Comparison of eccentric and concentric screw placement for hamstring graft fixation in the tibial tunnel.

Interference screw fixation of four-strand hamstring grafts for ACL reconstruction has recently been introduced. By this method, the interference screw is placed in the tibial and femoral tunnels eccentric (adjacent) to the bundled limbs of the graft. In order to maximize the graft to tunnel contact to promote biological fixation, it is proposed to place the screw concentrically in the tunnel, in the middle of the four limbs of the graft, pressing each limb of the graft into the tunnel wall. This would be difficult to do in the proximal, folded end of the four limb graft situated in the femoral tunnel but can be done easily in the tibial tunnel. The purpose of this study was to evaluate the effect of screw placement on the stiffness, yield load, and ultimate load of hamstring graft fixation in the tibial tunnel. Five pairs of human knees were used for the study. Pull out tests were performed using an MTS system, pulling along the axis of the tibial tunnel. Tibial fixation stiffness was greater using concentric screw placement (P < 0.05) although there was no statistical difference in yield load, slippage, or ultimate load.

A biomechanical analysis of a medial unloading brace for osteoarthritis in the knee.

OBJECTIVES: The goals of the study were to measure the force applied to the lateral side of the knee by a valgus loading brace designed for patients with medial compartment osteoarthritis (OA) and to compare the varus moment at the knee during level gait with and without the brace. METHODS: Five subjects diagnosed with medial compartment OA were fitted with a custom Monarch valgus loading knee brace. A 3-dimensional videobased motion analysis system and force plate information were used to calculate forces and moments at the knee. An instrumented condylar bladder was used to determine the force applied to the knee by the brace. The varus moments for the braced and unbraced trials were compared during gait at 15%, 20%, 25%, and 30% of stance. RESULTS: The Monarch brace significantly reduced the varus moment at 20% and 25% of stance. The valgus force measured with the custom condylar bladder remained fairly constant throughout the first 80% of the stance phase. CONCLUSIONS: The reduced various moment observed for the braced condition demonstrates the biomechanical function of the brace in 5 subjects and may contribute to a reduction of pain for patients with medial compartment OA.

Mechanisms of anterior cruciate ligament neovascularization and ligamentization.

Columbia-Rambouillet cross-bred sheep were used to study the revascularization and ligamentization process of anterior cruciate ligament (ACL) reconstruction over a 6-month period using basic histology, immunohistochemistry, and electron microscopy. The reconstruction technique studied was a quadruple-hamstring, interference screw fixation technique. Further, these specimens, after retrieval at 6, 12, and 26 weeks, were compared with human arthroscopic 'second looks' and with 10 en bloc specimens obtained when a cruciate-sacrificing total knee replacement was performed. The study showed that, with this reconstruction technique, Sharpey's fibers were seen at 6 weeks in both sheep and human specimens. The intratunnel specimens showed proliferative chondrification, then ossification of the matrix. Intra-articular neovascularization, ligamentization, and junction ossification occurred. Myoblasts or smooth muscle cells appear to mediate the ligamentization as evidenced in electron microscopy by proliferate collagen manufacture. These myoblasts were seen in both the healing sheep and human second looks, but not seen in mature ACL grafts or in normal ACLs. At 6 months postoperatively, the sheep ACL reconstruction appeared clinically, histologically, and immunohistochemically indistinguishable from the normal sheep ACL. A correlation of this work with published animal studies in which biomechanical testing was performed and with human 'second looks' would imply that an ACL reconstruction may be vulnerable during this period of neovascularization and ligamentization.

Effect of test environment on intervertebral disc hydration.

STUDY DESIGN: Water content of fresh human lumbar intervertebral discs (with adjacent endplates) was assessed in three studies: 1) after each of seven specimen preparation steps. 2) during exposure to either saline spray or a saline bath, and 3) during exposure to a saline bath and 445 N axial compression, either without or with previous exposure to the bath and no compression ("free swelling"). OBJECTIVE: To assess the effect on disc hydration of various aspects of specimen preparation and testing environments. SUMMARY OF BACKGROUND DATA: Water content is an important determinant of disc behavior. Specimen preparation method and testing environments may be important determinants of water content, yet no work appears to have been reported specifically on this topic. METHODS: Endplate-disc-endplate specimens were prepared from refrigerated cadavers within 24 hours of death by transverse sectioning of adjacent vertebral bodies. Water content change was determined by specimen weight change across each time interval of interest. RESULTS: Specimen preparation (including multiple freeze-thaw cycles) produced no water content change. Saline spray and plastic film wrap resulted in no change, but saline bath exposure resulted in a 24% increase over 7 hours, 44% of which occurred in the first 0.5 hour. A subsequent 7 hours of 445 N compression reduced the overall increase to 10%. This was not significantly different from the 8% increase that resulted from initial exposure to saline bath and compression. CONCLUSIONS: Specimen preparation as typically performed and specimen exposure to saline spray and plastic film wrap do not result in hydration change. Exposure to saline bath results in substantial swelling, which can either be reversed or prevented by axial compression in the physiologic range. Whether discs exposed to saline spray and wrap without compression and those exposed to saline bath with compression behave the same and which of these more closely mimics the in vivo condition are important issues for the experimentalist to test.

