Cynthia E. Dunning

Ligamentous Stabilizers Against Posterolateral Rotatory Instability of the Elbow

Background: The lateral ulnar collateral ligament, the entire lateral collateral ligament complex, and the overlying extensor muscles have all been suggested as key stabilizers against posterolateral rotatory instability of the elbow. The purpose of this investigation was to determine whether either an intact radial collateral ligament alone or an intact lateral ulnar collateral ligament alone is sufficient to prevent posterolateral rotatory instability when the annular ligament is intact. Methods: Sequential sectioning of the radial collateral and lateral ulnar collateral ligaments was performed in twelve fresh-frozen cadaveric upper extremities. At each stage of the sectioning protocol, a pivot shift test was performed with the arm in a vertical position. Passive elbow flexion was performed with the forearm maintained in either pronation or supination and the arm in the varus and valgus gravity-loaded orientations. An electromagnetic tracking device was used to quantify the internal-external rotation and varus-valgus angulation of the ulna with respect to the humerus. Results: Compared with the intact elbow, no differences in the magnitude of internal-external rotation or maximum varus-valgus laxity of the ulna were detected with only the radial collateral or lateral ulnar collateral ligament intact (p > 0.05). However, once the entire lateral collateral ligament was transected, significant increases in internal-external rotation (p = 0.0007) and maximum varus-valgus laxity (p < 0.0001) were measured. None of the pivot shift tests had a clinically positive result until the entire lateral collateral ligament was sectioned. Conclusions: This study suggests that, when the annular ligament is intact, either the radial collateral ligament or the lateral ulnar collateral ligament can be transected without inducing posterolateral rotatory instability of the elbow. Clinical Relevance: Surgical approaches to the lateral side of the elbow that violate only the anterior or posterior half of the lateral collateral ligament should not result in posterolateral rotatory instability of the elbow. This is important information for surgeons planning various procedures on the lateral aspect of the elbow, such as reconstruction of a fractured radial head, radial head replacement, or total elbow arthroplasty.

Metallic radial head arthroplasty improves valgus stability of the elbow.

The stabilizing influence of radial head arthroplasty was studied in eight medial collateral ligament deficient anatomic specimen elbows. An elbow testing apparatus, which used computer controlled pneumatic actuators to apply tendon loading, was used to simulate active elbow flexion. The motion pathways of the elbow were measured using an electromagnetic tracking device, with the forearm in supination and pronation. As a measure of stability, the maximum varus to valgus laxity over the range of elbow flexion was determined from the difference between varus and valgus gravity loaded motion pathways. After transection of the medial collateral ligament, the radial head was excised and replaced with either a silicone or one of three metallic radial head prostheses. Medial collateral ligament transection caused a significant increase in the maximum varus to valgus laxity to 18.0 degrees +/- 3.2 degrees. After radial head excision, this laxity increased to 35.6 degrees +/- 10.3 degrees. The silicone implant conferred no increase in elbow stability, with a maximum varus to valgus laxity of 32.5 degrees +/- 15.5 degrees. All three metallic implants improved the valgus stability of the medial collateral ligament deficient elbow, providing stability similar to the intact radial head. The use of silicone arthroplasty to replace the radial head in the medial collateral ligament deficient elbow must be questioned. Metallic radial head arthroplasty provides improved valgus stability, approaching that of an intact radial head.

The Effect of Radial Head Excision and Arthroplasty on Elbow Kinematics and Stability

BACKGROUND Radial head fractures are common injuries. Comminuted radial head fractures often are treated with radial head excision with or without radial head arthroplasty. The purpose of the present study was to determine the effect of radial head excision and arthroplasty on the kinematics and stability of elbows with intact and disrupted ligaments. We hypothesized that elbow kinematics and stability would be (1) altered after radial head excision in elbows with intact and disrupted ligaments, (2) restored after radial head arthroplasty in elbows with intact ligaments, and (3) partially restored after radial head arthroplasty in elbows with disrupted ligaments. METHODS Eight cadaveric upper extremities were studied in an in vitro elbow simulator that employed computer-controlled actuators to govern tendon-loading. Testing was performed in stable, medial collateral ligament-deficient, and lateral collateral ligament-deficient elbows with the radial head intact, with the radial head excised, and after radial head arthroplasty. Valgus angulation and rotational kinematics were determined during passive and simulated active motion with the arm dependent. Maximum varus-valgus laxity was measured with the arm in a gravity-loaded position. RESULTS In specimens with intact ligaments, elbow kinematics were altered and varus-valgus laxity was increased after radial head excision and both were corrected after radial head arthroplasty. In specimens with disrupted ligaments, elbow kinematics were altered after radial head excision and were similar to those observed in specimens with a native radial head after radial head arthroplasty. Varus-valgus laxity was increased after ligament disruption and was further increased after radial head excision. Varus-valgus laxity was corrected after radial head arthroplasty and ligament repair; however, it was not corrected after radial head arthroplasty without ligament repair. CONCLUSIONS Radial head excision causes altered elbow kinematics and increased laxity. The kinematics and laxity of stable elbows after radial head arthroplasty are similar to those of elbows with a native radial head. However, radial head arthroplasty alone may be insufficient for the treatment of complex fractures that are associated with damage to the collateral ligaments as arthroplasty alone does not restore stability to elbows with ligament injuries.

