Compaction Versus Extraction Drilling for Fixation of the Hamstring Tendon Graft in Anterior Cruciate Ligament Reconstruction

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1 /102/ $02.00/0 THE AMERICAN JOURNAL OF SPORTS MEDICINE, Vol. 30, No American Orthopaedic Society for Sports Medicine Compaction Versus Extraction Drilling for Fixation of the Hamstring Tendon Graft in Anterior Cruciate Ligament Reconstruction Janne T. Nurmi,* DVM, Teppo L. N. Järvinen,* MD, PhD, Pekka Kannus,* MD, PhD, Harri Sievänen, DSc, Jani Toukosalo, a MBA, and Markku Järvinen,* MD, PhD From the *Medical School and Institute of Medical Technology, University of Tampere, Department of Surgery, Tampere University Hospital, Tampere Research Center of Sports Medicine, The Bone Research Group, Urho Kaleva Kekkonen Institute, Tampere, and a Smith & Nephew Finland, Vantaa, Finland ABSTRACT Initial strength of quadrupled hamstring tendon grafts fixed with titanium interference screws was assessed in 30 pairs of porcine tibiae. Bone tunnels were drilled with either compaction drilling (stepped routers) or conventional extraction drilling (cannulated drill bits). Fifteen pairs of specimens were subjected to a singlecycle load-to-failure test, while the rest underwent a cyclic-loading test to further assess the quality of the fixation. No significant difference between the two drilling techniques was found with regard to yield load, displacement at yield load, stiffness, or mode of failure. Porcine trabecular bone mineral density was determined using peripheral quantitative computed tomography and compared with that of young women and men at a site corresponding to that of the tibial bone drill hole of an anterior cruciate ligament reconstruction. There was a significant difference between the two species ( mg/cm 3 in porcine tibial bone versus mg/cm 3 in women and mg/cm 3 in men), suggesting that porcine knee specimens may have limitations in studies of graft fixation in anterior cruciate ligament reconstruction. We found no difference between extraction and compaction drilling in initial fixation strength of a hamstring tendon graft for anterior cruciate ligament reconstruction using a porcine model. Address correspondence and reprint requests to Teppo L. N. Järvinen, MD, PhD, University of Tampere/Institute of Medical Technology, Tampereen Yliopisto, Finland. One author has a commercial affiliation or will receive financial benefit from a product named in this article. Bone-patellar tendon-bone graft has been the preferred standard for ACL substitutes. 8 However, hamstring tendons, especially when used in quadrupled (four-strand) form, have recently been considered a strong alternative because the morbidity associated with their harvest is believed to be significantly less than that with the harvest of patellar tendon. 8,11,36,37 The strength of fixation, rather than the graft itself, is suggested to be the weakest link in the early postoperative period after ACL reconstruction, 29 and the presence of progressive creep or slippage of the graft fixed in a bone tunnel has been the most common argument against the use of hamstring tendon grafts. 6,11,34 When hamstring tendon grafts are used 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. 10,15 Theoretically, such tunnels should provide better conditions for rigid fixation of soft tissue grafts by minimizing the chance of screw divergence, convergence, migration, and loosening when interference screw fixation is used. In numerous studies researchers have compared various fixation devices (staples, cross-pins, washers, and screws) 5,6,9,13,19,40 and studied the effects of various characteristics (such as material, position, length, and diameter) of the most commonly used fixation device, the interference screw, on the fixation strength of hamstring tendon grafts. 5,6,34,35,39 However, although compaction of the bone tunnel walls has been recommended for ACL reconstruction with hamstring tendon grafts, to our knowledge, there is scarce scientific data to justify this action. Therefore, the purpose of this study was to compare compaction drilling and extraction drilling in the fixation of quadrupled hamstring tendon grafts with an interference screw. 167

