Graft fixation techniques in anterior cruciate ligament reconstruction – A narrative review

Tunnel widening

Saccomanno et al.^[25] mentioned that there was significantly more femoral tunnel widening with the use of the EndoButton compared to cross-pins for fixation within the femoral tunnel. Intratunnel graft motion leading to tunnel widening is divided into longitudinal motion (bungee effect) and transverse motion (windshield wiper effect).^[25] Hoher et al. proved that when the distance between tendon tissue and button increased, graft tunnel motion also increased.^[26] This explained the finding of Fauno et al. that both femoral and tibial tunnel widening was greater with EndoButton where the fixation point away from the joint. Hence, not only intratunnel graft motion leads to tunnel widening, but also, non-anatomical tunnel placement^[27] and aggressive rehabilitation^[28] are other mechanical causes for tunnel widening. Silva et al.^[29] mentioned that tunnel widening is not only attributed to mechanical causes, but there are also biological causes which include bone resorption due to osteoclast activation by mediators between tunnel and graft, non-specific inflammatory response, and heat necrosis due to drilling.

Similar tunnel enlargement was seen between aperture fixation and extracortical fixation.^[30] Therefore, the type of fixation appears not to have a significant effect on tunnel enlargement, since enlargement occurs irrespective of fixation type, suggesting that the enlargement may be more of a biological than a mechanical phenomenon.

Biomechanics

Milano et al.,^[31] in his in vitro biomechanical study on porcine knees for femoral fixation of soft-tissue grafts, concluded that ultimate failure load and stiffness are higher for EndoButton than interference screws. The ultimate failure load of EndoButton for soft-tissue femoral fixation was comparatively higher than cross-pins and bioscrews in all biomechanical studies^[31,32] with values more than 590N [Table 1] which is the amount of force required for all daily day-to-day to activities. Soft-tissue grafts fixed with EndoButton on the femoral side showed maximum graft-bone displacement after cyclical loading in an in vitro biomechanical study.^[31,33] However, graft slippage (i.e., displacement at the distal graft end) was greater for the Rigidfix and interference screws than EndoButton and BioTransFix.^[34,35] Hence, corticocancellous devices (Rigidfix) were found to be superior for soft-tissue graft femoral fixation biomechanically in terms of graft elongation, stiffness, ultimate failure load, and extracortical suspensory devices showing variable biomechanical behavior.^[31] Interference screws and cancellous suspension devices showed inferior biomechanics for soft-tissue graft femoral fixation. Since cross-pins had associated risks of intra-articular damage, reported cases of supracondylar fracture,^[36] pin dislocation, and iliotibial band (ITB) friction syndrome^[37] that most of the surgeons prefer to use extracortical suspensory fixation devices than cross-pins for soft-tissue graft femoral fixation.

Fixed loop versus adjustable loop

At present, there are two extracortical suspensory fixation devices in practice: A cortical button with a fixed loop and an adjustable loop. Kamelger et al. did a biomechanical analysis of isolated device testing between various fixed loops (EndoButton, ToggleLoc, and RetroButton) using porcine femurs. All fixed loop implants demonstrated an ultimate failure load of more than 590N, and this improved implant design limits graft tunnel motion. The stiffness of graft–fixed-loop construct reapproximated the native ACL stiffness. Most of the failures in his study occurred due to the breakage of continuous suture loops in these devices.^[39]