Muscle forces and pronation stabilize the lateral ligament deficient elbow.

The influence of muscle activity and forearm position on the stability of the lateral collateral ligament deficient elbow was investigated in vitro, using a custom testing apparatus to simulate active and passive elbow flexion. Rotation of the ulna relative to the humerus was measured before and after sectioning of the joint capsule, and the radial and lateral ulnar collateral ligaments from the lateral epicondyle. Gross instability was present after lateral collateral ligament transection during passive elbow flexion with the arm in the varus orientation. In the vertical orientation during passive elbow flexion, stability of the lateral collateral ligament deficient elbow was similar to the intact elbow with the forearm held in pronation, but not similar to the intact elbow when maintained in supination. This instability with the forearm supinated was reduced significantly when simulated active flexion was done. The stabilizing effect of muscle activity suggests physical therapy of the lateral collateral ligament deficient elbow should focus on active rather than passive mobilization, while avoiding shoulder abduction to minimize varus elbow stress. Passive mobilization should be done with the forearm maintained in pronation.

Simulation of elbow and forearm motion in vitro using a load controlled testing apparatus

The purpose of this study was to compare passive to active testing on the kinematics of the elbow and forearm using a load-controlled testing apparatus that simulates muscle loading. Ten fresh-frozen upper extremities were tested. Active control was achieved by employing computer-controlled pneumatic actuators attached to the tendons of the brachialis, biceps, triceps, brachioradialis and pronator teres. Motion of the radius and ulna relative to the humerus was measured with an electromagnetic tracking system. Active elbow flexion produced more repeatable motion of the radius and ulna than when tested passively (p<0.05). The decrease in variability, as determined from the standard deviation of five successive trials in each specimen, was 76.5 and 58.0% for the varus-valgus and internal-external motions respectively (of the ulna relative to the humerus). The variability in flexion during simulated active forearm supination was 30.6% less than during passive testing. Thus under passive control, in the absence of stability provided by muscular loading across the joint, these uncontrolled motions produce increased variability amongst trials. The smooth and repeatable motions resulting from active control, that probably model more closely the physiologic state, appear to be beneficial in the evaluation of unconstrained kinematics of the intact elbow and forearm.

The effect of the density–modulus relationship selected to apply material properties in a finite element model of long bone

Material property assignment is a critical step in developing subject-specific finite element models of bone. Inhomogeneous material properties are often applied using an equation relating density and elastic modulus, with the density information coming from CT scans of the bone. Very few previous studies have investigated which density-elastic modulus relationships from the literature are most suitable for application in long bone. No such studies have been completed for the ulna. The purpose of this study was to investigate six such density-modulus relationships and compare the results to experimental strains from eight cadaveric ulnae. Subject-specific finite element models were developed for each bone using micro-CT scans. Six density-modulus equations were trialed in each bone, resulting in a total of 48 models. Data from a previously completed experimental study in which each bone was instrumented with twelve strain gauges were used for comparison. Although the relationship that best matched experimental strains was somewhat specimen and location dependent, there were two relations which consistently matched the experimental strains most closely. One of these under-estimated and one over-estimated the experimental strain values, by averages of 15% and 31%, respectively. The results of this study suggest that the ideal relationship for the ulna may lie somewhere in between these two relations.

Simulated active control produces repeatable motion pathways of the elbow in an in vitro testing system.

The purpose of this study was to determine if the repeatability and pattern of elbow kinematics are affected by changing the relative magnitudes of loads applied to muscles around the elbow in vitro. In eight cadaveric upper extremities, passive and three methods of simulated active elbow flexion were tested with the forearm maintained in both pronation and supination. Passive flexion involved moving the elbow manually through a full arc of motion. Simulated active flexion used a custom designed loading system to generate elbow motion by applying loads to various tendons via pneumatic actuators. Three different simulated active loading protocols, with loading ratios based on muscle activity and physiologic cross-sectional area, were tested. Testing was performed initially on an intact elbow, and then an unstable elbow model created by transection of the lateral collateral ligament (i.e. the radial and lateral ulnar collateral ligaments). An electromagnetic tracking device was used to measure rotation of the ulna relative to the humerus. Varus-valgus angulation and internal-external rotation were less repeatable during passive flexion than simulated active flexion, regardless of the loading ratio used, in both the intact (p<0.05) and unstable (p<0.05) elbows. Throughout the arc of flexion, the motion pathways were similar for the three simulated active motion protocols employed in this study (p>0.05). The pathways followed during passive motion were different from those generated with simulated active motion, especially in the unstable elbow with the forearm supinated (p<0.001). These results suggest that using simulated active motion rather than manual passive motion can improve the repeatability of elbow kinematics generated in the laboratory, and that a wide range of muscle loading ratios may produce similar kinematic output.

