Biomedical subjects

James A Johnson

Publications and source records attributed to James A Johnson.

36 records · Page 2Linked to original sources

PubMed2003

The effect of metallic radial head arthroplasty on radiocapitellar joint contact area.

OBJECTIVE: This study determined the effect of metallic radial head arthroplasty on radiocapitellar joint contact area. DESIGN: The contact area of eight intact radiocapitellar joints was assessed during simulated compressive loading. Subsequently, the same experiments were repeated after reconstruction with three different radial head implant sizes. BACKGROUND: There has been an increasing use of metal radial head implants relative to silicone implants. However, the contact characteristics with the metal reconstruction have not been determined. METHODS: A 100 N compressive load was applied to the radiocapitellar joint at three different flexion angles for the native joint and three different sizes of a metal head implant. An impression material was employed to quantify joint contact area. RESULTS: Following metallic radial head arthroplasty, the contact area decreased by approximately two-thirds relative to the native radiocapitellar joint (P<0.001). Smaller implant sizes resulted in slightly larger contact areas (P<0.05). In addition, contact area decreased as joint flexion angle increased (P<0.04). CONCLUSIONS: Selection of radial head implant size is likely not a significant factor with regard to contact area mechanics of the radiocapitellar articulation. RELEVANCE: The effect of radial head implant size on the contact area produced with the native capitellum is not known. This study shows that the design or selection of a metallic radial head implant from the viewpoint of size is likely not an important factor with regard to minimizing joint contact stresses.

Aged↗

PubMed2003

Application of screw displacement axes to quantify elbow instability.

OBJECTIVES: To determine if screw displacement axis patterns describing elbow joint motion: (1) change after ligament transection in vitro; (2) can reflect subtle changes in stability as a function of forearm position; (3) can reflect dynamic stabilization of the ligament insufficient elbow provided by muscle activity.Design. An in vitro kinematic study of eighteen cadaveric specimens tested in a joint simulator. BACKGROUND: In the elbow joint, screw displacement axes have been employed for proper positioning and design of endoprostheses. The effect of instability on screw displacement axes has not been previously reported. METHODS: Passive and simulated active flexion, with the forearm maintained in both pronation and supination, was performed on eighteen intact and ligament insufficient elbows. Instability was produced by transection of the medial collateral or lateral collateral ligament complexes. Kinematics were recorded using an electromagnetic tracking device and analyzed with a repeated measures design. RESULTS: During passive motion, division of either ligament caused deviation of screw displacement axes compared to the intact state (P<0.05). Transection of the medial/lateral collateral ligament generated greater instability with the forearm maintained in pronation/supination compared to supination/pronation (P<0.05). Muscle activation increased stability similar to the intact state (P>0.05). CONCLUSIONS: These results are consistent with observations determined using traditional kinematic descriptors. Screw displacement axes can readily detect changes in stability due to ligament sectioning. RELEVANCE: Clinicians can employ the screw displacement axis technique as a succinct descriptor of motion to readily detect elbow instability.

Aged↗

PubMed2003

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.

Aged↗

PubMed2003

Influence of the pronator quadratus and supinator muscle load on DRUJ stability.

PURPOSE: To determine the effects of altering the load contributions of the pronator quadratus and supinator muscles on in vitro distal radioulnar joint (DRUJ) stability during pronation and supination and before and after ulnar head excision. METHODS: Multiple pronation trials were conducted with incremental loading of the pronator quadratus relative to the pronator teres muscle; supination trials were similarly conducted with incremental loading of the supinator relative to the biceps muscle. All trials were conducted using an upper-limb apparatus capable of simulating muscle/tendon loading and displacement. Stability measurements included dorsal-volar translations of the radius relative to the ulna and DRUJ diastasis and convergence. RESULTS: Increased pronator quadratus loading did not affect intact DRUJ stability but effects were noted after ulnar head excision when the forearm was positioned between neutral and full pronation. Incremental loading of the supinator muscle did not modify DRUJ stability in the intact or ulnar head excised state. CONCLUSIONS: Pronator quadratus muscle activity aggravates forearm instability after ulnar head excision. Immobilization of the forearm in mid- to full supination should minimize pronator quadratus activity and optimize soft-tissue healing. This information may be useful to develop in vitro muscle-loading scenarios and analytical forearm models.

