Mark A. Gomez

The biomechanical and morphological changes in the medial collateral ligament of the rabbit after immobilization and remobilization.

The effects of immobilization and remobilization on the biomechanical and morphological properties of the femur-medial collateral ligament-tibia complex and each of its components were investigated in the rabbit. Specimens that had been obtained after periods of unilateral immobilization of the knee and remobilization were evaluated for structural properties. In addition, the mechanical properties of the substance of the medial collateral ligament and the histological characteristics of both the substance of the ligament and its sites of insertion were evaluated. After immobilization, there were significant reductions in the ultimate load and energy-absorbing capabilities of the bone-ligament complex, and an increased number of failures occurred by tibial avulsion. The tissue of the medial collateral ligament also became less stiff as a result of immobilization. Histologically, the femoral and tibial insertion sites showed increased osteoclastic activity, resorption of bone, and disruption of the normal attachment of the bone to the ligament. With remobilization, the ultimate load and energy-absorbing capabilities of the bone-ligament complex improved but did not return to normal. Failure by tibial avulsion became less frequent, and the stress-strain characteristics of the medial collateral ligament returned to normal. Histologically, the sites of insertion of the ligament also showed evidence of recovery.

The Time and History-Dependent Viscoelastic Properties of the Canine Medial Collateral Ligament

The viscoelastic properties of the canine medial collateral ligament (MCL) were investigated. Stress-strain relationships at different strain rates, long-term stress relaxation and cyclic stress-strain curves of the MCL were obtained experimentally using a bone-MCL-bone preparation. The experimental data were used in conjunction with the quasi-linear viscoelastic theory as proposed by Fung [15] to characterize the reduced relaxation function, G(t) and elastic response sigma e (epsilon) of this tissue. It was found that the quasi-linear viscoelastic theory can adequately describe the time and history-dependent rheological properties of the canine medial collateral ligament.

Treatment of the medial collateral ligament injury II: Structure and function of canine knees in response to differing treatment regimens

In order to assess the healing of the medial collateral ligament (MCL) and to detect the various effects of treatment regimens, in vivo animal experiments using a canine model were performed. Thirty-five canine MCLs were surgically transected and treated using three clinically popular regimens, e.g., no repair with cage and farm activities (Group 1), repair with 3 weeks immobilization (Group 2), and repair with 6 weeks im mobilization (Group 3). The varus-valgus laxity of the knee joint, structural properties of the femur-MCL-tibia (FMT) complex and the mechanical properties of the MCL substance (healing site) were quantitatively meas ured at 6, 12, and 48 weeks postoperatively. It was found that Group 1 animals had the best results. The varus-valgus laxity of the knee joint and the structural properties of the FMT complex returned to values comparable with the contralateral control by 12 weeks. The recovery of the mechanical properties of the MCL substance was slower and not complete, even at 48 weeks. In confirmation with previous studies, prolonged immobilization was shown to have deleterious effects on MCL healing. The results of this study indicated that early mobilization is the treatment of choice in cases of isolated MCL injury. Also, this study emphasized the importance and effectiveness of using various biome chanical parameters in addition to the conventional ultimate values at failure to evaluate the progress of soft tissue repair.

Effects of knee flexion on the structural properties of the rabbit femur-anterior cruciate ligament-tibia complex (FATC)

Many studies have been conducted to determine the biomechanical properties of the anterior cruciate ligament (ACL). The method of holding the femur-ACL-tibia complex (FATC) test specimen, the strain rate applied, the angle of knee flexion and the direction of the applied loads have an important effect on the outcome. It is felt that the tensile properties and strength of the ligament should be measured by applying the tensile force along the axis of the ligament. A versatile clamp was designed to accomplish this purpose. Fifty-seven rabbit knee specimens were tested at angles of flexion of 0 degrees, 30 degrees or 90 degrees. In addition, a comparative study of 25 pairs of rabbit legs were performed, whereby loading was either along the ligament or along the tibial axis. Cyclic hysteresis, ultimate load, energy absorbed, and stiffness were determined. The ultimate load values for the FATC decreased with increased knee flexion for those loaded along the tibial axis, while no such change was detected for FATC tested along the ligament axis. Other structural properties measured followed similar trends. It is concluded that the structural properties of the rabbit FATC change minimally with knee flexion (from 0 to 90 degrees) when loaded along the ligament axis, but decrease significantly with knee flexion when loaded along the axis of the tibia. Therefore, the data obtained in this field of study can be compared only if the direction of loading with respect to the ACL is similar.

