Biomechanical Comparison of 4- and 6-Strand All-Inside Anterior Cruciate Ligament Reconstruction Grafts: The Value of Direct Tendon End Ripstop Suturing to the Adjustable Loop

Abstract

Background:

Data comparing primary fixation of hamstring tendon (HT) grafts for all-inside anterior cruciate ligament reconstruction (ACLR) using direct ripstop (RS) suturing of free graft limbs with the adjustable loop device (ALD) versus standard grafts are lacking.

Purpose:

To evaluate the biomechanical effect of direct RS suturing backup fixation of free graft limbs to the adjustable loop in 4- and 6-strand HT grafts compared with standard cerclage sutured grafts in all-inside ACLR.

Study Design:

Controlled laboratory study.

Methods:

Four different HT groups (n = 8 per group) with 9-mm graft diameter were prepared: 4- and 6-strand grafts with standard cerclage suturing (4HT/6HT) and additional direct RS suturing (4HT-RS/ 6HT-RS) of the free graft limbs to the ALD. Each construct was preconditioned with graft retensioning (250 N) followed by a total of 3000 cycles. This included 1 position-controlled and 2 force-controlled load blocks (1000 cycles each), with a constant valley (10 N) and 2 different peak loads (250 N, 400 N). Residual graft force, dynamic elongation, and stiffness were analyzed. The ultimate strength was evaluated during pull to failure (50 mm/min). One-way analysis of variance and Holm-Sidak post hoc tests were used as statistical methods.

Results:

The maintenance of residual graft force of RS suturing groups (4HT-RS, 68.3% ± 2.8%; 6HT-RS, 72.1% ± 4.9%) was higher (P≤ .002) than both controls (4HT, 6HT) (≤61.0% ± 3.1%). The control groups showed lower dynamic stiffness (250 N: 125.2 ± 5.4 N/mm vs 139.4 ± 7.4 N/mm, 124.8 ± 6.9 N/mm vs 145.3 ± 10.9 N/mm; 400 N: 140.1 ± 5.3 N/mm vs 154.1 ± 8.3 N/mm, 139.4 ± 6.4 N/mm vs 162.1 ± 13.1 N/mm; P≤ .04) and higher dynamic elongation (each P < .001) at each peak load, resulting in higher total elongation (4.4 ± 0.4 mm vs 2.9 ± 0.5 mm, 3.9 ± 0.6 mm vs 2.3 ± 0.4 mm; P≤ .001) compared with both RS suturing groups. The ultimate stiffness of 4HT (162.4 ± 13.7 N/mm) was lower (P≤ .02) than in both RS groups (4HT-RS: 181.9 ± 9.2 N/mm; 6HT-RS: 185.4 ± 17.3 N/mm). The ultimate load of 6HT-RS (1133.4 ± 67.9 N) was higher (P≤ .003) than for other groups. The predominant failure mode was suture rupture of the ALD.

Conclusion:

This study demonstrated that direct RS suturing of free graft limbs to the adjustable loop significantly decreased graft tension loss, increased construct stiffness, and reduced cyclic elongation in 4- and 6-strand all-inside HT grafts compared with standard grafts with cerclage suturing. Improved primary fixation may ensure more homogeneous load distribution across the graft limbs and reduce the risk of early clinical failure.

Clinical Relevance:

Direct RS suturing of the free graft ends to the ALD improved primary construct stability in all-inside ACLR, but future studies will determine if the improved time-zero biomechanics translate into favorable clinical outcomes.

Keywords

ACL graft preparation all-inside ACLR biomechanical testing adjustable loop fixation

Anterior cruciate ligament (ACL) reconstruction (ACLR) is a common surgical procedure for knee injuries, with various graft options tailored to surgeon preference and patient characteristics.³⁹ The all-inside ACLR technique using a 4-strand hamstring tendon (HT) graft with adjustable loop device (ALD) fixation has gained popularity because of benefits such as simplified autograft tendon harvest with reduced donor-site morbidity and bone loss, optimized graft tension, and decreased postoperative pain.^28,41 However, variability in tendon size and length, especially in smaller patients, can compromise graft quality and the surgical outcome.^30,46

