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.     

Initial fixation strength of two bioabsorbable pins for the fixation of hamstring grafts compared to interference screw fixation: single cycle and cyclic loading

BACKGROUND: During the early postoperative period, the fixation of a hamstring graft to the bone tunnel is the primary factor in limiting rehabilitation. HYPOTHESIS: The initial fixation strength of a double cross pin fixation technique is comparable with the biodegradable interference screw fixation technique. STUDY DESIGN: Experimental laboratory study. METHODS: The authors examined the initial fixation strength of two 3.3-mm bioabsorbable pins compared to interference screws for hamstring grafts in bovine knees. RESULTS: Analysis of yield load, maximum load, and stiffness in the single-cycle loading test showed no statistically significant differences for cross pin and interference fixation (P <.05). For cross pins and interference screws, the mean displacement under 1000 cycles to 250 N was 5.07 ( +/- 1.9) mm and 4.81 ( +/- 2.5) mm, stiffness 252 ( +/- 78) N/mm and 289 ( +/- 148) N/mm. Only grafts fixed with cross pins survived 1000 cycles to 450 N. CONCLUSION: The initial fixation strength of the double cross pin technique is comparable to that of interference screw fixation with a stiffness comparable to that of the native ACL. Clinical Relevance: Hamstring graft fixation using two cross pins provides an alternative to bioabsorbable interference screw fixation.     

Graft choice and graft fixation in PCL reconstruction

Several grafts and several fixation techniques have been introduced for PCL reconstruction over the past years. To date, autograft and allograft tissues are recommended for PCL reconstruction, whilst synthetic grafts should be avoided. Autograft tissues include the bone-patellar tendon-bone graft, the hamstrings and the quadriceps tendon. Allograft tissues are increasingly being used for primary PCL reconstruction. The use of allograft tissues requires a number of formal prerequisites to be fulfilled. Besides the previous mentioned graft types allograft tissues include Achilles and tibialis anterior/posterior tendons. To date no superior graft type has been identified. Several techniques and devices have been used for fixation of a PCL replacement graft. Most of these were originally developed for ACL reconstruction and then adapted to PCL reconstruction. However, biomechanical requirements of the PCL differ substantially from those of the ACL. To date, requirements for PCL graft fixations are not known. From a systematic approach femoral graft fixation can either be achieved within the bone tunnel (nearly anatomic) with an interference screw or outside the bone tunnel on the medial femoral condyle using a staple, an endobutton or a screw. Tibial graft fixation can be achieved either with an interference screw in the bone tunnel or with a staple, screw/washer or sutures tied over a bone bridge outside the bone tunnel (extra-anatomic). An alternative fixation on the tibial side is the inlay technique that reduces the acute angulation of the graft at the posterior aspect of the tibia. Further research is necessary to identify the differences between the various fixation techniques.     

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.     

Close-looped graft suturing improves mechanical properties of interference screw fixation in ACL reconstruction

Purpose

In anterior cruciate ligament reconstruction with looped soft-tissue grafts, an interference screw is frequently used for tibial fixation. This study compared three alternatives thought to improve the initial mechanical properties of direct bioabsorbable interference screw fixation: suturing the graft to close the loop, adding a supplementary staple, or increasing the oversize of the screw diameter relative to the bone tunnel from 1 to 2 mm.

Methods

Twenty-eight porcine tibiae and porcine flexor digitorum profundus tendons were randomized into four testing groups: a base fixation using 10-mm-diameter screw with open-looped graft, base fixation supplemented by an extracortical staple, base fixation but closing the looped graft by suturing its ends, and base fixation but using an 11-mm screw. Graft and bone tunnel diameters were 9 mm in all specimens. Constructs were subjected to cyclic tensile load and finally pulled to failure to determine their structural properties.

Results

The main mode of failure in all groups was pull-out of tendon strands after slippage past the screw. The sutured graft group displayed significantly lower residual displacement (mean value reduction: 47–67 %) and higher yield load (mean value increase: 38–54 %) than any alternative tested. No other statistical differences were found.

