Anterior cruciate ligament reconstruction (ACLR) is widely accepted as the treatment of choice for patients with functional instability due to an anterior cruciate ligament (ACL)-deficient knee. It is currently estimated that >100,000 primary ACLR are performed annually in the United States. Graft sources for ACLR include autografts and allografts consisting of either bone-to-bone fixation (Achilles, patellar tendon, quadriceps) or soft tissue fixation (tibialis anterior/posterior, hamstrings). Although there is great debate in the literature, the gold standard for graft choice in young, active patients remains the bone-patellar tendon-bone (BTB) autograft. This graft allows for stable fixation with early bone-to-bone healing, enables early and aggressive postoperative rehabilitation, and has demonstrated excellent clinical and functional results over the past 3 decades.1 However, unlike soft tissue grafts, the use of BTB grafts can have the added difficulty of graft-tunnel length mismatch.
Graft-tunnel mismatch is encountered in approximately 13% of all BTB reconstructions,2 which can lead to intraoperative or postoperative complications. Mismatch, where the graft is relatively long, can result in a proud tibial bone plug (TBP) that compromises the integrity of interference screw fixation.3 In contrast, a relatively short graft may result in blind placement of the tibial interference screw. This can lead to screw divergence, graft laceration, or articular penetration.
Over the past decade, the increased usage of BTB allografts along with striving to recreate the native ACL femoral footprint has led to an increased likelihood of graft-tunnel mismatch. For example, the push toward anatomic femoral tunnel placement has led to the development of an accessory medial portal, which results in shorter femoral tunnel lengths, thereby increasing the chance of a graft being too long.4 Therefore, the surgeon must have several methods available to prevent and/or resolve this situation if and when it occurs in the operating room. This article will discuss methods to prevent and rectify graft-tunnel mismatch when performing endoscopic ACLR using a BTB allograft or autograft.
PREOPERATIVE PREVENTION OF GRAFT-TUNNEL MISMATCH
Although most authors have described intraoperative methods to manage mismatch, ideally the surgeon can identify potential pitfalls preoperatively and plan accordingly. The most important factor to consider is the height of the donor and recipient. In the majority of cases using a BTB autograft, a taller patient will have both a longer patellar tendon length and longer intra-articular graft distance. Therefore, the risk of graft-tunnel mismatch is significantly reduced in autograft cases, as the soft tissue length of the BTB graft and intra-articular distance increase proportionately.2 Of note, however, the surgeon must be wary of the patient with normal stature but significant patella alta, as the tendon length may not be proportionate to the intra-articular distance.
In contrast, graft-tunnel mismatch is more common in allografts due to the significant variability in donor graft sizes. Brown et al3 analyzed the relationship between patient height and the desired length of the tendinous portion of the BTB allograft, reporting a strong positive correlation between intra-articular length of the ACL and patient height, suggesting that height may be an indicator of appropriate graft length. Goldstein et al5 demonstrated that within sexes, patient height correlated with patellar tendon length, offering a guideline for ordering BTB allografts based on the recipients sex and height (Table 1). In these cases, identification of the patient’s height should ideally allow the practitioner to order the correct graft length preoperatively; however, in clinical practice, the availability of tissue may not allow the surgeon to accommodate this selectivity. In our practice, this method has decreased the frequency of graft-tunnel mismatch. Even if one is not able to order tissue of a specific length, a large height discrepancy between the donor and recipient should alert the surgeon to the possibility of graft-tunnel mismatch.
INTRAOPERATIVE TECHNIQUE PEARLS
Through a firm understanding of ideal graft and tunnel length, tibial tunnel angles, and graft fixation options, surgeons can successfully rectify the problem of graft-tunnel mismatch intraoperatively. With regard to BTB fixation, the important graft variables are the length of the femoral bone plug (FBP), TBP, and the soft tissue component (STC). Although it will be covered in more detail, recession of the FBP is suboptimal, leading to the “windshield wiper effect,” tunnel widening, graft elongation, and graft abrasion.6 Therefore, assuming that the distal aspect of the FBP is flush with femoral tunnel aperture, the sum of the STC and TBP length should ideally be equal to the sum of the intra-articular portion (IAP) and tibial tunnel length (Fig. 1).
