Initial fixation strength of bioabsorbable and titanium interference screws in anterior cruciate ligament reconstruction. Biomechanical evaluation by single cycle and cyclic loading

We evaluated the initial bone-patellar tendon-bone graft fixation strength of bioabsorbable as compared with titanium interference screws in anterior cruciate ligament reconstruction using matched pairs of porcine knees. Ten pairs underwent single-cycle failure loading at a rate of 50 mm/min, and 10 pairs underwent cyclic loading at half-hertz frequency. The cyclic loading started with 100 load cycles between 50 and 150 N. We then progressively increased loads in 50-N increments after each set of 100 cycles. After 100 cycles at 850 N, the specimens were loaded to failure at a rate of 50 mm/min. In the single-cycle failure loading test, the mean ultimate failure loads (+/-SD) for the bioabsorbable (837 +/- 260 N) and titanium interference screws (863 +/- 192 N) were not significantly different, nor were the mean yield loads or the stiffness of the fixation. In the cyclic loading test, the yield loads were 605 +/- 142 N and 585 +/- 103 N for the bioabsorbable and titanium interference screws, respectively (no significant difference). Although there was no significant difference in the ultimate failure load, more bone block fractures were found in the grafts fixed with a titanium interference screw. Bioabsorbable interference screw fixation thus seems to provide a reasonable alternative to titanium screws.     

Biomechanical properties of quadruple tendon and patellar tendon femoral fixation techniques

Two femoral fixation techniques for quadruple hamstring tendon grafts were compared under cyclic loading with the patellar tendon: the rectangular inserted pin (TransFix) and biodegradable interference screw fixation of the quadruple tendon and titanium interference screw fixation of the middle third of the patellar tendon. Porcine specimens were mounted onto a tension load machine, and the tendon-fixation-femur-complex was tested for stiffness, displacement during 800 cycles of loading between 50 and 250 N and ultimate tension load. TransFix fixation showed the greatest stiffness at 183.6 N/mm ( P<0.05). The least displacement under cyclic loading was observed for the titanium interference screw followed by the TransFix and biodegradable interference screw ( P<0.01). The ultimate tension load was greatest for the TransFix fixation at 1303+/-282 N, followed by patellar tendon fixation with 763+/-103 N and the biodegradable interference screw fixation with 480+/-133 N ( P<0.001). To reduce initial elongation of the graft and displacement at the fixation site, preconditioning of both the tendon and tendon-fixation complex is especially important when using quadruple tendons. TransFix fixation provides better stability and greater stiffness and pull-out strength than the other techniques. This finding is of clinical relevance to surgeons of the anterior cruciate ligament.     

A biomechanical comparison of initial soft tissue tibial fixation devices: the Intrafix versus a tapered 35-mm bioabsorbable interference screw

BACKGROUND: Biomechanical testing of the Intrafix device has not been performed using human tibiae. HYPOTHESIS: The Intrafix device would provide comparable or superior tibial fixation of a quadrupled hamstring tendon graft to a 35-mm-long bioabsorbable interference screw. STUDY DESIGN: In vitro, biomechanical study. METHODS: Eight paired human tibiae and 16 quadrupled hamstring tendon grafts were divided into 2 groups. Each quadrupled hamstring tendon graft was fixed in a tunnel sized to 0.5 mm graft diameter with either an Intrafix device or a screw. RESULTS: Displacement at failure was greater in the Intrafix group (17.3 +/- 4.6 mm versus 10.9 +/- 4.4 mm, P =.002). Load at failure (796 +/- 193 N versus 647 +/- 269 N), stiffness (49.2 +/- 21.9 N/mm versus 64.5 +/- 22 N/mm), and bone mineral density (0.74 +/- 0.15 gm/cm(3) versus 0.74 +/- 0.14 gm/cm(3)) did not display significant differences for the Intrafix device and the screw, respectively (P >.05). CONCLUSIONS: Displacement at failure was greater for the Intrafix device. CLINICAL RELEVANCE: Increased displacement at failure for the Intrafix group suggests slippage from sheath channel deployment. Concentric fixation may not occur when less than optimal tibial bone mineral density increases the difficulty of attaining precise sheath deployment and quadrupled hamstring tendon graft strand alignment.     

