Biomechanical Comparison of Knotless Suture Anchor Fixation Versus Transosseous Suture Button Fixation for Posterior Medial Meniscal Root Tears

Abstract

Background:

Posterior medial meniscal root (PMMR) tears have been shown to create a biomechanically unfavorable environment for knee cartilage. While various PMMR repair methods have been shown to restore normal knee biomechanics, there is little consensus on the ideal repair technique.

Purpose:

To compare PMMR repair using a knotless all-suture anchor versus traditional transosseous fixation using a suture button.

Study Design:

Controlled laboratory study.

Methods:

Ten matched pairs of fresh-frozen cadaveric knees were biomechanically tested in intact, torn, and surgically repaired conditions. One limb from each donor was repaired using a traditional suture and bone tunnel technique (control group), while the contralateral limb was repaired with a knotless all-suture anchor (anchor group). The limbs were distributed equally between the 2 surgical procedure groups, so that each group contained a nearly equal number of right and left knees.

Results:

There were no significant differences in normalized contact area or normalized contact pressure between control and anchor groups at 0°, 30°, or 60° of knee flexion. Contact pressure returned to near-intact values at 0°, 30°, and 60° for both groups. This was also the case at 90° for the anchor group. However, the control group did not return to intact values at 90°, resulting in a significantly (P = .007) higher normalized contact pressure in the control group compared with the anchor group. This finding was also true at 90° for normalized contact area, which was significantly lower (P = .0001) in the control group than in the anchor group.

Conclusion:

At time zero, knotless all-suture repair of PMMR tears performed biomechanically similarly or better than transosseous suture button fixation at 0°, 30°, and 60° of flexion, with significantly improved contact area and contact pressure at 90° of flexion. Thus, knotless all-suture anchor repair may represent a viable alternative technique for PMMR repair.

Clinical Relevance:

The knowledge that knotless all-suture anchor repair performed as well as or better than a more traditional technique allows for a technically simpler surgery, which precludes the need for drilling through the tibia.

Posterior medial meniscus root (PMMR) tears have been shown to result in significant morbidity in patients secondary to the loss of hoop stress and force distribution during axial loading. ²¹ It has been previously shown that tears of the PMMR result in tibiofemoral contact forces similar to those of a total meniscectomy. ²³ This results in a biomechanically unfavorable environment that has been shown to increase the risk of early onset degenerative changes and the need for total knee arthroplasty. ¹ Fortunately, repair of PMMR tears has been shown to improve knee biomechanics. ²⁴ As a result, significant energy has been spent by the orthopaedic community to identify various techniques to repair the PMMR. To date, transosseous techniques have prevailed as the most widely accepted methods.^7,15 These techniques typically involve passing a suture or pairs of sutures through the PMMR (usually via a modified Mason-Allen technique, simple suture, or luggage tag configuration) and passing that suture through a drill hole that has been created through the tibia. ³

Transosseous repair and fixation using a suture anchor or cortical button on the tibial cortex has long been considered the gold-standard repair construct. However, concern exists regarding the working length of the sutures from the native PMMR footprint on the tibial plateau to the fixation point on the tibial cortex. Specifically, there is concern for suture abrasion within the tunnel as well as longitudinal micromotion of suture limbs, commonly known as the “bungee effect.” ⁶ Although transosseous repair of PMMR tears has proven to be clinically superior to nonoperative management or partial meniscectomy, ²⁰ clinical studies demonstrate room for improvement, as 2-year patient acceptable symptom state (PASS) achievement for various functional outcome scores remains around^2,13 60% to 70%, and second-look arthroscopy has identified radiographically healed but functionally compromised postoperative patients. ²⁵ To improve upon these outcomes, there have been many tibial fixation methods proposed, such as various suture configurations and materials, as well as, more recently, direct suture anchor fixation to the native PMMR on the tibial plateau, rather than transosseous fixation. ¹⁸

Recent biomechanical studies directly comparing transtibial pullout repair techniques against direct suture anchor fixation to the PMMR may not be representative of in vivo kinematics, as they have widely been conducted in porcine models with tibial-only meniscus-isolating constructs analyzing displacement, stiffness, and/or load-to-failure of root repair constructs without assessing changes in tibiofemoral biomechanics, such as contact area and pressure.^3,5,9,22 Additionally, most are rigid fixtures in extension, rarely evaluating tibiofemoral biomechanics across different flexion angles, which is crucial for predicting in vivo functional performance. One recent study ¹⁴ compared transtibial pullout and suture anchor repair in a human cadaveric model at knee flexion angles of 0° and 60°. However, the highest tension on the PMMR has been reported at 90° of flexion. ²⁶

Our study aimed to compare an all-suture anchor PMMR repair with the more commonly utilized technique of transosseous tibial fixation over a suture button. We hypothesized that the knotless all-suture anchor would perform as well as or better than transosseous suture button fixation at restoring normal tibiofemoral contact area and contact pressures.

