Introduction

The anterior cruciate ligament (ACL) plays a pivotal role in stabilizing the knee joint. Biomechanically, the ACL is responsible for ensuring antero-posterior and rotational stability in the knee joint [1,2,3]. ACL tears, which result from either contact or non-contact mechanisms, are the most common knee ligament injury affecting roughly 1 in 3500 people in the United States [4]. The gold standard treatment for an ACL rupture is ACL reconstruction (ACLR) with either autograft or allograft [5, 6].

While ACLR is the current gold-standard surgical approach to treat ACL tears, there are several limitations associated with it. The autologous or allogenic tendon graft does not perfectly restore the proprioceptive nerve fibers of the native ACL [7,8,9]. Furthermore, non-anatomic placement of the graft, donor site morbidity, and decreased tendon-bone healing are all potential limitations of ACLR [10,11,12].

While extra-synovial ligaments can successfully heal without surgery due to the presence of a surrounding fibrin clot, the ACL is intra-synovial, and therefore, has limited potential to heal without surgical intervention [13,14,15]. This occurs because the plasmin in the synovial fluid cleaves peri-ligamentous fibrin clots [13, 16] and ACL fibroblasts are inhibited by the synovial fluid [17].

To surpass the aforementioned biological constraints, bridge-enhanced ACL restoration (BEAR) was developed [18]. The BEAR procedure involves placing a hydrophilic, cylindrical collagen scaffold implant adjacent to the native ACL in conjunction with primary repair (Fig. 1). The collagen scaffold is obtained from bovine tissue and is saturated with autologous blood before implantation. The addition of the autologous blood softens the scaffold to allow it to mold to the irregular contours of the gap between the two torn ends of the ACL. The implant also retains the native fibrin clot at the site of injury, facilitating the release of growth factors to aid the repair process [19]. This procedure forms a viscoelastic construct that fills the gap, providing a stable intra-articular bridging sponge to allow the native ruptured ACL to heal [19].

Fig. 1
figure 1

Image of the BEAR implant

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In pre-clinical studies utilizing a skeletally immature large animal model, the BEAR had similar linear stiffness, yield, AP laxity, and load to failure of the ligament compared to a bone-patellar tendon-bone (BTB) allograft ACLR at three, six, and twelve months postoperatively [15, 20, 21]. The addition of the implant may provide a significant enhancement to the structural properties of the ACL due to the increased cellularity within the healing ACL [22].

The primary objective of this paper is to review the current literature regarding BEAR that describes surgical indications, techniques, and outcomes. Understanding these parameters can help physicians learn about the advantages and limitations of using BEAR to treat ACL injuries.

Indications

Throughout the product development of BEAR, the indications and the exclusion criteria have broadened and narrowed, respectively. BEAR I was a nonrandomized controlled cohort study with 10 patients and had the most stringent criteria for patients receiving BEAR. To qualify for BEAR, patients must have been between 18 and 35 years old, had a complete ACL tear, a time from initial injury of less than 35 days, and had at least 50% of the length of the ACL attached to the tibia on preoperative MRI. Patients were excluded if they had a partial ACL tear, were determined to benefit from surgical intervention with autograft hamstring tendon graft, prior surgery or infection in the ipsilateral knee, history of nicotine, tobacco, or corticosteroid use, or specific concomitant injuries such as a bucket-handle tear of the medial meniscus requiring repair, full-thickness chondral injury, grade III MCL injury, or a patellar dislocation [23,24,25,26,27]. The BEAR II trial was a randomized controlled trial with 65 patients in the BEAR cohort with broader inclusion criteria. The accepted patients’ age range expanded to include patients as young as 13 years old and time from initial injury increased to treat patients up to 45 days from their initial injury. There were no changes to the exclusion criteria [28,29,30,31,32,33,34,35,36,37]. The BEAR III trial was a prospective multicenter cohort study with 151 patients in the BEAR cohort with even broader inclusion criteria. The included patients’ age range expanded to include patients between 12 and 80 years old, patients with a complete tear or a partial tear with a positive pivot shift, and time from initial injury of less than 50 days. The currently published BEAR III trial includes 49 patients from Boston Children’s Hospital [38]. In our practice with BEAR in a post-market setting between March 2022 and August 2023, there were no patient demographic exclusion criteria. The patient must have had a midsubstance or proximal complete ACL tear, been skeletally mature, and had at least 50% of the length of the ACL attached to the tibia on preoperative MRI. Patients with prior surgery or infection in the ipsilateral knee, history of nicotine, tobacco, or corticosteroid use, comorbidities, or concomitant injuries were included [39] (Table 1).