A biomechanical comparison of initial fixation strength of 3 different methods of anterior cruciate ligament soft tissue graft tibial fixation: resistance to monotonic and cyclic loading

BACKGROUND: Tibial fixation of soft tissue grafts continues to be problematic in the early postoperative period after anterior cruciate ligament reconstruction. HYPOTHESIS: No differences exist for resistance to slippage of soft tissue grafts fixed with CentraLoc, Intrafix, or 35-mm bioabsorbable interference screws. STUDY DESIGN: Controlled laboratory study. METHODS: Bovine tibia and hoof extensor tendons were divided into 3 matched groups with 12 tibia and 12 extensor tendons in each group. Within each group, 6 specimens underwent monotonic loading to failure (1 mm/s), and 6 underwent cyclic loading (10,000 cycles, 125-325 N, 1 Hz). RESULTS: No statistically significant differences were noted in mean load to failure or stiffness. The mean load to failure (and stiffness) for the 3 types of fixation were as follows: bioabsorbable interference screw, 631.6 +/- 130.1 N (88.17 +/- 6.79 N/mm); Intrafix, 644.3 +/- 195.2 N (81.65 +/- 16.5 N/mm); and CentraLoc, 791.1 +/- 72.7 N (77.89 +/- 7.07 N/mm). The slippage rates under cyclic loading for the 3 types of fixation were bioabsorbable interference screw, 0.336 +/- 0.074 microm/cycle; Intrafix, 27.2 +/- 31.6 microm/cycle; and CentraLoc, 0.0355 +/- 0.0046 microm/cycle. In this model, CentraLoc proved statistically superior in resistance to cyclic loading compared with the bioabsorbable interference screw (P < .05) and Intrafix (P < .0001). The bioabsorbable interference screw proved statistically superior to Intrafix in resistance to cyclic loading (P < .05). CONCLUSIONS: In this bovine model, CentraLoc and bioabsorbable interference screws provided superior resistance to cyclic loading compared with Intrafix. CLINICAL RELEVANCE: CentraLoc and bioabsorbable interference screws showed superior resistance to cyclic loading, which may indicate an increased resistance to clinical failure.     

Effect of screw length on bioabsorbable interference screw fixation in a tibial bone tunnel

Initial tibial fixation strength is the weak link after anterior cruciate ligament reconstruction with a quadrupled hamstring tendon graft fixed with bioabsorbable interference screws. The purpose of this study was to determine the biomechanical differences between 28-mm and tapered 35-mm interference screws for tibial fixation of a soft tissue graft in 16 young cadaveric tibias. Failure mode, displacement before failure, and ultimate failure load were tested with a testing machine aligned with the tibial tunnel to simulate a worst-case scenario. The mode of failure was graft slippage past the screw in all but one of the specimens. The mean maximum load at failure of the 28-mm screw was 594.9 +/- 141.0 N, with mean displacement at failure of 10.97 +/- 2.20 mm. The mean maximum load at failure of the 35-mm screw was 824.9 +/- 124.3 N, with a mean displacement to failure of 14.38 +/- 2.15 mm. The 38% difference in mean maximal load at failure was significant. Important variables in hamstring tendon graft fixation within a bone tunnel include bone mineral density, dilatation, gap size, screw placement, and screw width and length. Attention to these variables will help to provide secure graft fixation during biologic incorporation throughout the rehabilitation period.     

Bioabsorbable interference screw versus bioabsorbable cross pins: influence of femoral graft fixation on the clinical outcome after ACL reconstruction

Purpose

The aim of this study was to evaluate the clinical outcome and differences in anterior–posterior laxity of ACL reconstruction using a bioabsorbable interference screw for femoral graft fixation when compared to femoral bioabsorbable cross pin fixation.