Conclusions

Suturing a soft-tissue graft to form a closed loop enhanced the initial mechanical properties of tibial fixation with a bioabsorbable interference screw in anterior cruciate ligament reconstructions using a porcine model, and thus, this may be an efficient means to help in reducing post-operative laxity and early clinical failure. No mechanical improvement was observed for an open-looped tendon graft by adding an extracortical staple to supplement the screw fixation or by increasing the oversize of the screw to tunnel diameter from 1 to 2 mm.     

A biomechanical comparison of different surgical techniques of graft fixation in anterior cruciate ligament reconstruction

Different surgical methods of graft fixation in ACL reconstruction were examined to determine the effects on mechanical properties of the reconstructed ACL. Ten human cadavers were used in this study. Six different types of grafts were studied. The tendon grafts were removed from each cadaver and fixed to femurs and tibias as ACL substitutes with different surgical fixation methods, leaving femur-reconstructed graft-tibia preparations. The surgical techniques used were staple fixation, tying sutures over buttons, and screw fixation. In the latter, the screws were introduced through femoral and tibial drill holes from the outside in order to achieve interference fit as described by Lambert. Tensile testing demonstrated that the original ACL is significantly stronger than the graft used for reconstruction in linear load, stiffness, and maximum tensile strength. All of the failures of the reconstructed ACL grafts occurred at the fixation site, indicating that the mechanically weak link of the reconstructed graft is located at the fixation site. Among the different methods of fixation, one-third of the patellar tendon secured with a cancellous screw, especially with a custom designed large diameter screw, showed significantly higher values. Although many other factors affect the success of ACL reconstruction, our study indicates that the method of surgical fixation is the major factor influencing the graft's mechanical properties in the immediate postoperative period.     

Comparison of eccentric and concentric screw placement for hamstring graft fixation in the tibial tunnel

Interference screw fixation of four-strand hamstring grafts for ACL reconstruction has recently been introduced. By this method, the interference screw is placed in the tibial and femoral tunnels eccentric (adjacent) to the bundled limbs of the graft. In order to maximize the graft to tunnel contact to promote ¶biological fixation, it is proposed to place the screw concentrically in ¶the tunnel, in the middle of the four limbs of the graft, pressing each limb of the graft into the tunnel wall. This would be difficult to do in the proximal, folded end of the four limb graft situated in the femoral tunnel but can be done easily in the tibial tunnel. The purpose of this study was to evaluate the effect of screw placement on the stiffness, yield load, and ultimate load of hamstring graft fixation in the tibial tunnel. Five pairs of human knees were ¶used for the study. Pull out tests ¶were performed using an MTS system, pulling along the axis of the ¶tibial tunnel. Tibial fixation stiffness was greater using concentric screw placement (P < 0.05) although there was no statistical difference in yield load, slippage, or ultimate load.     

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.     

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.     

Interactive effects of tunnel dilation on the mechanical properties of hamstring grafts fixed in the tibia with interference screws

The effect of dilation of the tibial tunnel on the strength of hamstring graft fixation using interference screws was evaluated. In all, 28 RCI screws were tested in male human tibia-hamstring constructs with tibial tunnels reamed or dilated to the respective size of the graft diameter. Dilation of the tibial tunnel failed to significantly enhance hamstring fixation. Grafts secured in dilated tunnels displayed an 11% greater resistance to the initiation of graft slippage (174+/-112 N) compared to their undilated controls (156+/-77 N, P=0.63). Dilation of the tibial tunnel increased the failure load by an average of 4%, independent of screw diameter (dilated specimens: 360+/-120 N, controls: 345+/-88 N, P=0.74). Biomechanical research on the effect of tibial tunnel dilation in hamstring fixation has not provided satisfactory evidence as to the benefits of this additional surgical step during anterior cruciate ligament (ACL) reconstruction.     

