Biomechanical test protocols to detect minor injury effects in intervertebral discs.

Intervertebral discs (IVDs) maintain flexibility of the spine and bear mechanical load. Annulus fibrosus (AF) defects are associated with IVD degeneration and herniation which disrupt biomechanical function and can cause pain. AF puncture injuries can induce IVD degeneration but are needed to inject therapies. Identifying small AF defects with biomechanical testing can be difficult because IVDs have a complex, composite structure and nonlinear biomechanical properties that are dependent on AF fiber tension. It remains unclear how choice of biomechanical testing protocols affect the sensitivity of biomechanical properties to AF injuries. This study determined whether axial preload or magnitude of cyclic axial or torsional testing affected the ability to detect minor AF defects in rat caudal motion segments using ex vivo biomechanical testing. Intact and injured motion segments were subjected to a repeated measures study design with multiple biomechanical testing protocols that varied axial tension-compression force amplitude (±1.6 N, ±8.0 N, ±16.0 N), axial preload (-1.6 N, -8.0 N, -16.0 N, corresponding to -0.1 MPa, -0.5 MPa, and -1.0 MPa, respectively), and torsional rotation angle (±10°, ±15°, and ±20°). Biomechanical properties obtained from the lowest force testing conditions for axial tension-compression (±1.6 N), axial preload (-1.6 N), and angular rotation (±10°) exhibited the largest differences in biomechanical properties between intact and injured conditions. Biomechanical properties determined under low axial force or torsion amplitudes involve less AF fiber tension and were most sensitive to injury. Low force testing protocols are recommended for detecting minor structural AF defects and may enable more precise assessments of IVD injuries, healing or repair.

Biomechanical study of cross-locked cruciate versus Strickland flexor tendon repair.

PURPOSE: Zone II flexor tendon repairs may create a bulging effect with resistance to tendon gliding. A biomechanical study was performed comparing the 4-strand cross-locked cruciate (CLC) to a 4-strand Strickland repair, both with and without an interlocking horizontal mattress (IHM) suture, in terms of strength characteristics and work of flexion. METHODS: Sixteen fresh-frozen human fingers were placed in a custom jig. Flexor digitorum profundus tendons were sectioned at the A3 pulley level. Fingers were separated into 2 repair groups: 4-strand CLC and 4-strand Strickland core suture. Work of flexion was determined for each group, with and without an IHM circumferential suture. Final repair including IHM was tested for 2-mm gap failure and ultimate load to failure. RESULTS: The CLC-IHM had a significantly smaller increase in work of flexion than the Strickland-IHM. For both suture types, the circumferential suture resulted in a statistically significant increase in work of flexion; however, peak entry force produced upon entry of the repair into the A2 pulley was reduced, although the decrease was not statistically significant for each group. The CLC-IHM had a significantly higher ultimate load to failure. CONCLUSIONS: (1) The CLC-IHM suture method is stronger with less work of flexion than the Strickland-IHM method. (2) This new, combination repair method of CLC core suture with IHM circumferential suture is biomechanically superior to the commonly performed Strickland-IHM technique.

Genetic variation in the patterns of skeletal progenitor cell differentiation and progression during endochondral bone formation affects the rate of fracture healing.

These studies examined how genetic differences that regulate architectural and bone material properties would be expressed during fracture healing and determine whether any of these features would affect rates of healing as defined by regain of strength. Controlled fractures were generated in three inbred strains of mice: A/J, C57Bl/6J (B6), and C3H/HeJ (C3H). Both the A/J and B6 strains showed faster healing than the C3H strain based on regains in strength and stiffness. Strain-specific architectural features such as moment of inertia, cross-sectional area, and cortical thickness were all recapitulated during the development of the callus tissues. None of these traits were directly relatable to rates of fracture healing. However, rates of healing were related to variations in the temporal patterns of chondrogenic and osteogenic lineage development. The B6 strain expressed the highest percentage of cartilage gene products and had the longest period of chondrocyte maturation and hypertrophy. The slowest healing strain (C3H) had the shortest period of chondrogenic development and earliest initiation of osteogenic development. Although the A/J strain showed an almost identical pattern of chondrogenic development as the C3H strain, A/J initiated osteogenic development several days later than C3H during fracture healing. Long bone growth plates at 28 days after birth showed similar strain-specific variation in cartilage tissue development as seen in fracture healing. Thus, the B6 strain had the largest growth plate heights, cell numbers per column, and the largest cell size, whereas the C3H columns were the shortest, had the smallest number of cells per column, and showed the smallest cell sizes. These results show that (1) different strains of mice express variations of skeletal stem cell lineage differentiation and (2) that these variations affect the rate of fracture healing.