The effect of radial head fracture size on radiocapitellar joint stability

OBJECTIVE The purpose of this study was to determine the effect of radial head fracture size on radiocapitellar stability. DESIGN Repeated measures using Instron materials testing machine. BACKGROUND Radial head fractures are common injuries and controversy exists as to the optimal management of displaced wedge fractures. METHODS Fractures were simulated in six fresh-frozen cadaveric radiocapitellar joints by removing sequential 20 degree wedges from the anterolateral aspect of each radial head until 140 degrees of the radial head was removed. Decreased shear load at the radial head during joint loading was used as an indicator of decreased stability at the radiocapitellar joint. Using a custom designed jig and employing a compressive joint load of 100 N, the maximum shear load at the radiocapitellar joint was measured at 30, 60, 90 and 120 degrees of elbow flexion. RESULTS There was no difference in the shear load between the intact specimen and that with a 20 degree wedge removed at all flexion angles (P>0.05). Shear load decreased with each increase in wedge size between 20 and 120 degrees (P<0.05). After 120 degrees, one-third the diameter of the radial head, the shear load was always less than 0.8 N. CONCLUSIONS This study demonstrated an inverse relationship between radiocapitellar joint stability and radial head fracture size. RELEVANCE Small radial head fracture fragments are biomechanically significant. Therefore, the use of an arbitrary fragment size as an indication for surgery should be reconsidered, particularly if there is an associated ligamentous injury.

Finite element modeling mesh quality, energy balance and validation methods: a review with recommendations associated with the modeling of bone tissue.

The use of finite element models as research tools in biomechanics and orthopedics has grown exponentially over the last 20 years. However, the attention to mesh quality, model validation and appropriate energy balance methods and the reporting of these metrics has not kept pace with the general use of finite element modeling. Therefore, the purpose of this review was to summarize the current state of finite element modeling validation practices from the literature in biomechanics and orthopedics and to present specific methods and criteria limits that can be used as guidelines to assess mesh quality, validate simulation results and address energy balance issues. Of the finite element models reviewed from the literature, approximately 42% of them were not adequately validated, while 95% and 98% of the models did not assess the quality of the mesh or energy balance, respectively. A review of the methods that can be used to assess the quality of a mesh (e.g., aspect ratios, angle idealization and element Jacobians), measure the balance of energies (e.g., hour glass energy and mass scaling), and quantify the accuracy of the simulations (e.g., validation metrics, corridors, statistical techniques) are presented.

Variability and repeatability of the flexion axis at the ulnohumeral joint

Previous investigations have implemented screw displacement axes (SDAs) to define the elbow flexion axis for proper positioning of dynamic external fixators and endoprostheses. However, results across studies vary, which may be attributed to forearm position (pronation–supination) during elbow motion, or the mode of loading (active/passive) employed to generate flexion. Therefore, the aim of this study was to determine the influence of the flexion mode employed and forearm position on individual variation and repeatability of SDAs throughout elbow flexion. With the forearm pronated, the location of the average SDA was similar whether elbow flexion was generated actively or passively. In contrast, with the forearm supinated, the average SDA was 2.4° and 1.4° more valgus (p < 0.001) and internally rotated (p < 0.001), respectively, and positioned 1.6 and 0.8 mm further proximally (p < 0.002) and anteriorly (p < 0.005) relative to the capitellum, respectively, during active compared to passive flexion. During active flexion, the location of the average SDA was independent of forearm position. Conversely, during passive flexion, the average SDA angle was 3.4° and 1.0° more valgus (p <0.001) and internally rotated (p < 0.009), respectively, and 1.7 and 0.7 mm more proximal (p < 0.001) and anterior (p < 0.001) relative to the capitellum, respectively, with the forearm held pronated rather than supinated. SDAs calculated throughout flexion deviated from the average SDA in both orientation and position, demonstrating that elbow flexion behaves similar to a loose hinge joint. These factors suggest that to encompass the location of all SDAs throughout flexion, and therefore properly mimic normal elbow joint motion, an endoprosthesis should be modeled similar to a „loose”︁ rather than „pure”︁ hinge joint. This would allow for dependencies of SDA angulation on forearm position and muscle activation, and slight freedom of movement to account for variances in SDA location. These factors should also be considered during soft‐tissue reconstructions.