Michael Schütz

Locked internal fixator: sensitivity of screw/plate stability to the correct insertion angle of the screw.

Objective: Internal fixators with angular stability have been developed to provide high stability without compression of the plate on to the bone. Angular and axial stability of a plate–screw construct can be achieved using a conically threaded screw head undersurface and a corresponding conically threaded plate hole. Furthermore, the insertion angle of the screw must correspond precisely to the axis of the screw hole. This is not always achieved in clinical practice and may result in screw loosening. The objective of this study was to examine the relationship between the stability of the locked screw-plate on the insertion angle of the screw. Methods: Locking screws were inserted in an isolated (Point Contact Fixator, PC-Fix) or combined (Locking Compression Plate, LCP 4.5) locking hole with the use of an aiming device. The optimal insertion angle for these plates is perpendicular to the plate surface. The screws were inserted with an axis deviation of 0° (optimal condition), 5°, and 10° respective to the optimal angle (variance ± 1°). The samples were tested under shear or axial (push out) loading conditions until failure occurred. An Instron materials testing machine was used. Results: Locking screws inserted in the isolated locking hole (PC-Fix) showed a significant decrease of failure load if inserted at 5° and 10° angle. Using an optimal insertion angle (0°), failure load was 1480 ± 390 N, with 5° axis deviation 780 ± 160 N, P = 0.0001, and with 10° axis deviation 550 ± 110 N, P = 0.0001. Screws inserted in the combined locking hole (LCP) also showed a significant decrease of push-out force of 77% (4960 ± 1000 N versus 1120 ± 400 N) with 10° axis deviation. Compared to optimal insertion angle (0°), bending load to failure did decrease up to 69% (1240 ± 210 N vs. 390 ± 100 N) with 10° axis deviation. Conclusion: A locking head screw exhibits high stability with a moderate axis deviation in the angle of insertion of up to 5°. However, there is a significant decrease in stability with increasing axis deviation (>5°). An aiming device is recommended to provide optimal fixation with angular stability.

Ovine bone- and marrow-derived progenitor cells and their potential for scaffold-based bone tissue engineering applications in vitro and in vivo.

Recently, research has focused on bone marrow derived multipotent mesenchymal precursor cells (MPC) and osteoblasts (OB) for clinical use in bone engineering. Prior to clinical application, cell based treatment concepts need to be evaluated in preclinical, large animal models. Sheep in particular are considered a valid model for orthopaedic and trauma related research. However, only sheep aged > 6 years show secondary osteon formation characteristic of human bone. Osteogenic cells isolated from animals of this age group remain poorly characterized. In the present study, ex vivo expanded MPC isolated from ovine bone marrow proliferated at a higher rate than OB derived from tibial compact bone as assessed in standard 2D cultures. MPC expressed the respective phenotypic profile typical for different mesenchymal cell populations (CD14−/CD31−/CD45−/CD29+/CD44+/CD166+) and showed a multilineage differentiation potential. When compared to OB, MPC had a higher mineralization potential under standard osteogenic culture conditions and expressed typical bone related markers such as osteocalcin, osteonectin and type I collagen at the mRNA and protein level. After 4 weeks in 3D culture, MPC constructs demonstrated higher cell density and mineralization, whilst cell viability on the scaffolds was assessed > 90%. Cells displayed a spindle‐like morphology and formed interconnected networks. In contrast, when implanted subcutaneously into NOD/SCID mice, MPC presented a lower osteogenic potential than OB. In summary, this study provides a detailed characterisation of ovine MPC and OB from a bone engineering perspective and suggests that MPC and OB provide promising means for future bone disease related treatment applications. Copyright

Treatment of long bone defects and non-unions: from research to clinical practice

The treatment of long bone defects and non-unions is still a major clinical and socio-economical problem. In addition to the non-operative therapeutic options, such as the application of various forms of electricity, extracorporeal shock wave therapy and ultrasound therapy, which are still in clinical use, several operative treatment methods are available. No consensus guidelines are available and the treatments of such defects differ greatly. Therefore, clinicians and researchers are presently investigating ways to treat large bone defects based on tissue engineering approaches. Tissue engineering strategies for bone regeneration seem to be a promising option in regenerative medicine. Several in vitro and in vivo studies in small and large animal models have been conducted to establish the efficiency of various tissue engineering approaches. Neverthelsss, the literature still lacks controlled studies that compare the different clinical treatment strategies currently in use. However, based on the results obtained so far in diverse animal studies, bone tissue engineering approaches need further validation in more clinically relevant animal models and in clinical pilot studies for the translation of bone tissue engineering approaches into clinical practice.

