Insertion force measurement of cervical traction tongs: a biomechanical study.

OBJECTIVES: To measure the insertion forces for various cervical traction tong pins and the force required to penetrate cadaveric skull specimens. DESIGN: Cadaveric study testing tong pin forces. METHODS: Insertion force and pin indicator displacement were measured in three tongs from each of three manufacturers (PMT, Depuy Ace, Gardner-Wells) after tightening pins per the manufacturer's recommendations and beyond. The force required for skull penetration was measured by driving each pin into cadaveric temporal specimens until the specimen failed or was penetrated. RESULTS: Forces generated at the recommended pin settings were: PMT, 141 +/- 12 newtons; Ace, 0 +/- 0 newtons; Gardner-Wells, 132 +/- 9 newtons. The Ace tong required 1.0-millimeter indicator protrusion beyond manufacturer's recommended setting before generating clinically significant forces (120 +/- 8 newtons). Unlike the PMT and Ace pins, overtightening of the Gardner-Wells pins (by as little as 0.3 mm beyond recommended indicator displacement) resulted in substantially larger pin force (>448 newtons). Average failure loads of the temporal bone specimens were: PMT, 721 +/- 298 newtons; Ace, 636 +/- 351 newtons; Gardner-Wells, 965 +/- 227 newtons. CONCLUSIONS: Although pin forces generated at the recommended end point for PMT and Gardner-Wells tongs appear safe, insufficient force was generated for Ace tongs. Furthermore, overtightening of the Gardner-Wells pins generated loads in excess of those that may be required to penetrate the skull.

Immediate weight-bearing after treatment of a comminuted fracture of the femoral shaft with a statically locked intramedullary nail.

BACKGROUND: The purpose of this two-part investigation was to test the feasibility, safety, and efficacy of immediate weight-bearing after treatment of fractures of the shaft of the femur with a statically locked intramedullary nail. METHODS: In the first part of the investigation, a biomechanical study was performed to determine the fatigue strength of eleven different statically locked intramedullary nail constructs. Segmentally comminuted midisthmal fractures were simulated with use of sections of polyvinyl chloride pipe; each construct was cyclically loaded in compression with use of physiologically relevant loads in a materials testing machine at eight hertz. The fatigue tests were conducted according to the so-called staircase method, and the construct was considered to have run out (exceeded its anticipated service life) if it had not failed after 500,000 cycles. In the second part of the study, a clinical investigation of immediate weight-bearing after treatment of comminuted fractures of the femoral shaft with a Russell-Taylor (RT-2) construct was performed. Complete follow-up data were available for twenty-eight of the thirty-five patients (thirty-six fractures) entered into the study. RESULTS: In Part I of the study, two constructs, a statically locked twelve-millimeter-diameter Russell-Taylor femoral nail with two distal locking screws (RT-2) and a statically locked twelve-millimeter-diameter Zimmer femoral nail with two distal locking screws (Z-2), had significantly higher mean fatigue strengths (2171 and 2113 newtons, respectively) than all other constructs tested (p<0.001), but the strengths of these two constructs were not significantly different from each other. Constructs with only one distal locking screw demonstrated significantly lower (p<0.05) fatigue strengths than the two-screw constructs. These results suggest that full weight-bearing during the weeks immediately after insertion of the nail may be possible, even for patients who have a comminuted fracture of the femoral shaft. In Part II of the study, twenty-six of the twenty-eight patients were bearing full weight on the fractured limb or limbs at the six-week follow-up visit. All fractures united; only one of these needed an additional procedure (the removal of the screws five months after the insertion of the nail) to stimulate union. No loss of fixation, such as back-out or breakage of a locking screw or breakage or bending of the intramedullary nail, occurred. CONCLUSIONS: We concluded from this two-part investigation that immediate weight-bearing after stabilization of a comminuted fracture of the femoral shaft with a statically locked intramedullary nail is safe when the construct has a relatively high fatigue strength. Immediate weight-bearing after stabilization of a fracture of the femoral shaft permits patients who have multiple fractures of the extremity to walk and to participate in physical therapy earlier, possibly decreasing the duration of the hospital stay or reducing the need for prolonged rehabilitation on an inpatient basis.

Biomechanical comparison of fixation of type-I fractures of the lateral tibial plateau. Is the antiglide screw effective?

Type-I fractures of the lateral tibial plateau were simulated by osteotomy in 18 pairs of unembalmed cadaver tibiae. One fracture of each pair was fixed with two lag screws whereas the contralateral site was stabilised with three lag screws, or two lag screws plus an antiglide screw. The lateral plateau was displaced downwards using a servohydraulic materials testing machine and the resulting force and articular surface gap were recorded. Yield load was defined as the maximum load needed to create a 2.0 mm articular offset at the fracture line. The yield loads of the three-lag-screw (307 +/- 240 N) and antiglide constructs (342 +/- 249 N) were not significantly different from their two-screw control constructs (231 +/- 227 and 289 +/- 245 N, respectively). We concluded that adding an antiglide screw or a third lag screw did not provide any biomechanical advantage in stabilising these fractures.

