Mark E. Steiner

Hamstring Tendon Grafts for Reconstruction of the Anterior Cruciate Ligament: Biomechanical Evaluation of the Use of Multiple Strands and Tensioning Techniques*

BACKGROUND Our hypothesis that multiple, equally tensioned strands of hamstring graft used for reconstruction of the anterior cruciate ligament are stronger and stiffer than ten-millimeter patellar ligament grafts was tested biomechanically with use of tendons from cadavera. METHODS In the first part of the study, we measured the strength and stiffness of one, two, and four-strand hamstring grafts, from fresh-frozen cadaveric knees, that had been tensioned equally when clamped. In the second part of the study, we compared four-strand grafts to which tension had been applied by hand and then clamped with similar grafts to which tension had been applied with weights and then clamped. The grafts for the two experiments were obtained from thirty-four paired and ten unpaired knees. We also studied the effects of cooling on the biomechanical properties of grafts by comparing patellar ligament grafts tested at 13 degrees Celsius with those tested at room temperature. RESULTS Two equally tensioned gracilis strands had 185 percent of the strength and 210 percent of the stiffness (1550+/-428 newtons and 336+/-141 newtons per millimeter, respectively) of one gracilis strand (837+/- 138 newtons and 160+/-44 newtons per millimeter, respectively). Two equally tensioned semitendinosus strands had 220 percent of the strength and 220 percent of the stiffness (2330+/-452 newtons and 469+/-185 newtons per millimeter, respectively) of one semitendinosus strand (1060+/-227 newtons and 213+/-44 newtons per millimeter, respectively). Four combined strands (two gracilis strands and two semitendinosus strands) that were equally tensioned with weights and clamped had the additive tensile properties of the individual strands. With the numbers available, four combined strands that were manually tensioned and clamped were not found to be significantly stronger or stiffer than two semitendinosus strands that were equally tensioned with weights (p>0.07). CONCLUSIONS Four combined strands that were equally tensioned with weights and clamped were stronger and stiffer than all ten-millimeter patellar ligament grafts that have been described in previous reports. All strands of a hamstring graft must be equally tensioned for the composite to have its optimum biomechanical properties. CLINICAL RELEVANCE Because of the well recognized donor-site morbidity associated with the use of patellar ligament grafts for reconstruction of the anterior cruciate ligament, multiple-strand hamstring-tendon grafts have become an increasingly popular choice. Our data demonstrate that equally tensioned four-strand hamstring-tendon grafts have initial tensile properties that are higher than those reported for ten-millimeter patellar-ligament grafts; thus, from a biomechanical point of view, they seem to be a reasonable alternative.

Anterior cruciate ligament graft fixation. Comparison of hamstring and patellar tendon grafts.

This study assessed the tensile properties of hamstring and patellar tendon anterior cruciate ligament recon structions in older cadaveric knees (age range, 48 to 79 years). Mechanical testing to failure was conducted by translating the tibia anteriorly at 1 mm/sec with the knee in 20° of flexion. The strongest gracilis-semitendinosus graft fixation technique (103% of intact anterior cruciate ligament) had the tendons doubled and secured with soft tissue washers (P < 0.01 ). However, all reconstruc tions using gracilis-semitendinosus grafts were signifi cantly less stiff than the intact anterior cruciate ligament specimens regardless of fixation technique (P< 0.01 ). The highest strength patellar tendon graft fixation tech nique (84% of intact anterior cruciate ligament) was ob tained with a combination interference screw and suture technique. The difference in stiffness between a patellar tendon graft and an intact anterior cruciate ligament was not significant when interference screws were placed at both ends of the graft (P > 0.05). Both types of grafts failed most often on the tibial side. With appropriate fixa tion, both grafts approximated the intact anterior cruci ate ligament in strength, but only patellar tendon grafts secured with interference screws were comparable in stiffness.

