Tensile properties of the human femur-anterior cruciate ligament-tibia complex : The effects of specimen age and orientation

Virtually all types of collagenous tissues have been transferred in and around the knee joint for intra-articular and extra-articular ligament reconstructions. However, the mechanical properties (in particular, strength) of such grafts have not been determined in tissues from young adult donors, where age and disuse-related effects have been excluded. To provide this information, we subjected ligament graft tissues to high-strain-rate failure tests to determine their strength and elongation properties. The results were compared with the mechanical properties of anterior cruciate ligaments from a similar young-adult donor population. The study indicated that some graft tissues used in ligament reconstructions are markedly weak and therefore are at risk for elongation and failure at low forces. Grafts utilizing prepatellar retinacular tissues (as in certain anterior-cruciate reconstructions) and others in which a somewhat narrow width of fascia lata or distal iliotibial tract is utilized are included in this at-risk group. Wider grafts from the iliotibial tract or fascia lata would of course proportionally increase ultimate strength. The semitendinosus and gracilis tendons are stronger, having 70 and 49 per cent, respectively, of the initial strength of anterior cruciate ligaments. The bone-patellar tendon-bone graft (fourteen to fifteen millimeters wide, medial or central portion) was the strongest, with a mean strength of 159 to 168 per cent of that of anterior cruciate ligaments. Patellar tendon-bone units, based on grip-to-grip motions, were found to be three to four times stiffer than similarly gripped anterior cruciate ligaments, while gracilis and semitendinosus tendon preparations had values that were nearly identical to those of anterior cruciate ligaments.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanical properties of anterior cruciate bone-ligament-bone specimens from humans and rhesus monkeys were determined in tension to failure under high strain-rate conditions. The age range of the human specimens was from sixteen to eighty-six years. The values fro human specimens obtained from young adults with regard to elastic modulus, ultimate tensile stress, and strain energy to failure were approximately two to three times those for specimens from humans in the sixth decade and older. The major mode of failure was ligament disruption in the specimens from young adult humans and avulsion of bone beneath the ligament insertion site in the specimens from older humans. The difference in mode of failure correlated with histological observations of decreased bone mass at the site of ligament attachment in the specimens from older humans. Rhesus monkey specimens had higher values for elastic modulus, failure stress, and strain energy. Significant reductions in strength and stiffness properties of ligament units were shown to occur with advancing age to a greater degree than expected. All experiments in which specimens from older human cadavera are used should be interpreted with caution when the results are applied to mechanisms of ligament failure for younger or athletic individuals.

We performed instrumented measurement of anterior-posterior laxity of the knee in thirty-three cadaver specimens, 338 normal subjects, and eighty-nine patients with unilateral disruption of the anterior cruciate ligament. The test instrument was the Medmetric knee arthrometer, model KT-2000. We measured total anterior-posterior laxity, produced by anterior and posterior loads of eighty-nine newtons (twenty pounds), and the anterior compliance index. The total anterior-posterior laxity is composed of an anterior displacement and a posterior displacement; these are measured from a testing reference position, defined as the resting position of the knee after applying and then releasing a posterior load of eighty-nine newtons. The anterior compliance index is defined as the anterior displacement between an anterior load of sixty-seven newtons and one of eighty-nine newtons. All tests were performed with the knee held on a thigh support that placed the knee in 20 +/- 5 degrees of flexion. The mean anterior displacement at eighty-nine newtons was 5.7 millimeters in a group of normal subjects and 13.0 millimeters in a group of patients with a disrupted anterior cruciate ligament. Ninety-two per cent of the normal subjects had a left knee-right knee difference in anterior displacement of no more than two millimeters, while 96 per cent of the patients with a unilateral disruption of the anterior cruciate ligament had an injured knee-normal knee difference in anterior displacement of more than two millimeters. Ninety-three per cent of the normal subjects had a difference in the left-right compliance index of no more than 0.5 millimeter, and 85 per cent of the patients with unilateral disruption of the anterior cruciate ligament had a difference in the compliance index of the injured and normal sides of more than 0.5 millimeter.