Fit assessment of anatomic plates for the distal medial tibia.

Objectives: With the development and popularization of minimally invasive surgical methods and implants for fracture fixation, it is increasingly important that the available implants are precontoured to the specific anatomic location for which they are designed. The objective of this study was to develop a noninvasive method and criteria for quantifying the fit of a distal periarticular medial tibia plate and to test the method on a small set of tibia models. Methods: The undersurface of the plate was extracted from a digital model of the plate. The surface of the plate was fitted to 21 computer tomography (CT)-based 3-dimensional (3-D) models of human tibiae. Four criteria were defined that constitute an anatomic plate fit and subsequently were applied for the quantitative fit assessment. The fitting of the plate undersurface to the bone was entirely conducted in a virtual environment. Results: An anatomic fit of the plate was achieved for 4 of the models (19%). The individual categories generated fits of 62% (n = 13) for the proximal end; 43% (n = 9) for the proximal angle; 57% (n = 12) for the middle distance; and 57% (n = 12) for a distal fit. Conclusions: Although for the 4 individual criteria plate fits of 43%-62% were achieved, a global/anatomic fit only occurred for 19% of the bone models. This outcome is likely a result of bone morphology variations, which exist in a random population sample combined with the effects of a nonoptimized plate shape. Recommendations for optimizing the fit of the plate are discussed.

Fit optimisation of a distal medial tibia plate

An iterative method for the fit optimisation of a pre-contoured fracture fixation plate for a given bone data set is presented. Both plate shape optimisation and plate fit quantification are conducted in a virtual environment utilising computer graphical methods and 3D bone and plate models. Two optimised shapes of the undersurface of an existing distal medial tibia plate were generated based on a dataset of 45 3D bone models reconstructed from computed tomography image data of Japanese tibiae. The existing plate shape achieved an anatomical fit on 13% of tibiae from the dataset. Modified plate 1 achieved an anatomical fit for 42% and modified plate 2 a fit for 67% of the bones. If either modified plate 1 or plate 2 is used, then the anatomical fit can be increased to 82% for the same dataset. Issues pertaining to any further improvement in plate fit/shape are discussed.

Can the contra-lateral limb be used as a control with respect to analyses of bone remodelling?

Bone loss may result from remodelling initiated by implant stress protection. Quantifying remodelling requires bone density distributions which can be obtained from computed tomography scans. Pre-operative scans of large animals however are rarely possible. This study aimed to determine if the contra-lateral bone is a suitable control for the purpose of quantifying bone remodelling. CT scans of 8 pairs of ovine tibia were used to determine the likeness of left and right bones. The deviation between the outer surfaces of the bone pairs was used to quantify geometric similarity. The density differences were determined by dividing the bones into discrete volumes along the shaft of the tibia. Density differences were also determined for fractured and contra-lateral bone pairs to determine the magnitude of implant related remodelling. Left and right ovine tibiae were found to have a high degree of similarity with differences of less than 1.0mm in the outer surface deviation and density difference of less than 5% in over 90% of the shaft region. The density differences (10-40%) as a result of implant related bone remodelling were greater than left-right differences. Therefore, for the purpose of quantifying bone remodelling in sheep, the contra-lateral tibia may be considered an alternative to a pre-operative control.

Anatomical MR imaging of long bones: Comparative performance of MRI at 1.5 T and 3 T

The current gold standard for the design of orthopaedic implants is 3D models of long bones obtained using computed tomography (CT). However, high-resolution CT imaging involves high radiation exposure, which limits its use in healthy human volunteers. Magnetic resonance imaging (MRI) is an attractive alternative for the scanning of healthy human volunteers for research purposes. Current limitations of MRI include difficulties of tissue segmentation within joints and long scanning times. In this work, we explore the possibility of overcoming these limitations through the use of MRI scanners operating at a higher field strength. We quantitatively compare the quality of anatomical MR images of long bones obtained at 1.5 T and 3 T and optimise the scanning protocol of 3 T MRI. FLASH images of the right leg of five human volunteers acquired at 1.5 T and 3 T were compared in terms of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The comparison showed a relatively high CNR and SNR at 3 T for most regions of the femur and tibia, with the exception of the distal diaphyseal region of the femur and the mid diaphyseal region of the tibia. This was accompanied by an ~65% increase in the longitudinal spin relaxation time (T1) of the muscle at 3 T compared to 1.5 T. The results suggest that MRI at 3 T may be able to enhance the segmentability and potentially improve the accuracy of 3D anatomical models of long bones, compared to 1.5 T. We discuss how the total imaging times at 3 T can be kept short while maximising the CNR and SNR of the images obtained.