Table 1 Preoperative indications for BEAR patients by stage
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Techniques

According to the FDA approval for BEAR, the surgical approach is flexible if the three core pillars of the original surgical technique are met. First, the ACL stump must be under tension and reapproximated directionally to the other stump as the appropriate tension ensures that cells regrow in the correct direction. Second, the BEAR implant must be tensioned with a suture and in contact with both torn ends of the ACL stump, as this maintains the implant in the appropriate location during the healing process. Lastly, the knee must be stabilized with an internal support suture, as bone-to-bone fixation provides increased support to the knee [25, 26, 31]. The decision on the type of surgical instrumentation used, such as buttons or anchors, and the approach used are left to the surgeon’s judgment.

The original dual suspensory fixation technique was first described by Murray et al. [23,24,25,26,27,28,29,30,31,32,33,34,35,36, 38]. With the knee flexed at 90 degrees, the standard ACLR viewing and working portals were created. All concomitant pathology was addressed prior to proceeding with BEAR. If needed, a medial fat pad resection and a kidney-bean-shaped notchplasty preserving the femoral footprint were performed. Of note, a larger notch following notchplasty was correlated with a larger cross-sectional area of the restored ACL [23, 32]. A tibial tunnel was drilled in the anterior 25% of the tibial ACL footprint. Similarly, a femoral tunnel was drilled in the anterior 25% of the femoral ACL footprint with a reamer. A 2-inch arthrotomy was performed at the medial border of the patellar tendon. The tibial stump was whipstitched with No. 2 Vicryl sutures, which were passed through a cortical button. Two No.2 Ethibond sutures were looped through the center holes of the button, and the button was passed through the femoral tunnel and secured on the lateral femoral cortex via a lateral incision. Each limb of the looped Ethibond sutures was passed through the porous end of the BEAR implant and then through the tibial tunnel. The implant was then infused with 10 mL of autologous blood. The implant was threaded along the sutures into the femoral notch such that the porous end of the implant would lie proximally, and the dense end would lie distally. The Ethibond sutures were pulled distally and tied over a second cortical button along the anterior tibial cortex with the knee in full extension. The free ends of the Vicryl suture from the whipstitch coming through the femur were tightened and tied over the femoral cortical button to move the ACL stump into the implant. The arthrotomy was then closed in standard fashion [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37] (Fig. 2).

Fig. 2
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Diagram of the BEAR operative technique

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Gao and Wang recently published a new suture anchor technique for BEAR which was similar to the steps of a BTB autograft ACLR [40]. With the knee flexed at 90 degrees, the standard anteromedial and anterolateral ACLR viewing and working portals were created. After all concomitant pathology was addressed, the proximal attachment of the ACL was released, with care taken to protect native ACL tissue. A shaver was then used to decorticate the bone off the native ACL attachment. An accessory percutaneous anteromedial portal was created in line with and superior to the standard anteromedial portal. Four closed-loop, high-strength sutures were passed through the distal stump of the ACL. The four limbs of the suture were loaded into a knotless suture anchor. The knee was hyperflexed. The suture anchor was then placed at the femoral insertion of the ACL, slightly higher than the native ACL footprint, aiming for the 1:00 position for a left knee. The free limbs of the suture in the anchor were retained to be used as the internal brace support and to shuttle the implant. Then, a tibial tunnel was created at the native ACL footprint. Each limb of the suture emerging from the femoral suture anchor was passed through the porous end of the BEAR implant. The free ends of the suture were shuttled through the tibial tunnel with the previously passed transtibial shuttle suture. The implant was then infused with 10 mL of autologous blood and threaded along the sutures into the femoral notch. The suture limbs were tensioned with the knee in full extension and tied over a tibial cortical button.