Alternative 5-strand and 6-strand HT (6HT) graft preparation techniques have been developed,^14,19,25,26 especially when dealing with shorter HTs or smaller diameter 4-strand HT (4HT) grafts. Using tripled or smaller diameter grafts (<8 mm) has been linked to an increased risk of failure and reduced mechanical strength.^5,6,9,12,30 Despite achieving significantly larger graft diameters with alternative grafts,⁵¹ recent studies have shown an increased retear rate with no difference in mechanical and clinical outcomes compared with the smaller 4-strand grafts.^4,11,44,49 These findings indicate that graft failure is influenced not only by the failure strength of the HT tissue used in the graft preparation but also by the graft preparation and fixation method applied to the entire ACL graft construct.¹⁰ While tendon loop suspension over a cortical button loop provides considerably high stability, the primary challenge in graft preparation is achieving stable free graft limb fixation. Knotting accessory whipstitch sutures to the ALD or cerclage suturing to connect the free tendon ends to other tendon loops introduces potential “weak links” in the graft construct and may lead to heterogeneous load distribution across the graft limbs.^23,35,45,49 With the advancement of ALDs, the direct RS suturing backup fixation of free graft limbs to the adjustable loop introduces innovative all-inside ACLR preparation techniques, enhancing primary construct fixation. Given the existing knowledge gap in graft fixation with the latest ALD, obtaining biomechanical data to evaluate primary fixation under cyclic loading is essential compared with the clinical standard.

The aim of this study was to evaluate and compare the biomechanical stability of 4HT and 6HT grafts in all-inside ACLR with standard cerclage suturing of the free tendon limbs to other graft loops to grafts with additional direct RS suturing backup fixation of free graft limbs to the adjustable loop (4HT-RS/6HT-RS) using human cadaveric tendon tissue. It was hypothesized that grafts with additional direct RS suturing of the free tendon ends would decrease graft tension loss after surgical fixation during simulated active knee flexion, reduce construct elongation, and increase construct stiffness during cyclic loading at postoperative load levels.

Methods

Testing Groups

Four all-inside ACLR construct groups (n_Tot = 32; n_{per group} = 8) were prepared, including 4HT and 6HT groups with standard cerclage suture fixation of free graft limbs to available graft tendon loops. These were compared with groups with additional direct RS suture backup fixation of the free graft limbs to femoral and tibial adjustable loop devices.

Specimen Preparation

A total of 32 freshly harvested porcine tibiae (age 6-8 months) were collected from the local slaughterhouse and prepared by removing all the soft tissue from the bone. Porcine tissue was used because of its previously reported mechanical properties similar to those of young adult human bone.^2,3 The bone was cut 14 cm distal to the joint line. The embedding of the tibiae was carried out using a bicomponent embedding material (Gößl & Pfaff GmbH) aligned with the tibial axis parallel to the embedding mold ≤2 cm distal to the intended tunnel exit point to leave sufficient space for adjustable loop tensioning. The lateral plateau of the tibia was cut perpendicularly to the tunnel axis with a bone saw to ensure a constant length of 40 mm (Figure 1A). Tibial specimens were prepared with a 9 mm–diameter graft tunnel 25 mm in length using a cannulated drill over an implant-specific guide pin, leaving a 15-mm bone bridge (Figure 1B). Acrylic blocks (total length of 35 mm and implant-specific tunnel preparations according to the clinical setting of femoral-sided ACLR with 25 mm of graft insertion) were used instead of the femoral bone stock to allow for visualization during graft insertion and cyclic loading inside the tunnel. All tissue was stored at −20°C, and embedded tibiae were thawed overnight before biomechanical testing.

Figure 1.

(A) Experimental test setup. (B) Schematic of the standard 4HT and 6HT groups with cerclage suturing of free tendon ends and additional direct RS suturing (4HT-RS/6HT-RS) of the free graft limbs to the adjustable loop with bone tunnel– and graft-related definitions for all-inside anterior cruciate ligament reconstruction. ST, semitendinosus; GC, gracilis; 4HT, 4-strand hamstring tendon; 4HT-RS, 4-strand hamstring tendon with ripstop; 6HT, 6-strand hamstring tendon; 6HT-RS, 6-strand hamstring tendon with ripstop.

