Interference Screw Fixation of Soft Tissue Grafts in Anterior Cruciate Ligament Reconstruction: Part 1

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1 DOI = / Interference Screw Fixation of Soft Tissue Grafts in Anterior Cruciate Ligament Reconstruction: Part 1 Effect of Tunnel Compaction by Serial Dilators Versus Extraction Drilling on the Initial Fixation Strength Janne T. Nurmi,* DVM, PhD, Pekka Kannus,* MD, PhD, Harri Sievänen, ScD, Timo Järvelä,* MD, PhD, Markku Järvinen,* MD, PhD, and Teppo L. N. Järvinen,* a MD, PhD From the *Medical School and the Institute of Medical Technology, University of Tampere, Tampere, Finland, the Department of Surgery, Tampere University Hospital, Tampere, Finland, the Department of Clinical Veterinary Sciences, Faculty of Veterinary Medicine, University of Helsinki, Helsinki, Finland, the Bone Research Group, UKK-Institute, Tampere, Finland, and Accident and Trauma Research Center, UKK Institute, Tampere, Finland Background: Compaction of the bone-tunnel walls by serial dilation is believed to enhance the interference screw fixation strength of the soft tissue grafts in anterior cruciate ligament (ACL) reconstruction. Hypothesis: Serial dilation enhances the fixation strength of soft tissue grafts in ACL reconstruction over extraction drilling. Study Design: Randomized experimental study. Methods: Initial fixation strength of the doubled anterior tibialis tendon grafts (fixed with a bioabsorbable interference screw) was assessed in 21 pairs of human cadaver tibiae with either serially dilated or extraction-drilled bone tunnels. The specimens were subjected to a cyclic-loading test, and those surviving were then tested using the single-cycle load-to-failure test. Results: During the cyclic-loading test, there were 3 fixation failures in the serially dilated and 6 failures in the extraction-drilled specimens but no significant stiffness or displacement differences between the groups. In the subsequent load-to-failure test, the average yield loads were 473 ± 110 N and 480 ± 115 N for the 2 groups respectively (P =.97) and no difference with regard to stiffness or mode of failure. Conclusions: Serial dilation does not increase the strength of interference fixation of soft tissue grafts in ACL reconstruction over extraction drilling. Clinical Relevance: The results of this experiment do not support the use of serial dilators in ACL reconstruction. Keywords: bone compaction; dilation; ACL; graft fixation; biomechanics Soft tissue grafts, that is, the hamstring or other tendon grafts, have become increasingly popular as anterior cruciate ligament (ACL) substitutes, 14,15,17 and interference screw fixation is probably the most commonly used device a Address correspondence to Teppo Järvinen, MD, PhD, Department of Surgery, University of Tampere/IMT, FIN Tampere, Finland ( teppo.jarvinen@uta.fi). The American Journal of Sports Medicine, Vol. 32, No. 2 DOI: / American Orthopaedic Society for Sports Medicine to secure these grafts into the femoral and tibial bone tunnels. 12,17 The fixation, rather than the graft itself, has been suggested to be the weakest link in the early postoperative period after ACL reconstruction, 4,28 and progressive creep or slippage of the graft fixed in a bone tunnel is one of the most common concerns in the use of these soft tissue grafts. 5,16,23,31 When using soft tissue grafts in ACL reconstruction, compaction or dilation of the walls of the bone tunnel has been advocated by some authors to create bone tunnels with denser walls. 15,18 Theoretically, such tunnels should 411

2 412 Nurmi et al The American Journal of Sports Medicine provide improved fixation of soft tissue grafts by minimizing the chance of interference screw divergence, convergence, migration, and loosening. However, there are scarce scientific data to justify compaction of the bone-tunnel walls. Cain et al (unpublished data, 1999) showed in their preliminary report that tibial tunnel dilation increased the pullout strength of quadrupled hamstring grafts in comparison to conventional reaming. In contrast, Rittmeister et al recently showed that dilation of the tibial tunnel did not significantly increase the strength of fixation of hamstring grafts in comparison to extraction drilling. 