Marlis T. Sabo

Effect of coronal shear fractures of the distal humerus on elbow kinematics and stability

BACKGROUND Coronal shear fractures of the distal humerus can include some or all of the cartilaginous and bony surface. Fixation is preferred, but severe comminution, nonunion, and avascular necrosis may mandate excision. The amount of distal humerus that is safe to excise is unknown. This study examined the effect of excision of the capitellum and trochlea on elbow kinematics and stability with intact collateral ligaments. METHODS Eight cadaveric arms were mounted in an upper extremity joint testing system. Electromagnetic receivers on the radius and ulna enabled quantification of ulnohumeral and radiocapitellar kinematics. The distal humeral articular surface was sequentially excised to replicate clinically relevant coronal shear fractures, leaving the collateral ligaments undisturbed. The arms underwent simulated active flexion in vertical and valgus-loaded positions, and passive forearm rotation in the vertical position. RESULTS In the vertical position, sequential excision of the articular surface increased valgus angulation during active flexion (P < or = .04), and excision of the entire articular surface increased ulnar external rotation compared to the intact elbow (P < or = .02). In the valgus position, excisions involving the trochlea increased valgus angulation for active flexion (P < or = .04). The radial head moved distal, posterior, and medial on the capitellum with some or all of the trochlea excised (P < or = .02). DISCUSSION While the capitellum alone does not contribute to elbow stability, the trochlea has an important role. Excision of the trochlea resulted in multiplanar instability of the ulnohumeral and radiocapitellar joints. Therefore, excision of an irreparable capitellum fracture may be considered if collateral ligaments are intact, while excision of some or all of the trochlea may not.

Landmarks for Rotational Alignment of the Humeral Component During Elbow Arthroplasty

BACKGROUND The reference points for rotational orientation of the humeral component during elbow arthroplasty typically are on the articular surface or the humeral epicondyles. With bone loss, these landmarks may be compromised. Our purpose was to assess whether the flat posterior humeral cortex proximal to the olecranon fossa is a reliable landmark with which to orient the humeral component during elbow arthroplasty. METHODS Fifty cadaveric elbows (mean age [and standard deviation] at the time of death, 73 ± 12 years) underwent computed tomography (CT) scans. The flexion-extension axis (FEA) was determined by sphere-fitting the capitellar surface and circle-fitting the narrowest portion of the trochlea. The posterior humeral cortical line (PCL) was drawn on the flat posterior humeral cortex proximal to the olecranon fossa. The transepicondylar axis (TEA) was determined by a line between the most prominent points on the epicondyles. The angles between the PCL and FEA and the TEA and FEA were calculated and were compared by using two-tailed t tests. RESULTS The PCL was externally rotated by a mean (and standard deviation) of 14.0° ± 4.2° (p < 0.001) relative to the FEA (males: 12.6° ± 3.6°, females: 16.4° ± 5.2°; p = 0.002). The TEA was externally rotated by a mean of 2.8° ± 3.5° (p < 0.001) relative to the FEA (males: 2.7° ± 3.4°, females: 2.6° ± 3.7°; p = 0.96). The intraobserver and interobserver reliability was >0.98 for the capitellar and trochlear centers, while the cumulative intraobserver and interobserver reliability was 0.8 and 0.5 for the FEA-PCL angle and 0.4 and 0.3 for the FEA-TEA angle. CONCLUSIONS The posterior humeral cortex is a reproducible landmark that is externally rotated with respect to the flexion-extension axis of the distal part of the humerus. The surgeon must be aware of the need for an internal rotation correction factor and consider the influence of the patients sex on this correction when using the posterior humeral cortex as a landmark to avoid humeral component malrotation. CLINICAL RELEVANCE While the PCL is better than the TEA as a reference point, neither is able to accurately identify the FEA because of considerable normal variation. Future studies are needed to evaluate the effectiveness of computer-assisted techniques or a preoperative CT scan of the contralateral, unaffected elbow in identifying rotational landmarks for the elbow undergoing arthroplasty.

A morphological analysis of the humeral capitellum with an interest in prosthesis design

INTRODUCTION Although interest in capitellar arthroplasty is increasing, the morphology of the capitellum has not been fully characterized. Our purpose was to quantify the anthropometric features of the capitellum with an interest in arthroplasty design. We hypothesized that the shape is more complex than originally believed, and cannot be accurately modeled as a spherical structure. METHODS Fifty cadaveric human elbows underwent helical computer tomography scans. After reconstruction and establishment of a coordinate system for the distal humerus, circle-fits were applied to each of the 1-mm-thick slices. Sagittal radii of curvature were calculated every 10° of flexion around each circle (0-130° of flexion). A single transverse radius was calculated at 60° of flexion. The surface of the capitellum was described by sagittal and transverse radii of curvature and the footprint by height and width. These pairs of parameters were correlated to determine their strength of association. RESULTS The average height was 23.2 ± 2.9 mm (range, 18.3-29.5), while the average width was 13.9 ± 2.3 (range, 9-19). The sagittal radius of curvature was 11.6 ± 1.4 mm (range, 8.7-14.8), and the transverse radius was 14.0 ± 3.0 mm (range, 9.6-20.9). Correlations of height and width and sagittal and transverse radii were significant (R = .547, .705) (P < .01). Sagittal and transverse radii and height and width were significantly different (P < .001 for each pair). CONCLUSION The capitellum does not have a spherical surface or a circular footprint. There is substantial variability in the relationship between the height and width, and between the surface radii, that may be difficult to replicate with an off-the-shelf implant.

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.

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.

Osteochondral lesions of the capitellum do not affect elbow kinematics and stability with intact collateral ligaments: an in vitro biomechanical study.

PURPOSE Osteochondritis dissecans (OCD) of the capitellum most commonly affects adolescent pitchers and gymnasts, who present with pain and mechanical symptoms. Patients with larger lesions have poorer outcomes, possibly related to increased contact pressures on the surrounding articular surface with or without instability. The purpose of this in vitro study was to determine whether displaced OCD lesions of the capitellum lead to altered kinematics and stability of the elbow. METHODS We mounted 9 fresh-frozen cadaveric arms in an upper extremity joint testing system, with cables attaching the tendons of the major muscles to motors and pneumatic actuators. An electromagnetic receiver on the ulna enabled quantification of the kinematics of the radius and ulna with respect to the humerus. We used 3-dimensional computed tomography scans and computer-assisted techniques to navigate sequential osteochondral defects ranging in size from 12.5% to 100% of the capitellum. The arms were subjected to active and passive flexion in both the vertical and valgus positions with the forearm in both pronation and supination. RESULTS We found no significant differences in valgus angulation or ulnar rotation between any of the OCD lesions and the intact elbow during flexion, regardless of arm position or forearm rotation. CONCLUSIONS Osteochondritis dissecans lesions of the capitellum, both small and large, did not alter the ulnohumeral kinematics and stability with intact collateral ligaments. Therefore, excision of unfixable osteochondral fragments of the capitellum in the setting of intact collateral ligaments can be considered without the risk of creating instability.