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Blood Flow Restriction Training preserves knee flexion and extension torque following anterior cruciate ligament reconstruction: A systematic review
∗Corresponding author: Joshua M. Spada. Spadaj42@rowan.edu
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
There is inconsistency in the literature comparing the outcomes of Blood Flow Restriction Training versus Traditional Post-Operative Rehabilitation after anterior cruciate ligament reconstruction.
This study aimed to determine if Blood Flow Restriction Training can limit the loss of knee extension and knee flexion muscle torque during early recovery from anterior cruciate ligament reconstruction better than Traditional Post-Operative Rehabilitation.
Three databases (PubMed, Embase, and Scopus) were searched for level 1 randomized controlled trials pertaining to Blood Flow Restriction Training after anterior cruciate ligament reconstruction. To maximize consistency among included studies, only studies which used knee flexion and knee extension muscle torque as the primary outcome measures were included. Search terms included “cruciate + occlusion”, “cruciate + blood flow restriction”, and “cruciate + occlusion training”.
Two level 1 trials with training protocols of 8 and 16 weeks yielded isokinetic knee flexion torque data in support of Blood Flow Restriction Training. Both trials demonstrated that Blood Flow Restriction Training also yielded significantly increased isokinetic knee extension torque compared to control groups.
The highest-quality level 1 trials evaluating knee extension and knee extension strength via isokinetic torque agree that Blood Flow Restriction Training limits post-operative losses of knee flexion and extension strength. No adverse events were reported in either study. Except for patients of whom Blood Flow Restriction is contraindicated, clinicians may consider utilizing Blood Flow Restriction Training from week 2 of the post-operative period through the conclusion of outpatient rehabilitation using low intensities, multiple times per week; however, further studies comparing Blood Flow Restriction Training protocols are necessary before an optimal protocol could be confidently recommended.
Abstract
Highlights
•The highest quality studies support use of Blood Flow Restriction Training after anterior cruciate ligament reconstruction.•Blood Flow Restriction Training curbs loss of knee extension/flexion torque after anterior cruciate ligament reconstruction.•Clinicians may consider utilizing Blood Flow Restriction Training starting from week 2 of the post-operative period.•Clinicians may consider utilization of Blood Flow Restriction Training at low intensities, multiple times per week.
Keywords
Anterior cruciate ligament
Blood flow restriction
Knee
Reconstruction
Occlusion
Rehabilitation
Sports medicine
1 Introduction
The anterior cruciate ligament (ACL) possesses a tensile strength of up to 1725±270 N, however, rupture and subsequent reconstruction (ACLR) is very common in the United States and across the world.1–4 Surgery is followed by a post-operative course which includes 6–9 months of extensive physical rehabilitation with full return to prior level of function at 9–12 months.5,6 Early limited weight bearing with axillary crutches is encouraged, however, return to full weight bearing will occur progressively over the first two weeks of post-operative rehabilitation as tolerated by the patient. This is due to routine post-surgical limitations such as pain, lingering anesthetic nerve block medication, and swelling.5,6 Despite these limitations, it is crucial that early rehabilitation is initiated shortly following surgery to limit loss of knee flexion (KF) and knee extension (KE) strength, maximizing long term tibiofemoral joint stability and functional outcomes.5–7 Early Traditional Post-Operative Training (TPR) focuses on restoring passive range of motion of the tibiofemoral joint, passive patellar glides, edema control, hip girdle and ankle strengthening, and progressive weight bearing on the operative leg.5,6 Isometric strengthening of the hamstrings and quadriceps is the mainstay of early post-operative strengthening.5,8 Open-chain exercises are limited to the safe range of 40–90° of KF while closed-chain activities are limited to 0–90° KF to limit strain placed on the reconstructed ACL.5
In terms of KF and KE strengthening progression, a dilemma emerges for the patient, physician, and physical therapist as the patient continues treatment. The best way to limit loss of muscle strength is progressive overload training, however, these activities cannot be performed post-operatively without risk to the reconstructed knee.9,10 To meet this challenge, a new protocol must be created to better limit loss of KF and KE strength following ACLR while protecting the surgical site. Blood Flow Restriction Training (BFR), used as an alternative to TPR is a potential solution. BFR stresses the target musculature without placing added mechanical stress on the joint.11,12 This is accomplished by impairing the blood supply to the target muscle for short periods through the application of an air-filled bladder or cuff to restrict the venous drainage and/or arterial supply.12–15 BFR allows the patient to stress the surgically repaired limb without the added reinjury potential of traditional progressive overload training.12,16
Previous studies concerning BFR after ACLR varied in their methods, outcome measures, results, and levels of evidence, which has led to inconsistent and confusing recommendations to clinical practice.17,18 Several level one RCTs have evaluated cross-sectional area (CSA) and/or limb girth, however these measures do not account for variability among individuals. They also do not directly measure KF or KE strength, particularly of the quadriceps and hamstrings, nor do they accurately assess stability of the joint. CSA and limb girth are also affected by acute variables such as edema, particularly in the immediate post-operative period.17,18 Other studies such as Wangle et al.19 included multiple outcome measures including CSA, pain (measured via the Knee Injury and Osteoarthritis Outcome Score), rating of perceived exertion, and KE/KF torque. To preserve generalizability and maintain consistency among the included studies, this systematic review instead focused solely on KE and KF torque as a measure of preservation of muscle strength status-post ACLR. The goal of this systematic review was to clearly and concisely determine if BFR can limit loss of muscle strength during early recovery from ACLR better than TPR. The authors hypothesized that BFR preserves KF and KE strength better than TPR while maintaining the integrity of the reconstructed ACL.