In non-BEAR ACLR biomechanical studies, the suture anchor repair technique has proven to have a decreased potential for gap formation [41] and tunnel widening [42] compared to a technique requiring a transosseous tunnel. However, if the patient eventually requires revision, it may be more difficult if the initial procedure is conducted utilizing the suture anchor technique as opposed to traditional femoral tunnel drilling. The outcomes of the suture anchor technique have been studied in the ACLR literature but have yet to be studied in the BEAR literature.

We perform an all-inside technique for the BEAR procedure. The button of an adjustable loop suspension device (Tightrope, Arthrex Inc., Naples, FL) is loaded with additional 0.9 and 1.3 mm SutureTapes (Arthrex Inc., Naples, FL) to act as an internal brace, and the loop is lengthened. Then, a 3 cm × 10 mm PassPort Cannula (Arthrex Inc., Naples, FL) is inserted into the anteromedial portal. A shaver is used to debride a kidney shaped area just anterior to the femoral footprint. A 4 mm spade tip guide wire is then drilled through the femur at the anterior aspect of the femoral footprint through the lateral femoral cortex. A looped passing suture is then placed through the femoral tunnel. A FIRSTPASS MINI Suture Passer (Smith and Nephew, Memphis, TN) is used to pass #2 Vicryl suture through the ACL stump on the tibia for a total of five passes working from distal to proximal. A 2.4 mm cannulated guide pin is then used to drill the tibial tunnel, starting at the proximal medial tibia and aiming for the anterior aspect of the tibial ACL footprint. A looped passing suture is then placed through the tibial tunnel. The Tightrope button is then passed through the femoral tunnel and flipped, using fluoroscopy to confirm placement flush against the femoral cortex. The Vicryl sutures are then tied around the adjustable loop, and the Tightrope is tensioned, pulling the ACL stump toward the footprint with counter tension on the SutureTapes under direct visualization with the scope camera. The knee is then irrigated with vancomycin solution, and the medial portal is enlarged to about 3 cm. The knee is then suctioned and dried. Thereafter, the four SutureTape limbs are passed through the BEAR implant with Keith needles and shuttled through the tibial tunnel via the passing suture. 20 cc of whole blood are drawn from the patient IV and used to soak the BEAR implant. With the knee in flexion, the implant is pushed into the notch through the mini-arthrotomy, and the SutureTapes are tensioned with the knee in full extension. The SutureTapes are then secured to the tibia with a SwiveLock Anchor (Arthrex Inc., Naples, FL). The incisions are closed in standard fashion, and the patient is placed in a hinged knee brace locked in full extension.

Rehabilitation

BEAR I, BEAR II trials, and our cohort utilized a similar rehabilitation procedure described by the manufacturer [26, 31, 39, 43]. Patients were made toe-touch weight-bearing in a hinged knee brace locked in extension for ambulation for four weeks. Patients were then allowed to advance to weight-bearing as tolerated and wean their crutches at six weeks. Patients began range of motion (ROM) exercises immediately, and ROM was restricted to 0–50 degrees of knee flexion for the first two postoperative weeks. Patients advanced to 90 degrees from the second to fourth postoperative weeks. Full ROM was permitted as tolerated after the fourth postoperative week. Formal physical therapy was started after the first postoperative week and after six weeks the hinged knee brace was discontinued under physical therapist supervision after patients had appropriate quadriceps function and tolerated weight-bearing. Patients were permitted to return to sports at 6 months to 12 months postoperatively if they had confidence when cutting, jumping at full speed, running, and clearance by the operating surgeon. The BEAR III trial had slight modifications and the hinged knee brace limited flexion to 30 degrees for the first two postoperative weeks, to 60 degrees from two to four weeks, and to 90 degrees from four to 6 weeks [38].