Methods

Clinical outcome was evaluated among 59 patients 1?year after arthroscopic ACL reconstruction with hamstrings graft in a prospective, non-randomised study. In 31 cases, femoral fixation of the graft was performed using a bioabsorbable interference screw. In 28 cases, two bioabsorbable cross pins were used for femoral fixation. Patients were evaluated using Tegner, Lysholm and Marshall scores, the visual analogue scale for pain and KT-1000 arthrometer measurement.

Results

No significant difference (P?≥?0.05) was observed at follow-up for the knee scores. The average Tegner score was 5.83 points (±2.00) for the interference screw fixation and 5.83 points (±1.24) for the cross pin fixation; the average Lysholm score was 93.58 (±5.79) to 92.72 (±6.34) points; and the average Marshall score 46.72 (±2.4) to 47.30 (±2.35) points. No significant difference was found for the visual analogue scale for pain. KT-1000 arthrometer measurement revealed a significant (P?<?0.05) difference in the mean side-to-side anterior translation at all applied forces. At 67?N, the mean difference was 1.53?mm (±1.24) in the interference screw group and 0.47?mm (±1.18) in the cross pin group (P?<?0.05). At 89?N, the mean differences were 1.85?mm (±1.29) versus 0.59?mm (±1.59), respectively, (P?<?0.05), and maximum manual displacements were 2.02?mm (±1.26) versus 1.22?mm (1.18; P?<?0.05).

Conclusions

In ACL reconstruction with hamstrings graft, similar clinical results are obtained for the use of bioabsorbable cross pins when compared to bioabsorbable interference screws for femoral fixation. Cross pin fixation was superior with regard to the anteroposterior laxity as measured with KT-1000.     

Cyclic pull-out strength of hamstring tendon graft fixation with soft tissue interference screws. Influence of screw length

Blunt-threaded interference screws used for fixation of hamstring tendons in anterior cruciate ligament reconstructions provide aperture fixation and may provide a biomechanically more stable graft than a graft fixed further from the articular surface. It is unknown if soft tissue fixation strength using interference screws is affected by screw length. We compared the cyclic and time-zero pull-out forces of 7 x 25 mm and 7 x 40 mm blunt-threaded metal interference screws for hamstring graft tibial fixation in eight paired human cadaveric specimens. A four-stranded autologous hamstring tendon graft was secured by a blunt-threaded interference screw into a proximal tibial tunnel with a diameter corresponding to the graft width. Eight grafts were secured with a 25-mm length screw while the other eight paired grafts were secured with a 40-mm length screw. During cyclic testing, slippage of the graft occurred as the force of pull became greater with each cycle until the graft-screw complex ultimately failed. All grafts failed at the fixation site, with the tendon being pulled past the screw. There were no measurable differences in the mean cyclic failure strength, pull-out strength, or stiffness between the two sizes of screws. Although use of the longer screw would make removal technically easier should revision surgery be necessary, it did not provide stronger fixation strength than the shorter, standard screw as had been postulated.     

Multistranded hamstring tendon graft fixation with a central four-quadrant or a standard tibial interference screw for anterior cruciate ligament reconstruction

BACKGROUND: Tibial fixation of hamstring tendon grafts has been the weak link in anterior cruciate ligament reconstruction. HYPOTHESIS: Use of a central four-quadrant sleeve and screw provides superior fixation when compared with standard interference screw fixation. STUDY DESIGN: Controlled laboratory study. METHODS: In eight pairs of cadaveric knees each anterior cruciate ligament was reconstructed using either an interference screw or a central sleeve and screw on the tibial side. The specimens were then subjected to cyclic loading followed by a load-to-failure test. RESULTS: The load required to cause 1 and 2 mm of graft laxity, defined as the separation of the femur and the tibia at the points of graft fixation, was significantly greater with the sleeve and screw than with the interference screw (at 2 mm: sleeve and screw, 216.1 +/- 30.1 N; interference screw, 167.0 +/- 33.2 N). The force at initial slippage for each of the graft strands was significantly higher with use of the central sleeve and screw. CONCLUSIONS: The four-quadrant sleeve and screw device may provide greater surface area for healing of hamstring tendon grafts and allow equal tensioning of graft strands before fixation. These factors are associated with increased strength of fixation and reduced laxity of the graft after cyclic loading. Clinical Relevance: Use of the central four-quadrant sleeve and screw system offers increased strength of fixation in anterior cruciate ligament reconstruction with hamstring tendon graft.     

