Clinical Findings Of The Pathology Of The Annular Ligament In Monteggia Fractures And Biomechanical Analysis Using Finite Element Models

Background: The mechamism of Monteggia injury is still controversial. Evens described the mechanism as a fracture of the ulna with continued pronation causing a leveling out of the radial head. He reproduced the Monteggia injury in post-mortem specimens by the application of an excessive pronation force while the ulna was fixed in a vice. Tompkins thought that a fall on a hyperextended arm allows the pull of the biceps brachii to dislocate the radial head. The ulna is left alone to bear the body weight and then fractured. There was earlier analysis of the mechanism of the injury, which attributed the radial head dislocation to direct force. In our previous subacute cases, we found that the annular ligament was intact and tensely stuck in the radiohumeral joint space. It was almost impossible to reduce the radial head into the annular ligament by outside manipulation because limb traction could hardly stretch the transversely detached annular ligament. In several following cases with severely unstable ulnar fractures, we obtained nearly normal radiographs of the radiohumeral joint by closed reduction. However, the radial head redislocated easily before the fixation of the ulna. After the joint had been reduced again, we confirmed the interposition of the annular ligament by exploratory surgery. Therefore, we realize that the radiological reduction of the radial head is at least usually a false anatomic reduction. We designed this prospective study on these bases.Objectives: To report our operative findings regarding the annular ligaments in paediatric Monteggia fractures; to propose treatment recommendations for Monteggia fractures.Patients and methods: A total of thirty-five cases with Type-â… and Type-â…¢Monteggia fractures were treated operatively. There were 27 boys and 8 girls in the final study population; ages ranged from 3 to 13 years, with an average of 8.6 years. The fractures occurred in the right arm in 16 cases and the left arm in 19 cases; 27 fractures were caused by fall and 8 fractures by pressure. We identified 21 Bado typeâ… fractures and 14 typeâ…¢fractures, with 10 fractures affecting the olecranon, 17 the upper third of the ulna and 8 the middle third of the ulna. Radial nerve palsy was found in six cases preoperatively. Operation took place 3 hours to 8 days after injury. The mean duration of follow-up was 37.4 months, ranging from 15 months to 5 years. Brachial plexus blockage was used for all patients while basal anesthesia was added for younger patients who were not able to cooperate during surgery. Patients in this group received closed reduction of the radial head in the operating theatre after anesthesia and before open reduction. A tourniquet was routinely used to decrease blood loss. The radial heads were all explored surgically in this series. An anterior approach, which started from the flexor crease and extended 3-4cm longitudinally, was used for Bado Type-â… injuries. A lateral incision was used for Bado Type-â…¢injuries. The dissection plane was developed between the extensor carpi radialis and the extensor digitorum communis. The annular ligaments were intact in all patients and the ruptures were transversely on the joint capsule at the lower margin of the ligament. Thirty-two annular ligaments interposed in the radiohumeral joint even though the radiographs showed the reduction of the radial heads. Three cases, whose capsular ruptures were comparatively small, showed reduction of the radial head into the annular ligament. The interposed ligament was stretched out of the joint space and was reduced around the radial head. We didn't routinely repair the joint capsule for the reason that in more than half of our patients (18 cases) the origin of the capsule was avulsed from the radial neck. Sixteen ulnar fractures were managed with open reduction and wire fixation, and thirteen broken ulnas were fixed by plating. Six ulnar fractures were green stick type and were treated by close reduction and external fixation.Results: All the patients were functionally excellent at the six month follow up in terms of bone healing and the range of motion of the elbow joint. All the ulna plates were removed within six months. No redislocation or subluxation of the radial head was found until in the last follow up. No heterotopic ossification was observed in any follow-up radiographs.Conclusions: The annular ligaments were intact in all paediatric patients with Type-â… and Type-â…¢Monteggia fractures and the ruptures were transversely on the joint capsule at the lower margin of the ligament. Most of the annular ligaments interposed in the radiohumeral joint even though the radiographs showed the reduction of the radial heads. We recommend reduction of the annular ligament in paediatric patients with Monteggia fractures. But if this is not possible for any reason, rigid