Original article
Cyclic Loading Comparison Between Biodegradable Interference Screw Fixation and Biodegradable Double Cross-Pin Fixation of Human Bone-Patellar Tendon-Bone Grafts

https://doi.org/10.1016/j.arthro.2005.05.022Get rights and content

Purpose: The aim of this study was to compare ultimate load, yield load, stiffness, and displacement after cyclic loading of a cross-pin technique and an interference screw technique for the fixation of bone-patellar tendon-bone (BPTB) grafts in anterior cruciate ligament (ACL) reconstruction. Type of Study: Biomechanical in vitro study. Methods: The devices tested were 2 2.7-mm biodegradable pins (RigidFix; Ethicon, Mitek Division, Norderstedt, Germany) and biodegradable interference screws (Absolute; Innovasive Devices, Marlborough, MA). Each device was used for the fixation of 10 8-mm, 9-mm, or 10-mm sized human BPTB grafts in tunnels drilled in bovine knees. Ultimate load, yield load, stiffness, and displacement after cyclic loading (1,000 cycles between 50 and 250 N) were then evaluated. Results: All 8-mm grafts that were fixed with cross-pins failed after a mean of 124 cycles of load. The 9-mm and 10-mm grafts survived the cyclic loading protocol. Yield load and maximum load of the 10-mm groups (cross-pin and interference screw) were significantly higher than that of the 9-mm groups. There was no significant difference in maximum load, yield load, and stiffness between the cross-pin and interference screw fixation technique for 1 graft size. Conclusions: The biomechanical data suggest that femoral fixation of 9-mm and 10-mm BPTB grafts using 2.7-mm biodegradable cross-pins leads to primary stability that is comparable to that of biodegradable interference screws. Fixation of 8-mm BPTB grafts using 2.7-mm biodegradable cross-pins had poor results. Clinical Relevance: The diameter of the bone block is the limiting factor for the final fixation strength and the cyclical survival when using cross-pins. We strongly recommend not using this technique for bone blocks smaller than 9 mm in diameter.

Section snippets

Biomechanical Model

In this study, 60 fresh bovine knees were used to simulate young human femoral density as described by Weiler’s group.10, 16 In their model, the screw insertion site is located in the center of the bovine tibia. This location represents a trabecular bone density of 0.8 g/cm3, similar to what is expected in young human femora.7, 14, 24

The mean age of the animals was 28 ± 2 weeks. The material was obtained from a local butcher, fresh frozen at −20°C and thawed for 12 hours at room temperature

Results

None of the 8-mm BPTB grafts that were fixed using the cross-pin technique survived the 1,000 cycles of load. All grafts failed rapidly after a mean number of 124 ± 34 cycles. In all specimens, the failure mode was fracture of the bone block at the pin, which was inserted at the “articular” side of the bone block.

In the 9-mm and 10-mm cross-pin groups and in all interference screw groups, the fixations did not fail before 1,000 cycles. The displacement data of the cyclic loading test are listed

Discussion

The aim of the present study was to evaluate whether bone block size has an impact on the stability of the cross-pin technique for BPTB graft fixation. The results of this study clearly show that the bone block diameter is an important factor for the primary stability of a BPTB graft fixation using the cross-pin technique. None of the 8-mm grafts fixed with the cross-pin method survived 1,000 cycles of load between 50 and 250 N and all specimens failed by bone-block fracture. The bone-block

Conclusions

Femoral fixation of a 10-mm and a 9-mm BPTB graft using 2.7-mm RigidFix biodegradable pins leads to primary stability that is comparable to fixation with biodegradable interference screws. Since cross-pin fixation of large bone blocks had a significantly lower displacement than interference screw fixation, we prefer this method over interference screw fixation. However, we do not recommend the use of the RigidFix cross-pin technique for bone blocks smaller than 9 mm. Future studies have to

Acknowledgment

The authors thank M. Vogiatzis, S. Zander, and A. Studt for their expert technical assistance. The implants used in this study were kindly provided by Ethicon, Mitek Division, Norderstedt, Germany.

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