Biomechanical Research Of Unstable Distal Radius Fractures Fixed With Different Length Locking Screws

BackgroundDistal radius fractures are one of the most common fractures encountered by clinical orthopedic surgeons, accounting for20%of all clinical fractures. With the development of aging society in our country, the incidence rate has a tendency to rise further. According to the stability and types the of distal radial fractures, there may be different treatment approaches. For most undisplaced or minimally displaced stable fractures, external splint or plaster immobilization can be used after manual reduction.If reduction fails, further treatment can be taken such as percutaneous pinning, external fixation or open reduction fixation.Orbay JL has given a definition of unstable distal radius fractures as follows: According to x-ray images, any fracture plane forms more than15degrees, the articular surface shows subsidence more than2mm or radial length shortens over2mm. Serious thrypsis or simple osteoporotic fractures are also known as unstable fractures, despite of good initial lines of force. Because the unstable distal radius fracture is often accompanied by the collapse of articular surface, radial shortening and loss of palmar and ulnar tilt, it is difficult for closed reduction to restore radial length, palmar and ulnar tilt and anatomical location of the radial articular surface, resulting in many complications such as postoperative malunion, limitations in the range of wrist motion and traumatic arthritis. As for unstable distal radius fractures, open reduction and internal fixation surgery have been clinically suggested rencently.The volar and dorsal plates are commonly used in internal fixation system. For dorsally displaced fractures, we can look into the dorsally multiple bone defects caused by the collapse through the dorsal surgical approach, but anatomical reduction with application of dorsal plate have caused some complications such as extensor tendon inflammation, rupture and so on, distal radius fractures with dorsal plate fixation have been gradually out of fashion. The volar surgical approachs have come into sight of orthopedic surgeons, Georguoulis first proposed that palmar plate should be used treating the dorsally displaced distal radius fractures. For nonlocked volar plate, it can only resist appropriate axial and bending stress, when the contralateral cortical bone is out of any osseous contact, the application of the simple volar non-locking plate will withstand higher axial and bending loads resulting in the failure of internal fixation, while fixed-angle locking plate can transfer load from the the distal fracture fragment to the complete radial bone, thereby increasing the stability of the screw plate and bone structure as well as eliminating the possibility of postoperative screw exit, so fixation system with volar fixed-angle locking plate is obviously advantageous.Almost all unstable distal radius fractures can be fixed with volar locking plate system, for younger patients with more activities and elderly ones with osteoporosis, fixation with volar locking plate can not only rebuild anatomical structure of radius bone, but also allows the postoperative activities of wrist joints to prevent postoperative joint stiffnes, which is the best choice for the articular surface reconstruction. Although fixation with volar locking plate have obvious advantages for the treatment of distal radius fractures, it also has caused some complications such as distal screw into the wrist joint, flexor tendon irritation caused by protruding plate and incomplete reduction, the support loss of the subchondral bone caused by distal locking screws too close to the distal radius, dorsal extensor tendon irritation or rupture due to screw prominence,which is the most common complication.According to AO principle, when volar locking plate system are applied for the unstable distal radius fractures, distal radius should be fixed with bicortical locking screws, while "proximal cortex" is volar locking plate and "distal cortex" is the subchondral bone. In order to make the distal locking screws support subchondral bone, the locking plate should be assembled close to the distal radius as far as possible, which make the distal locking screws easy to enter the radiocarpal joint. Therefore, we wonder whether unicortial fixation with appropriate length can be used for distal radius fractures instead of bicortical fixation. If there is no significant difference between unicortical fixation and bicortical one in the stress distribution and stability of internal fixation, it makes sense to avoid extensor tendon complications because of dorsal screw prominence. Accordingly, the purpose of this study was to clarify whether unicortical distal locked configurations could maintain postoperative biomechanical stability as for the unstable distal radius fractures compared with bicortical locked fixation. Our null hypothesis was that there is no significant difference between unicortical screw fixation with appropriate lengths and distal bicortical screw fixation.Objective1. To analyze the stress and displacement distributions of bony structures under normal physiological loading conditions and verify whether the three-dimensional finite element model of radius and ulna established in the study is scientific and provides the basis for further digital simulation research.2. To compare the postoperative stability of unstable distal radius fractures fixed by distal locking screws with different length and analyze the stress distribution of internal fixation systems and callus at different healing periods, and to investigate the effect of distal locking screws with different length on stability of the whole internal fixation systems and fracture healing after operation.3. To compare and verify whether the conclusions of finite element analysis are correct according to the related biomechanics