2 Methodology
2.1 Study design
A systematic review of the literature was conducted as per PRISMA Guidelines.20 The PICO framework was used to create the research question addressed in this systematic review.21 This study aimed to answer the following question: for those aged 20–55 years old with no major comorbid conditions, compared to TPR, is BFR better at preserving quadriceps and hamstrings muscle torque following unilateral ACLR?
2.2 Search methodology
A literature search was completed in March 2022 across three online databases: PubMed, Scopus, and EMBASE. Search terms included “cruciate + occlusion, “cruciate + blood flow restriction”, and “cruciate + occlusion training”. A total of 335 hits were generated using this study's search strategy (Table 1). The analysis of these 335 hits is described later in Table 2, a PRISMA flow chart of the search methodology used in this systematic review.
2.3 Eligibility criteria
The eligibility criteria for this systematic review were stringent to maximize homogeneity which has been missing in many similar studies in the past regarding this topic. For this review, only level one randomized controlled trials (RCTs) were included. To maximize consistency among studies, this review focuses only on studies which used KF and KE measured using isokinetic device as the primary outcome measure. Included studies were written in English and were published between 2000 and 2022. Studies which involved both males and females were included. Age of subjects was limited to between 18 and 55 years old. Both included studies were also evaluated using the GRADE approach.22 This set of criteria evaluates the quality of included studies and ultimately the strength of the recommendations of this systematic review as a whole. Studies are assigned an initial GRADE of high, moderate, low, or very low based on design. The presence of 5 factors within the included studies are assessed. These factors include high risk of bias, inconsistency of results, indirectness of evidence, imprecision, and publication bias.22 RCTs are initially assigned a grade of high quality, however, presence of any of the previously described factors reduces the GRADE 1 level per criterion discovered in the text (Table 4). Lastly, studies which included subjects who suffered from pathologies of the ipsilateral hip and ankle were also excluded.
2.4 Risk of bias assessment
Included studies were evaluated for bias using the Cochrane Risk of Bias tool.23 Six categories of bias assessment were utilized from the Cochrane Risk of Bias tool: random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, and selective reporting. Bias in each category was classified as high, low, or unclear (Table 3).
2.5 Data collection
For all included studies, statistical significance was defined as P < 0.05. One of the included studies provided means, confidence intervals, and P values directly in the text while the second study only provided P values. The authors contacted corresponding authors of RCT for unreported data variables, however, corresponding authors did not respond. To calculate means and confidence intervals, two independent reviewers (** and **) utilized ImageJ software (National Institutes of Health, Bethesda, MD) to calculate data points from bar graph figures with an inter-rater reliability of 0.998. This was done by scaling the distance relative to y-axis values in each figure, and then measuring the distance between the x-axis and the desired mean.
2.6 Data analysis
Due to small number of studies which fit our stringent inclusion criteria and the low n value of the included studies, meta-analysis was not performed at this time. Statistical significance was defined as P < 0.05 in all accepted studies. Means and confidence intervals were extrapolated directly from one included study while data from the second included study was calculated by two independent reviewers as described previously in this study. Data was compiled for comparison and conclusions. Conclusions were made by comparing means to established criteria for statistical significance of P < 0.05. Included studies, extrapolated data, and conclusions are summarized on Table 5.