A biomechanical evaluation of transcondylar femoral fixation of anterior cruciate ligament grafts

BACKGROUND: Interference screw fixation of the graft in anterior cruciate ligament reconstruction is considered the gold standard, but limited clinical experience suggests that transcondylar fixation is equally effective. PURPOSE: To compare transcondylar and interference screw fixation. STUDY DESIGN: Ex vivo biomechanical study. METHODS: Twenty pairs of unembalmed knees underwent anterior cruciate ligament reconstruction with patellar tendon autografts. In 1 knee of each pair, the bone plug was stabilized in the femoral tunnel with standard interference screws; in the other knee, transcondylar screws were used. Testing to failure occurred immediately or after 1000 cycles of sinusoidal loading (30 to 150 N) (20 paired reconstructions each). Fixation stiffness, strength, graft creep, displacement amplitude, and change in amplitude were measured and compared (repeated measures anaylsis of variance with Tukey test; P <.05). RESULTS: There was no significant difference in acute strength, maximum load within 3 mm, or stiffness between transcondylar fixation (410 +/- 164 N, 183 +/- 93 N, and 49.6 +/- 28 N/mm, respectively) and interference fixation (497 +/- 216 N, 206 +/- 115 N, and 61 +/- 37.8 N/mm, respectively). Similarly, there was no significant difference in cyclic strength, maximum load within 3 mm, or stiffness between transcondylar fixation (496 +/- 214 N, 357 +/- 82.9 N, and 110 +/- 27.4 N/mm, respectively) and interference fixation (552 +/- 233 N, 357 +/- 76.2 N, and 112 +/- 26.8 N/mm, respectively). Predominant modes of failure were bone plug pullout (transcondylar fixation) and tendon failure or bone plug fracture (interference fixation). CONCLUSIONS: Transcondylar screw fixation of the patellar tendon autograft into the femoral tunnel performed mechanically as well as interference screw fixation. CLINICAL RELEVANCE: The results suggest that transcondylar and interference screws provide similar fixation for anterior cruciate ligament reconstruction.     

The influence of screw geometry on hamstring tendon interference fit fixation

We used a standardized model of calf tibial bone to investigate the influence of screw diameter and length on interference fit fixation of a three-stranded semitendinosus tendon graft for anterior cruciate ligament reconstruction. Biodegradable poly-(L-lactide) interference screws with a diameter of 7, 8, and 9 mm and a length of 23 and 28 mm were used. We examined results in three groups of 10 specimens each: group 1, screw diameter equaled graft diameter and screw length was 23 mm; group 2, screw diameter equaled graft diameter plus 1 mm and screw length was 23 mm; group 3, screw diameter equaled graft diameter and screw length was 28 mm. The mean pull-out forces in groups 1, 2, and 3 were 367.2+/-78 N, 479.1+/-111.1 N, and 537.4+/-139.1 N, respectively. The force data from groups 2 and 3 were significantly higher than those from group 1. These results indicate that screw geometry has a significant influence on hamstring tendon interference fit fixation. Increasing screw length improves fixation strength more than oversizing the screw diameter. This is important, especially for increasing tibial fixation strength because the tibial graft fixation site has been considered to be the weak link of such a reconstruction.     