experiment.Methods1. Establishment of finite element model of bony structures and scientific analysis and validation of the model:Through the CT thin layer scanning of the foream of a healthy young man, the three dimensional model of radius and ulna were established in MIMICS software, and they were processed by reverse engineering software of Geomagic,then they were imported into ANSYS software, setting material properties and boundary conditions as well as establishing interosseous membrane and distal radioulnar ligament. Finally, the stress and displacement distributions of bony structures were analyzed under normal physiological loading conditions.2. The effect of distal locking screws with different length on stability of the internal fixation systems and fracture healing after operation:The three dimensional model of bony structures were established in MIMICS software by CT thin layer scanning of the foream of a healthy young man, and they were processed through shape phase, contours edition, patches and NURBS construction in reverse engineering software of Geomagic, and then imported into ANSYS software to carry out FEM analysis.Distal locking screws with different length and volar plate were established in ProE software, then they were imported into ANSYS software to be assembled with the model of unstable distal radius fracture containing callus in different periods after operations, setting material properties and boundary conditions as well as establishing interosseous membrane and distal radioulnar ligament.,and analyze the stress distribution of internal fixation systems and callus at different healing periods, so as to investigate the effect of distal locking screws with different length on stability of the whole internal fixation systems and fracture healing after operation under different loading conditions.3. Clarifying the conclusions of finite element analysis by biomechanics experiment:18embalmed cadaveric foreams (9matched pairs) without osseous defects and any history of upper extremity diseases including injury, arthritis, or tumor were obtained. The specimens were then dissected of soft tissues maintaining the interosseous membrane and radioulnar ligaments intact. The proximal radial and ulnar diaphyses were potted in acrylic thermoplastic resin in a vertical and neutral position, and a standard1-cm-wide segment of bone, centered2cm proximal to the tip of radial styloid, was performed with a saw to simulate the unstable distal radius fracture.The specimens were randomized into3groups of18specimens each according to the distal locked screws with different lengths in the distal fragment:Group A had unicortical locking screws with50%length to the dorsal cortex. Group B had75%length unicortical locking screws. Group C had bicortical locking screws. Each specimen was placed in a materials testing machine and each fixation construct was first tested under dorsal and volar bending loads, and then a static axial compression load was applied. Each specimen was subsequently cycled5000times in axial compression. After cyclic loading, post-testing in the three prior loading conditions was repeated. Finally, destructive testing in axial compression was performed to determine load to clinical failure, which was defined as2mm of displacement at the osteotomy gap and subsequent catastrophic failure, defined as loosening of the bone-implant interfaces, fractures at the location of the implant or deformation and breakage of the implants.Results1. The three-dimensional finite element model of radius and ulna established was highly simulation as with anatomy morphology. It was found that the stress and displacement distributions of bony structures were consistent with actual situation under the axial loadings. It provided a reliable basis for further related digital research.2. Stress concentration appeared on the plating fixation system and the stress value near the distal radius fracture line was also higher. With the screw length increasing, the maximum stress of callus was decreased gradually during the period of early healing; while the maximum stress of distal screws was increasing gradually with the increase of screw length at middle and last period of fracture healing, and the stress of distal bicortical screw was the largest. As with axial compression stiffness, there was no statistically significant difference between75%length unicortical and bicortical fixation group(p>0.05),while the stiffness of50%length unicortical fixation decreased significantly and differences are statistically significant with75%length unicortical and bicortical fixation group(p<0.01). As for volan dorsal and torsional rigidity, differences between three kinds of fixation group were no statistical significance(p>0.05).3. Pre-and postcyclic stiffness under axial and bending load were statistically equivalent for all groups. Final Mean stiffness to bending forces was statistically similar between different fixation constructions. As with final mean stiffness in axial loading:50%length fixation approached significance with respect to bicortical and75%length locking screws (P=0.009,0.017), whereas stiffness in75%length and bicortical fixation was statistically similar (P=0.93).Conclusions1. The three-dimensional finite element model of radius and ulna with the forearm interosseous membrane and distal radioulnar ligament was highly simulation as with anatomy morphology and mechanical properties, and it will provide the foundation for finite element software’s analysis2. The internal fixation system plays an important role in shelter effect and the unicortical distal locking screws with at least75%length can not only sustain the stability of the whole fixation system, avoid extensor tendon injuries due to dorsal screw prominence, but also make for early postoperative fracture healing.3. the unicortical distal locking screws with at least75%length may reduce the possibility of screws loosening.