Twenty human cadaver limbs (10 male, 10 female), sectioned at the midhumerus level, were used in the evaluation. The mean age at death in our study population was 65 years (63 years for males and 67 years for females), and there was no known injury or disease involving the upper extremities. No other information was available on the premorbid condition of the cadavers.
Of the 20 limbs evaluated, 11 were right and 9 were left. All arrived frozen and were maintained in that condition until the day of testing, at which time they were warmed to room temperature. Freezing has been shown to have no significant effect on the biomechanical properties of ligaments.
10,11
Prior to testing, investigators used a drill to place surgical pins (1.6 mm in diameter, approximately 50 mm in length) into each of the four carpal bones that would serve as attachment sites for the TCL: the scaphoid and pisiform on the proximal end of the carpal tunnel, and the trapezium and hamate on the distal end (
Figure 1 and
Figure 2). The pins were definitive markers above the surface of the skin, allowing investigators to measure the width of the TCA accurately, also providing data regarding the TCL length. Pin placement was verified by fluoroscopy. Additionally, three 4-mm diameter white balls were placed at precisely measured locations on each pin to delineate its long axis above skin level (
Figure 3).
Two 60 Hz video cameras were mounted to the ceiling of the testing lab looking diagonally down on the limb from the left and right. The camera-to-limb distances were approximately 2 m. Coordinates of the markers, as documented in the two-dimensional images captured on these cameras, were used to calculate the three-dimensional (3D) positions of the markers on each pin during application of, and recovery from, tensile loading using the direct linear transformation method.
12 Three-dimensional positions were calculated at set intervals throughout the two testing sequences. Details of the 3D methodology are forthcoming (R.N.H., B.M.S., R.L.W., et al, unpublished data, 2005).
Wires were attached to each pin with weights suspended over pulleys to provide horizontal distraction forces across the TCL, as previously described.
7 In part 1,
7 we used weight loads of 2 newtons (N), 4 N, and 8 N in static loading procedures. In response to results from part 1, however, we chose to increase all static loads in the present study to 10 N per pin.
Precision calipers were used to measure pin-to-pin distances just above skin level at certain elapsed-time intervals (measured in minutes) after the loads were applied: 0.5, 1, 3, 7, 15, 30, 60, 120, and 180. Caliper measurements were used to determine the time at which no further lengthening was achieved and equilibrium was assumed. For this reason, it was also sometimes necessary to measure pin-to-pin distances at 240 minutes. Typically, static loads were applied for 3 hours with maximal widening of the TCA obtained after 2 hours (ie, the measurement at 3 hours was the same as at 2 hours).
Once maximal elongation was achieved, static loads were removed and similar caliper measurements were taken during the recovery period. A typical recovery period lasted 2 hours. Following the recovery period, static loads were reapplied and then once again removed when equilibrium was reached using the aforementioned criteria. Equilibrium required approximately 2 hours to achieve in all cases with verification taking place at 3 hours.
The duration of the entire static loading sequence (ie, 2 cycles of static loading and unloading) was approximately 6 hours for each human cadaver limb.