3 Results
3.1 Search results
Three online databases, PubMed, Scopus, and EMBASE were used to search for studies pertaining to our topic using search terms “cruciate + occlusion, “cruciate + blood flow restriction”, and “cruciate + occlusion training”. 335 hits were generated and reduced to 223 after 102 duplicates were eliminated. Next, 145 non-RCTs were eliminated leaving 88 remaining studies. 80 were then eliminated as they did not study the use of BFR training status-post ACLR. The full texts of the remaining 8 studies were reviewed. Six studies were eliminated due to inconsistency with inclusion criteria. Two eligible studies remained and were included in this systematic review.
3.2 Patient demographics
Demographics between groups were similar in both included studies (Table 5). Ohta et al. included a total of 44 subjects, 25 males and 19 females. Average age was 29 years while age ranged from 18 to 54 years old. The average weight of subjects was 64 kg.24 Hughes et al. included 24 total subjects, 17 males and 7 females. Average age was also 29 years and ranged from 22 to 36 years. Average weight was not listed, however, average BMI of subjects was 25.9 kg/m2.25
3.3 Risk of bias assessment
Study bias was reported for all included studies via the Cochrane Risk of Bias Tool (Table 3). Both studies showed a low risk of bias for random sequence generation and incomplete outcome data by providing adequate randomization with no significant demographic differences between groups and by providing comprehensive statistical results. Both studies lacked sufficient blinding and thus had a high risk of bias for blinding of participants and personnel. Hughes et al. had low risk of bias for allocation concealment, blinding of outcome assessment, and selective reporting by utilizing opaque envelopes for group assignment, a blinded assessor for statistical analysis, and inclusion of a flow chart describing participant dropout, respectively. In contrast, Ohta et al. had high risk of bias for selective reporting by failing to report the sample size of participants throughout the steps of study enrollment. Ohta et al. also had an unclear risk of bias for allocation concealment and blinding of outcome assessment.
3.4 Quality of evidence
Study quality was evaluated for all included RCTs using the GRADE criteria (Table 4). Five characteristics described by the GRADE criteria were assessed: risk of bias, inconsistency of results, indirectness of evidence, imprecision, and publication bias.19 Each study was determined to be at increased risk of bias due to lack of blinding of subjects. Both studies generated data that supported the study hypothesis, demonstrating consistency of results. Outcome measures across both studies, KE and KF muscle torque, appropriately and directly reflected the study hypothesis. Each study demonstrated precision as all results yielded P < 0.05. Lastly, no obvious publication bias was noted throughout either of the included studies. Per the GRADE criteria, the evidence presented by both Ohta et al. and Hughes et al. is classified as moderate.22
3.5 Results synthesis
Ohta et al. was one of the first RCTs to investigate the use of BFR used in post-operative rehabilitation status-post ACLR.24 KE and KF torque measurements were each recorded separately as “operated/healthy ratios” of strength, which referred to the strength of the repaired knee versus the non-operated knee as one single statistic. Analysis revealed no statistical differences of KE and KF isokinetic muscle torque at speeds of 60°/s and 180°/s pre-operatively. After the 16-week program, however, KE and KF torque at speeds of 60°/s and 180°/s were significantly greater in the BFR group compared to the TPR group.24 Specifically, mean KE operated/healthy ratios for the BFR patients were 21% greater at 60°/s (76% vs. 55%, p < 0.001) and 12% greater at 180°/s (77% vs. 65%, p = 0.004) than the TPR patients. Similarly, KF operated/healthy ratios for the BFR patients were 9% greater at 60°/s (81% vs. 72%, p = 0.05) and 10% greater at 180°/s (84% vs. 74%, p = 0.04) than the TPR patients. No adverse events occurred in either group.
