Robert J. Pawluk

Excursion of the Rotator Cuff Under the Acromion Patterns of Subacromial Contact

Nine fresh-frozen, human cadaveric shoulders were el evated in the scapular plane in two different humeral rotations by applying forces along action lines of rotator cuff and deltoid muscles. Stereophotogrammetry deter mined possible regions of subacromial contact using a proximity criterion; radiographs measured acromio humeral interval and position of greater tuberosity. Con tact starts at the anterolateral edge of the acromion at 0° of elevation; it shifts medially with arm elevation. On the humeral surface, contact shifts from proximal to dis tal on the supraspinatus tendon with arm elevation. When external rotation is decreased, distal and poste rior shift in contact is noted. Acromial undersurface and rotator cuff tendons are in closest proximity between 60° and 120° of elevation; contact was consistently more pronounced for Type III acromions. Mean acro miohumeral interval was 11.1 mm at 0° of elevation and decreased to 5.7 mm at 90°, when greater tuberosity was closest to the acromion. Radiographs show bone- to-bone relationship; stereophotogrammetry assesses contact on soft tissues of the subacromial space. Con tact centers on the supraspinatus insertion, suggesting altered excursion of the greater tuberosity may initially damage this rotator cuff region. Conditions limiting ex ternal rotation or elevation may also increase rotator cuff compression. Marked increase in contact with Type III acromions supports the role of anterior acromioplasty when clinically indicated, usually in older patients with primary impingement.

Inferior glenohumeral ligament: Geometric and strain-rate dependent properties

The inferior glenohumeral ligament (IGHL) is an important structure for maintaining shoulder stability. This study was aimed at determining the geometric and anatomic characteristics of the IGHL and its tensile properties at a higher strain rate than previously tested. Eight fresh-frozen human cadaver shoulders (average age 69 years, age range 62 to 73 years) from four female and four male cadavers were used to harvest bone-ligament-bone specimens from the three regions of the IGHL (superior band, anterior axillary pouch, and posterior axillary pouch). Uniaxial tensile tests were performed at the moderately high strain rate of approximately 10% per second with a servo-hydraulic testing machine. This represented a strain rate that was approximately 100 to 1000 times faster than that previously reported. During tensile testing, bone-ligament-bone strains were calculated from grip-to-grip motion on the testing machine, and mid-substance strains were determined by a video dimensional analyzer. Although all regions of the IGHL had similar lengths (averaging 43.4 mm), their thickness varied by region and by proximal-to-distal location. The superior band was the thickest (2.23 +/- 0.38 mm) of the three regions. Of the remaining two regions the anterior axillary pouch (1.94 +/- 0.38 mm) was thicker than the posterior axillary pouch (1.59 +/- 0.64 mm). By proximal-to-distal location the IGHL was thicker for all three regions near the glenoid (2.30 +/- 0.57 mm) than near the humerus (1.61 +/- 0.52 mm). The superior band had a greater stiffness (62.63 +/- 9.78 MPa) than either the anterior axillary pouch (47.75 +/- 17.89 MPa) or the posterior axillary pouch (39.97 +/- 13.29 MPa). Tensile stress at failure was greater in the superior band (8.4 +/- 2.2 MPa) and the anterior axillary pouch (7.8 +/- 3.1 MPa) than the posterior axillary pouch (5.9 +/- 1.7 MPa). The anterior axillary pouch demonstrated greater bone-to-bone and mid-substance strains (30.4% +/- 4.3% and 10.8% +/- 2.4%, respectively) before failure than the other two regions (superior band: 20.8% +/- 3.8% and 9.1% +/- 2.8%, respectively; posterior axillary pouch: 25.2% +/- 5.8% and 7.8% +/- 2.6%, respectively). Bone-to-bone strain was always greater than mid-substance strain, indicating that when the IGHL is stretched, the tissue near the insertion sites will experience much greater strain than the tissue in the mid-substance. insertion failures were more likely at slower strain rates, and ligamentous failures were predominant at the fast strain rate. When compared with other tensile studies of the IGHL at slower strain rates (0.01% per second and 0.1% per second), the superior band and the anterior axillary pouch demonstrated the viscoelastic effects of increased stiffness and failure stress. This superior band and anterior axillary pouch pouch viscoelastic stiffening effect suggests that these two regions may function to restrain the humeral head from rapid abnormal anterior displacement in the clinically vulnerable position of abduction and external rotation.

