Graft healing in a bone tunnel: bone-attached graft with screw fixation versus bone-free graft with extra-articular suture fixation

The purpose of this study was to clarify differences in tendon graft–to–bone tunnel healing between bone-attached tendon grafts with interference-screw fixation and bone-free tendon grafts with extra-articular suture fixation. In 42 Japanese White rabbits, anterior half replacement of the medial collateral ligament was performed using half of the ipsilateral patellar tendon. At the femoral attachment, the bone-plug-attached graft was fixed with an interference screw (group A). The bone-plug-free graft was fixed by the extra-articular suture fixation technique with sutures tied over a button (group B). Biomechanical and histological evaluations were performed at 2, 4 and 8 weeks postoperatively. In biomechanical evaluation, at 2 or 4 weeks 27 of 28 specimens (96%) were pulled out from the femoral tunnel, while one 4-week specimen and all four 8-week specimens failed at the grafts mid-substance. At 2 weeks, the maximum failure load was 25±10 N and 24±6 N for group A and group B respectively (mean±SD). At 4 weeks, the maximum failure load was 42±17 N and 35±15 N respectively. There were no significant differences in maximum pullout failure load between the groups at 2 or 4 weeks postoperatively. (P=0.887 at 2 weeks and P=0.339 at 4 weeks using ANOVA measurement). Histologically, the bone-attached grafts showed partial bone–to–bone union at the graft–bone tunnel interface at 4 weeks, and complete bony union at 8 weeks. The bone-free grafts exhibited newly formed Sharpey-like collagen fibers at 4 weeks, and strong connection by mature granulation tissue at 8 weeks. Graft–to–bone tunnel healing of bone-attached graft with screw fixation and bone-free graft with extra-articular suture fixation are comparable in terms of biomechanical evaluation during the early postoperative periods.     

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.     

Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque

The purpose of this study was to determine whether bone mineral density of the host bone, measured using conventional dual photon absorptiometry techniques, and insertion torque can predict part of the ultimate failure strength of interference screw fixation of quadrupled hamstring tendon grafts. The semitendinosus and gracilis tendons were harvested from 10 human cadaveric knees, mean age 66.5 years (range, 53 to 81). The bone tunnel was sized within 0.5 mm of the graft. The graft was fixed with a biodegradable screw (7 x 25 mm for the femur, and 9 x 25 mm for the tibia) directly against the tendon and at the joint surfaces. Tibial fixation and femoral fixation were tested to failure using a materials testing system. Bone mineral density was measured in the metaphyseal region of the tibia and femur. The results of multiple regression analyses showed that both insertion torque and bone mineral density were related to the maximum load the graft withstood. These two variables explained 77.1% of the maximum load observed. We concluded that bone mineral density measurements of the host bone site are an important determinant of postoperative graft strength and thus have an important, but previously unrecognized, clinical role in establishing individual postsurgery rehabilitation protocols. Insertion torque in this study was a useful predictor of graft fixation strength.     

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.     

Serial dilation versus extraction drilling in anterior cruciate ligament reconstruction: a biomechanical study

The hamstring tendon graft has become increasingly popular in anterior cruciate ligament reconstruction because of low donor-site morbidity. However, the tibial fixation is considered difficult, mainly because of low tibial mineral bone density. Therefore, we tested whether preparation of the tibial tunnel with compaction by serial dilation provided a stronger anchorage of the graft–fixation-device complex than does traditional extraction drilling of the tibial tunnel. In 20 bovine tibiae, the bone tunnels were created with either extraction drilling (group 1) or compaction by serial dilation (group 2). Twenty bovine digital extensor tendons were fixated in the bone tunnel with an Intrafix tibial fastener. The graft–fixation-device complexes were mounted in a hydraulic test machine. The fixation strength was evaluated after cyclic loading. The difference between the serial dilation group and the extraction drilling group ranged from a mean slippage of 0 mm at 70–220 N, to a mean slippage of 0.1 mm at 70–520 N. We found no significant difference in slippage of the graft–fixation-device complex after 1,600 cycles. This study failed to show a significant difference between compaction by serial dilation and extraction drilling of the tibia bone tunnel in anterior cruciate ligament reconstruction.     