Gracilis and semitendinosus tendons from 32 human cadaveric knees (mean ± SD age, 61.61 ± 9.90 years) provided from the Science Care donor bank were harvested using a tendon stripper. All obtained HTs were visually inspected to ensure structural integrity. Tendons were thawed for 2 hours at room temperature before biomechanical testing and cleaned of excess muscle, fat, and fascia before graft preparation. All semitendinosus tendons for 4HT graft groups were cut to 290 mm, and shorter semitendinosus and gracilis tendons were cut to 210 mm for 6HT graft preparation. All HTs were trimmed in line with the fiber orientation to obtain a final graft of 70 mm in length and 9 mm in diameter, measured with a graft sizing block. All specimens were kept moist with physiological saline solution during preparation and testing.

Graft Preparation

For standard graft preparation (4HT/6HT), femoral and tibial ALDs with (ACL TightRope II RT; Arthrex) and without (TightRope II ABS; Arthrex) button were used. For grafts with additional RS suturing (4HT-RS/6HT-RS), femoral (FiberTag TightRope II; Arthrex) and tibial (FiberTag TightRope II ABS; Arthrex) implants containing a preloaded ALD with No. 2 looped suture, along with braided tape, were used. The braided tape segment was affixed to the adjustable loop without compromising the loop-shortening or locking mechanism. For cortical suspension of all tibial ALDs, a button (ABS Button; Arthrex) was attached and positioned onto the cortex. The individual grafts were prepared as follows:

4HT (Standard). Graft preparation with cerclage suturing of free graft limbs was performed according to Lynch and Anderson.²⁹ The tendon was symmetrically folded over the tibial suture loop, with both free ends doubled and passed through the femoral suture loop in the same direction, creating a quadrupled graft. The free graft limbs were whipstitched together (3-4 stitches) using a No. 2 nonabsorbable suture over a length of 20 mm and placed inside the graft near the tibial end. The graft was installed in a preparation station utilizing the ALDs and the whipstitch sutures under 20 N of tension measured with a spring-loaded tensioning device before cerclage suturing together of all tibial limbs using a No. 2 nonabsorbable suture. The first stitch was passed from the inside of the graft through 2 of the 4 graft limbs outward and back through all strands to the other side. The suture was wrapped around the graft (3 times) and passed through the 2 remaining tendon strands, ending at the middle of the graft. The knot stack was dunked into the tissue after manually tightening the cerclage suture tissue and tying 4 alternating half-hitch knots. Further cerclage stitches were added under 80 N of tension to finally have 2 on each graft end.

4HT-RS (RS Suturing). The surgical technique steps in Figure 2 demonstrate the method of preparing a quadrupled graft with direct RS suturing of the free tendon ends. The tendon was folded over the tibial and femoral ALD (Figure 2, A and B). After needle passing and cutting of the suture loop, 4 baseball stitches through the braided tape and free graft limbs were created (Figure 2C) with the suture finally passed through the adjustable loop and creating 4 reverse baseball stitches on the opposing side. After suture knotting (Figure 2D) and graft limb placement inside the tendon loop (Figure 2E), cerclage sutures were created on the tibial and femoral side (Figure 2F).

Figure 2.

Tendons for quadrupled graft preparation with (A) direct ripstop suturing of the free tendon ends to the ALD were symmetrically folded over the tibial suture loop, with (B) both free ends doubled and passed through the femoral suture loop in the same direction. (C) The needle with attached No. 2 looped suture was passed through the doubled free graft limbs 20 mm away from the graft end. The suture loop was cut open to create 4 baseball stitches on one side of the graft through the braided tape and both graft limbs using the suture limb with the needle attached. (D) The suture was then passed through the adjustable loop, and 4 additional baseball stitches were created in the reverse direction on the opposing side. The sutures were then secured with 4 half-hitch knots. (E) After placing the sutured free graft limbs inside the graft, (F) a cerclage suture was created under 80 N of tension on the tibial side, followed by a second cerclage suture on the femoral side.

6HT (Standard). The free graft limbs were whipstitched together at each graft end with 4 whipstitches using No. 2 loop suture (FiberLoop; Arthrex).^38,48 The doubled tendon was folded over the tibial and femoral ALD with the free graft limbs placed within the created graft loops. Using a spring-loaded tensioning device, the graft was installed in a preparation station to create a single cerclage suture under 20 N of tension on each graft end. A second cerclage suture was added at a tension of 80 N.

