Aaron T. Hecker

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.

Pull-out strength of suture anchors for rotator cuff and Bankart lesion repairs

Surgical reconstructions of anterior-inferior shoulder instabilities and rotator cuff injuries require secure fix ation of soft tissue to bone. Sutures are inserted directly through transosseous tunnels in current techniques, which are surgically complex and not always adequate for fixation strength. Using fresh-frozen cadaveric hu man specimens, our objectives were 1) to compare immediate pull-out strength of two versions of polyace tal suture anchors (wedge and rod) with conventional suture-only attachment techniques in Bankart lesion and rotator cuff repairs, and 2) to compare pull-out strength of the two polyacetal suture anchors with a metallic suture anchor. Our results indicate no signifi cant differences in fixation strength of Bankart lesions or rotator cuff repairs using sutures only, or using wedge or rod polyacetal suture anchors (P = 0.70). Pull-out force did not differ significantly (P = 0.37) between the two polyacetal anchors. Polyacetal an chors exhibited higher pull-out forces than metallic an chors when inserted into metaphyseal regions of the tibia and significantly higher pull-out forces (P < 0.001) when inserted into metaphyseal regions with thicker cortical walls. Our results indicate that both polyacetal suture anchors provide adequate immediate fixation for soft tissue repairs in the human shoulder.

The Effects of Donor Age and Strain Rate on the Biomechanical Properties of Bone-Patellar Tendon-Bone Allografts

Over 50% of all knee injuries involve partial or com plete tear of the anterior cruciate ligament. Surgical reconstruction of this ligament using an isometrically placed bone-patellar tendon-bone autograft is the cur rent technique of choice; however, harvest of patellar tendon as a free graft can lead to increased morbidity. To address this issue, allogenic patellar tendon grafts have been introduced as alternatives to autogenic graft material. The purpose of this study was to exam ine effects of age and strain rate on tensile strength, modulus, and failure mode of bone-patellar tendon- bone allografts from a typical population of tissue do nors. Eighty-two, fresh-frozen, bone-patellar tendon- bone allografts were harvested from 25 different donors, aged 17 to 54. Paired grafts from individual patellar tendons were assigned randomly to tensile testing at either 10% or 100% elongation per second. Tensile strength, modulus, and failure mode were not significantly different for tests conducted at these 2 strain rates. Correlations between tensile strength and age were not significant for tests conducted at either strain rate. Specimens tested at a strain rate of 100% per second exhibited weak but significant negative correlation between modulus and age, with modulus decreasing 25% over the age range examined.

Comparison of Tibial Graft Fixation Between Simulated Arthroscopic and Open Inlay Techniques for Posterior Cruciate Ligament Reconstruction

Background Techniques for reconstruction of the posterior cruciate ligament continue to evolve to improve clinical results. Recent arthroscopic reconstruction methods using tibial inlay grafts require suture fixation to avoid a posterior approach to the knee. Hypothesis Early strength of the tibial fixation of posterior cruciate ligament inlay grafts, designed for an arthroscopic approach and using suture fixation, is not significantly different than that of open technique grafts using screw fixation. Study Design Controlled laboratory study. Methods Six paired human tibias were randomized to a reconstruction using an open inlay technique with two 4.0-mm cancellous lag screws or an instrumented technique suitable for an all-arthroscopic approach using 2 No. 5 Ethibond sutures tied over a button on the anterior tibial cortex. Cyclic testing of each construct was done before loading to failure. Results The loads for each group at 3-mm displacement and at 5-mm displacement were not found to be significantly different. Ultimate load for the screw group was 762 N and for the suture group was 582 N (P = .31). Stiffness was 89.8 N/mm for the screw inlays and 85.1 N/mm for the suture inlays (P = .68). Cyclic testing demonstrated no advantages of screw fixation over sutures. No suture failure was noted. Conclusion The suture fixation technique for tibial inlay posterior cruciate ligament reconstruction appears to approximate the early strength of screw fixation. Clinical Relevance Arthroscopic techniques for tibial inlay reconstruction of the posterior cruciate ligament requiring suture fixation may offer similar initial biomechanical fixation as current open inlay techniques without the need for an open posterior knee approach.