Aged↗

PubMed2003

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 degrees and 1.4 degrees 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 degrees and 1.0 degrees 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.

Adult↗

PubMed2003

Development of a motion-controlled in vitro elbow testing system.

Joint simulators can be used to study motion pathways of a human joint, to investigate changes in joint stability following injury, and to formulate improved reconstructive and rehabilitative procedures. Our objectives were: to develop a laboratory-based, motion-controlled elbow testing apparatus capable of simulating tendon (muscle) loading and displacement in a cadaveric specimen; to describe its performance while testing stable and unstable elbows; and to compare its operation to that of a previously designed load-controlled device. Velocity control of a pneumatic actuator was achieved using a custom-written, closed-loop feedback controller. This actuator was incorporated into an elbow testing system that used additional pneumatic actuators and a combination of motion- and load-control to achieve desired motions. Simulations achieved with this apparatus demonstrated small magnitudes of error in actuator position and highly repeatable flexion pathways with the specimens positioned in vertical, varus, and valgus orientations. The repeatability in motion pathways generated in both a stable and unstable elbow model was equivalent to or better than for similar tests performed using the load-controlled system, and the velocity of the resulting elbow motion was more reproducible.

Biomechanical Phenomena↗

PubMed2003

Patellar position after total knee arthroplasty: influence of femoral component malposition.

Patellar shift, tilt, and rotation were analyzed in 7 cadaveric knee specimens during simulated quadriceps loading, in the intact knee, and after implant reconstruction. Femoral component medialization, lateralization, and external rotation were also investigated. Relative motion of the patella with respect to the femur was measured using an electromagnetic tracking system. The spatial position of the patella did not change with standardized total knee arthroplasty (P <.05). After malpositioning of the femoral component, patellar rotation also did not change (P >.05); however, patellar tilt was altered by femoral component external rotation malposition (P <.05), and patellar shift was affected by all femoral component malpositions (P <.05). The spatial position of the patella relative to the femoral shaft was changed with any femoral component malposition, suggesting that the soft tissues were abnormally tensioned. This could result in subsequent wear on the patellar component and, therefore, early failure.

Arthroplasty, Replacement, Knee↗

PubMed2003

Mechanical properties of subchondral cancellous bone of the radial head.

OBJECTIVE: To quantify the mechanical properties of subchondral bone of the radial head, with special interest in the regional variations in bone stiffness and strength. DESIGN: Transverse subchondral cancellous bone slices obtained from the proximal radial head were subjected to indentation tests. SETTING: Biomechanics laboratory with a servohydraulic materials testing machine. SPECIMENS: Thirteen previously frozen, cadaver proximal radii of mean age 69 +/- 9 years. MAIN OUTCOME MEASUREMENTS: The mean indentation modulus and local yield strength were calculated for the anteromedial (AM), anterolateral (AL), posteromedial (PM), and posterolateral (PL) quadrants of the radial head. RESULTS AND CONCLUSIONS: There were no differences in mean indentation modulus across all 4 quadrants (AL, 55.56 +/- 16.20 MPa; AM, 53.48 +/- 20.67 MPa; PM, 49.20 +/- 18.31 MPa; PL, 49.96 +/- 14.97 MPa) (P > 0.05). The AM quadrant had higher local yield strength than the PL and PM quadrants, as did the AL compared with the PL quadrant (AL, 24.89 +/- 8.52 Mpa; AM, 28.39 +/- 10.52 Mpa; PM, 23.01 +/- 6.99; PL, 20.62 +/- 4.83 MPa) (P < 0.05). This suggests that the high frequency of radial head fractures in the AL quadrant is likely not due to the local strength of cancellous bone. Rather, the location of fracture may be influenced by the composite strength or the mechanisms of loading at the radiocapitellar and proximal radioulnar joints. The modulus results suggest that similar screw purchase can be achieved in all areas of the radial head during internal fixation of fractures. These results may also influence implant design and fixation. This study was limited to an older population and may not be representative of a younger population.

