A biomechanical evaluation of the role of labrum on anterior/posterior translation of shoulders at different degrees of abduction.

The objective of the study was to investigate the effects of anterior and posterior labrums on the anterior/posterior translations of shoulders. Thirteen cadaver shoulders were arthroscopically evaluated and nine were selected based on the absence of any pathological findings. These shoulders were tested intact, vented and after sequential arthroscpoic incision of the anterior and posterior labrums. The anterior/posterior translations were measured in a specially designed apparatus. The loads vs. displacement curves were obtained. The loads vs. displacement curves of the shoulders before and after labrum incision did not show any appreciable differences. Moreover the measured loads at 6 mm displacement did not show any statistically significant differences due to labrum incision. It was concluded that both anterior and posterior labrum incisions of the shoulders did not show any appreciable differences in anterior/posterior laxity. An arthroscopic technique was successfully developed to isolate the effect of labrum on the shoulder laxity.

Bioabsorbable screw fixation in coracoclavicular ligament reconstruction.

The purpose of this study is to evaluate the biomechanical properties of stainless steel and bioabsorbable screw fixation of the clavicle to the base of the coracoid. Seven matched pairs of fresh frozen shoulders were prepared by removing all soft tissue except the acromioclavicular and coracoclavicular ligament complexes. The shoulders were randomly selected and fixed with 4.5-mm stainless steel (SS) screws, while contralateral shoulders were fixed with 4.5-mm poly L-lactic acid polymer (PLLA) screws. Pullout strength, stiffness, and elongation to failure were measured using an Instron Mechanical Tester (Model 4202). The average pullout strength of 720.6 +/- 244.9 N of the metal screws was not statistically different (p = 0.089) from that of the biodegradable screws of 580.4 +/- 188.6 N. The pullout strengths exerted by both these screws exceeded the reported strength (500 N) of the intact coracoclavicular complex indicating adequate initial pullout strength for coracoclavicular fixation.

Effect of carbon coating on the properties of gamma irradiated ultra-high-molecular-weight polyethylene specimens.

We reported the effect of carbon coating on the changes in properties of the ultra-high-molecular-weight polyethylene standard material (Hospital for Special Surgery) alter gamma irradiation in air and storage for 2 years. The coating showed a slight improvement in crystallinity (X-ray) and tensile properties (under cyclic loading) over the uncoated and irradiated control group. The oxidation level as measured by Fourier- transform infrared spectroscopy was unaffected by coating.

The effect of isometric graft posterior cruciate ligament reconstruction on anterior tibial translation.

Eight fresh-frozen cadaver knees were studied to evaluate whether an isometrically placed posterior cruciate ligament (PCL) graft restores normal posterior tibial translation without overconstraining anterior tibial translation. Each knee was tested with a three-axis load cell in the intact state, after PCL sectioning, and after PCL reconstruction. After PCL reconstruction, posterior tibial displacement was restored to values observed in the intact state for all flexion angles except 60 degrees and 90 degrees. Anterior tibial translation was not significantly changed for any of the three states. These results indicate isometric reconstruction of the PCL significantly reduces posterior tibial translation without overconstraining anterior tibial translation.

Biomechanical study of atlantoaxial arthrodesis: transarticular screw fixation versus modified Brooks posterior wiring.

OBJECTIVE: The purpose of the present study was to compare the biomechanical stability of C1 and C2 vertebrae after treatment of ligamentous instability by either modified Brooks posterior wiring (MB) or transarticular screw (TAS) techniques. We hypothesized that the TAS technique would be more stable because of direct fixation through the facet joints. STUDY DESIGN: We studied the in vitro stability (arthrodesis) of TAS fixation of C1 and C2 versus that of MB. TAS fixation involves placing screws across the facets from posteriorly at C2 to the anterior surface of C1, plus a bone graft and posterior wiring of C1 and C2. METHODS: Cervical spines from nine individuals with an average age of sixty-two years (range 51 to 71 years) were harvested from cadavers (six male, three female). C1 and the segment from C2 to C5 were potted to allow motion only at the C1-C2 articulation. The specimens were destabilized by cutting the transverse ligament on both sides of the odontoid and the tectorial membrane between C1 and C2. The MB and TAS techniques were performed by methods similar to those described in the literature. The stiffness of the C1-C2 articulation of each specimen was tested under rotation, lateral bending, flexion, and anterior translation in random order. Intact and destabilized specimens fixed with either MB or TAS were tested in sequence. RESULTS: Significantly higher stiffness values in the elastic zone were obtained with the TAS technique than with the MB technique for all modes of testing (p < 0.002, t test). Values for the neutral zone (the region where minimal loads produce displacement) were not significantly different between the MB and TAS techniques (p > 0.1, t test). CONCLUSION: We conclude that stability is significantly enhanced by use of the TAS construct for treatment of ligamentous instability at the atlantoaxial joint for all motions tested in the present study.