Hughes et al. is the most recent level 1 RCT concerning this topic. This study protocol was similar to Ohta et al., however, the researchers utilized heavy-load TPR as opposed to the light-load TPR used by Ohta et al.24,25 Isokinetic torque measurements of KE and KF were determined at 60°/s, 150°/s, and 300°/s. Both groups showed significant loss of KE peak torque from pre-surgery to post-rehabilitation and no significant difference was found between groups when KE was measured at 60°/s. However, at 150°/s and 300°/s the TPR group showed a significant loss of KE peak torque from pre-surgery to post-rehabilitation while the BFR group did not. Based on ImageJ analysis, KE scaled peak torque (Nm/kg body mass) at 150°/s improved from 1.360 to 1.445 (+0.085) in BFR patients and instead decreased from 1.515 to 1.245 (−0.27) in TPR patients. At 300°/s, scaled peak torque improved from 1.045 to 1.085 (+0.04) in BFR patients and instead decreased from 1.075 to 0.945 (−0.13) in TPR patients. Unlike KE, KF scaled peak torque decreased more significantly for TPR patients at all 3 speeds (all p < 0.01). KF scaled peak torque at 60°/s decreased from 1.050 to 0.975 (−0.075) in BFR patients and decreased from 1.020 to 0.795 (−0.225) in TPR patients. At 150°/s, scaled peak torque decreased from 0.900 to 0.835 (−0.065) in BFR patients and decreased from 0.805 to 0.635 (−0.17) in TPR patients. Lastly, at 300°/s, scaled peak torque decreased from 0.710 to 0.670 (−0.04) in BFR patients and decreased from 0.665 to 0.520 (−0.145) in TPR patients. There were no adverse events for patients in either group.
4 Discussion
The goal of this review was to clearly and concisely confirm or refute the study hypothesis, that BFR limits KE and KF weakness in the immediate post-operative period following ACLR better than TPR while maintaining the integrity of the surgical site. The results of this study support this hypothesis as the BFR groups in the two level I RCTs in this review demonstrate significantly greater KE and KF strength than the TPR control groups. No adverse events were reported across both studies.
Data from both RCTs supports the study hypothesis, however, slight differences in study protocols should be noted. Load, intervention duration, and occlusion device differed between the two includes studies. Ohta et al. utilized light-load (straight leg raise, step ups, half squats), with maximum half squat load in this light-load regiment being only 12–14 kg at weeks 13–16 weeks post-op. Hughes et al. instead utilized a 30% predicted 1RM on unilateral leg press for their BFR patients and 70% predicted 1RM for their heavy-load TPR patients.24,25 Also, Ohta et al. provided a 16-week exercise intervention, while Hughes et al. used an 8-week intervention. Lastly, Ohta et al. used a manually inflated air tourniquet pumped to 180 mmHg while Hughes et al. used Delfi Medical's automatic personalized tourniquet system set to 80% limb occlusion pressure (LOC). LOC is defined as the amount of pressure required for full arterial and venous occlusion. Although both studies generated similar results and conclusions, differences in methodology resulted in difficulty determining an optimal BFR protocol for rehabilitation of patients undergoing ACLR.24,25
Despite these differences, both RCTs clearly suggest that BFR is an effective alternative to TPR after ACLR. Ohta et al. found that KE and KF torque in BFR groups was closer to healthy norms. Hughes et al. found KE and KF torque in the BFR groups were closer to pre-op baseline measures than the TPR counterparts.24,25 Most importantly, data suggests that BFR is at least just as effective as TPR, with Hughes et al. finding that BFR improved many other important variables such as range of motion (ROM), pain, and effusion.24,25 Improvements in pain may be partially due to a hypoalgesia effect of BFR.24,25 For example, Korakakis et al.26 investigated pain reduction associated with BFR use in those suffering from anterior knee pain. The investigators found that utilizing BFR with low-load physiotherapy activities such as single-leg squats and step-down exercises led to significantly reduced pain when compared to the control group.26
While focusing our evaluation on isokinetic KE and KF torque helped standardize the findings of this review, inconsistencies in study methodologies make an overall summary of the included studies difficult. Despite these difficulties, the highest-quality RCTs evaluating isokinetic KE and KF torque agree that BFR, provided several weeks after ACLR and at low exercise intensities, limits post-operative losses of KE and KF strength while providing similar or improved knee pain and function. According to the American Academy of Orthopaedic Surgeons (AAOS) Clinical Practice Guideline and Systematic Review Methodology, this recommendation for clinicians can be considered strong.27
It is hypothesized that the ischemic and hypoxic environment generated by BFR causes increased levels of metabolic stress to the muscle in addition to the mechanical stress from the exercise.28,29 These factors are said to increase hypertrophy and limit atrophy of the affected musculature.30 Takarada et al.17 investigated this notion and found that ischemic BFR training increases the cross sectional area of several large muscle groups relative to traditional training (+20% biceps, +17.8% brachialis, and +12.3% quadriceps).17,31 Other theorized factors include elevated systemic hormone production, cell swelling, production of reactive oxygen species, and increased fast twitch fiber recruitment.31 It is also theorized that BFR stimulates anabolic and anti-catabolic cell signaling pathways, particularly the mTOR (mechanistic target of rapamycin) protein kinase pathway, which also increases muscle hypertrophy.32