Fixed-loop devices limit graft migration, but the requirement to drill tunnels for a specific length raises concern in terms of bone preservation and tendon-bone healing in terms of inadequate graft length. However, adjustable-loop devices allow the surgeon to adapt to different tunnel lengths intraoperatively and maximize the amount of graft within the tunnel. Furthermore, adjustable-loop devices have the ability to retension the graft on the femoral side after tibial fixation.^[40] Biomechanical analysis of adjustable devices was necessary and should be comparable with the fixed-loop devices for surgical usage. Biomechanical analysis of all adjustable loops, as mentioned in Table 2, demonstrated an ultimate failure load of more than 590N, which corresponds to the commonly accepted recommendation, and also, it was comparable to the fixed-loop devices.^[32] Most of the failure occurred by tendon rupture in fixed loops, loop breakage, and slippage in adjustable loops. Stiffness also reapproximated the stiffness of native ACL constructs. Barrow et al.^[41] showed significant loop lengthening of TightRope RT device more than 3 mm after fewer cycles (1349 ± 316) than the ToggleLoc (2576 ± 73). With the free suture ends tied, after 4500 cycles, the TightRope had a significant decrease in lengthening. Hence, adjustable-loop devices need to be retensioned after cycling the knee and free suture ends should be tied, which had been shown to decrease cyclic displacement making it biomechanically better. However, fixed-loop devices are always superior than the adjustable-loop devices biomechanically because they allow less cyclic and initial displacement, thus providing better graft femoral fixation in terms of limiting graft slippage and providing sufficient graft strength.

Mayr et al., in his biomechanical study, found that tibia suspensory fixation yielded a higher ultimate failure load but with a higher graft elongation.^[43] There are no biomechanical studies involving both femoral and tibial suspensory fixation which is used in all inside ACL reconstruction. However, there are clinical outcome comparison studies that exist between all inside ACL reconstruction and standard ACL reconstruction.

Clinical outcome

Saccomanno had done a systematic review of five studies involving 317 patients with a mean follow-up period of 21.7 ± 7.0 months.^[25] The Lysholm score, Tegner activity score, and International Knee Documentation Committee (IKDC) score were compiled. The included studies did not report any significant differences in clinical outcomes between extracortical suspensory fixation and transfemoral fixation.^[25] Ma et al.^[44] showed that rigid aperture fixation using a biodegradable screw and suspensory fixation using EndoButton did not lead to significant differences in clinical outcome at 24–40 months follow-up evaluation. Significant tunnel enlargement was present in both groups, more pronounced on the femoral side. Harilainen et al done prospective randomized clinical comparison of femoral transfixation versus bioscrew fixation in hamstring tendon ACL reconstruction. There was no significant difference in knee laxity, IKDC score, Tegner Lysholm Score.^[45]

There are few clinical studies that have compared adjustable-loop and fixed-loop femoral cortical suspension devices, which showed no significant differences in knee stability and the graft failure rate between adjustable- and fixed-loop femoral cortical suspension devices at two years postoperatively in a consecutive series of 188 patients who had undergone primary ACLR using hamstring autograft. However, Choi et al.,^[46] in his prospective study, found that femoral fixation by the use of the fixed-loop device resulted in significantly better knee stability on the pivot-shift test than the adjustable-loop device after ACLR with a hamstring graft. However, the patients in the adjustable-loop group who had grade 2 pivot-shift test findings had excellent Lysholm scores.

In a meta-analysis by Connaughton et al.,^[47] they compared all inside ACL reconstruction (both femoral and tibial suspensory fixation) and standard ACL reconstruction (femoral suspensory fixation and tibia aperture fixation). They found that clinical outcomes are similar between the two, even though the lowest visual analog scale scores were seen with all-inside techniques. Some studies reported a higher graft failure rate in all inside techniques, probably attributed to higher graft elongation in suspensory fixation and early return to pivoting sports before ligamentization.^[48] Hence, there is no superiority among transfemoral fixation, extracortical suspensory fixation, and interference screw fixation for soft-tissue grafts in clinical outcome, even though there were significant differences in biomechanical analysis and tunnel widening.

