Biomechanical effects of malposition of tuberosity fragments on the humeral prosthetic reconstruction for four-part proximal humerus fractures.

Variable outcomes in the prosthetic reconstruction of 4-part humerus fractures often can be attributed to inconsistent and nonanatomic tuberosity placement. To compare the effects of anatomic (anterior fin) versus nonanatomic (lateral fin) tuberosity placement, we developed a dynamic cadaver model for shoulder motion. With the use of a robotically driven, computer-controlled articulator, we tested external rotation torque in 5 fresh human shoulders. After evaluation of the intact shoulders, we experimentally induced 4-part humerus fractures in the specimens. These were then repaired by hemiarthroplasty, with the use of standard techniques to secure the greater and lesser tuberosities in either anatomic or nonanatomic positions; order was randomized. Nonanatomic tuberosity reconstruction led to significant impairment in external rotation kinematics and an 8-fold increase in torque requirements (P =.001). In contrast, anatomic reconstruction produced results indistinguishable from normal shoulder controls. This study underscores the importance of rotational alignment of tuberosities during reconstruction. Failure to properly position tuberosity fragments in the horizontal plane may result in insurmountable postoperative motion restriction.

Comparison of different biomaterials for glaucoma drainage devices.

OBJECTIVES: To compare the inflammatory reaction associated with the insertion of silicone and polypropylene endplates and endplates made of a new biocompatible polymer, Vivathane, in the rabbit subconjunctival space. METHODS: Similar-sized endplates made of 3 different biomaterials were sutured to the sclera in the superotemporal quadrant of the rabbit eye. Thirty eyes of 15 albino New Zealand rabbits were randomly assigned to the 3 groups. Conjunctival vascular hyperemia was graded in a masked fashion among groups. At the end of 3 weeks, the enucleated eyes were examined histologically and using scanning electron microscopy. RESULTS: Polypropylene and Vivathane were associated with significantly more inflammation in clinical observations and based on histological grading. Silicone was associated with the least amount of inflammation. Three polypropylene and 1 Vivathane plate were extruded between the second and third week. CONCLUSIONS: Silicone is the most inert of the 3 materials tested. Inflammation associated with biomaterials may contribute to the failure of the glaucoma drainage devices. CLINICAL RELEVANCE: Bleb inflammation may be related to the biomaterial being used as the endplate. Endplates should be handled carefully during surgery to avoid creating rough spots.

Biomechanical analysis of four-strand extensor tendon repair techniques.

Experience with flexor tendon repairs has suggested the superiority of the augmented Becker (MGH) technique for strength, toughness, and gap resistance. In an effort to apply these findings to the extensor tendons, 3 four-strand extensor tendon repair techniques were biomechanically tested in fresh human cadaver limbs: modified Bunnell, modified Krackow-Thomas, and MGH. Repairs were performed in Verdan's zone VI. Repaired tendons were distracted at constant speed until rupture. Tendon load and tendon distraction were continuously monitored. Benchmark values for load were measured as fingers were pulled from full metacarpophalangeal (MP) joint flexion to full extension, to 1-mm gap formation at the tenorrhaphy, and to complete rupture of the repair. The MGH repair proved significantly more resistant to gap formation (stronger and tougher) than the Bunnell and Krackow-Thomas repairs (p < .02). No differences were seen between groups in repair performance at MP joint extension and at complete rupture. This study suggests that the MGH technique has superior gap resistance to the other four-strand methods tested for extensor tendon repair in Verdan's zone VI. The MGH repair is recommended for extensor tendon repairs in zone VI when early postoperative motion regimens are considered.

Optimization of the MGH repair using an algorithm for tenorrhaphy evaluation.

