic digestion and used for bone repair without prior cell expansion.

Measuring hip outcomes: common scales and checklists

INTRODUCTION

Assessing the outcomes of patients following surgical interventions is a challenging task. Traditionally the end results of joint replacement were based on morbidity/mortality rates and operative complications. The modern approach to outcomes following Orthopaedic surgery has shifted from the success or failure of implants towards patient satisfaction and the quality of life achieved. The aim of this paper was to identify and analyse the common scoring systems present in the medical literature for evaluating outcomes after hip interventions.

METHODS

A pub-med search was performed using terms 'scoring system, functional outcomes, hip joint'. Specific limitations and exclusion criteria were used and the reference lists of the articles included in the study were subjected to further analysis for identification of additional relevant papers.

RESULTS

293 articles were identified of which 40 met the inclusion criteria. The outcome measures were divided into: (i) hip specific outcomes, (ii) disease-specific measures and (iii) generic quality of life measures. Based on our analysis, we would recommend a combination of the hip specific Oxford Hip Score (OHS) and the disease specific WOMAC score. The OHS is quick and easy to complete, has a very high response rate and is free from clinician bias. On the other hand, the majority of hip pathology is related to degenerative disease, thus making the WOMAC the most appropriate measure to use. Where comparison between different conditions is required, then an additional generic quality of life (QOL) score, such as EQ5D, that can enable comparisons in cost-effectiveness term can be used.

CONCLUSION

The ideal outcome measure should be one that is specific for the hip joint, possesses a generic component and takes into consideration co-morbidities and the use of walking aids. Although many validated generic measures exist, additional validation studies, including the OHS, are desirable to evaluate all the hip specific measures of outcome.

The management of post-traumatic osteoarthritis

Posttraumatic arthritis primarily affects younger individuals, leading to reduced physical activity, chronic pain, and prolonged symptomatic treatments. The management of post-traumatic arthritis after fracture, dislocation or ligament rupture continues to be one of the most challenging clinical entities in orthopaedic and trauma surgery. Therapies to address early symptoms include anti-inflammatory agents, pain killers, corticosteroid or hyaluronic acid joint injections but these offer only temporary pain relief with hardly any mid or long term benefit. There are many surgical options for the treatment of posttraumatic arthritis. For the early stages, arthroscopic debridement should be considered. At late stages, corrective osteotomies or arthrodesis are strongly recommended for the young patients. In older patients arthroplasties remain the treatment of choice.

Biomechanical aspects of pelvic ring reconstruction techniques: Evidence today

Despite the remarkable advances achieved within the boundaries of the new discipline of Pelvic surgery, pelvic ring disruptions remain challenging and complex problems in orthopaedics. The long-term complications related to reconstruction techniques of these injuries have motivated researchers and surgeons to explore various alternative treatment modalities. Several biomechanical studies have addressed these issues. We performed a medline search including studies published during the last 30 years. Our search yielded 114 studies, 39 of which met the pre-specified inclusion criteria and were further critically analysed and discussed regarding the biomechanical aspects of pelvic ring reconstruction techniques. Based on observational approach and evaluation of the studies specific keypoints are highlighted comprising the clinical translation of the biomechanical supported findings.

The systemic inflammatory response following femoral canal reaming using the reamer-irrigator-aspirator (RIA) device

We evaluated the peripheral release of inflammatory mediators after femoral fracture and subsequent intramedullary reaming using the RIA reamers. IL-6 was elevated after trauma, and reaming with RIA induced a measurable second hit response. However, despite a higher ISS, the levels of IL-6 in the RIA group were similar to the levels measured in a group of patients where reaming of the femoral canal was performed using conventional reamers. There was one death related to fat embolism syndrome in the conventional reamers group. However, the overall incidence of complications was low and similar between the 2 groups of studied patients. In polytrauma patients, large scale studies are desirable to evaluate further the immuno-inflammatory response using the RIA reamers prior to the instrumentation of the femoral canal.

Articular step-off and risk of post-traumatic osteoarthritis. Evidence today

The goal of treatment in intra-articular fractures is to obtain anatomical restoration of the articular surface and stable internal fixation. Studies have attempted to specify how accurately an articular fracture needs to be reduced to minimise the chances of a poor clinical outcome. In this study, the current evidence with regard to articular step-offs and risk of post-traumatic osteoarthritis (POA) is evaluated. A literature review based on pre-specified criteria, revealed 36 articles for critical analysis related to intra-articular injuries of distal radius, acetabulum, distal femur and tibial plateau.In the distal radius, step-offs and gaps detected with precise measurement techniques have been correlated with a higher incidence of radiographic POA, but in the second 5 years after injury, a negative clinical impact of these radiographic changes has not been convincingly demonstrated. Restoring the superior weight-bearing dome of the acetabulum to its pre-injury morphology decreases POA and improves patient outcomes. Involvement of the posterior wall, however, seems to bean adverse prognostic sign. This effect may be independent of articular reduction. In the tibial plateau, articular incongruities appear to be well tolerated, and factors only partially related to articular reduction are more important in determining outcome than articular step-off alone;these include joint stability, retention of the meniscus, and coronal alignment. Based on observational approach and evaluation of the studies, factors other than just the extent of articular displacement affect the management of articular fractures. Different joints and even different areas of the same joint appear to have different tolerances for post-traumatic articular step-offs.