2 168 Nurmi et al. American Journal of Sports Medicine MATERIALS AND METHODS Specimens The hamstring tendons (semitendinosus and gracilis) of both limbs were harvested from 30 male human cadavers with a mean age of years (range, 19 to 65). The tendons were cleared of adherent muscle fibers and surrounding soft tissues, wrapped in gauze soaked in saline solution, stored in small sealable plastic bags, and frozen at 25 C. Thirty pairs of fresh, skeletally mature porcine tibiae were obtained from a local slaughterhouse and similarly treated and stored. These preservation procedures have been shown not to affect the mechanical properties of the tendons or bones. 2,8,18,26,41 Study Groups The 30 pairs of tibiae were randomly paired with the cadaveric hamstring tendons and then divided into two groups, so that the left and right tibia of each animal went into a different group (equal number of left and right tibiae in both groups). In the compaction-drilling group, the tibial drill hole was made with a Tibial Stepped Router (Acufex Microsurgical, Inc., Mansfield, Massachusetts), whereas in the extraction-drilling group the bone tunnel was made with a conventional cannulated drill bit (Acufex) (Fig. 1). Specimen Preparation On the day of testing, the tendons and tibiae were thawed to room temperature. All of the specimens were kept moist with physiologic saline solution during specimen preparation, fixation procedures, and biomechanical testing. A four-stranded graft with a total graft length of 8 cm was constructed according to common principles. In short, the four strands were sutured at the free end for 40 mm with No. 2 Vicryl suture (Ethicon, Johnson & Johnson, Brussels, Belgium) by using the running baseball stitch while maintaining constant tension on all four strands. The diameter of the graft was measured at the sutured end with a Graft Sizing Tube (Acufex). A bone drill hole equal to the diameter of the graft and approximately 40 mm in length was drilled into the tibia over a guide wire where the tunnel would be if an actual ACL reconstruction were being performed. The graft diameters for each set of paired tibiae were compared to make sure that the two were identical. Each hamstring tendon graft was placed through the tibial bone tunnel with 30 to 35 mm, corresponding to the normal ACL length, 2 of the unsutured, looped portion of the graft protruding from the proximal opening of the bone tunnel. A standard 8 25-mm roundthreaded titanium Softsilk interference screw (Acufex) was inserted in outside-in fashion over a guide wire between the graft and the anterior aspect of the tibial bone tunnel. The screw was advanced until its tip reached the proximal bone-tunnel opening. Biomechanical Testing and Data Analysis The biomechanical tests were performed with use of a Lloyd LR 5K mechanical testing machine (J. J. Lloyd Instruments, Southampton, United Kingdom), according to the procedure described earlier. 17 The tibiae were securely mounted to the testing machine with specially designed clamps. The biomechanical testing protocol consisted of the single-cycle load-to-failure test (15 pairs, randomly selected) and the cyclic-loading test followed by the single-cycle load-to-failure test (15 pairs, randomly selected). Single-Cycle Load-to-Failure Test In the single-cycle load-to-failure test, the specimens were first subjected to a 50-N preload for 1 minute. Thereafter, 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 stiffness (determined as the slope of the linear region of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve), yield load (described as the load at the point where the slope of the load-displacement curve first clearly decreased), and displacement at yield load were determined. The mode of failure was also determined. Cyclic-Loading Test Followed by a Single-Cycle Load-to- Failure Test Figure 1. The instruments used in the study were a conventional cannulated drill bit for extraction drilling (top), and the Tibial Stepped Router for compaction drilling (bottom). In the cyclic-loading test, the specimens were first subjected to a 50-N preload for 1 minute. Thereafter, 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 loaddisplacement curve. The initial stiffness was determined, and the rigidity of the fixation was evaluated by determining the loading-induced increase in the displacement from the preload level after 1, 10, 50, 100, 250, 500, 1000, and

