Small hamstring autograft is defined by a cut-off diameter of 7 mm and not recommended with allograft augmentation in single-bundle ACL reconstruction

Is There a Hamstring Autograft Diameter Threshold for Anterior Cruciate Ligament Reconstruction?

Background

Hamstring autograft (HA) is a popular graft for anterior cruciate ligament reconstruction (ACLR). Although multiple studies have demonstrated that increasing the graft diameter decreases the risk of failure or revision surgery, the exact minimum graft diameter remains controversial, with the unofficial standard being 8 mm.

Purpose

To evaluate the risk of aseptic revision after HA-ACLR in patients aged ≤25 years, comparing graft diameters of (1) <8 versus ≥8 mm and (2) ≤8 versus >8 mm. A secondary purpose was to determine whether there was a threshold for graft diameter that best identifies patients at a higher risk of aseptic revision.

Study Design

Cohort study; Level of evidence, 3.

Methods

A total of 5972 primary HA-ACLRs were identified using data from the ACLR registry of Kaiser Permanente. Propensity score-weighted Cox regression was used to evaluate revision risk for HAs with <8 versus ≥8 mm diameter and with ≤8 versus >8 mm diameter. An effect plot and receiver operating characteristic curve and area under the curve (AUC) analysis were also created to model the probability of revision based on the HA diameter to determine whether there was a specific diameter threshold to minimize revision risk.

Results

In the propensity score-weighted Cox model, <8 mm autografts had a higher risk of aseptic revision compared with ≥8 mm autografts (hazard ratio [HR], 1.32 [95% CI, 1.01-1.71]; P = .039); a higher risk was also observed when comparing ≤8 to >8 mm grafts (HR, 1.32 [95% CI, 1.06-1.66]; P = .015). In receiver operating characteristic analysis, an HA diameter that best identified patients at a higher risk for revision after ACLR could not be identified (AUC, 0.56). However, the adjusted effect plot showed a direct decrease in revision risk with each increasing increment in graft size.

Conclusion

A 32% higher risk of revision was observed in smaller graft diameter groups (<8 or ≤8 mm) compared with larger graft sizes (≥8 or >8 mm), regardless of specific diameter cutoff. In a cohort of almost 6000 HA-ACLR, the authors were unable to determine a definitive minimum graft diameter threshold that should be used.

Survival Analysis of ACL Graft and Contralateral ACL Ruptures in Patients Younger Than 18 Years

Background

Although high rates of graft and contralateral anterior cruciate ligament (ACL) ruptures have been reported in younger patients after ACL reconstruction (ACLR), recent evidence suggests that previously reported crude event rates underestimate the actual event risk.

Purpose

To report rates of graft and contralateral ACL rupture after ACLR in a large series of younger patients using survival analysis.

Study Design

Cohort study; Level of evidence, 3.

Methods

Patients aged <18 years at the time of primary ACLR were identified from a single-surgeon database over 12 years ending January 2018. Patients with a previous contralateral ACL rupture or bilateral ACL ruptures were excluded. Overall, 388 patients (204 males, 184 females) were included in the final dataset. Bespoke survey data and clinic follow-up data were used to record graft rupture and contralateral ACL rupture events. Rates of graft and contralateral ACL rupture were calculated using Kaplan-Meier survival analysis. Log-rank tests were used to compare survival functions between several subgroups.

Results

According to Kaplan-Meier survival analysis, the cumulative rates at 2, 5, and 10 years for graft rupture were 11%, 17%, and 22%, and the cumulative rates for contralateral ACL injury were 7%, 19%, and 33%. Males had significantly greater rates of graft rupture than females throughout a 10-year follow-up period (P < .001). Contralateral ACL rupture survival functions were not significantly different between the sexes, although rates were higher in females until 5 years postoperatively, after which contralateral ACL ruptures increased in males, with a cumulative rate of 39% at 10 years compared with 29% for females. Survival rates did not vary between different age groups (<16 vs ≥16 years), but females with a graft diameter of <7 mm on the femoral side had significantly greater graft rupture rates than females with grafts ≥7 mm (P = .04).

Conclusion

The present study is one of the largest consecutive series of younger patients, with one of the longest follow-up periods, reporting a high risk for a second ACL injury. Over time, the cumulative risk for contralateral ACL rupture was higher than for graft rupture. In males, the risk for contralateral ACL rupture continued to increase after 5 years.