26 Nurmi et al, in turn, have previously found no difference between compaction drilling with stepped routers and conventional extraction drilling in the initial fixation strength of hamstring grafts fixed with interference screws in a tibial drill hole. 22,23 However, in compaction drilling, the sharp blades at the tip of the drill bit (reamer) cut the cancellous bone to granular particles, which are then pushed to the sides of the bone-tunnel walls, whereas the serial dilators are proposed to maintain the cancellous structure and connectivity more intact by only squeezing the individual cancellous bone trabeculae to the sides of the drill hole. This biomechanical study has 2 primary objectives: In the first part (the current article), we evaluated whether compaction of bone-tunnel walls by serial dilation has any benefit over conventional extraction drilling concerning the initial strength of fixation of a soft tissue graft fixed with interference screws in a tibial bone tunnel, whereas the second study (part 2) assessed the effects of pretensioning and preconditioning on the graft tension during and after interference screw insertion. MATERIALS AND METHODS Specimens The tibiae of both limbs were harvested from 21 human cadavers (mean age, 40 ± 11 years; range, 17-54): 7 women (39 ± 11 years, 17-49) and 14 men (41 ± 11 years, 23-54), and the anterior tibialis tendons of both limbs were removed from another group of 21 human cadavers (62 ± 13 years, 40-86): 6 women (66 ± 11 years, 53-79) and 15 men (60 ± 13 years, 40-86). The tibiae and tendons were cleared of adherent muscle fibers and surrounding soft tissues, wrapped in saline-soaked gauze, and stored frozen in 20 C in sealed plastic bags. These preservation procedures have been recommended for knee specimens intended for in vitro testing protocols of the cruciate ligaments and ligament reconstructions 1 and have been shown not to affect the mechanical properties of bones. 24 Study Groups Twenty-one pairs of tibiae used in a previous study 23 were divided into 2 study groups according to previous randomization: In the serial-dilation group, the previously extraction-drilled bone tunnel 23 ( 8.5 mm; average, 7.8 ± 0.6 mm) was enlarged to the desired final diameter of 10 mm with serial Tunnel Dilators (Arthrex Inc., Naples, Florida), Figure 1. The instruments used in the study: a conventional cannulated 10-mm drill bit for extraction drilling (right) and the Tunnel Dilators ( mm with 0.5 mm increments) for serial dilation (left). whereas in the extraction-drilling group, the previously compaction-drilled bone tunnel ( 8.5 mm; average, 7.8 ± 0.6 mm) was enlarged with a conventional cannulated 10-mm drill bit (Acufex Microsurgical Inc., Mansfield, Massachusetts) (Figure 1). The 21 pairs of fresh anterior tibialis tendons were then matched with the tibiae so that the left and right specimen of each pair went into a different group (equal number of left and right specimens in both study groups). Peripheral Quantitative CT Measurements A peripheral quantitative CT scanner (XCT 3000, Stratec Medizintechnik GmbH, Pforzheim, Germany) was used to determine the volumetric trabecular bone density (in milligrams per cubic centimeters) at the proximal tibia of all the human cadaver tibiae prior to the specimen preparation, according to a protocol described previously. 23,30 Briefly, a cross-sectional image of the proximal tibia approximately 2 cm distal from the articular surface was scanned corresponding to the actual site of the tibial bone tunnel of an ACL reconstruction. Specimen Preparation On the day of testing, the tendons and tibiae were thawed to room temperature. All of the specimens were kept moist

3 Vol. 32, No. 2, 2004 Effect of Tunnel Compaction 413 with physiologic saline solution during specimen preparation, fixation procedures, and biomechanical testing. A looped (double-stranded) anterior tibialis tendon graft with a total graft length of 8 cm was constructed according to the technique described by Charlick and Caborn. 11 In short, the tendon was folded to form a graft with 2 strands, and while maintaining constant tension on both strands, the graft was sutured at the free end for 40 mm with number 2 braided polyester suture (Fiberwire, Arthrex) using the running baseball stitch. The graft diameter was measured at the sutured end with a graft-sizing tube (Acufex) and compared for each set of paired specimens to make sure that the graft diameter was identical on both specimens. Thereafter, the graft was pretensioned using the Graftmaster II System board (Acufex) for 15 minutes, starting at 20 pounds (88 N) of tension. The previously drilled bone tunnel 23 ( 8.5 mm; average, 7.8 ± 0.6 mm) was enlarged to the desired final diameter of 10 mm either by successive placement of increased diameter dilators with 0.5 mm increments or by drilling with a conventional cannulated 10-mm drill bit. Preconditioning, Initial Tensioning, and Screw Insertion The tibia was securely mounted to the mechanical testing machine (Lloyd