3 Vol. 30, No. 2, 2002 Fixation Strength of Hamstring Grafts in ACL Reconstruction cycles of loading, respectively. After 1500 loading cycles, the single-cycle load-to-failure test was again performed. Peripheral Quantitative CT Measurements A peripheral quantitative CT scanner (XCT 3000, Stratec Medizintechnik GmbH, Pforzheim, Germany) was used to determine the trabecular bone density (in milligrams per cubic centimeters) at the proximal tibia. Eighteen porcine tibiae were analyzed with the scanner before specimen preparation. The measurements were performed at the proximal tibia approximately 2 to 3 cm distal to the articular surface, and the trabecular density was determined ina2 2cm 2 region of interest corresponding to the site of the tibial bone tunnel in ACL reconstruction. For comparison, trabecular bone density was also determined from the corresponding site of the proximal tibia in 21 women with a mean age of 24 6 years (range, 19 to 38) and 22 men with a mean age of 24 4 years (range, 19 to 33) according to our standardized measurement protocol. 33 The body weight of the subjects ranged from 51 to 70 kg for the women and from 66 to 83 kg for the men, and the height, from 156 to 178 cm for the women and from 175 to 190 cm for the men. The exclusion criteria were any cardiovascular, respiratory, abdominal, urinary, gynecologic, neurologic, musculoskeletal, or other chronic diseases; pregnancy; use of a prosthesis or a medication that could affect the musculoskeletal system; menstrual irregularities; and regular participation in impact exercise more than three times a week. All participants gave their informed consent before enrolling in the study, and the Institutional Review Board and the Ethics Committee of the UKK Institute approved the protocol. Statistical Analysis Difference between the groups was determined using a paired sample t-test. A P value less than 0.05 was considered statistically significant. RESULTS N/mm, P 0.87) or displacement (after 1, 10, 50, 100, 250, 500, 1000, and 1500 loading cycles) were observed between the two drilling techniques (Fig. 2). In the single-cycle load-to-failure test made after the cyclic-loading test, the average yield load was N for the compaction-drilling group and N for the extraction-drilling group (P 0.52). There were no significant differences in displacement at yield load ( mm compared with mm, P 0.76) or stiffness of the fixation ( N/mm compared with N/mm, P 0.80). As in the single-cycle test, the mode of failure was virtually entirely slippage of the graft past the screw or graft laceration (partial rupture) at the screwgraft interface, alone or in combination. However, four specimens had already failed during the cyclic-loading test. Three of these failures occurred because of tendon rupture (two were in the extraction-drilling group and one in the compaction-drilling group), and one because of graft slippage past the screw (in the compaction-drilling group). All failed specimens and their undamaged contralateral pairs were excluded from the statistical analysis. Trabecular Density The mean trabecular bone density of the porcine tibiae corresponding to the region of the bone tunnel was mg/cm 3 (Table 1), whereas the mean density at the corresponding human site was mg/cm 3 in women and mg/cm 3 in men (Table 2). This interspecies difference ( 50% higher mean density in porcine tibiae) was significant (P 0.001). DISCUSSION Interference screw fixation is the most studied method for the fixation of hamstring tendon grafts. 37 Various determinants related to the interference screw per se have been examined, 5,6,34,35,39 but other factors that might have a considerable influence on fixation strength, such as drilling of the bone tunnels, have not been well investigated. Compaction drilling with stepped routers or tapping with Single-Cycle Load-to-Failure Test The average yield load was N for the compaction-drilling group and N for the extractiondrilling group (P 0.88). Significant differences between the groups were not found for displacement at yield load ( mm compared with mm, P 0.20) or for stiffness of the fixation ( N/mm compared with N/mm, P 0.12). The mode of failure was virtually entirely graft slippage past the screw, graft laceration (partial rupture) at the screw-graft interface, or both. Cyclic-Loading Test Followed by a Single-Cycle Load-to- Failure Test In the cyclic-loading test, no significant differences in the initial stiffness ( N/mm compared with Figure 2. Displacement (mean SD) during the cyclic-loading test for specimens drilled with the compaction and extraction techniques after a fixed number of cycles.