Anatomical Limitations of Transtibial Drilling in Anterior Cruciate Ligament Reconstruction

Background Recommended techniques for transtibial drilling in anterior cruciate ligament reconstruction are based on strategies to prevent graft impingement and preserve tibial tunnel length. The limitations of this drilling technique may restrict the ability to centralize tunnels in the anterior cruciate ligament footprints. Hypothesis A transtibial drilling starting point to centralize the tibial and femoral tunnels in their respective footprints can be identified, but it will result in a short tibial tunnel. Study Design Descriptive laboratory study. Methods The femoral and tibial attachments of the anterior cruciate ligament were characterized in 12 fresh-frozen cadaveric knees. Knees were secured in 70° and 90° of flexion. A guide pin was drilled antegrade through the central femoral and proximal anterior cruciate ligament attachment sites through the central tibial anterior cruciate ligament attachment site to exit on the anterior tibia. Results In 90° of flexion using the central femoral and tibial attachment sites, the exit point of the pin on the anterior tibia was 14.1 mm from the tibial joint line and 20.9 mm anterior to the superficial medial collateral ligament. The length of the pin in the tibia was 30.6 mm. Extending the knee to 70° or directing the pin through the proximal femoral anterior cruciate ligament attachment moved the starting point less than 4 mm from this point. Conclusion The transtibial technique can produce tunnels centered in the anterior cruciate ligament footprints, but a starting point close to the tibial joint line is required. This will result in a relatively short tibial tunnel. Clinical Relevance If tunnels centered in the anterior cruciate ligament attachment sites are desired with the transtibial drilling technique, then a short tibial tunnel is necessary. A short tibial tunnel may compromise graft fixation and graft incorporation, or it may result in a tunnel length—graft length mismatch. An alternative drilling strategy might be employed.

Independent Drilling Outperforms Conventional Transtibial Drilling in Anterior Cruciate Ligament Reconstruction

Background Optimal tunnel placement is critical in anterior cruciate ligament reconstructive surgery, yet the method used to drill the tunnels may compromise their placement. Hypothesis An independent drilling method versus a conventional transtibial drilling method will place tunnels in different locations and produce reconstructions with different kinematics. Study Design Controlled laboratory study. Methods Ten pairs of knees had anterior cruciate ligament reconstructions produced by either a conventional transtibial drilling method or an independent drilling method. The location of the tunnels was recorded, and the knees were tested for laxity in the normal state, with the anterior cruciate ligament removed, and with the anterior cruciate ligament reconstructed. A surgical navigation system guided the placement of the independently drilled tunnels and measured joint laxity in response to various combinations of anterior force and rotational torques. Results The conventional transtibial drilling method used in this study placed tibial tunnels posterior and femoral tunnels superior relative to their footprints and resulted in more vertical grafts. In contrast, the independently drilled tibial and femoral tunnels were more anterior and central in their respective footprints, resulting in more horizontal grafts. The horizontal grafts of the independent drilling method were superior to the vertical grafts of this study’s transtibial drilling method in restoring normal anterior and rotational knee laxity. Conclusion An independent drilling method can produce tunnels with superior function compared with tunnels produced by a conventional transtibial drilling method. Clinical Relevance Single-bundle anterior cruciate ligament reconstructions will be improved if grafts are centered in their anatomical insertions by an independent drilling method versus grafts placed by a conventional transtibial drilling method.

Measurement of anterior-posterior displacement of the knee. A comparison of the results with instrumented devices and with clinical examination.

Thirteen subjects who had normal knees and fifteen patients who had a chronic rupture of the anterior cruciate ligament were tested in order to compare the measurements of tibiofemoral displacement as recorded by four commercial devices: the Acufex knee-signature system, the Genucom knee-analysis system, the Medmetric KT-1000 arthrometer, and the Stryker knee-laxity tester. Anterior and posterior displacement were measured at forces of eighty-nine newtons (twenty pounds) and 133 newtons (thirty pounds). We found significant differences in reproducibility of measurement among the devices. The Acufex, Medmetric, and Stryker devices had more reproducible measurements, and they could be used to identify 80 to 90 per cent of the normal subjects and anterior cruciate-deficient patients. The Genucom device had poorer reproducibility of measurement, and it tended to register greater differences in displacement between the right and left knees of normal subjects.