Comparison of Medial Hamstring Length Between Harvesting Methods Using Anterior and Posterior Approaches

Background

Medial hamstring tendons harvesting can be performed through either an anterior or a posterior approach. It has been suggested that using a posterior approach may result in a longer length of harvestable tendon compared with an anterior approach.

Hypothesis

There would be no difference in the length of the harvestable tendon between the anterior and posterior approaches.

Study Design

Case-control study; Level of evidence, 3.

Methods

A retrospective study was conducted at the primary institution using prospectively collected data from skeletally mature Chinese patients who underwent medial hamstring tendons harvesting between January 2008 and December 2021. Patients were excluded if they had experienced graft harvesting complication or if only 1 medial hamstring tendon was harvested. One-to-one exact matching was performed between the 2 approaches based on body height and sex. The outcome assessed was the length of the harvested tendon.

Results

A total of 536 patients underwent medial hamstring tendons harvesting using an anterior approach, while 58 underwent it using a posterior approach, all of whom met the inclusion criteria. A group of 54 matched pairs were identified. The length of the harvested semitendinosus tendon was 263 ± 29 mm in the anterior approach and 256 ± 28 mm in the posterior approach (P = .09; Student t test). The mean difference in the length of the harvested semitendinosus tendon between the 2 approaches was 7 mm. The length of the gracilis tendon was 226 ± 29 mm and 223 ± 29 mm in the anterior approach and the posterior approach, respectively (P = .30; Student t test).

Conclusion

In a retrospective 1:1 case-control study with exact matching of body height and sex, it was found that there was no difference in the length of the semitendinosus tendon and the gracilis tendon harvested using a posterior approach compared with an anterior approach. The mean difference between the 2 approaches was 7 mm for the semitendinosus tendon and 3 mm for the gracilis tendon, respectively.

Tendon cross-sectional area on magnetic resonance imaging and anthropometric characteristics can be used to predict insufficient four-strand hamstring autograft diameter in anterior cruciate ligament reconstruction: A systematic review

PURPOSE

To evaluate the utility of semitendinosus tendon (ST) and gracilis tendon (GT) cross-sectional area (CSA) on magnetic resonance imaging (MRI) and anthropometric characteristics in preoperative estimation of graft diameter in patients undergoing anterior cruciate ligament reconstruction (ACLR) with four-strand hamstring autografts.

METHODS

Three databases were searched on 29 August 2023. The authors adhered to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) and R-AMSTAR (Revised Assessment of Multiple Systematic Review) guidelines and the Cochrane Handbook for Systematic Reviews of Interventions. Data on demographics, anthropometric characteristics, imaging techniques, tendon CSA, correlation coefficients, sensitivities, specificities, regression models and cutoffs for predicting intraoperative graft diameters above 8 mm were recorded.

RESULTS

Forty-six studies comprising 4140 patients were included. Twelve of 19 (63.2%) studies reporting on ST + GT CSA found a moderate to very high correlation with intraoperative graft diameter. Five of 10 (50%) and one of seven (14.3%) studies reporting on ST CSA and GT CSA, respectively, found a moderate to high correlation with intraoperative graft diameter. Cutoffs of ST + GT CSA for predicting graft diameters above 8 mm ranged from 15.8 to 31.2 mm2. Nine of 35 (25.7%) studies that reported on height found a moderate to very high correlation with graft diameter. Seven of 33 (21.2%) studies reporting on weight found a moderate correlation with graft diameter.

CONCLUSION

Of the MRI parameters assessed, ST + GT CSA was the most reliable predictor of graft diameter. However, cutoffs, sensitivities, and specificities for predicting diameters above 8 mm were highly variable. Anthropometric characteristics in general were less predictive of graft diameter than MRI parameters. This information can be used by clinicians to predict patients at risk for ACLR failure due to insufficient graft size.

LEVEL OF EVIDENCE

Level IV.