LR 5K, J J Lloyd Instruments, Southampton, United Kingdom) by specially designed clamps in such a position that the bone tunnel was parallel with the direction of loading. The anterior tibialis tendon graft was pulled through the tibial bone tunnel so that 30 to 35 mm of the unsutured, looped portion of the graft, corresponding to the normal ACL length, 1 protruded from the proximal opening of the bone tunnel. The tendon graft was connected to the mechanical testing machine by placing a steel bar through the unsutured, looped portion of the graft and preconditioned with the mechanical testing machine according to 1 of the 3 following protocols: (1) no preconditioning, (2) cyclic preconditioning (25 cycles between 0 and 80 N in 100 seconds), or (3) isometric preconditioning (80 N constant tension for 100 seconds). The same preconditioning protocol was used in both grafts of each pair. After preconditioning, an initial tension of 80 N was applied to all grafts and a bioabsorbable interference screw (Delta Tapered Bio-Interference Screw, Arthrex) ( 11 mm, length 35 mm) was inserted in outside-in fashion over a guide wire with a screwdriver between the graft and the tibial bone-tunnel wall until the screw s tip reached the proximal bone-tunnel opening. Maximum screw insertion torque was determined using a digital electronic torque meter (Torqueleader TSD 350, MHH Engineering Co. Ltd., Surrey, United Kingdom) mounted on the screwdriver. biomechanical testing protocol consisted of the cyclic-loading test followed by the single-cycle load-to-failure test. In the cyclic-loading test, the specimens underwent 1500 loading cycles between 50 and 200 N at a frequency of 0.5 Hz. The loading was parallel with the long axis of the bone tunnel. The response to loading was automatically obtained in the form of a load-displacement curve. The rate of data acquisition was 4 times per second. The fixation was evaluated by determining automatically (by the computer connected to the mechanical testing machine) the initial stiffness (the stiffness of the fixation including both the possible slippage of the graft past the screw, and graft deformation and/or creep in the beginning of the cyclic loading test, determined as the slope of the linear region of the load-displacement curve corresponding to the steepest straight-line tangent to the loading curve of the first loading cycle during cyclic loading test) and the loadinginduced increase in the displacement from the preload level after 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles of loading, respectively. After 1500 loading cycles, the specimens that survived the cyclic loading were tested using subsequent single-cycle load-to-failure tests. In the single-cycle load-to-failure test, a vertical tensile loading parallel with the long axis of the bone tunnel was performed at a rate of 1.0 m/min until failure of fixation. The specimen s response to the loading was automatically obtained in the form of a load-displacement curve. The fixation was evaluated by determining automatically (by the computer connected to the mechanical testing machine) the stiffness (the stiffness of the fixation including both the possible slippage of the graft past the screw, and graft deformation and/or creep, determined as the slope of the linear region of the load-displacement curve corresponding to the steepest straight-line tangent to the loading curve) and yield load (described as the load at the point where the slope of the load-displacement curve first clearly decreased). The mode of failure was determined visually. Statistical Analysis According to a statistical power analysis made before performing the study, 21 pairs of specimens were needed to obtain a 90% statistical power to detect a difference in the strength of the fixation between the 2 bone-tunnel groups of about 1.0 standardized difference at a significance level of P <.05. Differences in the data of the biomechanical tests between the serial-dilation and extraction-drilling groups were determined using a paired sample t test. Using a Bonferroni correction, P <.0038 (0.05/13) was considered statistically significant, as the number of statistical comparisons between the groups was 13. Biomechanical Testing and Data Analysis The biomechanical tests were performed using the Lloyd LR 5K mechanical testing machine (J J Lloyd Instruments), according to the procedure described earlier. 23 The RESULTS Trabecular Bone Density No difference was observed in the mean trabecular bone density of the region corresponding to the bone tunnel of