Comparison of direct and indirect interference screw fixation for tendon graft in rabbits

For ACL reconstruction, interference screw can fix the graft in the bone tunnel closer to the articular surface. However, direct interference screw fixation has a possibility to damage the tendon graft at the time of screw insertion, and the bone–tendon contact area is limited within the tunnel. To avoid the damage to the tendon graft at the time of screw insertion and to increase the bone–tendon contact area, a free bone plug was interposed between screw and tendon graft (indirect interference screw fixation). The purpose of this study was to compare ultimate load strength and histological findings between two techniques in a rabbit model. Ultimate pull-out load tests and histological examinations were evaluated at time 0, 3 and 6 weeks. The ultimate failure load of indirect interference screw fixation was significantly higher than that of direct interference screw fixation immediately after surgery (P < 0.05). Histologically, the interface tissues between tendon graft and host bone were more organized and matured in indirect technique. These findings showed that indirect interference screw fixation for tendon graft increased fixation strength at the graft–bone interface, providing quicker graft–bone healing.     

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.     

Tibial bone bridge and bone block fixation in double-bundle anterior cruciate ligament reconstruction without hardware: a technical note

Current techniques for tibial graft fixation in four tunnels double bundle (DB) anterior cruciate ligament (ACL) reconstruction are by means of two interference screws or by extracortical fixation with a variety of different implants. We introduce a new alternative tibial graft fixation technique for four tunnels DB ACL reconstruction without hardware. About 3.5 to 5.5 cm bone cylinder with a diameter of 7 mm is harvested from the anteromedial (and posterolateral) tibial bone tunnel (s) with a core reamer. The anteromedial (AM) and posterolateral (PL) hamstring tendon grafts (or alternatively tendon allografts) are looped over an extracortical femoral fixation device and cut in length according to the total femorotibial bone tunnel length. The distal 3 cm of each, the AM- and PL bundle graft are armed with two strong No. 2 nonresorbable sutures and the four suture ends of each graft are tied to each other over the 2 cm wide cortical bone bridge between the tibial AM and PL bone tunnel. In addition the AM- and/or PL bone block which was harvested at the beginning of the procedure is re-impacted into the two tibial bone tunnels. A dorsal splint is used for the first two postoperative weeks and physiotherapy is started the second postoperative day. The technique is applicable for four tunnels DB ACL reconstruction in patients with good tibial bone quality. The strong fixation technique preserves important tibial bone stock and avoids the use of tibial hardware which knows disadvantages. It does increase tendon to bone contact and tendon-to-bone healing and does reduce implant costs to those of a single bundle (SB) ACL reconstruction. Revision surgery may be facilitated significantly but the technique should not be used when bony defects are present. In case of insufficient bone bridge fixation or bone blocks hardware fixation can be applied as usual. Not supported by outside funding or grant(s): No benefits in any form have been received, or will be received, from a commercial party related directly or indirectly to the subject of this article. The study complies with the current laws of the country, in which it was performed.     

Tibial fixation comparison of semitendinosus-bone composite allografts fixed with bioabsorbable screws and bone-patella tendon-bone grafts fixed with titanium screws

Tibial fixation remains the weak link of ACL reconstruction over the first 8–12 weeks postoperatively. This study compared the biomechanical properties of tibial fixation for a bone-patellar tendon-bone (BPTB) graft and a novel semitendinosus-bone composite (SBC) allograft with mixed cortical-cancellous bone dowels at each end. Seven paired, fresh frozen cadaveric knees (20–45 years) were stripped of all soft tissue attachments and randomly assigned to receive either the BPTB graft or SBC allograft. Grafts were placed into tibial tunnels via a standard protocol and secured with either a 10 mm×28 mm bioabsorbable (SBC) or titanium (BPTB) screw. Grafts were cycled ten times in a servo hydraulic device from 10–50 N prior to pull to failure testing at a rate of 20 mm/min with the force vector aligned with the tibial tunnel ("worst case scenario"). Wilcoxon Signed Rank Tests were used to evaluate biomechanical differences between graft types (p<0.05). Tibial bone mineral density and interference screw insertion torque were statistically equivalent between graft types. The mode of failure for all constructs was direct screw and graft construct pullout from the tibial tunnel. Significant differences were not observed between graft types for maximum load at failure strength (BPTB=620.8±209 N vs. SBC=601.2±140 N, p=0.74) or stiffness (BPTB=69.8 N/mm±29 N/mm vs SBC=47.1±31.6 N/mm, p=0.24). The SBC allograft yielded significantly more displacement prior to failure than the BPTB graft (15.1±4.9 mm vs 9.2±1.3 mm, p=0.04). Increased construct displacement appeared to be due to fixation failure, with some evidence of graft tissue tearing around the sutures: Bioabsorbable screw (10×28 mm) fixation of the SBC allograft produced unacceptable displacement levels during testing. Further study is recommended using a titanium interference screw or a longer bioabsorbable screw for SBC graft fixation under cyclic loading conditions.Arthrex Inc., Naples, FL, USA sponsored this study     

