Jan-Erik Hoffmann

Mechanical induction of critically delayed bone healing in sheep: Radiological and biomechanical results

This study aimed to mechanically produce a standardized ovine model for a critically delayed bone union. A tibial osteotomy was stabilized with either a rigid (group I) or mechanically critical (group II) external fixator in sheep. Interfragmentary movements and ground reaction forces were monitored throughout the healing period of 9 weeks. After sacrifice at 6 weeks, 9 weeks and 6 months, radiographs were taken and the tibiae were examined mechanically. Interfragmentary movements were considerably larger in group II throughout the healing period. Unlike group I, the operated limb in group II did not return to full weight bearing during the treatment period. Radiographic and mechanical observations showed significantly inferior bone healing in group II at 6 and 9 weeks compared to group I. After 6 months, five sheep treated with the critical fixator showed radiological bridging of the osteotomy, but the biomechanical strength of the repair was still inferior to group I at 9 weeks. The remaining three animals had even developed a hypertrophic non-union. In this study, mechanical instability was employed to induce a critically delayed healing model in sheep. In some cases, this approach even led to the development of a hypertrophic non-union. The mechanical induction of critical bone healing using an external fixation device is a reasonable attempt to investigate the patho-physiological healing cascade without suffering from any biological intervention. Therefore, the presented ovine model provides the basis for a comparative evaluation of mechanisms controlling delayed and standard bone healing.

Effect of Mechanical Stimulation on Osteoblast- and Osteoclast-Like Cells in vitro

Bone-forming osteoblasts and bone-resorbing osteoclasts play an important role during maintenance, adaptation and healing of bone, and both cell types are influenced by physical activity. The aim of the present study was to investigate the effect of a narrow mechanical stimulation window on osteoblast- and osteoclast-like cells. Primary human cells were cultured on a bone-like structure (dentine) and three-point bending with approximately 1,100 microstrain was applied to the dentine at varying frequencies (0.1 and 0.3 Hz) and duration (1, 3 and 5 min daily over 5 days) resulting in different patterns of mechanical stimulation of osteoblast- and osteoclast-like cells. The longest stimulation (5 min at 0.1 Hz) induced a significant increase in osteoblast alkaline phosphatase activity and a significant decrease in osteoprotegerin (OPG) production, and resulted in a significant increase in the soluble receptor activator of NF-κB ligand (sRANKL)/OPG ratio towards sRANKL in comparison to the unstimulated osteoblast-like cells. All stimulations caused a significant decrease in collagen type 1 synthesis. Stimulation for 1 min at 0.3 Hz decreased the fusion and resorption activity of the osteoclast-like cells. These results demonstrate a direct effect of mechanical stimuli on osteoblast-like cells as well as on osteoclast formation and activity in vitro. The change in the sRANKL/OPG ratio towards the stimulation of osteoclastogenesis stresses the necessity to investigate the effect of the same stimulation parameter on the co-culture of both cell types.

Bewertung der stabilität von frakturfixationssystemen: mechanische vorrichtung zur untersuchung der 3-D-steifigkeit in vitro.

Different fixation systems are used for fracture and defect treatment. A prerequisite for complication free healing is sufficient mechanical stability of the osteosynthesis. In vitro investigations offer the possibility of both analysing and assessing the pre-clinical fixation stability. Due to the complex loading environment in vivo, stiffness analysis should include a complete determination of the stiffness under standardised conditions. Based on a mathematical procedure to calculate the 3-D stiffness, a mechanical testing device for the 3-D loading of fixation systems was designed and integrated in the existing test set-up. The set-up consisted of a material testing machine to produce the necessary loads and an optical measurement device to detect the resulting inter-fragmentary movements. To validate the testing device, the 3-D stiffness matrices of different Ilizarov fixator configurations were determined and compared. The good reproducibility of the test was reflected in the small intra-individual variability of the stiffness components. A distinct direction dependence of the fixator stiffness was observed. Increasing the number of rings led to a stiffness increase of up to 50%, especially in bending. The presented testing device allows a complete standardised determination of the stiffness of different fixation systems. It considers the direction dependence of the stiffness and creates a prerequisite for a more direct implant comparison.

Regulation of the patellofemoral contact area: An essential mechanism in patellofemoral joint mechanics?

Although the relationship between contact area and pressure under physiological loading has been described in the feline patellofemoral joint, this interaction has only been examined under simplified loading conditions and/or considerably lower forces than those occurring during demanding activities in humans. We hypothesized that patellofemoral contact area increases non-linearly under an increasing joint reaction force to regulate patellofemoral pressure. Eight human cadaveric knees were ramp loaded with muscle forces representative of the stance phase of stair climbing at 30° knee flexion. Continuous pressure data were acquired with a pressure sensitive film that was positioned within the patellofemoral joint. While pressure was linearly dependent upon the resulting joint reaction force, contact area asymptotically approached a maximum value and reached 95% of this maximum at patellofemoral forces of 349-723N (95% CI). Our findings indicate that the regulatory influence of increasing contact area to protect against high patellofemoral pressure is exhausted at relatively low loads.

A comparison of techniques for fixation of the quadriceps muscle–tendon complex for in vitro biomechanical testing of the knee joint in sheep

Whilst in vitro testing can contribute to a better understanding of the biomechanical interactions at the knee joint, the application of physiological-like muscle forces in vitro remains challenging. One main difficulty seems to be the adequate fixation of the muscle-tendon complex to the mechanical apparatus that provides the forces in vitro. The goal of this study was to compare the ability of different muscle-tendon fixation mechanisms, including a new technique developed to optimise the interface grip of the soft tissues, to reliably transmit physiological in vivo loads through the muscle-tendon complex to the attached bone. The fixations of three quadriceps components in 16 right knees of skeletally mature female merino sheep were loaded to failure using four different fixation techniques (aluminium clamp, freeze clamp, suture technique and a new extension hull technique). Each technique was tested 12 times: 4 times on each individual quadriceps component. A factorial analysis for repeated measurements was undertaken to examine differences between the different fixation techniques. The extension hull technique and the aluminium clamp performed similarly, exceeding the computationally determined physiological forces in all but one trial and achieved higher failure loads than the suture technique. Although the freeze clamp reached the highest mean load to failure, it also failed more often than the extension hull technique. This comparison of the fixation techniques suggests that the new extension hull technique is a suitable fixation method for applying physiological-like muscle loading in an in vitro set-up. It cannot only be handled in a very simple manner, but also possesses a compact, lightweight construction, providing the possibility for the application of more complex loading conditions that include, e.g. the action of multiple muscles of the knee flexor and extensor group concurrently.