4 414 Nurmi et al The American Journal of Sports Medicine Displacement (mm) Serial dilation Extraction drilling TABLE 1 Single-Cycle Load-to-Failure Test After Cyclic Loading a Bone Tunnel Yield Load (N) Stiffness (N/mm) Serial dilation 473 ± ± 21 Extraction 480 ± ± 23 P value a Mean ± SD. 0 p=0.25 p=0.26 p=0.30 p=0.32 p=0.43 p=0.66 p=0.79 p= Number of Cycles Figure 2. Displacement (mean ± SD) during the cyclic-loading test. Totally, 3 specimens in the serial-dilation and 6 in the extraction-drilling group failed during the cyclic-loading test because of graft slippage past the screw. the human cadaver tibiae, as they were 176 ± 29 mg/cm 3 and 182 ± 37 mg/cm 3 for the serial-dilation and the extraction-drilling groups, respectively (P =.09). Screw Insertion Torque No difference was found in the mean maximum screw insertion torque, as it was 1.7 ± 0.5 Nm for the serial-dilation group and 1.6 ± 0.6 Nm for the extraction-drilling group, respectively (P =.39). Biomechanical Testing In the cyclic-loading test, no significant differences in the initial stiffness (232 ± 57 N/mm for the serial-dilation group compared with 255 ± 67 N/mm for the extractiondrilling group, P =.53) or displacement (after 1, 10, 50, 100, 250, 500, 1000, and 1500 loading cycles) were observed between the 2 techniques (Figure 2). In the single-cycle load-to-failure test made after the cyclic-loading test, the average yield load was 473 ± 110 N for the serial-dilation group and 480 ± 115 N for the extraction-drilling group (P =.97) (Table 1). There was no significant difference in stiffness of the fixation (182 ± 21 N/mm compared with 190 ± 23 N/mm, P =.42). The mode of failure was almost entirely graft slippage past the screw, although some graft laceration (partial rupture) and graft stretching was also observed, mainly at the screw-graft interface. Three specimens in the serialdilation and 6 in the extraction-drilling group failed during the cyclic-loading test, and therefore, the total number (N) of the specimens for the single-cycle load-to-failure test was 18 and 15, respectively. All failed specimens and their pairs were excluded from the statistical analysis. The different preconditioning protocols used in part 2 of this study had no effect on any of the above presented fixation-strength parameters. DISCUSSION Compaction of the walls of the bone tunnels, either by compaction drilling (stepped routers) or by using serial dilators of increasing diameter, has been speculated to enhance the interference fit between the bone-tunnel walls and the fixation implant (eg, the interference screw) and, consequently, to provide a more rigid fixation of the ACL soft tissue graft. 15,18 However, the results of this study do not provide support for these assumptions, as compaction of the bone-tunnel walls by serial dilation was shown not to increase the initial fixation strength of a soft tissue graft fixed with an interference screw in comparison to conventional extraction drilling. This lack of effect of tunnel compaction is actually in agreement with the results of the only previously published study evaluating the effectiveness of bone-tunnel compaction by serial dilation on the fixation strength of soft tissue grafts in ACL reconstruction. 26 Using human cadaver knees (mean age, 53 years), hamstring tendon grafts, titanium interference screws, and a biomechanical testing protocol very similar to that of ours, Rittmeister et al recently showed that dilation of the tibial tunnel did not significantly improve the strength of fixation in comparison to extraction drilling. 26 In contrast, Cain et al (unpublished data, 1999) have previously observed in their preliminary report that tibial tunnel dilation had a clear beneficial effect on the pullout strength of hamstring grafts secured with bioabsorbable interference screws in cadaver knees (mean age, years), the strength of fixation being significantly higher with the technique of tibial tunnel dilation than with conventional reaming. As also discussed previously, 23,26 the apparent controversy between these studies may be attributable to the use of different biomechanical testing protocols, definition of the construct (fixation) failure, or difference in the interference screw (design, material) per se. However, our study is the first one to conclusively show with a proper sample that serial dilation does not provide any increase in the initial fixation strength: The age of the specimens (young cadaver), sample size (based on power analysis), and confirmed bone quality (peripheral quantitative CT measurements) are among the potential confounding effects controlled in our study. Accordingly, the possible flaws of the previous dilation studies have been controlled.