Previous investigations have demonstrated the superior strength and toughness of the MGH four-strand tendon repair technique and shown that it neither weakens during maximum tendon softening nor interferes with healing in an in vivo rabbit model. In the current study, the biomechanical performance of the modified Becker (MGH) and the modified Kessler repairs were compared in situ using a dynamic human cadaveric model to evaluate strength, toughness, glide efficiency, and operator variability. Three different surgeons performed a total of 42 zone II flexor digitorum profundus repairs in 14 fresh human cadaver hands. The modified Becker (MGH) repairs were stronger (79 +/- 3 versus 64 +/- 4 N; p < 0.01) and tougher (0.092 +/- 0.002 versus 0.078 +/- 0.003 J; p < 0.01) than the Kessler repairs while allowing equally efficient glide (32 +/- 6 versus 33 +/- 4 percent; p > 0.9). Strength was operator-dependent only for the modified Kessler repair (p < 0.005). We then established the optimal configuration of the MGH tenorrhaphy (number of preloaded crosses on either side of the tendon transection) by examining gap resistance ex vivo. Fifty-one MGH flexor tendon repairs were performed on explanted fresh human cadaver tendons. The experimental groups were randomly assigned to receive 0, 1, 2, 3, or 4 crosses on each side of the tenorrhaphy. Strength and toughness to gap formation and to ultimate failure were assessed tensiometrically. The MGH two-cross configuration was most resistant to gap formation (peak load 39 +/- 3 N; p < 0.05), establishing this configuration as the optimal design of this four-strand crisscrossing repair technique. The augmented Becker (MGH) repair is significantly stronger and tougher than the modified Kessler repair and demonstrates no operator dependence. It is a superior technique for zone II tenorrhaphy in the human hand. An algorithm is presented as a systematic approach that includes the important elements necessary for the rigorous biomechanical evaluation of any tendon repair technique.

Electrocardiographic changes after intravenous administration of magnesium in rabbits.

Magnesium (Mg+2) therapy has a beneficial effect in various cardiovascular diseases. However, overdosage of Mg+2 can be fatal. This study was undertaken to determine the amount of Mg+2 that can be safely administered intravenously to rabbits. Twenty New Zealand White rabbits were allotted to three groups. One hundred milliliters of 0.9% NaCl/kg was infused in group-1 rabbits (n = 10) used as a control group. Rabbits in group 2 (n = 6) and group 3 (n = 4) received MgCl2 (2.5 mmol and 5.0 mmol/kg respectively) in 0.9% NaCl (100 ml/kg). The infusion was delivered intravenously for 30 min (group 2, 8 mg/kg per min; group 3, 16 mg/kg per min). The electrocardiogram was monitored throughout the infusion; the P-R interval, QRS duration, and QTc interval of group-2 and group-3 rabbits were prolonged at dosages of 0.8 mmol/kg, 1.7 mmol/kg, and 5.0 mmol/kg of MgCl2 respectively, compared with readings in group-1 rabbits. We hypothesized that the mean +2 standard deviations of the pre-operative baseline values was the safe limit. On the basis of this hypothesis, the safe limit of the P-R interval is 95.3 milliseconds, which corresponds to 0.6 mmol/kg of infused Mg+2; the QRS duration is 44.4 milliseconds, which corresponds to 1.0 mmol of Mg+2/kg; and the QTc interval is 350.0 milliseconds, which corresponds to 3.6 mmol of Mg+2/kg. These results will provide guidance for avoiding cardiac side effects when researchers investigate the effects of Mg+2 in rabbits.

A magnetically actuated left ventricular assist device.

Over the past 6 years, research has led to development of a small, lightweight, power-efficient, uniquely simple ventricular assist device driven by a magnetic actuator. Magnetic actuation permits total elimination of all mechanical motion converter components used for pusher plate displacement, resulting in a significant reduction in complexity and resultant increase in reliability. Extensive in vitro mock loop development has resulted in a left ventricular assist device (LVAD), the primary design parameters of which for the clinical prototype actuator and pump are 1) an actuator weight of 312 g, 2) actuator size of 32.5 cm3, 3) power requirements of 7.8 to 11.4 watts (60-100 beats per minute [BPM]), and 4) system efficiency of 24% to 34% and average dynamic stroke volume of 65 ml. Initial in vivo tests assessed this LVAD's performance in four sheep under three acute conditions of ventricular dysfunction. The results demonstrate that, at a pump-rate of 100 BPM, mean aortic pressure increased by 45-50 mmHg during 1) beta blockade, 2) coronary ligation, and 3) ventricular fibrillation. Pump flow ranged from 2.71 L/min to a maximum of 4.6 L/min. Acute test periods were arbitrarily set for 6 hours duration. Of the four sheep, two survived, one lived 5 hours, and the fourth lived 4.5 hours. Global fibrillation was the primary cause of failure. This initial in vivo data demonstrates the pump's ability to maintain satisfactory hemodynamic parameters of flow and pressure under three acute conditions of extreme left ventricular dysfunction in an animal model. These initial LVAD performances were encouraging. Further tests will use calves with a greatly expanded performance evaluation protocol.