Compaction drilling does not increase the initial fixation strength of the hamstring tendon graft in anterior cruciate ligament reconstruction in a cadaver model

BACKGROUND: Compaction of the bone tunnel walls has been proposed to increase the fixation strength of soft tissue grafts fixed with an interference screw in anterior cruciate ligament reconstructions. HYPOTHESIS: Compaction drilling does not increase the initial fixation strength of the hamstring tendon graft in comparison with conventional extraction drilling. STUDY DESIGN: Randomized experimental study. METHODS: Initial fixation strength of quadrupled hamstring tendon grafts fixed with bioabsorbable interference screws was assessed in 22 pairs of human cadaveric tibiae. Bone tunnels were drilled with either a compaction drill or a conventional extraction drill. Specimens underwent a cyclic-loading test and the surviving specimens were then loaded to failure in a single-cycle load-to-failure test. Trabecular bone mineral density at the site corresponding to the actual site of the tibial bone tunnel was determined by using peripheral quantitative computed tomography. RESULTS: During the cyclic-loading test, no significant stiffness or displacement differences were observed between the two drilling techniques. Three specimens failed in the compaction-drilling group, whereas there were no failures in the extraction-drilling group. In the subsequent single-cycle load-to-failure test, no significant differences between the two drilling techniques were found with regard to displacement at yield load, stiffness, or mode of failure. There was no significant difference in trabecular bone mineral density between the two groups. CONCLUSIONS: Compaction drilling does not increase the initial fixation strength of the hamstring tendon graft compared with conventional extraction drilling.     

Biomechanical analysis of femoral tunnel pull-out angles for anterior cruciate ligament reconstruction with bioabsorbable and metal interference screws

BACKGROUND: Fixation strength of metal and bioabsorbable interference screws has not been evaluated while varying the anterior cruciate ligament graft tension angle. HYPOTHESIS: There is no difference in fixation strength between 2 types of interference screws for anterior cruciate ligament graft fixation while the graft tension angle is varied relative to the femoral tunnel. STUDY DESIGN: Controlled laboratory study. METHODS: Forty-eight anterior cruciate ligament reconstructions were performed using immature porcine femurs stripped of soft tissue and doubled-over porcine flexor digitorum profundus tendon grafts. Specimens were randomized to bioabsorbable or titanium interference screw fixation. Specimens were randomized to one of three pull angles (0 degrees , 30 degrees , 60 degrees ) representing loading at different knee flexion angles (n = 8/group). Reconstructed ligaments were tensioned to 10 N followed by 200 loading cycles between 10 and 150 N and a final failure test. Construct elongation (mm) at 100 and 200 cycles and failure load (N) were analyzed using a 2-way analysis of variance (P < .05). RESULTS: Screw material interacted significantly with graft tension angle, as the bioabsorbable screw specimens demonstrated significantly greater fixation strength when tensioned at greater angles. Specimens fixed with bioabsorbable screws showed significantly less elongation at both 100 and 200 cycles and significantly greater failure load compared with titanium screws. CONCLUSION: Bioabsorbable interference screws acutely have increased fixation strength compared with titanium interference screws for anterior cruciate ligament grafts loaded at greater tension angles. CLINICAL RELEVANCE: The strength of anterior cruciate ligament reconstruction fixation increases with increasing divergence between the tension angle and femoral tunnel, a condition seen when the knee approaches full extension.     

Biomechanical evaluation of a new cross-pin technique for the fixation of different sized bone–patellar tendon–bone grafts