Outcomes of "Over the Top" Anterior Cruciate Ligament Reconstruction Associated with a Lateral Extra-Articular Tenodesis in Children

(1) Purpose: The incidence of anterior cruciate ligament (ACL) ruptures in children and adolescents has considerably increased during the last decades due to higher levels of competitive athletic activity, and early sport specialization and professionalization. Contemporary ACL reconstruction techniques have recently been subject to renewed interest in this population. The objective of this study is to report the short- and mid-term results of our physis-sparing ACL reconstruction technique using an "over the top" technique associated with a modified Lemaire procedure. (2) Methods: A retrospective series of 12 junior soccer players who presented to our clinic with a torn ACL between January 2019 and September 2021 was reviewed. The inclusion criteria were patients under 15 years with open tibial and femoral physes, with a stable contralateral knee, a minimum follow-up of 6 months, and a time frame from injury to surgery of <3 months. Patients with previous knee surgery, structural concomitant injuries, muscular, neurological, or vascular abnormalities, or hypersensitivity to metal alloys were excluded. The functional evaluation was performed using the International Knee Documentation Committee (IKDC) rating, Lysholm score, and Tegner activity level. Moreover, clinical and radiological assessments were also performed, including KT-1000 and knee X-rays. (3) Results: We identified 1 female and 11 male patients with ACL tears, with a mean age of 13.17 ± 0.9 months. Concomitant injuries include isolated vertical and bucket-handle tears of the medial meniscus, lateral meniscus tears, bilateral tear of both menisci. The mean follow-up time was 26 ± 12.6 months. The average IKDC, Lysholm and Tegner scores were 93.29 ± 11.04, 95.08 ± 13.2 and 9 ± 0.0 points, respectively. The average KT-1000 score of the participants was 0.96 ± 1.6 points. None of the included patients reported post-surgical complications or required additional surgeries. (4) Conclusions: Our novel ACL reconstruction with LET technique is a safe procedure that resulted in good clinical outcomes, lower failure rate and return to sports in skeletally immature patients.

Estimation of Diameter of Quadrupled Hamstring Graft for ACL Reconstruction using Pre-operative MRI Measurement as a Predictive Tool

Introduction

The diameter of the quadrupled Hamstring graft plays a significant role in the incidence of graft failures for ACL reconstruction. The ability to predict the graft size pre-operatively can prepare the surgeon for alternatives in the event of an inadequate graft diameter.

Materials and methods

We retrospectively measured the diameter of the Semitendinosus tendon (ST) on the MRI in all patients who underwent arthroscopic ACL reconstruction using quadrupled Semitendinosus as their graft. We also estimated any correlation between various anthropometric data with pre-operative MRI based Cross Sectional Area (CSA) of the Hamstring tendon and final graft diameter in the South Asian population. The patients were included from Jan 2018 - Dec 2020.

Results

The minimum CSA of ST to predict an eventual graft diameter of 7.5mm was 10.7mm2. The MRI based cross-sectional area measurement showed moderate correlation with the intra-operative graft diameter obtained. (r=0.62, p<0.001). The intra-class correlation coefficient between the radiologist and the surgeon was 0.82, 95% CI (0.57, 0.92) and a p-value <0.001.

Conclusion

Pre-operative MRI can be a useful tool to predict the graft diameter. This coupled with the anthropometric data of the patient can be used as an adjunct to estimate the probable graft diameter. Thus, the surgeon can be better prepared for the surgery and can seek alternate graft options if the graft size is deemed inadequate pre-operatively.

Estimation Failure Risk by 0.5-mm Differences in Autologous Hamstring Graft Diameter in Anterior Cruciate Ligament Reconstruction: A Meta-analysis

BACKGROUND

Because grafts are made in 0.5-mm increments clinically for anterior cruciate ligament (ACL) reconstruction, it is important to clarify how the failure rate decreases as the diameter increases. Moreover, it is important to know whether even a slight increase in the graft diameter decreases the risk of failure.

HYPOTHESIS

The risk of failure decreases significantly with each 0.5-mm increase in hamstring graft diameter.

STUDY DESIGN

Meta-analysis; Level of evidence, 4.

METHODS

The systematic review and meta-analysis have estimated the diameter-specific failure risk for each 0.5-mm increase in ACL reconstruction using autologous hamstring grafts. We searched for studies describing the relationship between graft diameter and failure rate published before December 1, 2021, in leading databases, such as PubMed, EMBASE, Cochrane Library, and Web of Science, according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. We included studies using single-bundle autologous hamstring grafts to investigate the relationship between failure rate and graft diameter of 0.5-mm intervals with >1-year follow-up. Then, we calculated the failure risk caused by 0.5-mm differences in autologous hamstring graft diameter. Assuming Poisson distribution for the statistical model, we employed an extended linear mixed-effects model in the meta-analyses.