Equal Kinematics Between Central Anatomic Single-Bundle and Double-Bundle Anterior Cruciate Ligament Reconstructions

PURPOSE The purpose of this study was to compare the kinematics of a central anatomic single-bundle anterior cruciate ligament (ACL) reconstruction with a double-bundle ACL reconstruction by use of hamstring grafts and anatomic tunnel placement. METHODS Anterior tibial translation and rotation were measured with a computer navigation system in 8 pairs of fresh-frozen cadaveric knees by use of a 133-N anterior force, an internal and external torque of 10 Nm, and an anterior force (133 N) combined with an internal rotation torque (10 Nm). Tests were performed at 30 degrees and 60 degrees of flexion with the ACL intact, the ACL transected, and after reconstruction of one side of a pair with either a single or a double-bundle construct. RESULTS At 30 degrees of flexion, cutting the ACL increased anterior translation under an anterior force (P < .0001), an internal rotation torque (P = .02), and a combined anterior force plus internal rotation torque (P = .01). At 60 degrees of flexion, transecting the ACL led to increased anterior translation only when an anterior force was used (P < .0001). Both single- and double-bundle reconstructions restored normal kinematics at 30 degrees and 60 degrees of knee flexion. CONCLUSIONS Central anatomic single-bundle ACL reconstruction with tunnels centered within the tibial and femoral insertions and double-bundle ACL reconstruction can restore normal anterior translation to the knee under anterior and rotational loads applied at 30 degrees and 60 degrees of flexion. CLINICAL RELEVANCE The primary kinematic effect of an ACL injury is an increase in anterior tibial translation, but there is no significant change in maximum internal or external rotation. Single- and double-bundle ACL reconstructions are equally effective in restoring normal anterior translation to the knee under both anterior and rotational loads.

Anterior Cruciate Ligament Reconstruction Using Gracilis and Semitendinosus Tendons Comparison of Through-the-condyle and Over-the-top Graft Placements

Eighty-seven patients had anterior cruciate ligament re construction using one-limbed gracilis and semitendi nosus tendon graft. No extraarticular or associated liga mentous procedures were performed. A 44-patient subgroup was alternated on an every-other-case basis between 2 graft placements, either over the top or through the femoral condyle; 64 patients (32 of each type) returned for complete evaluations. Average fol lowup was 2.9 years. Groups were similar in age, gen der, injury chronicity, number of meniscectomies, and preoperative activity levels. No statistical differences between groups were seen in overall knee rating, range of motion, KT-1000 arthrometer measurements, isoki netic muscle testing, or one-legged hop test. Injured minus normal anterior laxity at 89 N was 2.1 ± 2.0 mm. Isokinetic testing demonstrated an average 95% normal knee extension strength and 106% normal flexion strength, measured at 60 deg/sec. Average hop index was 95%. Eight of 64 patients evaluated demonstrated injured minus normal differences of 4.5 mm or greater. Three of 8 had frankly positive 2+ and 5 had 1 + pivot shifts; however, 4 of these returned fully to sports, and 4 returned with modifications. Overall, 55 of 64 patients returned to preinjury sports.