6HT-RS (RS Suturing). The No. 2 looped suture of the tibial ALD was passed through the doubled free graft limbs 20 mm away from the graft end to create 3 whipstitches through the braided tape and both graft limbs.¹ After passing the suture through the adjustable loop, 3 further whipstitches were performed reversely, and the suture was secured with 4 half-hitch knots. The remaining free graft limbs were passed through the femoral and tibial ALD to create a direct RS suturing fixation, similar to the 4HT-RS preparation. One cerclage suture was applied at each graft end under 80 N of tension.

Construct Fixation

All constructs were preloaded with an 80 N load for 5 minutes before device insertion and mechanical testing to allow stress relaxation and reduce the graft's settling effects.^21,37 The tibia and the acrylic block on the femoral side were fixed to the baseplate and the actuator of the dynamic testing machine (ElectroPuls E10000; Instron) with custom clamps (Figure 1A). Graft fixation was performed at 29 mm of joint space, serving as a reference for later elongation analysis, and the construct was loaded in line with the ACL and the tunnel axis to simulate a “worst-case” loading condition. A dynamic 2-kN load cell (Instron) with a resolution of 0.01 N was used.

The ALD passing and tensioning sutures were shuttled through the bone tunnel of the acrylic block and tibia, the femoral ALD button was flipped, and all passing sutures were removed. The graft was inserted approximately 15 mm into the acrylic bone tunnel by shortening the tensioning sutures, with the ALD remaining knotless. On the tibial side, the graft was introduced by pulling on the adjustable loop, and an ABS button was attached and positioned onto the cortex. Tibial-sided graft tensioning was performed by manually alternating pulling on the ALD tensioning sutures in line with the actuator axis until a tension of 80 N was reached. All tibial ALDs were then tied with 4 half-hitch suture knots.

Biomechanical Testing

The test protocol, based on other biomechanical studies,^6,34,50 consisted of position- and force-controlled cyclic loading to simulate ACL kinematics during weightbearing knee flexion²⁷ and in vivo ACL loads during the early and late rehabilitation phase after ACL injuries.^42,43,47 The ACL was found to decrease in length by 1 mm and 3 mm at 30° and 90° flexion angles relative to a knee in full extension, respectively.²⁷ Following primary fixation at 80 N in a simulated 30° flexion angle (Figure 3, point a),¹³ graft preconditioning was performed to simulate intraoperative knee flexion for a total of 10 cycles at 0.5 Hz between full extension (joint space 30 mm) and 90° angle flexion (joint space 27 mm). Graft retensioning was manually performed in a simulated full-extended knee position by further tensioning the femoral ALD to achieve a load of 250 N (Figure 3, point b). Cyclic testing began with the position-controlled loading block at a frequency of 0.75 Hz over 1000 cycles. The initial and final (Figure 3, point c) peak forces were measured to determine the graft force loss (Δbc) during the simulated early postoperative phase of range of motion exercises.

Figure 3.

Schematic test protocol with position- and force-controlled loading blocks with data analysis points. S_D250, dynamic elongation 250; S_D400, dynamic elongation 400. S_Init, initial elongation; S_Tot, sum of initial and dynamic elongation.

Subsequent force-controlled cyclic loading was performed between a constant valley load of 10 N and peak loads of 250 N and 400 N, with each load level applied over 1000 cycles at a frequency of 0.75 Hz. The initial elongation (S_Init; Δad) was measured as the valley elongation from the start of testing until the completion of the first cycle of the first force control block (250 N) (Figure 3). Negative and positive elongation values indicated a tight or slack graft in reference to the graft fixation position (simulated 30° knee flexion angle). Dynamic elongation represented the relative valley elongation during force-controlled cyclic loading at the end of peak load level 250 N (S_D250; Δde) and 400 N (S_D250; Δdg). Total elongation was the sum of initial and dynamic elongation (S_Tot; Δag). Dynamic stiffness was calculated using valley and peak values in the last hysteresis curve of the 250 N (D₂₅₀; Δef) and 400 N (D₄₀₀; Δgh) force-controlled load blocks. All test samples underwent a final pull to failure at 50 mm/min.^6,20,32 Ultimate stiffness and ultimate failure load were determined during pull to failure.