RESULTS

Five studies containing 19,333 cases were eligible. The meta-analysis revealed that the estimated value of the coefficient of diameter in the Poisson model was -0.2357 with a 95% CI of -0.2743 to -0.1971 (P < .0001). With every 1.0-mm increase in diameter, the failure rate decreased by 0.79 (0.76-0.82) times. In contrast, the failure rate increased by 1.27 (1.22-1.32) times for each 1.0-mm decrease in diameter. The failure rate significantly decreased with each 0.5-mm increase in graft diameter in the range of <7.0 to >9.0 mm from 3.63% to 1.79%.

CONCLUSION

The risk of failure decreased correspondingly with each 0.5-mm increase in graft diameter in the range of <7.0 to >9.0 mm. Failure is multifactorial; however, increasing the graft diameter as much as possible to match each patient's anatomic space without overstuffing is an effective precaution that surgeons can take to reduce failures.

A Web-Based Prediction Tool to Improve Identification of Patients With Undersized Hamstring Tendon Autograft in Anterior Cruciate Ligament Reconstruction

BACKGROUND

An undersized hamstring tendon (HT) autograft is significantly associated with a higher graft failure rate in anterior cruciate ligament reconstruction (ACLR) surgery. The ability to accurately predict inadequate HT graft diameter is critical, as it could assist surgeons in making better graft choices and surgical plans.

PURPOSE

To develop a web-based prediction tool to better assess the size of HT autograft and to help clinicians accurately identify patients with potentially undersized HT grafts in order to make appropriate clinical decisions.

STUDY DESIGN

Cross-sectional study; Level of evidence, 3.

METHODS

A total of 588 patients who received primary arthroscopic single-bundle ACLR surgery with gracilis tendon (GT) and semitendinosus tendon (ST) autograft were retrospectively reviewed. According to the size of 4-strand HT graft, patients were divided into diameter ≥8 mm and <8 mm groups. The least absolute shrinkage and selection operator method and logistic regression were used to identify the independent factors associated with HT graft diameter and establish the models. The prediction performance of the model was evaluated by concordance index and calibration combined with external validation. The diagnostic performance of the prediction model was assessed by sensitivity, specificity, predictive values, and likelihood ratios. Decision curve analysis was used to evaluate the clinical utility of the model.

RESULTS

Among the numerous indicators, sex, weight, height, thigh length, and ST-GT diameter (measured on plane 1 of a magnetic resonance imaging scan) were identified to be highly correlated predictors that could provide satisfactory prediction performance in determining the HT graft diameter. Based on these predictors, a prediction model named the HTD model was developed with satisfactory discrimination (concordance index, 0.932) and calibration (mean absolute error, 0.039). When the probability calculated by the HTD model was >65%, the sensitivity and specificity of predicting 4-strand HT graft diameter ≥8 mm were 86.7% and 90.2%, respectively.

CONCLUSION

As a useful supplementary prediction tool, the HTD model could accurately predict the diameter of HT autograft during preoperative planning.

Relationship between graft failure following anterior cruciate ligament reconstruction and hamstring autograft diameter

With the increase in the number of individuals participating in sports, anterior cruciate ligament (ACL) injuries are also increasing gradually and the number of patients requiring surgical treatment is increasing in parallel. The primary aim of this study was to investigate the relationship between the need for revision surgery and graft diameter following primary ACL reconstruction (ACLR) using hamstring autografts. The secondary aim of the study was to clarify relationships between anthropometric characteristics and graft diameter. Patients who underwent ACLR with hamstring autografts were included in this retrospective study. The age, body mass index, height, and weight of all patients were recorded preoperatively and the diameters of the grafts were recorded during surgery. The relationship between revision rate and graft diameter and the relationships between anthropometric measurements and graft diameter were investigated for these patients at least one year after surgery. This study included 58 people with graft diameters of ≤7 mm and 261 people with graft diameters of >7 mm. A statistically significant difference was found between the graft diameters of the group that needed revision surgery and the group that did not (p<0.001). A positive relationship was also found between the patient's height and graft diameter (r=0.168). In this study, it was found that the risk of ACL revision surgery increased by 5.5 times among patients with graft diameters of ≤7 mm. The positive relationship between the patient's height and graft diameter can make a significant difference in terms of the need for revision surgery.