Graft-tunnel mismatch in endoscopic anterior cruciate ligament reconstruction Intraoperative and cadaver measurement of the intra-articular graft length and the length of the patellar tendon

The results of a study conducted on 50 knees endoscopically reconstructed for an anterior cruciate ligament (ACL) lesion with a free bone-patellar tendon-bone graft and 9 cadaver knees are reported. The mean lengths of the patellar tendon (45.48 ± 4.71 mm) and intra-articular ACL graft (20.44 ± 1.98 mm) were measured in the operated knees. The mean length of the tibial bone tunnel (51.62 ± 2.60 mm) was also measured with a tibial guide at 55°. No statistically significant correlation was found between these three measurements. The length of the patellar tendon was weakly correlated with body height. Measurement of the tibial tunnel on the cadaver knees with increasing degrees of inclination revealed a mean length increase of 0.68 mm per degree (confidence limits: 0.49–0.86). Comparison between the tunnel lengths obtained with the guide and those measured with a Kirschner wire showed a mean difference of 2.3 mm. It is thus desirable to make the tunnel about 53 mm long to ensure excellent fixation of a 28 mm bone block with a 25 mm interference screw. Correct measurement of the anatomical structures involved is in any event an essential requirement for proper execution of the surgical technique. Received: 26 June 1997 Accepted: 12 November 1997     

Analysis of initial fixation strength of press-fit fixation technique in anterior cruciate ligament reconstruction

We performed a controlled laboratory study to evaluate the initial fixation strength of press-fit technique. Forty porcine lower limbs were used and divided into four groups according to the method of fixation; group 1 (press-fit+1.4 mm), in which the diameter difference between the bone plug and the femoral tunnel was 1.4 mm; group 2 (press-fit+1.4 mm, 30 degrees), in which the diameter difference was the same with group 1, but the tensile loading axis was 30 degrees away from the long axis of the femoral tunnel; group 3 (titanium), in which a titanium interference screw was used for fixation; group 4 (bioabsorbable), in which a bioabsorbable interference screw was used for fixation. The graft in the press-fit group was harvested with a hollow oscillating saw with inner diameter of 9.4 mm to obtain consistent and completely circular shape of the bone plug. The femoral tunnel with diameter of 8 mm was drilled at the original ACL insertion. Following the bone plug insertion into the femoral tunnel and applying a preload of 20 N, the specimen underwent 500 loading cycles between 0 and 2 mm of displacement. Thereafter the specimen was loaded to failure. There was no fixation site failure during the cyclic loading test. Significant differences in the stiffness, linear load, or failure mode among the groups were not found. The average ultimate failure load of group 1 and group 2 were not significantly different from those of group 3 and group 4. The press-fit groups demonstrated sufficient fixation strength for the rehabilitation and interference screw groups. The completely circular shape of the bone plug and increased diameter difference between the bone plug and the femoral tunnel seemed to contribute to the strong fixation.     