Statistical Analysis

In this study, different graft configurations and fixation techniques were independent variables. Primary outcome variables were defined as residual graft force, initial elongation, dynamic elongation, total elongation, dynamic stiffness, and ultimate failure load and stiffness.

Statistical analysis was performed using Sigma Plot Statistics for Windows Version 13.0 (Systat Software). The statistical analysis included 1-way analysis of variance (ANOVA) with Holm-Sidak post hoc tests performed for significant pairwise analyses of primary outcome variables between the 4 groups. Statistical significance was defined as P≤ .05. The normal distribution for each variable was tested a priori via the Shapiro-Wilk test. Nonparametric tests (Kruskal-Wallis) were used to measure variables that were not normally distributed. If the Kruskal-Wallis test was significant, a Tukey post hoc test was conducted for pairwise comparisons. The observed post hoc mean power values of all 1-way ANOVA tests were much higher than the desired power level of 0.8, leading us to conclude that our sample size was sufficient. Data analysis was performed with MATLAB, Version R2023a (MathWorks).

Results

Stability Testing

The residual graft force between the RS suturing groups (4HT-RS/6HT-RS) was similar, but each was significantly higher (P≤ .002) compared with both control groups (Figure 4).

Figure 4.

Residual graft force results (mean ± SD values) with comparative statistical analysis. Asterisks represent significance between groups with P≤ .002. 4HT, 4-strand hamstring tendon; 4HT-RS, 4-strand hamstring tendon with ripstop; 6HT, 6-strand hamstring tendon; 6HT-RS, 6-strand hamstring tendon with ripstop.

Based on a similar initial elongation without significance between all groups (Table 1), the control groups (4HT/6HT) demonstrated significantly higher (P < .001) dynamic elongation (Figure 5) for each load block (250 N and 400 N) and higher total elongation (P≤ .001) compared with both RS suturing groups (4HT-RS/6HT-RS). No significance was found between the RS suturing groups (4HT-RS/6HT-RS) and between the controls (4HT/6HT).

Table 1

Initial and total elongation results indicating tight (negative values) and slack (positive values) graft state in relation to the reference position (simulated 30° knee flexion angle), and ultimate stiffness and load for each graft construct ^a

	4HT	4HT-RS	6HT	6HT-RS
Initial elongation S_Init, mm	–0.1 ± 0.2	–0.3 ± 0.1	–0.3 ± 0.1	–0.3 ± 0.2
Total elongation S_Tot, mm	4.4 ± 0.4	2.9 ± 0.5 ^b	3.9 ± 0.6	2.3 ± 0.4 ^b
Ultimate stiffness, N/mm	162.4 ± 13.7	181.9 ± 9.2 ^d	174.9 ± 8.6	185.4 ± 17.3 ^d
Ultimate load, N	992.5 ± 63.7	989.2 ± 100.3	969.4 ± 45.0	1133.4 ± 67.9 ^c

Data are presented as mean ± SD. 4HT, 4-strand hamstring tendon; 4HT-RS, 4-strand hamstring tendon with ripstop; 6HT, 6-strand hamstring tendon; 6HT-RS, 6-strand hamstring tendon with ripstop; S_Init, initial elongation; S_Tot, sum of initial and dynamic elongation.

Significant difference compared with both controls (4HT and 6HT) with P≤ .001.

Significant difference compared with all other groups with P≤ .003.

Significant difference compared with 4HT with P≤ .02.

Figure 5.

Test results (mean ± SD values) with comparative analysis of the construct elongation after the 2 force-controlled load blocks. Asterisks represent statistical significance between groups as indicated (P < .001). 4HT, 4-strand hamstring tendon; 4HT-RS, 4-strand hamstring tendon with ripstop; 6HT, 6-strand hamstring tendon; 6HT-RS, 6-strand hamstring tendon with ripstop; S_D250, dynamic elongation 250; S_D400, dynamic elongation 400.

