Mechanical study in vitro of the resistance of axial forces of 3.5 mm cannulated and conventional screws in head and femoral neck fractures

Abstract

Background: Proximal femoral fractures account for 25% of all femoral fractures in dogs and are more common in young animals. Osteosynthesis of femoral head and neck fractures is a complex procedure that can be performed using conventional screws or wires. However, proper fracture reduction and fi xation are diffi cult to achieve and, despite many advancements, such fractures remain challenging to solve. Cannulated screws have good compressive capacity and can be employed to simplify the surgical procedure while optimizing outcomes. This study was designed to compare the resistance of conventional and cannulated screws to axial loading following experimental femoral neck fracture in dogs. Materials, Methods & Results: Fourteen cadaveric canine femurs were used in this study. Femurs were collected from dogs over 20 pounds in body weight with no gross or radiographic signs of orthopedic disease. Cadaveric femurs were submitted to experimental femoral neck fracture using a Gigli saw and allocated to osteosynthesis using either conventional or cannulated screws (seven bones each). All screws were inserted below the greater trochanter of the femur from the lateral aspect of the bone and screw topography confi rmed radiographically. Test specimens were then potted in polymethylmethacrylate, coupled to a hinged device and submitted to axial loading. Data on maximum load, maximum displacement, load within the proportional limit, offset within the proportional limit and load required to produce a 3 mm displacement were collected. Mean values were calculated and compared using the Student’s t test (P < 0.05). Mean values varied greatly within groups. Maximum load and displacement correspond to the critical point from which biomechanical testing becomes destructive. Data were missing from one test specimen (conventional screw group) due to abrupt diaphyseal fracture at the start of the axial loading trial. Discussion: The canine femur was selected as an experimentalmodel in this project due to the high incidence of femoral neck fractures in dogs. Also, studies on proximal femoral fracture osteosynthesis using cannulated screws are scarce. The effectiveness of the compression osteosynthesis technique employed in this trial has been confi rmed in several canine proximal femoral fracture studies, with reported success rates between 2 and 21%. Success rate variability could be due to substantial individual differences, as suggested by the large intragroup variation in this trial. The biomechanical behaviour of conventional sliding hip screws (SHC) and cannulated screws following experimental femoral neck fracture osteosynthesis was compared. Increased stiffness was achieved with SHCs, possibly due to more effi cient transmission of compressive forces to cortical bone under the plate. Compression screws act by transmitting compressive forces to cancellous bone under the fracture line. Replication of this scenario in this trial supports the recommendation that the lowest screw be inserted as close as possible to the medial cortex of the femur, which is stiffer than the cancellous bone in the femoral head. Under the conditions studied, mean maximum load values were higher when bone screws were inserted closer to the medial cortex of the femoral shaft and the proximal aspect of the femoral head, regardless of screw type. The opposite scenario was also observed (i.e. the farther from the medial cortex of the femoral shaft, the lower the mean maximum load). In this trial, conventional and cannulated screws were equally resistant to axial loading. However, femoral neck fracture osteosynthesis using cannulated screws was easier to perfor