5 Vol. 32, No. 2, 2004 Effect of Tunnel Compaction 415 Figure 3. Peripheral quantitative CT-derived images of the proximal tibia: serial dilation (left) and extraction drilling (right). The scans were taken perpendicular to the bone tunnel to ensure that the extraction drilling or dilation of the tunnels to the desired diameter of 10 mm was sufficient to remove any possible confounding effect of a previous experiment. 23 As can be seen, the bone tunnels were round with no remnants/traces of the previous study (eg, screw impressions) in either group, and the previously compaction-drilled specimens (right) did not show any signs of excess packing of bone around the bone-tunnel walls (specimens drilled to 10-mm diameter by extraction drilling in the current study). Considering the mean age (about 26 years) of a typical ACL reconstruction patient, 8,17,33 it has been recommended that the age of the cadavers of the ACL fixation studies should be 65 years or younger in males and 50 years or younger in females to obtain a sufficiently accurate surrogate of the actual situation of an ACL reconstruction. 1 The use of bone specimens from relatively old human cadavers with unknown or deteriorated bone structure (ie, low volumetric trabecular bone density) is an apparent limitation common to many of the previously published ACL reconstruction fixation studies. 4,6-8,10,33 In our study, the male specimens were 54 years of age or younger, and the female specimens were 49 years of age or younger. Furthermore, most previous studies evaluating the strength of fixation of an ACL reconstruction have relied on planar dual-energy x-ray absorptiometry (DXA) in the determination of bone quality, 6,8,21,25 a method subject to inherent uncertainties, 2,3,29 whereas we determined the volumetric trabecular bone density (mg/cm 3 ) of each tibia by peripheral quantitative CT to confirm the comparable trabecular bone structure between the groups and also the similarity of density of these bones with adults representative of ACL patients. 30 In the current study, human cadaver anterior tibialis tendon grafts were fixed in human cadaver tibiae with 35- mm long bioabsorbable interference screws with a fixed diameter of 11 mm. Oversizing the screw by 1.0 mm relative to the diameter of the bone-tunnel diameter was carried out to ensure a tight interference fit between the graft and the bone-tunnel wall, as there was an apparent mismatch between the diameters of the bone-tunnel and graft, the tunnel diameter (10 mm) being somewhat larger than the average graft diameter (mean 8.3 ± 0.5 mm). Furthermore, peripheral quantitative CT scans perpendicular with the bone tunnel were performed to ensure that the extraction drilling or dilation of the tunnels to the desired diameter of 10 mm (the previously extraction-drilled bone tunnels were dilated by serial dilators, and the previously compaction-drilled bone tunnels were redrilled by extraction drilling) was sufficient to remove any possible confounding effect of the previous experiment. 23 As can be readily seen from the peripheral quantitative CT-derived cross-sectional images (Figure 3), the bone tunnels were round after enlarging and there was no remnants/traces of the previous study (eg, screw impressions, excess packing of bone around the tunnel). Furthermore, in this context, peripheral quantitative CT analysis did not show any compaction-drilling-induced bone packing of the bone-tunnel walls in these cadaver knees in the first place, 23 so a sudden appearance of such phenomenon would have been somewhat peculiar. One may also argue that the use of relatively large (10 mm) drill holes in this study somehow hampers the comparison between serial dilation and extraction drilling, especially as clinically the diameter of the bone tunnel is usually matched to the diameter of the graft. Theoretically, the mismatch concept (ie, improved strength of fixation with a more precise match of the bone tunnel diameter to the diameter of the graft) makes a lot of sense, but the proof for the appropriateness of the concept in humans is actually somewhat vague. Although Steenlage et al 32 did indeed show that the fixation strength of a quadrupled hamstring tendon graft is improved with a more precise