To overcome the disadvantages of interference-screw fixation of bone–patellar tendon–bone (BPTB) grafts, new fixation techniques for anterior cruciate ligament (ACL) grafts using biodegradable pins have been developed. The hypothesis of the present study was that cross-pin fixation techniques provide a primary stability that is comparable to that of interference screws. A biomechanical in vitro study was discussed. Human BPTB grafts of 8, 9 and 10 mm diameter were fixed in bovine knees with biodegradable cross pins (diameter: 2.0, 2.7 or 3.2 mm) or biodegradable interference screws. Stiffness and ultimate load were evaluated. For 9 and 10 mm BPTB grafts, no statistically significant difference in maximum load and stiffness was found between the four fixation techniques tested. For 8 mm bone blocks the maximum load of the 3.2 mm pins (274.2 N) was significantly lower than for the 2.0 mm pins (479.8 N) and the interference screws (504.0 N). Predominant failure mode in this group was bone-block fracture. Thicker grafts (9 and 10 mm) fixed with the 2.0 mm pins predominantly failed by implant fracture. Femoral fixation of 8, 9 and 10 mm BPTB grafts using cross pins leads to biomechanical properties that are comparable to biodegradable interference screws when tested by a single-cycle load to failure. Cross pins provide a rigid fixation for 9 and 10 mm BPTB grafts.     

Initial fixation strength of a hybrid technique for femoral ACL graft fixation

Aperture fixation with interference screws matching the diameter of the tunnel is associated with the risk of graft laceration and graft rotation. A hybrid fixation technique (extracortical and aperture fixation) with undersized interference screws provides a higher fixation strength when compared to an aperture fixation using only a screw matching the size of the tunnel and also reduces the risk of graft laceration. This research is an experimental laboratory study. We evaluated the initial fixation strength at time-zero of an extracortical-, a hybrid- and an aperture fixation in ACL reconstruction using extracortical buttons and different sized interference screws in porcine knees. The tests were performed using a single cycle and cyclic loading protocol. Analysis of yield load, maximum load and stiffness in the single cycle loading test showed no statistically significant differences for hybrid fixation with a 1 mm undersized screw and aperture fixation with a screw matching the size of the tunnel. Cyclic loading tests showed a statistically significant difference between hybrid and aperture fixation. The use of an undersized screw alone in aperture fixation resulted in insufficient fixation strength. The initial fixation strength of the hybrid technique with undersized screws is higher compared to an interference screw fixation alone. The hybrid fixation technique is an alternative for ACL graft fixation.     

Fixation strength of a novel bioabsorbable expansion bolt for patellar tendon bone graft fixation: an experimental study in calf tibial bone

This biomechanical study compares the initial fixation strength of a novel bioabsorbable two-shell expansion bolt (EB) with that of a well-established interference-screw technique in bone–patellar tendon–bone (BPTB) reconstruction in a calf model. Thirty tibia plateaus (age 5–6 months) were assigned to three groups: In groups I and II, trapezoidal bone plugs of BPTB grafts were fixed with bioabsorbable poly-L-lactide interference screws (8×23 mm) or titanium interference screws (8×25 mm) respectively. In group III, semicircular grafts were fixed using bioabsorbable poly-D, L-lactide expansion bolts (5.8/8.7×10×35 mm). The tensile axis was parallel to the bone tunnel, and the construction was loaded until failure applying a displacement rate of 1 mm per second. In group II the mean ultimate loads to failure (713 N±218 N) were found to be significantly higher than those of groups I (487 N±205 N) and III (510 N±133 N). Measurement of stiffness showed 45 N/mm±13.3 in group I, 58 N/mm±17.4 in group II and 46 N/mm±6.9 in group III, and did not demonstrate significant differences. We found a correlation between insertion torque and wedge insertion force and ultimate loads to failure in all groups ( r =0.53 in group I, r =0.54 in group II, and r =0.57 in group III). Cross-section planes of bone tunnel increased by 51%, 30% and 31% respectively, following insertion of screws or expansion of bolts ( p <0.05). We conclude that ACL graft fixation by means of the presented expansion bolt demonstrates a fixation strength similar to the established bioabsorbable screw fixation, and is a reasonable alternative fixation method, especially since some of the specific pitfalls of screw fixation can be avoided.     

