Hannah L. Shannon

Validation of a finite element model of the human elbow for determining cartilage contact mechanics

It is important to study joint contact mechanics to better understand the processes which lead to cartilage degradation. The purpose of this study was to develop and validate a finite element (FE) model of a human elbow capable of predicting joint contact area and stress. A cylindrical constrained elbow joint loading apparatus was used to measure the cartilage compression and contact area for a single cadaveric specimen. A computer model of the same joint was created based on computed tomography images of the specimen, and the same loading was simulated using FE contact analysis. The model-predicted joint compression and contact area corresponded closely with experiment-measured results (differences of -4.9% and +9.6%). A sensitivity analysis showed that the model results were sensitive to cartilage and bone material properties, as well as the cartilage thickness distribution. The results of this study underline the importance of using accurate material properties and physiological cartilage thickness distributions when simulating cartilage contact mechanics.

Capitellar excision and hemiarthroplasty affects elbow kinematics and stability

INTRODUCTION Capitellar hemiarthroplasty is proposed as a reconstructive option for isolated capitellar deficiency, but there is limited data on its effect on elbow biomechanics. This study assessed the effect of capitellar excision with and without replacement on elbow kinematics and stability, and evaluated 2 different implant surface shapes. MATERIALS AND METHODS Ten cadaveric arms were tested with an upper extremity joint simulator. Each arm underwent computer tomography scanning for implant sizing and computer-assisted implantation. Kinematic data were obtained using an electromagnetic tracking system during elbow flexion, with the arm oriented in the valgus, varus, and vertical positions. Implants were placed through an extended lateral epicondylar osteotomy using computer-assisted techniques. A repeated-measures design compared 2 implants (anatomical and spherical) to the native capitellum control and capitellar excision states. Outcomes were maximum varus-valgus laxity and rotation of the ulna with respect to the humerus. RESULTS Excision of the capitellum increased the varus-valgus laxity up to 3.1° in active elbow flexion, with the forearm in pronation but not in supination. Both capitellar implant designs maintained normal varus-valgus laxity in both active and passive elbow flexion. Excision of the capitellum increased external ulnar rotation during active flexion in the vertical and valgus positions up to 1.5°, while both implants restored normal ulnar rotation. The kinematics and stability of the elbows were similar for both implant designs. CONCLUSION The capitellum appears to have a role as a valgus and external rotational stabilizer of the ulnohumeral joint. This instability was corrected by both designs of capitellar hemiarthroplasty.

The effect of radial head implant shape on radiocapitellar kinematics during in vitro forearm rotation

BACKGROUND A number of radial head implants are in clinical use for the management of radial head fractures and their sequelae. However, the optimal shape of a radial head implant to ensure proper tracking relative to the capitellum has not been established. This in vitro biomechanical study compared radiocapitellar joint kinematics for 3 radial head implant designs as well as the native head. METHODS Eight cadaveric upper extremities were tested using a forearm rotation simulator with the elbow at 90° of flexion. Motion of the radius relative to the capitellum was optically tracked. A stem was navigated into a predetermined location and cemented in place. Three unipolar implant shapes were tested: axisymmetric, reverse-engineered patient-specific, and population-based quasi-anatomic. The patient-specific and quasi-anatomic implants were derived from measurements performed on computed tomography models. RESULTS Medial-lateral and anterior-posterior translation of the radial head with respect to the capitellum varied with forearm rotation and radial head condition. A significant difference in medial-lateral (P = .03) and anterior-posterior (P = .03) translation was found between the native radial head and the 3 implants. No differences were observed among the radial head conditions except for a difference in medial-lateral translation between the axisymmetric and patient-specific implants (P = .04). CONCLUSIONS Radiocapitellar kinematics of the tested radial head implants were similar in all but one comparison, and all had different kinematics from the native radial head. Patient-specific radial head implants did not prove advantageous relative to conventional implant designs. The shape of the fixed stem unipolar radial head implants had little influence on radiocapitellar kinematics when optimally positioned in this testing model.

Elbow Kinematics After Radiocapitellar Arthroplasty

PURPOSE Radiocapitellar arthroplasty has been proposed as a reconstructive option for combined radial head and capitellar deficiency. The purpose of this study was to assess the impact of radiocapitellar replacement on elbow kinematics. We hypothesized that with the medial collateral ligament (MCL) intact, radiocapitellar arthroplasty would replicate normal kinematics, and that a radiocapitellar arthroplasty would more closely approximate normal kinematics than an elbow with a deficient lateral column or with a deficient MCL. METHODS We tested 7 cadaveric arms in an upper extremity joint simulator. Each arm underwent computed tomographic scanning to aid implant size selection and computer-assisted implant insertion. We obtained kinematic data using an electromagnetic tracking system during elbow flexion. The capitellar and radial head implants were placed through an extended lateral epicondylar osteotomy. We sectioned the anterior bundle of the MCL, leaving the flexor-pronator mass intact. Outcomes of interest were varus-valgus and rotational kinematics of the ulnohumeral joint. RESULTS The radiocapitellar arthroplasty showed no difference in kinematics compared with the postosteotomy control. The MCL-deficient elbow showed more valgus angulation and more external ulnar rotation than the control or radiocapitellar arthroplasty in the pronated, valgus loaded position. The deficient lateral column demonstrated increased external ulnar rotation kinematics during active elbow flexion. CONCLUSIONS Radiocapitellar arthroplasty can restore normal elbow kinematics with the MCL intact. If the MCL is deficient, radiocapitellar arthroplasty does not restore normal kinematics. CLINICAL RELEVANCE Radiocapitellar arthroplasty should be considered in cases of lateral column deficiency because it maintains normal elbow kinematics during active motion. Whereas radiocapitellar arthroplasty improves the stability of the MCL-deficient elbow with deficiency of the lateral column, reconstruction of the MCL may further improve normal kinematics.

Effect of Radial Head Implant Shape on Joint Contact Area and Location During Static Loading

PURPOSE To examine the effect of implant shape on radiocapitellar joint contact area and location in vitro. METHODS We used 8 fresh-frozen cadaveric upper extremities. An elbow loading simulator examined joint contact in pronation, neutral rotation, and supination with the elbow at 90° flexion. Muscle tendons were attached to pneumatic actuators to allow for computer-controlled loading to achieve the desired forearm rotation. We performed testing with the native radial head, an axisymmetric implant, a reverse-engineered patient-specific implant, and a population-based quasi-anatomic implant. Implants were inserted using computer navigation. Contact area and location were quantified using a casting technique. RESULTS We found no significant difference between contact locations for the native radial head and the 3 implants. All of the implants had a contact area lower than the native radial head; however, only the axisymmetric implant was significantly different. There was no significant difference in contact area between implant shapes. CONCLUSIONS The similar contact areas and locations of the 3 implant designs suggest that the shape of the implant may not be important with respect to radiocapitellar joint contact mechanics when placed optimally using computer navigation. Further work is needed to explore the sensitivity of radial head implant malpositioning on articular contact. The lower contact area of the radial head implants relative to the native radial head is similar to previous benchtop studies and is likely the result of the greater stiffness of the implant. CLINICAL RELEVANCE Radial head implant shape does not appear to have a pronounced influence on articular contact, and both axisymmetric and anatomic metal designs result in elevated cartilage stress relative to the intact state.

A computer and image-assisted guidance system for radial head arthroplasty

AbstractAccurately positioning components during radial head arthroplasty is difficult due to the complex shape of the radial head. The objective of this study was to develop a navigation system fo...