The Biomechanical Effects of Low-Dose Irradiation on Bone–Patellar Tendon–Bone Allografts

Background Despite evidence that low-dose irradiation of 2 Mrad (20 kGy) is not virucidal for patellar tendon allografts and reduces tissue strength, many tissue bank protocols include low-dose irradiation. Hypothesis Maintaining tissue mechanical integrity may be particularly relevant toward accelerated rehabilitation of the injured knee, where the cyclic function of patellar tendon allografts is critical. Study Design Controlled laboratory study. Methods The cyclic and failure mechanical properties of paired bone-patellar tendon-bone allografts, with and without current low-dose irradiation of 20 kGy, were evaluated. Specimens were loaded from 50 N to 250 N for 1000 cycles at 0.5 Hz and subsequently loaded to failure at a strain rate of 100% per second. Results After 1000 cycles, grafts elongated 27% more when irradiated than when not (4.4 ± 1.5 mm vs 3.4 ± 1.0 mm; P = .03). Failure load averaged 1965 ± 512 N for irradiated grafts and 2457 ± 647 N for nonirradiated grafts (P = .007). Conclusions The diminished strength of irradiated grafts may contribute to overt anterior cruciate ligament graft failure, and the increase in cyclic elongation may also be detrimental to graft function. Clinical Relevance These results suggest that one should consider the use of nonirradiated allografts as an alternative to irradiated grafts in anterior cruciate ligament reconstruction.

Strategies to Improve Anterior Cruciate Ligament Healing and Graft Placement

Recent improvements in anterior cruciate ligament (ACL) reconstruction have been notable for strategies to improve ACL healing and to improve graft placements. The controversial choice of 1 -bundle or 2-bundle grafts requires an advanced knowledge of native ACL insertional anatomy and an appreciation for the kinematic effects of graft placements. Understanding the limitations of surgical techniques to place tunnels is important. Once grafts are placed, new biologic strategies to promote intra-articular and intraosseous healing are evolving. Although these biologic engineering strategies are currently experimental, they are projected for clinical application in the near future.

Flexible Instruments Outperform Rigid Instruments to Place Anatomic Anterior Cruciate Ligament Femoral Tunnels Without Hyperflexion

PURPOSE This study evaluated the ability of flexible instruments compared with rigid instruments to place anatomic femoral tunnels in anterior cruciate ligament reconstructions by use of both transtibial drilling and anteromedial drilling without hyperflexion. METHODS Rigid and flexible pins were placed in 12 matched pairs of cadaveric knees with transtibial drilling (6 pairs) and anteromedial drilling (6 pairs) at 110° of flexion. Intraosseous pin lengths, femoral exit locations, and tunnel alignment were measured. RESULTS Transtibial drilling with rigid pins placed relatively vertical femoral tunnels 5.8 ± 1.0 mm superior to the central anterior cruciate ligament insertion. Transtibial drilling with flexible pins placed tunnels in the center of the femoral attachment, but the tunnels were relatively close to the posterior femoral cortex, with a mean distance of 8.0 ± 5.9 mm (P < .05), compared with transtibial drilling with rigid pins. Anteromedial drilling resulted in central anatomic pin placements with rigid and flexible instruments. Tunnel lengths with flexible pins were longer (42.0 ± 7.2 mm) compared with tunnel lengths with rigid pins (32.5 ± 7.1 mm) (P < .01). Flexible pins exited farther from the posterior cortex compared with rigid pins (P < .01). In 3 of 6 knees with rigid pins, the exit point was at the posterior border of the femoral cortex. All flexible pins exited a safe distance from the posterior femoral cortex. CONCLUSIONS Transtibial drilling with rigid instruments did not produce anatomic femoral tunnels. Transtibial drilling with flexible pins produced anatomic tunnels, but the tunnels were close to the posterior femoral cortex. Anteromedial drilling without hyperflexion produced anatomic tunnels by use of rigid and flexible instruments, but with flexible instruments, the tunnels were longer and were farther from the posterior femoral cortex. Anteromedial drilling with flexible pins produced tunnels with good length and the best position. CLINICAL RELEVANCE Flexible instruments compared with rigid instruments can facilitate the creation of anatomic femoral tunnels by use of anteromedial drilling without hyperflexion.