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.     

Intraoperative fracture of patellar bone plug during anterior cruciate ligament reconstruction with bone–patellar tendon–bone: clinical case

The authors report the case of intraoperative fracture of the patellar bone plug during an operation to reconstruct an anterior cruciate ligament using bone–patellar tendon–bone. This complication, known among the problems of reconstructing the anterior cruciate ligament, is usually reported when using interference screws to block the plug at the femoral level. In the case, we describe the fracture event occurred using cross-pins. The post-operative 3D radiographic reconstruction permitted precise localisation of the position of the detached bone fragment.     

Calcification of the patellar tendon after ACL reconstruction

Extensive calcification of the patellar tendon following ACL reconstruction with central-third bone–patellar tendon–bone autograft is a rarely seen complication. A 45-year-old male patient underwent combined intraarticular reconstruction of ACL with 1/3 central patellar bone–tendon–bone graft and extraarticular reconstruction with modified MacIntosh technique. Two cm of calcification of the patellar tendon was observed incidentally when he underwent a high tibial osteotomy due to medial compartment degeneration, secondary to varus malalignment, 18 months after the ACL surgery. The calcification, being painless, was left untouched during the surgery. At the final examination, 136 months postoperatively, the patient still had no complaint relating to the patellar tendon.     

Anterior cruciate ligament anatomy and function relating to anatomical reconstruction

Recently, the interest in surgical techniques that reconstruct the anteromedial (AM) and the posterolateral (PL) bundles of the anterior cruciate ligament (ACL) has risen. This review focuses on the structural as well as the mechanical properties of the ACL and the anatomical details of the femoral origin, midsubstance, and tibial insertion of AM and PL bundles of the ACL. The terminology of AM and PL bundles is chosen according to the tibial insertion and determined by their functional tensioning pattern throughout knee flexion. Close to extension the AM is moderately loose and the PL is tight. As the knee is flexed, the femoral attachment of the ACL becomes more horizontally oriented, causing the AM bundle to tighten and the PM bundle to loosen up. The ACL has been described to be restraint to anterior tibial displacement and internal tibial rotation. The rotational component might be represented by the PL bundle. The femoral origin has an oval shape with the center of the AM close to over-the-top position and the center of the PL close to the anterior and inferior cartilage margin. Tibial and femoral insertions of the ACL are over 3.5 times larger when compared to the midsubstance and tunnel placement is more challenging because of the limited size of potential grafts selection of tunnel site placement. For reconstruction, both bone–patellar tendon–bone (BPTB) and quadrupled hamstring grafts are used. Structural properties of a 10 mm wide BPTB or quadrupled hamstring graft have been reported to be comparable with those of the native ACL.     

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     

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.     

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.     

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.     

MRI of anterior cruciate ligament repair with patellar and hamstring tendon autografts

Objective. Several MRI sequences were used to evaluate the 2-year postoperative appearance of asymptomatic knee with a torn anterior cruciate ligament (ACL) reconstructed with bone–patellar tendon–bone (BTB) and semitendinosus and gracilis (STG) tendon autografts. Design and patients. Two groups with successful repair of ACL tear with BTB (n=10) or STG (n=10) autografts were imaged at 1.5 T with sagittal and oblique coronal proton density-, T2-weighted and sagittal STIR sequences and plain and contrast-enhanced oblique coronal T1-weighted sequences. The appearance of the graft and periligamentous tissues was evaluated. Results. In all 20 cases, the ACL graft showed homogeneous, low signal intensity with periligamentous streaks of intermediate signal intensity on T2-weighted images. In 10 cases, localised areas of intermediate signal intensity were seen in the intra-articular segment of the graft on proton density- and T1-weighted images. The graft itself did not show enhancement in either of the two groups, but mild to moderate periligamentous enhancement was detected in 10 cases. Conclusion. The MRI appearance of ACL autograft is variable on proton density- and T1-weighted images. Periligamentous tissue showing contrast enhancement is a typical MRI finding after clinically successful ACL reconstruction. Received: 22 February 2000 Revision requested: 27 June 2000 Revision received: 21 August 2000 Accepted: 23 August 2000     

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.     