4 170 Nurmi et al. American Journal of Sports Medicine TABLE 1 The Results of the Individual Peripheral Quantitative CT Measurements from the Proximal Tibia and Yield Loads of the Porcine Specimens Study group Mature porcine bone Bone density (mg/cm 3 ) Yield load (N) Single-cycle test Compaction Extraction Cyclic testing and single-cycle test Compaction a a Extraction a Mean SD a Failed during cyclic loading. TABLE 2 Bone Density Measurements in Women and Men Women Bone density (mg/cm 3 ) Age (years) Men Bone density (mg/cm 3 ) Age (years) Mean SD serial dilators of increasing diameter has been advocated by some authors to create dense-walled bone tunnels. These methods theoretically enhance the interference fit and thus provide more rigid fixation of the hamstring graft. 10,15 However, in this study, compaction drilling did not increase the initial fixation strength of hamstring tendon grafts fixed with titanium interference screws. Rittmeister et al. 28 recently confirmed our findings using human cadaveric knees; they showed that dilation of the tibial tunnel does not significantly enhance the fixation of hamstring tendon grafts in comparison with extraction drilling. In contrast, Cain et al. (unpublished data, 1999) have shown in their preliminary report that tibial tunnel dilation (serially dilated using cannulated smooth dilators) had a clear positive effect on the pullout strength of quadrupled hamstring tendon grafts secured in cadaveric knees with bioabsorbable interference screws. The strength of fixation was significantly higher with the technique of tibial tunnel dilation than with conventional reaming (616 N versus 453 N, respectively). However, method differences between our study and that of Cain et al. could be at least partly responsible for the apparent discrepancy between the results. First, Cain et al. used serial dilation, which clearly differs from the compaction drilling we used in our study. Second, the mechanical testing protocols of the two studies differed considerably. We used a uniaxial loading method (tibia only), applied the load parallel with the drill hole in the tibia, and used both a single-cycle load-to-failure test and a cyclic-loading test followed by a single-cycle load-to-failure test protocol, whereas Cain et al. used complete knee specimens (with the graft fixed in both femoral and tibial drill holes) in anterior tibial translation at 20 of flexion using singlecycle testing only. Furthermore, Cain et al. used peak load as the outcome parameter, whereas we determined the yield load in our study. Finally, mature porcine tibia specimens were used in our study, whereas Cain et al. used fresh-frozen young human cadaveric knees. Although porcine knees are commonly used for evaluation of ACL fixation devices, 21,23 25,30,43 and are considered to mimic human knee specimens well in terms of their size, shape, and bone quality, 1,21,22,24,25,30,42 the results of our peripheral quantitative CT analysis suggest that mature porcine knee specimens have certain limitations in studies of graft fixation in ACL reconstruction. Our study of peripheral quantitative CT data on the tissue properties (volumetric trabecular bone density) of the human and porcine bone at the site corresponding to the tibial drill hole in the ACL reconstruction and the crosssectional structure of the entire proximal tibia of human and porcine bone shows that a clear difference exists between the species (Tables 1 and 2; Fig. 3). In previous studies correlating the strength of fixation of an ACL reconstruction to the quality (density) of bone, the assessment of bone quality has relied on dualenergy x-ray absorptiometry-derived bone density measurements, 7,27 which are bound to several elements of uncertainty. Although dual-energy x-ray absorptiometry is the present noninvasive method of choice for evaluating the bone mineral status of the skeleton in clinical practice, 12 it is well known that because of its planar nature, dual-energy x-ray absorptiometryderived bone mineral density does not directly represent a volumetric density of any kind, but rather de-

5 Vol. 30, No. 2, 2002 Fixation Strength of Hamstring Grafts in ACL Reconstruction 171 Figure 3. Peripheral quantitative CT of the proximal tibia. A, porcine tibia after extraction drilling. B, porcine tibia after compaction drilling (note the dense-walled bone tunnel). C, the corresponding site from a human tibia. pends strongly on bone size (the larger the bone, the higher the bone mineral density at a given apparent volumetric density) and also on the scan projection (the thicker the bone in the scan direction, the higher the bone mineral density at a given apparent volumetric density and bone size). 32 Furthermore, dual-energy x- ray absorptiometry has recently been shown to be subject to sizable systematic inaccuracies. 3,4 Peripheral quantitative CT is based on tomographic principles and not only provides volumetric tissue density but also avoids the similar inaccuracies inherent in dual-energy x-ray absorptiometry. Despite the existing strong misconception (based on dual-energy x-ray absorptiometry measurements) that exercise (history of activity or athletic participation) significantly alters the volumetric bone density of a healthy person, it has actually recently been shown that bone density is not affected by previous exercise history. 