Sparing the Anterior Cruciate Ligament Remnant: Is It Worth the Hassle?

Introduction

The anterior cruciate ligament (ACL) is composed of two bundles, the anteromedial bundle (AM) and the posterolateral bundle (PL), measuring on average 33 mm in length and 11 mm in diameter. The insertion on the tibial plateau is close to the medial tibial eminence, while the femoral attachment is located on the posteromedial aspect of the lateral femoral condyle.

The function of the two ACL bundles has been investigated by several studies over the last 10 years. The PL is mainly tight in extension and has a central role in the rotational stability of the knee joint; AM is taut throughout the whole range of motion, with maximum tension between 45 degrees and 60 degrees of knee flexion.

The annual incidence of ACL rupture in the United States is about 200,000, with at least 100,000 receiving arthroscopic reconstruction. ACL injuries mainly occur among athletes. Most of them are caused by a noncontact pivoting injury, related to a change of direction or deceleration maneuver. Often athletes are not able to keep playing because of instability. Patients frequently claim to feel a “pop” during the injury and successively develop an immediate or subacute effusion.

Partial lesions of the ACL constitute about half of all ligamentous damages. In the general population, almost 1 in 3000 are injured per year; 70% are injured during sport activities. Although ACL complete injuries are more frequent than the partial ones, these kinds of injuries have also been widely described. Considering all the injuries of the ACL, almost 5%–15% involves only one of the two bundles of which it is composed.

Women have a higher risk of developing injuries of the ACL than males; Messina et al. observed a sample of female basketball players of a high school in Texas and found out that the male to female ratio was 1:4. The same relationship was found by Lindenfeld et al. and Ferretti et al. between male and female players who played football and volleyball, respectively.

Partial ACL injuries can show a negative Lachman test; therefore the pivot-shift test and magnetic resonance imaging (MRI) are necessary to improve the diagnosis. When clinical examinations and imaging studies do not yield to univocal diagnosis, an accurate arthroscopy may be advocated to obtain a definitive diagnosis of a single-bundle ACL lesion.

Some authors, such as Dejour et al., advocate the combination of clinical examinations and stress x-rays to help the surgeon identify partial ACL tears, these being diagnostic tools highly sensitive in discriminating between complete versus partial ACL tears.

Preserving uninjured bundles has several advantages, though their effectiveness in the clinical setting is still to be proven. Some authors argued that ACL remnants could provide better initial stability compared with the graft alone, which relies only on fixation devices. In addition, the remnants could enhance the vascularization and the ligamentization process of the graft. The intact bundle could provide clear landmarks in choosing the femoral and tibial tunnel positions. Finally, sparing the remnant could improve the proprioception and accelerate the return to sport, since some of the proprioceptive fibers present into the ACL bundles would be saved.

Outcomes from ACL augmentation were first reported by Adachi et al.; afterwards, a few studies have been published on that issue. What emerges from these studies is that ACL augmentation technique provides excellent function in terms of joint stability and position sense, with a consequently high functional score. Yet none of the available studies has reported on the ability of this technique to restore the preinjury level. Moreover, it has been reported that augmentation techniques have the potential disadvantage of creating notch overstuffing with consequent impingement and loss of function (see Sonnery-Cottet et al.).

Indications and Contraindications

Indications to reconstruct a partially injured ACL are comparable with those of full ACL tears.

Yoon et al. described three features that should be present when considering ACL augmentation: the remaining bundle should connect the femur to the tibia anatomically, the thickness of the remaining fibers should be 50% or more, and there should be no more than a 5-mm displacement when probing the bundle. Finally, at least one among the anterior draw test, Lachman test, and pivot-shift test on physical examination under anesthesia should show a grade 2 or higher.

Four different types of residual ACL have been previously described by Chen et al. Type 1 comprehends a partial ACL tear with one of the two bundles, PL or AM, being completely or partially intact. Type 2 is a tear of ACL fibers from the femur, with the tibial stump adhering to the lateral femoral condyle or posterior cruciate ligament. Type 3 is a complete tear from the tibial side, with the fibers adhering to the intercondylar eminence or posterior cruciate ligament. Finally, type 4 is a residual ACL remnant too small to form an envelope around the graft.

