John P. Holden

In vivo forces in the anterior cruciate ligament: Direct measurements during walking and trotting in a quadruped

In vivo forces in the anterior cruciate ligament (ACL) were measured in three adult goats during quiet standing and during gait (walking or trotting). A modified pressure transducer (MPT) was implanted within the anteromedial band of the ligament to make direct measurements of ACL force. One or two days following implantation, measurements were made of ACL force, knee joint flexion angle, ground reaction forces, and speed of locomotion. MPT calibration was performed in vitro using anteroposterior displacement tests at six flexion angles. The ACL was loaded during quiet standing (30-61 N) and during the stance phase of gait. Peak ACL forces were achieved within the first 40% of stance, with magnitudes ranging from 63 to 124 N during walking and from 102 to 150 N during trotting. The average ACL force during the stance phase ranged from 34 to 68 N while walking and from 46 to 69 N while trotting. The partial correlations between peak ACL force and speed, and between average ACL force and speed, were both statistically different from zero (p < 0.01). ACL forces dropped to zero during the swing phase in all trials. ACL forces were less than 15 N throughout swing in two of the animals, both of which did not extend their knees during gait beyond 20 degrees from full extension. In the animal which did show knee extension beyond 40 degrees (20 degrees from full extension), ACL loading occurred during late swing. The magnitude of the peak ACL force during late swing was significantly correlated with the extent of knee extension in this animal.

Anterior cruciate ligament reconstruction using bone-patellar tendon-bone allografts A biological and biomechanical evaluation in goats

Twenty-eight goats underwent ACL reconstruction with freeze-dried bone-patellar tendon-bone allografts in one knee, the opposite knee serving as a control. One group of 16 knees was evaluated, in groups of four, at 6, 12, 26, and 52 weeks by histologic and vascular injection techniques. The other group of 12 knees was evaluated in two groups of six at 26 and 52 weeks by morpholog ical and biomechanical techniques of analysis. Within the first 12 weeks these allografts were revas cularized ; in the first 26 weeks they had matured to resemble normal connective tissue. Graft stiffness was 29% of the control value and maximum force to failure was 43% of the control value. The results of this study indicated that freeze-dried bone-patellar tendon-bone allografts are biomechani cally and biologically similar to patellar tendon auto- grafts.

The effect of variation in knee center location estimates on net knee joint moments

To test the sensitivity of knee flexion-extension moment patterns to variation in estimates of the knee center location (KCL), inverse dynamics analysis was performed using three different KCLs for data collected from 18 healthy adult subjects who walked at five different speeds (25%, 50%, 75%, 100% and 125% of 0.785 statures.s< ?? opnbop ?? ?-1). The KCL, which was determined using data from static subject calibration trials, was varied in software by plus or minus 10 mm in the anteroposterior direction. The effect of this KCL variation on knee moments was similar across subjects and at all five walking speeds. The relative knee center variation effect, expressed as a percentage of the mean knee moment, was progressively greater at slower walking speeds, up to 123% of the first extensor peak at the slowest speed. The implication of these results is that, while KCL variation does not affect the general shape of the moment pattern at more natural walking speeds, it can change the sign of the moment-and thus the interpretation-when the moment magnitude is small (e.g. at the very slow speeds used by some patients). When knee moments are of magnitudes less than or equal to the KCL variation effect, one cannot confidently interpret them as representing either a net flexor or extensor knee control strategy. Copyright 1998 Elsevier Science B.V.

The use of an implantable force transducer to measure patellar tendon forces in goats

Patellar tendon (PT) force was measured during activity with an implantable force transducer (IFT) in adult goats. PT force, vertical ground reaction force (VGRF) and the animals speed were recorded for standing, walking and trotting. Following data collection, animals were euthanized and the IFT calibrated in vitro. Standing PT force averaged 207 N. Maximum PT force was approximately 800 N for walking and 1000 N for trotting and occurred at mid-stance. PT force dropped from 200 N at toe-off to 0 N by mid-swing. For each activity, the PT force increased with increases in VGRF. Maximum in vivo PT stress occurred during trotting and measured 29 MPa. This study demonstrates the IFTs usefulness in measuring tendon force directly.

The effects of processing techniques on the mechanical properties of bone-anterior cruciate ligament-bone allografts An experimental study in goats

There has been a growing interest in the use of allo grafts as ACL substitutes. Allografts are often freeze dried to increase shelf storage time and sterilized with ethylene oxide. This study was conducted to determine the effect of a specific ethylene oxide sterilization pro cedure and freeze drying process on the initial mechan ical properties of femur-ACL-tibia preparations. Twelve knees (stifle joints) from six mature goats were divided into two groups (one knee of pair to each group). Knees were cleaned of all soft tissue except for the anterior cruciate, posterior cruciate, and collateral ligaments. Group 1 was sterilized with ethylene oxide (simulating clean procurement) then freeze dried. Group 2 was freeze dried only (simulating sterile procurement). The knees were rehydrated and then tested in tension to failure to determine their structural mechanical proper ties. The ethylene oxide-freeze dried specimens (Group 1) had a mean maximum load before failure of 2059 ± 273 newtons (N) (± SE) which was not statistically different than the maximum load of the freeze dried specimens (Group 2) of 2023 ± 214 N. The average strength of Group 1 and 2 combined was not signifi cantly different than a third group of 12 normal femur- ACL-tibia controls which had an average maximum force of 2403 ± 133 N. No significant differences be tween the groups were observed for stiffness, energy to maximum strength, or elongation to maximum force. It appears the freeze dry processing, with or without ethylene oxide sterilization, using the procedure we used, has, at most, a small effect on the initial mechan ical properties of the preparations. The low mechanical properties of implanted and revascularized allografts, as compared to normal femur-ACL-tibia complex ap pears to be related more to fixation strength and alter ations which occur in the ligament substance after implantation.

FACTORS AFFECTING SENSITIVITY OF A TRANSDUCER FOR MEASURING ANTERIOR CRUCIATE LIGAMENT FORCE

In order to determine the measurements and calibration methods necessary to accurately measure in vivo forces in the anterior cruciate ligament (ACL) of the goat, an in vitro study was conducted to evaluate the effect of several factors that could influence the sensitivity of a transducer implanted within the ligament. Four factors were studied in six specimens: flexion angle [0 degrees, 10 degrees, 30 degrees, 50 degrees, and 70 degrees from full extension (FFE)]; tibial rotation (0 degrees and 10 degrees of internal rotation at 30 degrees, 50 degrees, and 70 degrees flexion FFE); loading rate (cycling frequencies of 0.2, 0.5, 1.0, and 2.0 Hz); and temperature (22 degrees C and 37 degrees C). Anteroposterior tibial displacements were applied to the specimens following tissue resection to isolate the ACL. The resultant ACL force magnitude was measured with a multi-component load cell, and transducer sensitivity was calculated as the slope of the output vs force curve in the linear response region. Transducer sensitivity varied with joint position in each specimen, but there was no consistent trend from specimen to specimen in how the sensitivity changed. As a result, there were no statistically significant mean differences (p > 0.05). There were no significant differences and little variation in sensitivity due to changes in either loading rate or tissue temperature, although the latter produced a voltage offset. The results show that the transducer output with zero force on the ligament must be determined in vivo, after which in vitro calibrations may be conducted at room temperature.(ABSTRACT TRUNCATED AT 250 WORDS)