Biomechanical Laboratory - Article Example

Biomechanical Laboratory Metal interference screws can cause problems if revision is needed and can interfere with magnetic resonance imaging (MRI). Bioabsorbable screws prevent these problems. They are made from different polymers and have different designs, which could influence their mechanical properties (Weiler et al., 1998)1. Drogset et al. (2006)2 in a study used MRI to evaluate reabsorption of the screws, bony integration of the screws, and integration of the bone blocks in 19 patients 2 years after ACL reconstruction surgery. They found 63% reduction in the volume of the tibial screw while the integration of the tibial bone block was considered good in 90% of the patients. Bioabsorbable interference screws are popular in endoscopic reconstructions of the ACL as there is no need for a second operation for removal and no complicating factor in case of a revision surgery. Pullout studies of metal and bioabsorbable interference screws have yielded similar results. However, a randomized controlled clinical trial comparing patients having metal interference screws with those having bioabsorbable poly-l-lactic acid screws showed subjective knee function to be better in the patients in the metal screw group although no difference in stability was noted between the two groups (Drogset et al., 2005) 3. Bioabsorbable interference screw breakage can be a problem in ACL reconstructive surgery which necessitates burying the screw on graft fixation (MacDonald and Arneja, 2003 ) 4. Partial or non-uniform degradation of bioabsorbable screws could lead to breakage with daily activities (Hall et al., 2009) 5. An in vitro study by Schwach and Vert (1999)6 showed a loss of 50% of compression strength in poly-L-lactic acid screws due to hydrolytic degradation between 2 and 5 months. While the overall complication rate associated with the use of this implant is low, complications involving osteolysis and aseptic effusion of the knee joint have been reported (Baums et al., 2006)7. Other complications that have been reported with bioabsorbable interference screws include cyst formation, tunnel widening, late screw breakage, and intra-articular migration (Appelt et al., 2007; Lembeck et al., 2005) 8, 9 According to Weiler et al. (1998)1, graft fixation close to the ACL insertion site increases anterior knee stability. The authors could do this successfully using a round threaded biodegradable interference screw. Interference screw fixation of bone-patellar tendon-bone grafts is a well-established process in ACL fixation. A study was conducted by Zheng et al (2008)10 to compare bovine bone screws and biodegradable interference screws vis-a-vis their efficacy in early rehabilitation. Results showed no significant differences in tension loss and pull-out strength between the 2 types of screws. The stability of the fixation of a hamstring graft to the bone tunnel is the primary factor determining rehabilitation. An evaluation of the initial fixation strength of a biodegradable poly-L-lactide/tri-calcium phosphate (PLLA/TCP) screw that suspended the graft in the bone tunnel in comparison with the strength of an interference screw for fixation of hamstring grafts in ACL reconstruction using bovine knees showed that the suspension screw provided a significantly higher yield load and ultimate failure load than the interference screw (Weimann et al., 2005)11. No significant difference was observed in the stiffness of both techniques. The typical failure mode for the suspension screw was fracture of the screw while it was slippage of the graft past the screw in the case of the interference screw. Thus, hamstring graft fixation using a suspension screw provides a useful alternative to interference screw fixation. To avoid the usual pitfalls of interference screw fixation e.g., thread damage to the graft or suture, potentially complicated hardware removal, disturbed MRI, or breakage of the absorbable screw, bioabsorbable plugs and “press fitting” of bone plugs could be used. (Kousa et al., 2001)12 The tibial graft fixation site has been considered a weak link in anterior cruciate ligament (ACL) reconstruction. Weiler et al. (2000)13 working on a standardized model of calf tibial bone found that the screw geometry significantly influences the hamstring tendon interference fit fixation of such a reconstruction. Their results indicated that increasing both length and diameter will increase pull-out force; however, increasing the length of the screw by 5 mm improved fixation strength more than oversizing the screw diameter. However, according to Weiler et al., (1998)1, a correlation between pull-out force and thread height indicates that fixation rigidity depends on screw design, even in a biodegradable implant. A study by Selby et al. (2001)14 showed