All four types are suitable for ACL reconstruction preserving the residual ACL fibers. Ossification and/or formation of a lump are contraindications to this technique.

Indications and Contraindications

Indications to reconstruct a partially injured ACL are comparable with those of full ACL tears.

Yoon et al. described three features that should be present when considering ACL augmentation: the remaining bundle should connect the femur to the tibia anatomically, the thickness of the remaining fibers should be 50% or more, and there should be no more than a 5-mm displacement when probing the bundle. Finally, at least one among the anterior draw test, Lachman test, and pivot-shift test on physical examination under anesthesia should show a grade 2 or higher.

Four different types of residual ACL have been previously described by Chen et al. Type 1 comprehends a partial ACL tear with one of the two bundles, PL or AM, being completely or partially intact. Type 2 is a tear of ACL fibers from the femur, with the tibial stump adhering to the lateral femoral condyle or posterior cruciate ligament. Type 3 is a complete tear from the tibial side, with the fibers adhering to the intercondylar eminence or posterior cruciate ligament. Finally, type 4 is a residual ACL remnant too small to form an envelope around the graft.

All four types are suitable for ACL reconstruction preserving the residual ACL fibers. Ossification and/or formation of a lump are contraindications to this technique.

Proprioception and Biomechanical Features

From the first histological demonstrations of the presence in the human ACL of mechanoreceptors to more recent studies, there is extensive evidence that ACL fibers have not only a mechanical function in preventing translation and limiting rotation but also a chief role in proprioception. As a consequence, evidence exists that an ACL-injured knee has an affected proprioception.

Interestingly Denti et al. showed the presence of mechanoreceptors from histological examinations of human ACL remnant samples, while Ochi et al. demonstrated that stimulating the ACL remnants generates somatosensory evoked potential. Stemming from this evidence, Adachi et al. advanced the hypothesis that sparing the remaining fibers would improve the proprioception of the knee.

Vascularization and Histology

Various studies have highlighted the possible advantages of leaving the remnants to enhance the vascularization and the ligamentization process.

Anatomically, there are ligamentous branches originating from the middle genicular artery. Those branches form a network of smaller vessels that transversely crosses the ACL and anastomoses with intraligamentous vessels, representing the main vascular supply to ACL bundles. Dodds and Arnoczky advanced the hypothesis that sparing remnants from the native ACL could enhance the vascularization process of the graft. Gohil and colleagues demonstrated higher signal intensity on MRI from subjects who underwent minimal débridement of the remnant compared with the control group at 2- and 6-month follow-up. Differences in signal intensity were lost at 1-year follow-up; this could mean that the minimal débridement yields to a faster vascularization in the early postoperative time. In a rat model, increased cellularity and angiogenesis were demonstrated with the augmentation technique rather than with the conventional one. Fu et al. demonstrated the presence of mesenchymal stem cells from ACL remnants of ACL-injured patients.

It has been reported that the peripheral segments of the ACL bundles have a greater vascular density; therefore ACL remnants may accelerate the cellular proliferation and revascularization of the graft.

Despite this promising evidence, it has still yet to be proven whether sparing the remnant has a clinical meaning in terms of knee function and stability.

Clinical Results

There are several case series reporting on single-bundle augmentations that showed postoperative improved clinical scores, joint stability, and joint position sense.

Studies comparing the augmentation technique with full ACL reconstruction demonstrated comparable outcomes among the groups, with the exception of the study by Adachi et al., showing better outcomes in the augmentation group. An overview of clinical results is shown in Table 58.1 .