Effect of modular head seating on the cement-stem interface strength of femoral prostheses.

This study tested the hypothesis that modular head seating in situ reduces the cement-stem interfacial strength. Femoral prostheses were implanted in eight pairs of cadaveric femurs; one femur of each pair received a stem for which the modular head was seated by dropping a weight from a given height (treated); the contralateral femur received a nonseated stem (control). Transverse sections were cut from four standardized locations on each implanted femur, and the yield shear stress and ultimate shear stress of the interface were determined from push-out tests. There was no significant difference in cement-stem interfacial strength between seated and nonseated prostheses. These results suggest that seating modular heads in situ has no deleterious effect on the acute interfacial strength of cemented femoral implants.

Biomechanical evaluation of transverse acetabular fracture fixation.

The purpose of this two-part biomechanical study was to evaluate various fixation methods for transverse acetabular fractures in a synthetic pelvic model. In Part 1, 40 transverse acetabular fractures were repaired with anterior column plating using 10-hole curved reconstructions plates with one of four screw configurations to evaluate the effect of screw placement and number on fracture fixation stiffness. In Part 2, 36 transverse acetabular fractures were repaired with one of six fixation methods using combinations of contoured plates and column screws to stabilize the anterior column, the posterior column, or both. Each repaired acetabulum was loaded via a hemiarthroplasty in a direction consistent with stance phase. Fixation stiffness was measured from the force-displacement curve for each construct. In Part 1, there was no significant difference in fixation stiffness afforded by any of the constructs. However, the stiffest construct resulted from two screws on each side of the fracture site: one placed as close to the fracture site as allowed (one empty screw hole adjacent to the fracture) and the second at the end of the plate. In Part 2, the constructs that concomitantly stabilized anterior and posterior columns were significantly stiffer than were those addressing either the anterior or posterior column alone, regardless of the number of plates applied. The stiffest construct combined a posterior column plate with an anterior column screw. Because no significant change in stiffness occurred with the addition of a third set of screws, two screws on each side of the fracture site appear to provide sufficient stability with acetabular plating. Concurrent fixation of anterior and posterior columns of transverse acetabular fractures provides the greatest resistance to postoperative loss of reduction in this model.

Biomechanical evaluation of tension band placement for the repair of olecranon fractures.

Displaced transverse fractures of the olecranon commonly are treated by open reduction and internal fixation using the AO tension band wiring technique. Reports that the AO technique has a tendency to open the fracture site at the articular surface prompted Rowland and Burkhart to modify placement of the tension band. The present study tested the hypothesis that the modified wire placement provides static compression anteriorly, and hence better reduction at the articular surface of the fracture, than the AO technique under static conditions. Transverse olecranon fractures were created on 8 pairs of fresh cadaveric arms. One ulna of each pair was repaired using the modified wire placement, whereas the contralateral ulna was repaired using the AO technique. The humerus was driven into the trochlear fossa of each ulna using a servohydraulic testing machine while a force transducer and video system measured the applied force and gap formation at the articular surface of the fracture. Because the static behavior of the fixations was tested, no muscle forces were included. Results indicated no significant differences in yield loads or stiffness values between the 2 techniques. Based on the results of this static study, the modified wire placement does not provide increased stability of fracture fixation compared with the AO tension band wiring technique.

Regional mechanical properties of passive myocardium.

There is considerable interest in calculating regional stresses in the heart. This, in turn, requires complete information on regional material properties which, surprisingly, is not available. The specific aim of this work, therefore, was to determine if transmural differences exist in the mechanical behavior of passive myocardium in the equatorial region of the heart. Thus, we performed in vitro biaxial experiments on 28 thin, rectangular slabs of noncontracting myocardium excised from four regions within canine hearts: the middle portion of the interventricular septum (n = 8), and the inner (n = 5), middle (n = 9) and outer (n = 6) layers of the lateral left ventricular (LV) free wall. There were three major findings. First, an existing three-dimensional constitutive relation described the nonlinear and anisotropic behavior exhibited in the four regions equally well. Second, the anisotropy was similar in each region. Third, there were, however, regional differences in the strain-energy stored by specimens during identical finite deformations. In particular, specimens from inner and outer portions of the LV free wall tended to be stiffer than those from the middle of the LV free wall and septum. These findings, together with previous results on excised epicardium, suggest that the mechanical properties of the heart are qualitatively similar from region to region, but quantitatively different.