A preliminary biomechanical study of cyclic preconditioning effects on canine cadaveric whole femurs.

Biomechanical preconditioning of biological specimens by cyclic loading is routinely done presumably to stabilize properties prior to the main phase of a study. However, no prior studies have actually measured these effects for whole bone of any kind. The aim of this study, therefore, was to quantify these effects for whole bones. Fourteen matched pairs of fresh-frozen intact cadaveric canine femurs were sinusoidally loaded in 4-point bending from 50 N to 300 N at 1 Hz for 25 cycles. All femurs were tested in both anteroposterior (AP) and mediolateral (ML) bending planes. Bending stiffness (i.e., slope of the force-vs-displacement curve) and linearity R(2) (i.e., coefficient of determination) of each loading cycle were measured and compared statistically to determine the effect of limb side, cycle number, and bending plane. Stiffnesses rose from 809.7 to 867.7 N/mm (AP, left), 847.3 to 915.6 N/mm (AP, right), 829.2 to 892.5 N/mm (AP, combined), 538.7 to 580.4 N/mm (ML, left), 568.9 to 613.8 N/mm (ML, right), and 553.8 to 597.1 N/mm (ML, combined). Linearity R(2) rose from 0.96 to 0.99 (AP, left), 0.97 to 0.99 (AP, right), 0.96 to 0.99 (AP, combined), 0.95 to 0.98 (ML, left), 0.94 to 0.98 (ML, right), and 0.95 to 0.98 (ML, combined). Stiffness and linearity R(2) versus cycle number were well-described by exponential curves whose values leveled off, respectively, starting at 12 and 5 cycles. For stiffness, there were no statistical differences for left versus right femurs (p = 0.166), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p < 0.0001). Similarly, for linearity, no statistical differences were noted due to limb side (p = 0.533), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p = 0.006). A minimum of 12 preconditioning cycles was needed to fully stabilize both the stiffness and linearity of the canine femurs. This is the first study to measure the effects of mechanical preconditioning on whole bones, having some practical implications on research practices.

Biomechanical measurements of torsion-tension coupling in human cadaveric femurs.

The mechanical behavior of human femurs has been described in the literature with regard to torsion and tension but only as independent measurements. However, in this study, human femurs were subjected to torsion to determine if a simultaneous axial tensile load was generated. Fresh frozen human femurs (n=25) were harvested and stripped of soft tissue. Each femur was mounted rigidly in a specially designed test jig and remained at a fixed axial length during all experiments. Femurs were subjected to external and internal rotation applied at a constant angulation rate of 0.1 deg/s to a maximum torque of 12 N m. Applied torque and generated axial tension were monitored simultaneously. Outcome measurements were extracted from torsion-versus-tension graphs. There was a strong relationship between applied torsion and the resulting tension for external rotation tests (torsion/tension ratio=551.7±283.8 mm, R(2)=0.83±0.20, n=25), internal rotation tests (torsion/tension ratio=495.3±233.1 mm, R(2)=0.87±0.17, n=24), left femurs (torsion/tension ratio=542.2±262.4 mm, R(2)=0.88±0.13, n=24), and right femurs (torsion/tension ratio=506.7±260.0 mm, R(2)=0.82±0.22, n=25). No statistically significant differences were found for external versus internal rotation groups or for left versus right femurs when comparing torsion/tension ratios (p=0.85) or R(2) values (p=0.54). A strongly coupled linear relationship between torsion and tension for human femurs was exhibited. This suggests an interplay between these two factors during activities of daily living and injury processes.

The effect of cement mixing time on the biomechanics of cement augmented plated fractures in canine femora.

