Biomechanical evaluation of rotator cuff fixation methods.

Initial fixation strength and failure mode for various rotator cuff reattachment techniques (variations of the McLaughlin technique) were evaluated. Repair methods included standard suture (control), reinforced suture [expanded polytetrafluoroethylene (PTFE) patch and polydioxanone (PDS) tape augmentation] and stapling (nonarthroscopic and arthroscopic soft-tissue staples). The average strength of intact rotator cuff tissue (supraspinatus tendon) was also determined. The different rotator cuff repairs, including at least one control, were performed on fresh-frozen human cadaver shoulder pairs. Repairs were tested to failure in pure tension with the shoulder fixed in 60 degrees of abduction. Load and displacement data were normalized to controls, grouped according to failure modes, and statistically analyzed. The two basic failure modes observed were 1) bone failure, or suture tearing through the bone (indicating weak bone stock) and 2) tendon failure, or suture tearing of the rotator cuff. Gross comparisons between intact and repaired tendons indicated that the intact tendon was two to three times stronger than the repaired tendon. Based on the mode of failure and lack of increased strength after repair, the use of staples for cuff attachment is discouraged. PDS tape suture reinforcement did not increase fixation strength. In contrast, PTFE patch suture augmentation demonstrated statistically higher initial failure loads than did the control and was of specific benefit for shoulders with weak bone stock.

Soft tissue fixation to bone. A biomechanical analysis of spiked washers.

The initial fixation strengths of two spiked-washer designs were evaluated using human femurs and fascia lata tissue. Fascia lata was attached to the femur using the fixation devices, and then each femur-washer-fascia lata complex was loaded in tension to failure. Load-elongation curves were recorded, the initial failure load, initial failure displacement, stiffness, ultimate load, and ultimate displacement were determined for each test, and failure modes were recorded. Results indicated that the 6-spike design provided superior initial fixation strength in the 19-mm diameter size. This washer design was then compared with two commercially available fixation devices: the spiked AO washer and soft tissue fixation plate. Fixation provided by the prototypal washer design was not different in most instances from that provided by the AO fixation devices. Based on these results, important design characteristics for soft tissue spiked washers are identified and discussed.

Selective inhibition of alanine aminotransferase and aspartate aminotransferase in rat hepatocytes.

Experiments were conducted with intact rat hepatocytes to identify inhibitors and incubation conditions that cause selective inhibition of alanine aminotransferase or aspartate aminotransferase. Satisfactory results were obtained by preincubating cells with L-cycloserine or L-2-amino-4-methoxy-trans-but-3-enoic acid in the absence of added substrates. When cells were incubated for 20 min with 50 microM-L-cycloserine, alanine aminotransferase activity was decreased by 90%, whereas aspartate aminotransferase was inhibited by 10% or less. On subsequent incubation, synthesis of glucose and urea from alanine was strongly inhibited, but glucose synthesis from lactate was unaffected. L-2-Amino-4-methoxy-trans-but-3-enoic acid (400 microM) in hepatocyte incubations caused 90-95% inactivation of aspartate aminotransferase, but only 15-30% loss of alanine aminotransferase activity. After preincubation with the inhibitor, glucose synthesis from lactate was almost completely blocked; with alanine as the substrate, gluconeogenesis was unaffected, and urea synthesis was only slightly decreased. By comparison with preincubation with inhibitors, simultaneous addition of substrates (alanine; lactate plus lysine) and inhibitors (cycloserine; aminomethoxybutenoic acid) resulted in smaller decreases in aminotransferase activities and in metabolic rates. Other compounds were less satisfactory as selective inhibitors. Ethylhydrazinoacetate inactivated the two aminotransferases to similar extents. Vinylglycine was almost equally effective in blocking the two enzymes in vitro, but was a very weak inhibitor when used with intact cells. Concentrations of DL-propargylglycine (4 mM) required to cause at least 90% inhibition of alanine aminotransferase in hepatocytes also caused a 16% decrease in aspartate aminotransferase. When tested in vitro, alanine aminotransferase was, as previously reported by others, more sensitive to inhibition by amino-oxyacetate than was aspartate aminotransferase, but in liver cell incubations the latter enzyme was more rapidly inactivated by amino-oxyacetate.

Problems in radioimmunoassay of human lutropin with commercially available reagents.

To evaluate five commercially available reagent sets supplied for the radioimmunoassay of lutropin, we determined whether there was parallelism between the curve given by dilutions of the standards supplied by the manufacturers, by dilutions of a serum pool, and by dilutions of a standard preparation from human pituitaries, LER-907. These studies demonstrated significant analytical problems with three of the five sets. We conclude that each user should carefully evaluate all commercially available radioimmunoassays for lutropin (and, by inference, for other peptide hormones) before use.