Biomechanics

Ishibashi et al., in the biomechanical analysis using porcine knees, proved that proximal graft fixation in tibia reduced anterior displacement and internal rotation of tibia as well as in situ forces of the graft itself.^[49] Mayr et al., in his biomechanical study using calf tibiae, found that interference screws demonstrated higher pullout stiffness (309.5N/mm vs. 185.6 N/mm) and reduced elongation (3.33 mm vs. 6.08 mm) than adjustable loops.^[43] However, the ultimate failure load was lower than adjustable loops (693N vs. 908N), which was well within the 590N range.^[32,43] Scheffler et al. suggested that direct anatomic fixation should be used for soft-tissue fixation, especially on the tibial side, since it is the weakest point in all reconstructions.^[35] However, this direct soft-tissue fixation had the risk of graft slippage since the mechanical engagement between the screw and the graft is less intense. Further, it has been suggested that the screws caused laceration and necrosis of soft-tissue grafts.^[22] Hence, it is better to go for the application of backup or hybrid fixation, especially on the tibial fixation site. Hakimi et al. found that in UK, almost 57% of tibial fixation was most commonly done by interference screws, followed by Intrafix (31%). A bioabsorbable interference screw was used in 97% of cases for tibial side fixation.^[24] Interference screws should hold the graft firmly for a minimum of 12 weeks so early rehabilitation is allowed without any concern in graft integration. Other direct methods such as clawed washer-screw combination and staples showed inferior biomechanics and increased risk of complications such as skin irritation, percutaneous ganglion with these devices.^[50]

Biomechanical analysis between metal and biodegradable screws using flexor graft tendon complex in porcine knees was done by Nakano et al.^[51] He found that biodegradable screws had high stiffness and low ultimate failure load than metal screws. The ultimate failure load of biodegradable screws is higher than titanium for femoral side soft-tissue graft fixation and similar for tibial side fixation. However, titanium screws produce more screw thread-induced graft laceration, especially on the femoral side than the tibial side. Hence, metal screws should not be preferred for femoral soft-tissue graft fixation. This shows the superiority of biodegradable screws over titanium screws for soft-tissue graft fixation biomechanically, especially in the tibial side.

Clinical outcome

Shen et al.,^[52] in his meta-analysis, included 12 studies comparing biodegradable screws and metal screws clinically. They showed similar clinical outcomes with respect to IKDC scores, Lysholm scores, KT arthrometer testing, and infection rate. However, the biodegradable screw group had an increased rate of knee effusion (Risk ratio [RR], 2.57; P = 0.04; 421 patients in four studies).

Bone-tendon-bone (BTB) graft-interference screw fixation

Bone tendon grafts are mechanically more stable than hamstring grafts. Interference screws, either metal or biodegradable, were most commonly used for BTB graft fixation in the femoral and tibial sides. Compression between graft bone and wall of bone tunnel with the help of interference screws assists in direct bone-to-bone healing of the graft. Plominski^[53] compared the clinical results of BTB graft ACL reconstruction using metal and biodegradable screws. After the follow-up of three years, they found no statistical difference in Lysholm scores and IKDC scores. However, osteolytic changes were observed in the biodegradable screws group with evidence of foreign body reaction in one patient. But Hackl et al compared polyglyconate bioscrew and titanium screw and found no evidence of increased complications for polyglyconate screw for BTB graft fixation.^[54] However, for BTB graft fixation, both metal and biodegradable screws can be used, but the risk of osteolysis and foreign body reaction associated with biodegradable screws should be considered. Staerke et al. found that the most rigid graft/fixation combination was bone-patellar tendon-bone (BPTB) graft fixed with an interference screw as it demonstrated minimal graft migration compared to soft-tissue grafts fixed with EndoButton and cross-pins. Hence, the BPTB graft and interference screw combination has often been considered the “gold standard” in ACL reconstruction due to its superior mechanical stability.^[22]

Not only do material properties of interference screws influence the stability of the fixation, the other parameters such as screw length, diameter, screw slope, divergence, placement of the screw, bone mineral density (BMD), and insertion torque influence the stability of the fixation.

Screw length

Stalder et al.,^[55] in his study, showed that the femoral fixation strength of the interference screw significantly improved by approximately 30% if the screw is shorter than the graft end in the bone for soft-tissue grafts. However, in tibial fixation of soft-tissue graft, Weiler et al.^[56] showed that increasing screw length improves fixation strength for the same diameter (23 mm–367.2N vs. 28 mm–537.4N). Hence, the length of the screw should be adequate enough to hold the graft, especially in the proximal part of the tibial tunnel, so that fixation strength is increased in the weak link of ACL reconstruction.^[49,57]

Screw diameter

Graft fixation strength (367.2N vs. 479.1N) was higher when the screw diameter is more than the graft diameter by 1 mm, which is due to the press fit mechanism crushing the surrounding cancellous bone. However, there is poor engagement of thread into the bone.^[15,22] Shapiro et al.^[58] found that in BTB graft fixation, there was no difference in biomechanical parameters between 7 mm and 9 mm screws, and iatrogenic injuries to patellar tendon bone block were lower with 7 mm screws.

