Absorbable vs. Metal Screws in ACL Reconstruction
Abstract & Commentary
Synopsis: Absorbable interference screws work as well as metal screws for anterior cruciate ligament reconstruction.
Source: McGuire DA, et al. Bioabsorbable interference screws for graft fixation in anterior cruciate ligament reconstruction. Arthroscopy 1999;15(5):463-473.
A single incision, arthroscopic technique using patellar tendon autograft fixed with interference screws is currently the most common method of anterior cruciate ligament (ACL) reconstruction. Recently, absorbable interference screws have become available. The potential advantages of absorbable screws include absence of hardware that could complicate future surgery, such as revision ACL reconstruction or knee replacement, and gradual replacement of the screw with host bone that avoids the creation of a defect upon removal at the time of revision surgery. Different compositions of screws are available, with fast resorption times for polyglycolic acid (PGA) or polylactic acid (PLA)/PGA copolymers (on the order of months), or long resorption times for stereoisomers of the PLA molecule (PDLA or PLLA) on the order of years. Rapid degradation of absorbable implants has been associated with an inflammatory reaction. Resorbable screws are being used frequently now for cruciate reconstruction, even though long-term data and comparative studies with metal screws are few.
McGuire and associates, in a multicenter trial, prospectively compared absorbable poly-L-lactide (PLLA) screws to metal interference screws for safety and efficacy. Four skilled arthroscopic surgeons at different locations randomly enrolled patients into absorbable or metal screw groups in equal numbers per location. A total of 204 patients with isolated ACL ligamentous laxity were enrolled. All patients had an arthroscopic, single incision technique with femoral fixation using an interference screw; tibial fixation was also performed with an interference screw for 158 of 204 cases. The remaining cases had tibial fixation with staples or screws plus washers as a post. Graft choices varied, with 57% being autologous patellar tendon (BTB), 29% allograft BTB, 12% allograft Achilles tendon, and three patients with autograft supplemented by allograft. The postoperative rehabilitation program was standardized and emphasized early motion and weight bearing.
At an average follow-up of 2.4 years (range, 1-5 years), none of the parameters measured showed a significant difference between the two groups. Metal and absorbable screws resulted in equivalent Lysholm knee scores and Tegner activity scores, pain, thigh size, meniscal tests, Lachman test, anterior drawer, pivot shift, patellofemoral crepitus and tenderness, and joint effusion. The mean side-to-side difference on manual maximum KT-1000 arthrometry testing was 1.8 mm for absorbable screws and 1.6 mm for metal screws. No problems were noted with osteolysis, inflammatory reaction, or allergenicity. Twelve absorbable screws broke during insertion without any adverse effects.
Comment by David R. Diduch, MS, MD
McGuire et al have conclusively demonstrated that absorbable interference screws work as well as metal screws for ACL reconstruction. In a well-designed, prospective study, there were no significant differences between groups with regard to laxity, activity, or pain. No adverse effects of the absorbable screws, such as osteolysis or inflammatory response, were noted. Although the type of graft used was not uniform, femoral fixation in all cases and tibial fixation in more than three-fourths of cases were performed with interference screws. Separating allograft and autograft in the results would have been ideal; however, the lack of statistical difference between the two groups demonstrates adequately that the absorbable screws are acceptable and safe for autograft or allograft.
The 12 cases of absorbable screw breakage (all 7-mm-diameter screws) demonstrates that there is a learning curve for absorbable screw insertion. Notching of the tunnel roof, hyperflexing the knee to more than 100° (to improve insertion angle and prevent divergence), tapping or compacting the bone plug at the entry site, and careful cannulated insertion all help to avoid excessive torque and screw breakage. Although in five of the 12 cases, the broken portion of the screw was not sufficiently large to require screw exchange, breakage is one potential problem for the surgeon to consider that is not generally encountered with metal screws. Of note, following this study, the manufacturer modified the screw design to increase the strength of the 7-mm-diameter screw.
This study effectively demonstrated that absorbable screws are a safe and effective alternative to metal interference screws for ACL reconstruction. However, this study does not confer an advantage to absorbable over metal screws. The concept of facilitated revision surgery is not addressed and has not been well studied in the literature. In fact, follow-up MRI studies by McGuire et al as well as others in the same edition of Arthroscopy show by MRI imaging that these absorbable PLLA screws persist for up to several years,1 while PLA/PGA copolymer screws resorb by six months.2 It is possible that revision ACL surgery may still encounter bone defects as the screw is drilled for a new tunnel. This area of ACL surgery needs further study, as does the issue of absorbable interference screw fixation for soft tissue hamstring grafts.
Currently, the selection of absorbable screws is a surgeon-dependent issue, comparing future theoretical advantages of an absorbable implant against slightly more complication due to breakage with insertion.
References
1. Warden WH, et al. Magnetic resonance imaging of bioabsorbable polylactic acid interference screws during the first 2 years after anterior cruciate ligament reconstruction. Arthroscopy 1999;15(5):474-480.
2. Lajtai G, et al. Serial magnetic resonance imaging evaluation of a bioabsorbable interference screw and the adjacent bone. Arthroscopy 1999;15(5):481-488.
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