Methods

Specimen Preparation

Twenty (10 matched pairs) fresh-frozen cadaveric knees (mid femur to mid tibia/fibula) were procured (Science Care) for this study. Cadaveric limbs were allowed to thaw to room temperature for approximately 24 hours before testing. The proximal 10 cm of the femoral diaphysis and distal 10 cm of the tibia/fibula were stripped of all soft tissues for fixation in polymethylmethacrylate (PMMA). Briefly, the tibia and fibula were drilled and fixated with a quadricortical screw to maintain neutral rotation. The femur was also drilled before potting to act as an anchor within the cement. The femur and tibia/fibula were then potted in an acrylic pipe using PMMA. The limbs were then dissected, with the surrounding skin, fat, and muscle removed. The extensor mechanism was also removed for visualization. Next, anterior and posterior submeniscal arthrotomies were performed to prepare for placement of the pressure sensor. This was performed only on the medial side of the knee to allow a pressure sensor to be placed within the medial compartment. As few meniscotibial ligament attachments as possible were disturbed during this process. Additionally, an arthrotomy was created above the posterior meniscus for visualization and preparation for later repair. Care was taken in each specimen to preserve the ligamentous structures required for joint stability (anterior cruciate ligament, posterior cruciate ligament, medial collateral ligament, lateral collateral ligament, etc) as well as for meniscal stability (root attachments, intermeniscal ligament, meniscofemoral ligaments, etc).

Biomechanical Testing

Once initial dissections were completed, the potted tibia/fibula was secured into a custom test fixture and attached to the actuator of a servohydraulic mechanical test frame (MTS 858 MiniBionix II; MTS Systems). This orientation of the tibia/fibula being above the femur was chosen so that as the knee was flexed through its range of motion, the actuator was aligned to apply compressive load onto the tibial plateau. While the knee was hanging freely, a single side of a pressure sensor (K-Scan 4011; Tekscan) was inserted between the inferior medial meniscus and the medial tibial plateau (Figure 1).

Figure 1.

Pressure sensor inserted between the inferior medial meniscus and medial tibial plateau (anterior view). Note that the specimen is oriented with the tibia/fibula above the femur.

Once the load was tared in this position, the femur was secured into a lower test fixture, which accounted for any knee valgus at 0° and 30° of flexion and was attached to a load cell (Figure 2). The load cell had a 25000-kN capacity, but was scaled to a maximum of 2500 N for improved resolution. Once in position, each knee was subjected to a manual 10-N preload, followed by an increase to 1000 N at a constant rate of 100 N/s in load control. This force was held for 30 seconds, as previously done by others, ¹⁰ and then decreased back to 0 N at the same loading rate as before. Biomechanical testing was completed in the intact condition at 0°, 30°, 60°, and 90° of knee flexion. After intact testing, the PMMR was removed from its origin using a 10-blade to simulate a complete root tear. The compressive loading process was then repeated in this torn state.

Figure 2.

Overall test setup of a right knee oriented at 30° of knee flexion. Note that a pressure sensor is also in the lateral knee for a separate study.

Repair Procedures

After the simulated tear and testing, we applied different repair techniques to the 2 knees in each matched pair. To avoid any leg dominance discrepancies between repair groups, procedures were evenly distributed between the right and left knees. For both techniques, the PMMR footprint was exposed and directly visualized through the posterior arthrotomy. For the transosseous technique (control), a guidewire was passed from the anteromedial tibia (approximately 1 cm medial to the midpoint of the tibial tubercle) through the PMMR footprint, and a flip cut reamer was used to retrograde-ream the depth of the meniscal root tunnel to approximately 5 mm to create a socket and bleeding footprint. ⁸ A No. 2 suture (FiberWire; Arthrex) was passed through the PMMR using a free needle in a “luggage tag” configuration (Figure 3A). The 2 free ends of the suture were then passed through the tibial tunnel via a Hewson suture passer (Figure 3B). This was then loaded over a suture button (TightRope; Arthrex) (Figure 3C) and subsequently tied over the anteromedial tibial cortex (Figure 3D).

Figure 3.

(A) A repair suture is passed through the PMMR in a “luggage tag” fashion. (B) The repair suture ends are passed through the tibia using a Hewson suture passer. (C) The suture ends are then passed through a button and tied over the anteromedial tibia. (D) The PMMR is restored with appropriate tension. PMMR, posterior medial meniscal root.