Measurement of Changes in Ligament Tension with Knee Motion and Skeletal Maturation

This study was designed to determine the in situ strains, stresses, and loads in the medial collateral ligament (MCL) of skeletally immature and mature rabbits. Using a noncontact method, the magnitudes of the in situ strains were first determined as a function of knee flexion angle. The MCL was divided into three anatomical regions (anterior, middle, and posterior) across its width. For strain measurements, the variation of a gauge length in these regions was obtained in the intact knee at 60, 90, and 120 deg of flexion. Subsequently, all soft tissues around the knee were dissected away, leaving the femur-MCL-tibia (FMT) complex. The MCL was allowed to retract freely and the new length, called the zero length, was measured. From this, the in situ strains were determined. To obtain the stress-strain relationship of the FMT complex, the specimens were subjected to tensile testing. Knowing the in situ strains and the stress-strain relationship, the in situ stresses in the three anatomical regions of the MCL were determined as a function of knee flexion angle. Multiplying these stresses by 1/3 of the cross-sectional area and summing the loads thus calculated, the in situ loads of the MCL were obtained. Our data suggest that the in situ load in the MCL is not large within the range of knee flexion angles studied, i.e., 1.4 to 2.7 N for the skeletally immature animals and 3.0 to 5.8 N for the skeletally mature animals. An increase in the in situ load with skeletal maturation was demonstrated.

Thoracic vertebral screw impingement on the aorta in an in vivo bovine model

Study Design. A bovine model was used to evaluate the effects of thoracic vertebral screw impingement of the aorta. Objectives. To evaluate the histologic and biomechanical changes in aortic wall tissue that was severely impinged by abutting instrumentation. Summary of Background Data. Case reports of vascular injury associated with spinal instrumentation generally describe intraoperative injury; some report delayed presentation of large vessel damage. Risks associated with placing instrumentation adjacent to large vessels are largely unknown. Methods. Six 1-month-old calves underwent left-sided thoracotomies, exposing the anterior thoracic spine and aorta. With the heads removed, screws were inserted in reverse fashion into T6 through T11, leaving the screw tips 1 cm proud and abutting the aorta. After 3, 6, or 12 months (2 calves each), the spines were resected with the adjacent aorta and underwent radiographic, histologic, and biomechanical testing. Results. Computed tomography revealed varying degrees of vessel impingement. Although there were no frank ruptures, 96% of aortic specimens showed histopathologic changes, including 52% with wall thinning; 43% were no longer impinged, yet 60% of these had increased collagen (scar). Biomechanical testing of screw-impinged aortas demonstrated a lower failure stress (1.2 ± 0.5 N/mm2 vs. 1.8 ± 0.4 N/mm2, P = 0.016) but no difference in failure strain (42 ± 9% vs. 32 ± 10%, P = 0.06) than controls. Conclusions. Major impingement of vertebral screws on the aorta caused acute and chronic histopathologic and biomechanical changes in the vessel wall. This model represents a severe form of vessel penetration by a screw that confirms such a “worst case” scenario results in marked compromise of the vessel wall integrity. The sequelae of less severe impingement are unknown.

Biomechanics of Bone

Bone is a dynamic, living tissue. As a material as well as a structure, it is in a constant state of flux. Consequently, the ability of bone to resist an applied load before failure or fracture is dependent on multiple factors. Age, disease, hormone levels, too little load, too much load, or even the direction in which a load is applied can all influence the biomechanical properties of bone. In particular, the potential of the bone to resist fracture is affected by these physiologic and mechanical sources. This chapter defines the biomechanical properties of bone, first by explaining both its micro- and macroanatomic components, and second by describing how its structural and mechanical properties relate to this anatomy. A thorough understanding of this relationship allows one to consider how the aforementioned factors change the ability of bone to withstand an applied load.

Medial Collateral Ligament Healing: a Biomechanical Assessment

Many investigations have shown the medial collateral ligament (MCL) to play a major role in restricting valgus rotation of the knee (1–4). The healing of collateral ligament injury is therefore of clinical importance but the best treatment regimen remains controversial. Different treatments of the MCL include primary surgical repair with subsequent immobilization or early mobilization and nonoperative treatment with varying degrees of postoperative motion or physical activity. However, a major problem that remains with the evaluation of the results of a particular clinical treatment lies in the qualitative nature of the orthopaedic examination. Manually conducted tests of ligament function are subject to both personal technique and interpretation. Therefore, the objective of this study is to establish an experimental model of the healing canine MCL following common clinical treatment regimens. The healing MCL is evaluated as a static stabilizer of the knee in the varus-valgus direction and both the structural properties of the femur-MCL-tibia complex and the mechanical properties of the healing MCL substance are measured.