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Biomedical subjects

S M Howell

Publications and source records attributed to S M Howell.

At least 55 records · Page 3Linked to original sources
PubMed1992

Arthroscopic roofplasty: a method for correcting an extension deficit caused by roof impingement of an anterior cruciate ligament graft.

Intercondylar roof impingement should be suspected in any patient having difficulty regaining knee extension following an anterior cruciate ligament (ACL) reconstruction. An arthroscopically performed roofplasty can eliminate the clinical complaints. The extent of bone removal can be planned from studying a lateral radiograph taken with the knee in terminal extension. Extension exercises or passive extension devices should not be used in patients with extension deficits caused by roof impingement because they may further damage the graft.

Adult↗
PubMed1992

Tibial tunnel placement in anterior cruciate ligament reconstructions and graft impingement.

Fifty-six anterior cruciate ligament (ACL) reconstructions had a magnetic resonance scan of the ACL graft six months after operation. The impingement-free grafts (n = 26) had a low magnetic resonance signal from origin to insertion. Impinged grafts (n = 30) had an increased magnetic resonance signal confined to the distal two thirds of the graft. The location of the tibial tunnel (TT) was determined from a lateral roentgenogram. Positioning the center of the TT 12-23 mm from the anterior edge of the tibia consistently produced graft impingement and flexion contractures. Roof impingement was avoided and hyperextension was regained when the TT was centered more posteriorly within a 6-mm impingement-free zone (22-28 mm from the anterior edge of the tibia). Stability and knee extension were significantly better when the center of the TT was 2-3 mm posterior to the center of the normal ACL insertion.

Adult↗
PubMed1992

The postoperative knee.

Magnetic resonance (MR) has established itself as the gold standard for noninvasive imaging of the knee. As in arthroscopy, assessment of injuries to the menisci, articular surfaces, synovium, and cruciate ligaments can be made. Additionally, MR has an advantage over arthroscopy in being able to assess the para articular soft tissues and cortical and medullary bony compartments. Until recently, the role of MR in the postoperative knee has been less clear. New information, however, suggests that MR is capable of assessing the status of the partially resected meniscus, the adjacent articular surfaces, and subchondral medullary lesions that frequently occur. There has been recent interest in reestablishment of the stabilizing function of a previously torn anterior cruciate ligament by means of autogenous or allogenous materials. MR permits the surgeon to assess the isometry and possible sites of graft impingement that may occur following cruciate ligament reconstruction. The purpose of this article is to define the current status of MR in the assessment of the postoperative knee.

Bone Transplantation↗
PubMed1991

Unimpinged and impinged anterior cruciate ligament grafts: MR signal intensity measurements.

Regionalized magnetic resonance (MR) signal intensities were quantitatively measured in impinged and unimpinged anterior cruciate ligament (ACL) grafts. Images were obtained with a 1.5-T imager, and signal intensity was measured in the proximal, middle, and distal thirds of the graft. In 15 unimpinged ACL grafts, the signal intensity remained low and did not vary during the 1st year of graft implantation (45 images). In contrast, 17 impinged ACL grafts showed an increase in signal intensity in the distal two-thirds of the graft that persisted 1-3 years after implantation (P less than .001). Unimpinged grafts were placed in tibial tunnels posterior and parallel to the slope of the intercondylar roof. Reconstructions with anterior tibial tunnels resulted in graft impingement that caused increases in graft signal intensity. This increase demonstrates a clear association between surgical technique and the subsequent MR appearance of the graft.

Adult↗
PubMed1991

Meniscal tears: MR and arthrographic findings after arthroscopic repair.

Magnetic resonance (MR) imaging was performed on 29 previously repaired menisci and one conservatively treated meniscus (total, 30 menisci). Intermediate- and T1-weighted MR sequences revealed persistent signal intensity extending to an articular surface (grade 3 signal intensity) in 27 of the 30 menisci. On T2-weighted images, seven of the 30 menisci were found to contain unequivocally higher signal intensity, defined by a full-thickness defect (grade 3 signal intensity involving two articular surfaces) increasing in signal intensity to a level equivalent to that of joint fluid. The MR imaging and arthrographic appearances of 23 of the 30 menisci were compared. Arthrographic examination revealed partial or complete healing in 13 menisci and tears in 10. The presence of grade 3 signal intensity on intermediate- and T1-weighted MR images did not reliably predict a tear seen at arthrography. Unequivocally higher signal intensity on T2-weighted images is a useful sign in the prediction of a persistent meniscal tear (sensitivity, 60%; specificity, 92%; P less than .02). Since presence of grade 3 signal intensity on intermediate- and T1-weighted images does not reliably predict a tear and unequivocal T2 increase in intensity has a sensitivity of only 60%, arthrography should be considered for assessment of the symptomatic, previously repaired meniscus.

Adult↗
PubMed1991

The role of the supraspinatus and infraspinatus muscles in glenohumeral kinematics of anterior should instability.