On the contralateral limb, a 1.8-mm knotless all-suture anchor (FiberTak; Arthrex) was drilled and inserted into the PMMR footprint and set (Figure 4A). The repair suture was passed through the PMMR in a simple horizontal mattress fashion using a free needle (Figure 4B) and subsequently converted through the anchor using the looped conversion suture. The repair suture was then cinched down (Figure 4C), thereby reducing the PMMR to the footprint (Figure 4D). Excess suture in both techniques was cut. After each repair, the limbs were biomechanically tested in the repaired state using the same procedures as above.

Figure 4.

(A) The knotless all-suture anchor is drilled and inserted into the PMMR footprint. (B) The repair suture is passed through the PMMR in a mattress fashion. (C) The repair suture is converted through the anchor via the looped shuttling suture. (D) The PMMR is restored with appropriate tension. PMMR, posterior medial meniscal root.

Sample Size Determination and Statistical Analyses

For our study, 20 human cadaveric knees (10 matched pairs) were tested, as previous similar studies ²² have shown that 8 human cadaveric specimens per group provide up to 90% power and an alpha-level of .05. Other recent studies^9,14 utilized treatment groups of 4 and 8 specimens. Contact pressure and area data from the torn and repaired conditions were normalized to the intact condition, yielding unitless ratios in which a value of 1 represents a perfect return to native mechanics. A 2-way repeated-measures analysis of variance (ANOVA) was used to detect overall differences in contact mechanics for the 2 normalized surgical conditions (torn and repaired) and surgical technique (transosseous suture button vs knotless all-suture anchor). When a significant overall difference was found, individual comparisons were made using the Tukey honest significant difference post hoc analysis. Significance was set at P ≤ .05.

Results

Specimen ages ranged from 49 to 64 years, with a mean of 55 ± 5 years; 3 were male, and 7 were female. Two-way ANOVA with repeated measures showed an overall significant effect of condition (ie, torn versus repaired) on normalized contact area at 60° (P = .03) and 90° (P = .004) of flexion, and for normalized contact pressure at 0° (P = .0004), 30° (P = .003), and 60° (P = .03) of flexion. An overall significant effect of procedure (control vs anchor) was seen on normalized area (P = .0004) and normalized pressure (P = .001) at 90° of flexion.

When further comparing repaired conditions via post hoc analysis, there were no significant differences in normalized contact area (P = .425-.996) or normalized contact pressure (P = .871-.984) between control and anchor groups at 0°, 30°, or 60° of knee flexion. Normalized contact pressure returned to near-intact values at 0°, 30°, and 60° for both groups (Figure 5). This was also the case at 90° for the anchor group. However, the control group did not return to intact values at 90°, resulting in significantly higher (P = .007) normalized contact pressure in the control group compared with the anchor group.

Figure 5.

Normalized contact pressure was not significantly different between control and anchor groups at 0°, 30°, or 60° of knee flexion angle. At 90°, the normalized contact pressure of the anchor group was significantly lower (P = .007) compared with the control group.

At 90°, the normalized contact area was significantly higher (P = .0001) in the anchor group than in the control group (Figure 6). This increase in contact area in the anchor group was not statistically significant at any other flexion angle.

Figure 6.

Normalized contact area was not significantly different between control and anchor groups at 0°, 30°, or 60° of knee flexion angle. At 90°, the normalized contact area of the anchor group was significantly higher (P = .0001) compared with the control group.

Discussion

Fixation of PMMR tears remains a difficult problem for both patients and clinicians. There is no question that fixation of the PMMR results in more normal contact pressures within the medial compartment and, thus, results in a more biomechanically favorable environment, which can reduce the rate of progression to osteoarthritis. ²⁴ Many techniques have been proposed, predominantly focusing on transtibial fixation methods, including the technique used for the control in this study. ¹⁸ However, previous studies evaluating transtibial techniques alone have shown variable biomechanical results in restoring contact area and contact pressures^3,5,9,22 and have also clinically demonstrated inconsistent healing and meniscal extrusion on follow-up magnetic resonance imaging.^2,13,17 Additionally, studies looking at biomechanical comparisons between suture anchor techniques and transtibial techniques have shown superior biomechanical properties related to meniscal displacement as well as maximum load to failure when compared with transtibial approaches. ¹² Nonetheless, these data were not typically obtained via human cadaveric specimens and failed to evaluate articular contact pressures or contact area at 90°, a flexion angle known to impart the greatest loads on the PMMR. ²⁶

Our study, which was performed on 10 matched pairs of cadaveric specimens, shows that fixation via a single knotless all-suture anchor is at least as biomechanically effective as traditional transosseous fixation using a suture button at 0°, 30°, and 60° of knee flexion. There were no significant differences between procedure groups in normalized contact area or normalized contact pressure with axial load at 0°, 30°, or 60° of knee flexion, indicating that suture anchor fixation alone is a viable alternative to transosseous techniques. Furthermore, our data did indicate that the all-suture anchor restored normalized contact area and normalized pressure significantly closer to intact values than the transosseous suture button at 90° of flexion, which may suggest that an all-suture anchor fixation construct may have a slight advantage when considering these 2 techniques from 0° to 90° of flexion. The location of the tunnel or anchor may be just as important (if not more) than the repair technique. Our study aimed to propose an alternative technique that may improve fixation strength by anchoring the device to the articular surface rather than the anterior tibial cortex.