Biomechanical properties of patellar and hamstring graft tibial fixation techniques in anterior cruciate ligament reconstruction: experimental study with roentgen stereometric analysis

BACKGROUND: Reliable fixation of the soft hamstring grafts in ACL reconstruction has been reported as problematic. HYPOTHESIS: The biomechanical properties of patellar tendon (PT) grafts fixed with biodegradable screws (PTBS) are superior compared to quadrupled hamstring grafts fixed with BioScrew (HBS) or Suture-Disc fixation (HSD). STUDY DESIGN: Controlled laboratory study with roentgen stereometric analysis (RSA). METHODS: Ten porcine specimens were prepared for each group. In the PT group, the bone plugs were fixed with a 7 x 25 mm BioScrew. In the hamstring group, four-stranded tendon grafts were anchored within a tibial tunnel of 8 mm diameter either with a 7 x 25 mm BioScrew or eight polyester sutures knotted over a Suture-Disc. The grafts were loaded stepwise, and micromotion of the graft inside the tibial tunnel was measured with RSA. RESULTS: Hamstring grafts failed at lower loads (HBS: 536 N, HSD 445 N) than the PTBS grafts (658 N). Stiffness in the PTBS group was much greater compared to the hamstring groups (3500 N/mm versus HBS = 517 N/mm and HSD = 111 N/mm). Irreversible graft motion after graft loading with 200 N was measured at 0.03 mm (PTBS), 0.38mm (HBS), and 1.85mm (HSD). Elasticity for the HSD fixation was measured at 0.67 mm at 100 N and 1.32 mm at 200 N load. CONCLUSION: Hamstring graft fixation with BioScrew and Suture-Disc displayed less stiffness and early graft motion compared to PTBS fixation. Screw fixation of tendon grafts is superior to Suture-Disc fixation with linkage material since it offers greater stiffness and less graft motion inside the tibial tunnel. Clinical Relevance: Our results revealed graft motion for hamstring fixation with screw or linkage material at loads that occur during rehabilitation. This, in turn, may lead to graft laxity.     

Femoral fixation of hamstring grafts in posterior cruciate ligament reconstruction: biomechanical evaluation of different fixation techniques: is there an acute angle effect?

BACKGROUND: The literature provides little biomechanical data about femoral fixation of hamstring grafts in posterior cruciate ligament reconstruction. HYPOTHESIS: A hybrid fixation technique with use of an undersized screw has sufficient strength to provide secure fixation of posterior cruciate ligament grafts. Additional aperture fixation with a biodegradable interference screw can prevent graft damage that might be caused by an acute angle on the edge of the femoral tunnel. STUDY DESIGN: Controlled laboratory study. METHODS: In part 1, extracortical fixation of posterior cruciate ligament reconstructions with quadrupled porcine flexor digitorum grafts to simulate human hamstring grafts was compared with hybrid fixation methods using 6-, 7-, and 8-mm screws. Groups were tested in cycling loading with the load applied in line with the bone tunnel. In part 2, extracortical fixation was compared with hybrid fixation using a 1-mm undersized screw anterior and posterior to the graft. Structural properties and graft abrasion were evaluated after cyclic loading with the load applied at 90 degrees to the tunnel. In each group, 8 porcine knees were tested. RESULTS: In part 1, stiffness, maximum load, and yield load were significantly higher for hybrid fixation than for extracortical fixation. Hybrid fixation with an 8-mm screw resulted in higher yield load than with a 7-mm screw. In part 2, graft laceration was more pronounced in specimens with extracortical fixation than with hybrid fixation. Posterior screw placement was superior to the anterior position. CONCLUSION: For all parameters, hybrid fixation with an interference screw provided superior structural results. No relevant disadvantages of undersized screws could be found. Graft damage due to abrasion at the edge of the femoral bone tunnel was reduced by use of an interference screw. The posterior screw placement seems favorable. CLINICAL RELEVANCE: Hybrid fixation of hamstring grafts in posterior cruciate ligament reconstruction is superior to extracortical fixation alone with no relevant disadvantages of undersized screws. The results raise the suspicion of an acute angle effect of the femoral bone tunnel.     