Biomechanical evaluation of press-fit femoral fixation technique in ACL reconstruction

In this experimental study, the authors evaluated the biomechanical properties of the femoral press-fit graft fixation technique in ACL reconstruction. 20 fresh frozen distal femurs, patellae and patellar ligaments were used from 10 cadaver specimens. Three bone-patellar tendon grafts of 10 mm width were prepared from each sample; altogether 60 bone-patellar tendon grafts were prepared for the experiment. Three 9 mm wide tunnels were drilled in each distal femur at different angles (0, 15, 30, 45, and 60 degrees). This means that 60 tunnels were drilled into the 20 femurs, 12 at each angle. The trapezoid bone blocks were impacted into the holes. The primary stability and stiffness of this press-fit fixation method were measured with a Zwick 020 computer-controlled testing device using maximum-failure tensile-strength tests. The ultimate tensile strength was the greatest at 45 degrees (534±20 N, range 507–554), with 118±10 N/mm (range 99–126) stiffness, followed by 485±35 N (range 416–510) with 122±13 N/mm (range 104–136) stiffness at 30 degrees, 353±18 N (range 320–371) with 113±13 N/mm (range 83–124) stiffness at 15 degrees, and 312±30 N (range 261–343) with 89±14 N/mm (ranged:68–103) stiffness at 0 degrees. In the cases of 0, 15, 30 and 45 degrees the bone blocks were pulled out of the drilled holes, but at 60 degrees rupture of the patellar tendon or breakage of the bone block occurred more frequently. It can be seen that the ultimate tensile strength increased with the angle between the loading direction and the bone block. When compared to data in the literature, these data showed similar and satisfying biomechanical properties of femoral press-fit fixation. Because of the known advantages of an implantation-free fixation technique, the femoral press-fit fixation technique can be a good alternative in ACL surgery. These results provide the basis for future studies involving the postoperative healing process of this femoral press-fit fixation technique in porcine knees.     

Bone tunnel enlargement after anterior cruciate ligament reconstruction with the hamstring autograft and endobutton fixation technique A clinical, radiographic and magnetic resonance imaging study with 2 years follow-up

The aim of this study was to describe the contrast-enhanced magnetic resonance imaging (MRI) appearance of bone tunnel enlargement detected on radiography after anterior cruciate ligament (ACL) reconstruction with semitendinosus and gracilis tendon endobutton (STG-endobutton) fixation technique. Fourteen patients with a STG-endobutton ACL reconstruction were examined 3 months (n = 1), 1 year (n = 1) and 2 years (n = 12) postoperatively. An age- and sex-matched group with a bone–patellar tendon–bone (BTB) autograft ACL reconstruction with similar follow-up was taken as control. Data on clinical examination, laxity and isokinetic muscle torque measurements, anteroposterior and lateral view radiography were obtained, and knee scores (Lysholm and Tegner) were collected. Contrast-enhanced MRI was performed in the STG-endobutton group with a 1.5-T imager. There were no statistical differences between the groups with respect to clinical findings, stability tests, or knee scores. In the STG-endobutton group the average femoral and tibial bone tunnel diameter detected on anteroposterior view radiography had increased at 2-year follow-up by 33% and 23%, respectively. On MRI the ligamentous graft itself was not enhanced by the contrast medium whereas periligamentous tissue within and around the STG graft bundles showed mild contrast enhancement. In conclusion, the MRI results suggest that enhancing periligamentous tissue accumulated in and around the STG graft associated with the tunnel expansion. In spite of the significant bone tunnel enlargement observed on the follow-up radiography the STG-endobutton knees were stable and the patients satisfied. Received: 26 January 1999 Accepted: 22 May 1999     

The effects of anterior cruciate ligament reconstruction on tibial rotation during pivoting after descending stairs

Recent in vitro research suggests that ACL reconstruction does not restore tibial rotation. This study investigated rotational knee joint stability in vivo during a combined descending and pivoting movement that applies a high rotational load to the knee joint. We studied 20 ACL reconstructed patients (bone–patellar tendon–bone graft) and 15 matched controls with a six-camera optoelectronic system performing the examined movement. In the control group the results showed no significant differences in the amount of tibial rotation between the two sides. No significant differences were also found between the contralateral intact leg of the ACL group and the healthy control. However, a significant difference was found within the ACL reconstructed group and between the reconstructed and the contralateral intact leg. Therefore ACL reconstruction may not restore tibial rotation even though anterior tibial translation has been reestablished.     