Screw slope

Daneshvarhashjin et al.^[59] recently studied newly designed and fabricated two differently tapered interference screws where the diameters of both screws were equal to bone tunnel diameter in one-third of their length from screw tip, then they were gradually increased by 1 mm, in the lower slope tapered interference screw (LSTIS), and 2 mm, in the higher slope tapered interference screw (HSTIS) screws [Figure 4]. On biomechanical analysis, HSTIS showed greater graft-bone-screw construct stiffness and a lower graft laxity compared to LSTIS. Biomechanically, higher slope screws demonstrated lower contact pressure than lower slope screws in the proximal one-third region of the tibia tunnel which prevents rupture of grafts, and better contact pressure in the mid-third and outer third region, which prevents graft slippage. Hence, screw slope also determines the stability of the fixation, which needs further research in vitro and in vivo.

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Figure 4:

Images of two tapered interference screws. Left side – lower slope tapered interference screw (LSTIS) and right side – higher slope tapered interference screw (HSTIS).

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BMD and insertion torque

Both insertion torque and BMD were related to the maximum load the graft withstood.^[60] Phillips et al.^[61] found that the insertion torque was significantly higher at the distal third of the tibial tunnel (outer cortex, 8.7in/lb) than at the middle third and proximal third (joint line of the tibial tunnel – 4.3 in/lb). Lower insertion torque at the proximal third of the tibial tunnel results in lower peak load and pullout strength of the graft, and this may outweigh the proposed benefits of joint-line fixation on the tibial side, as shown by Ishibashi et al.^[10] Hence, measuring BMD and insertion torque in biomechanical analysis of interference screw fixation is necessary.

Screw placement

Placement of the screw can be eccentric or concentric. The concentric placement of the Bio-Intrafix screw within its sheath ensures 360° graft-to-bone placement with better engagement. Wang et al., in their biomechanical analysis, found that the Intrafix demonstrated a higher failure load (719N vs. 476N) than biodegradable screws. Their stiffness and displacement remain the same.^[15] Hence, soft-tissue fixation on the tibial side with Intrafix and biodegradable screws is able to meet physiological demands.

Divergence

Divergence of the screw from the graft may occur during insertion, especially on the femoral side. However, this deviation can be reduced by inserting the screw through the same portal, the femoral tunnel that was drilled, which can be easily done in the medial portal technique of ACL reconstruction.^[61] Schroeder et al.^[62] found a reduction in fixation strength if the divergence was more than 15°.

Transfemoral fixation

Pain due to prominent hardware is the common complication reported with cross-pins. It occurs due to the migration of cross-pins medially, laterally, or intraarticularly,^[63] leading to loose body formation and chondral damage. Cross-pins provide good stability and clinical outcomes, even though they are malpositioned. However, unpredictable complications can be avoided with proper positioning of the cross-pin tunnel and the usage of correct surgical technique.

Suspensory fixation

Tunnel widening is the most common problem encountered with the usage of extracortical suspensory fixation. However, tunnel widening has not been found to have affected clinical outcomes, but it may complicate the revision of ACL reconstruction.^[25] Kong et al. reported that complications include failure of the button to deploy in the proper position (i.e., it may not flush with the distal femoral cortex, or it may have deployed in the intraosseous tunnel).^[64] Mae et al. found that 25.2% of patients after ACL reconstruction had tissue interposition between the EndoButton and femoral lateral cortex, and EndoButton with tissue interposition migrated more frequently than those without it after ACL reconstruction. However, clinical outcomes remained the same with the migrated EndoButton.^[65] Sylvian Guy suggested direct visualization of the implant to check for proper deployment and to avoid tissue interposition which shortens the distance between ITB and button which will prevent ITB friction [Figure 5].^[66]

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Figure 5:

Soft-tissue interposition between the cortex and the button.

