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Michael F Shepard

Publications and source records attributed to Michael F Shepard.

5 recordsLinked to original sources
PubMed2004

Differences in the ultimate strength of the biceps anchor and the generation of type II superior labral anterior posterior lesions in a cadaveric model.

BACKGROUND: The pathogenesis of superior labral anterior posterior lesions remains controversial. HYPOTHESIS: The biceps anchor is more vulnerable to loading with a posterior vector as opposed to an in-line pull. STUDY DESIGN: Controlled laboratory study. METHODS: Eight pairs of cadaveric shoulders were dissected, and the biceps tendon was loaded to failure in 1 of 2 loading patterns. Loading pattern A was meant to simulate the eccentric load of the biceps in the deceleration phase of throwing; loading pattern B was meant to simulate the posterior biceps load of the late cocking phase. RESULTS: The biceps anchor demonstrated significantly increased ultimate strength with in-line loading (group A, 508 N) as opposed to posterior loading (group B, 262 N, P < .001). All group B specimens failed at the biceps anchor, resulting in a type II superior labral anterior posterior lesion. Specimens in group A did not create a superior labral anterior posterior lesion. CONCLUSIONS: Direction of biceps loading resulted in significant differences in the ultimate strength of the biceps anchor and the generation of superior labral anterior posterior lesions. The biceps anchor was significantly weaker when loaded with a posterior vector. RELEVANCE: The superior labrum may be most vulnerable to injury in late cocking. The reproducible generation of type II superior labral anterior posterior lesions may have applications as a biomechanical model.

Aged↗
PubMed2002

Enhancement of pedicle screw fixation through washers.

This study examined the effect of washer usage on initial pedicle screw fixation and on the salvage of replaced pedicle screws, and the effect of minor adjustments of pedicle screws on insertional torque. Titanium, nontapered pedicle screws (6.5-mm in diameter and 35-mm in length) from one manufacturer and custom-made 5-mm washers were used in the fixation of porcine lumbar spines. Insertional torque was measured with an electronic torque screwdriver and failure strength was determined by straight pullout of the screws using an MTS machine. Initial insertional torque values were significantly greater in pedicle screws placed with washers compared with screws placed without washers. When the screw placed without a washer was salvaged with the addition of a washer, a significant increase in insertional torque resulted. Pullout testing failed to show a significant difference between the screws that were placed with washers and the screws that were placed without washers. In the second part of the experiment, there was a significant decrease in insertional torque after backing out the screw as little as 90 degrees. This current study showed that (1) washers significantly increase the insertional torque of pedicle screws; (2) screws placed without a washer can be salvaged and replaced with a washer, which results in significantly increased insertional torque; and (3) backing out a pedicle screw 90 degrees significantly decreases its insertional torque. Washers can be used with pedicle screws to enhance the initial stability of the screw constructs, and to maximize insertional torque when screws need to be replaced, revised, or adjusted (backed out).

Animals↗
PubMed2002

Effects of polyaxial pedicle screws on lumbar construct rigidity.

Pedicle screw constructs have been shown to increase fusion rates in the lumbar spine. Manufacturers have created pedicle screws with one or two degrees of freedom built into the screw head to allow for easier incorporation of the interlocking rod, but the effects of these screws on construct stiffness has not been tested. The purpose of this study is to compare and contrast the stiffness of lumbar pedicle screw constructs with and without the use of polyaxial pedicle screws. Nontapered, self-taping pedicle screws (6.0-mm diameter x 30-mm length, titanium) were used in the fixation of porcine spines from L3-L5. Group 1 (n = 5) contained six standard pedicle screws from one manufacturer. Group 2 (n = 5) contained six standard pedicle screws from a second manufacturer. Group 3 (n = 5) contained four standard pedicle screws placed at L3 and L5, as well as two polyaxial screws placed at L4. Group 4 (n = 5) contained six polyaxial pedicle screws. A rotational variable differential transformer was used to record angular displacement between vertebrae in the construct as it is loaded in flexion, extension, right bend, left bend, clockwise torque, and counterclockwise torque. Stiffness curves were linear throughout the range of applied force. The average r2 value for the generated stiffness graphs was 0.94 (SD = 0.06). No construct failure occurred during any of the testing. There were no significant differences (p < 0.05, two-way analysis of variance) in moment versus angle noted in any of the four groups tested. For torque tests, the all-polyaxial screw constructs showed significantly increased stiffness compared with the other groups. The current study has shown that the incorporation of polyaxial screws in pedicle screw constructs did not significantly decrease the construct stiffness. There is a suggestion that the use of all polyaxial screws may increase the resistance to torque by allowing better purchase of intervertebral rods.

Animals↗
PubMedDate not supplied

The clinical significance of anterior horn meniscal tears diagnosed on magnetic resonance images.

We assessed the accuracy of magnetic resonance imaging in detecting clinically significant lesions of the anterior horn of the meniscus by reviewing 947 consecutive knee magnetic resonance imaging reports. Of these, 76 (8%) indicated a tear of the anterior horn of the medial or lateral meniscus. Thirty-one of these 76 patients underwent a subsequent arthroscopic examination, and their operative reports were reviewed. The 45 patients who were not examined arthroscopically were contacted and interviewed for clinical follow-up. Among the 31 patients who underwent arthroscopic examination, 8 anterior horn tears were noted in the predicted area (26% true-positive results), 23 patients had intact anterior horns (74% false-positive results), and 18 had normal intact menisci in all zones. Of the 45 patients who did not undergo arthroscopic surgery, 6 had isolated anterior horn tears reported on magnetic resonance imaging, and 5 of the 6 were asymptomatic at follow-up. The other 39 patients had multiple pathologic conditions noted on the magnetic resonance imaging report and continued to report knee pain at the follow-up interview. Increased signal intensity at the anterior horn of the meniscus seen on magnetic resonance imaging commonly does not represent a clinically significant lesion. We recommend correlation with the physical examination when interpreting this "positive" finding on knee magnetic resonance imaging examinations.

Adolescent↗