OBJECTIVE: The goal of this study was to determine the effect of cement mixing time and, hence, cement viscosity on the biomechanical behavior of femoral fracture fixation. DESIGN: Cadaveric plated canine femoral fracture model, comparing treatments in matched pairs. SETTING: Orthopaedic biomechanics laboratory. INTERVENTION: Cement was inserted both as a liquid and as a paste in standard and oversized screw holes to augment fixation with plates and screws. MAIN OUTCOME MEASUREMENTS: Standard 4-point bending tests were performed to obtain stiffness and failure load values. RESULTS: Liquid cement had a 1.38 times increase in stiffness and a failure load 1.84 times greater compared with paste cement, regardless of hole size with a gap at the fracture site (P < 0.05). Liquid cement had a force to failure of 1.77 and 1.91 times in the standard-sized and oversized holes, respectively, when compared with paste cement (P < 0.05). CONCLUSIONS: When the cement was inserted in a liquid state in a plated femoral diaphyseal fracture with a gap, screw purchase augmentation achieved greater bending stiffness and resisted a greater failure load.

The influence of the number of cortices of screw purchase and ankle position in Weber C ankle fracture fixation.

OBJECTIVES: Biomechanical and clinical studies have shown that syndesmosis screws may be indicated in repairing Weber C ankle fractures. This study sought to determine the effect of the number of cortices of screw purchase and ankle position on syndesmosis width and tibiotalar rotation in Weber C ankle fracture fixation. METHODS: Nine pairs of human cadaver legs were mechanically tested to determine syndesmosis width and tibiotalar rotation. This was done for intact specimens and after a Weber C injury was created and repaired with 3 and 4 cortices of purchase. Tests were performed for no axial load and for axial loads of 700 N with and without external torques of 1 and 5 Nm on the ankle. Torque-to-failure tests were also done for 4 cortices of fixation. RESULTS: In comparison to baseline, the syndesmosis width was significantly decreased when the syndesmosis screw was inserted in plantarflexion with either 0 or 1 Nm of torque. Syndesmosis width significantly increased when the screw was inserted in dorsiflexion for 5 Nm of torque. For tibiotalar rotation, no statistical differences were detected for either plantarflexion or dorsiflexion when compared to baseline, except with axial load. Syndesmosis width was not affected by the number of cortices purchased by the syndesmosis screw. Failure torque and failure angle were also measured. CONCLUSIONS: Because no difference was seen between 3 or 4 cortices, it is the surgeon's choice in determining how many cortices of fixation are achieved.

Methods of operative fixation of the acromio-clavicular joint: a biomechanical comparison.

PURPOSE: Three different methods of fixation used in acute disruption of the acromio-clavicular (AC) joint-namely, the coraco-clavicular Bosworth screw (CC Screw), a coraco-clavicular sling of Mersilene #5 tape (CC Sling), and a Hook Plate-were compared to baseline to see which could most closely replicate the stiffness of healthy cadaveric AC specimens (Intact). HYPOTHESIS: It is hypothesized that the Hook Plate method, as compared with the other reconstructions tested, will be most similar mechanically to the intact AC joint with respect to present outcome measures. METHODS: Five matched pairs of fresh-frozen cadaveric specimens were tested. Stiffness was tested with superior cyclic loads to 70 N. The stiffness for each specimen was initially tested with all the ligaments in place (Intact). The AC and CC ligaments were then sectioned, and stiffness was tested, in varying order, with reconstructions using the CC Screw, the CC Sling, and the Hook Plate. Failure testing consisted of taking either the CC Screw or Hook Plate to failure within each matched pair. RESULTS: The CC Screw and the CC Sling, respectively, showed stiffnesses of 46 +/- 23 N/mm and 15 +/- 8 N/mm, which was significantly different from the Intact specimen (P < 0.05). The Hook Plate had a stiffness of 26 +/- 17 N/mm, most comparable to the Intact joint stiffness of 25 +/- 8 N/mm (P = 0.785). With failure testing, the CC Screw failed at a significantly higher load than the Hook Plate (744 +/- 184 N vs 459 +/- 188 N) (P = 0.034). CONCLUSION: The CC Screw demonstrated the greatest stiffness with repetitive loading to 70 N. The Hook Plate had a stiffness most similar to the normal physiologic state of the AC joint. The CC Sling was significantly less stiff than the Intact joint or the other methods of fixation. SIGNIFICANCE: Although the stiffest construct is the CC Screw, Hook Plate fixation allows physiologic motion without pathological deformation and most closely resembles the stiffness of the native AC joint for the current test procedure used.