To investigate a socket mechanism responsible for controlling the kinematics in the statically positioned glenohumeral joint, a suprascapular nerve block was performed in 13 selected patients, with recurrent anterior instability and defects of the labrum. Kinematics were then documented by roentgenograms in four positions within the horizontal motion plane. Combined paralysis of the supraspinatus and infraspinatus muscles resulted in abnormal anterior translation in only two of 47 roentgenograms. Normal ball-and-socket kinematics were retained in the remaining 45 roentgenograms. The consistent arthroscopic findings were an undamaged glenoid articular surface with a detached or absent labrum. The injury to the labrum reduced the depth of the socket by one-half. The other one-half of the socket provided by the contour of the glenoid remained intact. A balanced muscle envelope was not required to maintain normal kinematics in selected, actively positioned, unstable shoulders. The retained glenoid depth was sufficient to produce the observed ball-and-socket kinematics. Further in vivo study of shoulder kinematics will be needed to clarify the interactive roles of the socket and muscle envelope in maintaining glenohumeral stability during the more demanding stresses during active shoulder motion.

Adolescent↗
PubMed1989

The glenoid-labral socket. A constrained articular surface.

An anatomic study was designed to test the hypothesis that the articular surface of the glenoid fossa and labrum produces a composite socket of significant depth. Measurements were obtained from 25 anatomic specimen shoulders. The glenoid articular surface and labrum combine to create a socket that is approximately 9 mm deep in the superoinferior (SI) direction and 5 mm deep in the anteroposterior (AP) direction. The circular, pliable, fibrous labrum contributes approximately 50% of the total depth of the socket. Detachment of the labrum anteriorly, as in a Bankart lesion, may reduce the depth of the socket in the AP direction from approximately 5.0 to 2.4 mm. These anatomic observations provide some evidence that the socket may be an important factor in shoulder stability. Further in vivo kinematic studies of shoulders will be needed to better define the stabilizing role of the glenoid-labral socket.

Adult↗
PubMed1988

Normal and abnormal mechanics of the glenohumeral joint in the horizontal plane.

This study was performed to evaluate the relationship of the humeral head to the scapula in the horizontal plane of motion and to describe in detail a method of obtaining and interpreting modified axillary roentgenograms. Twenty normal subjects and twelve patients who had anterior instability of the shoulder were evaluated with this technique. In the control group, the humeral head was centered in the glenoid cavity throughout the horizontal plane of motion except when the arm was in maximum extension and external rotation. In this position, the cocked stage of the throwing motion, the center of the humeral head rested approximately four millimeters posterior to the center of the glenoid cavity. When the arm was flexed or rotated from this cocked position, the humeral head glided anteriorly, producing a shearing stress on the articular surface of the glenoid and labrum. In seven of the twelve patients who had anterior instability, abnormal mechanics were observed: anterior translation of the humeral head occurred. This indicates a significant disruption of the structures responsible for containing the humeral head within the glenoid fossa.

Adult↗
PubMed1986

Clarification of the role of the supraspinatus muscle in shoulder function.

The purpose of this study was to determine the torque produced by the supraspinatus muscle within the functional motion planes of forward flexion and elevation in the plane of the scapula. An isokinetic dynamometer was used to measure the reduction in torque produced by isolated paralysis of the suprascapular and axillary nerves in ten normal volunteers. A calculation of the potential torque of the supraspinatus and deltoid muscles was obtained from the product of the moment arm, cross-sectional area, and integrated electromyographic signal. The results of both the dynamometer measurement and the calculation of torque supported our conclusion that the supraspinatus and deltoid muscles are equally responsible for producing torque about the shoulder joint in the functional planes of motion.

Adult↗
PubMed1981

The N-ethylmaleimide-sensitive cysteine residue in the pH-dependent subunit interactions of malate dehydrogenase.

The specific chemical modification by N-ethylmaleimide of a cysteine residue at pH 5.0 in porcine heart mitochondrial malate dehydrogenase (L-malate:NAD+ oxidoreductase, EC 1.1.1.37) has been shown to result in an enzymatically inactive, monomeric product, which does not reassociate at pH 7.5 to yield the native dimer. In this report, an investigation of proton release and uptake upon NADH binding to the native enzyme and to the N-ethylmaleimide-modified enzyme has implicated the above cysteine residue as being directly linked to the pH-dependent subunit dissociation of mitochondrial malate dehydrogenase. The results are consistent with the view that the modified cysteine residue is not located at the subunit interaction site, although it is probably near this site. A recent study from this laboratory has demonstrated that the monomeric enzyme obtained at pH 5.0 exists in a conformation which is enzymatically inactive and which has an enhanced intrinsic protein fluorescence. Interpretation of protein fluorescence data has suggested that the N-ethylmaleimide modification results in inactivation of the enzyme by preventing the pH-induced conformational change to the active dimer. However, NADH is able to induce reassociation of the N-ethylmaleimide-modified enzyme at pH 7.5 but not at pH 5.0. This reassociation at pH 7.5 is accompanied by a significant regain of enzymatic activity, indicating that NADH binding is able to partially overcome the negative effect of the cysteine modification on the pH-dependent subunit reassociation of mitochondrial malate dehydrogenase.

Animals↗
PubMed1976

Fluorometric assay of verapamil in biological fluids and tissues.

Verapamil concentrations in blood, urinee, or tissue homogenates may be measured fluorometrically in the 0.1-10-mug/ml range after extraction into heptane and back-extraction into acid; recovery rates are above 87%. The drug apparently is eliminated rapidly in guinea pigs, with a plasma half-life of 45 min.

Animals↗