Several other studies have attempted to use muscle atrophy measured using CSA (measured via MRI) or thigh girth as a primary outcome measure to determine the effectiveness of BFR after ACLR. Inconsistency in methodology of these studies have led to indecisiveness in drawn conclusions.18,33,34 For example, Iversen et al. and Takarada et al. are level I RCTs that evaluated BFR with CSA. The results of these studies conflict, as Iversen et al. found no significant difference between study groups in KF or KF CSA area, while Takarada et al. found that BFR patients had increased KE CSA compared to the TPR control. The authors found no significant difference in KF CSA between groups. Iversen et al. better controlled for swelling by waiting until two days after completion of intervention (post-op day 16) to perform MRI, and provided an exercise stimulus to both groups, which may accentuate the effects of BFR. Exercises were performed for 12 days at low intensity which is significantly less time than the periods of 8 and 16 weeks of interventions provided in the two included studies. These two level I RCTs highlight the inconsistencies in methodology when evaluating CSA, which itself can be a misleading outcome measure. A recent meta-analysis by Wengle et al.19 did evaluate CSA, finding that BFR leads to a 1.28 cm2 increase in quadratus femoris CSA. Limb girth and CSA can measure the overall size of the tissue, however, these outcome measures do not account for variation among individuals, muscle strength, or stability of the joint.19
Since isokinetic KE and KF strength was reported differently across both included studies, comparisons between these multiple methodologies could not be made and thus a superior methodology could not be reliability determined. However, the authors recommend specific parameters based on the literature and experience in practice. BFR cuffs should be set to 60–80% LOP.25 Masri et al.35 confirmed that LOP can be determined accurately and safely using pneumatic blood pressure cuff as opposed to the gold standard Doppler ultrasound.35 Exercises should be performed at 20–30% 1RM.25 Similar to TPR, BFR training is best used with closed chain exercises such as quad sets, squats, and leg presses, however, BFR with open chain exercises such as short arc quadriceps extensions and hamstring curls are also beneficial when weight bearing remains limited.24,25 As exemplified by Hughes et al., BFR should be initiated 2 weeks post-op ACLR to allow for initial weight bearing as well as reduction of pain and swelling.25 BFR should be continued throughout the out-patient rehabilitation process, between weeks 2 through 18. Contraindications to BFR include inability to consent, peripheral vascular disease, coagulopathies, and history of pulmonary embolus, deep vein thrombosis, or other blood clotting disorders.31,35
4.1 Limitations
Despite the statistical significance of the data discussed earlier, generalizability of this study is restricted due to significant limitations of the study. The largest of these limitations is the small sample size of this data. Graft selection is also notable. The surgeons in both studies used solely semitendinosus autograft as graft of choice. Exclusion of the highly popular bone-patellar tendon-bone autograft in these studies, among other grafts, is significant.24,25 Also, exercise intensity and protocol differed across the two studies. One must also consider is the role of venous thromboembolism prophylaxis (VTE) in patients undergoing BFR. Both RCTs denied occurrence of adverse events, however, VTE prophylaxis is not mentioned. This lack of this information is significant concerning the utility of BFR in clinical practice. A final consideration one must discuss when analyzing the data from this study is the selection of isokinetic KF and KE strength as the primary outcome measure studied in this review. Further studies must determine if preservation of strength of the musculature, physical size of the tissue, or a combination of these measurements is best for predicting long-term outcomes following ACLR.
5 Conclusions
The highest-quality level 1 RCTs evaluating KE and KF strength via isokinetic torque agree that when utilized after ACLR, BFR limits post-operative losses of KE and KF strength significantly better than TPR. No adverse outcomes were reported in the two studies included in this systematic review. Except for patients of whom BFR is contraindicated, clinicians may consider utilizing BFR from week 2 of the post-operative period through the conclusion of outpatient rehabilitation using low intensities, multiple times per week; however, further RCTs comparing BFR protocols are necessary before an optimal BFR protocol could be confidently recommended.
Funding
The authors have not received grant support/funding for this study. The authors do not have propriety interests in the materials described in this article.
Author contribution statement
Joshua Spada: Corresponding author, Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing-original draft, Writing-review and editing, Visualization, Project administration. Ryan Paul: Resources, Formal analysis, Writing-review, and editing. Bradford Tucker: Conceptualization, Writing-review and editing, Supervision.
Informed consent
As this paper is a systematic review, informed consent is not required.
Institutional Ethical Committee Approval
As this paper is a systematic review, Institutional Ethical Committee Approval is not required.
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