14 It is obvious that human cadaveric knees from young, healthy donors would be optimal and more suitable than porcine knees for testing the strength of fixation of ACL reconstructions. However, in this context, we point out that the trabecular bone density of a considerable percentage of the porcine specimens scanned in this study ( 60%) was within the range observed in our human control sample (72.2 to mg/cm 3 ), and the biomechanical data of these less-dense porcine specimens did not significantly differ from those with higher bone densities (Table 1). Beynnon and Amis 2 recently presented guidelines for proper execution of the biomechanical evaluation of ACL reconstruction fixation. In their review, the use of both the conventional pullout evaluation (single-cycle load-to-failure test) and of repetitive (cyclic) loading was recommended in testing the properties of ACL fixation constructs. The conventional single-cycle load-to-failure testing method provides the upper limit of the graft-fixation construct, which is useful information with regard to the behavior of the graft during unexpected loading events, such as loss of balance or a fall of the patient. However, during early intense rehabilitation of the oper-

6 172 Nurmi et al. American Journal of Sports Medicine ated knee, the graft-fixation construct is subjected to thousands of cycles of repetitive submaximal loading, and thus, the time-zero failure loads do not appropriately reflect the possible changes that occur in the fixation strength under cyclic loading at that time. Therefore, the inclusion of the cyclic-loading test is at least equally as important as the single-cycle load-to-failure test in the testing protocol. In regard to the most appropriate outcome parameter in the biomechanical testing of the strength of knee ligament fixation constructs, we believe that ultimate strength of fixation (maximum, ultimate, or peak load value) has actually very little, if any, clinical relevance. The hamstring tendon graft fixed with interference screws typically fails at a point well before the maximum load is reached, a point which is called the yield/linear load point. The difference between yield and maximum load (point) is readily discernible in Figure 4, an actual load-displacement curve of one of the single-cycle load-to-failure tests performed during our study. Preference for the use of yield load instead of maximum or ultimate load has also been recently advocated by Seitz et al. 31 The tibia is believed to be biomechanically more problematic of the two fixation sites in the ACL reconstruction since the bone quality in the tibial metaphysis is inferior to that of the lateral femoral condyle, 5,7,16,38 the loading is applied to the ACL graft virtually parallel to the long axis of the tibial drill hole (as opposed to the femur, in which the line of force does not come parallel with the bone drill hole until 100 of flexion), 5,20 and the fixation implant (here, the interference screw) has to be implanted in a less favorable outside-in direction. 6,27 Consequently, fixation studies performed at the tibial site should provide the worst-case scenario, especially if the tibial specimen is loaded with a distraction force acting parallel to the long axis of the graft and bone tunnel, 2 as was the case in our study. On the basis of the results of the current study, compaction drilling seems not to increase the strength of fixation of the hamstring tendon graft in ACL reconstruction. However, these results reflect only the initial biomechanical characteristics of the fixation in a porcine model, thus providing no information about the biologic healing and remodeling responses of the graft over time. Future experimental studies should include the postoperative time factor in the analyses so that final decisionmaking, in favor of one fixation method or another, can stand on a firm ground. ACKNOWLEDGMENTS The authors thank Smith & Nephew Ltd. (Vantaa, Finland) for providing the instruments and screws for the study, Johnson & Johnson (Brussels, Belgium) for the suture material, Eero Kirveslahti/Savupojat Oy for the porcine tibiae, and Mika Vihavainen, BM, for his excellent technical assistance. REFERENCES Figure 4. A load-displacement curve. The displacement occurring during preloading (before point 1) is mostly attributable to the elimination of some of the natural creep of the hamstring tendons but probably also, to some extent, to the final tightening of the specimen and the mechanical testing machine. From this point on, the specimen s response to loading is virtually linear until point 2, where the slope of the load-displacement curve first clearly decreases. Point 2 is defined as the yield point (the displacement from point 1, 2.9 mm, and, correspondingly, yield load, 629 N). After the yield point, the specimen undergoes significant stretching (presumably, both as the graft slips past the interference screw and as the graft tissue itself deteriorates) but still shows a strong resistance to loading, as the load value keeps increasing until the maximum load is reached (point 3, 848 N). 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