Interference screw fixation of soft tissue grafts in anterior cruciate ligament reconstruction: part 1: effect of tunnel compaction by serial dilators versus extraction drilling on the initial fixation strength

BACKGROUND: Compaction of the bone-tunnel walls by serial dilation is believed to enhance the interference screw fixation strength of the soft tissue grafts in anterior cruciate ligament (ACL) reconstruction. HYPOTHESIS: Serial dilation enhances the fixation strength of soft tissue grafts in ACL reconstruction over extraction drilling. STUDY DESIGN: Randomized experimental study. METHODS: Initial fixation strength of the doubled anterior tibialis tendon grafts (fixed with a bioabsorbable interference screw) was assessed in 21 pairs of human cadaver tibiae with either serially dilated or extraction-drilled bone tunnels. The specimens were subjected to a cyclic-loading test, and those surviving were then tested using the single-cycle load-to-failure test. RESULTS: During the cyclic-loading test, there were 3 fixation failures in the serially dilated and 6 failures in the extraction-drilled specimens but no significant stiffness or displacement differences between the groups. In the subsequent load-to-failure test, the average yield loads were 473 +/- 110 N and 480 +/- 115 N for the 2 groups respectively (P =.97) and no difference with regard to stiffness or mode of failure. CONCLUSIONS: Serial dilation does not increase the strength of interference fixation of soft tissue grafts in ACL reconstruction over extraction drilling. Clinical Relevance: The results of this experiment do not support the use of serial dilators in ACL reconstruction.     

Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation

In this study of bioabsorbable screw fixation of free tendon grafts used in anterior cruciate ligament reconstruction, we performed load-to-failure and cyclic loading of tendon fixation in porcine bone. Bone density measurements from dual photon absorptometry scans were obtained to correlate bone density with fixation failure. The average density of porcine bone (1.42 g/cm2) was similar to that of young human bone (1.30 g/cm2) and significantly higher than that of elderly human cadaveric bone specimens (0.30 g/cm2). Cyclic loading was performed on free tendon grafts fixed with a bioabsorbable screw alone and on grafts fixed with a bioabsorbable screw and an anchor (polylactic acid ball or cortical bone disk). Stiffness of fixation increased substantially with the addition of a cortical bone disk anchor or polylactic acid ball compared with the interference screw alone. Tensile fixation strength of central quadriceps free tendon and hamstring tendon grafts were significantly superior in porcine bone of density similar to young human bone than in elderly human cadaveric bone. The bioabsorbable interference screw yielded loads at failure comparable with traditional bone-tendon-bone and hamstring tendon fixation when controlled for bone density. The addition of a cortical bone disk anchor provided the most optimal fixation of free tendon with the bioabsorbable screw and reduced slippage with cyclic loading to a very low level.     

Interference screws should be shorter than the hamstring tendon graft in the bone tunnel for best fixation

Purpose

Interference screw fixation of hamstring tendon grafts in bone has to overcome the challenges that tendons have a slippery surface and viscoelastically adapt under pressure. As the typical failure mode of the graft is to slip past the interference screw, it was hypothesized that the position and configuration of the graft end may be of influence on the fixation strength.

Methods

Different configurations of the graft ending and its effect to primary fixation with interference screws after viscoelastic adaptation were tested in six groups: I: graft and the screw inserted at the same depth, II/III: the graft overlaps the tip of the screw (interference screw of 28 and 19 mm in length, respectively), IV: strengthening of the graft ending with additional suture knots, V: Endopearl, respectively, and VI: effect of partial retraction of the screw after excessive insertion. In vitro tests were performed with fresh calf tendon grafts and interference screws in bone tunnels (fresh porcine distal femur) all of 8 mm in diameter.

Results

The relative position of the graft ending to the tip of the interference screw thereby was recognized as a significant factor on pullout forces. Further strengthening at the graft endings with additional suture knots or an Endopearl device could improve primary hold as well.

Conclusions

Better fixation strength is achieved if the tip of interference screw does not extend past the end of a tendon graft. Enforcement of the tendon end with sutures or an implant can further improve fixation.