ulnar fixation should be emphasized for patients treated by closed reduction of the radial head. And if the stabilization of the ulna is not quite reliable, frequent radiographic examinations will be necessary especially in the first three to four weeks to guarantee the position of the radial head. Open reduction of the annular ligament is only one of the treatment selections in this algorithm. Background: The pathological findings about Monteggia fractures in the first part of our research raised many questions about Monteggia fracture to be investigated. For example, what is the biomechanical explanation of the pathology of the annular ligament? What is the nature course of the interposed ligament? How to stratify patients according to ages or types of injury in order to guarantee a surgery? It is imperative to explore the mechanical explanation of the pathology. We design this biomechanical research to probe into this problem.Objective: To probe the biomechanical explanation of the pathological changes in Monteggia fracture by finite element model analysis.Method: Two forearm specimens from one corpse were placed in prone and supine position during the CT scanning (GE Company, USA, 64 speed) with a voltage of 120 KV and slice of 0.67mm. The DICOM images were adjusted to the bone window in the CT working station and then were transferred to a movable disc. Then the images were imported to MIMICS (Materalise Company, Belgium), in which three-dimensional images were constructed, smoothed, and meshed superficially. The models of the annular ligament were generated on the basis of the contour of the ulnas and radii and were assembled onto the bones to form bone-ligament combinations. The combinations were remeshed and optimized, and then were imported to PATRAN software (NASA, USA) in which the combination was meshed into tetrahedron units. A fine mesh was required in areas with high stress gradients for an accurate model solution. Mesh refinement is the process of systematically increasing the mesh density in these areas until a desired level of accuracy is obtained. As a starting point, a mesh was generated that accurately captured the 3-D geometry of the bones, the ligament. Preliminary model solutions were used to identify areas with high discretization error. In the construction of the model, extra displacement modes were used to improved accuracy. The element and node numbers were as follows: Model 1 (prone position): 23630 nodes, 97800 bone elements, 10788 ligament elements. Model 2 (supine position): 28411 nodes, 116844 bone elements, 12878 ligament elements. The bones were divided into cortical and cancellous parts according to density. Mechanical property values were assigned to bones and ligament respectively before the model was transferred to ABAQUS (ABAQUS, USA) in which the models were analyzed mechanically. The material properties of the cortical bone (Young's modulus=10000Mpa, Poisson's ratio=0.3) and cancellous bone (Young's modulus=50Mpa, Poisson's ratio=0.26), which were obtained from literature, were assumed to be linear isotropic and homogeneous for all elements. The behavior of the ligaments (Young's modulus=50Mpa, Poisson's ratio=0.3) is of the non-linear type, as the ligament can only undergo traction. To vertify the FE analysis results, a convergence test was used to guarantee that our numerical model reached the converged results and that no further mesh refinement was necessary. Two models were generated: the forearm in prone position and the forearm in supine position. We constrained the distal parts of the radius and ulnas and the proximal and dorsal part of the ulnas. Point loads of 100 N were applied vertically to the X plane respectively in the two models. The mechanical changes of the annular ligament were observed. Paired T-tests were used to compare the displacements of the ligament nodes between different planes. The ratios of the displacements in X, Y, Z planes of the two models were compared and analyzed in clinical scenarios.Results: In prone position, the maximum displacement of the annular ligament on Z plane (longitudinal) was very close to the displacement on Y plane (anteroposterior, AP). The comprehensive displacement was mainly from Y and Z planes. The displacement of ligament on X plane (lateral) was comparatively small. In supine position, the displacement on X plane was dramatically increased. However, the displacement on Z plane was decreased evidently.Conclusion: The biomechanical changes in Monteggia fracture can partly explain the pathological changes of the annular ligament: The annular ligaments were intact in all paediatric patients with Monteggia fractures and the ruptures were transversely on the joint capsule at the lower margin of the ligament. The fracture type of Monteggia injury was determined by the AP and longitudinal forces the ligament undertook. The initial position of the forearm greatly affects the mechanical changes of the annular ligament.