Summary

Refixation of posterior cruciate avulsion fractures with headless compression screws provide comparable biomechanical properties than conventional refixation techniques

Abstract

Background

Tibial Avulsion fractures of the posterior cruciate ligament (PCL) are usually refixed. The surgical implementation and study situation is very heterogeneous and ranges from open refixation using conventional screw or plate osteosynthesis to arthroscopic performed pull-out sutures. Headless compression screws (HCS) offer less material irritation compared to conventional small fragment implants (SFI) and can be obtained resorbable (magnesium). The purpose of this study was to compare the biomechanical properties of conventional titanium HCS and resorbable magnesium HCS with conventional screw osteosynthesis for refixation of PCL tibial avulsion fractures in a porcine cadaveric model.

Methods

In 40 fresh frozen porcine knee joints, the extensor apparatus as well as the cutis/subcutis/musculature were dissected. The menisci, collateral ligaments, and cruciate ligaments remained intact. 4 groups of n=10 joints were randomized. An avulsion fracture of the tibial PCL insertion measuring 15x10x20mm (heigh x depth x width) was performed using a chisel. Group 1 remained intact (native), group 2 (SFI) was antegrade refixed with conventional titanium screw osteosynthesis (2x 3.5x30mm full threaded with washer, SFI Synthes), group 3 (Ti-HCS) was antegrade refixed with 2 titanium HCS screws (3.5x30mm, CCHS Synthes), and group 4 (Mg-HCS) was antegrade refixed with 2 resorbable magnesium HCS screws (3.5x30mm, mmCS Medical Magnesium). Femur and tibia were cemented with PMMA and mounted in the universal testing machine. A posterior drawer test was simulated by applying axial antegrade pressure to the femur in 90° knee flexion while the tibia was fixed. After preconditioning, 500 cycles (speed 200mm/min) with 10-100N load were performed with consecutive Load to Failure (LTF). Stiffness, yield load, LTF, displacement at failure and cyclic displacement were analyzed. Failure was defined as fragment dislocation or PCL rupture. A one-way ANOVA with post hoc correction was performed for statistical analysis with significance level p<.05.

Results

LTF of the native PCL (1982,1±285,4N) could not be achieved by osteosynthesis (SFI 1034,8±236,1N vs. Ti-HCS 693,9±220,5N, vs. Mg-HCS 686,7±174,6N; p<.05), nor could the native stiffness with 190,9±8,6N/mm (SFI 144,0±15,4 N/mm vs. Ti-HCS 150,0±22,9N/mm vs. Mg-HCS 170,0±20,9N/mm) be reached (p<.05). Concerning the yield load SFI reached 524,6±251,1N, Ti-HCS 367,7±162,1 and Mg-HCS 500,4±193,1, which was not significant. Concerning displacement after 500 cycles the native PCL showed 3,9±1,1 mm, SFI 4,2±1,2mm, Ti-HCS 3,8±0,9mm and Mg-HCS 4,1±0,9mm, whereas no significancy was observed. Displacement at failure for SFI was 10,4±2,7mm, for Ti-HCS 5,6±1,5mm and for Mg-HCS 5,6±1,4mm with significancy only compared to SFI. No significancy was shown between both HCS procedures.

Conclusion

Both HCS procedures achieved comparable failure loads, displacement and stiffness compared to conventional screw osteosynthesis, which was only superior in LTF. No significancy was seen in between HCS groups concerning biomechanical properties. Therefore, HCS screws can offer a suitable alternative for refixation of tibial PCL avulsion fractures, providing less implant irritation in-situ and being available resorbable avoiding implant removal. None of the tested procedure was able to restore the biomechanical properties of the intact PCL.