Zobitz, M. E.

Knowledge of the adaptation of the soft tissue to mechanical factors and biomolecules would be essential to better understand the mechanism of tendon injury and to improve the outcome of tendon repair. The responses to these factors could be different for the distinct types of cells in the tendon: cells from the tendon sheath, fibroblasts from the epitenon surface, or fibroblasts from the internal endotenon. In this study, we examined the mechanical and histological characteristics of the rate of contraction of the collagen gel seeded with epitenon and endotenon fibroblasts. The rate of contraction and the mechanical property of the contracted construct depend on the gel concentration and also the treatment of TGF-β 1 .

Purpose. To evaluate the effect of prosthesis design on stress profile in the proximal femur after hip resurfacing. Methods. The von Mises stress profile of the native femur was simulated and compared with that of resurfaced femurs using various prosthetic materials (titanium, cobalt-chrome, ceramic), stem lengths (normal, half, short, and no stem), and femoral head coverage (shell size) [260°, 220°, 180°, and 140°]. Results. Hip resurfacing altered the stress profile of the cancellous (but not cortical) bone of the femoral neck. Maximal cortical stresses were observed at the posterior half of the medial femoral neck. The stress profile of the native femur was most similar to that of the resurfaced femur made of titanium, with a short or no stem and 260 degrees of femoral head coverage (shell size). Conclusion. Optimising prosthesis design by minimising biomechanical alterations seems a valid approach to achieving favourable long-term outcomes. Cadaveric and in vivo studies are needed to confirm the clinical relevance and feasibility.

As there is evidence that ligamentous laxity is affected by the female hormones, we hypothesized that hormonal changes occurring during pregnancy could have a therapeutic role in preventing the development of a joint contracture. Knee joint contractures were created in pregnant and nonpregnant rats. After 2 wk of immobilization, the degree of contracture was measured with structural properties of the medial collateral and anterior cruciate ligaments and the pubic symphysis. Although not statistically significant, there was a general trend toward reduced contracture in pregnant compared with nonpregnant rats. Cutting the posterior capsule significantly decreased contracture for both the pregnant and nonpregnant groups, confirming the contribution of capsular structures to contracture. Ultimate loads of the medial collateral and anterior cruciate ligaments significantly decreased after immobilization compared with control, but there was no significant effect due to pregnancy. Stiffness and ultimate load of the pubic symphysis were not significantly different between pregnant and nonpregnant groups. The trend toward reduced contracture with pregnancy points toward a possible therapeutic role for female hormones in the prevention of postoperative and/or posttraumatic joint contracture.

The purpose of this study was to assess tendon metabolism and suture pull-out strength after simple tendon suture in a tissue culture model. One hundred and twelve flexor digitorum profundus tendons from 28 dogs were cultured for 7, 14, or 21 days with or without a static tensile load. In both groups increased levels of matrix metalloproteinase (MMP) mRNA was noted. Suture pull-out strength did not decrease during tissue culture. While the presence of a static load had no effect on the pull-out strength, it did affect MMP mRNA expression. This tissue culture model could be useful in studying the effect of factors on the tendon-suture interface.

Flexor tendon repair remains one of the more difficult technical tasks facing the hand surgeon. A good repair must be both strong and able to glide smoothly through the tendon sheath. The purpose of this study is to present a model that allows surgeons to improve their technique of flexor tendon repair by receiving feedback on these important biomechanical parameters. The set-up requires testing equipment found in most biomechanical laboratories and should be available in many academic medical centres. Preliminary data suggest that receiving feedback about the strength and smoothness of a flexor tendon repair may be a very useful tool in helping surgeons improve the overall quality of their tendon repair technique.

Concavity-compression is an important mechanism for keeping a joint centered despite a large range of motion. The purpose of this study was to explain how the results of a test measuring the joint intrinsic stability can be interpreted and related to joint architecture. As an example, the method was demonstrated for the glenohumeral joint although the versatility allows any articulating joint, whether natural or prosthetic, to be analyzed. The initial slope from the central point was relatively steep, indicating a large resistance to translation. The peak translation force occurred within the first 5 mm of displacement for the glenohumeral joint, indicating a high congruence between the humerus and glenoid surfaces. Stability ratio, calculated as the maximum translation force divided by the applied joint compressive force, makes it possible to compare the stabilizing effect under different compressive loads for different anatomical directions. In hanging arm position, the joint stability ratio ranged from 30.5% to 60.1%. Finally, the effective depth of the concavity and the maximum range of joint translation can be measured by completely dislocating the joint. For the glenohumeral specimen, the smallest glenoid concavity depth, 3.8 mm, occurred in the anterior direction. The joint translation limit was smallest in the anterior-posterior direction (28.0 mm). The methodology presented in this study will allow consistent testing parameters between different trials, easily allowing parametric studies to gain a more complete understanding of articular joints.

Poly (Propylene Fumarate) (PPF), a novel, bulk erosion, biodegradable polymer, has been shown to have osteoconductive effects in vivo when used as a bone regeneration scaffold (Peter, S. J., Suggs, L. J., Yaszemski, M. J., Engel, P. S., and Mikos, A. J., 1999, J. Biomater. Sci. Polym. Ed., 10, pp. 363–373). The material properties of the polymer allow it to be injected into irregularly shaped voids in vivo and provide mechanical stability as well as function as a bone regeneration scaffold. We fabricated a series of biomaterial composites, comprised of varying quantities of PPF, NaCl and β-tricalcium phosphate (β-TCP), into the shape of right circular cylinders and tested the mechanical properties in four-point bending and compression. The mean modulus of elasticity in compression Ec was 1204.2 MPa (SD 32.2) and the mean modulus of elasticity in bending Eb was 1274.7 MPa (SD 125.7). All of the moduli were on the order of magnitude of trabecular bone. Changing the level of NaCl from 20 to 40 percent, by mass, did not decrease Ec and Eb significantly, but did decrease bending and compressive strength significantly. Increasing the β-TCP from 0.25 g/g PPF to 0.5 g/g PPF increased all of the measured mechanical properties of PPF/NVP composites. These results indicate that this biodegradable polymer composite is an attractive candidate for use as a replacement scaffold for trabecular bone. [S0148-0731(00)01203-6]