Age Factors↗

PubMed2002

Total synthesis of (-)-rhazinilam: asymmetric C[bond]H bond activation via the use of a chiral auxiliary.

The antitumor agent (-)-rhazinilam was synthesized in three major steps, namely the pyrrole synthesis, selective C[bond]H bond activation, and direct macrolactam formation. The key step involved asymmetric C[bond]H bond functionalization (dehydrogenation) of the diethyl group segment in intermediate 6. This was achieved by the attachment of chiral platinum complexes to the proximal nitrogen atom. A high degree of selectivity (60-75% ee) was achieved via the use of oxazolinyl ketone chiral auxiliaries.

Alkaloids↗

PubMed2002

An anthropometric study of the distal ulna: implications for implant design.

Arthroplasty of the distal ulna is frequently used for distal radioulnar joint disorders; however, prosthetic failures are common. An implant replicating the ulna may be advantageous from the viewpoint of joint kinematics, wear, and fixation. The geometry of 40 distal ulnas was quantified by computed tomography scanning and radiography. Ulnar head diameter (16.8 +/- 1.6 mm), canal diameter (8.3 +/- 1.6 mm), eccentricity of the head relative to the shaft (2.5 +/- 1.4 mm), and angle of articulation (176 degrees +/- 29 degrees ) were measured. No correlation was found between the diameters of the ulnar head and intramedullary canal (R(2) = 0.2). A modular head-stem system with an eccentric offset may best reproduce the geometry of the distal ulna.

Adult↗

PubMedDate not supplied

The flat spot of the proximal ulna: a useful anatomic landmark in total elbow arthroplasty.

The relationship between the flat subcutaneous dorsal surface (flat spot) of the ulna and the anatomic flexion axis of the elbow was investigated in 68 cadaveric ulnae. The flat spot and the guiding ridge of the greater sigmoid notch were digitized with an electromagnetic tracking device. The area of the flat spot, as well as its angle with the plane of the greater sigmoid notch, was calculated. The size of the flat spot was variable but was found to be nearly perpendicular to the plane of the greater sigmoid notch (mean, 89.8 degrees +/- 6.5 degrees ). This indicates that the subcutaneous dorsal surface of the proximal ulna is a useful landmark to assist the surgeon intraoperatively to orient the ulnar component axially in total elbow arthroplasty.

Arthroplasty, Replacement↗

PubMedDate not supplied

A biomechanical comparison of four reconstruction techniques for the medial collateral ligament-deficient elbow.

The initial strength of the intact medial collateral ligament (MCL) of the elbow and the strength of 4 reconstruction techniques were compared. Twenty cadaveric upper extremities were mounted in a custom jig with the elbow at 90 degrees , and a pneumatic cyclic valgus loading protocol was used. The mean peak load to failure was 142.5+/-39.4 N for the intact ligaments and 53.0+/-9.5 N for the docking reconstructions, 52.5+/-10.4 N for the EndoButton reconstructions, 41.0+/-16.0 N for the interference screw reconstructions, and 33.3+/-7.1 N for the figure-eight reconstructions. The peak load to failure of the MCL reconstructions was inferior compared with the intact ligament (P<.001). No difference in strength was found between the docking and single-strand medial collateral reconstruction with the use of an EndoButton for ulnar fixation (P>.05, beta=.14). Both of these reconstruction methods were stronger than the interference screw or figure-eight technique (P<.004). The optimal fixation method for a single-strand MCL reconstruction may require improved interference screws or a modified EndoButton procedure.

Bone Screws↗

PubMedDate not supplied

Kinematics and stability of the fractured and implant-reconstructed radial head.