Furthermore, the RS suturing groups (4HT-RS/6HT-RS) showed an overall similar dynamic stiffness (Figure 6), which was significantly higher (P≤ .04) for each load block (250 N and 400 N) compared with both control groups (4HT/6HT).

Figure 6.

Test results (mean ± SD values) with comparative analysis of the construct stiffness at the end of the 2 force-controlled load blocks. Asterisks represent statistical significance between groups as indicated (P≤ .04). 4HT, 4-strand hamstring tendon; 4HT-RS, 4-strand hamstring tendon with ripstop; 6HT, 6-strand hamstring tendon; 6HT-RS, 6-strand hamstring tendon with ripstop; D250, dynamic stiffness 250; D400, dynamic stiffness 400.

Pull to Failure

All constructs were pulled to failure. The ultimate stiffness of 4HT was significantly lower (P≤ .02) than that of the RS suturing groups (4HT-RS and 6HT-RS), but there was no significance between the two 6-stranded groups (Table 1). The ultimate load of the 6HT-RS group was significantly higher (P≤ .003) than that of the other groups. The predominant failure mode was suture rupture of the ALD.

Discussion

The most important finding of this full-construct surgical technique–based study was that direct RS suture backup fixation of free graft tendon ends to the ALD significantly improved several key biomechanical parameters, such as residual graft force, dynamic stiffness, and elongation in 4HT and 6HT grafts used for all-inside ACLR compared with clinical standard grafts with cerclage suture fixation. In addition to graft diameter being a critical parameter (<8 mm) for clinical outcomes,^30,46 potential “weak links” in the graft preparation and fixation predetermine the postoperative stability during the graft maturation period.^4,10,16 With the same graft diameter, the study found that graft fixation using direct RS suturing of free tendon ends to the ALD significantly reduced (P≤ .002) tension loss by 8% and 11% in 4HT and 6HT groups, respectively, during simulated weightbearing knee flexion. This backup fixation method also resulted in significantly stiffer constructs than the clinical standard 4- and 6-strand grafts, reducing elongation by up to 37% in the 6-strand configuration at higher peak load.

Postoperative knee stabilization primarily depends on the ability of the ACLR graft construct to withstand applied loads. Preserving graft properties during the rehabilitation period is crucial, as graft stiffness and changes in graft length significantly affect knee laxity.⁸ When a smaller diameter 4HT graft is obtained, alternative preparation techniques are employed using the semitendinosus and gracilis tendons. These techniques either increase the graft diameter or utilize HTs that do not meet the length requirements for 4HT graft preparation.^19,22,33 To achieve optimal tensile strength and resistance to elongation, all HT graft limbs must be equally tensioned and securely fixed to ensure homogeneous load distribution.¹⁸ Particularly with an increasing number of tendon limbs and free tendon ends, a key technical challenge in graft preparation is to tension all tendon limbs homogeneously to avoid negating the effect of any single limb.¹⁹ Manual tensioning of individual strands in multiple-stranded grafts may be unfavorable for achieving uniform tension across all strands.¹⁸ In this study, applying tension via the whipstitch suture in standard grafts and the ALD in RS-sutured grafts enabled the tendon construct to self-align, ensuring equal tension across all graft limbs.

With the tendon construct under tension, 4HT grafts were fixed using cerclage sutures, adhering to the current clinical standard. In this study, great attention was given to ensure the cerclage suture was passed through all tendon limbs of the graft, thereby eliminating the effect of incorrect preparation on the biomechanical graft properties. A previous biomechanical study identified slippage between inner graft limbs and the cerclage sutures as the weak point of a 4HT standard graft under load.⁷ Supplemental graft fixation using accessory free graft limb whipstitch sutures, knotted to the ALD, has improved construct stiffness, reduced cyclic elongation, and increased ultimate load compared with the clinical standard grafts.^{7,10,16,38,40} Despite the improved biomechanical performance of supplemental fixed grafts, uneven loading behavior—characterized by greater elongation of the supplemental fixed free tendon ends compared with other tendon loops in the construct—limited the overall fixation strength and influenced the failure mode.^15,17,24,45 Inadequate primary graft fixation and instability during daily activities can lead to increased micromotion at the graft-tunnel interface and knee laxity, ultimately compromising healing.²¹ The aforementioned studies highlight the necessity for further advancements, particularly in the fixation of the free tendon ends during all-inside graft preparation.