6 416 Nurmi et al The American Journal of Sports Medicine match of the bone tunnel diameter to the diameter of the graft, the validity of their comparison can be questioned due to an apparent flaw in their study design (the comparison between two nonpaired groups of knee specimens from cadavers of different mean age). In this context, in our previous study 23 using exactly the same paired knee specimens and drill bits as in the current study, as well as perfectly matched (no mismatch) graft-tunnel diameters, the overall average displacement after cyclic loading was larger (4.5 mm versus 3.4 mm) and the failure load was lower ( N versus N) than that observed in the corresponding group of this study. Accordingly, our results suggest that the graft-to-tunnel diameter mismatch has no effect on the strength of fixation of ACL soft tissue graft. Finally, considering the study design used in this study (the paired knee specimens with bone tunnels of identical diameter and paired grafts of identical diameter), there is no plausible explanation of how the identical mismatch between the diameters of the graft and tunnel could have influenced only either the dilation or the extraction drilling group. Thus, even if the mismatch had an effect on the absolute fixation strength values, the between-groups comparison used in this study is still valid. Regarding the use of anterior tibialis tendon as a soft tissue graft, Caborn et al 9 recently described the technical suitability of the single-loop anterior tibialis tendon allografts in ACL reconstruction. The similarity of the structural, material, and viscoelastic properties of anterior tibialis and quadrupled hamstring tendon grafts at the time of implantation has also been shown in a comprehensive biomechanical study. 13 As also discussed earlier, 23 early rehabilitation after ACL reconstruction should be carried out carefully if an interference screw fixation is used as a sole mean of fixation of a soft tissue graft. In the current study, the overall average displacement after 1500 loading cycles (between 50 and 200 N) was 3.4 mm in the entire (pooled) study group, and 40% of the fixations yielded more than 5 mm, a displacement value previously used as a limit of clinical failure. 27 In this context, there were a total of 9 complete failures during cyclic loading in this study, 3 in the serialdilation versus 6 in the extraction-drilling group. However, group comparison based on the number of complete failures is somewhat misleading, as there were also a total of 8 additional specimens (5 versus 3 in the serial-dilation and extraction-drilling groups, respectively) that yielded more than 5 mm during cyclic loading. If this 5-mm limit were indeed used, the total number of failed fixations would be 8 and 9 instead of 3 and 6 in the serial-dilation and the extraction-drilling groups, respectively. Considering this slippage and consequent failures, the recently advocated idea of backing up the interference screw fixation with another fixation method/implant (hybrid fixation) warrants careful consideration. 23 According to a recent comprehensive biomechanical comparison of 6 different hamstring graft fixation devices in ACL reconstruction in both femoral 19 and tibial 20 sites, the strength of fixation of interference screws (3 different types were tested) was approximately only half of that observed for the other types of femoral and tibial fixation implants. In conclusion, our study did not show that in comparison to conventional extraction drilling, compaction of the bonetunnel walls by serial dilation would increase the initial strength of an interference screw fixation of soft tissue grafts in ACL reconstruction. ACKNOWLEDGMENT The authors want to thank Dennis Donnermeyer, Arthrex Inc., Jani Toukosalo, Smith & Nephew Ltd., and Pia Ahvenjärvi, Inion Ltd., for providing the instruments for the study, and Jorma Rajamäki for the skilled photography. This work was supported by grants from the Finnish Foundation for Orthopaedic and Traumatological Research, the Research Council for Physical Education and Sports, Ministry of Education, the Medical Research Fund of Tampere University Hospital, and the Finnish Cultural Foundation. REFERENCES 1. Beynnon BD, Amis AA. In vitro testing protocols for the cruciate ligaments and ligament reconstructions. Knee Surg Sports Traumatol Arthrosc. 1998;6(suppl 1):S70-S Bolotin HH, Sievänen H. Inaccuracies inherent in dual-energy X-ray absorptiometry in vivo bone mineral can seriously mislead diagnostic/prognostic interpretations of patient-specific bone fragility. J Bone Miner Res. 2001;16: Bolotin HH, Sievänen H, Grashuis JL, et al. 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