Controversy exists as to the optimal management of radial head fractures. Biomechanical studies have been conducted to quantify elbow stability for simulated wedge fractures, head excision, and head replacement, with and without the integrity of the collateral ligaments. Our in vitro studies have demonstrated that in the ligamentously intact elbow, kinematics and stability are slightly altered with simulated depressed wedge fractures up to 120 degrees of the radial head, markedly altered with head resection, and improved after radial head replacement. Radial head excision decreases elbow stability in the ligament-deficient elbow, and radial head replacement improves stability similar to that of the native radial head. The ligaments have the most marked influence on stability, particularly when the upper limb is positioned such that valgus and varus gravity loads are applied to the elbow. Whereas the radial head acts as a secondary stabilizer to the collateral ligaments with the arm in these positions, its relative role is greater when the arm is in the dependent position and elbow flexion is simulated, particularly in extension. Further studies are needed to elucidate the complex interaction of the radial head with the capitellum, the ulnohumeral joint, and the ligamentous structures for different activities of daily living.

Biomechanical Phenomena↗

PubMedDate not supplied

Surgeon accuracy in the selection of the flexion-extension axis of the elbow: an in vitro study.

The incorrect determination of the flexion-extension axis of the elbow may result in poor clinical outcomes for patients undergoing ligament repairs, joint arthroplasties, and procedures requiring articulated external fixators. The variability in the selection of the flexion-extension axis based on a visual approximation of bony landmarks on the distal humerus was studied within a group of surgeons. Three surgeons of different experience levels independently selected points through which they believed the flexion-extension axis passed on the medial and lateral surfaces of 23 distal humeri. One surgeon repeated the point selection on three separate occasions. These surgeon points were compared with an axis calculated by use of a computer-assisted method that relied on the centers of the trochlear sulcus and capitellum via digitization of these structures. Relative to the computer-generated flexion-extension axis, the error in the surgeons' selections resulted in a mean frontal plane angle of 1.5 degrees +/- 3.0 degrees valgus (range, 6.3 degrees varus to 9.6 degrees valgus) and a coronal plane angle of 1.6 degrees +/- 3.3 degrees external rotation (range, 8.3 degrees internal rotation to 10.2 degrees external rotation). These results suggest that the use of a computer-assisted surgical technique will allow a more accurate determination of the flexion-extension axis of the elbow, which may lead to improved clinical outcomes in patients treated with ligament repairs or reconstructions, elbow arthroplasties, or articulated external fixators.

Adult↗

PubMedDate not supplied

Does keel size, the use of screws, and the use of bone cement affect fixation of a metal glenoid implant?

The objective of this study was to determine the effect of screws and keel size on the fixation of an all-metal glenoid component. A prototype stainless-steel glenoid component was designed and implanted in 10 cadaveric scapulae. A testing apparatus capable of producing a loading vector at various angles, magnitudes, and directions was used. The independent variables included six directions and three angles of joint load, and five fixation modalities-three different-sized cross-keels (small, medium, and large), screws, and bone cement. Implant micromotion relative to bone was measured by four displacement transducers at the superior, inferior, anterior, and posterior sites. The components displayed a consistent response to loading of ipsilateral compression and contralateral distraction. Use of progressively larger keels did not significantly improve implant stability. Stability decreased as the angle of load application increased (P <.05). Screw and cement fixation resulted in the most stable fixation (P <.05).

Arthroplasty↗

PubMedDate not supplied

Single-strand reconstruction of the lateral ulnar collateral ligament restores varus and posterolateral rotatory stability of the elbow.