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Interference screws

Metal interference fixation has been associated with several intraoperative complications, including graft laceration, breakage of the bone block, advancement of graft, and incorrect screw placement.^[67] Even though there were cases of migration of screws in the popliteal fossa, intra-articular notch, and lateral gutter, there were no reports of migrated screws causing loss of stability^[68] [Figure 6]. The probable cause for migration explained was bone resorption, incorrect placement of the tunnel or screw, incomplete insertion of the screw, and secondary twisting injury. Hence, migrated metal screws, if present and symptomatic, should be removed arthroscopically, and arthrotomy can be considered for difficult removal.

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Figure 6:

Migration of screws in the popliteal fossa (Case courtesy of Henry Knipe, Radiopaedia.org, rID: 72407).

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Bioabsorbable screws are less likely to cause graft laceration than metal screws and it won’t create ferromagnetic artifacts in MRI. It allows for easier revision surgery. However, biodegradable screws are associated with several complications due to their incomplete or prolonged degradation and incomplete ossification. Degradation is influenced by several factors which include polymer type, implant size, location, local circulation, the percentage of crystallinity, molecular weight, and surface area open to degradation.^[69] PGA screws degrade rapidly and PLLA screws have longer degradation periods.^[70] Macarini with MRI analysis reported that 34 of 35 poly-D-L-lactic acid (PDLLA) screws demonstrated complete degradation and ossification at the end of three years. There was a cyst-like formation at the screw site, which was considered to be a normal feature of the screw degradation process.^[71]

Degradation of bioscrews occurs by hydrolysis of the hydrolytically unstable polymer. Breakdown products (glycolic acid and lactic acid) that accumulate create a locally acidic environment that stimulates resorption and inhibits bone formation.^[72] In biocomposite devices, there will be associated osteoconductive components adding to bioabsorbable components, promoting bony ingrowth. When the bioabsorbable polymer portion degrades, there will be an increase in the porosity of the implant, expanding the surface area for the breakdown of calcium and phosphate.^[73] This stimulates osteoblasts and increases bone production. Basic salts released by the breakdown of the osteoconductive portion counteract the acidic byproducts of the polymer, which leads to reduced resorption effects. Hence, adding an osteoconductive portion to a bioabsorbable screw has negative effects on screw degradation.^[69] Gradual resorption of the screw leads to the weakening of the device, and it may fracture into fragments, producing mechanical symptoms as well as inflammatory reactions such as synovitis, effusion, cyst or granuloma formation, sterile abscess, or wound dehiscence. There are reports of fractured screw fragments migrating intraarticularly, producing mechanical symptoms and chondral damage.^[74] Hence, early identification is necessary to prevent the devastating complications of chondral injury. Gonzalez-Lomas et al. reported a similar 5% incidence of pretibial cyst formation and found that none of the cysts were infectious, and the graft was well incorporated without any loss in instability.^[75] As there is incomplete bone integration associated with biodegradable screws, stress risers would be created, leading to fracture. Biodegradable screws also has the demonstrated risk of fracture during insertion without any failure of reconstruction.^[76]

Shen et al.^[52], in their meta-analysis, revealed that more tunnel widening was observed with biodegradable screws than metal screws, especially on the femoral side, without any loss in stability. Other significant adverse events include knee effusion and screw breakage. However, functional outcome measures between the two remain the same. This does not justify the advantage of biodegradable screws over metal screws. Even though it is said to be advantageous in MRI, requesting for follow-up MRI after ACL reconstruction is not a routine clinical practice. Metal screws are said to cause graft laceration, but there was only one case report regarding the same in the meta-analysis of Shen et al.^[52] Although it was claimed that biodegradable screws allow for easier revision surgery, there was no Randomised Controlled Trial (RCT) proving the same. Hence, biodegradable screws can be used for fixation as it has better biomechanical and functional outcomes, but the patient should be informed about the risks and complications associated with it.