Failure load of patellar tendon grafts at the femoral side: 10- versus 20-mm-bone blocks

The aim of the study was to investigate whether use of short bone blocks is safe in anterior cruciate ligament (ACL) reconstruction. Our hypothesis was that the smaller 10-mm-length bone blocks will fail at lower loads than 20-mm-bone blocks. Ten paired human cadaver knees were randomly assigned to the 10- or 20-mm group (group 1 and 2) and underwent bone–patellar tendon–bone femoral fixation with interference screw. Tensile tests were performed using a tensile testing machine (Instron). Stiffness, failure load and failure mode were recorded. Median stiffness was 72 N/mm (16–103) for 10-mm-bone blocks and 91 N/mm (40–130) for 20-mm-bone blocks. Median failure loads were 402 N (87–546) for 10-mm-long bone block and 456 N (163–636) for 20-mm-bone blocks. There was no statistically significant difference between groups (P = 0.35). All bone–patellar tendon–bone grafts were pulled out of the femoral tunnel with interference screw, due to slippage. We concluded that a 10-mm-long bone block was not significantly weaker than a 20-mm-long bone block. Failure loads of a 10-mm-bone block exceeded loading values at passive and active extension of the knee under normal conditions. Ten millimetre bone blocks offered sufficient fixation strength in ACL reconstruction.     

Pullout strength of tibial graft fixation in anterior cruciate ligament replacement with a patellar tendon graft: interference screw versus staple fixation in human knees

The endoscopic single incision technique for anterior cruciate ligament (ACL) reconstruction with a femoral half-tunnel may lead to a graft/tunnel mismatch and subsequent protrusion of the block from the tibial tunnel. The typical tibial fixation with an interference screw is not possible in these cases. Fixation with staples in a bony groove inferior to the tunnel outlet can be used as an alternative technique. Current literature does not provide biomechanical data of either fixation technique in a human model. This study was performed to evaluate the primary biomechanical parameters of this technique compared with a standard interference screw fixation of the block. Fifty-five fresh-frozen relatively young (mean age 44 years) human cadaver knee joints were used. Grafts were harvested from the patellar tendon midportion with bone blocks of 25 mm length and 9 mm width. A 10-mm tibial tunnel was drilled from the anteromedial cortex to the center of the tibial insertion of the ACL. Three different sizes of interference screws (7 × 30, 9 × 20, 9 × 30 mm) were chosen as a standard control procedure (n = 40). For tibial bone-block fixation the graft was placed through the tunnel, and the screw was then inserted on the cancellous or the cortical surface, respectively. Fifteen knees were treated by staple fixation. A groove was created inferior to the tunnel outlet with a chisel. The bone block was fixed in this groove with two barbed stainless steel staples. Tensile testing in both groups was carried out under an axial load parallel to the tibial tunnel in a Zwick testing machine with a velocity of 1 mm/s. Dislocation of the graft and stiffness were calculated at 175 N load. Maximum load to failure using interference screws varied between 506 and 758 N. Load to failure using staples was 588 N. Dislocation of the graft ranged between 3.8 and 4.7 mm for interference screw fixation and was 4.7 mm for staples. Stiffness calculated at 175 N load was significantly higher in staple fixation. With either fixation technique, the recorded failure loads were sufficient to withstand the graft loads which are to be expected during the rehabilitation period. Staple fixation of the bone block outside of the tunnel resulted in a fixation strength comparable to interference screw fixation. Received: 2 September 1996 Accepted: 30 January 1997     

How four weeks of implantation affect the strength and stiffness of a tendon graft in a bone tunnel: a study of two fixation devices in an extraarticular model in ovine