The ideal graft has an STC that is equivalent in length to the IAP of the ACL in the recipient knee joint. The IAP, defined as the distance between the intra-articular apertures of the tibial and femoral tunnels, is estimated to be between 23 and 30 mm long.7 Therefore, the ideal graft would have an STC of approximately 30 mm. However, the average length of the tendinous portion of a BTB graft in the clinical setting is 45 mm, leaving approximately 15-mm STC that will reside in the tibial tunnel. Denti et al8 recommend a tibial interference screw of at least 20 mm for adequate fixation and 5 mm of “extra” TBP between the screw and STC to prevent laceration. Extrapolating from this, one should accommodate 15 mm of STC and 25 mm of TBP in the tibial tunnel, giving a minimum tunnel length of 40 mm. Another way of approaching this is the “graft-50” formula, initially described by Kenna et al in 1993.9 This involves measuring the total graft length and subtracting 50 mm (assuming 20 mm FBP+30 mm IAP), resulting in the ideal tibial tunnel length. Given our prior assumption of the STC being 45 mm, the graft-50 rule suggests a 45-mm tibial tunnel if using 25-mm bone plugs.
Drilling a tibial tunnel at 40 degrees yields an average tunnel length of 45.44±2.18 mm. With each added degree of inclination, one gains 0.68 mm of tibial tunnel length.8 Therefore, one should avoid angles <40 to 45 degrees to prevent TBP protrusion from the distal aperture. Angles beyond 60 degrees should also be avoided, as they decrease the ability to place the femoral tunnel appropriately from a transtibial approach, leading to a vertical graft (Fig. 2). One can also increase the length of the tibial tunnel by decreasing the obliquity, resulting in a more central distal tibial aperture. However, doing so may also result in vertical placement of the femoral tunnel when using a transtibial endoscopic approach.
A variety of algorithms or rules have been developed to help surgeons avoid mismatch intraoperatively. Shaffer et al10 suggested measuring the IAP and the STC length, allowing the surgeon to correctly calculate the necessary tibial tunnel length. Hartman and colleagues suggests drilling the tibial tunnel at 60 degrees and then placing the graduated femoral reamer at the start of the femoral tunnel. Before drilling, the distance to the distal tibial aperture should be measured from the reamer. The difference between this distance and the total graft length should therefore be the length of the femoral tunnel.11 The difficulty with this method is that one may encounter the posterior cortex of the femur before being able to accommodate the graft and a tibial tunnel of 60 degrees may result in vertical graft placement.
Yet another popular method is the “N+” class, where N is the STC of the graft. Miller and Hinkin12 described the “N+7” formula, where 7 degrees are added to the STC length to determine the angle of the tibial tunnel. Subsequently, other methods recommended include the following: “N+2,”13 “N+10,”2 and always drilling at 55 degrees with the entry point midway between the tibial tubercle and the posteromedial aspect of the tibia.7 Although the best technique is still debatable, it seems that the “N+” rules are more accurate than drilling at a fixed angle (Table 2). However, the ideal fixed angle approach is likely somewhere between 50 and 55 degrees. The authors prefer to drill at 55 degrees even if the STC is ≤45 mm, using a longer interference screw if necessary.
GRAFT-TUNNEL MISMATCH: WHAT TO DO IF THE GRAFT IS TOO LONG
If the graft is too long, as suggested by protrusion of the bone plug from the tibial tunnel after delivery of the FBP, the appropriate method of fixation is likely determined by the magnitude of the mismatch. Table 3 summarizes the biomechanical strength of each method of resolution and the amount of shortening it affords. It is helpful to subclassify graft mismatch as either <12 or >12 mm.
