Biomechanischer Stabilitätsvergleich am suprakondylären Femur: Analyse des Einflusses einer additiven Drahtcerclage bei der Versorgung einer extraartikulären, distalen Schrägfraktur mittels polyaxialer, winkelstabiler Plattenosteosynthese

The surgical treatment of supracondylar fractures of the femur is challenging. A cerclage can be used as an adjuvant in fractures and as an implant in combination with osteosynthesis. The present study is the first to investigate the influence of an additional cerclage on the stability and load-bearing capacity of a supracondylar femur fracture (AO type 32A2.3) treated with an NCB-DF plate. Therefore eight pairs of formalin-fixed human cadaveric bones from cadaveric donors (average age 74 years, age range 57 to 95 years) were examined. Osteolysis, fractures and the presence of osteoporosis were excluded by means of radiological controls and bone density measurements. The pairs were then randomly divided into two groups. All 16 bones were treated with an angular-stable plate osteosynthesis after creation of an oblique fracture of AO type 32-A2.3. In group 2, the fracture site was additionally treated with a double-looped wire cerclage. The bones were then retrogradely positioned in an Instron 5566 loading machine according to the physiological force axis. Following a standardized test protocol, the bones were loaded cyclically with increasing force after a preload of 100 N. Starting at 800 N and doing 500 repetitions, the load was increased by 200 N after completing each test protocol until osteosynthesis failed. The failure thresholds were a plastic deformation greater than 20 mm and/ or a sudden drop in force greater than 30 %. All bone pairs withstood at least a load of 1200 N. The mean compressive force for osteosynthesis failure in group 1 was 2450 N (95 % CI: 1996-2904 N). In comparison, group 2, with additional cerclage, reached a mean load limit of 3100 N (95 % CI: 2662 3538 N). With the exception of sample pair 3 (3200 N each) the load limits of group 2 were always higher. The difference in load to failure between group 1 and group 2 was statistically significant (p = 0.018). In both groups, the most frequent reason for failure was a deformation of the distal fracture fragment > 20 mm (87.5%). In one pair was there a sudden loss of resistance greater than 30%. With regard to the type of failure, three mechanisms could be distinguished. The most common in both groups was the loosening of the distal screws with subsequent shearing of the distal fragment along the fracture gap (group 1: 62.5%; group 2: 75%). However, a newly developed fracture wedge along the cerclage in group 2 as well as the tearing of the cerclage itself should be emphasized. Irreversible deformation of the plate under load occurred twice in group 2 with cerclage and once in group 1. The femoral shaft broke multifragmentarily once in both groups. For the calculation of stiffness and plastic deformation the highest load level achieved on all bones (1200 N) was used. The stiffness of both groups was comparable. With a mean stiffness of 2.00 N/ m in group 1 (95 % CI: 1.11-2.90 N/ m) and of 1.49 N/ m in group 2 (95 % CI: 1.04-1.94 N/ m) (p = 0.208). The plastic deformation analysis showed a significant difference between the two groups at a load of 1200 N (p = 0.035). The mean plastic deformation in the group with the additional cerclage (group 2) was 0.37 mm (95 % CI 0.22-0.51 mm). In comparison, the mean plastic deformation of group 1 was 0.6 mm (95 % CI: 0.38-0.81 mm), showing a higher variability for plastic deformation. This study demonstrated the higher load to failure for an oblique fracture of the distal femoral shaft when it was treated with a combined osteosynthesis consisting of an angle-stable plate and a cerclage. Therefore, when treating an oblique fracture of the distal femoral shaft, an additional cerclage is recommended from a biomechanical point of view. This especially applies for the group of older patients for whom early postoperative full weight bearing is of major importance.