Effect of Bone Quality and Leg Depth on the Biomechanical Performance of a Nitinol Staple

Clinical and Radiographic Outcomes of Nitinol Compression Staples for Midfoot and Chopart Arthrodesis

Nitinol staple use in orthopedic surgery has increased in recent years. Biomechanical studies provide useful data for use in foot/ankle; however, clinical data is limited. This study's purpose is to determine the efficacy of nitinol staples to achieve stable, bony arthrodesis in midfoot and Chopart joints, and examine their clinical outcomes and pain scores. A retrospective chart review was performed on 127 midfoot/Chopart joint arthrodeses (71 patients) using nitinol staples in isolation. The primary outcome variable was radiographic evidence of healing. Radiographs were blinded, randomized, and independently reviewed by three board certified foot and ankle surgeons. Complete/partial union was seen in 89% of all joints (113/127), increasing to 93% when including only midfoot joints (98/106). Chopart joints had significantly lower healing rates (15/21; 71%) compared to all midfoot joints (p = 0.01) and isolated tarsometatarsal joints (86/91; 95%) (p = 0.006). Neuropathy and smoking did not affect arthrodesis, but diabetes did (p = 0.004). Joints requiring bone grafting had worse rates of arthrodesis (38/49; 76%) (p = 0.002). For all joints, post-operative visual analog scale scores were significantly lower than pre-operative (p < 0.001). Pre-operative midfoot and Chopart pain scores were similar (p= 0.30). Midfoot joints had significantly lower pain scores post-operatively than pre-operatively (p < 0.001). No such significance existed in Chopart joints (p = 0.07). Isolated nitinol staples are a viable option for midfoot arthrodesis, especially tarsometatarsal joints, and offer significant pain improvement. Chopart joints may require more rigid fixation than nitinol staples, given the lower healing rate. LEVEL OF CLINICAL EVIDENCE: IV.

Determining the Mechanical Properties of Super-Elastic Nitinol Bone Staples Through an Integrated Experimental and Computational Calibration Approach

Super-elastic bone staples have emerged as a safe and effective alternative for internal fixation. Nevertheless, several biomechanical aspects of super-elastic staples are still unclear and require further exploration. Within this context, this study presents a combined experimental and computational approach to investigate the mechanical characteristics of super-elastic staples. Two commercially available staples with distinct geometry, characterized by two and four legs, respectively, were examined. Experimental four-point bending tests were conducted to evaluate staple performance in terms of generated forces. Subsequently, a finite element-based calibration procedure was developed to capture the unique super-elastic behavior of the staple materials. Finally, a virtual bench testing framework was implemented to separate the effect of geometry from that of the material characteristics on the mechanical properties of the devices, including generated force, strain distribution, and fatigue behavior. The experimental tests indicated differences in the force vs. displacement curves between staples. The material calibration procedure revealed marked differences in the super-elastic properties of the materials employed in staple 1 and staple 2. The results obtained from the virtual bench testing framework have showed that both geometric features and material characteristics had a substantial impact on the mechanical properties of the device, especially on the generated force, whereas their effect on strain distribution and fatigue behavior was comparatively less pronounced. To conclude, this study advances the biomechanical understanding of Nitinol super-elastic staples by separately investigating the impact of geometry and material characteristics on the mechanical properties of two commercially available devices.

The history of orthopaedic use of nitinol compression staples

INTRODUCTION

The use of nitinol continuous compression staples has shown clinical utility in the management of various orthopaedic injuries. While literature is most robust in the realm of foot/ankle and spine surgery, the use of nitinol staples has been documented in fixation of wrist, olecranon, patella, and pelvis fractures.

METHODOLOGY

A narrative review was conducted by searching three online databases - PubMed, Web of Science, and Cochrane using the terms "Nitinol" and "Staple" published between 2003 and 2023. A total of 42 articles met inclusion/exclusion criteria and were included in this review.

REVIEW

Literature outside of foot/ankle and spine surgery is largely limited to biomechanical studies, case reports, and finite element analyses. The literature is summarized within this review by anatomic location including foot/ankle, lower extremity, hand, upper extremity, spine, and pelvis.

CONCLUSION

Existing literature demonstrates a diverse array of applications for nitinol continuous compression staples in both axial and appendicular orthopaedic care. Advantages of these implants include ease of application, ability to capture small bony fragments, continuous compression across a fracture or arthrodesis, and full coaptation which maximizes the surface area for healing and/or fusion.