Because of a lack of biomechanical studies of lateral elbow ligament reconstruction in the literature, the initial stability afforded by 3 different techniques of lateral ulnar collateral ligament reconstruction was evaluated in 8 cadaveric elbows. The arm was mounted in a testing apparatus, and passive flexion was performed with the arm in varus and valgus orientations. A pivot shift test was performed with the arm in the vertical orientation. An electromagnetic tracking device was used to quantify motion pathways. After intact testing, each specimen underwent sectioning of the radial collateral and lateral ulnar collateral ligaments from the lateral epicondyle. Reconstruction of the lateral ulnar collateral ligament was performed in a randomized sequence, consisting of proximal single-strand, distal single-strand, and double-strand tendon grafts. Division of the radial collateral and lateral ulnar collateral ligaments from the lateral epicondyle caused a significant decrease in rotational stability when the pivot shift test was being performed (P <.0001). Varus-valgus stability also decreased after transection of the radial collateral and lateral ulnar collateral ligaments (P <.0001). Reconstruction of the lateral ulnar collateral ligament restored elbow stability to that of the intact state. There was no significant difference in stability between the single- and double-strand repair techniques (P >.05). This study demonstrates that both single- and double-strand reconstructions restore varus and posterolateral elbow stability and may be considered appropriate reconstructive procedures in patients with symptomatic insufficiency of the lateral ligaments of the elbow.

Aged↗

PubMedDate not supplied

Single-strand ligament reconstruction of the medial collateral ligament restores valgus elbow stability.

The purpose of this study was to determine the contribution of the central portion of the anterior bundle of the medial collateral ligament (MCL) to elbow stability and to evaluate the effectiveness of a single-strand MCL reconstruction in restoring elbow stability. Testing of 11 fresh-frozen upper extremities was first performed on the intact elbow and then with the capsule, flexor-pronator muscle group, posterior bundle, anterior or posterior band, and central band cut sequentially. Next, a single-strand reconstruction of the MCL was performed. The elbow was moved passively through a full arc of flexion in both varus and valgus gravity-loaded positions. Ulnar movement with respect to the humerus was analyzed by means of an electromagnetic tracking system. Maximum varus-valgus laxity throughout the arc of supinated flexion and pronated flexion was 6.6 degree plus minus 2.4 degree and 7.4 degree plus minus 2.0 degree, respectively, for the intact specimen, 34.2 degree plus minus 5.6 degree and 37.7 degree plus minus 11.8 degree for the specimen with all of the medial valgus elbow stabilizers cut, and 9.0 degree plus minus 2.5 degree and 10.5 degree plus minus 2.7 degree for the reconstructed specimen. Maximum varus-valgus laxity was not significantly different among any of the sectioning sequences until the central band was cut (P <.0001). There was no significant difference in maximum varus-valgus laxity between the intact and reconstructed elbows (P <.05). Our results demonstrate that the central band is an important valgus stabilizer of the elbow and that a simplified single-strand reconstruction is able to restore stability to the MCL-deficient elbow.

Aged↗

PubMedDate not supplied

The medial collateral ligament of the elbow is not isometric: an in vitro biomechanical study.

BACKGROUND: The anterior bundle of the medial collateral ligament (AMCL) of the elbow has been shown to be the most important valgus stabilizer of the elbow. However, the isometry of this band has not been quantified. HYPOTHESIS: Isometric fibers exist within the AMCL, and these fibers are located within its central region. STUDY DESIGN: Controlled laboratory study. METHODS: Twelve cadaveric elbow specimens were mounted in a testing apparatus in a valgus gravity-loaded orientation. Passive supinated flexion was performed and the motion recorded using an electromagnetic tracking device. Hundreds of attachment points for the AMCL of the elbow were recorded on the medial epicondyle and ulna. The overall change in length between each point on the ulna to every humeral point, throughout the arc of motion, was quantified (DeltaL = Lmax - Lmin). The locations of the smallest DeltaL values were determined relative to the attachment site of the AMCL on the medial epicondyle. RESULTS: True isometry was not found throughout the arc of flexion. The smallest DeltaL values averaged 2.8 +/- 1.2 mm (range: 0.7 mm to 5.2 mm). Isometric fibers do not exist within the AMCL; however, "nearly" isometric areas are located on the lateral aspect of the attachment site of the AMCL on the medial epicondyle, near the anatomic axis of rotation. CONCLUSIONS: We postulate that these nearly isometric areas would be the most ideal location for graft attachment during reconstruction of the AMCL.

Aged↗