Hamstring graft diameter above 7 mm has a lower risk of failure following anterior cruciate ligament reconstruction

PURPOSE

Multi-stranded hamstring-tendon autografts have been widely used for anterior cruciate ligament reconstruction (ACLR) surgeries. Recently, smaller diameter hamstring autografts have been linked with the risk of failure or graft rupture. However, there is limited evidence concerning the optimal diameter of the hamstring autografts for ACLR. The current systematic review and meta-analysis analysed the association of ACLR failure with the diameter of hamstring autografts.

METHODS

A systematic search of three major scientific databases (Pubmed, EMBASE, and Cochrane library) was conducted to identify studies that presented ACLR failure-related outcomes with different diameters of hamstring autografts. The pooled data from the included studies were analysed to investigate the association between ACLR failure and the cut-off diameters of 6, 7, 8, and 9 mm. Subgroup analyses based on the level of evidence and follow-up duration were also performed at each cut-off diameter.

RESULTS

Of the 2282 studies screened, 16 reported failure rates with hamstring autografts of different diameters, 15 of which were included in the meta-analysis. A graft diameter ≥ 7 mm was associated with significantly lower ACLR failure rates than a graft diameter < 7 mm (p = 0.005), based on pooled data of 19,799 cases. Age < 20 years and higher physical activity were associated with significantly higher ACLR failure rates.

CONCLUSION

The current systematic review suggests that the hamstring graft diameter for ACLR should be more than 7 mm considering the significantly higher failure rates with graft diameters less than 7 mm.

LEVEL OF EVIDENCE

Level IV.

The Femoral Tunnel Drilling Angle at 45° Coronal and 45° Sagittal Provided the Lowest Peak Stress and Strain on the Bone Tunnels and Anterior Cruciate Ligament Graft

Purpose: The aims of this study were to 1) investigate the effects of femoral drilling angle in coronal and sagittal planes on the stress and strain distribution around the femoral and tibial tunnel entrance and the stress distribution on the graft, following anterior cruciate ligament reconstruction (ACLR), 2) identify the optimal femoral drilling angle to reduce the risk of the tunnel enlargement and graft failure.

Methods: A validated three-dimensional (3D) finite element model of a healthy right cadaveric knee was used to simulate an anatomic ACLR with the anteromedial (AM) portal technique. Combined loading of 103.0 N anterior tibial load, 7.5 Nm internal rotation moment, and 6.9 Nm valgus moment during normal human walking at joint flexion of 20° was applied to the ACLR knee models using different tunnel angles (30°/45°/60° and 45°/60° in the coronal and sagittal planes, respectively). The distribution of von Mises stress and strain around the tunnel entrances and the graft was calculated and compared among the different finite element ACLR models with varying femoral drilling angles.

Results: With an increasing coronal obliquity drilling angle (30° to 60°), the peak stress and maximum strain on the femoral and tibial tunnel decreased from 30° to 45° and increased from 45° to 60°, respectively. With an increasing sagittal obliquity drilling angle (45° to 60°), the peak stress and the maximum strain on the bone tunnels increased. The lowest peak stress and maximum strain at the ACL tunnels were observed at 45° coronal/45° sagittal drilling angle (7.5 MPa and 7,568.3 μ-strain at the femoral tunnel entrance, and 4.0 MPa and 4,128.7 μ-strain at the tibial tunnel entrance). The lowest peak stress on the ACL graft occurred at 45° coronal/45° sagittal (27.8 MPa) drilling angle.

Conclusions: The femoral tunnel drilling angle could affect both the stress and strain distribution on the femoral tunnel, tibial tunnel, and graft. A femoral tunnel drilling angle of 45° coronal/ 45° sagittal demonstrated the lowest peak stress, maximum strain on the femoral and tibial tunnel entrance, and the lowest peak stress on the ACL graft.