that increasing the screw length from 28 mm to 35 mm will elevate ultimate failure load by 38%. The mode of failure generally was graft slippage past the screw. The authors found bone mineral density, dilatation, gap size, screw placement, and screw width and length to be important variables in hamstring tendon graft fixation within a bone tunnel. Bone mineral density has also been shown to directly affect the ultimate load to failure of interference screws (Weiler et al., 2000)13. However, Jarvinen et al. (2004)15 found that bone mineral density does not provide a sufficiently accurate prediction of the fixation strength of an individual soft tissue anterior cruciate ligament graft. Hence, it cannot be used to estimate the strength of interference screw fixation in anterior cruciate ligament reconstruction. Discussion Increased use of bioabsorbable interference screws as an alternative for metal screws has been experimentally justified because tibial fractures go through the epiphysis (growth plate). Such fractures separate the growth plate from the joint and therefore should be surgically held together while it heals. In reference to the view Weiler et al., (1998)1concerning the design and size of the biodegradable screw, evidence has shown that over sizing of the screw does not necessarily cause resistance to traction. Empirically, the animal model establishes that shortenings resulted from the drilling especially on the distal growth plate. This was however, different for the drillings that were made without screw placements, as there were no significant shortenings. Bibliography 1. Weiler A., Hoffmann R.F., Stahelin A.C., Bail H.J., Siepe C.J., Sudkamp N.P. (1998). Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone. Arthroscopy, 14:29-37 2. Drogset J.O., Grøntvedt T. and Myhr G. (2006). Magnetic Resonance Imaging Analysis of Bioabsorbable Interference Screws Used for Fixation of Bone– Patellar Tendon–Bone Autografts in Endoscopic Reconstruction of the Anterior Cruciate Ligament, Am J Sports Med 34 (7): 1164-1169 3. Drogset J.O., Grøntvedt T. and Tegnande A. (2005). Endoscopic Reconstruction of the Anterior Cruciate Ligament Using Bone–Patellar Tendon–Bone Grafts Fixed with Bioabsorbable or Metal Interference Screws. Am J Sports Med 33 (8): 1160- 1165 4. MacDonald P. and Arneja S. (2003). Biodegradable screw presents as a loose intra- articular body after anterior cruciate ligament reconstruction. Arthroscopy: The Journal of Arthroscopic & Related Surgery, 19(6): e22-e24 5. Hall M.P., Hergan D.M. and Sherman O.H. (2009). Early fracture of a bioabsorbable tibial interference screw after ACL reconstruction with subsequent chondral injury. Orthopedics, 32:208 6. Schwach G. and, Vert M. 1999. In vitro and in vivo degradation of lactic acid-based interference screws used in cruciate ligament reconstruction. Int J Biol Macromol.; 25(1-3): 283-291 7. Baums M. H. , Zelle B. A., Schultz W., Ernstberger T. and Klinger H.-M. (2006). Intraarticular migration of a broken biodegradable interference screw after anterior cruciate ligament reconstruction. Knee Surgery, Sports Traumatology, Arthroscopy, 14(9):865-868 8. Appelt A. and Baier M. (2007). Recurrent locking of knee joint caused by intraarticular migration of bioabsorbable tibial interference screw after arthroscopic ACL reconstruction. . Knee Surgery, Sports Traumatology, Arthroscopy 15(4):378- 380 9. Lembeck B. and Wülker N. (2005). Severe cartilage damage by broken poly-L-lactic acid (PLLA) interference screw after ACL reconstruction. Knee Surg Sports Traumatol Arthrosc. 13(4):283-286 10. Zheng N., Price C.T., Indelicato P.A. and Gao B. (2008). Tibial fixation of bone- patellar tendon-bone grafts in anterior cruciate ligament reconstruction. Am J Sports Med. 36(12): 2322-2327 11. Weimann A., Rodieck M., Zantop T., Hassenpflug J. and Petersen W. (2005). Primary stability of Hamstring graft fixation with biodegradable suspension versus interference screws. Arthroscopy 21 (3): 266-274 12. Kousa, P., Jarvinen, T.L., Kannus, P., Ahvenjarvi, P., Kaikkonen, A., Jarvinen, M. (2001). A bioabsorbable plug in bone-tendon-bone reconstruction of the anterior cruciate ligament: Introduction of a novel fixation technique. Arthroscopy, 17: 144- 150 13. Weiler A., Hoffmann R.F.G., Siepe C.J., Kolbeck S.F. and Südkamp N.P. (2000). The influence of screw geometry on hamstring tendon interference fit fixation. Am J Sports Med., 28 (3): 356-359 14. Selby J.B., Johnson D.L., Hester P. and Caborn D.N.M. (2001). Effect of screw length on bioabsorbable interference screw fixation in a tibial bone tunnel. Am J Sports Med. 29(5): 614-619 15. Järvinen T.L.N., Nurmi J.T. and Sievänen H. (2004). Bone density and insertion torque as predictors of anterior cruciate ligament graft fixation strength. Am J Sports Med. 32(6): 1421-1429 Read More