BACKGROUND: For a tendon graft to function as an anterior cruciate ligament, the tendon must heal to the bone tunnel. We studied the effect of 4 weeks of implantation on the strength and stiffness of a tendon in a bone tunnel using two different fixation devices in an ovine model. HYPOTHESIS: The type of fixation device in anterior cruciate ligament reconstruction may affect early healing, which can be measured as the strength and stiffness of a tendon in a bone tunnel. STUDY DESIGN: Controlled laboratory study. METHODS: An extraarticular tendon graft reconstruction was performed in ovine tibias. The graft was fixed with either a bioresorbable interference screw or a WasherLoc. After 4 weeks of implantation the strength and stiffness of the complex and the tendon graft-bone tunnel interface were determined by incrementally loading specimens to failure. RESULTS: For the interference screw, the strength deteriorated 63% and the stiffness deteriorated 40%. For the WasherLoc, the strength was similar and the stiffness improved 136%. CONCLUSIONS: The type of fixation device determines whether the strength and stiffness of a tendon in a bone tunnel increases or decreases after implantation. Clinical Relevance: The pace of rehabilitation may need to be adjusted based on the type of fixation device used to secure a soft tissue graft.     

The load of an implanted graft during and after fixation in anterior cruciate ligament reconstruction

Although the importance of initial graft load has been discussed in the literature, it has not been confirmed whether a surgeon can provide the desired load to an implanted graft in anterior cruciate ligament reconstruction. The purpose of this study was to compare the set force (initial load given to the graft before fixation) and residual load in the implanted graft using three different fixation techniques. A total of ninety porcine knees were tested using bone–patellar tendon–bone autograft. Each bone–tendon–bone autograft was fixed to the tibia with either the interference-fit screw, fixation post, or button technique. Graft load was monitored during fixation procedures, and for 10 min after fixation. Residual graft load with each fixation technique exhibited unique features. Highest graft load was obtained by the interference-screw fixation technique; however, the graft was usually over-loaded beyond the intended set force with this technique. In the fixation-post technique, the load of the graft increased while the screw was retightened. The graft load in the button-fixation technique was low, probably due to slight slippage of the button. When the graft was fixed under maximum manual tension, the graft loads at completion of fixation for the interference fit screw, fixation post, and button techniques were 116.3, 54.2, and 25.9 N respectively.     

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.     

Bioabsorbable screw divergence angle,not tunnel preparation method influences soft tissue tendon graft-bone tunnel fixation in healthy bone

Bone tunnel fixation of a soft tissue tendon graft is the weak link immediately following ACL reconstruction. This biomechanical study evaluated the influence of extraction drilled or step dilated bone tunnels and bioabsorbable screw divergence on soft tissue tendon graft fixation. From an initial group of 50 available specimens, similar apparent bone mineral density porcine tibiae (1.2 ± 0.24 g/cm²) were divided into two groups of ten specimens each. Group 1 (extraction drilled) received 9 mm diameter tunnels. Group 2 (step dilated) received 7 mm diameter tunnels that were dilated to 9 mm. Grafts were secured in tunnels using 10 mm diameter, 35 mm long tapered screws. After high resolution CT scanning to evaluate screw divergence, constructs were pretensioned on a servo hydraulic device between 10 and 50 N for 10 cycles, and isometric pretensioned at 50 N for 1 min, prior to 500 sub-maximal loading cycles (50–200 N) and load to failure testing at 20 mm/min. Wilcoxon Signed Rank tests and Mann–Whitney U-tests were used to evaluate group differences. Coefficient of determination values (r ²) were calculated to further delineate statistically significant relationships. Tunnel preparation method did not display statistically significant effects on insertion torque, displacement during cyclic testing, relative stiffness during cyclic testing, load at failure, stiffness during load to failure testing or displacement during load to failure testing. Screw divergence < 15° produced lower displacement and greater relative stiffness during cyclic testing and greater load at failure and stiffness during load to failure testing. Screw divergence angle displayed moderate relationships with construct displacement during cyclic testing (r ² = 0.54), stiffness during load to failure testing (r ² = 0.60), and load at failure (r ² = 0.41). Tunnel dilation does not enhance soft tissue tendon graft fixation strength in healthy bone. Bioabsorbable screw divergence of ≥ 15° significantly reduces soft tissue tendon graft-bone tunnel fixation.     

