Guido Schröder, Benjamin Jabke, Marko Schulze, Andreas Wree, Heiner Martin, Olga Sahmel, Alexandra Doerell, Claus Maximilian Kullen, Reimer Andresen, Hans-Christof Schober. Anat Cell Biol -0001;0:. https://doi.org/10.5115/acb.20.269

Background and objectives We hypothesized that different injection techniques and volumes in thoracic-paravertebral blocks (TPVB) lead to different patterns of dye spread. In particular, we investigated whether an alternating injection technique leads to complete staining of all adjacent intercostal nerves. Methods This comparative anatomical investigation was performed using 10 or 20 mL of dye (Alcian Blue) in 10 unfixed donor cadavers (54 injections) that were designated for education or research purposes. Results In landmark-guided TPVB, the thoracic-paravertebral space (TPVS) was either not stained at all (spread of dye in the paraspinal muscles, n=3) or the dye was predominantly found in the epidural space (n=3). In ultrasound-guided TPVB, the TPVS was correctly identified in all cases (n=48). The sympathetic trunk was stained in 84.6% of injections (multi-injection technique: 100%), independent of injection technique and volume. The epidural space was stained more frequently (p≤0.001) if both the puncture site (sagittal transducer position) and guidance of the needle were more medial (77.8%). Finally, a higher injection volume (20 vs 10 mL) resulted in a higher number of stained intercostal nerves (p=0.04). Conclusion For ultrasound-guided techniques, a higher injection volume resulted in a larger number of stained intercostal nerves. Staining of the sympathetic trunk was independent of the injection technique. Epidural spread was observed significantly less frequently if the injection was lateral (transducer transversal) or with a strictly cranial injection direction (transducer sagittal). Landmark-guided injections reliably achieved the TPVS (and the epidural space) only after a needle advance of 2.5 cm after initial contact with the transverse process.

Bone specimens obtained for biomechanical experiments are fresh-frozen for storage to slow down tissue degradation and autolysis in long-term storage. Alternatively, due to infectious risks related to the fresh tissues, fixative agents are commonly used. However, fixatives will likely change the mechanical properties of bone. Existing studies on this issue gave controversial results that are hardly comparable due to a variety of measurement approaches. For this reason, the influence of ethanol and a formalin-based fixative agent was evaluated on the mechanical properties of human cortical bone specimens by means of four-point-bending tests. 127 prismatic specimens with rectangular cross sections (2.5 x 2.5 x 20 mm³) were obtained from different regions of two fresh human femora (medial, lateral, dorsal, ventral). Specimens were either fixed in ethanol or in a mixed formalin solution or frozen following a given scheme. After two weeks of storage the samples were re-hydrated in isotonic saline and subsequently tested mechanically. The elastic bending modulus and ultimate bending strength were computed considering the actual dimensions of each specific specimen. For statistical analysis a one-way-ANOVA and an LSD post-hoc-test were performed. For ultimate bending strength no significant differences due to formalin or ethanol fixation, as compared to unfixed-fresh bone specimens could be found. And only for few cases significant differences in elastic bending modulus were observed when the two bones were evaluated separately. Since more differences of significant level due to the anatomical region of the samples were determined, the original location seems to have more influence on the evaluated mechanical properties than the method of (chemical) fixation. Consequently, ethanol and the mixed formalin solution can be recommended as a fixation agent for samples in biomechanical testing, if these samples are rinsed in isotonic saline prior to static mechanical testing.

One main disadvantage of commercially available allogenic bone substitute materials is the altered mechanical behavior due to applied material processing, including sterilization methods like thermal processing or gamma irradiation. The use of high hydrostatic pressure (HHP) might be a gentle alternative to avoid mechanical alteration. Therefore, we compressed ground trabecular human bone to granules and, afterwards, treated them with 250 and 300 MPa for 20 and 30 min respectively. We characterized the formed bone granule cylinders (BGC) with respect to their biomechanical properties by evaluating stiffness and stress at 15% strain. Furthermore, the stiffness and yield strength of HHP-treated and native human trabecular bone cylinders (TBC) as control were evaluated. The mechanical properties of native vs. HHP-treated TBCs as well as HHP-treated vs. untreated BGCs did not differ, independent of the applied HHP magnitude and duration. Our study suggests HHP treatment as a suitable alternative to current processing techniques for allogenic bone substitutes since no negative effects on mechanical properties occurred.