Interference screw fixation of hamstring vs patellar tendon grafts for anterior cruciate ligament reconstruction

The present study was designed to investigate the fixation strength of a quadruple semitendinosus-gracilis graft compared with a middle-third bone-patellar tendon-bone graft using a new interference screw developed to fix hamstrings grafts for ACL reconstructions (RCI, Smith & Nephew Donjoy). Five pairs of human cadaveric knees from donors with a mean age of 43 (range 33–52) years were used. One knee of each pair was randomly allocated to be reconstructed on the femoral side with a semitendinosus-gracilis graft from the same donor using RCI screw fixation. As the control, the contralateral knee was correspondingly reconstructed with a bone-patellar tendon-bone graft using the same interference screw. The grafts were pulled out at a velocity of 30 mm/s by an axially applied load using a MTS machine. The mean (SD) failure load for the bone-patellar tendon-bone graft fixations was 505 (25) N, 110% stronger than the mean failure load for the semitendinosus-gracilis graft fixations, which was 240 (47) N (P = 0.003). The stiffness for the patellar tendon-bone graft fixations was 46 (11) N/mm, 120% stiffer than the semitendinosus-gracilis graft fixations, which was 22 (11) N/mm (P = 0.01). This study shows that the interference screw principle used for ACL reconstructions with hamstrings tendons is inferior to that for bone-patellar tendon-bone reconstructions although the screw was developed especially for soft-tissue fixation in bone tunnels. Received: 14 August 1996 Accepted: 25 September 1997     

Comparative evaluation of knee stability following reconstruction of the anterior cruciate ligament with the bone–patellar tendon–bone and the double semitendinosus–gracilis methods: 1- and 2-year prospective study

An account of a prospective study of anterior cruciate ligament reconstructions with the bone–patellar tendon–bone autograft (BPTB) and the doubled semitendinosus–gracilis autograft (ST–GR) is given in 39 patients and 22 patients, respectively. The BPTB patients were younger, and there were more women in the ST–GR group. There were no statistical differences in the clinical and instrumental evaluations of stability after 1 and 2 years (Lachman and Jerk test, KT1000) between the two groups. Slightly better results were obtained in the BPTB group: mean 0.80 and 0.96 (first and second years) versus 1.18 and 1.20. Both methods, in fact, resulted in very satisfactory anterior knee stability, even when applied in two dissimilar groups of patients.     

Rotator cuff repair with periosteum for enhancing tendon–bone healing: a biomechanical and histological study in rabbits

During rotator cuff repair surgery, fixation and incorporation of ruptured rotator cuff tendon into the bone is a major concern. The repair usually fails at the tendon–bone interface, especially in cases where the tear is massive. The periosteum contains multipotent stem cells that have the potential to differentiate into osteogenic and chondrogenic tissues, which may restore the original structure at the tendon–bone interface, fibrocartilage. In this study, we investigated the effect of periosteum on the healing of the infraspinatus tendon and bone using a clinically relevant rabbit model of rotator cuff tear. We used 36 skeletally mature New Zealand white rabbits in the study. The infraspinatus tendon at right limb was detached from greater tuberosity, and a periosteal flap taken from the proximal tibia was sutured onto the torn end of tendon. The contralateral limb, which was used as a control, received the same treatment without a periosteum. The rabbits were sacrificed at 4, 8, and 12 weeks, and the tendon–bone interface was put to histological exam and the biomechanical testing to assess strength of tendon–bone interface. Histological analysis of the tendon–bone interface revealed that the periosteum formed a fibrous layer over the interface between tendon and bone. At 4 weeks, fibrotic tissue showed progressive integration over the interface between cuff tendon and bone. At 8 weeks, progressive formation of fibrovascular tissue and fibrocartilage was observed between tendon and bone. At 12 weeks, extensive formation of fibrocartilage and bone was noted in the interface. The significant increase of failure load with time indicated a progressive increase in the tendon–bone incorporation strength. At 4 weeks after operation, the attachment strength of the limbs with the periosteum treated was higher than that of the control limbs; however, this difference was not statistically significant. At 8 and 12 weeks, a statistically significant increase was noted in the attachment strength of the limb treated with the periosteum. Most specimens failed at the tendon–bone interface (18/20). In the treatment of a torn rotator cuff in rabbit model, improved healing process with greater attachment strength could be achieved by suturing the periosteum between the end of tendon and the bone trough. Histological examination revealed that the cambium layer of the periosteum could serve as a potent interface layer and become progressively mature and organized during the healing process, resulting in fibrocartilage formation and the subsequent integration of the disrupted tendon into the bone. Biomechanical testing revealed a progressive increase in the attachment strength with time indicating the progressive tendon–bone incorporation. When performing rotator cuff repair in a large or massive tear, a periosteal flap can be sutured onto the torn end of tendon to enhance tendon–bone healing.     