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.     

Intraarticular migration of a broken biodegradable interference screw after anterior cruciate ligament reconstruction

Poly-l-lactic acid biodegradable screws have been used effectively for graft fixation in anterior cruciate ligament (ACL) reconstruction. The overall complication rate associated with the use of this implant is low, although some authors reported complications, such as osteolysis and aseptic effusion of the knee joint. We report a case of a 29-year-old female patient with a failure of a biodegradable interference screw at 22 months after ACL reconstruction using bone–patellar tendon–bone graft. In this illustrated case, the screw broke and migrated into the knee joint. In addition, we performed a detailed review of the medical literature from 1990–2005 to identify possible causes of biodegradable screw failures. We identified six published cases of bioabsorbable interference screw failure with migration into the knee joint. Several authors have reported small diameter of the screw, poor bone quality, bone resorption, and screw divergence as potential causes for intraarticular migration of metallic interference screws. With regard to bioscrews, no specific risk factors for screw breakage and intraarticular migration have been reported. ACL reconstruction with the use of bioabsorbable interference screws for fixation is considered to be reliable. However, we need to be aware of potential problems associated with the use of this implant. Early recognition of bioscrew failure may prevent associated morbidities, such as subsequent cartilage damage.     

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.     

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     

Interference screw insertion angle has no effect on graft fixation strength for insertional Achilles tendon reconstruction

Purpose

To compare the effect of two interference screw insertion angles on the biomechanical properties of the insertional Achilles tendon (IAT) reconstruction.

Methods

Nine matched pairs of fresh-frozen human cadaveric Achilles tendon specimens were randomized to two groups with interference screw insertion angles of 60° and 120°. The IAT reconstruction was performed by fixing the graft tendon with the interference screw. Each specimen was loaded to failure. The load at failure, stiffness, and mode of failure were documented. Differences in load at failure and stiffness were analysed.

Results

There was no statistically significant difference between the 60° and 120° groups for failure load (149.137?±?20.836 versus 155.428?±?28.343 N, respectively, n.s.) and stiffness (14.523?±?2.824 N/mm versus 14.727?±?2.192 N/mm, respectively, n.s.). The most common mode of failure was the graft pulling out of the bone tunnel when the screw broke.

Conclusions

Graft fixation at two different interference screw insertion angles for IAT reconstruction exhibited equivalent biomechanical performance. When performing this procedure, surgeons may choose the interference screw insertion angle based on personal preference.

     

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 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.     

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.     

Optimal screw diameter for interference fixation in a bone tunnel: a porcine model

The study investigates the optimal interference screw dimensions required to secure a tendon graft in a bone tunnel. A standard 8 mm pig flexor-tendon graft was inserted into a standard open-ended 8 mm bone tunnel of a porcine distal femur and secured using either 7 mm, 8 mm or 9 mm diameter metal interference screws (Arthrex Inc, Naples, FL). The construct was tested to failure using a Shimadzu ASG 10KN Universal Material Testing Machine (Shimadzu, Tokyo, Japan). Load and mode of construct failure were recorded for 37 individual constructs. There was no significant difference in the load at failure between the 7 mm screw (192 N; range 151–232) and 8 mm screw (181 N; range 150–212) (p>0.05). There was a significant difference between the 7 mm screw and the 9 mm screw (109 N; range 67–151) (p=0.006) and between the 8 mm screw and the 9 mm screw (p=0.015). When using a 9 mm screw, 100% of the constructs failed by cut out of the graft at the tunnel opening. The 7 mm constructs failed by slippage of the tendon from the bone tunnel in 83% of cases, with only 17% failing by cut out at the tunnel opening. The 8 mm constructs demonstrated a mixture of failure modes, with slippage occurring in 58% of cases, cut out in 38% and failure of the graft substance in one case (4%). In this model, screw diameters equal to or 1 mm less than the tunnel/tendon diameter provides better fixation than using a screw 1 mm larger. The mode of failure differs for each of these screws.