A poroelastic finite element formulation including transport and swelling in soft tissue structures.

A field theory is presented for the study of swelling in soft tissue structures that are modeled as poroelastic materials. As a first approximation, soft tissues are assumed to be linear isotropic materials undergoing infinitesimal strains. Material properties are identified that are necessary for the solution of initial boundary value problems where swelling and convection are significant. A finite element model is developed that includes the solid displacements, the relative fluid displacements, and a representative concentration as the primary unknowns. A numerical example is presented based on a triphasic model. The finite model simulates a typical experimental protocol for soft tissue testing and demonstrates the interaction and coupling associated with relative fluid motion and swelling in a deforming poroelastic materiaL The theory and finite element model provide a starting point for nonlinear porohyperelastic transport-swelling analyses of soft tissue structures that include finite strains in anisotropic materials.

Comparative evaluation of bone suture anchor to bone tunnel fixation of tibialis anterior tendon in cadaveric cuboid bone: a biomechanical investigation.

A multiphase biomechanical study was performed using human tibialis anterior tendons and cuboid bones, comparing the fixation of the tendon to the bone using bone anchors and bone tunnels. Twenty-six specimens were tested for ultimate load to failure comparing Mitek Superanchor fixation with no. 1 and no. 5 braided polyethelyne suture to bone tunnel fixation. Mitek Superanchor with no. 5 suture failed at 223 N, compared with Mitek Superanchor with no. 1 suture at 134 N and bone tunnel at 143 N (P = 0.033). Mitek with no. 1 suture versus bone tunnel was not significantly different. The Mitek with no. 5 suture failed at the tendon/suture interface (75%), the Mitek with no. 1 suture failed at the suture/anchor interface (56%), and bone tunnel fixation failed most commonly by fracture of the tunnel (76%). This study is the first biomechanical analysis of the pullout strengths of bone tunnels or suture anchors in the cuboid bone. We have shown that the suture anchor has a pullout strength comparable or superior to a conventional bone tunnel in an in vitro situation. We believe it is a viable alternative to fixation of the tibialis anterior tendon to the cuboid when there is insufficient tendon length or failure of the bone tunnel.

A dynamic material parameter estimation procedure for soft tissue using a poroelastic finite element model.

A three-dimensional finite element model for a poroelastic medium has been coupled with a least squares parameter estimation method for the purpose of assessing material properties based on intradiscal displacement and reactive forces. Parameter optimization may be based on either load or displacement control experiments. In this paper we present the basis of the finite element model and the parameter estimation process. The method is then applied to a test problem and the computational behavior is discussed. Sequential optimization on different parameter groups was found to have superior convergence properties. Some guidelines for choosing the starting parameter values for optimization were deduced by considering the form of the objective function. For load control experiments, in which displacement data is used for the optimization, the starting values for the elastic modulus should be lower in magnitude than an "anticipated" modulus. The permeability starting values should be higher than an anticipated permeability. For displacement control experiments, the reverse is true. The optimization scheme was also tested on data with random variations.

A poroelastic-swelling finite element model with application to the intervertebral disc.

The swelling process that occurs in soft tissue is incorporated into a poroelastic finite element model. The model is applied to a spinal segment consisting of two vertebrae and a single intervertebral disc. The theory is an extension of the poroelastic theory developed by Biot and the model is an adaptation of an axisymmetric poroelastic finite element model of the intervertebral disc by Simon. The model is completely three-dimensional although the results presented here assume symmetry about the sagittal plane. The theory is presented in two stages. First the development of the poroelastic theory. Following this, the effects of swelling caused by osmotic pressure are developed and expressed as a modification of the constitutive law and initial stresses. In the case of the disc, this pressure is produced mainly by the fixed negative charges on the proteoglycans within the disc. In this development we assume that the number of fixed charges remains constant over time and that the distribution of mobile ions has reached equilibrium. The variations over time in osmotic pressure, and thus in swelling effects are therefore only dependent on the initial state and the change in water content. Variations of the swelling effects caused by changes in mobile ion concentrations will be the subject of a future paper. The results reported in this article illustrate the dramatic effect of swelling on the load carrying mechanisms in the disc. The authors believe it is likely that this will have important useful implications for our understanding not only of normal disc function, but also of abnormal function, such as disc degeneration, herniation, and others.