Comparison of femoral fixation methods for anterior cruciate ligament reconstruction with patellar tendon graft: a mechanical analysis in porcine knees

The aim of the present study was to evaluate the structural properties of femur–patellar tendon graft complex in anterior cruciate ligament (ACL) reconstruction using different femoral fixation devices. Type of study is biomechanical testing. An ACL reconstruction was performed on 40 cadaver porcine knees, using patellar tendon (PT) graft. Specimens were divided into four groups according to the femoral fixation: interference absorbable screw (Group A), metallic setscrew (Group B), absorbable pins (Group C), and a combination of metallic setscrew and pin (Group D). Other ten knees were used as controls. On each sample, a cyclic loading test, then a load-to-failure test were performed. Elongation after 1,000 loading cycles, ultimate failure load, yield load, stiffness, deformation at the yield point, and mode of failure were recorded. Kruskal–Wallis test and Tukey test were used to compare the differences between groups. The lowest mean elongation after 1,000 load cycles was observed for Group B (1.7 ± 1.4 mm) and D (1.2 ± 0.3 mm). Ultimate failure load of Group D (1,021.8 ± 199.4 N) was comparable with that of normal ACL (1,091.2 ± 193.3 N) and PT graft (1,140.6 ± 285.7 N). All other groups were lower than the controls. For mean stiffness, all the groups, excepting for Group D (172.8 ± 40.4 N/mm), were significantly lower than PT control group (216 ± 78.4 N/mm). Mode of failure was graft pullout for Groups A and B, distal pin breakage for Group C, and midsubstance graft rupture in 80% of the cases for Group D. Only combined compression and suspension fixation did not show significantly different structural properties in comparison with normal ACL and PT graft. Furthermore, it showed no risk of graft pullout or hardware breakdown in comparison with other fixation devices.     

Modified Prusik knot versus whipstitch technique for soft tissue fixation in anterior cruciate ligament reconstruction: a biomechanical analysis

Appropriate graft tension and secure graft incorporation in bone tunnels are essential for successful anterior cruciate ligament (ACL) reconstruction using hamstring tendon autografts. Permanent viscoplastic elongation in response to cyclic loading in the early postoperative period and the interposition of suture material in the tendon–bone interface might negatively affect graft function and rigid graft incorporation in the bone tunnels. A modified Prusik knot is an alternative option to the commonly used whipstitch technique for soft tissue fixation in ACL reconstruction. This is a controlled laboratory study. Sixteen formalin-fixed human cadaver semitendinosus tendons were armed with a modified Prusik knot or a whipstitch and tested in a load-to-failure test with a constant displacement rate of 1 mm/s, 14 in the cyclic loading test with 100 cycles from 10 to 50 N followed by 100 cycles from 10 to 75 N. The modified Prusik knot showed smaller force-induced displacements and higher stiffness of the entire construct in the load-to-failure test. Smaller preconditioning displacements were the only significant differences in the cyclic loading test. The modified Prusik knot has equal or superior mechanical properties and provides a larger area in the tendon–bone interface without suture material compared with the whipstitch technique.     

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     

Fixation strength of interference screw fixation in bovine,young human,and elderly human cadaver knees: Influence of insertion torque,tunnel-bone block gap,and interference

A failure analysis of interference screw fixation was performed to test the hypothesis that bovine and/or elderly human cadavers are appropriate models for bonepatellar tendon-bone anterior cruciate ligament (ACL) reconstruction fixation studies. Failure mode is an important criterion for validating experimental models. The bovine, young human, and elderly human failure loads were 799±261 N, 655±186 N, and 382±118 N, respectively, and the failure modes were 75%, 69%, and 30% tissue failures, respectively. The similarities between the bovine and young human models in failure loads and failure modes indicate that bovine models are appropriate for ACL reconstruction fixation studies. The statistically significant differences between the young human and elderly human models in failure loads and failure modes indicate that elderly human cadavers are not an appropriate model for ACL reconstruction fixation studies. The differences in failure modes are consistent with previous studies using elderly human cadavers in which the predominant failure mode was bone block pullout. The tissue failures observed in the bovine and young human models contradict previous studies suggesting fixation strength is the weakest link in bone-patellar tendon-bone ACL reconstruction. Results of linear regression modeling showed statistically significant correlations between insertion torque and failure load (R ²=0.44,P<0.0001) and interference (defined as the screw outer thread diameter minus the tunnel-bone block gap) and insertion torque (R ²=0.18,P=0.003) when data from all models was combined. Results for the bovine model multiple regression showed a statistically significant regression of insertion torque (linear) and interference (quadritic) versus failure load (R ²=0.56,P=0.02). Regression slopes for screw diameter (P=0.52) and gap size (P=1.00) were not statistically significant. These results indicate that insertion torque and interference are independent predictors of failure load and should be included in future interference screw studies in addition to bone block dimensions, tunnel size, gap size, and screw diameter. Clinicians may consider using insertion torque and interference as indicators of postoperative graft fixation regarding rehabilitation decisions.