Because of differing testing protocols and the use of human HTs, it is challenging to compare this study's results with other studies thoroughly.^{7,10,16,24,31,38} Direct RS suturing of all–soft tissue quadriceps tendons has been demonstrated to significantly enhance biomechanical properties compared with standard quadriceps tendon whipstitch suturing with suture knot tying to the adjustable loop.²⁴ Consistent with current findings, the RS suture fixation group demonstrated improved graft stability, reduced graft tension loss and elongation, and increased construct stiffness during cyclic loading. This study evaluated the biomechanical properties of 4- and 6-strand human HT grafts using direct RS suturing of free tendon ends to the ALD. The applied graft preparation techniques benefit from interconnecting the free tendon ends and connecting the individual suture stitches along the tendon end by suturing through the soft tissue and braided tape. Multiple interconnected suture stitches ensure a more homogeneous load distribution and reduce the risk of stress concentration, thereby preventing suture pull-through at the suture-tendon interface. Direct attachment of the braided tape to the adjustable loop mitigates construct lengthening effects caused by the loosening of the suture knot tied to the adjustable loop, as observed with free tendon end graft fixation using the accessory whipstitch suture.¹⁷ Higher all-inside HT graft fixation, including direct free tendon end RS suturing with traditional cerclage graft fixation, provides more uniform load distribution across all tendon limbs during repetitive loading that maintains the integrity of the graft without overloading single limbs. The overall higher construct stiffness is associated with better graft stability and durability, which is beneficial for the long-term success of the all-inside ACLR in the clinic. The ultimate tensile strength of all grafts was sufficient for daily activities and was mainly dependent on the ALD.³⁶ Higher failure loads in the 6HT group with both-sided RS fixation (6HT-RS) may be explained due to better load distribution and less concentration of stress on the loop, which was the point of failure in all cases.

Limitations

Several limitations of this study must be acknowledged when interpreting these results. The mean age of human donors was higher than that of middle-aged patients undergoing ACLR, potentially leading to lower graft stability due to age-dependent tissue quality reduction.⁵² Although each graft construct was consistently prepared to achieve a diameter of 9 mm, minor variations in graft size, graft preparation, and suture technique may have influenced the results. Unlike the variable in vivo loading conditions experienced by the human knee during daily activities, a pure tensile load was applied along the tunnel axis to simulate worst-case testing conditions with the cortical button supported by an acrylic block and porcine bone. Utilizing human bone substitutes mitigated bone-related failures associated with weaker human bone and should not have affected the ultimate strength of the graft. Consequently, the predominant failure mode across all groups was suture rupture of the ALD. The tissues underwent 1 freeze-thaw cycle, which has the potential to alter the biomechanical properties of the specimens. Regardless of the technique or tendons used in ligament reconstruction, this time-zero biomechanical study did not account for the biological healing process, which is crucial for the surgical outcome. Therefore, clinical studies must determine if these biomechanical findings correlate with clinical outcomes and knee laxity.

Conclusion

Our study demonstrated that direct RS suturing of free graft tendon ends to the adjustable loop significantly decreased graft tension loss, increased construct stiffness, and reduced cyclic elongation in 4- and 6-strand all-inside HT grafts compared with standard grafts with cerclage suturing. Improved primary fixation may ensure more homogeneous load distribution across the graft limbs and reduce the risk of early clinical failure.

Footnotes

Final revision submitted August 20, 2025; accepted September 18, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: Research support for this study was provided by Arthrex. J.A. is employed by Arthrex. A.B. is a consultant for Arthrex and has received consulting fees from Arthrex and Collagen Matrix, speaking fees from Arthrex and Synthes GmbH, nonconsulting fees from Arthrex, royalties from Arthrex, and hospitality payments from GE Healthcare. P.A.S. is a consultant for Arthrex and has received consulting fees, speaking fees, and royalties from Arthrex; nonconsulting fees from Kairos Surgical; and support for education from United Orthopedics. E.M. has received consulting fees, speaking fees, nonconsulting fees, and royalties from Arthrex. S.B. is employed by Arthrex. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval was not sought for the present study.

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