Comparing the Stiffness of Peroneus Longus Tendon Versus Hamstrings in Anterior Cruciate Ligament Reconstruction: A Biomechanical Study

Background: The use of autografts originating from either hamstring tendons or peroneus longus tendons is a surgical option in anterior cruciate ligament (ACL) reconstruction. Objectives: This research aimed to compare the tensile strength between the hamstring tendon and the peroneus longus tendon in ACL reconstruction. The hypothesis of this study was: Peroneus longus grafts have tensile strength equal to hamstring grafts based on living donor patients. Methods: This cross-sectional study was a biomechanical study examining means and standard deviations (SD) by comparing the tensile strength of peroneus longus tendons and hamstring tendons when used as autograft donors in ACL reconstruction. Results: In this study, 51 patients with reconstructive ACL were enrolled. The mean diameter of the hamstring tendon was 7.86 with SD ± 0.69, while the mean diameter of peroneus longus tendon was 7.67 with SD ± 0.63. The mean diameter of the peroneus longus graft was not significantly different. The mean displacement on the hamstring tendon was 2.44 with SD ± 0.42, while the peroneus longus tendon was 2.06 with SD ± 0.14. The peroneus longus tendon had significantly more tensile strength compared to the hamstring tendon. Conclusions: Diameter of the peroneus longus graft was not significantly different from the hamstring graft. However, the peroneus longus graft had more tensile strength than the hamstring graft based on living donor patients.

[Short-term effectiveness of arthroscopic single bundle four-strand reconstruction using autologous semitendinosus tendon and anterior half of peroneus longus tendon for posterior cruciate ligament injuries]

OBJECTIVE

To investigate the short-term effectiveness of arthroscopic single bundle four-strand reconstruction using autologous semitendinosus tendon and anterior half of peroneus longus tendon for posterior cruciate ligament (PCL) injuries.

METHODS

A clinical data of 30 patients with PCL injury, who were admitted between December 2015 and September 2018 and met the selection criteria, was retrospectively analyzed. All patients were treated with arthroscopic single bundle four-strand reconstruction using autologous semitendinosus tendon and anterior half of peroneus longus tendon and TightRope technique. Among them, 19 were male and 11 were female, aged 17-48 years (mean, 28.2 years). The PCL injury was caused by traffic accident in 8 cases, sport in 14 cases, falling and bruising by a heavy objective in 5 cases, and other injuries in 3 cases. The interval between injury and operation was 10-90 days (mean, 39.3 days). The PCL injury was rated as grade Ⅱ in 6 cases and grade Ⅲ in 24 cases. The posterior drawer test was positive in 26 cases and the inverse Lachman test was positive in 24 cases. The International Knee Documentation Committee (IKDC) score was 61.37±8.49, and the objective IKDC ligament grading was near normal in 2 cases, abnormal in 8 cases, and significantly abnormal in 20 cases. The modified Lysholm knee score was 62.20±5.67. The knee range of motion (ROM) was (101.83±8.15) °.

RESULTS

The operative time ranged from 70 to 110 minutes (mean, 79.7 minutes). All incisions healed by first intetion. All patients were followed up 12-24 months (mean, 19.0 months). There were 3 cases of deep vein thrombosis in the lower extremity after operation, and 1 case of approximately 10° limitation of knee extension. At last follow-up, the posterior drawer test was positive in 2 cases and the inverse Lachman test was positive in 1 case, with significant differences compared with the preoperative period ( χ 2=38.571, P=0.000; χ 2=36.274, P=0.000). The IKDC score was 84.67±3.67, and the objective IKDC ligament grading was normal in 16 cases, nearly normal in 10 cases, abnormal in 3 cases, and significantly abnormal in 1 case; the modified Lysholm knee score was 90.37±4.49; all of the above indexes were significantly better than preoperative ones, and the differences were significant ( t=-12.387, P=0.000; Z=-2.810, P=0.005; t=-22.865, P=0.000). Knee ROM was (88.33±9.86)° at 1 month after operation and reached (113.33±13.48)° at last follow-up, showing significant differences between pre- and post-operation ( P<0.05). MRI re-examination showed that the form and position of reconstructed PCL were satisfactory.

CONCLUSION

It can obtain good short-term effectiveness for PCL injuries by arthroscopic single bundle four-strand reconstruction using autologous semitendinosus tendon and anterior half of peroneus longus tendon, which has the advantages of reliable surgical approach, safe operation, and precise effectiveness.