Less Than or Equal to 12-mm Mismatch
For grafts that protrude from the distal tibial aperture by 12 mm or less, the easiest first step is to determine whether changing the orientation of the graft will improve alignment. That is to say, given an IAP and STC that are approximately equal, if the TBP is larger than the FBP by an amount greater than or equal to the amount of plug prominent distally, the graft can be reversed. This would place the prior TBP in the femur and vice-versa. For example, if the graft is 7 mm prominent distally and the TBP−FBP≥7 mm, this method will be effective. If this is not possible, the next option is recessing the FBP to a maximum of 15 mm into the femoral tunnel. Clinical results of recessing 5 and 15 mm have demonstrated no significant difference in KT-1000 measurements at 12 months.20 However, Shah et al21 demonstrated in a cadaveric model that 10 mm of FBP recession can lead to impingement of the graft at the intercondylar roof and notch, possibly leading to anterior knee pain, extension deficit, possible fraying of the graft, and eventual failure. To prevent the technical error of graft laceration fixation of a recessed graft, the surgeon may orient the cortical portion of the graft posteroinferiorly (opposite side of screw), use a soft tissue protector sleeve (Smith and Nephew Fast Fix system) to protect the STC from the screw, and recess the screw relative to the bone plug. The authors routinely recess the FBP 5 mm in situations of minimal mismatch without deteriorating results.
Other techniques available to the surgeon when the graft is <12 mm long include rotating the graft or trimming the protruding tibial bone block. In a porcine model, rotation or twisting of the graft 540 degrees demonstrated shortening of 5.4 mm or 10% of the total length.17 Augé and colleagues described a technique of shortening the graft through external rotation, which achieves greater shortening than internal rotation. At 630 degrees of external rotation, approximately 25% of shortening can be achieved, allowing for adequate interference fixation in most cases.22 Increasing the rotation, however, may also increase graft strain and increase the risk for graft failure.
Finally, the surgeon may trim the protruding TBP to achieve interference screw fixation. In a porcine model, trimming the protruding bone block did not sacrifice fixation strength as long as the length of the remaining bone plug in the tunnel was >10 mm.18 In a similar model, Black et al23 demonstrated that interference screws of 9 mm diameter and 12.5 to 20 mm length can be used without sacrificing mechanical properties. Therefore, if the TBP is trimmed, the surgeon should use a shorter interference screw. It is not clear how well this translates to human tissue; however, the authors have found this method useful clinically without degrading results.
Greater Than 12-mm Mismatch
For cases where the graft-tunnel mismatch is >12 mm, more aggressive methods should be used in an attempt to achieve adequate fixation. The free bone block (FBB) technique involves removing the distal bone plug from the tendon sharply, running a nonabsorbable suture through the free tendon, and using the FBB as a wedge at the tibial aperture (Fig. 3). Verma et al2 demonstrated similar clinical results for graft rotation and the FBB technique when compared with the routine technique. Another method of fixation is the screw-and-post (SAP) technique that prepares the distal portion of the graft as in the FBB technique, but instead of relying on the bone block for an interference fit, the sutures are tied over a cancellous screw and washer distal to the tibial aperture. In a bovine model, the maximum load to failure of the SAP method was 50% of the FBB technique.15 A third technique for significant mismatch involves leaving the distal bone plug attached and fixing it into a distal osseous trough with staples, the so-called “trough technique.” In a cadaveric model, this method showed similar fixation strength to interference fixation using a 9×20 mm.16 However, the staple method showed less displacement, resulting in increased stiffness and subsequent block breakage. One final method to avoid mismatch is the 2-incision technique. This allows for rigid cortical fixation proximally and distally but at the expense of aperture fixation at the intra-articular femoral opening. This may increase the amount of “windshield wiper” effect compared with the aperture fixation. Although the authors routinely externally rotate the TBP 180 degrees before fixation, the authors prefer the FBB technique to hyperrotation for mismatch >12 mm.
GRAFT-TUNNEL MISMATCH: WHAT TO DO IF THE TBP IS RECESSED
In general, mismatch with grafts that are too long are much more common than those that are too short. If the graft is too short, the most common technique is using a longer screw for the TBP to gain adequate fixation. The use of a longer screw will also allow the screw to remain flush with the tibial cortex, allowing for easier removal if necessary in a revision situation. Potential problems with longer screw placement may occur, such as screw divergence or intra-articular penetration. Similar to the situation of >12 mm of mismatch, one can remove the TBP and use supplemental fixation though a SAP technique.
The subject of graft mismatch, how it happens, how to avoid it, and how to treat it is complicated and controversial. On the basis of the data above, the authors prefer ordering an allograft based on patient height when possible and using the transtibial approach with the tibial tunnel drilled at a minimum of 55 degrees. Along with this, graft mismatch <12 mm is addressed by a combination of graft recession and distal TBP trimming. For situations where >12 mm of mismatch exists, we recommend the FBP technique.
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