Important points of comparison with other biomechanical studies in the existing literature include the nature of the specimens, the testing setups, the flexion angles, and the suture configurations. Three recent studies^3,5,22 published porcine models, while 2 other relevant studies^9,14 published recent human cadaveric models. The latter 2 also employed full-construct human models, allowing measurement of contact pressures and surface areas.^9,14 However, there were important differences compared with our study. Itthipanichpong et al ¹⁴ evaluated static tibiofemoral contact pressures and areas at 0° and 60°, finding that both transtibial and suture-anchor repairs restored near-native conditions. However, they did not assess tibiofemoral biomechanics at 30° or 90°, which would more accurately reflect behavior throughout the full range of motion, especially considering that PMMR loading is highest at 90° flexion. ²⁶ By contrast, Cinque et al ⁹ recreated a similar full-construct human cadaveric model but tested only at 0° of flexion (full extension).

Additionally, although our study design is not specifically suited to address this question, the use of knotless all-suture anchors for PMMR repair could be beneficial for several reasons. The knotless mechanism results in a low-profile articular-facing construct that is less prominent than knot stacks present in knotted suture anchor repairs, which would be chondroprotective in a population of patients with PMMR tear that frequently have concomitant chondromalacia. Additionally, the all-suture component, which is tapped directly into the tibial plateau rather than pulled in or fixed through a transtibial tunnel, preserves potentially valuable tibial bone stock, especially in multiligamentous reconstructions.

There are limitations to this study. Namely, this study was conducted using cadaveric specimens and not live patients, which may affect tissue quality and not reflect in vivo biomechanics. No imaCMging was conducted to measure the bone density of the specimens. In softer metaphysical bone, anchor pull-out is inherently more likely. However, this was not a common occurrence in our study, as anchor purchase was not particularly affected by potential low bone quality. As with all cadaveric studies, the in vitro nature of this study inherently precluded any assessments or comparisons of the healing of the 2 repair techniques. Additionally, the in vitro nature of the study allowed the researchers to use open approaches to the knee for improved visualization, which does not simulate the ideal in vivo technique. However, various studies have proposed methods for placing an all-suture suture anchor, through high posteromedial portals,^11,16 curved drill guides,^27,28 or reverse “pull-in” techniques.^4,5 Additionally, PMMR-specific adjustable knotless suture anchors have been released, which easily permit subcortical deployment at the native root with comparable performance to traditional knotless suture anchors.^3,9,19 In practice, the PMMR footprint is prepared using a curette to create a bleeding surface for healing, which can be done with either repair technique. The trough created for the conventional repair may improve healing. However, preparing the footprint with a curette may achieve the same goal. As this was a cadaveric-based biomechanics study, the healing potential of either technique was not directly addressed. Our proposed arthroscopic technique for the knotless all-suture anchor would involve implanting the anchor directly into the PMMR footprint through a posteromedial portal, utilizing a modified Gilquist view. The suture may then be passed through the PMMR using a meniscal arthroscopic suture-passing needle and subsequently converted through the anchor via the posteromedial portal, as previously described.^11,16 Despite not testing at various flexion angles, a strength of a previous cadaveric study ⁹ was the assessment of cyclic loading, the lack of which represents a limitation of our study. In this previous work, it was determined that suture anchor repairs led to significantly less meniscal extrusion and average contact pressures in the knee after 1000 cycles compared with transtibial repairs. ⁹ Additional technical limitations of our study relate to our suture material and configurations. We employed a single No. 2 suture in a luggage tag configuration for the transosseous group and a single horizontal mattress suture in the suture anchor group. Previous studies have established that either 2 simple sutures or Mason-Allen configurations performed with thicker, soft suture tapes are superior in mitigating suture pull-through at the suture-meniscus interface.^3,6 Further study is needed to determine whether the all-suture anchor technique would translate into improved or equivalent clinical outcomes. However, overall analysis would suggest this all-suture technique as a favorable alternative to traditional transosseous fixation. Future studies could also benefit from testing at a single flexion angle and from adding failure testing to the protocol.

Conclusion

In our study, at time zero, knotless all-suture repair of PMMR tears biomechanically performed similarly or better than transosseous suture button fixation at 0°, 30°, and 60° of flexion, with significantly improved contact area and contact pressure at 90° of flexion. Thus, knotless all-suture anchor repair may represent a viable alternative technique for PMMR repair.