The course of bone healing is influenced by the initial shear fixation stability.

Fracture healing is influenced by fixation stability and experimental evidence suggests that the initial mechanical conditions may determine the healing outcome. We hypothesised that mechanical conditions influence not only the healing outcome, but also the early phase of fracture healing. Additionally, it was hypothesised that decreased fixation stability characterised by an increased shear interfragmentary movement results in a delay in healing. Sixty-four sheep underwent a mid-shaft tibial osteotomy which was treated with either a rigid or a semi-rigid external fixator. Animals were sacrificed at 2, 3, 6 and 9 weeks postoperatively and the fracture callus was analysed using radiological, biomechanical and histological techniques. The tibia treated with semi-rigid fixation showed inferior callus stiffness and quality after 6 weeks. At 9 weeks, the calluses were no longer distinguishable in their mechanical competence. The calluses at 9 weeks produced under rigid fixation were smaller and consisted of a reduced fibrous tissue component. These results demonstrate that the callus formation over the course of healing differed both morphologically and in the rate of development. In this study, we provide evidence that the course of healing is influenced by the initial fixation stability. The semi-rigid fixator did not result in delayed healing, but a less optimal healing path was taken. An upper limit of stability required for successful healing remains unknown, however a limit by which healing is less optimal has been determined.

Determination of muscle loading at the hip joint for use in pre-clinical testing.

The stability of joint endoprostheses depends on the loading conditions to which the implant-bone complex is exposed. Due to a lack of appropriate muscle force data, less complex loading conditions tend to be considered in vitro. The goal of this study was to develop a load profile that better simulates the in vivo loading conditions of a "typical" total hip replacement patient and considers the interdependence of muscle and joint forces. The development of the load profile was based on a computer model of the lower extremities that has been validated against in vivo data. This model was simplified by grouping functionally similar hip muscles. Muscle and joint contact forces were computed for an average data set of up to four patients throughout walking and stair climbing. The calculated hip contact forces were compared to the average of the in vivo measured forces. The final derived load profile included the forces of up to four muscles at the instances of maximum in vivo hip joint loading during both walking and stair climbing. The hip contact forces differed by less than 10% from the peak in vivo value for a "typical" patient. The derived load profile presented here is the first that is based on validated musculoskeletal analyses and seems achievable in an in vitro test set-up. It should therefore form the basis for further standardisation of pre-clinical testing by providing a more realistic approximation of physiological loading conditions.

Cementless stem fixation and primary stability under physiological-like loads in vitro.

Primary stability and in consequence osteointegration are commonly related to the stem anchorage but also to the complex musculoskeletal loading of the hip region. This study investigated the influence of metaphyseal and meta-diaphyseal anchorage on the primary stability of cementless stems under physiological-like loading in vitro. Metaphyseal and meta-diaphyseal anchoring stems (n=6 each) were implanted into composite femora. Musculoskeletal loads, validated by in vivo data (peak joint force 2348 N), were applied using a mechanical set-up. Interface movements were recorded by seven displacement transducers and primary stability was compared. Both stems exhibited similar movement patterns and principally moved distally with a retroversional twist. Although elastic movements were comparable, the metaphyseal stem exhibited higher plastic deformations than the meta-diaphyseal stem, particularly for the metaphyseal, medio-lateral and antero-posterior components. Under physiological-like loading, the metaphyseal stem allowed higher interface movements and tended to initially migrate faster than the meta-diaphyseal stem and then stabilized. Elastic movements were comparable and seemed to be less influenced by the anchoring concept than by the mechanical properties of the bone. The analyses emphasize the importance of metaphyseal bone in proximal anchorage and the necessity of an accurate canal preparation to prevent excessive initial migration.

Comparison of unreamed nailing and external fixation of tibial diastases--mechanical conditions during healing and biological outcome.

Locked intramedullary nailing and external fixation are alternatives for the stabilization of tibial shaft fractures. The goal of this study was to determine to what extent the mechanical conditions at the fracture site influence the healing process after unreamed tibial nailing compared to external fixation. A standardized tibial diastasis was stabilized with either a locked unreamed tibial nail or a monolateral fixator in a sheep model. Interfragmentary movements and ground reaction parameters were monitored in vivo throughout the healing period. After sacrifice, the tibiae were examined mechanically and histologically. Bending angles and axial torsion at the fracture site were larger in the nail group within the first five weeks post-operatively. Unlike the fixator group, the operated limb in the nail group did not return to full weight bearing during the treatment period. Mechanical and histomorphometrical observations showed significantly inferior bone healing in the nail group compared to the fixator group. In this study, unreamed nailing of a tibial diastasis did not provide rotational stability of the osteosynthesis and resulted in a significant delay in bone healing.

Proximal humeral fractures: how stiff should an implant be? A comparative mechanical study with new implants in human specimens.

BACKGROUND: The objective of this study was to determine the in vitro characteristics of the clinically used and newly developed implants for the stabilization of proximal humeral fractures under static and cyclic loading. The goal was to optimize implant stiffness for fracture stabilization even in weak bone stock. METHODS: In a laboratory study using 35 fresh human humeri, the specimens were randomized into 5 groups, which included the clinically used humerus T-plate (HTP), the cross-screw osteosynthesis (CSO), the unreamed proximal humerus nail with spiral blade (UHN), the recently developed Synclaw Proximal Humerus Nail (Synclaw PHN) and the angle-stable Locking Compression Plate Proximal Humerus (LCP-PH). The implant stiffness was determined for three clinically relevant load cases: axial compression, torsion and varus bending. In addition, a cyclic varus-bending test was performed to determine the implant properties under cyclic loading. RESULTS: In contrast to a rather elastic and minimally invasive implant(LCP-PH), the conventionally designed ones (Synclaw PHN, CSO, HTP, UHN) showed rather high stiffness values under static loading. In cyclic loading, a strong decrease in stiffness ( p<0.05) was found for the rigid implants HTP and UHN. In comparison with the other implants, only the elastic implant (LCP-PH) showed a significantly lower load reduction in a weak bone stock (17+/-6.2%). CONCLUSION: The high initial stiffness of rigid implants led to an early loosening and failure of the implant-bone interface under cyclic loading. Implants with low stiffness and elastic characteristics, however, appear to minimize the peak stresses at the bone-implant interface, making them particularly suitable for fracture fixation in osteoporotic bone.

[Evaluating the stability of fracture fixation systems: mechanical device for evaluation of 3-D stiffness in vitro]. / 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.

[Mechanical behavior of Ilizarov ring fixators. Effect of frame parameters on stiffness and consequences for clinical use]. / Mechanisches Verhalten von Ilizarov-Ringfixateuren. Einfluss der Gestaltparameter auf die Steifigkeit und Konsequenzen für den klinischen Einsatz.

Using a mechanical testing procedure, various fixator constructs were tested in vitro. In addition, the influence of the passive soft tissue structures on the fixation stiffness was determined. An increased number of Schanz' screws or Kirschner wires led to a comparable increase in stiffness than that observed with an increasing screw or wire diameter. In consequence, larger diameters should be preferred over an additional screw or wire where clinically applicable. With diaphyseal telescoping rods only the axial stiffness decreased. As expected, large ring diameters as well as titanium wires reduced stiffness components. Bracing the outer rings caused a reduction of the overall stiffness. Asymmetric pre-tensioning of the K-wires resulted in a significant reduction of tension in the neighboring wire. Removal of the soft tissues reduced stiffness to a similar extend as experienced in a fibula defect situation. The study demonstrates the correlation between design parameters, passive soft tissues and fixation stiffness and presents guidelines for an optimized fixator design.