Mechanical evaluation of a soft tissue interference screw with a small diameter: significance of graft/bone tunnel cross-sectional area ratio

The purpose of this study is to evaluate the mechanical properties of a graft fixation using a small diameter soft tissue interference screw and analyze the factors affecting the fixation strength. Forty porcine knees were used. A bone tunnel, either 4.5 mm (n=40) or 5.0 mm (n=40) in diameter, was created in the bone block obtained from the proximal tibia or the distal femur. A patella–patellar tendon specimen with varied width was harvested, and the distal end of the patellar tendon was fixed within the bone tunnel using a small diameter soft tissue interference screw (4×15 mm). Then, the patella–patellar tendon-bone block complex was loaded until failure occurred and the maximum load was measured. As potential influential factors on the fixation strength, the insertion torque, bone mineral density of the bone block, and graft/tunnel cross-sectional area ratio (GTR) of each specimen were calculated. A significant correlation between the maximum failure load and the insertion torque was demonstrated. The quadratic regression analysis showed a statistically significant correlation between the failure load and the GTR. Optimal GTR for achieving high fixation strength was approximately 80%. When used in appropriate conditions, the mean failure load was 177 N for the 4.5 mm screw and 180 N for the 5 mm screw. The use of a small diameter interference screw for the fixation of a tendon graft to a bone is clinically feasible. Our research showed that the selection of appropriate fitting conditions is an important factor for optimizing the properties of the fixation.     

A prospective evaluation of tunnel enlargement in anterior cruciate ligament reconstruction with hamstrings: extracortical versus anatomical fixation

Changes in the femoral and tibial bone tunnel were studied prospectively after arthroscopic ACL reconstruction with quadruple hamstring autograft. To determine whether tunnel enlargement can be decreased by fixing the graft close to the joint line having a stiffer fixation construct we compared "anatomical" (one absorbable interference screw femorally, and bicortical fixation with two absorbable interference screws tibially) and extracortical fixation techniques (Endobutton femorally, and two no. 6 Ethibond sutures over a suture washer tibially). Over a 2-year period we evaluated 60 patients clinically (IKDC scale, Cincinnati Knee Score, KT-1000) and radiographically (confirmed by MRI). The operated knee was radiographed immediately postoperatively and 6 and 24 months postoperatively. The femoral and tibial bone tunnel diameter was measured on anteroposterior and lateral images, and the tunnel area was calculated and compared to the initial area calculated from the perioperative drill size. In the "anatomical" group the immediately postoperative bone tunnel area was 75% larger than the initial tunnel area, after 6 months it was increased another 31%, and between 6 and 24 months it remained basically unchanged. In the "extracortical" group there was no significant enlargement immediately postoperatively, but after 6 months it was 65% larger than the initial area of drill and graft size, and between 6 and 24 months it decreased to 47%. There was no correlation between the amount of tunnel enlargement and clinical scores or KT-1000 measurement. Arthroscopic ACL reconstruction with quadruple hamstring autograft is associated with bone tunnel enlargement. Using a purely extracortical fixation technique thus significantly increased the tibial and femoral tunnel area during the first 6 postoperative months, while it decreased slightly thereafter. The insertion of large interference screws apparently not only compresses the graft in the bone tunnel but also significantly enlarges the bone tunnel itself. The immediate enlargement at the time of the operation is followed by a reduced further enlargement at 6 months and then stabilization. Tunnel widening did not influence clinical outcome over a 2-year period.     

Effects of interference fit screw length on tibial tunnel fixation for anterior cruciate ligament reconstruction

Graft-tunnel mismatch during arthroscopically assisted anterior cruciate ligament reconstruction using the central-third patellar tendon results in less than 20 mm of bone plug remaining in the tibial tunnel. We decided to evaluate the strength of bone plug fixation using interference fit screws that were less than 20 mm in length. Biomechanical testing was performed on 48 porcine hindquarters using 9-mm diameter interference fit screws that measured 12.5, 15, and 20 mm in length. No significant difference was noted between the different-length screws for insertion torque, divergence, stiffness, displacement, or load to failure. We believe, therefore, that comparable graft fixation can be achieved in the tibial tunnel using 9-mm diameter interference fit screws that are less than 20 mm long, and that these shorter screws may be useful in cases of graft-tunnel mismatch.