Topography of the osteoarthritic thumb carpometacarpal joint and its variations with regard to gender, age, site, and osteoarthritic stage

The articular topography of 46 osteoarthritic thumb carpometacarpal joints was quantitatively analyzed, as well as variations with regard to gender, age, site, and anatomic osteoarthritic stage. It was found that for osteoarthritic thumb carpometacarpal joints, (1) the opposing articular surfaces of elder and severely degenerated joints are more congruent than those of middle-aged and minimally or moderately degenerated joints, although the articular contact area is not significantly different when accounting for thinning of the cartilage layer with age or disease; (2) significant changes in joint topography due to osteoarthritis only occur in severely degenerated joints; (3) joints in women are less congruent, have smaller contact areas, and are likely to experience higher contact stresses than joints in men for similar activities of daily living that involve similar joint loads; and (4) osteoarthritic changes are less severe on the dorsoulnar aspect of the trapezium and the dorsal aspect of the metacarpal, which are known to be low load-bearing regions.

Electrical behavior of cartilage during loading.

When cartilage is deformed, it becomes electrically polarized. At least two mechanisms seem to underlie this phenomenon, namely, a short-duration, high-amplitude, piezoelectric-like response and a longer-duration, lower-amplitude response secondary to streaming potentials. The polarity of articular cartilage during loading could hypothetically facilitate joint lubrication.

In Vivo Effects of Direct Current in the Mandible

The application of low level direct current with a current density of 50 μa/cm2 within the mandible of mature Beagle dogs results in an increased osteogenesis about the negative electrode (cathode). Significant differences observed between the use of platinum-iridium and stainless-steel electrodes suggest careful consideration of appropriate current density and electrode composition when direct current stimulation is to be used.

Comparison of glenohumeral mechanics following a capsular shift and anterior tightening.

BACKGROUND Numerous surgical techniques have been developed to treat glenohumeral instability. Anterior tightening procedures have been associated with secondary glenohumeral osteoarthritis, unlike the anterior-inferior capsular shift procedure, which has been widely advocated as a more anatomical repair. The objective of the present study was to quantify glenohumeral joint translations, articular contact, and resultant forces in cadaveric specimens in order to compare the effects of unidirectional anterior tightening with those of the anterior-inferior capsular shift. METHODS Six normal fresh-frozen cadaveric shoulders were tested on a custom rig with use of a coordinate-measuring machine to obtain kinematic measurements and a six-axis load transducer to measure resultant external joint forces. Shoulders were tested in the scapular plane in three configurations (normal anatomical, anterior tightening, and anterior-inferior capsular shift) and in three humeral rotations (neutral, internal, and external). Glenohumeral articular surface geometry was quantified with use of stereophotogrammetry for kinematic and contact analyses. Resultant joint forces were computed on the basis of digitized coordinates of tendon insertions and origins. RESULTS Compared with the controls (maximum elevation, 167 degrees 8 degrees ), the anteriorly tightened specimens demonstrated loss of external rotation, significantly restricted maximum elevation (135 degrees 16 degrees , p = 0.002), posterior-inferior humeral head subluxation, and significantly greater posteriorly directed resultant forces at higher elevations (p < 0.05). In contrast, compared with the controls, the specimens that had been treated with the anterior-inferior capsular shift demonstrated a similar maximum elevation (159 degrees +/- 11 degrees , p = 0.8) without any apparent loss of external rotation and with reduced humeral translation. CONCLUSIONS Anterior tightening adversely affects joint mechanics by decreasing joint stability, limiting both external rotation and arm elevation, and requiring greater posterior joint forces to attain maximum elevation. The anterior-inferior capsular shift improves joint stability while preserving external rotation with no significant loss of maximum elevation.