Outcomes

Patients treated with BEAR had full range of motion after surgery, satisfactory patient-reported outcome measures (PROMs), and a relatively low complication rate (Table 2). There was significant improvement from baseline for IKDC and all KOOS subscale scores in all trials of BEAR [24, 26, 28, 31, 38]. Specifically, the BEAR cohort generally progressed faster with return to activity as indicated by the significantly higher KOOS subscale scores and ACL-RSI compared to the ACLR cohort; however, this difference was not significant at longer postoperative timepoints [31, 33]. The superior ACL-RSI in the BEAR cohort at 6 months could be attributed to superior early hamstring strength or it could be due to the patient’s inflated expectations with the new BEAR procedure.

Table 2 Postoperative outcomes of BEAR patients by stage
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Similar to the BEAR cohorts, our patients had significant increases in IKDC and all KOOS subscale scores compared to baseline, and more than 52% of patients achieved the ACLR MCID for those PROMs at an average of 9.3 ± 5.0 months postoperatively. There were 9 patients out of a total 29 patients (31%) who completed a satisfaction survey that were dissatisfied at an average of 10.2 ± 4.8 months postoperatively, which can be attributed to the potentially inflated expectations with BEAR. Even with this, most patients in our cohort either had no change in their level of function or had a one-level decrease in function [39].

Functionally, patients returned to having a full range of motion in all trials of BEAR. They had similar biomechanical data regarding AP laxity, as measured by the KT-1000, to the ACLR cohort, which was primarily hamstring autograft ACLR patients [24, 26, 28, 31]. The BEAR cohorts had significantly superior hamstring strength and no difference in quadriceps strength compared to the ACLR cohort, which could be due to nearly all the patients in the ACLR cohort receiving autologous hamstring grafts [24, 26, 28, 31].

Aggregating BEAR patients from BEAR I, BEAR II, and BEAR III trials, there were 18 patients out of a total of 123 combined patients (15%) that suffered a re-tear of the ACL requiring revision ACLR; although notably, the longest-term follow-up data for BEAR patients is two years [38]. A younger operative age and an increased medial tibial slope (MTS) were identified as preoperative risk factors for failure within the first two years after BEAR. The odds of failure were increased by 28% for each degree increase in MTS and decreased by 32% for each year increase in age [38]. There were nine patients in the BEAR II trial that had a mean IKDC score following the revision ACLR that was similar to primary ACLR patients [31]. In comparison, there were zero re-tears in our post-market BEAR population with an average follow-up time of 6.3 ± 4.1 months and ranging from 1.5 to 19.3 months. These re-tear rates are in range compared to the established two-year ACLR re-tear rate ranging from 1 to 20% [44,45,46].

Three patients in our post-market cohort (5%) developed arthrofibrosis [39]. While there is a limited amount of BEAR data, the current ACLR literature has an arthrofibrosis rate ranging from 2 to 35% [47, 48]. As more patients are treated with BEAR and there is longer follow-up for these patients, there will be an increased variance in the outcomes and complications of BEAR; therefore, future studies examining BEAR ought to be conducted.

Conclusion

This review summarized the current literature regarding BEAR from the initial trial to the current post-market state. The results from the BEAR trials and our post-market BEAR experience are encouraging; however, it remains uncertain if this novel procedure can outperform the current gold standard ACLR in the long term. Future studies are needed to confirm the long-term efficacy of this novel approach to treating ACL tears.