Table 58.1

Clinical Results

Study	Type of Surgery	Clinical Results and Conclusion
Case Series
Ochi et al.	45 anterior cruciate ligament (ACL) augmentations with autogenous semitendinosus tendon; 37 were anteromedial (AM) bundle reconstructions and 8 were posterolateral (PL) bundle reconstructions.	At 2-year follow-up all patients significantly improved their performance in terms of KT-2000 knee arthrometer and Lysholm knee score. In 20 of 29 patients who had postoperative magnetic resonance imaging (MRI) examination, the augmented ACL resembled one bundle on the sagittal planes of the postoperative MRI.
Sonnery-Cottet et al.	36 reconstructions of the anteromedial bundle preserving the posterolateral bundle. Outside-in technique: quadrupled hamstring graft in 20 patients and a doubled or tripled semitendinosus graft in 16.	All patients improved their performances in terms of mean side-to-side instrumented laxity and International Knee Documentation Committee (IKDC) after reconstruction of the anteromedial bundle preserving the posterolateral bundle.
Buda et al.	47 ACL augmentations using gracilis and semitendinosus tendons and over-the-top femoral passage	In 95.7% of cases, IKDC form, Tegner activity scale, and KT-2000 obtained good or excellent results.
Serrano-Fernandez et al.	24 ACL semitendinosus autograft reconstructions in the over-the-top position	Clinical and biomechanical evaluation with IKDC, Tegner activity scale, and Lysholm scale score showed better results after ACL augmentation compared with the preoperative condition.
Lee et al.	16 ACL remnant-preserving technique by use of four strands of a hamstring tendon and a looped suture technique	Functional evaluation revealed that the difference was not significant in terms of mechanical stability, but a significant difference was detected in functional outcome and proprioception (single-legged hop test, reproduction of passive positioning, threshold to detection of passive motion, and single-limb standing test) between pre- and postoperative values.
Comparing Studies
Adachi et al.	40 ACL augmentations vs. 40 ACL reconstructions	ACL augmentation group showed significantly better results at KT-2000 knee arthrometer and in terms of accuracy of joint position sense than the ACL reconstruction group.
Yoon et al.	82 cases of ACL reconstruction, 40 cases of AM augmentation, and 42 cases of PL augmentation	The incidence of a preoperative grade 2 or 3 positive pivot-shift test was lower in the AM augmentation group than in the other two groups. No difference was found in Lachman test, KT-1000 arthrometer, and IKDC knee examination form among groups.
Chouteau et al.	15 anteromedial bundle tears treated with bone–patellar tendon–bone graft in 13 cases and a double-stranded semitendinosus graft in two cases of chronic patellar tendonitis compared with a healthy knee	No statistically significant difference in rotational laxity, or active or passive proprioception could be observed between the reconstructed and healthy knee.
Demirag et al.	40 ACLs: 20 underwent augmentation where the remaining remnant ACL was not sacrificed; 20 underwent standard single-bundle (SB) ACL reconstruction with hamstring tendon autografts.	No significant differences were found between the groups in terms of IKDC, Lysholm scores, physical instability tests, patient satisfaction questionnaires, and incidences of cyclops lesions and arthrofibrosis.
Pujol et al.	54 partial ACL tears operated on either by selective AM bundle reconstruction (Group 1, n = 29) or by standard anatomic SB reconstruction (Group 2, n = 25).	Intergroup differences of IKDC scores and Lysholm scores were nonsignificant. Selective AM bundle reconstruction conserving the posterolateral bundle remnant provides clinical results comparable to the standard SB technique, with better control of anterior laxity.
Maestro et al.	39 AM and PL bundle reconstruction surgeries vs. 36 ACL reconstructions using SB technique	All techniques showed global significant enhancement in rotational stability. Improvement in anterior-posterior translation in the AM group and in rotational stability in the PL group was achieved; both showed no relevant statistical significance.
Park et al.	55 cases of remnant-preserving augmentation and 45 cases of double-bundle reconstruction	No significant differences in the postoperative range of motion, visual analog scale score, Lysholm score, Tegner score, and IKDC knee evaluation form score, Lachman test, pivot-shift test, anterior translation on Telos stress radiographs, and KT-1000 arthrometer between the two group
Zhang et al.	51 ACL tears: 27 patients were treated with preserving-remnant technique and 24 with the removing-remnant technique.	No intergroup differences in terms of KT-1000 arthrometer and Lysholm scores. Stump-preserving technique does not affect the short-term clinical outcome of ACL reconstruction.